The T-type three-level inverter has been widely applied to grid integration system due to its salient features. In this digest, a three-phase four-wire T-type inverter is used as an active power filter. Due to inconsistency of capacitances and unbalanced three-phase operation, the capacitor voltages deviate. The unbalanced capacitor voltages lead to distorted grid-side currents and unstable of T-type inverter. Existing voltage-balancing algorithms inject offset into modulation reference or require complicated analysis of switching states under various operating conditions, which are not suitable for three-phase four-wire T-type inverters. To simplify and improve voltage-balancing algorithms, firstly, the T-type inverter is equivalent to two-level inverter based on its switching states. Then, the proposed voltage-balancing adjust the fundamental frequency component of ac-side voltage using a proportional controller to distribute energy between upper capacitor and low capacitor. The parameter of voltage-balancing controller is analyzed and design in this digest. Finally, the proposed analysis and voltage-balancing algorithm are verified by experimental results.
