Control Strategy Research For HVDC Transmission System Based On MMC

VSC-HVDC has significant advantages in solving new energy power generation, offshore wind power connected to grid, large city power supply and ac magnetic ring network operation, which will have broad prospects for development. As modular multilevel converter (MMC) has many advantages, such as lower voltage change rate and current rate, less output harmonic, high degree of modularization, easy to inverter of redundant design, it became one of the main development directions of voltage source type inverter, and will have wide application prospects in HVDC.At present, MMC-HVDC is still in the engineering technology research and the preliminary application stage. Many scientific problems and the inherent law of MMC-HVDC had not been fully revealed. A large number of basic theorys and engineering problems remain to be in-depth study. This article started with the basic theory of MMC, carried out in-depth research with internal energy balance control of MMC, MMC-HVDC control strategy under normal operating condition of the system, MMC HVDC control strategy under ac system fault cases.This paper established the relationship between component parameters and the capacitor voltage fluctuation, the arm current. On this basis, it designed the MMC sub-modules capacitance and arm reactor parameters selection method. It deduced the electric parameters relationship between AC side and DC side, and gives the MMC-HVDC operation features under various kinds ac/dc side work mode. The simulation result verified that the mathematical model is effectiveness.The inherent Sub-module inputs, remove cause of the capacitor voltage imbalance, for this problem, it established arm current relationship with the sub module capacitance voltage, revealed the sub-modules capacitance voltage fluctuation mechanism and the regularity of capacitance voltage fluctuation. It proposed the capacitor voltage balance control method. As a criterion for sub-module voltage and arm current, it take sub-module adjustments.This method takes the capacitance voltage balance, at the same time, it effectively reduces the frequency of switching devices. The simulation results verify the feasibility and effectiveness of the proposed method.It proposed calculate method of equivalent value of capacitance current and arm current. It established mathematical model about relationship between sub-modules capacitance voltage and arm circulation. It revealed the mechanism of MMC arm circulation. It proposed the arm circulation suppression method. It increase a compensation controller in the inverter control system, which used for circulation inhibition. The controller controlled the amount of arm reference voltage by the difference between arm voltage expectations and the actual value. It make the output voltage of arm to based sinusoidal wave, thereby inhibiting circulation arm bridge. The simulation results show that the method can effectively restrain arm circulation, at the same time reduce the sub-modules capacitance voltage fluctuation.Under dq coordinate system, d axis component and q axis component of the MMC-HVDC system cross coupling, for this problem, it established the MMC HVDC control system model based on state feedback decoupling control. On this basic, it is designed inside and outside control system of MMC-HVDC based on state feedback control. Inner loop control used state feedback decoupling control to achieve inverter dq decoupling control, outer ring control used PI control to achieve the MMC-HVDC system steady state without control.It set up MMC-HVDC equivalent circuit and mathematical model under the ac fault. On the ac converter three phase current imbalance which caused by the failures, it proposed the unbalance current suppression method based on sequence component method. For Inverter DC voltage fluctuation problems which caused for both sides power unbalanced, it proposed a stable DC voltage current limiting control strategy. The simulation results show that the method can effectively inhibit the unbalanced three phase current and fault arm circulation, at the same time, it can ensure the DC voltage stability.