Dynamic compensation of reactive power in Various Faults in Power System

The STATCOM (Synchronous Static Compensator) based on voltage source converter (VSC) is used for voltage regulation in transmission and distribution system. The STATCOM can rapidly supply dynamic VARs required during system faults for voltage support. Strict requirements of STATCOM losses and total system loss penalty preclude the use of PWM (Pulse-Width Modulation) for VSC based STATCOM applications. This constraint of implementing VSC without PWM functionality, results in over-currents and trips of the STATCOM during and after system faults, when its VAR support functionality is most required. In this paper, we propose and develop an "emergency PWM" strategy to prevent over-currents (and trips) in the VSC during and after single line to ground system faults, LLLG faults and to ensure that the STATCOM supplies required reactive power. The Simulation results are shown for a 48-pulse VSC based ± 100 MVAR STATCOM connected to a 2- bus power strategy to prevent VSC over-currents and to supply required reactive power under line to ground system faults. Keywords: STATCOM, Voltage Source Converter (VSC), Pulse-width Modulation (PWM), Single line to ground fault. I. Introduction The Flexible AC Transmission systems (FACTS) controllers are emerging as an effective and promising alternative to enhance the power transfer capability and stability of the network by redistributing the line flow and regulating the bus voltages. Static VAR compensator (SVC) and Thyristor controlled series compensator (TCSC) are some of the commonly used FACTS controllers, The developments in the field of power electronics, particularly Gate Turn-off (GTO) based devices, have introduced a new family of versatile FACTS controllers, namely static synchronous compensator (STATCOM), The STATCOM is one of the custom power devices that received much attention for improving system stability, with the development of power electronics technology, custom power devices play important role in bringing unprecedented efficiency improvement and cost effectiveness in modern electrical power system (1,2). The custom power is relatively new concept aimed at achieving high power quality, operational flexibility and controllability of electrical power systems (3-5). The possibility of generating or absorbing controllable reactive power with various power electronic switching converters has long been recognized (6-8). The STATCOM based on voltage source converter (VSC) is used for voltage regulation in transmission and distribution systems (8-12). The STATCOM can rapidly supply dynamic VAR's during system faults for voltage support. In this paper, we propose and develop an "emergency PWM" strategy to prevent over-currents (and trips) in the VSC during line to ground faults, all though PWM technique results in higher switching losses but it recompense total system loss. This limitation of implementing VSC with PWM functionality, results in avoiding over-currents and trips of the STATCOM supplies required reactive power. With "emergency PWM" strategy STATCOM gains capability to prevent over-currents and trips in the VSC based STATCOM. Simulation results are presented for a 48-pulse VSC based ±100 MVAR STATCOM connected to a 2-bus power system. The operating characteristic of compensator during steady state, capacitive and inductive modes validate "emergency PWM" strategy (13) to prevent VSC over-currents and to supply required reactive power under line to ground system faults (9-12).