Scientific Papers

Conference 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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Conference Papers:

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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Journal Article:

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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Conference Papers:

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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Journal Article:

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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Conference Papers:

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: 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: 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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Journal Articles:

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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Conference Papers:

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: 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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Journal Article:

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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Conference Paper:

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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