Parallel operation of capacity-limited three-phase four-wire active power filters

Three-phase four-wire active power filters (APFs) are presented that can be paralleled to enlarge the system capacity and reliability. The APF employs the PWM four-leg voltage-source inverter. A decoupling control approach for the leg connected to the neutral line is proposed such that the switching of all legs has no interaction. Functions of the proposed APF include compensation of reactive power, harmonic current, unbalanced power and zero-sequence current of the load. The objective is to achieve unity power factor, balanced line current and zero neutral-line current. Compensation of all components is capacity-limited, co-operating with the cascaded load current sensing scheme. Multiple APFs can be paralleled to share the load power without requiring any control interconnection. In addition to providing the theoretic bases and detailed design of the APFs, two 6 kVA APFs are implemented. The effectiveness of the proposed method is validated with experimental results