Modeling,Stability Analysis And Fault Ride-through Of MMC-HVDC Systems

With the emergence of new transmission technologies such as modular multilevel converter(MMC)based HVDC grid and hybrid multi-terminal HVDC transmission system,large-scale AC/DC power systems are facing many new problems in terms of practical operations,as well as the security and stability analyses.Under this background,this thesis systematically studies the electromechanical transient modeling,static security analysis,small disturbance stability constrained minimum short circuit ratio,and passive controller design with fault ride through capability of MMC-HVDC system:(1)An improved MMC electromechanical transient model is derived mathematically,and it is theoretically and experimentally shown that more accurate DC-side model of the MMC should be established as a second-order differential circuit which considering the equivalent capacitance and equivalent arm inductance dynamic characteristics.Based on the derived MMC model,an HVDC grid modeling method based on preset DC fault information is proposed,and the impact of different types of DC faults on the transient stability of AC system is studied.The results show that the effect of DC fault on the transient stability of AC system is very limited as long as the faulted branch is isolated rapidly,even if more than one DC short-circuit fault occurs within the HVDC grid.Based on the derived MMC model,the electromechanical transient modeling of LCC-MMC HVDC system is studied.Considering the firing angle limit of the LCC,a sequential power flow algorithm for hybrid multi-terminal HVDC system is proposed,ensuring the reliability of model initialization.The simulation results of China Southern Power Grid show that the proposed power flow calculation and electromechanical transient modeling method for hybrid multi-terminal HVDC system satisfy the transient stability analysis of large-scale AC/DC power system.(2)A static security analysis method for MMC-HVDC system with droop control is proposed.Compared with existing methods,study shows that the proposed method based on "bisection algorithm" can better evaluate the steady-state operation of HVDC grid in the situation of converter outage and converter overload:namely the steady-state variations of converter active power,deviations of DC voltage,and variations of DC line loss.Based on the expressions of line loss variation,the steady-state current deviation of each line can be further estimated to figure out whether the DC line current is in safe and reasonable ranges.(3)The minimum short circuit ratios(SCRs)of MMC station connected to sending-end and receiving-end system are studied.Firstly,a small signal modeling method of MMC-HVDC system is proposed for system-level study.Then,by using the eigenvalue analysis method,the impacts of different AC system strength,system impedance angles,and different types of direct current vector controls on the small signal stability of the MMC-HVDC system are analyzed,together with the minimum SCR of the access system and the corresponding dominant influencing factors are obtained.The research shows that the minimum SCR required for MMC station connected to the sending-end system under the rated active power transmission is between 1.51 and 1.95,and the minimum SCR decreases with the increase of the equivalent impedance angle.For the MMC station connected to the receiving-end system,the minimum SCR required is between 1.34 and 1.51,and the SCR becomes larger with the increase of the equivalent impedance angle.Therefore,in order to ensure that the current vector controlled MMC-HVDC system can operate stably under rated conditions,it is recommended that the SCR should be greater than 2.0 for the sending-end system and 1.5 for the receiving-end system.(4)For the scenario that MMC-HVDC system supplies power to passive island system,a passive controller is designed to improve the AC fault ride through ability of the MMC.The research shows that the key to improve the fault ride through capability is to maintain the DC voltage at the rated level as much as possible during the AC fault.With.the help of the passive controller,the receiving-end MMC station can regulate the voltage amplitude of the island system automatically according to the fault condition of the sending-end system,without relying on communications.It is also shown that the proposed controller effectively alleviates the discharging process of MMC capacitors,ensuring fast recovery of the MMC-HVDC system as well as the passive island system.