A New Approach to the Online Estimation of the Loss of Generation Size in Power Systems

Following an unintended disconnection of a synchronous generator from the power system, what is also known as a loss of generation (LoG), it is not trivial to precisely estimate the post-event power system's inertia and the LoG size. One of the reasons for that is that both of them are a function of the unknown inertia reduction. To solve this challenging problem, this paper presents an analytical method based on the rate-of-change-of-frequency (RoCoF). The method relies on a modified swing equation, allowing a simultaneous estimation of both unknowns. To this end, the values of mechanical starting time, apparent power, and loading of lost generator are formulated for the power system under study. In a practical application, the method can use RoCoF measured by phasor measurement units. The paper discusses the impact of various frequency estimation approaches to the proposed LoG estimation. Furthermore, a new method for LoG size estimation, based on the interpolated estimated inertial response, is proposed. The efficiency of the proposed approach is validated through extensive simulations with Matlab/Simulink using a simple power system and the IEEE 39-bus test network.

I. Introduction

Loss of generation (LoG) can cause frequency instability and this phenomenon is of particular interest for future power systems characterized by high penetration of power-electronic-interfaced renewable generation that produces a reduction of the system's inertia [1]. Enhanced frequency control mechanisms are expected to provide fast frequency response to mitigate the consequences associated to the system inertia reduction and these control mechanisms do require a suitable coupling with wide-area monitoring and control systems [1], [2]. As well-known, the inertial response (IR) in power systems comes from the kinetic energy stored in the rotating shaft of SGs and rotating electrical motors from the demand side. Immediately after a LoG, the lost power is supplied by the IR and after that by the SGs governors [3]. In the other hand, under-frequency load shedding (UFLS) has been commonly used for preventing frequency instability following severe contingencies. In conventional UFLS schemes, non-critical loads are sequentially shed, based on a-priori assumptions and past experience [4]. However, it is essential to minimize the amount of load to be shed using adaptive mechanisms [5]–[9]. In the framework introduced in [5], the optimal load shedding plan can be created based on the estimated power imbalance using the swing equation. In [6], the LoG size is estimated by considering voltage dependency of the loads for adjusting the adaptive UFLS schemes in order to utilize as much as possible the primary frequency support. In [7] a strategy for updating the estimated LoG during load shedding process through the system's inertia constant estimation is introduced. In [8], load shedding amount is analytically derived by solving the swing equation considering ramp-shape governor response to achieve minimal load shed. Recently, an improved UFLS is presented. It is based on the calculation of power imbalance during load shedding process using equivalent swing equation of power system [9].