STATCOM Operation under single line-ground system faults with magnetic saturation in series connected transformers based 48-pulse voltage-source converter

In this paper, we investigate an "emergency PWM" method to prevent over-currents in VSC based STATCOM and control transformer magnetic saturation under single line-ground (SLG) power system faults. The 100 MVA 48-pulse VSC topology is based on four series connected transformers on the primary side and each transformer secondary is connected to a 3 Phi, 3-level Neutral Point Connected (NPC) VSC, with a common dc bus. A practical issue of impact of slightly different saturation (B-H) characteristics of series connected transformers, on VSC performance is investigated for SLG faults in the power system. The detailed simulation results for a 48-pulse GTO thyristor based VSC +100 MVA STATCOM connected to a 2-bus power system under SLG system faults validate that the proposed "emergency PWM" enables STATCOM operation under transformer saturation without over-currents in the GTO devices. The proposed "emergency PWM" strategy prevents STATCOM tripping under SLG power system faults.