The Refinement of Generation Scheduling and Underfrequency Load Shedding Protection Scheme for Nangan-Beigan Power System in Taiwan

The offshore Nangan-Beigan power system in Taiwan was suffered frequently from generator tripping contingency. Previous underfrequency load shedding (UFLS) protection scheme could shed load properly to undergo many N-1 contingencies. However, due to surplus generation capacity, Nangan-Beigan power system was operated under half generation ratings for a long time, and was inclined to trigger consecutive generator tripping N-1-1 contingencies which led to several complete blackouts in Nangan-Beigan power system. To mitigate the negative effects of under half rating operation, system operators sometimes operates single generation unit over 70 percents of rating, but unfortunately such an operation also increases the risk of complete blackout. This paper describes the refinement of generation schedule and UFLS protection scheme for Nangan-Beigan power system that can avoid complete blackout. In addition, the refined UFLS protect scheme is superior in that it can shed load less than the previous one. Moreover, the refinement considers generation schedules that can satisfy not only normal operation but also when Nangan-Beigan power system performs over 70 percents rating operation. Comprehensive computer simulations and practical operating records are presented to demonstrate the effectiveness of the refinement. The experience gained in the refinement can assist power engineers in designing UFLS protection scheme.

I. Introduction

Under frequency load shedding (UFLS) protection appears to be the last resort to protect electric power system from blackout [1]-[4]. Normal power system operation hinges on the balance of power supply and load demand. If generation units trip suddenly, power supply becomes insufficient and power system frequency starts to decline. Such a decline may trip more power generation units and, just like a vicious circle, may cause further frequency decline, and eventually may lead to complete blackout. To break the vicious circle, the UFLS protection must be summoned up quickly to shed part of system load instantly. The idea behind UFLS is to shed part of system load for the sake of saving the remaining part of power system in operation so as to avoid complete blackout and to assist restoration [5]-[7].