Protection of DC grids is seen as one of the major challenges to be resolved before the full integration of large scale DC and AC grids can be realized. The appropriate detection and fault clearing of DC side faults is essential to safely and reliably operate meshed DC grids. Fault current interruption is much more complex here, as the prospective fault current rapidly reaches high values and exhibits no natural periodical zero-crossing. Clearing of DC faults in DC grids may interact with the established methods for control and dispatch of the AC grids. Therefore, DC fault clearing must become an integrated part of the combined AC and DC power systems, which are likely to be developed and/or operated by different entities.
In this respect, work package 4 (WP4) of the PROMOTioN project aims at the development of DC grid protection systems. A first step was to determine the functional requirements of DC grid protection in the internal deliverable D4.1. This deliverable also highlights the need for a risk-based approach when developing DC grid protection systems and provides small, medium and large impact benchmark networks for future research within the project.
In a second step undertaken in WP4, different possible protection philosophies and fault clearing strategies are assessed and benchmarked for application in each of the systems defined in D4.1. This research is the subject of deliverable D4.2a.
Current academic literature offers several strategies to clear DC faults in HVDC grids. Examples of these strategies are: de-energizing the entire DC grid (through AC breakers or fault blocking converters), splitting the grid using DC breakers or DC/DC-converters or protecting each line individually using DC breakers. With these strategies, the objectives of protection, e.g., guaranteeing system reliability, are fulfilled to a different degree and at differing costs. Furthermore, the choice of fault clearing strategy will lead to fundamentally different system designs. An in-depth analysis of the consequences of choosing a certain fault clearing strategy and a benchmark exercise is unique and has never been done before.
The first objective of D4.2 is to benchmark the fault clearing strategies found in the existing literature. The strategies are benchmarked in terms of expected cost drivers, feasibility for a certain type of system, performance with respect to grid extension and duration and probability of power outages, considering both primary and backup protection. The benchmarking of the fault clearing strategies thus takes into account the feasibility of the strategy while considering the constraints imposed by the connected AC systems, robustness with respect to addition of lines or converters in the DC system, and an assessment of the outages during successful operation or failure of the primary protection.
Along with the fault clearing strategy, protection algorithms are needed to detect and identify faults. These protection algorithms place different requirements with respect to, e.g., measurement equipment, measurement locations and communication. Certain protection algorithms are specifically tailored to a certain fault clearing strategy, whereas others can be applied under several strategies.
The second objective of D4.2 is to provide an overview of protection algorithms found in the existing literature. The main principles of the protection algorithms are provided, and they are categorized according to the protection philosophy for which they are designed.
Intermediate results:
A number of intermediate conclusions are reached at this point. First, treating the DC grid as a single entity, depending on AC breakers, is the cheapest option, but offers least protection and is not acceptable when the DC system has a significant impact on the AC system. Protecting each line individually in a DC grid provides superior performance with respect to AC system impact, but the feasibility of this option depends on the cost of equipment to interrupt the fault current, which must be installed at each line terminal. Alternative methods, i.e., using fault blocking converters or splitting the DC grid, offer good performance in AC system impact at intermediate cost, but might require greater design efforts when extending the DC grid or modifying the AC grid.
Download Deliverable 4.2: Broad comparison of fault clearing strategies for DC grids (PDF 15 MB)