Fast and adaptive under Frequency Load Shedding and restoration technique using rate of change of frequency to prevent blackouts

Abstract

This paper explains a fast acting Under Frequency Load Shedding and restoration scheme. This load shedding technique has an added advantage of selecting the amount of load which is to be dropped at a particular frequency threshold. The amount of load dropping is selected based on the rate of change of the frequency. Automatic restoration of loads is discussed with engaging a command signal from the system control centre. On/Off control of the power factor correction capacitor is also explained based on the magnitude of its terminal voltage. A three phase voltage vector Phase Lock Loop (PLL) is developed in EMTDC/PSCAD environment to measure the frequency of the power system. The PLL is tuned together with a filter to smooth the frequency measurement. This PLL measures the frequency online and based on phase angle deviation. Therefore it has a fast initial response. A load shedding factor is selected based on the rate of change of frequency between the specified thresholds. According to the load shedding factor the amount of load is selected. Load shedding and automatic load restoration operations are triggered accordingly when the frequency passes its relevant thresholds. The proposed distributed load shedding and restoration control technique was tested for IEEE 12 bus system at two different grid substations. The simulation results confirmed the fast, proper and smooth operations of the proposed technique.