Multiple converter performance and active filtering

The application of active power filtering to power systems in limited by the low switching rate of available high power inverter switches. In this paper, parallel and series connection of multiple voltage source inverter bridges are examined to increase their effective switching rate. The analysis of the inverter voltage spectrum for double edge modulation shows the modulation desired for the separate bridges in open loop. Intuitively “i” bridges in series or parallel increases the effective switch rate by a factor “i”. However the modulation process for each power converter maintains its pattern for its switching period T which gives a roll off at high frequencies and reduces effective bandwidth. The experimental system of two 10 kVA voltage source inverters demonstrated how the closed loop control strategies for the active filter can be applied for multiple bridges implementing periodic optimized error sawtooth feedback control. The double bridge closed loop system showed that the lowest switch frequency terms were double that of the separate bridges. The higher frequency switching lines that should have been cancelled in theory were still discernible due to the finite precision of the edge timing