Scientific Papers

Authors: M. A. Hernández-Mejías, E. Rodriguez-Diaz, A. Sala, R. Blasco-Gimenez, G. Chaqúes-Herraiz, J. M. Guerrero

This paper presents an H∞ control design for a DC-grid, considering limited information of the rest of the grid. Proposed strate gy is compared with previously reported plug-and-play and droop controllers. The paper shows both PSCAD/EMTDC simulation and experimental results in a scaled prototype.

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Authors: G. Chaqués-Herraiz, S. Bernal-Perez, J. Martínez-Turégano, S. Añó-Villalba, R. Peña, R. Blasco-Gimenez

Distributed diode rectifier units have been proposed as a robust and cost efficient solution for the connection of large off-shore wind farms to HVDC links. However, since the diode rectifier platforms are connected in series with cables of varying distances, their dc-side voltage might not be the same in all diode rectifier platforms. This paper introduces a diode rectifier platform dc-voltage balancing control to cancel the aforementioned unbalanced. Balanced operation has been shown by means of detailed PSCAD simulations, considering a wide range of power generation unbalance between the wind power plants connected to the HVDC diode rectifier platforms. Moreover, the proposed balancing control has shown a very good dynamic performance.

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Authors: R. Vidal-Albalatey, R. Penaz, S. Añó-Villalba, E. Belenguery, R. Blasco-Gimenez

The connection of large off-shore wind farms using diode rectifier units presents important advantages due to the simplicity of the diode rectifier converter, its robustness and weight and loss reduction.Moreover, series connected diode rectifier units allow for increased reliability as the system is capable of reduced power operation in the case of the failure of one unit. However, a DR unit outage requires a reduced DC voltage, consequently, the use of full bridge MML power converters at the on-shore station. Such converters allow for reduced HVDC-link voltage operation when one diode rectifier unit is faulty and also help to improve transient response during faults. However, the full bridge MML is more complex than its half bridge counterpart and has higher losses. In this paper a study has been carried out to ascertain the advantages and disadvantages of using a mixed full-bridge half-bridge MML power converter for the diode rectifier connection of offshore wind farms, highlighting the operational limits of each type of MML converter under a reduced DC voltage.

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Authors: P. Ruffing, C. Brantl, M. Stumpe, A. Schnettler

Modern Modular Multilevel Converters (MMCs) equipped with full-bridge submodules are capable of controlling and interrupting DC fault currents by blocking all the converter’s submodules. The first HVDC link using this technology will be the Ultranet project in Germany. Main advantage of full-bridge blocking is the almost instantaneous DC fault current interruption. Depending on the protection strategy DC fault blocking is used as primary or backup protection. Moreover, converter blocking is usually used for internal converter protection. However, a major drawback of the standard total converter blocking is the loss of the converter’s reactive power generation capability. Especially during DC fault situations, where the active power infeed is interrupted, an additional loss of the reactive power supply can lead to further stress for the stability of the AC system. Consequently, a continuous support of reactive power to the AC system is a desirable functionality and might be a requirement set by grid operators.
Within this work a partial blocking concept for full-bridge based converters is proposed, which enables the interruption of DC fault currents without interruption of the reactive power support to the adjacent AC networks. For this purpose, only the submodules of the arms directly connected to the fault are blocked, while the other arms provide the requested reactive power to the adjacent AC grid. In electromagnetic transient simulations using PSCAD|EMTDC™ partial blocking shows a similar performance in comparison to standard blocking in regard to DC fault current interruption. With the proposed concept single conductor DC faults, as well as DC faults with multiple conductor involvement, can be interrupted, while reactive power is continuously supplied to the adjacent AC grid. Moreover, internal voltages and currents can be maintained within the converter’s limits. The concept is tested for overhead line and cable based transmission systems within this publications.

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Authors: C. Brantl, P. Ruffing, P. Tünnerhoff, R. Puffer

Multi-terminal HVDC systems based on Modular Multilevel Converters (MMCs) are envisaged for  the large scale integration of renewable energy sources worldwide. There are several system designs in discussion for theses future grids. Two of the discussed realisation aspects being the HVDC system configuration and the protection system. Existing point-to-point systems are built using the symmetrical monopole configuration. For future multi-terminal systems, bipolar configurations are in discussion as they provide redundancy in case of pole-to-ground faults. Due to the different grounding schemes associated with the system configurations, the steady state fault currents differ significantly for pole-to-ground faults. The proposed protection systems for multi-terminal systems, however, may have to identify, locate, and clear faults based on the first travelling waves within a few milliseconds. This paper therefore compares the system behaviour of a high impedance grounded monopole with a bipolar configuration with dedicated metallic return regarding a selective HVDC protection system applying HVDC circuit breakers. Based on the analysis, the differences in requirements on the protection system for the two system configurations are evaluated.

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Authors: P. Ruffing, C. Brantl, R. Puffer

Future multi-terminal DC networks are envisioned for the large-scale integration of renewable energy sources into today’s power systems. To minimise the downtime of HVDC networks and thereby reduce the impact of DC contingencies on surrounding AC networks, faulted DC lines must be separated quickly and reliably from the remaining part of the network.

Even though the fast separation of faults in HVDC networks is often associated with DC circuit breakers, protection systems based on fault blocking converters and DC high-speed switches have been proposed as a reasonable alternative for the protection of DC networks in the recent past.

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Authors: A. Bidadfar, O. Saborio-Romano, M. Altin,  E. Prieto-Araujo, O. Gomis-Bellmunt, N. A. Cutululis, P. E. Sørensen

Offshore HVDC grids are envisaged to render more reliable and economical solutions for transferring more offshore wind energy to onshore power systems. From the reliability point of view, the most promising operational/control strategy of HVDC grids is the dc voltage droop control. This control provides autonomous power-sharing among converters and promotes the frequency support to onshore power systems from other ends of HVDC grids without using fast communication links. The simultaneous use of frequency and dc voltage droops in an HVDC converter can transform frequency deviations into dc voltage changes. Other converters with DC voltage droop can react to the changes by regulating their power flow. Therefore, autonomous frequency support can be established through an HVDC grid. Although this method is reliable, fast, and economical, it gives rise to interactions between different droops, which can result in a reduction in the HVDC power that system operators can expect during frequency events. The present study investigates the effectiveness and challenges of the droop-based (communication less) control for providing frequency support in offshore HVDC grids.

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Authors: A. Bidadfar, O. Saborio-Romano, J.N. Sakamuri, M. Altin, N.A. Cutululis, P.E. Sørensen

Contribution to the power systems’ frequency support is expected to be one of the essential ancillary services that wind power plants (WPPs) shall provide. The high-voltage DC (HVDC) connected offshore WPPs may provide this service with and without using fast communication links between onshore and offshore. In the case of offshore HVDC grid, implementing the communication-less frequency support is challenging. Although it increases the reliability of the frequency control, among other challenges, it is not straightforward to comply with relevant grid code requirements. In this paper, this issue is mathematically described and a static model is developed to calculate the deviation of various electrical parameters of an HVDC grid in case of frequency drop on the land ac systems. A solution for the aforementioned problem is presented and its associated concerns are addressed. The study is verified by simulations of a four terminals dc grid with two offshore WPPs and two inland AC systems.

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Authors: C. Brantl, P. Tünnerhoff, A. Peitz, A. Schnettler

Multi-terminal HVDC systems have been proposed as a promising solution to enable the large-scale integration of offshore wind power plants. Due to the envisaged amount of installed generation capacity in the range of several gigawatts, a high availability of these DC grids is required, as a loss of power transfer from the HVDC grid to the AC grid could endanger the overall system stability. Consequently, a fast, and secure fault handling strategy for faults in the DC grid is needed. So far, no standard DC grid protection system is available for HVDC grids. Several fault clearing strategies have been proposed, with most publications focussing on selectivity and breaker design and stresses for the different fault clearing strategies. This paper shifts the focus to the power flow recovery after fault clearing to identify recovery times and the main impact factors on it. To this end, a selective fault clearing strategy based on HVDC circuit breakers is evaluated in PSCAD | EMTDC for varying fault scenarios and breaker opening times with regard to the recovery of the DC voltage and the active power transfer to the AC side. The results indicate, that fast fault clearing leads to a faster recovery, where changes in the range of few milliseconds in the fault clearing may have a huge impact on the power flow deviations.

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Authors: C. Brantl, P. Ruffing, P. Tünnerhoff, R. Puffer

The introduction of an increasing number of converters in the existing AC systems has led to a series of challenges. One of them is securing the stability of the existing AC grids in case of faults. This work reviews existing contributions on the impact of modular multilevel converters (MMC) on the AC distance protection, especially on its second zone. Based on this analysis, study cases are derived to compare the impact of an MMC on the distance protection to other impact factors. Thereafter, a differentiation between the factors resulting from the chosen system configuration and the impact of the MMC is given. The results show that there is an impact of the MMC on the second zone due to the given system topology. However, the impact of the MMC on the impedance measurement does not exceed the variation due to other impact factors, because of its limited fault current contribution capability. Depending on the previously existing system conditions, the impact of an MMC inserted as intermediate infeed into an existing line is negligible in a meshed AC grid due to its limited short circuit current contribution.

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Authors: G.P.Adam, MD Rahman, Rui Li & Lie Xu

In the last decade, a number of academic and industry studies have identified diode rectifier unit (DRU) as a potential replacement for offshore modular multilevel converter (MMC) of DC connected offshore windfarms. However, side-by-side operation of DRU and MMC connected windfarms in multi-terminal DC grid will present new operational challenges. Therefore, this paper will study the interoperability of a minimum meshed DC grid, which includes MMC and DRU connected offshore windfarms. To identify any potential issues that may arise from introduction of DRU, the system performance during onshore AC faults are simulated using PSCAD models. Simulation results show that the DRU connected windfarm exhibits different behaviours with the MMC based equivalent, but does not adversely impact the DC grid performance. Instead, the use of DRU improves DC grid performance with its inherent sensitivity of active power transmission to DC voltage variation.

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Authors: C. Brantl, P. Ruffing, R. Puffer

The increase in the number of grid connected converters along with the decommissioning of conventional power generation integrated via synchronous generators will lead to a change of the transmission system behaviour under faults. This paper reviews the possible behaviour of Modular Multilevel Converters (MMCs) under AC faults, considering different grid conditions and control objectives as well as the resulting impact on the line protection functionality. One main result is that typical distance protection relays will mal-operate when installed at radial line ends terminated by MMCs due to the different short circuit powers of the MMC and the grid, the flexible phase angle of the MMCs AC side current and the changed current profiles under asymmetrical faults compared to HVAC systems fed by synchronous generators. For cases in which the MMC is connected at busbars with multiple outgoing lines, its impact depends on the short circuit power of the connected nodes. However, even for low short circuit powers of the surrounding grid, both distance and differential protection work reliably as the MMC does not significantly change the characteristic behaviour of the AC transmission grid.

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Authors: Philipp Ruffing , Christina Brantl, Cora Petino, Armin Schnettler

Within the framework of modernisation of the European electricity grid, multi-terminal high-voltage direct current (HVDC) offshore grids shall be integrated into future transmission systems. An essential aspect of multi-terminal HVDC systems is fast and selective DC-side fault handling and the separation of faulty lines. This study investigates the applicability of different control methods relying on full-bridge-based converters in combination with high-speed switches for a fast and selective separation of faulty line segments in a multi-terminal HVDC cable system in a symmetrical monopole configuration. It is shown that the proposed line current control method can significantly reduce the separation time of a faulty line compared with standard fault control methods. The analysis is based on simulations in PSCAD|EMTDC™ with a converter model based on the CIGRÉ WG B4.57, which is enhanced for the use of full-bridge converters with fault current control schemes.

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Authors: R. Li, L. Yu, and L. Xu,

Offshore ac fault protection of wind turbines (WTs) connecting with diode rectifier unit-based highvoltage dc (DRU-HVdc) system is investigated in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during offshore ac fault transients and quickly provide fault current for fault detection. Considering different fault locations, the fault
characteristics during symmetrical and asymmetrical faults are presented and the requirements for fault detection are addressed. A simple and effective offshore ac fault protection solution, combining both overcurrent protection and differential protection, is proposed by utilizing the
developed fast fault current providing control. To improve system availability, reduced dc voltage of the DRU-HVdc system is investigated, where one of the series-connected DRUs is disconnected and the onshore modular multilevel converter actively reduces dc voltage to resume wind
power transmission. The proposed scheme is robust to various offshore ac faults and can automatically restore normal operation. Simulation results confirm the proposed fault protection strategy.

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Authors: R. Li, L. Yu, and L. Xu,

Parallel operation of diode rectifier based high-voltage direct current (DR-HVDC) and modular multilevel converter (MMC) based HVDC (MMC-HVDC) for transmitting offshore wind energy is investigated in this paper. An enhanced active power control scheme of the offshore MMC station is proposed to improve the power flow distribution between the MMC-HVDC and DR-HVDC links which are both connected to the offshore wind farm AC network. By regulating the offshore voltage, all the wind powers are transmitted via the DR-HVDC link in low wind conditions while the offshore MMC power is controlled around zero to reduce transmission losses, considering the efficiency superiority of DR-HVDC over its MMC counterpart. When the DR-HVDC is out of service, wind energy is transferred via the MMC-HVDC and the wind turbine generated power is automatically limited by slightly increasing the offshore AC voltage to avoid potential MMC-HVDC overload. A power curtailment control is also proposed which slightly increases the DC voltage of the DR-HVDC to enable autonomous reduction of the generated wind power so as to avoid DR-HVDC overload during MMC-HVDC outage. The proposed coordinated control only uses local measurements and, without the need for communication, can seamlessly handle transitions including various faults. The proposed scheme enables fault ride-through operation and provides a high efficient solution with flexible operation for integrating large offshore wind farms. Simulation results confirm the proposed control strategy.

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Authors: L. Shi, G. P. Adam

The adoptions of medium-voltage in AC collection networks of large DC connected wind farms significantly increase the AC current magnitudes during normal and fault conditions. Controlling fault currents at zero during asymmetric AC faults is possible, but it has several drawbacks such as increased risk of protection mal-operation due to absence of fault currents, which also tends to prevent the recovery of AC voltage in post-fault. Therefore, this paper presents an enhanced control strategy that exploits the induced negative sequence voltages to facilitate controlled injection of negative sequence currents during asymmetric AC faults. The proposed control not only defines a safe level of fault current in the offshore AC network but also instigates immediate recovery of the AC voltage following clearance of AC faults, which avoid protection mal-operation. In addition, the positive sequence voltage set-point of the offshore modular multilevel converter (MMC) is actively controlled by considering the negative and zero sequence voltages, which effectively avoids the excessive overvoltage in the healthy phases during asymmetrical AC faults. The theoretical basis of the proposed control scheme is described, and its technical viability is assessed using simulations.

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Authors: Lujie Yu, Rui Li, and Lie Xu

This paper investigates the operation of offshore wind farm connected by parallel diode-rectifier based HVDC (DR-HVDC) and HVAC links. A secondary voltage control is proposed to control the offshore AC voltage amplitude by regulating the DC voltage of the DR-HVDC link. A secondary frequency control and a phase angle control are proposed to adjust the reactive power reference in the primary control, which synchronise the offshore point of common coupling (PCC) frequency and phase angle to those of the HVAC link. Such secondary voltage control, frequency control and phase angle control enable seamless transition from DR-HVDC mode to parallel mode. A tertiary power control scheme is further proposed to control the active power flow distribution between DR-HVDC and HVAC links through the regulation of PCC phase angle. To ensure smooth transition from HVAC mode to parallel mode, a virtual DC power control is proposed to control the virtual DC power at zero prior to the connection of the DR-HVDC link. A small-signal model of the parallel system is developed and the stability analysis is carried out for the proposed control scheme. Simulation results in PSCAD/EMTDC verify the proposed control under normal and fault conditions.

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Authors: Lei Shi, G.P. Adam, Rui Li & Lie Xu

In recent years, several DC fault clearance schemes have emerged, in which reduced number of fast acting DC circuit breakers (DCCBs) and AC circuit breakers (ACCBs) are used to clear DC faults. In offshore DC grids, such approach entails opening of the ACCBs that connect the wind farms to the offshore HVDC stations which control offshore AC voltages and frequencies, potentially leading to uncontrolled offshore voltage and frequency. Existing studies show that the loss of offshore converter due to blocking or sudden opening of ACCBs can cause significant over-voltage and over-frequency in the offshore AC grid, which could necessitate immediate shutdown of the wind farm. An enhanced control for wind turbine converters (WTCs) of the offshore wind farm is proposed to enable retention of AC voltage and frequency control when the offshore converter is lost, in which seamless transition of the WTCs between grid following and forming modes is facilitated. The viability of the proposed control is demonstrated in wider context of partially selective DC fault protection in an illustrative meshed DC grid, which includes detailed implementations of DC fault clearance, system restart and power transfer resumption. The presented simulation results confirm the effectiveness of the proposed WTC control in preventing excessive rise of offshore AC voltage and frequency and facilitating DC fault ride-through using reduced number of DCCBs.

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Authors: R. Vidal-Albalate, R. Pena, E. Beleguer, S. Añó-Villalba, S. Bernal-Perez, R. Blasco-Gimenez

This paper introduces a control strategy whcih can be used for the connection of both type-3 and type-4 wind turbines to HVDC-Diode Rctifiert (HVDC-DR) stations. In this way, wind turbines from different technologies could effectively be conneted to the same HVDC Diode Rectifier station. The proposed strategy is verified by means of PSCAD simulations including start-up operation, optimal power tracking with changing wind conditions and off-shore ac-grid frequency reference changes. Moreover, reactive power sharing between the wind turbines is proven from the aforementioned scenarios.

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Authors: O. Saborío-Romano, A. Bidadfar, Ö. Göksu, M. Altin, N. A. Cutululis, P. E, Sørensen

This paper provides an overview of two technologies for connecting offshore wind power plants (offshore WPPs,
OWPPs) to high-voltage direct current (HVDC) networks: voltage source converters (VSCs) and diode rectifiers (DRs). Current grid code requirements for the connection of such power plants are also addressed, and their implications when using such technologies are discussed.

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Authors: R. Vidal-Albalate, P. Peña, S. Añó-Villalba, E. Belenguer, R. Blasco-Gimenez

The connection of large off-shore wind farms using diode rectifiert units presents important advantages, due to the simplicity of the diode rectifier converter, its robustness and weight and loss reduction.

Moreover, series connected diode rectifier units allow for increased reliability as the system is capable of reduced power operation in the case of failure of one unit. Diode rectifier convertres require the use of full bride on-shore MML power converters. Such converters allow for reduced HVDC-link voltage operation when one diode rectifiert unit is fauly and alos help to imporve transient response during faults.

However, the full bridge MML is more complex than its hald bridge counterpart and has higher losses. Therefore a study has been carried out to ascertain the advantages of using a mixed full-bridge half-bridge MML power converter for the diode rectifier connection of off-shore wind farms.

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Authors: Ö. Göksu, N. A. Cutululis, P. Sørensen, L. Zeni

Short-circuit faults for HVDC connected Wind Power Plants (WPPs) have been studied mostly for dc link and onshore ac    grid    faults,    while    the    offshore    ac    faults, especially
asymmetrical faults, have been mostly omitted in the literature. Requirements  related  to  the  offshore  asymmetrical  faults  have been  kept  as  future  development  at national levels in the recent ENTSO-E HVDC network code. In this paper offshore ac faults are studied   using   the   classical   power   system   fault   analysis methods.  It  is  shown  that  suppression  of  negative  sequence current flow is not applicable and negative sequence current has to  flow  during  the  asymmetrical  offshore  faults,  which  implies that  the  offshore  WPP  and  the  HVDC  offshore  converter  are required  to  provide  flow  of  negative  sequence  current.  The steady-state    fault    analysis    is    verified    with    time-domain simulations.

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Authors: O. Saborío-Romano, A. Bidadfar, Ö. Göksu, N. A. Cutululis

This paper presents a study assessing the actual capability of an offshore wind power plant  (offshore WPPs, OWPPs) to provide frequency support (FS) to an onshore network, when connected through a high-voltage direct-current (HVDC) link having a diode rectifier (DR) offshore terminal and a voltage source converter (VSC) onshore terminal. Both primary and   fast frequency response (PFR and FFR, respectively) are studied, and both the power reserves from preventive curtailment and the kinetic energy stored in the rotating mases of the wind turbines (WTs) are considered as sources of additional power during onshore under-frequency events. Three methods are considered for overloading the WTs, including the proposed External Reference method, in which the base active power reference can be set externally. The performance of the controls is studied by means of electromagnetic transient (EMT) simulations, for which an aggregated model of the OWPP is used. The results suggest that such OWPPs can in principle provide onshore FS by means of plant-level active power control strategies already  developed for OWPPs connected to HVDC via VSCs. Some of the results also suggest that it may be unnecessary to overload the WTs if active power reserves from curtailed operation are available when providing both PFR and FFR.

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Authors: Ö. Göksu, OSaborío-Romano, N. A. Cutululis, P. E. Sørensen

Current practice of wind turbines’ start-up relies on the electricity available at the network that they are connected. Hence, in case of a long duration of electricity shortage, external auxiliary power supplies are needed for starting and running the wind turbines or at least for
powering their internal auxiliary loads. The state of the art wind turbines with power electronics based converters can be equipped with functions such that they can start and run without the need for external auxiliary power supplies; and furthermore wind power plants composed of the state of the art wind turbines can form grid voltage and operate as a stiff voltage source, as long as wind blows. This can help fast and environmental friendly black start solutions by wind turbines for power system restoration and also use of cost effective offshore HVDC converters (e.g. diode rectifier) as well. Additionally, this would help to avoid  today’s necessity of auxiliary diesel generators for offshore wind power plants, which in turn  would increase reliability and decrease cost. In this paper the background and existing solutions for wind turbine and wind power plant (self) start-up and island operation are presented, while the challenges are identified as future focus areas. 

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Authors: A. Bidadfar, O.Saborío-Romano, M. Altin, Ö. Göksu, N. A. Cutululis, P. E. Sørensen

The low impedance characteristics of DC transmission lines cause the voltage source converter (VSC) in HVDC networks to become electrically closer together and increase the risk of severe interactions between the converters. Such interactions, in turn, intensify the implementation of the grid control schemes and may lead the entire system to instability. Assessing the stability and adopting complex coordinated control schemes in an HVDC grid and wind farm turbines are challenging and require a precise model of the HVDC grid, wind farm, and the controllers. In this paper, a linear multivariable feedback control system (FCS) model is proposed to represent the dynamic characteristics of HVDC grids and their controllers. The FCS model can be used for different dynamic analyses in time and frequency domains. Moreover, using the FCS model the system stability is analyzed in both open- and closed-loop forms. The standard eigenanalysis identifies the modes of only the closed-loop system and detects the pertaining state variables. The open-loop model, in the frequency domain, is a complementary tool that helps to have more intuitive insight into the system stability. A four terminal HVDC grid with two OWPPs and two AC grids is used for simulations and verification of the proposed FCS model.

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Authors: J. Martínez-Turégano, S. Añó-Villalba, G. Chaqués-Herraiz, S. Bernal-Perez, R. Blasco-Gimenez

A large number of nodes is required for the detailed simulation of large off-shore wind farms. Therefore, the corresponding electromagnetic transient simulations require large simulation times. This paper compares a detailed model of an off-shore wind farm with three clusters of 50 wind turbines each on (i.e. a total of 150 wind turbines), with a simplified model consisting of three aggregated wind farms. The paper includes the aggregation criteria so equal power flows and voltages are obtained at the wind farms points of common coupling. The control is carried out using a centralized PI controller to control total active power, and a distributed droop control for active and reactive power control of each wind turbine. The proposed aggregation method leads to significant savings on computational times, with the aggregated models being more than three hundred and fifty times faster than the detailed ones. The result have been validated by means of detailed PSCAD/EMTDC simulations.

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Authors: Ömer Göksu, Nicolaos A. Cutululis, Poul Sørensen

One of the main short-circuit analysis for the HVDC connected offshore wind power plants is the asymmetrical faults at the offshore network between the offshore HVDC converter and the wind turbines. There have been few requirements specified for these offshore asymmetrical faults, e.g. in the recent ENTSO-E HVDC network code, and few studies in the literature, focusing on the control algorithms for the HVDC converter and the wind turbines during asymmetrical faults. However, a fundamental theoretical analysis focusing on the underlying physics behind the offshore asymmetrical faults has been missing. In this paper, physical phenomenon occurring in the offshore network with the HVDC converter and the wind turbine converters during asymmetrical faults has been explored. It has been showed that the negative sequence fault current flow via the HVDC converter is strictly necessary and there is an interaction between wind turbine positive sequence fault current and the HVDC negative sequence current. The classical fault analysis approach has been utilized and complemented with time-domain simulations. The output provides understanding for the offshore asymmetrical faults and insights for related future grid code requirements and converter control design.

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Authors: O. Saborío-Romano, A. Bidadfar, J.N. Sakamuri, Ö. Göksu, N.A. Cutululis

This study assesses the capability of an OWF to provide frequency support to an onshore AC network, when connected through a HVDC link having a diode-rectifier-based (DR-based) offshore terminal and a voltage-source-converter-based (VSC-based) onshore terminal.

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Authors: R. Li, L. Yu, L. Xu and G. Philip Adam

DC fault ride-through operations of the offshore wind farm connecting with diode rectifier unit (DRU) based HVDC link are presented in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during DC faults and to provide fault current. This contributes the control of the offshore AC voltage, which does not drop to zero but is remained relatively high to facilitate fast system recovery after clearance of a temporary DC fault. The WT converters operate on current limiting mode during DC faults and automatically restore normal operation after fault clearance. The full-bridge based modular multilevel converter (MMC) is adopted as the onshore station and its DC fault current control ability is explored to effectively suppress the fault current from the onshore station around zero, which reduces semiconductor losses and potential overcurrent risk of the MMC station. Simulation results confirm the robustness of the system to DC faults.

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Authors: L. Yu, R. Li and L. Xu

This paper investigates the integration of large offshore wind farms using parallel HVAC and diode-rectifier based HVDC (DR-HVDC) systems. Three different operation modes, i.e. HVAC operation mode, DR-HVDC operation mode and parallel operation mode are investigated. A wind turbine control scheme including distributed control and centralized control is proposed to ensure the stable operation of the offshore wind farms under different operation modes. The proposed control requires no switching of the distributed control strategy when operation mode is changed. Moreover, power flow between the DR-HVDC link and HVAC link under parallel operation can be well controlled with the centralized control. Simulation results in PSCAD/EMTDC verify the proposed control during transition among the three operation modes.

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Authors: O. Saborío-Romano , A. Bidadfar , J. N. Sakamuri , Ö. Göksu , N. A. Cutululis

Diode rectifiers have been gaining traction as a viable alternative for connecting offshore wind farms (OWFs) to HVdc networks. However, before technical connection requirements compatible with such solutions can be determined, more studies are needed to assess their capabilities to contribute to the secure operation of the networks linked to them. This study assesses the capability of such an OWF to provide support to an onshore ac network by means of primary frequency response (PFR). A semi-aggregated OWF representation is considered in order to examine the dynamics of each grid-forming wind turbine (WT) within a string when providing PFR. Simulation results corroborate that such an OWF can indeed provide PFR, while its grid-forming WTs share the reactive power and keep the offshore frequency and voltages within their normal operating ranges.

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Authors: L. Yu, R. Li and L. Xu

A distributed PLL-based frequency control is proposed in this paper for offshore wind turbine converters connected with diode-rectifier based high-voltage-direct-current (HVDC) systems. The proposed control enables a large number of wind turbines to work autonomously to contribute to the offshore AC frequency and voltage regulation. The proposed control also provides automatic synchronization of the offline wind turbines to the offshore AC grid. Stability of the proposed frequency control is analyzed using root locus method. Moreover, an active dc voltage control of the onshore modular multilevel converter (MMC) is proposed to ride-through onshore AC fault, where the onshore MMC converter quickly increases the dc voltage by adding additional submodules in each phase, in order to rapidly reduce wind farm active power generation so as to achieve quick active power re-balance between the offshore and onshore sides. Thus the overvoltage of the submodule capacitor is alleviated during the onshore fault, reducing the possibility of system disconnection. Simulation results in PSCAD verify the proposed control strategy during start-up, synchronization and under onshore and offshore fault conditions.

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Authors: R. Li, L. Yu, and L. Xu

Offshore AC fault protection of wind turbines (WTs) connecting with diode rectifier unit based HVDC (DRU-HVDC) system is investigated in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during offshore AC fault transients and quickly provide fault current for fault detection. Considering different fault locations, the fault characteristics during symmetrical and asymmetrical faults are presented and the requirements for fault detection are addressed. A simple and effective offshore AC fault protection solution, combining both overcurrent protection and differential protection, is proposed by utilizing the developed fast fault current providing control. To improve system availability, reduced DC voltage of the DRU-HVDC system is investigated, where one of the series-connected DRUs is disconnected and the onshore modular multilevel converter (MMC) actively reduces DC voltage to resume wind power transmission. The proposed scheme is robust to various offshore AC faults and can automatically restore normal operation. Simulation results confirm the proposed fault protection strategy.

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Authors: R. Li, L. Yu, and L. Xu

The diode rectifier unit (DRU) based high voltage DC (DRU HVDC) system is a promising solution for offshore wind energy transmission thanks to its compact design, high efficiency and strong reliability. Herein we investigate the feasibility of the DRU HVDC system considering onshore and offshore AC grid faults, DC cable faults, and internal DRU faults. To ensure safe operation during the faults, the wind turbine (WT) converters are designed to operate in either current limiting or voltage limiting mode to limit potential excessive overcurrent or overvoltage. Strategies for providing fault curren ts using WT converters during offshore AC faults to enable offshore overcurrent and differential fault protection are investigated. The DRU HVDC system is robust against various faults and it can automatically restore power transmission after fault isolation. Simulation results confirm the system performance under various fault conditions.

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Authors: O. Saborío-Romano, A. Bidadfar, J.N. Sakamuri, L. Zeni, Ö. Göksu  and N A. Cutululis

Before diode rectifier (DR) technology for connecting offshore wind farms (OWFs) to HVdc is deployed, indepth studies are needed to assess the actual capabilities of DR-connected OWFs to contribute to the secure operation of the networks linked to them. This study assesses the capability of such an OWF to provide communication-less frequency support (CLFS) to an onshore ac network. It is shown that the HVdc link’s offshore terminal direct voltage can be estimated from measurements at the OWF’s point of connection with the DR platform. Two different methods are proposed for implementing CLFS in the OWF active power controls. In Method 1, the estimated offshore terminal direct voltage is used for estimating the onshore frequency deviation. In Method 2, the actual offshore terminal direct voltage measurement is used instead. Unique features of the provision of CLFS from OWFs connected to HVdc via DRs are highlighted, and the dynamic and static performance of the CLFS control scheme is compared to that of the communication-based frequency support scheme. To assess the impact of parameter estimation errors on the provision of CLFS, a parametric sensitivity study is presented as well, and recommendations are given to increase accuracy.

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Authors: M. Wang, W. Leterme, J. Beerten, D. Van Hertem

To ensure a reliable fault clearing, a backup protection scheme is required for selective HVDC grid protection. One way to achieve this is to use both voltage and current measurements to distinguish uncleared faults from cleared ones during the fault clearing process of the primary protection. This paper studies the applicability of such a backup protection algorithm in meshed bipolar HVDC grids and evaluates the robustness of the algorithm against system operating conditions and breaker opening delays. The influence of different operating conditions on the fault waveforms is analysed using a three-terminal bipolar test system. The robustness of the fast breaker failure backup protection algorithm is evaluated via simulation studies on the bipolar test system in PSCAD. The simulation results show that the fast breaker failure backup protection algorithm can distinguish between uncleared and cleared faults with sufficient margin for all considered operating conditions.

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Authors: M. Abedrabbo, M. Wang, P. Tielens, F.Z. Dejene, W. Leterme, J. Beerten, D.v. Hertem

In order to reliably and safely operate future multi-terminal HVDC grids, different protection strategies are currently being developed and presented in the literature. Especially for large DC grids, it is important to investigate how the choice of the protection strategy influences AC system stability during DC fault clearance and system restoration. This paper contributes to the understanding of these issues by giving an overview of the different protection strategies and their influence on frequency and transient stability. DC grid protection requirements are defined, based on the AC system constraints related to the level and duration of power transfer loss.

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Authors: T. Augustin, I. Jahn, S. Norrga, H.-P. Nee

In future high-voltage direct current (HVDC) systems, a large number of HVDC breakers will be required. In this paper, the influence of HVDC breakers on the transient performance of point-to-point HVDC links in both asymmetrical and symmetrical monopolar configuration with half-bridge modular multilevel converters is studied with simulations in PSCAD. As HVDC breakers,  the active resonant breaker and ABB’s hybrid breaker are considered. The analyzed scenarios include DC line faults, DC bus faults, and AC faults between the converter and the transformer. The highest DC breaking capability is required during DC line faults in the asymmetric and symmetric monopole. The converter stress is highest for DC bus faults and unbalanced converter AC faults in the asymmetric monopole and for DC bus pole-to-pole faults in the symmetric monopole.  During DC pole-to-ground faults in the symmetric monopole, the HVDC breaker combined with DC side arrestors yields the lowest overvoltage stress on the cable of the healthy pole. The fault current shapes depend strongly on the interaction of the converter and the travelling waves on the lines, and differ from the fault current shapes in typical HVDC breaker test circuits.  Furthermore, the active resonant breaker and the ABB hybrid breaker perform similarly in the used benchmarks due to the very fast DC line fault detection.

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Authors: M. Wang, M. Abedrabbo, W. Leterme, D. Van Hertem, C. Spallarossa, I. Grammatikos, K. Kuroda, S. Oukaili

At present, HVDC links are built as turnkey projects by a single vendor, who optimizes control and protection based on the project requirements. However, future DC grids require multi-vendor solutions to ensure acompetition between vendors. Standards which aim at interoperability of protection equipment, and thus multi-vendor solutions, exist for AC systems whereas for DC systems such standards are currently lacking. This paper provides a review of AC and DC protection technologies, and assesses which aspects of AC protection can be applied to DC protection, especially from the view point of multi-vendor interoperability. In particular, this paper focuses on the review and comparison of recent developments of AC and DC protection equipment, such as measurement devices, relay, communication protocol and circuit breakers. As the result of this study, recommendations on standardization process of DC protection devices learned from AC protection are provided.

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Authors: M. Wang, W. Leterme, J. Beerten, D. Van Hertem

Fast DC breakers are essential components to realise future high voltage DC grids. Recent development in the industry shows great feasibility in achieving such DC breakers using various technologies. In particular, hybrid DC breakers with modular design have the potential to operate in fault current limiting (FCL) mode, which can provide added functionalities in DC grid protection. However, the degrees of freedom in breaker design and control, and their impact on the transients associated with the FCL operation has not yet been addressed in the literature. Understanding the characteristics of the FCL operation is crucial to achieve interoperability between various technologies in a multivendor environment. This paper investigates the impact of design and control parameters on  the transients during the FCL operation. Possible applications of the FCL operation in DC grid protection are discussed and demonstrated in a four-terminal test system.

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Authors: P. Ruffing, C. Brantl, C. Petino, A. Schnettler

Within the framework of modernisation of the European electricity grid, multi-terminal HVDC offshore grids shall be integrated in future transmission systems. An essential aspect of multi-terminal HVDC systems is fast and selective DC-side fault handling and the separation of faulty lines. This paper investigates the applicability of different control methods relying on full-bridge based converters in combination with high speed switches for a fast and selective separation of faulty line segments in a multi-terminal HVDC cable system in symmetrical monopole configuration. It is shown, that the proposed line current control method can significantly reduce the separation time of a faulty line compared to standard fault control methods. The analysis is based on simulations in PSCAD|EMTDC™ with a converter model based on the CIGRÉ WG B4.57, which is enhanced for the use of full-bridge converters with fault current control schemes.

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Authors: I Jahn, F Hohn, S Norrga

The increased demand for renewable energy generation requires higher flexibility of transmission systems. This requirement together with technical progress in high-voltage direct-current (HVDC) technology have resulted in the ambition to build large-scale multi-terminal DC (MTDC) grids. To achieve this goal, vendor interoperability is considered a key element. Standards exist for AC systems, but not for DC systems. This work discusses and evaluates the suitability of AC standards for DC systems. As a result, a different view on substation architecture is developed and two communication protocols are suggested for further investigation in this context.

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Authors: G.D. Freitas, B. Ismail, A. Bertinato, B. Raison, E. Niel, S. Poullain, B. Luscan

This paper presents an assessment methodology of protection strategies for meshed grids. It also proposes the computation of two performance indicators to evaluate protection strategies through a reliability and speed perspective. The Monte Carlo method is used to calculate the two indicators proposed. These two indexes can be used as criteria for comparison between protection strategies. Due to the increasing debate around the protection for HVDC grids, three proposals of protection of HVDC grids were chosen as application cases.

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Authors: Firew Zerihun Dejene, Mudar Abedrabbo, Jef Beerten and Dirk Van Hertem

This paper introduces a method that helps to reduce the DC fault current contribution of the HB MMC submodule capacitors using the converter controls of the MMCs within the HVDC grid in order to reduce the requirements of DCCB designs. The proposed method achieves this during a DC pole-to-pole fault on a symmetric monopolar HVDC system or a DC pole-to-ground fault on bipolar HVDC system through the use of modified circulating current controls of the MMC HVDC. The parameters of the proposed controller are selected taking into consideration the operational limits of the converters.

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Author: M. Wang, D. Jovcic, W. Leterme, D. van Hertem, M. Zaja, I. Jahn

This paper first classifies the DCCB functions into minimally required and auxiliary ones based
on reviewing the existing literature. Then, standardised interfaces between the IEDs and DCCBs
are proposed to enable both types of DCCB functions. An example of such IED is implemented
in PSCAD/EMTDC to demonstrate that the proposed interfaces are adequate to enable
both minimally required and auxiliary functions using a four-terminal test system. Auxiliary
functions of hybrid DCCBs, such as proactive opening, fault current limiting, fast reclosing and
reopening, breaker failure internal detection and repeated O-C-O operation are demonstrated by
simulations.

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Author: W. Leterme, Mian Wang, Dirk Van Hertem

HVDC grids are currently considered to accommodate the integration of renewable energy sources into the power system. Since HVDC circuit breakers might at present incur a large investment cost, fault clearing strategies with limited use of HVDC circuit breakers have been recently proposed. These strategies, here called partially selective, split the grid into a number of protection zones which encompass sub-grids of multiple converters, transmission lines or a combination of both. Although the application of such strategies within the HVDC grid has been studied recently, discrimination of faults between zones is not yet widely investigated. This paper analyzes the application of a communication-less protection algorithm introduced for fully
selective protection in a partially selective protection strategy. A sensitivity analysis using a reduced model shows that the reach of these algorithms is determined by, on the one hand, the length of the cables and complexity of the grid within a protection zone and, on the other hand, the inductance of the series inductor associated with the breaker between the protection zones. A test case implemented in EMT-type software is used to demonstrate the fault clearing sequence in a partially selective strategy.

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Author: A. Bertinato, J.C. Gonzales, D. Loume, C. Creusot, B. Luscan, B. Raison

Multi-terminal high voltage direct current (MTDC) grids are considered to be an interesting solution to integrate bulk renewable energies located far away the consumption areas. One of the most challenging issues for the development of such MTDC grids remains the protection against DC faults. Although several types of HVDC breaker technologies have already been proposed and demonstrators tested by manufacturers, the coordination of such devices to ensure a proper protection system has not yet been proved. Difficulties encountered during the development of such DC grid protection system address, among others, the reliability of the protection scheme and its ability to ensure the AC and DC systems stabilities during and after the fault clearance. Therefore not only a reliable primary protection scheme, but also a robust backup scheme, have to be developed.
This paper presents a novel protection scheme based on converter breaker; its key element, namely a low-speed mechanical DC breaker, is located at each DC converter output. This strategy belongs to the non-selective fault clearing philosophy whose protection priority is given to the fault current clearance by opening the converter circuit breakers without any discrimination. The selectivity of the protection is thereafter ensured by the opening of mechanical DC breakers located at each end of faulty line.
To validate the protection strategy, primary protection scheme as well as back-up solutions in case of main component failures have been considered. Protection schemes are validated through detailed EMT simulations for a 3-terminals DC grid in bipolar configuration and with cable links. Technical requirements of the main components such as DC circuit breakers are highlighted in the paper. Moreover, an optimized pre-insertion resistor for a fast grid voltage restoring after faulty line isolation is also described.
To demonstrate the adequacy of the proposed protection strategy a particular emphasis is given to the impact of a DC fault on the AC transient stability. Benchmark cases of a future MTDC grid with high renewable energy source penetration will be presented to identify advantages and limits of such protection scheme.

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Author: P. Torwelle, A. Bertinato, B. Raison, T.D. Le, M. Petit

This paper provides a fault analysis of bipolar overhead line based HVDC grids using HB-MMC converters. The impacts of fault type and fault resistance are shown and the physical behaviour of the transient fault return current is explained. A transformation in modal quantities enables a decoupling of the measured phase currents and voltages. The results are pre-sented in a point-to-point HVDC grid for both phase and modal quantities. It is shown that fault analysis in the modal domain is an auspicious method for fault detection and faulty line dis-crimination.

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Author: G.D. Freitas, A. Bertinato, B. Raison, E. Niel, O. Despouys

This paper provides a fault analysis of bipolar overhead line based HVDC grids using HB-MMC converters. The impacts of fault type and fault resistance are shown and the physical behaviour of the transient fault return current is explained. A transformation in modal quantities enables a decoupling of the measured phase currents and voltages. The results are pre-sented in a point-to-point HVDC grid for both phase and modal quantities. It is shown that fault analysis in the modal domain is an auspicious method for fault detection and faulty line dis-crimination.

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Author: M. Wang, J. Beerten, D. Van Hertem

Maintaining balanced voltage during normal operation and rebalancing pole voltages after a pole-to-ground fault are both necessary to prevent high insulation stresses on the DC cable in a HVDC system. The state-of-the-art solution requires two separate devices to deal with normal and post-fault operation separately. This paper proposes pole rebalancing solutions using a single device applicable to both normal and post-fault operations, thus leading to reduced cost and footprint for pole rebalancing. The proposed circuits use fast switches to dynamically adapt the connected impedance in order to fulfil the requirements for rebalancing voltages during both situations. The performance of the proposed circuits are verified in a four-terminal HVDC test network using EMTP simulations. 

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Authors: M. Wang, W. Leterme, G. Chaffey, J. Beerten, D. Van Hertem

Pole rebalancing in symmetrical monopolar HVDC grids is necessary to remove pole imbalances resulting from pole to-ground faults. For selective protection employing DC circuit breakers, the interaction between DC circuit breakers and pole rebalancing methods have not been studied. This paper proposes new strategies for pole rebalancing methods to deal with DC circuit breaker operation in HVDC grids. A complete analysis of pole rebalancing using equipment at DC or AC side is performed for all stages of the fault clearing process. Based on the analysis, new control strategies are proposed to optimize the use of the pole rebalancing equipment. The proposed control methods are shown to enable the pole rebalancing equipment to meet the required high protection speed and low losses. Both DC and AC side equipment such as dynamic braking systems and AC groundings are investigated and proven to be applicable for pole rebalancing in selective protection strategies. The impact of the breaker technology on the interaction between DC circuit breaker requirements and pole rebalancing needs is investigated in detail. The conclusions are validated using EMTP simulation on a four terminal test grid.

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Authors: M. Wang, J. Beerten, D. v. Hertem

This paper proposes a frequency domain based methodology to analyse the influence of High Voltage Direct Current (HVDC) configurations and system parameters on the travelling wave behaviour during a DC fault. The method allows us to gain deeper understanding of these influencing parameters. In the literature, the majority of DC protection algorithms essentially use the first travelling waves initiated by a DC fault for fault discrimination due to the stringent time constraint in DC grid protection. However, most protection algorithms up to now have been designed based on extensive time domain simulations using one specific test system. Therefore, general applicability or adaptability to different configurations and system changes is not by default ensured, and it is difficult to gain in-depth understanding of the influencing parameters through time domain simulations. In order to analyse the first travelling wave for meshed HVDC grids, voltage and current wave transfer functions with respect to the incident voltage wave are derived adopting Laplace domain based component models. The step responses obtained from the voltage transfer functions are validated by comparison against simulations using a detailed model implemented in PSCAD TM. Then, the influences of system parameters such as the number of parallel branches, HVDC grid configurations and groundings on the first travelling wave are investigated by analysing the voltage and current transfer functions

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Authors: W. Leterme, S. Pirooz Azad, D. Van Hertem

To meet the required operation speed for protection of meshed voltage source converter (VSC)  high voltage direct current (HVDC) grids, traveling wave-based algorithms operating in the sub-millisecond time-frame can be used. The domain in which these algorithms operate, i.e., modal or phase, determines their performance in fault discrimination, fault type classification and faulted pole selection. In the recent literature, high-speed algorithms have been proposed for various VSC HVDC grid configurations and transmission line types; yet the choice of domain has received insufficient attention. This paper offers recommendations for the choice of domain for protection algorithm design of HVDC overhead line or cable systems in symmetric monopolar and bipolar configurations. The theoretical analysis of this paper, which is based on fundamental wave  propagation theory, indicates that the preferred domain for protection algorithms for cable and overhead line systems are the phase and modal, respectively. Furthermore, the paper provides comprehensive guidelines to construct detection functions for both configurations and discusses the errors introduced by approximations. Finally, study results from a bipolar overhead line test system demonstrate the advantages of modal over phase domain for fast fault discrimination and classification and illustrate practical problems associated with non-ideal detection functions.

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Authors: P. Ruffing, N. Collath, C. Brantl , A. Schnettler,

Future multi-terminal dc networks are envisioned for the large-scale integration of renewable energy sources into today’s power systems. An essential aspect for the reliable operation of these systems is the fast and selective dc-side fault handling and separation of faulted lines. The objective of this paper is the development and evaluation of a multi-terminal dc protection strategy based on fault-blocking converters (e.g., full-bridge based modular multi-level converters) and high-speed switches. Both a suitable fault control and a new high-speed switch design for a fast separation of faulted dc lines are elaborated. The novel concept is intended to reduce the requirements on the high-speed switches compared to dc circuit breakers and to ensure a fast restoration of the active power transmission capability, as well as the dc voltage.
The fault-clearing strategy is analyzed using electromagnetic transient (EMT) simulations under variation of the fault location, type, and resistance. The investigations are carried out in a cable-based multi-terminal HVdc system with full-bridge modular multi-level converters in symmetrical monopole configuration.

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Author: W. Leterme, P. D. Judge, J. Wylie, T. C. Green

The fault current characteristics in dc systems depend largely on the response, and hence also the topology, of the ac-dc converters. The presently used ac-dc converter topologies may be categorized into those with controlled or uncontrolled fault blocking capability and those lacking such capability. For the topologies of the former category, generic models of the dc-side fault response have not yet been developed and a characterization of the influence of control and sensor delays is a notable omission. Therefore, to support accurate and comprehensive dc system protection studies, this paper presents three reduced converter models and analyzes the impact of key parameters on the dc-side fault response. The models retain accurate representation of the dc-side current control, but differ in representation of the ac-side and internal current control dynamics, and arm voltage limits. The models were verified against a detailed (full-switched) simulation model for the cases of a full-bridge and a hybrid modular multilevel converter, and validated against experimental data from a lab-scale prototype. The models behave similarly in the absence of arm voltage limits, but only the most detailed of the three retains a high degree of accuracy when these limits are reached.

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Authors: N. A. Belda, R. P. P. Smeets, C. Plet, R. Nijman

Testing of HVDC circuit breakers (CBs) is fundamentally different from that of AC CBs as both voltage across and current through the HVDC CB exist at the same time, leading to an energy absorption requirement. Meaningful validation of HVDC CB technology is achieved when the applied tests accurately reflect realistic fault conditions encountered in practical operation. Because of the absence of standards for testing HVDC CBs, it is necessary to first define generic test requirements and a test programme based on fault analysis of multi-terminal HVDC grids. 

Therefore, the paper first  provides detailed analysis of fault currents in a multi-terminal HVDC grid composed of bipolar converter configuration. The phenomena following the occurrence of a fault and the critical parameters contributing to the fault current are investigated. Then, the fault current interruption process by various concepts of HVDC CBs and the important stresses sssociated with these different technologies are identified. These stresses are translated into generic current interruption test requirements that are adequate representatives of practical operational conditions.

It is found that AC short-circuit generators running at low power frequency and having sufficient short-circuit power can supply the necessary electrical stresses needed for testing different technologies of HVDC CB. Experimental test  results demonstrating a test circuit based on AC short-circuit generators at 16.67Hz power frequency are presented. These results are compared with simulation results obtained from system studies. Some practical challenges of using AC short-circuit generators for testing HVDC CBs in a high-power laboratory are addressed via experimental validation.

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Authors: S. Tokoyoda, T. Inagaki, K. Tahata, F. Page, K. Kamei, T. Minagawa, H. Ito, C. Spallarossa

HVDC transmission has been expanding due to rapid development of power electronics technology and by the need for connection of offshore or remote wind farms and / or large hydro power generators. CIGRE Study Committee B4 established various WGs and leads the HVDC investigations especially for multi-terminal HVDC. For example, WG B4.52 published TB533 on “HVDC Grid Feasibility Study” and WG B4.46 published TB492 on “VSC HVDC for Power Transmission”. SC A3 and B4 established a JWG A3/B4.34 and published TB 683 “Technical requirements and specifications of state-of-the-art HVDC switching equipment”  investigating various DC switchgears potentially applicable to future HVDC grids and summarized available technical specifications required for DC circuit breakers applied
to multi-terminal HVDC.

Multi-terminal HVDC grid will be required to operate the healthy lines continuously, DC circuit breaker is required to avoid DC voltage collapse and operate the healthy lines continuously, even if a DC fault occurs. Rapid fault clearing is essential for DCCB even though the requirement varies depending on: 1) DC  transmission system configurations, 2) Voltage  Source Converter (VSC) design, 3) transmission capacity, 4) DC reactor connected in series with the line/cable, and 5) impedance of the line/cable.

In this paper, a prototype mechanical DC circuit breaker with active resonant current zero creation scheme was tested in a test circuit using a AC short circuit generator operating under a reduced power frequency. The mechanical DC circuit breaker can interrupt DC current up to 16 kA within several milliseconds after receiving a trip signal (command).

Since there are no international standards for HVDC circuit breakers to demonstrate the interrupting capability, an equivalent and economical testing circuits were first investigated  within a certain restriction of testing facilities. Several testing circuits including a synthetic test, multi-part test, and a unit test can verified to reproduce the full stress imposed on DC circuit breakers in the networks.

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Authors: N. A. Belda, C. A. Plet, R. P. P. Smeets, R. Nijman, S. Tokoyoda, K. Tahata, F. Page, H.Ito, C. Spallarossa

This paper presents test results of an active current injection mechanical HVDC circuit breaker. The design, operating principle and two possible ways to realize extra high voltage (EHV) ratings using this type of HVDC circuit breaker are discussed. In addition, the practical implementation of a test circuit capable of testing the DC current interruption performance, not only of the mentioned HVDC circuit breaker topology but also of other proposed technologies of HVDC circuit breakers, is discussed. In order to demonstrate the performance of a HVDC circuit breaker, a test circuit needs to provide the necessary stresses: current, energy and voltage (both during and after interruption). A test circuit based on AC short-circuit generators operated at reduced power frequency, capable of providing the mentioned stresses, is designed and implemented. Special provisions on protection of the testobject and the test circuit are demonstrated.

A prototype of mechanical HVDC circuit breaker module (80 kV rated voltage) based on active current injection is tested. To demonstrate DC current interruption performance at different  current magnitudes, test duties with bi-directional currents ranging from 2 kA to 16 kA are defined. The prototype mechanical HVDC circuit breaker with active current injection is then used to successfully interrupt the test current at each test duty.

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Authors: N. A. Belda, R. P. P. Smeets

High voltage direct current circuit breakers (HVDC CBs) are necessary for reliable and safe operation of future multi-terminal meshed HVDC grids. So far no commercially mature products of such equipment exist. A few industrial concepts as a result of advances in power electronics have been proposed and prototypes  have been built. However, performances of these concepts have never been demonstrated under realistic operation condition. Since these tests require construction of high power DC sources incurring significant investment costs, alternative test circuits providing equivalent stresses must be  considered. In this paper various test methods and circuits used for testing HVDC CBs are  investigated and their performance is evaluated against the stresses in a conceptual future HVDC grid. A novel method of testing an HVDC CB using existing installations in a high power ac test laboratory is proposed and its prospective fault current and voltage is experimentally demonstrated. It is concluded that adequate test circuits need to maintain the supply voltage not only up to the current zero in the main interrupter unit of the HVDC CB, but also during the entire energy dissipation phase of the interruption process. 

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Authors: N. A. Belda, C. Plet, R. P. P. Smeets

The paper provides a detailed analysis of the temporal development of fault currents in a  multi-terminal high voltage direct current (MT-HVDC) grid composed of bipolar converter configuration. The sequence of events following the occurrence of a pole-to-ground fault is identified, divided into three distinct periods; namely, sub-module capacitor discharge, arm current decay and ac in-feed periods. The critical parameters that have a significant impact on the fault current in each period are discussed. The impacts of various parameters of the HVDC grid such as the size of the current limiting reactor, ac grid strength as well as the location of the fault within the grid are studied through PSCAD/EMTDC simulation. Then, a fault current  nterruption process using models of various HVDC circuit breaker technologies and the resulting stresses are studied. Both serve as important inputs to define test procedures. It is found that the HVDC CBs are subjected to not only dc current and voltage stresses but also energy stress. These stresses are translated into test requirements.

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Authors: D. Jovcic, M.H. Hedayati

High voltage DC circuit breakers (DC CB) are essential components for the future DC transmission grid. One of the challenges after manufacturing the DC CB is testing it and confirming the working of the DC CB under different conditions. In this paper, a 320kV, 1.5kA test circuit is proposed for testing DC CB in conditions very close to real DC transmission systems. In the proposed circuit topology a chopper is used to regulate the DC voltage. A DC capacitor bank is used to provide the energy required during the fault replication. In the test circuit the initial DC voltage is boosted by 12% which results in a DC voltage drop of 88% at the end of discharge period. The test circuit keeps the voltage at nominal level as soon as the fault is cleared to enable testing dielectric stress. The roposed test circuit is modelled using PSCAD and the simulation results are shown which confirm the working principle of the proposed test circuit.

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Authors: A. J. Far, D. Jovcic

This paper presents model of hybrid direct current (DC) circuit breaker (CB) based on phase-control thyristors. The design aspects are shown and the CB parameters are calculated for a 120kV and 1.5kA DC CB assuming 10kA interrupting current. An internal control system for opening and closing is presented. The DC CB is modelled at system level in PSCAD. Simulations are used to evaluate the DC CB performance under different operating conditions such as opening and closing. The model represents well DC CB including main internal subsystems and can be employed for DC grid protection studies. It is also concluded that the thyristor-based hybrid DC CB potentially benefits in higher fault current interruption compared IGBT-based hybrid DC CB. However on the downside, simulations indicate that extinction time of phase-control thyristors results in DC CB longer opening time.

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Authors: F. P. Page, K. Kuroda, R. Uda, S. Tokoyoda, T. Minagawa, H. Ito, C. Spallarossa

The mechanical dc circuit breaker (DCCB), with active current injection, has been developed to protect against dc side faults in HVDC multi-terminal systems. To reduce its cost, volume and weight, as  well as operate in a short period, the natural frequency of current injection is high (typically several kilohertz). As such, a number of fast transients occur whilst commutating current between various branches within the circuit breaker.

Real-time simulation platforms can be used to assess protection strategies of multi-terminal HVDC networks (MTDC). To do so, suitable models of all major components of the MTDC are required, which are compatible with such platforms. Specifically, to facilitate real-time operations, the minimum time-step threshold must be considered.  This limitation makes it challenging to implement a detailed model of the mechanical circuit breaker, where transients take place in very short time-periods. Therefore, it is desirable to reduce the complexity of the circuit breaker model to allow implementation within a real-time environment.

From a system-level design perspective, it is important that the circuit breaker model replicate key  system-level  quantities  accurately, such as current, voltage and energy  dissipation. However, it is not critical to reproduce internal stresses (such as those inside the vacuum interrupter, for example) as accurately, which would typically require detailed modelling and the small simulation time-step associated.

In this paper a simplified model for the mechanical dc breaker is introduced, for the purposes of system-level studies on a real-time platform. The loss of accuracy between detailed and simplified models is assessed and justified by comparing simulation results  produced with a detailed model on EMTP-type software (which allows a wide-range of time-steps to be used). Finally, a simplified model is then implemented and demonstrated on the RTDS platform, in both normal and small time-step.

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Authors: M. Hedayati, D. Jovcic

Direct current Circuit Breakers (DC CB) are vital components for DC grids and the Hybrid DC CB is of high interest because of fast operating speed and low on state losses. The speed of operation of hybrid DC CB is crucially determined by the speed of Ultra-Fast Disconnector (UFD). This article presents in some detail design, fabrication and testing of a low voltage UFD at a University laboratory. The UFD is designed to operate in 2ms and separates the contacts by 3.0mm, which is adequate to withstand a voltage level of 7kV in air. The contacts of the UFD are designed to carry a rated current of 100A. The speed requirements demand a driver capable of supplying a current pulse of few kA, which is  achieved using capacitive storage.  The experimental measurements show that the UFD is capable of opening and closing in around 1.8ms. Higher opening speeds are possible but  bounce becomes pronounced. The paper also presents a method of magnetic braking which eliminates bounce and enables application of higher forces and the opening time of 1.3ms is demonstrated. The speed of UFD open-close-open cycle is dependent on the charging time of the UFD driver circuit which is quite long because of large stored energy.  It is concluded that at least two complete driver circuits are necessary.

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Authors: D. Jovcic, A. Jamshidifar, M. Popov, S. Liu

The performance of AC circuit breakers (CBs) has been well analyzed and standardized, but current interruption with HVDC CBs is very different and therefore its functionalities will be different. Considering also that several fundamentally different HVDC CB technologies are emerging (IGBT-based hybrid, thyristor-based hybrid and mechanical), there is a need for a universal set of modelling requirements. This paper investigates a simulation test circuit set up and a set of PSCAD-simulated scenarios which reveal essential performance for most common HVDC CB technologies. Universal test circuit and tests will enable comparisons between technologies and set the ground for interoperability and standardization. Demonstration of low current interruption is required since it leads to longer interruption time for some DC CBs. Not all DC CBs are capable of interrupting reverse current, while others have different performance compared with positive current interruption. The study shows that various DC CBs respond differently under high current in circumstances where there is no trip signal from the protection system in which case its self-protection activates. DC CBs may respond differently with change in system parameters like different cable.

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Authors: D. Jovcic, A. Jamshidi Far, A. Hassanpoor

This paper studies the integration of fault current limiting mode of hybrid HVDC breakers (HHB) with dc grid protections and HHB internal controls. Fault current limiting is particularly beneficial in case of delayed protection operation in outer zones. This might happen for a range of reasons like: slow HVDC breakers, breakers (temporary) not ready, or (temporary) protection failure.

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Authors: M. Hedayati, D. Jovcic

This paper reports on the development of laboratoryscale mechanical DC Circuit Breaker (DC CB) demonstrator of current injection type. A 500A, 900V DC CB is developed with
around 4ms operating time. We have used 3 series connected commercially-available mechanical contactors, with appropriate grading circuits, as the main switch. The resonant circuit has been designed with 3kHz resonance frequency. The experimental tests show successful interruption of 500A current at 1000V DC voltage in both positive and negative direction. Further tests with high impedance faults show successful interruption of low
fault current at 65 A. Some further analysis of impact of timing of current injection is shown.

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Author: D. Jovcic

DC Circuit Breakers are expected to play critical role in future DC transmission grids and in multiterminal HVDC lines. They will facilitate continued operation of complex DC systems, by providing fast isolation of faulted DC lines. This paper elaborates on a 1200V, 200A prototype for LC DC Circuit Breaker.

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Authors: A. Jamshidifar and D. Jovcic

This paper presents a systematic study on designing hybrid direct current (DC) circuit breaker (CB) based on fast thyristors. As an illustration, the DC CB main parameters are calculated for a 120kV, 1.5kA test breaker with interrupting current of 10kA. The studies indicate that the opening time of 2.3ms can be achieved only if fast thyristors are employed. It is further illustrated that there is a design tradeoff between minimum interrupting current capability and discharge time for the internal capacitors (reclosing speed). The DC CB control system for opening and closing is presented based on different levels of protection and the self-protection. The DC CB is modelled in PSCAD and simulation results are used to evaluate the breaker performance under different operating conditions. It is concluded that the model represents well the DC CB and can be employed for DC grid protection studies. It is further shown that opening time becomes longer as interrupting current reduces, and it is very long in case of load current interruption.

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Authors: D. Jovcic, M. Zaja and M. Hedayati 

This paper examines two new bidirectional hybrid dc circuit breaker topologies for application in meshed dc grids. The goal is to retain performance of hybrid DC CB with bidirectional current interruption, while reducing semiconductor count, DC CB size and weight. The fault current is routed to the unidirectional internal valve using multiple additional ultrafast disconnectors. Operation of both topologies is studied using a 320 kV, 16 kA simulation model, as well as demonstrated on a 900 V, 500 A lab prototype. The control systems are presented and discussed in detail. The hardware prototypes verify performance of several new technical and operating solutions. A comparison is made with the existing DC CB topologies and performance and reliability compromises of each topology are assessed. The conclusion is that it might be possible to halve the DC CB semiconductor count while retaining same 2 ms opening speed and bidirectional operation.

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Authors: D. Jovcic

The article proposes a mechanical DC circuit breaker (CB) based on a series LC circuit. It requires two switches (a fast disconnector and an AC circuit breaker), an inductor and a capacitor, and therefore the cost is expected to be low. A series LC circuit is analysed and it is concluded that fault current will always have natural zero-current crossings which enable use of simple AC CBs. The current commutation into a capacitor is investigated since this is important for successful operation. A number of analytical conditions are derived for the voltage stress across disconnector contacts which enable arc-less contact opening. Experimental results on a 900 V laboratory prototype LC DC CB illustrate successful DC fault clearing, with commutation of 130 A and peak DC current of 190 A. A detailed PSCAD model for 320 kV LC DC CB is developed and DC fault clearing is evaluated in order to understand the possible benefit for high-voltage direct current applications. Further comparisons with the commercialised hybrid DC CB and mechanical DC CB on 320 kV system illustrate some benefits in terms of performance and simplicity. The mechanical LC DC CB operates very fast because of early capacitor insertion, and this results in low peak current and energy dissipation.

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Authors: M. Hedayati, D. Jovcic

Peak fault current and energy dissipation in high voltage direct current (HVDC) circuit breakers (CBs) are very important parameters that impact dc grid protection development. This paper analyses a hybrid DCCB (HCB) control that reduces peak current and energy dissipation, by regulating the voltage across contacts of the ultra-fast disconnector (UFD). This is achieved by manipulating the number of inserted surge arresters while contacts of the UFD are moving apart. The controller is seamlessly integrated with the current controller of HCBs. Analytical model for current and energy calculation is presented, verified, and employed for parametric studies. PSCAD simulation with 320kV, 16kA test circuit confirms that the proposed voltage control reduces the peak current and energy dissipation by around 20-30%. A 900V, 500A HCB laboratory hardware is described and the experimental results are shown to corroborate simulation conclusions.

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Authors: S. Liu, M. Popov

The main goal of the paper is the modelling of the mechanical direct current circuit breaker (DC CB) with active current injection that includes different circuit breaker characteristics. System level models provide adequate representation of the circuit breakers for system analysis studies. The performance characteristics of the DC CB in those proposed models replicate the ones of the devices in practice. The developed mechanical circuit breaker model is realized for a 320 kV demonstration circuit in PSCAD environment and its limitations and robustness are analyzed. The performance of the model is investigated by different cases. The obtained results show that the DC CB model can be used with full success for both to simulate DC fault interruptions and to be used for different protection studies.

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Authors: L. Liu, S. Liu, M. Popov⁎

The widely acknowledged high-voltage direct current (HVDC) technology has now been accepted as a solution of connecting renewable energy sources. However, this technology is vulnerable when facing DC-side faults; due to the low DC impedance, the fault current can rise to an extremely high value in a short time. In addition, when building a multi-terminal DC (MTDC) system, the fault can make a worse failure or blackout of the system when it is not cleared or isolated in time. The urgent need to ensure reliable mentioned HVDC power system can be realized by making use of DC circuit breaker (DCCB). The vacuum CB, which is one division of active DCCBs, has its own operational limit; it can interrupt fault currents when the di/dt of injected current is lower than a critical value, otherwise the arc may reignite. Therefore, the designing and testing of a DCCB must consider this feature. On the other hand, because of the complex configuration of an MTDC system, one DC-side fault can result in different fault currents at faulty line’s terminals; thus, the DCCB needs to be calibrated based on its local fault information. This paper presents an algorithm to optimize the DCCB according to its critical di/dt and local fault current. Furthermore, the operational delay and chopping current of circuit breaker are also considered and modelled. The simulation results from PSCAD platform verify the effectiveness of the presented algorithm.

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Authors: S. Liua, Z. Liua, J. d. J. Chaveza, M. Popov

The main goal of the paper is the modeling of the mechanical circuit breaker (MCB) that can replicate the breaker characteristics in real time environment. The proposed MCB with active current injection is modelled for a system level, which provides adequate representation of the circuit breakers for system analysis studies. External current-voltage characteristics of the proposed MCB models replicate the ones of the devices in the real world. It is well known that the DC circuit breaker (DCCB) needs to interrupt DC faults very quickly in order to avoid converter damages. The total current interruption time consists of fault detection time, time needed for the DC protection to provide command to the DCCB, and DCCB arc clearing time. Thus, it is necessary to demonstrate the system performance of associated protective devices through real time simulation, before these devices can be implemented and commissioned in practice. This paper presents a detailed modeling of the mechanical DCCB in real time simulation environment based on RTDS. The performance of the model is verified by the simulations based on PSCAD and meaningful conclusions are drawn.

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Authors: M. Zaja, A. A. Razi-Kazemi, D. Jovcic

Ultra-fast disconnector (UFD) is a key component of hybrid DC circuit breakers and it is also studied as the main switch in some DC grid topologies. A UFD model suitable for DC grid studies and considering both normal operation and failure mode is presented. The dynamic motion of contacts is analysed in detail and it is concluded that Thomson coil inductances including parasitic parameters play an important role and it is recommended to use finite element modelling. The arcing mode of  UFD is repressed using a variable resistance in series with an ideal switch. The variable resistance is calculated analytically based on the instantaneous position of contacts and the circuit conditions. Two different arc models are recommended: for the air-insulated UFD and SF6 UFD, and in each case, two operating regimes should be considered: high and low currents. The UFD model is verified for both normal operation and failure mode using measurements on a 5 kV laboratory UFD and the results show very good matching. The 320 kV SF6 UFD model is evaluated using limited reported results from manufacturers.

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Authors: C. Wouters, W. van der Veen, P. Henneaux, M.J. Van Blijswijk

A meshed European offshore transmission grid connecting offshore wind farms to shore could provide significant financial, technical, economic and environmental benefits to the European electricity market. Launched in January 2016, PROMOTioN [1] aims to explore and identify these potential benefits. To fully understand the economic and societal consequences of such a grid, it is necessary to perform a societal cost benefit analysis (SCBA). Existing SCBA methodologies, however, are not designed to assess complex offshore systems. A study was performed within the PROMOTioN project to develop a suitable methodology. The study delivered a set of guidelines for performing an SCBA analysis for offshore grids, as reported in Deliverable 7.11 [2]. This paper presents the results of this study. It describes requirements for an SCBA for offshore grids, challenges in comparing offshore grid solutions and the choices made in developing an  appropriate methodology. The presented methodology aims to enable the comparison of alternative offshore grid configurations in a certain geographical area, given offshore wind  capacity development in this area. The methodology sets out the criteria and guidelines for the assessment of costs and benefits of a complex offshore energy system.

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Authors: P.C Bhagwat, T. Schittekatte, N. Keyaerts, L. Meeus

The application of cost-benefit analysis (CBA) for offshore electricity infrastructure projects with a pan-European impact is discussed. An analytical framework for the evaluation of CBA methodologies is presented. The framework is then applied to assess the CBAs of three offshore infrastructure projects (EWIC, COBRAcable and ISLES). Overall, the CBAs assessed already comply with several dimensions of the analytical framework. However, based on this assessment it is found that scope for improvement in quality exists in three  areas namely, in considering project interactions, in dealing with uncertainty and in making the results between CBAs comparable by ensuring full monetisation. Furthermore, the research also confirms the view that a common harmonised CBA methodology is essential for selection of PCIs.

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Authors: S. Seman, N. T. Trinh, R. Zurowski, S. Kreplin

This paper presents the detailed and simplified dynamic simulation models of Diode-Rectifier (DR) based HVDC  transmission solution which is also known as Siemens New  Grid  Access (NGA) technology. The new grid access solution was developed mainly to enable an efficient transmission of power produced by large offshore wind power plants  that  are located more than 100 kilometers away from the mainland. The model of DR HVDC transmission solution is including wind power plant (WPP), diode rectifier units (DRU), DC cable and onshore voltage-source converter (VSC) station. An introductory part describes the full-scale EMT model and its dynamic performance is demonstrated on two study cases showing the power flow initialization and the dynamic behavior during an onshore fault. Based on EMT model, a simplified RMS model was developed and it has been validated by several test  cases. The comparison of the EMT and RMS simulation results shows close agreement. Therefore, it could be concluded that the developed RMS model of Diode Rectifier based HVDC grid connection solution could be utilized for large scale dynamic power system studies.

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Authors: L. Ängquist, S. Nee, T. Modeer, A. Baudoin, S. Norrga, N. A. Belda

This paper presents the VSC assisted resonant current (VARC) direct current circuit-breaker concept, which comprises a vacuum interrupter, operated by an ultra-fast actuator, together with a power electronic converter that creates a zero-crossing in the arc current. A few main circuit topologies are shown and discussed and a dynamic model of the DC link voltage in the VSC is presented. A module rated for 10 kA against 40 kV transient interruption voltage has been built and tested at an independent test laboratory, and some test results are presented.

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Authors: S. Tokoyoda, T. Inagaki, R. Kamimae, K. Tahata, K. Kamei, T. Minagawa, D. Yoshida, H. Ito

The paper presents recent development progress of a mechanical EHV DC circuit breaker with current injection scheme. An EHV level DC circuit breaker composed of multi-break HV vacuum interrupters, a current injection circuit and a metal oxide surge arrester (MOSA) energy absorption unit. 320/400 kV DC and 525/600 kV DC circuit breakers consist of 4 and 6 series connected 100 kV vacuum interrupters, respectively. There are two fundamental design concepts employed with common (combined design) or separate (module design) current injection circuit with a bunch of charged capacitors[1]. The module design with the separate current injection circuits as well as separate MOSA energy absorption units has an advantage in terms of flexible design to meet different higher DC voltage ratings. However, some control coordination is required to operate each module of the current injection circuits simultaneously.

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Authors: N. Belda, R. Smeets, R. Nijman, M. Poikilidis and C. Plet

Recently, several manufacturers have proposed and developed prototypes of high-voltage direct current (HVDC) circuit breakers (CBs) based on various dc current interruption principles. However, due to a lack of operational experience with this type of equipment, no clearly defined requirements that these new developments should satisfy exist. To define and refine justified test requirements, a thorough  understanding of the interactions between the internal components of the HVDC CBs and the stresses on these components is necessary. For this purpose, an experimental dc CB based on the active current injection technique is setup in a high-power laboratory. The contribution of this paper focuses on the performance of vacuum interrupters (VIs) for dc CB application. The performances of three different VIs, designed for ac application, are investigated. About 200 tests, under different test conditions, have been conducted in which up to 850 current zero crossings (CZCs) are created. The test results are analyzed and presented in detail along with statistical information obtained from the analysis of the measured  parameters. The test results serve to identify the critical stages of current interruption process that need to be demonstrated during typical current interruption tests of HVDC CBs. It is found that the three types of VIs behave very differently; the rate-of-change of current near CZCs is not the only key parameter; and successful current interruptions can still be achieved after re-ignitions and restrikes.

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Authors: R. Smeets, N. Belda

This paper describes the interruption principles and technology of fault current interruption in HVDC systems. First, an overview on switching in HVDC systems is provided, followed by a description of the technology of fault current interruption using HVDC circuit breakers. Then, the actual state-of-the-art of HVDC circuit breaker technology and its application is highlighted. Finally,  recommendations on testing of HVDC circuit breakers and the actual status of standardization activities are discussed.

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Authors: R. Smeets, N. Belda

Nowadays considerable effort is put into planning/realizing multi-terminal, meshed HVDC transmission networks. With gaining attention one of the crucial components of a multi-terminal HVDC grid is the HVDC circuit breaker. The HVDC circuit breaker must be capable of clearing DC faults without de-energizing the DC side of the grid. Recently, a few HVDC circuit breaker solutions have been proposed, some are realized into prototypes and a few are in operation.

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Authors: N. A. Belda, C. A. Plet, R. P. P. Smeets,

This paper provides a pragmatic solution to the challenge of testing fault current interruption of high-voltage direct current (HVDC) circuit breakers (CBs). The critical parameters in the design of a test circuit capable of supplying the necessary stresses: current, energy and voltage (both during and after interruption) are discussed. In addition, a practical implementation of a test circuit based on ac short-circuit generators operated at low power frequency, which is capable of testing the current interruption performance of the proposed technologies of HVDC CBs, is discussed. Tests validating the proposed method and circuit have been conducted on a prototype of an HVDC CB and the test results are presented. Because the performance of some  technologies of HVDC CBs can depend on the magnitude of the interrupted current, four test duties are defined and demonstrated in the paper. Moreover, testing of HVDC CBs using ac short-circuit generators poses new challenges such as the application of dielectric dc stress after current interruption, and the protection of both the test-object as well as the test-circuit components when the HVDC CB fails to interrupt. Methods to overcome these challenges are developed and  practically demonstrated in a test laboratory. Finally, taking into account the available resources of the author’s test laboratory, the capability to test multiple series-connected modules of different technologies of HVDC CBs is verified and example cases are demonstrated. Six short-circuit generators (13500 MVA @ 50 Hz) and up to ten step-up transformers (up to 550 kV) were actually used.

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Authors: N. A. Belda, R. P. P. Smeets, R. M. Nijman

Recently a number HVdc circuit breakers (CBs) based on various dc current interruption principles have been developed and a few are put in operation. However, due to a lack of practical experience, no clearly defined requirements that the HVdc CBs should satisfy exist. To define and refine justified test requirements, a thorough understanding of the interactions between the internal components of the HVdc CB and the stresses on these components under real dc fault current interruption condition is necessary. In this paper, an experimental dc CB based on the active current injection technique is setup in a high-power laboratory to investigate the performances of the main components; namely, the vacuum interrupter (VI) and the metal oxide surge arrester (MOSA). The performances of three different designs VIs are investigated and it is found out that each of the VIs behave completely different. The key parameters having impact on current interruption performance of the VIs are identified and analyzed in detail.

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Authors: B. Nouri; A. Arasteh; Ö. Göksu ; J.N. Sakamuri; P.E. Sørensen

Developing an integrated pan-European energy system based on renewable energy sources (RES) has technical and economic benefits. In this way, harmonized rules for grid connection of RES are required at the international level. Wind energy is one of the most promising renewable energy worldwide. The integration of wind energy into the power system is overgrowing through onshore and offshore installations. The European network codes have been drafted and regulated for AC- and HVDC-connected power-generating modules (PGM) in two separate international network codes. This paper presents the main aspects of the regulated European network codes and compares them.

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Authors: Behnam Nouri, Ömer Göksu, Vahan Gevorgian, and Poul Ejnar Sørensen

The electrical test and assessment of wind turbines go hand in hand with standards and network connection requirements. In this paper, the generic structure of advanced electrical test benches, including grid emulator or controllable grid interface, wind torque emulator, and device under test, is proposed to harmonize state-of-the-art test sites. On the other hand, modern wind turbines are under development towards new features, concerning grid-forming, black-start, and frequency support capabilities as well as harmonic stability and control interaction considerations, to secure the robustness and stability of renewable-energy-based power systems. Therefore, it is necessary to develop new and revised test standards and methodologies to address the new features of wind turbines. This paper proposes a generic test structure within two main groups, including open-loop and closed-loop tests.

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Authors: U. Riechert, J. Josefsson, C. Plet, S. Mebrahtu-Melake, A. Hassanpoor

The increasing demand for HVDC technology requires the adaptation of gas insulated switchgear (GIS). Based on the development and research results combined with the service experience a new type test philosophy including insulation system tests was developed. If future offshore grids would be considered with multi-terminal or switching stations offshore, the gain would be considerably larger. Moreover, the gas-insulated components can be applied in various HVDC applications. This paper explains that the new components in an HVDC substation are far into the development phase and are on a clear path to an even higher Technology Readiness Level (TRL). The activities to increase the technical assurance to implement these components in the grid as described. Once the HVDC substation equipment has been implemented into HVDC systems, and the experience should be collected on how they are actually being used, to develop more cost efficient solutions.

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Authors: L. C. Castro Heredia and A. Rodrigo Mor

This work introduces an approach for partial discharge (PD) detection on GIS that makes use of HFCT. The novelty of this method is that the HFCT is installed at the bolts of the external-type spacers to measure, with a bandwidth in the range of hundreds of MHz, the induced PD surface currents flowing along the GIS compartments.  Experiments with calibrator signals were conducted to prove that the PD events induce currents in the  compartments and to understand their distribution. Next, measurements with test cells were carried out to estimate the spatial sensitivity of the measuring circuit and the effect of noise and disturbances. Finally, the  spectral power ratio clustering technique was applied to test results as a method to discriminate PD and non-PD signals.

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Authors: U. Riechert, M. Gatzsche, A. Hassanpoor, C. Plet and N. Belda

Although GIS components have been developed, their performance is today relatively unknown to the market. The paper shows that the new components in a HVDC substation are far into the development phase and are on a clear path to an even higher Technology Readiness Level (TRL). The activities to increase the technical assurance to implement these components in the grid are described. Based on the development and research results combined with the service experience a new type test philosophy including insulation system tests was developed. Standardization work has been started in committees like CIGRE and performance demonstrations are planned in the  PROMOTioN project aligned with this standardization work. The paper provides a comprehensive update on status of standardization and demonstration efforts and provides suggestions for future work.

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Authors: A. Rodrigo-Mor, F.A. Muñoz and L.C. Castro-Heredia

This paper presents a new concept for partial discharge measurements in gas insulated systems (GIS). The proposed technique uses a novel GIS magnetic antenna that measures the magnetic field produced by partial discharges (PD) propagating in GIS. The foundations of the measurement technique and the magnetic antenna design are presented together with laboratory measurements. The magnetic antenna performance and the sensitivity of the acquisition system are studied. The bandwidth of the measurement system is in the high frequency and very high frequency (HF–VHF) range. Laboratory experiments demonstrate the suitability of the novel magnetic antenna-based measuring system for PD in GIS for corona, surface discharges, and free moving
particles in SF6.

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Authors: A. Rodrigo Mor, L.C. Castro Heredia and F.A. Muñoz

This paper presents a novel measuring system for partial discharge (PD) measurements in Gas Insulated Systems (GIS) using high frequency current transformers (HFCT). The system is based on the measurement of the induced PD currents in the GIS enclosure. In opposition to the existing antenna technologies that measure the radiated energy in the very high frequency/ultra-high frequency (VHF/UHF) range, the proposed system  measures the PD conducted currents in the high frequency (HF) range and below. The foundation of the measurements together with a detailed explanation of the sensor installed conveniently at the bolts of the GIS spacer are presented. An experimental study on the current distribution in the GIS enclosure is described to evaluate the impact of the sensor on the measurements. Laboratory experiments have been performed that show the suitability of this method to properly measure particle discharges caused by corona, surface and free moving particle discharges in SF6. Discharges in the range of 1 to 4 pC have been properly measured. An analysis to evaluate the performance of the method is shown, in comparison to VHF/UHF antenna measurements. The potential benefits of this novel technique rely on the small attenuation of PD signals in the GIS components in the HF range and sample rate reductions. Finally, a discussion on the potential applicability of present cluster

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Authors: A. Rodrigo-Mor, F.A. Muñoz, L.C. Castro-Heredia

This paper describes a simplified model and a generic model of high-frequency current transformer (HFCT) sensors. By analyzing the models, a universal charge estimation method based on the double time integral of the measured voltage is inferred. The method is demonstrated to be valid irrespective of HFCT sensor, assuming that its transfer function can be modelled as a combination of real zeros and poles. This paper describes the mathematical foundation of the method and its particularities when applied to measure nanosecond current pulses. In practice, the applicability of the method is subjected to the characteristics and frequency response of the sensor and the current pulse duration. Therefore, a proposal to use the double time integral or the simple time integral of the measured voltage is described depending upon the sensor response. The procedures used to obtain the respective calibration constants based on the frequency response of the HFCT sensors areexplained. Two examples, one using a HFCT sensor with a broadband flat frequency response and another using a HFCT sensor with a non-flat frequency response, are presented.

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Authors: S. Seman, N. T. Trinh, R. Zurowski, S. Kreplin

This paper presents the detailed and simplified dynamic simulation models of Diode-Rectifier (DR) based HVDC  transmission solution which is also known as Siemens New  Grid  Access (NGA) technology. The new grid access solution was developed mainly to enable an    efficient transmission of power produced by large offshore wind power plants  that  are located more than 100 kilometers away from the mainland. The model of DR HVDC transmission solution is including wind power plant (WPP), diode rectifier units (DRU), DC cable and onshore voltage-source converter (VSC) station. An introductory part describes the full-scale EMT model and its dynamic performance is demonstrated on two study cases showing the power flow initialization and the dynamic behavior during an onshore fault. Based on EMT model, a simplified RMS model was developed and it has been validated by several test  cases. The comparison of the EMT and RMS simulation results shows close agreement. Therefore, it could be concluded that the developed RMS model of Diode Rectifier based HVDC grid connection solution could be utilized for large scale dynamic power system studies.

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