Research On Capacitor Voltage Balancing And Low-frequency Operation For Modular Multilevel Converter

Modular Multilevel Converter(MMC) has been widely applied especially in the high-voltage and high-power area because of its advantages: highly modular, high voltage and multilevel output achieving by low voltage switching devices, reliability, easy to redundancy design and so on. The work principle of MMC is level assembling, the more sub-module, the more level of output voltage and the better waveform quality which means cutting off the output transform. And it's mathematic model usually could be sorted by energy and capacitor voltage but which essentially could be classify as the same model because of the direct ratio relation between the square of capacitor voltage and energy. This paper establishes the simplified circuit model of MMC based on capacitor voltage and with further reduction achieves a model that clearly presents the power flow.Capacitor voltage unbalancing includes two aspects of intra-arm and inter-arm voltage unbalancing. Conventional intra-arm control strategies have mass of calculated amount when sub-module increased and the later would affect the output voltage waveform. This paper describe an intra-arm balancing control with two balancing controller based on CPSPWM which should be finished in a fundamental period. Conventional inter-arm control strategies could be classified into “common injection” method compared to arm output voltage. In railway traction or similar occasions the injected common current would flow into grid because of its unsystematic and lead to inferior grid quality. When MMC applied in Static Synchronous Compensator(STATCOM) area this method would result in control un-independently on account of DC link floating. Also this method increases the current stress of arm devices. This paper proposed a bidirectional, half-bridge DC/DC converter so called power channel that connects upper arm and lower arm with in one phase and this power channel transfers the average different power of one fundamental period between upper and lower arm.At present, the research of MMC when applied on middle-high voltage and high power ac traction domain was few mostly because of capacitor voltage huge pulsation when the motor starts up and low-frequency operation. Conventional method could be sort as “high frequency injection”, of which injected waveform was complicated, controller was hard to design and the waveform was difficult to track on. There are papers also attempting to transfer the voltage pulsation to other passive devices by changing the sub-module structure but hard to realize full-speed operation of MMC with motor. Taking note of the equal amplitude, opposite direction pulsation voltage between upper and lower arm, this paper presents a method by connecting DC/DC power channels among correspondent upper and lower arm sub-modules to transfer instantaneous pulsatile power and cancel out the main pulsatile component. Meanwhile, so as to reduce the switch losses, here also proposed a step-control when MMC applied in ac traction field. which makes full use of the installed capacity.In order to verify the availability of the discussed capacitor voltage balancing control strategy and the pulsation suppressing method of low-frequency operation of MMC. Simulation has been established based on Matlab/Simulink software and experimental prototype also has been set up. Both of the simulation and experimental verify the validity and correctness of these methods.