A Passive Islanding Method to Improve the Stability of Power Supply to Critical Loads in a Real Distribution Network

Abstract

This study proposed a modified passive islanding technique that combines over/under frequency (OUF), rate of change of frequency (ROCOF), rate of change of phase angle difference (ROCPAD), and rate of change of voltage (ROCOV) for sensing of islanding and nonislanding conditions. The technique was implemented in MATLAB/Simulink and applied to the distribution network of the Federal Capital Territory (FCT) of Nigeria containing 120 buses to maintain a stable power supply to its critical loads during grid outages. The method utilized the frequency, voltage, and current signals as input to the combined method. For each of the relays, the low threshold was for a fast operation while the high threshold was for a secure operation. First, the OUF, ROCOF, ROCOV, and ROCPAD relays were combined using an AND gate to limit the relay operation during short circuits since the ROCOV measures the voltage drop and determines whether it is in grid-connected or islanding mode. After that, the higher set threshold of the ROCOF was connected using a logical OR gate to limit the operation of the relay under transients and noises. Finally, the higher set threshold of the ROCPAD was used to sense islanding in case the lower threshold failed to initially detect it. The efficacy of the technique was tested on the FCT distribution network, and the result showed successful islanding for the loss of the grid supply and loss of power supply due to fault on the 330 kV line, while the loss of 33 kV feeder and loss of power supply at a single supply point were nonislanding events. After islanding due to an outage and loss of the 330 kV line, it took 0.01138 and 0.000316 s to switch to the DES, and the optimal capacities that supplied power to the critical loads were 33.2 MW at Bus 13, 18.9 MW at Bus 120, 48.4 MW at Bus 72, and 45.1 MW at Bus 46. The total connection of the DES was able to supply 144.8 MW out of the total peak load of 600 MW which represents about 24.13% of the total system load referred to as the critical loads. Therefore, connecting DES capable of islanding operation to the distribution network improves power supply security, supply continuity to consumers, and customer satisfaction.