Effect of residential solar PV penetration on distribution network protection schemes

Abstract

Electricity generation from solar PV has been increasing due to many reasons including heavy environmental considerations, reduction of burning fossil fuels, fast developments in the solar technology and the industry, consequent reduction in investment costs etc. Especially in Sri Lanka, the government took lot of steps to encourage the consumer end to install roof top Solar PV`s. As an initiation Government revised the tariff system and provided considerable financial support for the consumers to encourage PV generation. Especially, in the urban areas the number of rooftop solar PV connected to the LV distribution system (0.4 kV) has increased. Due to this heavy solar PV penetration, fault current seen by the grid side has been changing. Hence, it is required to analyze the changes in fault current levels and consequent impact on the protection coordination in the medium voltage (MV) distribution system. The main objective of this research is to assess the impact of increasing solar PV penetration levels on the MV level protection coordination and to propose a methodology for revising the protection settings for proper relay coordination with the increasing solar PV levels. For the analysis a MV feeder having high penetration of solar PV has been selected and simulated. Modelling of the low voltage (LV, 0.4 kV) distribution system with widely dispersed solar PV integration has been a key challenge in this research. As the protection coordination has been evaluated at the MV side, the total LV distribution system has been simulated as an equivalent system and the solar PV has been modelled as an equivalent voltage source inverter. The equivalent solar PV model has been validated using already verified built-in solar PV model in PSCAD/EMTDC version 4.5. Initially, performance of the MV level protection coordination has been analyzed in the selected feeder without solar PV being integrated. Then, the performance of the protection system has been analyzed based on the present PV penetration levels and fault current variation for different fault locations considering all probable fault types. Analysis indicated that, with the present PV penetration levels, existing protection settings can still be used, as they effectively maintain the required protection coordination. After analysing the system behaviour with increasing solar penetration levels, a generalized method has been proposed to evaluate the MV level protection system performance. If the any system relay parameters fail to perform the expected outcome, a technique has been proposed to identify a criterion to review the relay settings.