Harmonic compensation for nonlinear loads by active power filters

Abstract

Up to now most active power filters are designed for large-power applications, where complex digital control circuit and expensive batteries are often used. In this thesis, we propose a simple and low-cost active power filter circuit using an analog-based hysteresis current controller and a capacitive energy storage. The filter is intended to be a low-power add-on unit to reduce the AC harmonic currents of existing electronic equipment (e.g., personal computers), which impose nonlinear loads to the AC mains, to enable them to meet new regulatory requirements such as IEC61000-3-2. The operation principle, design criteria, and control strategy of the proposed filter are discussed. The limitations are identified. Simulation and experimental results are reported to verify the validity and practicability of the proposed active power filter circuit. In addition, the design, analysis, and limitations of a new method to sense the load current of active power filters are presented. By sensing the rectified load current along the return path of the load using a resistive current shunt, amplifying it with a proper gain and correcting it appropriately using a simple polarity-correction circuit, the waveform of the load current can be reconstructed accurately. This method offers low cost, small size, simple circuit, wide bandwidth and no electrical isolation required. The technique is suitable for low-power active power filtering applications. Besides, a wideband current sensor with extended low-frequency response (practically flat from 10Hz to 3MHz) is synthesized using a current transformer and wideband operational amplifiers. It can replace an expensive Hall-effect current sensor used in an active power filter. Experimental results confirm the usefulness of the synthesized current sensor. Finally, it is suggested that the integrated magnetics can be explored to integrate the filter current sensor and the load current sensor into the same magnetic core. The special winding technique enables the two current sensors to operate independently despite sharing a common core. In this way, the cost and the size of the current sensor can be reduced.