Control strategies for distributed generators operating on weak distribution networks

The application of DG in South Africa is dominated by schemes operating synchronous generators. Reports have indicated the possibility of overvoltage problems that result when operating DG on lightly loaded feeders. It was expected that the "weak" nature of South Africa's distribution networks might exaggerate these effects. A load-flow study was undertaken to investigate the impact of various system parameters on the allowable levels of generation by DG before the onset of network overvoltage conditions. Among the parameters considered are source impedance, feeder load and voltage level. The results of the load-flow simulations were used to develop a composite control strategy for synchronous DG that could be applied when the penetration limits for unity power factor generation are too restrictive. A generalised approach to selecting the control mode for synchronous DG is proposed, and is applied to two South African DG case studies with good results.