High Performance AC-AC Regenerative 5-Level NPC Converter with Capacitor Voltage Balancing

Regenerative AC-AC converters are required for serving AC loads that draw and return power during operation, when input is also taken from an AC source. AC motor drives, static VAR compensators, harmonic compensators and inverter testbeds are some of the examples using regenerative AC-AC converters. Generally, these are implemented as 2-level converters and operated with Pulse Width Modulated (PWM) control, incorporating higher switching frequencies. The use of higher switching frequencies leads to correspondingly higher switching losses and EMI issues. This paper describes the development of a 5-level regenerative AC-AC converter that can deliver equally good performance but operating at very low switching frequencies. It was developed as part of a testbed for performance testing of grid-tie solar PV inverters. The output specifications were accordingly drawn to comply with the requirements of IEEE1547. Input specifications were chosen to produce sinusoidal current and inphase with input voltage all the times. The converter employed back-to-back connected 5-level neutral point clamped (NPC) structure with middle DC-link. The input stage is operated with current control and the output stage with voltage control. DC-link capacitors were equipped with separate voltage-balancing circuits so that the voltage across each capacitor could be regulated at the nominal value all the times. The details of design, operation, control and simulation with operating waveforms are presented in the paper.