Magnetic Saturation in Transformers used for a 48-pulse Voltage-Source Converter based STATCOM under Line to Line System Faults

The STATCOM (Synchronous Static Compensator) based on voltage source converter (VSC) is used to supply dynamic VARs required during power system faults for voltage support. However, magnetic saturation of transformers used in VSC topology result in over-current and trips of the STATCOM during system faults, when dynamic VARs and voltage support are required the most. In this paper, we propose and develop an "emergency PWM" strategy to prevent over-currents in VSC and transformer magnetic saturation for line-to-line system faults. A plusmn 100 MVAR STATCOM with a 48-pulse VSC based on 3-level Neutral Point Clamped (NPC) topology and four series connected transformers with realistic saturation (B-H) characteristics are considered. Simulation results are presented for a 48-pulse VSC based plusmn 100 MVAR STATCOM connected to a 2-bus power system to validate the "emergency PWM" strategy to prevent VSC over-current and transformer saturation and supply reactive power under line-to-line system faults. A practical issue of impact of slightly different saturation (B-H) characteristics of the four series transformers, on VSC performance is shown by simulation results for line-to-line fault.