Alleviating the impact of DGs and network operation modes on the protection system

Abstract

Integration of distributed generations (DGs) into distribution networks leads to a significant change of fault current behaviour in magnitude and direction, causing loss of existing protection settings, as does the network operation in two modes. This study proposes an active protection scheme for digital protection system of medium-voltage distribution network to overcome those drawbacks. The method adds a new feature in the fault current detectable by digital relays and makes the relay settings independent of fault current contribution of DGs and upstream grid. The feature is created by injecting a current with frequency rather than the fundamental frequency only under fault conditions. The relays extract the injected current and ignore the DGs and upstream grid contributions. If the relay settings are determined based on the injected current, the relay can operate with a fixed setting despite the variations of fault current level. As a result of the proposed method, the possibility of connecting DGs and operating network in two modes without any effect on relay settings is provided. The proposed protection scheme does not require communication links. The effectiveness of the proposed method is verified by simulating IEEE 33-bus network in PSCAD/EMTDC software.