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Nowadays low voltage SVG (Static Var Generator) has got a certain amount of practical applications. Current PWM control is a key unit of SVG control process. Taking the three-phase four-leg inverter for research object, this paper designs the current controller base on one-cycle control algorithm and derivates the control equation in detail. Aimed at the problem of control shocks existing in traditional one-cycle control, the improved control unit is given out with the fundamental positive sequence active component of load current as control objectives. This improved control unit could realize reactive power, harmonics, and negative sequence imbalance current comprehensive compensation function, and duty cycle stability are improved. Control performance and response time of the control unit were significantly optimized. The proposed improved one-cycle control unit was validated in terms of simulation and experiment result, and has high practical value of low-voltage SVG application.

The DC bus voltage stability control is one key technology to ensure that Active Power Filter (APF) operates stably. The external disturbances such as power grid and load fluctuation and the system parameters changing may affect the stability of APF DC bus voltage and the normal operation of APF. The mathematical model of DC bus voltage is established according to power balance principle and a DC bus voltage piecewise reaching law variable structure control algorithm is proposed to solve the above problem, and the design method is given. The simulation and experiment results proved that the proposed variable structure control algorithm can eliminate the chattering problem existing in traditional variable structure control effectively, is insensitive to system disturbance, and has good robustness and fast dynamic response speed and stable DC bus voltage with small fluctuation. The above advantages ensure the compensation effect of APF.

Bases on present harmonic detection algorithms, single-phase load current is decomposed and a novel single-phase current detection algorithm, which is simple, reliable, response quickly and favors the hardware realization, is proposed, in order to solve complexity of algorithm and slow response of common used single-phase harmonic detection methods. Meanwhile, as the method need to use low pass filter, an improved method is is analysed. By increasing time delay unit, the effect of 2nd harmonic was eliminated, and the cut-off frequency of the low-pass filter was increased without adding the filter order. The response time of improved detection was shorten about half period. Correctness and validity of the proposed method were verified through simulating and experiment.

Accuracy and dynamic performance of Active Power Filter (APF) harmonic detection algorithm affect the effect of harmonic compensation directly. Commonly used harmonic current detection algorithms share the problem that certain hysteresis error and unsatisfactory dynamic performance. Harmonic current detection algorithm bases on instantaneous reactive power is analysed. Hysteresis error that affect the detection accuracy is studied, quantitative analysis model is built and computational formulas are deduced. Hysteresis of current detection is reduced via importing correction link is come up with. By means of bringing in feedback link to the system aim at unsatisfied dynamic performance of detection algorithm and improve it. Correctness and validity of the proposed improved harmonic detection algorithm are verified through simulation and experiment.