The STATCOM (Synchronous Static Compensator) based on voltage source converter (VSC) is used for voltage regulation in transmission and distribution systems. 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, and to ensure that the STATCOM supplies required reactive power. The operational results for a 48-pulse VSC based 150 MVAR STATCOM are presented to illustrate the problem during and after a system fault, and motivation for this work. Simulation results are presented for a 48-pulse VSC based 100 MVAR STATCOM connected to a 2-bus power system. Simulation results validate the "emergency PWM" strategy to prevent VSC over-currents and to supply required reactive power under a single line to ground system faults.
