Analysis Of Operating Characteristics Of LCC-MMC Hybrid HVDC Transmission System

With the development of DC transmission technology,the Line Commutated Converter based on High Voltage Direct Current,LCC-HVDC,has been widely used in power systems.The switching device of LCC-HVDC is a thyristor,which is a semi-controlled device.The LCC-HVDC fed into the weak AC system will have a commutation failure when the inverter side AC system has a short-circuit fault,and even a simultaneous commutation failure may occur in a multi-feed system.These fault events pose a threat to the safety and stability of the power grid.The Multiple Module Converter based on High Voltage Direct Current,MMCHVDC,uses a fully controlled device,which can avoid commutation failure from the device,but there are also deficiencies such as the high cost of project construction and large losses in system operation.The LCC-MMC hybrid HVDC transmission system,which combines the economic and technical advantages of both LCC-HVDC and MMC-HVDC,can not only solve the problem of commutation failure,but also reduce construction costs,and has broad application prospects.First,based on the basic principles and control strategies of LCC-HVDC and MMCHVDC,two control strategies of LCC-MMC system are designed.The control strategy of constant current control at the LCC rectifier station and constant voltage control at the MMC inverter station is referred to as Strategy One.The control strategy of constant voltage control at the LCC rectifier station and active power control at the MMC inverter station is referred to as Strategy Two.The LCC-MMC hybrid UHVDC transmission system was built on PSCAD/EMTDC software,and the steady-state operating characteristics of the system and the transient response characteristics of the two control strategies under AC system short-circuit fault were simulated and tested.The simulation shows that both control strategies can effectively restore the stable operation of the system after the fault,and the Strategy One has smaller power fluctuations and better transient response during the power recovery process.Then,it analyzes the cause of the unbalanced DC voltage in the high and low valve groups of the converter station,and proposes a control strategy for the equalization of high and low valve groups.When the LCC-MMC system adopts Strategy One and Strategy Two,there is a phenomenon of unbalanced DC voltage and active power between the high and low voltage valve groups of the LCC rectifier station and the MMC inverter station.This is because the DC offset voltage existing on the converter valve side of the high and low valve groups causes an unbalanced current in the DC current between the high and low valve groups connected in series,and then the DC voltage imbalance occurs between the high and low valve groups.Based on the analysis of the valve group voltage imbalance mechanism process,a pressure equalization control strategy based on the deviation of the actual voltage of the high and low pressure valve group from the rated voltage is proposed to modify the control target.The control strategy can make the DC voltage of the high and low valve groups of the LCC rectifier station and the MMC inverter station be balanced.Finally,the power operating characteristics of the LCC-MMC system are studied through quantitative analysis of the power operating range of the MMC inverter station.Under the dq rotating coordinate system,a 10 th order analytical model reflecting the harmonic dynamic characteristics of MMC was established,and the accuracy of the established analytical model was verified on PSCAD/EMTDC.Considering the constraints of safe and stable operation,the power operating range of the MMC converter station connected to the AC system was quantitatively analyzed.The influence of MMC internal harmonic characteristics and external AC system impedance characteristics on the power operating range is studied by changing the sub-module capacitance parameters,the status of the circulating current controller,the AC system strength and the system impedance angle.This paper believes that the transmission of MMC active power is mainly constrained by the maximum allowable fluctuation of the submodule capacitor voltage and the maximum value of the bridge arm current,and the transmission of reactive power is mainly constrained by the maximum and minimum modulation ratios and the maximum and minimum voltages of the converter bus.