Abstract
In the three phase four wire system, when systems are the unbalance load, it will cause the problems of zero sequence current and voltage. In order to improve this condition, the conventional method is to separate the DC-link capacitor, and inject the zero sequence current into the midpoint of the capacitor. However, this method will need the larger capacitor to eliminate the ripple of capacitor, and even worse, will reduce the lifetime of capacitor because of the effect of zero sequence current. Base on the above problems, this thesis proposes the method for a four-pole converter to eliminate the problems of zero sequence current which is caused by the unbalanced load of the system. The four-pole converter adopts three dimensional space vector modulation to control the positive sequence, negative sequence and zero sequence voltage. Based on the proposed method, the whole system can have more operational choices and increase reliability when facing problems of unbalancing loading, to maintain the quality of the output voltage. This thesis will introduce the control method of three dimensional space vector modulation and explain how to use space vector selection to construct voltage command of the four-pole converter in different operation states. In different unbalanced conditions, the four-pole converter will be introduced to control the zero sequence current, and maintain the system operation. In the end of the report, the simulation and experimental result will also be included to verify the zero sequence voltage elimination method.