Abstract
It is well-known that multiphase converters can be paralleled to increase the power rating, reliability, efficiency as well as decrease the cost and current/voltage ripples . In addition, the parallel structure is very suitable for modular system design and system reconfiguration, providing better flexibility. However, when converters are paralleled together, circulating currents will be generated automatically whenever closed loops between different converters are formed by the switching operations. This will result in current distortion, unbalanced load sharing and the overall performance will be degraded. However, up to the present, very few existing papers concerning with circulating current modeling of paralleled systems are available. Further research work about more complete modeling and better control of circulating currents remains to be done. Hence, in this dissertation, definition of circulating currents of multi-phase paralleled converters is first presented and the circulating current generating mechanism is clearly explained. Then, based on this definition an averaged model of the circulating current is proposed. It is seen from this model that the circulating current consists of not only the zero sequence but also the non-zero sequence components. The governing differential equation also shows explicitly the relation between the circulating currents and the affecting factors such as different PWM strategies. With this understanding, a simple coordinate control is then presented to achieve equal load sharing control and reduce the circulating current. The phenomenon of the intrinsic circulating current is also explained. In addition, in view of the gradually increased parallel systems in distribution power generation systems, the author also proposed a circulating current index and unbalance rate for the kth phase of converter j as a quantitative measure about the serious status of power quality. Furthermore, the proposed circulating current definition and model are also generalized to cover the case of paralleled converters with different loadings. Meanwhile, a generalized coordinate control is also proposed to achieve unequal load sharing control as well as to eliminate the circulating currents according to the desired distribution factors. A prototype system is also constructed and the proposed general coordinate control is implemented by using TMS320F2812 DSP to verify the validity. Both simulation and experimental results indeed show the effectiveness of the proposed general coordinate control.