A Probabilistic Multi-Stage UFLS Framework Considering Distributed Generation with RoCoF Relays

The under frequency load shedding (UFLS) is considered to be the ultimate action (automated) to reinforce the frequency stability of the power system following extreme disturbances resulting in a remarkable frequency drop in the interconnected system. This article proposes a probabilistic multi-stage UFLS scheme that considers uncertainties such as generation deficiency, system inertia, and load damping. Monte Carlo Simulation is used to model the uncertainties of the power system parameters. The rate-of-change-of-frequency relays are activated to ensure the anti-islanding protection of distributed generators. The optimization problem is solved using particle swarm optimization, accounting for several system constraints. In this paper, four different UFLS strategies are studied based on various frequency decline criteria. Finally, the proposed UFLS framework is assessed on the IEEE-39 bus system.

I. Introduction

It is crucial to sustain the power systems' frequency stability by maintaining the grid frequency within admissible limits, to ensure the stable operation of a power system [1]. The sudden load-generation imbalance results in a deviation in the system frequency. In unbearable frequency decline circumstances, under-frequency load shedding (UFLS) is the rearmost automated action to restrain the system from uncontrollable system isolation and total breakdown [2]–[3]. To restrain the transmission of cascading system outages, it is crucial to implement a robust and automatic UFLS plan. The UFLS study is carried out for optimal tuning of the UFLS relays, which are situated at several points of the power system [4].