Comparative Study of Protection Requirements of Active Distribution Networks Using Radial and Ring Operations

Abstract

Configuration, operation and management of existing distribution networks are based on passive networks, whereby the role of such networks is confined to transferring electricity from generation and transmission system to load centres. Under this situation, the main function of distribution network is delivering good quality electricity with minimum amount of network control to the load centres at different voltage level regardless of the most critical case. Most distribution networks are designed to operate in radial mode without having generation unit accommodate on them. Continued increase of distributed generation (DG) penetration into existing distribution networks in recent years has resulted in bi-directional power flow and changes in network voltage profile. These in turn caused important impact on the operation of conventional feeder protection schemes that are originally designed based on radial network operation assumption. The protection issues that had been identified in the presence of DG include (i) malfunction of protective devices due to increase of network fault current level, (ii) unnecessary tripping of non- directional relays due to significant fault contribution by DG, (iii) reduction of fault detection sensitivity of overcurrent relays due to fault contribution of DG, (iv) fault clearing time of graded overcurrent relays is affected and (v) operation of re-closer is affected with introduction of LOM relay. It has been reported that future distribution networks are likely to be operated in ring mode rather than radial in order to optimise the integrated capacity of DG. There are advantages in adopting ring mode operation but it could complicate the protection systems of networks with integrated DG. This paper reports an investigation into the impact of distributed generation on the setting of protective devices of distribution networks that originally operated in radial mode and then switched to ring mode and to compare the protection requirements for the two modes of operations.