Voltage Balance and Control in a Multilevel Unified Power Flow Controlier

The neutral-point-clamped multilevel converter is an attractive implementation of the unified power flow controller because it facilitates back-to-back operation, high-voltage operation (without direct series connection of devices), and low distortion (without the use of multipulse transformers). A UPFC using three converters is proposed. Two phase-shifted converters are required to provide a full range of voltage control of the series connection while ensuring low distortion and a balanced dc link. A single shunt converter is used. A commutation angle solution that balances the voltages of the multiple dc link capacitors is analyzed in terms of the active power balance at each node. Control of shunt reactive power requires a variable dc link voltage. Control schemes for both shunt and series converters are developed and verified in terms of voltage balancing and power flow control on a micro-scale experimental system using five-level converters.