Abstract
The aim of this research work is to develop a load shedding methodology to improve the frequency response of low inertia grids by attaining satisfactory voltage stability. In recent times, wind energy integration has considerably increased in many power grids. Consequently, conventional synchronous machines are being replaced from dispatch. Unlike traditional synchronous machines, variable speed wind turbine generators usually do not take part in frequency regulation without supplementary control mechanism. During substantial wind penetration, a power system may have a small number of online synchronous machines. As a result, synchronous inertia and governor responsive reserve significantly reduce. Under such situation, a system has to rely on load shedding as a last line of defense to rescue the system frequency following a large contingency. However, the conventional Under-Frequency Load Shedding (UFLS) strategy may lead to larger frequency deviation and higher amount of load cut in certain cases. A new load shedding methodology is presented in this paper to overcome this challenge. Unlike conventional UFLS technique, higher proportion of load shedding is applied to relatively weaker buses in terms of voltage stability in the proposed mechanism. Based on reactive power margin, which is an index to specify voltage stability, a general expression to quantify load shedding is derived. Also, the adaptability of the proposed strategy to various load levels is ensured. Later on, performances of the developed strategy are explored in a low inertia wind dominated test network. Simulations are executed considering various penetration levels of wind power and for two severe contingencies - loss of 550 MW interconnection and loss of 650 MW interconnection. Investigations reveal that the proposed load shedding methodology ensures satisfactory frequency response in all simulation cases. Also, the developed technique yields less frequency deviation and load cut compared to the conventional UFLS mechanism. Therefore, the reported load shedding scheme is found to be more competent to concurrently maintain frequency and voltage stabilities in renewable dominated power systems.
Highlights
Inadequate frequency response following a large contingency is a major concern in renewable dominated power systems [1]
Reactive power margin is deployed as a tool to determine the required amount of load to be shed at each P-Q bus
The reactive power margins of all P-Q buses are determined from the corresponding Q-V curves
Summary
Inadequate frequency response following a large contingency is a major concern in renewable dominated power systems [1]. Prolific penetration of renewable resources, especially wind power integration in transmission levels, causes the replacement of conventional fossil-fuel based synchronous generators from the generation mix [2]. Modern wind machines frequently utilize Type-III and Type-IV (Doubly-Fed Induction Generator: DFIG and Full Scale Converter: FSC respectively) wind turbine generators. These are variable speed machines, which are electrically detached from the corresponding host network via power electronics interfaces. As a result, these WTGs are incapable of offering inertial support and governor response without additional control strategy [3]. Frequency response of a power system is likely to deteriorate under high wind penetration [4]
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