Research On Reactive Power Coordination Control Strategy For Hybrid Dual-infeed HVDC Transmission System

The traditional high-voltage direct current(LCC-HVDC)transmission system has advantages such as long transmission distance,large capacity,and low line cost.However,LCC-HVDC has drawbacks such as the need for reactive power compensation during operation and the risk of commutation failure.The newly developed flexible direct current transmission system with voltage source converter based high voltage direct current(VSC-HVDC)does not have the problem of commutation failure during operation,nor does it need to consume reactive power like traditional direct current transmission,and can achieve simultaneous control of active and reactive power.However,VSC-HVDC also has disadvantages such as high cost of converter stations and smaller transmission capacity compared to LCC-HVDC.With the development of the power grid and the continuous construction of flexible DC transmission projects,a hybrid dual feed structure has emerged,which is formed by the transmission of traditional DC transmission and flexible DC transmission to the same AC system.When a fault occurs on the AC side of the hybrid dual feed DC system,it can lead to a drop in the AC bus voltage.As the voltage drops,it may cause commutation failure at the subsystem LCC-HVDC converter station.The VSC-HVDC converter station can adjust the control strategy to generate reactive power to compensate for the AC system and LCC converter station,providing voltage support for the AC system and reducing the risk of LCC-HVDC commutation failure.This article first introduces the converter station structure of LCC-HVDC,establishes a mathematical model of LCC-HVDC based on the brief equivalent circuit diagram of LCC-HVDC,and establishes a control strategy model of LCC-HVDC based on the standard test model.A mathematical model and control system model of VSC were established by analyzing the structure of VSC-HVDC converter station.Based on the above analysis,a structural model and simulation model of a hybrid dual infeed DC transmission system were constructed.The steady-state operation simulation of the simulation model was conducted,and the results met the set parameters.Finally,the mechanism of commutation failure and the reactive power characteristics after AC faults in the hybrid dual feed DC transmission system were analyzed.Secondly,when a minor fault occurs in the AC system that causes AC voltage drop and lag,from the perspective of providing support for the stability of AC voltage,based on the analysis of the control characteristics of the external loop fixed AC voltage control,a voltage coordination control strategy based on the AC bus voltage change rate added to VSC-HVDC is proposed.Through simulation and comparison of voltage coordination control and fixed AC voltage control,Verified the effectiveness of the proposed voltage coordination control in enhancing transient low voltage and reducing transient high voltage;When a serious AC fault causes a significant voltage drop and causes the first commutation failure of LCC,voltage coordinated control cannot meet the reactive power compensation requirements of LCC.In terms of providing the required reactive power compensation for LCC commutation failure,based on the analysis of the control characteristics of fixed reactive power control,a VSC-VDC additional reactive power coordinated control strategy based on the consumption of reactive power by LCC converter stations is proposed,When a fault occurs and the voltage begins to drop,voltage coordination control is first used.After detecting the first commutation failure of LCC,voltage coordination control switches to reactive power coordination control.The superiority of the proposed reactive power optimization control in suppressing subsequent commutation failures is verified through simulation and comparison.Then,in response to the problem of the maximum limit value of reactive power generated by VSC-HVDC,a coordinated control strategy for STATCOM based on the gap value between the reactive power consumed by LCC and the maximum reactive power generated by VSC was designed based on the analysis of the structure and control strategy of STATCOM,By coordinating control,STATCOM cooperates to generate additional reactive power above the maximum reactive power limit of VSC-HVDC to compensate for the reactive power consumption of LCC-HVDC converter station,in order to suppress subsequent commutation failures.The superiority of the proposed STATCOM coordinated control strategy was verified through simulation.Finally,an overall coordinated control strategy was established to determine the severity of the fault based on the fault diagnosis module,and corresponding coordinated control strategies were selected to improve the AC voltage regulation ability of the hybrid dual feed DC transmission system after a fault,enhance the system’s ability to withstand subsequent commutation failures,and ensure the safe and stable operation of the system.