Research On Power Angle Stability And Emergency Power Control Of AC/DC Hybrid Power Grid Containing LCC-MM

Traditional HVDC transmission based on Line Commutated Converter(LCC)is playing an increasingly important role in the long-distance,large-capacity and regional power grid interconnection.As a new transmission mode,high-voltage flexible DC using Modular Multilevel Converter(MMC)is widely used in DC power grid networking and multi-drop point operation of receiving power grid.The hybrid DC transmission(LCC-MMC)composed of traditional HVDC and flexible DC has both advantages.The ±800k V Wudongde hybrid DC project and ±800k V Baihetan hybrid DC project built in China provide successful experience for the future development of hybrid DC transmission technology.In the future,several LCC-MMC and LCC-LCC may be fed into the same receiving load center to form a new hybrid feed power grid of LCC and MMC.This paper focuses on the improvement effect of LCC-MMC on the transient stability of traditional AC/DC power grid after it is connected to the power grid,including the following two aspects:(1)Analysis of LCC-MMC improving the stability of generator power angle in the systemFor the load center with double LCC-LCC feeds,the AC system failure may lead to successive commutation failures on the DC inverter side,which will affect the transmission of electromagnetic power in the power grid at the sending end,and finally cause the generator power angle instability at the sending end.In this paper,it is proposed to change one of the inverter stations of the traditional DC transmission system into MMC converter station,build a simulation model of AC/DC power network containing LCC-MMC and LCC-LCC,adopt constant reactive power control in the reactive outer loop control at MMC side,and make full use of the characteristics of MMC to quickly adjust reactive power,which can not only improve the voltage quality of AC side bus,but also stabilize the transmission of DC power and reduce the probability of commutation failure of LCC-LCC inverter station.At the same time,the power angle stability of generator in LCC-MMC quick recovery system is analyzed based on the equal area rule.Finally,the simulation proves that the DC transmission system with LCC-MMC is effective in improving the stability of generator power angle.(2)Research on emergency power control of LCC-MMC in case of power shortage under large system disturbance.The active power transmitted in LCC-LCC system is closely related to the reactive capacity provided in converter station,so LCC-LCC emergency power control is usually restricted by the reactive capacity of the system.The inverter side of LCC-MMC can be regarded as a reactive power source.When the transmitted active power increases,it does not need to absorb reactive power from the system,and it can also provide dynamic reactive power support for the AC system.According to the characteristics of two different types of DC transmission systems,when there is a shortage of active power in the system,LCC-MMC is preferred for emergency power control of the receiving power grid.When the lost active power of the system exceeds the maximum power of LCC-MMC,LCC-LCC is selected to transmit the remaining power.Finally,the simulation verifies the correctness of the proposed strategy in the case of N-1,N-2 and LCC-LCC bipolar blocking faults of AC tie lines in complex AC/DC networks.