Combinational scheme for voltage and frequency recovery in an islanded distribution system

Abstract

In low-voltage autonomous micro-grids, preserving both frequency and voltage stabilities at event of severe contingencies is an unsolved technical challenge. The conventional control algorithms are designed based on either one of frequency and voltage stabilities and do not bring satisfactory performance for real world practices. As per experiences in the transmission level, load shedding (LS) can be an effective countermeasure to overcome this challenge. This study introduces an under voltage-frequency LS (UVFLS) scheme to achieve proper load shed amounts (LSAs). The voltage effect is accounted for in two online and real-time mode stages. A quick computational algorithm is proposed in the online stage to obtain `nose' curves of the network as a linearised piecewise function of voltage/load level. Then in the real-time operation stage, according to these curves and the present operating points of the system, optimum LSA is calculated. The frequency droop characteristics of distributed generation units are employed to incorporate the effect of frequency on the LSA. Simulation studies are carried out on two distribution systems, a 5-bus autonomous micro-grid and the modified standard IEEE 37-bus system. Validation of the proposed method shows that frequency and voltages are restored to their desired ranges using the novel UVFLS algorithm.