An Underfrequency Load Shedding Scheme for Hybrid and Multiarea Power Systems

The purpose of underfrequency load shedding (UFLS) is to maintain the frequency of an interconnected or island power system within permissible limits in case of severe power deficits. Thus, the proper performance of UFLS scheme is of great importance. In hybrid power systems, disconnecting and connecting the power generation sources and loads result in variations in the system parameters. Moreover, the output power of some distributed energy resources, such as photovoltaic (PV) or wind turbine generator (WTG), might change during the load shedding process. These factors may affect the performance of the load shedding scheme and decrease the reliability of power system. Hence, in this paper, a UFLS scheme is proposed that has two advantages: 1) it is independent of power system parameters; and 2) it considers power generation variations during the load shedding process. To verify the effectiveness of the proposed load shedding method, simulation studies are carried out on a hybrid power system including PV, WTG, fuel cell, diesel generator, and battery energy storage system. Finally, the performance of the proposed method on a multiarea power system is evaluated.

I. Introduction

In severe frequency decline situations, underfrequency load shedding (UFLS) is the only appropriate way to prevent a power system from collapsing. Underfrequency operation could be a huge threat to the secure and stable operation of a power system [1]–[4]. Therefore, the suitable amount of load should be shed as soon as possible to retain the power balance and prevent the frequency from falling below the specified value. The most common protection scheme used in power systems for this purpose is UFLS [5]. UFLS schemes use the frequency of system or its derivatives to determine the amount of power deficit and compensate it by several steps of load shedding [6]–[10].