| Along with the improvements of economy, science and technology, the society has become more electrified. While operating, nonlinear power electrical devices will continuously add various kinds of harmonic waves, which severely affected normal operation of other devices and the safe and stable working status of electrical systems. The harmonic problems inside electrical systems have gained high attention from electrical departments and researchers. Voltage source converter is one of the major harmonic source in electrical system, and its application is extremely wide. For instance, AC-DC-AC frequency converter, television, air-conditioner, energy-saving light and electric vehicle all include voltage source converters. In terms of electricity quality, the harmonic waves produced by voltage source converter have very high distortion rates, which brings high threats towards the safe and stable operation of electric systems. Up to now, the most popular model of harmonic source is traditional constant current source model. Despite its simple principles, the estimation of harmonic is inadequate, and it’s not available to further evaluate harmonic currents of widely used voltage source converters running in different conditions in the system. The harmonic research of voltage source converter containing uncontrolled rectifier has become the key problem that needs proper solution in this area.This paper performed deep study into the harmonic model of three-phase voltage source converter containing uncontrolled rectifier. The harmonically coupled matrix model of three-phase voltage source converter containing uncontrolled rectifier was established for both discontinued and continued modes. Its harmonic characteristics were also analyzed. One method was brought up to determine the operation mode of three-phase voltage source converter containing uncontrolled rectifier based on DC-side current, and to analyze the factors that affect its operation modes. The feasibility of simplifying AC-DC-AC frequency converter harmonic analysis with equivalent circuit of three-phase voltage source converter containing uncontrolled rectifier was verified and circuit parameters were estimated. Based on program calculation, time-domain simulation and experimental results, it was proved that the suggested harmonically coupled matrix model can accurately resemble harmonics produced by AC-DC-AC frequency converter including three-phase voltage source converter containing uncontrolled rectifier. Main research contents of this paper are listed as follows:(1) Based on working principles of rectifier, harmonically coupled matrix model was established for three-phase voltage source converter containing uncontrolled rectifier under both discontinuous and continuous modes. This model can convert non-linear characters in time domain into harmonically coupled admittance matrix, and elements inside matrix can directly show the coupling relationship of each harmonic voltage and current. Compared with traditional constant-current source model, harmonically coupled matrix model can show the production characteristics of harmonic waves from three-phase voltage source converter containing uncontrolled rectifier more directly. From the distribution of each matrix elements, it can be concluded that fundamental frequency voltage has major contribution to harmonic current. The contribution of harmonic voltage towards harmonic current decreases as the order increases. In the model, same order harmonic voltage’s effects to harmonic current is greater than those of different order harmonic voltage.(2) One method was suggested to determine the operation mode of three-phase voltage source converter containing uncontrolled rectifier based on DC-side current. The most significant advantage of this method is rectifier’s status can be determined just by circuit parameters and supply voltage. One more step into the analysis of effects from circuit parameters and THD of supply voltage towards operation mode of three-phase voltage source converters containing uncontrolled rectifier. The studies show that the resistor R on DC side has primary effects on operation mode of three-phase voltage source converter containing uncontrolled rectifier. Then comes inductor L and THD of supply voltage. Capacitor C has minor effects on operation mode, so it can be treated as constant in harmonic analysis of three-phase voltage source converter containing uncontrolled rectifier.(3) The harmonically coupled matrix model was applied to analysis the harmonic currents generated by AC-DC-AC frequency converter. The feasibility of simplifying harmonic analysis of AC-DC-AC frequency converter with equivalent circuit of three-phase voltage source converter containing uncontrolled rectifier was verified, circuit parameter calculation method was also provided. The empirical equation of DC-side resistor R considered torque shock happened during motor startup, which is powered by AC-DC-AC frequency converter. Other special cases like unstable rotation speed in actual operation were also considered. So it can better reveal the harmonic change of AC-DC-AC frequency converter and load operation. Three-phase voltage source converter containing uncontrolled rectifier can show low-order harmonic produced by AC-DC-AC frequency converter, and is capable to greatly simplify harmonic analysis process of frequency converter.(4) Three-phase voltage source converter containing uncontrolled rectifier was set up on the platform of Matlab/Simulink and the hardware circuit was built in the laboratory. Comparing the results of harmonically coupled matrix model programming calculation, time domain simulation and experiments, the suggested method of determining operation mode of three-phase voltage source converter containing uncontrolled rectifier is accurate and effective. The constructed harmonically coupled matrix model can accurately show the harmonic production characteristics of three-phase voltage source converter containing uncontrolled rectifier. Moreover, the harmonic currents generated by AC-DC-AC frequency converter were tested in the lab to co-verify that it is reasonable and feasible to analyze harmonic characteristics of AC-DC-AC frequency converter with three-phase voltage source converter containing uncontrolled rectifier. |
