单向MMC型DC-DC变换器的控制策略

Control Strategy of Unidirectional Modular Multilevel DC-DC Converter

直流电网是解决新能源发电并网稳定性问题的研究热点。DC-DC变换器是连接不同电压等级直流电网的关键器件。MMC型DC-DC变换器采用模块化的拓扑结构,因而适用于高压大功率场合,其中单向MMC型DC-DC变换器适用于功率传输方向一定的电压变换场合,造价低、控制方式简单。但是目前对该拓扑的研究还比较少,相应的控制策略仍主要沿用MMC传统电平逼近的控制策略,国内目前也尚未有实际的示范工程,因此还需对该拓扑进行更深入的研究,提出适用于该拓扑的控制策略,为实际的示范工程提供一定的参考。该文提出了一种新型的适用于单向M M C型DC-DC变换器的控制策略,该控制策略将传统的电压追踪转变为电流追踪,在每个周期开始时刻计算出子模块的开通个数,通过控制子模块的开通和关断,使输出电流跟随参考电流变化。最后搭建了单向MMC型DC-DC变换器的仿真模型,仿真结果表明该控制策略可以很好地实现电压变换,输入侧电流波动小,是一种有效的控制策略。

DC power grid is the research hotspot to solve the stability problem related to grid-connected renew able energy. DC-DC converters play the key role to connect DC grid at different voltage level. Modular multilevel DC-DC converters adopt modular topology which is suitable for high-voltage and high-power applications. The unidirectional modular multilevel DC-DC converter is suitable for voltage conversion with a certain direction of power transmission. It has low cost and simple control method. But research on this topology is still relatively less. Corresponding control strategy is still mainly following the nearest level modulation（ NLM） of MMC,and there is no practical demonstration project in China.So it is necessary to study the topology more deeply and put forw ard the control strategy which is suitable for the topology,and provide some reference values for practical demonstration projects. A new control strategy for unidirectional modular multilevel DC-DC converter is presented in this paper. The control strategy adopts current tracking instead of traditional voltage tracking,and calculates the numbers of the inserted sub-modules at the beginning of each cycle. By controlling the switch,the output current will follow the reference current. Finally,a simulation model of unidirectional modular multilevel DC-DC converter is built. The simulation results show that the control strategy can realize the voltage transformation well and current fluctuation at the input side is small. It is an effective control strategy.