Distributed Energy Resources Protection Requirements in Future Distribution Grids

One of the key protection functionalities in the future active distribution networks will be reliable detection of islanding. Non-detection zone (NDZ) near a power balance situation and maloperation due to other network events have been the major challenges with traditional passive islanding detection methods like voltage, frequency, rate-ofchange-of-frequency, voltage vector shift (VVS). However, in the new grid codes for distributed energy resources (DERs), for example the use of voltage, frequency, rate-of-change-of-frequency for defining DER unit fault-ride-through (FRT) becomes more common. Also when the number of DER units’ in distribution networks increases in the future, the possibility of achieving power balance in the distribution network will also increase. Therefore, the risk of distribution system segments operating in the NDZ of the traditional passive islanding detection methods will increase, too. In addition, some new grid codes state that islanding detection should not be based only on communication based transfer trip. Due to these issues proper co-ordination between traditional passive islanding detection methods and DER unit FRT / grid code requirements becomes very challenging in the future and new islanding detection schemes are needed. In this paper the new requirements for distribution network protection and islanding detection related to development of new DER unit grid code requirements will be presented. In the future active management of distribution networks can also simultaneously affect to protection settings or principles if for instance network topology is changed. Therefore, protection adaptation and DER grid code compatibility may be required increasingly in future distribution networks during normal grid-connected and intended island (microgrid) operation. This paper presents potential future grid-code compatible protection schemes i) combined scheme high-speed communication based transfer trip + VVS, ii) multicriteria based scheme. With new methods no selectivity issues between islanding detection and DER unit FRT / grid code requirements will exist. In addition, risk of false islanding detections due to other network disturbances will be negligible. Active network management (ANM) functionality could be also used to continuously control the reactive power unbalance over circuit-breaker in order to ensure reliable islanding detection (no NDZ) with VVS alone or as part of the combined scheme. This reactive power unbalance control to ensure reliable islanding detection could be one part of future ANM scheme which would be able to fulfil multiple MV network management targets simultaneously. More details related to this ANM scheme and new combined islanding detection scheme research and development in Sundom Smart Grid (smart grid pilot in Vaasa, Finland) will be also presented in the final paper.