In a power system, the goal of the protection system is to identify and clear fault conditions to minimise damage and disruption to the network and equipment, while maintaining reliability, security, sensitivity and selectivity. The ever increasing penetration of DG sources threatens to negatively impact the operation of the protection system, and affect each of the four aims of the protection system.
Tests are carried out on a 60 kV ring network, based on a real Danish system, with a 26 MW CHP plant and a 60 MW wind farm of full-rated VSC-connected wind turbine generators. A model is developed in DIgSILENT PowerFactory and tested according to the IEC 60909 standard for short-circuit currents, to determine the extent of the effect of DG sources on the protection system
The results find that the largest effect seen is the reduction in reach of the distance relays, particularly the back-up 3rd zone. Each of the reliability, security, sensitivity and selectivity are affected, through the direct effect of a reduced reach and through corrective measures that can be taken to attempt to restore the protection to its desired level. Problems encountered include: increased fault clearance time; loss of back-up protection for portions of circuit; reduced loadability.