Comprehensive and quantitative analysis of protection problems associated with increasing penetration of inverter-interfaced DG

In the last two decades there has been a continuous increase in the amount of Distributed Generation (DG) connected to the power network. This has resulted in a number of technical challenges for network protection including: sympathetic tripping of DG interface protection; blinding of feeder over-current (OC) protection; and deterioration of feeder OC protection grading. While the available literature typically analyses the impact of synchronous and induction DG (which is characterised by a high fault current contribution), this paper focuses on the impact of inverter-interfaced distributed generators (which are characterised by low fault current contributions). This paper quantifies the ‘tipping points’ at which protection problems occur by modelling a series of fault scenarios on a typical UK distribution network with inverter-interfaced DG, all simulations have been modelled in PSCAD. The results of this paper suggest that the occurrence of sympathetic tripping is highly dependent on the grid infeed fault level and the inverter-interfaced DGs fault location; protection blinding is unlikely to be a problem on UK power networks as it was not observed in any of the scenarios simulated in this paper; in all cases of protection grading deterioration the grading margin between the protection devices was found to increase meaning the protection devices never operated in an uncoordinated fashion. The tipping point at which these protection issues occurred was evaluated on a typical UK network. The conclusions drawn from these simulations may not be equally valid on future power networks that could employ novel network architectures and higher penetrations of DG.