| As more and more power electronic converters are applied in practice, harmonic problems in power system deteriorate and harmonic compensation has become a research focus. The article studies harmonic compensation of nonlinear loads of uncontrolled rectifier, which is one kind of the main harmonic sources in power system, and harmonic compensation is by shunt APF.According to the filter components of DC side, nonlinear loads of uncontrolled rectifier can be classified into three kinds: capacitor filtering, inductor filtering and LC filtering. The former two is called voltage-source nonlinear load and current-source nonlinear load respectively for the first one has a characteristic of voltage source and the second one has a characteristic of current source. For the last one, we call it resonance-type nonlinear load because it is often in a state of resonance of DC side. Resonance-type nonlinear load has both characteristics of voltage-source nonlinear load and current-source nonlinear load and all of the three have characteristic harmonics of 6ką1 order.On the base of this classification, the dissertation first researches on the compensation characteristics of series and shunt APF to voltage-source nonlinear load and current-source nonlinear load, from the aspects of load models and equivalent circuit of compensation systems. The research indicates that shunt APF is fit for compensating current-source nonlinear load and not for voltage-source nonlinear load, and series APF is fit for compensating voltage-source nonlinear load and not for current-source nonlinear load. Reasons have been analyzed. Further analysis implies the reason why shunt APF is not fit for compensating voltage-source nonlinear load is that harmonic current of load magnifies after compensation, which we call harmonic-amplifying effect.For the second, the dissertation models resonance-type nonlinear load according to load models of voltage-source and current-source nonlinear load. Effects of parameters of inductor, capacitor and load-resistance to load characteristics and compensating characteristics of shunt APF are analyzed. The article points out that the reason why shunt APF is not fit for compensating resonance-type nonlinear load is that harmonic-amplifying affect occurs. The harmonic-amplifying affect is related to impedance of power system, compensation rate of APF, fluctuation ratio of voltage-source in load model and parameters of inductor and capacitor of DC side.To improve compensating characteristic of shunt APF to resonance-type nonlinear load, the article designs the parameters of DC-side L and C mainly from the point of inductor and capacitor of DC side, besides selective harmonic-compensation control strategy is applied. The LC filter design aims to a nonlinear load of 14.15 kW and selects ripple voltage of load, surge voltage of capacitor, surge current of inductor and filter volume as constraints, constraint of harmonic-amplifying effect is also included. In accordance with equivalent circuit of compensation system of resonance-type nonlinear load, we use harmonic current as the real constraint representing harmonic-amplifying effect. Parameters of inductor and capacitor needed are got by calculating the numerical values of LC filter which satisfy all constraints and has the smallest volume, by the way of traversal and visualization. The designed inductor is 3.45 mH and capacitor is 1.25 mF.After compensating by shunt APF, current THD of power-grid side of resonance-type nonlinear load is reduced to 5.7%, by means of selective harmonic-compensation control strategy. The THD value basically meets the harmonic-current requirement of national regulations and simulation results agree with experiment well, which proves that strategies applied in the dissertation for improving compensation characteristics have worked to some extent. |
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