An Improved UFLS Scheme based on Estimated Minimum Frequency and Power Deficit

In the event of a power system disturbance, it is important that the decision to implement under frequency load shedding is based on both the minimum frequency and the magnitude of the disturbance. In this paper, we propose the use of higher order polynomial curve fitting to estimate the minimum frequency. If the prediction shows that the minimum frequency threshold will be violated, the magnitude of the total disturbance is estimated using the swing equation. In addition, the minimum amount of load that must be shed to restore the frequency just above the minimum value can also be directly calculated. Simulations are carried out for the considered Taiwan power system and the results prove the efficiency of the proposed technique.

I. Introduction

Modern power systems are usually loaded close to the steady state stability limit and this threatens their secure operation. Among the most common stability issues that arise in power systems, frequency stability is becoming very popular [1]. This is because when an online generator trips, or a heavily loaded transmission line is interrupted, the demand-generation balance is affected. In such cases the power systems frequency begins to decline also below the minimum thresholds if not quickly restored [2]. Under frequency load shedding (UFLS) technique comes as a tool to quickly curtail connected loads in order to restore the frequency. Through relaying action, trip signals are sent to load buses that participate in the load shedding. Traditional UFLS is a widely used technique to curtail load in emergency scenarios. However, this technique has proven to have challenges including not-optimum load shedding [3]. In this regard, researchers have come up with different teclmiques to address such problems. These teclmiques are generally classified as semi adaptive teclmiques, adaptive techniques and computational intelligent teclmiques [4]. Even if these teclmiques have shown great improvement from the old traditional technique, new research is still required to accurately define the indicators to trigger UFLS relays.