Selective Harmonic Compensation Control Algorithm For Shunt Active Power Filter

Active power filter(APF) is a electric device for harmonic filtering of the power system. It has a good effect in harmonic suppression, fast response speed, high reli-ability. In conventional APF, all harmonics of the load current are detected and com-pensated. However, in the power system distorted current are mainly composed of 6k±1th harmonics, such as 5th,7th,11th and so on. In fact, it is more significant to filter lower harmonics other than higher harmonics, because higher harmonics can be filtered easily by passive filter. This thesis designed characteristic harmonic compens ation APF control algorithm based on a diode clamped three-level circuit structure, a nd the following work has been carried out.Firstly, the control principle of the harmonic compensation is discussed, then the mathematical models of the shunt APF in abc coordinate and dq coordinates are established. Through the active/reactive power decoupling algorithm, the APF realized the dc-link voltage control. One kind of inductor-based auxiliary circuits for the voltages equalization of dc-link capacitors is discussed. By using this circuit, the unbalanced problem of neutral point voltage of diode clamped three-level circuit can be solved effectively.Secondly, based on dqO total harmonic detection arithmetic, the characteristic harmonic detection method has been derived. The nth harmonic regarded as reference of compensating current can be detected by this method. The PI controller has been introduced to get the nth harmonic current compensation control current-type command signal. According to the rela-tionship between voltage and current in dq coordinates, this current-type command signal can be turned into the voltage-type command signal as the modulation wave for SPWM module. Increasing other characteristics harmonic compensation module can realize APF outputs multiple characteristic harmonic compensation current at the same time.At last, a small scale experimental APF prototype based on diode clamped three-level circuit structure is built. A full-digital control system based on FPGA is designed by using Verilog-HDL code, The control system mainly includes the synchronous coordinate transformation, phase-lock-loop, harmonic detection control, active/reactive power decoupling control and the characteristic harmonic compensation control. All of these control algorithms are designed and verified. The experimental results show the total harmonic distortion (THD) factor of the line current has been reduced from 20% to 0.5%, with the highest compensated harmonic current of 31th.