Study On Control Of The Unified Power Flow Controller Based On Modular Multilevel Converter

With the expansion of China’s power grid, requirements for stability and flexibility of power system are increased. As the most versatile device of FACTS, the unified power-flow controller (UPFC) can control the active power, reactive power, voltage, impedance and power angle of power system separately or simultaneously, thus realizing the increase of transmission capacity and enhancement of power system stability. Compared with two-level, three-level, or the clamped multi-level converter, the Modular Multilevel Converter (MMC) has advantages such as modular design, easily expanding and low harmonic output voltage etc. A UPFC based on MMC will show its power in high power applications. In this thesis, control strategy of this MMC-UPFC has been carried out, and main works are listed as follows:1. The basic structure of the MMC-UPFC is analysed. Then the principle of MMC is reviewed from aspects of sub-module switch status, operation mechanism and modulation technique. After this, parameters of key equipments of MMC-UPFC are calculated.2. The detailed mathematical model of shunt part of MMC-UPFC is eatablished. Through simplifying of the model, current control system and capacitor voltage control system of shunt part are designed. Simulation results show that the control system can maintain the stability of shunt part in conditions which active power and reactive power changes greatly.3. A circulating current control strategy of MMC is proposed based on MMC arm model. Not only is the DC current flowing through shunt and series converter of UPFC controlled by the proposed control strategy, but also high frequency AC circulating current ripples flowing between positive and negative DC bus are suppressed significantly.4. The detailed mathematical model of series part of MMC-UPFC is eatablished. Based on the simplified model, output voltage control system of series converter is achieved using a single PID control loop. The control system not only controls output voltage rapidly and precisely, but also has a strong resistance to the disturbance caused by the changing of line current. Then SSSC operation mode of series part of MMC-UPFC is studied, and its performance is verified by simulating capacitors and inductors. 5. By controlling the DC current, this thesis investigates a way to control the power as well as the capacitor voltage of series part when two converters work jointly. Simulation results show that the proposed strategy is effective to coordinate active power and to maintain voltage stability during dynamic processing under four typical operation conditions of MMC-UPFC.