Impact of Solar PV Penetration on Overcurrent Relay Coordination in Radial Distribution Networks

The trend on the growth of the installation of solar photovoltaic (PV) systems in the current radial distribution networks radar has altered the system protection dynamics largely. This paper examines the quantitative effects of different PV penetration rates on over current relay (OCR) coordination in a system of differing grid strengths strong, medium, and weak by short circuit and relay coordination of MATLAB simulations. Two models of inverters INV1.2 and INV2.0 were used in order to test how the limit of current in inverters affects the performance of protection. The suffix indicates the per-unit limit of the short-circuit current contribution of the inverter to its actual current. In particular, the maximum current of faults is limited to 1.2 p.u. by INV1.2 and to 2.0 p.u. by INV2.0 before being saturated. Such limits attempt to reproduce the protection type logic of a current-day grid-following inverter when there is a fault. The inverter current-limiting models are represented by the acronyms INV1.2 and INV2.0, in which the short-circuit contribution of the inverter is limited to 1.2 and 2.0 per-unit of rated current give or take. INV1. The findings indicate that fault current in robust grids had risen by 55 per cent (5.1 kA to 7.9 kA) and in weak grids by ？20% (5.1 kA to 4.1 kA) with 100 % penetration of PV. The coordination time interval (CTI) between primary and backup relays decreased to a value of 0.18 s, which means that selectivity is partially lost. Further, INV2.0 decreased average operating time of the relays (15 - 25) when compared to INV1.2 at the expense of more miscoordination events. In all the simulated cases, the mean time of operation of the relays was 0.80 s (strong grids), 1.10 s (medium grids), and 1.50 s (weak grids). These findings affirm that, the level of protection coordination is becoming more sensitive to network impedance and relay settings as the level of PV penetration and inverter current capacity rises. The results highlight that conventional fixed-setting relays cannot be used in high-renewable systems and that adaptive or AI-aided protection coordination is needed to maintain system reliability and selectivity.