Study On Active Power Filter Based On Unity Power Factor Control

With the wide application of nonlinear loads in distribution network, the problems of three phase unbalance, reactive power and harmonics pollution are becoming increasingly serious. Active power filter (APF) is an important instrument to compensate harmonics and reactive component because of its excellent performance. Aim at the problem of the detect methods of harmonic current of existing active power filter, a new control method which is based on the unity power factor control is proposed, which only need to detect the line current of the power supply and the voltage of the capacitor at the DC side of the inverter, without detecting and calculating the harmonic current of the load. It can make the line current of the power supply to be a standard sine wave which has the same phase as the network voltage. Based on this control method, a single-phase active power filter with high performance is designed. Its steady and dynamic behaviors are analyzed in details. The control strategies of compensate current are the PI control and the triangle wave modulation, and the unipolar Pulse Width Modulation (PWM) of the H-bridge inverter is achieved.A three-phase four-wire active power filter with four bridge legs is designed, which is also based on the unity power factor control. It also only need to detect the line current of the power supply and the voltage of the capacitor at the DC side of the inverter, which can make the line current of the power supply to be standard sine wave which has the same phase as the network voltage, and compensate the neutral line current to eliminate the three phase unbalance.The whole system of single-phase APF and three-phase four-wire APF are simulated respectively on computer using PSCAD 4.0 simulating software. Thesimulating results have demonstrated that this single-phase active power filter can compensate the harmonics and reactive component of load current effectively as well as good dynamic performance. It can achieve the unit power factor. The three-phase four-wire active power filter with four bridge legs can compensate the neutral line current, harmonics, reactive component and three phase unbalance in transient state and steady state effectively. It has good dynamic performance.In order to further verify the accuracy of the proposed control method, a 5kVA experiment prototype of single-phase active power filter is prepared. The experimental results have proved that the active power filter can compensate the harmonics and reactive component of load current in transient state and steady state effectively. It is easy to control and has good dynamic performance.