A RoCoF-constrained underfrequency load shedding scheme with static voltage stability-based zoning approach

Abstract

The modern power system is proliferating in terms of size, capacity, and nature of integrated components. Such expansion engendered an increasing risk of system strength and security, posing a more significant operational challenge to power system operators (PSO). The frequency dynamics of the system under numerous possible contingencies serve as a metric to evaluate how efficiently the grid is being controlled. There are a couple of defensive mechanisms in case of inadvertent contingency events, namely - inertial response, governor control, underfrequency load shedding, and dispatch control. Inertial response and governor control institute the primary line of defense to arrest the frequency variation and Rate of Change of Frequency (RoCoF) and are triggered automatically from the dispatched machines. However, the final defense mechanism - load shedding (LS)– is designed and often controlled by the operators. There are numerous methodologies of how LS can be designed and implemented in a power system, many of which rely on some manipulation of historical operational data. Although it has been a widespread practice for PSOs to divide the grid into several zones for higher operation and control advantage, there has been very little research on how LS mechanisms can make use of the zonal allocation to tend to the local grid needs for more secure and reliable network performance. To this extent, this paper proposes a static voltage stability-based zoning mechanism using Fisher-Jenks natural breaks algorithm. The zoning scheme uses the reactive power margin of buses to cluster them into several zones. Subsequently, unique underfrequency load shedding (UFLS) schemes are devised for each zone stemming from the frequency response characteristics of that zone, particularly RoCoF. The proposed RoCoF-constrained zonal UFLS scheme is tested on IEEE Standard 39 bus network. The zonal scheme not only shows significant improvement in frequency response in individual zones over conventional UFLS schemes but also achieves that with less load shedding throughout the grid.