Research On An Improved Three-Phase Four-Switch Shunt Active Power Filter

Compared with passive power filter, shunt active power filter which have many advantages, for example, different order harmonic and reactive power compensation at the same time, performance not affected by grid parameters, high efficiency, rapid dynamic response, strong overload capacity and so on, is widely used in low-voltage power system occasions which need harmonic compensate or power quality control.When used in three-phase three-wire power system, SAPF’s main circuit topology is generally a three-phase half bridge of voltage source PWM rectifier. It contains six power switches. So the SAPF can be called as three-phase six-switch SAPF.Three-phase four-switch SAPF is one three-phase six-switch SAPF which changes one phase of its switch bridge into capacitance bridge. So the capacitance bridge input current is controlled by the other two phase input current. Meanwhile, the switch number of SAPF descend from6to4.Therefore, the switching loss has greatly reduced which means the total cost of heat dissipation and power switch highly reduced.So in low voltage applications, three-phase four-switch SAPF is a quite economic solution for harmonic suppression.At first, this paper respectively builds the mathematical models of three-phase four-switch SAPF under abc static coordinate system and dq synchronous rotating coordinate system. It provides theoretical supports for the selection of control strategy, the design of controller and the research of modulation strategy. At the same time, one of three-phase four-switches SAPF’s inherent defects, the SAPF’s capacitors in dc side exists voltage bias, is derived by three-phase four-switches SAPF’s mathematical model in abc stationary coordinate system. It will affect SAPF’s output voltage and the performance of harmonic suppression. Some corresponding measures must be taken to ensure three-phase four-switch SAPF’s normal performance.Then, this paper uses the indirect current control strategy on three-phase four-switch SAPF. Ensuring three-phase four-switch SAPF’s performance is not affected by the accuracy and speed of harmonic track algorithm in direct current control strategy of SAPF.In this way, the hardware of SAPF can reduce the current sensors which measure the load current. Simultaneously, thanks to the PI-VPI controller’s powerful control performance which used in the current inner loop presented in this paper, the three-phase four-switch SAPF can eliminate the current sensors which sample SAPF’s output current. From this aspect, it reduce the cost of SAPF.In addition, this paper puts forward a modulation strategy which is suitable for this improved three-phase four-switch SAPF. The modulation strategy is one kind of carrier modulation strategy based on SPWM modulation. It can effectively eliminate the effects of dc side capacitor voltage deviation.At the same time, this paper proposes a dead zone compensation algorithm which is easy to implement and can effectively eliminate the negative effect of the dead zone.Finally, a simulation and experiment platform is built in the laboratory to validate the feasibility of the proposed control strategy, controller and modulation strategy.