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 decoupliiig control approacli 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 coinpensation 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 neutralline current. Compensation of all components is capacity-limited, co-operating with tlie cascaded load current sensing schenic. Multiple APFs can be paralleled to share tlie load power without requiring any control interconnection. In addition to providing the theoretic bases and detailed design of tlie APFs, two 6 KVA APFs are implemented. The effectiveness of the proposed method is validated with expcrimcntal results.