| With continuous proliferation of nonlinear loads in grid, the problem of harmonic contamination and treatment attracts more and more attention. Because the hybrid active power filters(HAPF) can utilize the strongpoint of both passive filters(PPF) and active power filters(APF) and offset their weakness, it has been an important area of research and believed to have wider application. The primary purpose of this article is to find a cost-effective and practical solution for harmonic compensation in high-voltage, high-power situation, the studies carried out in the paper and the corresponding results are as follows. (1)This paper reviews the existing HAPF methods applying to high-voltage grid and points out their advantages and disadvantages. Since the APF in the HAPF of LC series resonance injecting type only support a little fundamental voltage and current, the present study therefore focus on this kind of HAPF. By establishing differential equations and processing coordinate conversion, the equivalence between the HAPF single phase equivalent circuit and the three phase system is discussed. (2)The C type HAPF scheme is proposed for the first time, in which the APF doesn't need to bear fundamental voltage, current and total harmonics. So the APF capacity can be small and its DC voltage level is decreased (The scheme is now applying for a Chinese patent, the applying number is 03128234.2).Two control strategies for the C type HAPF, which are the combined control method and the selective harmonic compensation one, are also explored and analyzed. (3)For the second order HAPF, the paper puts forward two combined control methods: one can compensate some appointed harmonics accurately, the other one can attenuate harmonics higher than 3rd to more than 20dB. The close loop control strategy of the APF's DC voltage is also presented and the approximate mathematical model is developed with averaging and small signal perturbation method. The state feedback and repetitive control algorithm are adopted to correct the output current waveform of the voltage source inverter in the HAPF. In this control strategy, the state feedback loop is used to guarantee system stability and the repetitive control loop is to improve current waveform quality. (4)When the HAPF is in parallel with the capacitor load, the resonance possibility of some typical connection mode is analyzed in detail by taking the second order HAPF for example. A efficient control algorithm is presented to suppress the resonance caused by load harmonics. Even in some certain circumstance that the available current measuring values are limited, the performance of the algorithm is still good if the disturbance observer is used. However for the grid voltage harmonics, it is shown that the capacity of APF would be unacceptably large to restrain respective resonance. (5)Through theoretically investigating into the convergence process of the two weighting factors in adaptive harmonic filter, two novel algorithms are presented to modify the least mean square(LMS) algorithm. The first one can estimate the steady-state weighting values from their initiative variety, and the second one update the step-size from the time-average of error signal to provide a fast convergence at early stages and ensure a small final maladjustment. Both of them can greatly shorten the transient period while keeping steady-state accuracy.
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