Effects on electrical distribution networks of dispersed power generation at high levels of connection penetration

Abstract

The advent and deployment of significant levels of photovoltaic and wind energy generation in the spatially dispersed mode (i.e. residential and intermediate load centres) may have deleterious effects upon existing protective relay equipment and its time-current co-ordination on radial distribution circuits to which power conditioning equipment may be connected for power sell-back purposes. Note that significant in this context is now generally agreed to mean greater than 50% of a feeder's minimum load capacity rating. The problems that may arise involve harmonic injection from power conditioning inverters that can affect protective relays and cause excessive voltage and current from induced series and parallel resonances on feeders and connected passive equipment. Voltage regulation, var requirements, and consumer metering can also be affected by this type of dispersed generation. The creation of “islands” of supply is also possible, particularly on rural supply systems. This paper deals mainly with the effects of harmonics and short-circuit currents from wind energy conversion systems (WECS) and photovoltaic (PV) systems upon the operating characteristics of distribution networks and relays and other protective equipment designed to ensure the safety and supply integrity of electrical utility networks. Traditionally, electrical supply networks have been designed for one-way power flow—from generation to load, with a balance maintained between the two by means of automatic generation and load-frequency controls. Dispersed generation, from renewables like WECS or PV or from non-renewable resources can change traditional power flow. These changes must be dealt with effectively if renewable energy resources are to be integrated into the utility distribution system. This paper gives insight into these problems and proposes some solutions.