The Research And Application Of HVDC Control And Its Interaction With AC Power Grid

Traditional LCC-HVDC technology is more mature and widely used in large capacity and long distance transmission. But LCC converter is effected by both ends of AC grid. And it needs a lot of filtering devices to reduce harmonics. At present, VSC-HVDC using full controlled power electronic devices has been rapid development. It overcomes a series of inherent defects of LCC-HVDC, and can independently control the reactive power and active power. In this paper, the control strategy of HVDC power system and its interaction with the AC power system are studied. The main issues are as following:The basic working principle of LCC-HVDC system is analyzed, and the corresponding equivalent circuit and mathematical model are obtained. At the same time, the traditional control strategy of the LCC-HVDC system is adopted, the rectifier side is controlled by constant current and low voltage current limiting control. The inverter side is controlled by the fixed arc angle and is equipped with the constant current control. The simulation model is built in RTDS. The results show that the direct current is output according to the given command in the steady state; When fault occurs in the inverter side AC bus, the AC voltage of the rectifier side is changed, and the DC voltage and current are violent fluctuation, resulting commutation failure. After the failure of the system, the system will still oscillate and the recovery is slow.The operating principle and working mode of VSC-HVDC transmission system are studied, and the mathematical model of VSC-HVDC system in dq0 reference frame is established. Double closed loop control is adopted: In the outer loop, the rectifier side is controlled by constant DC voltage and constant AC voltage, and the inverter side adopts constant active power and constant AC voltage control; The inner loop adopts current decoupling control of DQ axis. The simulation model is built in RTDS. The results show that the transmission power is a given reference value in the steady state; When fault occurs in the inverter side AC bus, the AC voltage of the rectifier side is still stable, the fluctuation of DC voltage and current is very small, VSC-HVDC plays a role in fault isolation. Compared with the traditional HVDC transmission system, the stability of VSC-HVDC system is higher, and the control is more flexible.In view of the imbalanced phenomenon of energy caused by the geographical location and seasonal variation, this paper proposes a normalized control strategy of two-way transmission, which contacting grid by partition based on VSC-HVDC system to realize the complementary advantages. The control system determines the reference value of transmission power between AC and DC system through normalizing the droop characteristic on both sides of voltage source converter; The transmission power of the DC transmission line is obtained by comparing the size and direction of the reference value of the transmission power between the two ends of the DC transmission line; Then it achieves two-way transmission by controlling the DC voltage at both ends of the DC line, reaching power balance of the two grids, which realizes the low carbon power dispatching. Three different work situations are designed in RTDS. The simulation results verify the effectiveness of the above control strategy.With the development of VSC-HVDC system, Hybrid multi-infeed HVDC system is becoming the development trenda. Considering the actual situation of two different placement of the converter station, this paper established an improved dual-infeed hybrid HVDC system. At the same time, the control of voltage regulation and current adjustment were introduced to reduce the probability of commutation failure of LCC-HVDC, accelerate the recovery of the transmission system after the failure, and then enhance the stability of the system. The simulation results verify the effectiveness of the above control strategy in RTDS. It finds that the voltage support ability of VSC-HVDC is limited by the distance between LCC and VSC and load level after the failure.According to the planned data of Shanxi power grid for the year of 2017, the safe and stable effect of UHV AC/DC operation in Shanxi power grid is simulated on BPA platform. The research results provide the certain theoretical guidance to Shanxi power grid for planning and security dispatch.