Study Of Strategy For Flexible HVDC Based On MMC

The modular multilevel converter (MMC) is a new multilevel cascade inverter.MMC is very suitable for HVDC for its characteritics of highly modular structure, easy cascade and having a common DC bus.At different voltage levels, MMC only needs to change the number of sub-modules to meet its requirements, and with the increase of the number of levels, the output harmonic current is smaller and the switching frequency can be decreased without additional AC filter.In this paper, the principle of operation of MMC is analyzed based on topology in detail,and introduced three states and six sub-modes of MMC. MMC modulation strategy is one of the core technology, And NLM modulation strategy and CPS-PWM modulation strategy is analyzed in the paper.The balance of the sub-modules capacitor voltage is premise of the normal operation of MMC. The fluctuation reason of sub-module capacitor voltage is analyzed. The balance of the sub-modules capacitor voltage is achieved by sorting through the sub-modules capacitor voltage and combining with NLM modulation strategy.The circulating current is caused because of imbalance of three phase voltage of MMC. To suppress the circulating curent, the circulating curent suppression method of general MMC and the circulating curent suppression strategy based on PR controller are introduced. It is proved that the circulating curent suppression strategy based on PR controller simplifies the structure of the controller.The MMC-HVDC core function is to achieve the power transmission.In terms of power control method,the MMC-HVDC system model based on dq decoupling control is established in dq coordinate system. The decoupling control of active and reactive power is achieved.Taking into account the dq decoupling control method is a double closed loop PI control structure,it requires a lot of PI controllers. In order to simplify the structure of controller, the MMC-HVDC model based on model predictive control were presented.The 21 level MMC-HVDC system simulation model of dq decoupling control and model predictive control is established in PSCAD. The three conditions of power step change, power flow reversal and DC voltage step change are compared and analyzed.The results show that model predictive control could accelerate the response speed, and the model predictive control is more easy to achieve digital control based on the discrete mathematical model.For asymmetric fault in AC system, the mathematical model of MMC-HVDC system under fault condition is established. An inner loop controller is designed to suppress negative sequence current in the αβ static coordinate system.The simulation model is built to simulate the single phase earth fault and two phase earth fault, and the effectiveness of the controller is verified.