FPGA Control of a Three-Phase Three-Level T-Type NPC Grid-Connected Inverter with LCL Filter

Abstract

This paper presents an FPGA control scheme for the active T-type neutral-point-clamped (AT-NPC) inverter with LCL filter in applications to three-phase grid-connected systems. The three-level AT-NPC inverter employs active bidirectional switches to produce an additional voltage level for its PWM generation and provide the advantages of lower conduction losses, lower switching losses, and lower common-mode voltage with superior output waveform quality. The LCL filter benefits from its significant harmonic attenuation capability but suffers from its resonance. A multi-loop control scheme is developed to control each state variable of LCL filer with predictive control applied for inner loop current regulation. Although fast dynamic response can be achieved by predictive control, it is very sensitive to measurement noises. To overcome these problems, multi-sampling technique is developed to improve the robustness of the control system. The proposed control scheme combines the advantages of digital control such as flexibility and accuracy with positive aspects of analog control such as real-time response and high bandwidth. The designed controller has been realized on a mixed-signal FPGA SmartFusion (R) A2F500. Experimental verification is carried out with a 9kW three-phase grid-connected inverter. The results show that the proposed control scheme can achieve both fast dynamic responses and high quality of current regulation.