Study of the settings of over speed protection for enhancing frequency stability : a case study for Lakvijaya power station

Abstract

The main target of the large interconnected Power Systems (PS) present today is to provide continuous and quality electricity supply to the consumers with highest possible reliability. “Fast Valving” (FV) action of Steam Turbines is one of the most effective elements for enhancing the transient stability of a power system in order to maintain a healthy power supply under the large and sudden disturbances. FV plays a significant role in mitigating the impact of sever disturbances by instantly decreasing steam turbine power, thus ensuring the Power System Stability (PSS). Sri Lankan power system currently comprises 4,043 MW generation capacity consisting of 900 MW of coal power, 1,215 MW of oil burning thermal power, 1,720 MW of hydropower and 208 MW of non-conventional renewable energy sources such as wind, mini hydro, biomass, and solar power plants. Lak Vijaya Power Plant (LVPP) is the first and only coal power plant which is connected to Sri Lankan Power System and it contributes almost 45% of total power requirement of the nation. LVPP has three identical generating units driven by steam turbines each having 300MW capacity. Main objective of this research was to study the behaviour of “Over Speed Protection Control” (OPC) unit of LVPP which has almost the same function of FV and to establish the main objective of the OPC function. This study further investigates the effects of FV scheme of LVPP on the transient stability of Sri Lankan Power System. The study further explores the procedure to obtain the optimum valve actuation timing of the Intercepting Valves (IVs) for the FV scheme while proving that FV has much better control over transient stability than OPC. The results of the study further unveil that proper selection of actuation timings of IVs for FV scheme of LVPP steam turbines has a significant effect on enhancing the power system frequency stability under transient conditions. As the main objective of this study, the optimum ranges for actuation timings of IVs for FV scheme of LVPP were found based on frequency stability of Sri Lankan power system under the transient conditions and a set of timings for FV scheme were recommended to set as actuation timings of IVs. The sensitivity of each time settings were also examined and the relationships were clearly established.