Response of Low Voltage Networks with High Photovoltaic Systems Penetration to Transmission Network Faults

Abstract

The need for fault ride-through (FRT) capability is not yet documented for the low voltage connected DG. The realization that an increase in the capacity of installed DG power in the specific voltage level can trigger disconnection, in case of transmission faults, of massive amounts of power from DG and jeopardize system stability is forming. In order to investigate the response of low voltage distribution systems with high penetration of PVs, in case of a fault in the transmission system, a complex network model is created consisting of different voltage levels, from eHV (220 kV) to LV (400 V). This model is implemented as a benchmark system with DG capacities chosen to match the German network. Also an elaborate PV model is used to simulate different behaviours during and after fault and assist in examining the impact of different sensitivity factors on network stability. The problem is analysed through root mean square (RMS) stability simulations performed on the models of the network and the DG. The projected year that this study takes place is 2022 and the DG capacity is chosen for that year. The commercially available DIgSilent PowerFactory software version 14.1 is used to carry out these simulations. A sensitivity analysis is performed for different control modes of the PV that range from disconnection in the event of a fault to full dynamic network support during fault. Also different pre-fault states of the network are used to examine the effect of the state of the network in the results; these states vary from normal top-down power flow to full reverse power flow when PVs are at full capacity. Moreover the effect of delayed post-fault active power recovery of the LV connected DG is discussed. The conclusions and analysis show the importance of introducing FRT criteria even for low voltage connected DG and give an insight on the behaviour of the system under the circumstances described. The differences between control modes are affecting stability of the system and the various phenomena that take place are analysed and explained. Finally corrective steps that need to be taken in order to avoid future stability problems for the network are outlined and justified and recommendations for future grid connection requirements for low voltage installed DG are given.