Fast Valving (FV) mechanism of steam turbines is considered to be one of the most effective actions for enhancing the security of a power system subsequent to disturbances. It plays a significant role in mitigating the impact of severe disturbances by instantly decreasing steam turbine power, thus ensuring the power system stability. There is a direct relationship between the settings of Over-Speed Protection Control (OPC) unit of steam turbines and the control functions of FV. This paper proposes a generalized simulation-based procedure to determine the optimum intermediate valve actuation timings of the FV control action. A coal-fired thermal power plant has been used as the case study and the accuracy of the settings made using the proposed approach has been demonstrated using actually happened contingencies in a real power system. Finally, it is concluded that the proper selection of actuation timings of intermediate valves for FV scheme has a significant effect on enhancing frequency stability under transient conditions.

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Contamination-driven insulator failure is a problem that incessantly plagues distribution systems. Contamination levels can continue to grow unless abated by natural cleaning or if not taken measures to wash insulators at the distribution level in a preventative maintenance mode. In order to assess the pollution behavior, 33kV pin type insulator was selected as a sample insulator and been subjected for natural pollution at three selected localities (zone 1, 2 and 3) for considerable period. The naturally polluted insulators have been subjected for conductivity test and by which the equivalent salt deposit density (ESDD) is calculated. Subsequently, artificially contaminated insulators of different pollution severities were tested for power frequency and impulse test in the High Voltage Laboratory of University of Moratuwa. Finally, it is recommended that insulators in Zone 1 have to be treated after 8 months from the date of last treatment and those in Zone 2 & 3 to be treated after 18 months under preventive maintenance to get away from flashover. It is also recommended to review the required specific creepage distance (SCD) of insulators installed in non-polluted areas due to the fact that the current practice of insulators placement in most of the places are with SCD of 25mm/kV which is recommended for high polluted zones as per IEC regulation.