Research On Interaction Mechanism Between Current And Voltage Source C Onverters In Hybrid Dual-infeed HVDC System

Line-Commutated-Converter based High Voltage Direct Current(LCC-HVDC)has been widely applied in large power transmission for long distance.Multi-infeed HVDC system,where multiple LCC-based inverters from different HVDC links locating in close proximity,has appeared in power grid.Recently,with the rapid development of Voltage Source Converter based High Voltage Direct Current(VSC-HVDC)technology,it is gradually applied in many areas such as renewable energy delivery,interconnection between different power grid etc.The hybrid multi-infeed HVDC system,where multiple LCC-HVDC and VSC-HVDC links feed into an AC system in close mutual proximity has appeared in power grid.The operating principles and control characteristics of VSC-HVDC and LCC-HVDC have significant differences.What's more,the control and electrical coupling effects between LCC-HVDC and VSC-HVDC,will bring new challenges to security and stabilization control of the power grid.Therefore,the interaction mechanism of hybrid multi-infeed HVDC system needs to be investigated deeply.In this paper,the interaction effects between current and voltage source converters in hybrid dual-infeed HVDC system is investigated.(1)The steady operating limit of the hybrid dual-infeed HVDC systemThe impact of VSC-HVDC(or LCC-HVDC)subsystem on the steady operating limit of LCC-HVDC(or VSC-HVDC)subsystem with the control system working at perfect state is one of the important parts for research on interaction effect between LCC and VSC in hybrid dual-infeed HVDC System.Firstly,the steady mathematic model of a hybrid dual-infeed HVDC system has been modeled.Then,the impact of VSC-HVDC subsystem based on constant active power control-constant reactive power control(APC-RPC)mode and constantactive power-constant AC voltage control(APC-AVC)mode on the maximum available power(MAP)of LCC-HVDC subsystem has been investigated.The results show that i)LCC-HVDC subsystem with VSC-HVDC based on APC-AVC mode could output more active power compared with VSC-HVDC based on APC-RPC mode;ii)with the increasing capacity of VSC or reducing the electrical distance between the two subsystems,the MAP value of the LCC-HVDC subsystem could be increased.Finally,an approach to determine the steady operating area of the VSC-HVDC subsystem in hybrid dual-infeed HVDC system is presented.Based on this method,the impact of the different LCC-HVDC control modes on the steady operating area of VSC-HVDC subsystem is studied.The results show that,the constant DC current-constant extinction angle(C-E)control mode and constant DC current-constant DC voltage(C-U)control mode are suggested for LCC-HVDC subsystem to improve the steady operating area of VSC-HVDC subsystem,when the AC system strength is relatively weak or the capacity rate between VSC and LCC is relatively low.And the electric distance between the two subsystems mainly has impact on the reactive power operating area of VSC-HVDC subsystem.(2)Control loops interaction of VSC-HVDC subsystemFirstly,a multi-input multi-output(MIMO)transfer function model of the single-infeed VSC-HVDC subsystem based on APC-RPC control mode and APC-AVC control mode has been modeled.The theoretical stability limit of active power control(APC)loop,reactive power control(RPC)loop,phase-locked-loop(PLL)and AC voltage control(AVC)loop are investigated,which shows that i)the coupling effect between PLL and APC loop will reduce the theoretical stability limit of APC loop,while ii)the coupling effect between AVC and APC loop will improve the theoretical stability limit of APC loop,iii)the coupling effect between RPC and APC has little effect on the theoretical stability of APC loop,when VSC react power reference is set as 0 p.u..Then,the distribution of zeros for different VSC individual controlled plants has been investigated,which shows that the APC loop for VSC working as rectifier and the RPC loop and PLL for VSC working as converter are prone to instability with high control bandwidth,as limited by right-half-plane(RHP)zeros in its controlled plants.What's more,an interaction factor has been defined to evaluate the control interaction strength between different VSC individual control loops.The results show that,i)PLL or reactive power controller with high control bandwidth will cause instability of VSC rectifier,and ii)AC voltage controller with high control bandwidth will also lead to instability of VSC inverter.Finally,a frequency-based synchronization approach is presented for the VSC-HVDC station connected to a weak AC grid,which could effectively improve the instability caused by PLL for VSC-HVDC based on classical vector current control.(3)Control loops interaction of LCC-HVDC subsystemFirstly,the steady operating limit of a single-infeed LCC-HVDC system based on C-E and C-U control mode are investigated.And the MIMO transfer function model of LCC-HVDC system is deduced.Then,the zeros of different controlled plants for LCC-HVDC under different AC system strength are analyzed.The results show that,i)the control performance of extinction angle control(EAC)loop and DC voltage control(DVC)loop are not limited by RHP zeros for LCC-HVDC connected to relatively strong AC system,while ii)the EAC and DVC loop are prone to lose stability with high control bandwidth for LCC-HVDC under weak AC system,iii)the control bandwidth of PLL is limited by RHP zeros,and the maximum available control bandwidth of PLL is decrease for LCC-HVDC connected to weak AC system.Finally,the interaction factor is defined to evaluate the control interaction strength between different LCC individual control loops,which shows PLL with high control bandwidth will cause instability of LCC-HVDC system under weak AC grid.(4)Interaction between VSC-HVDC and LCC-HVDC in hybrid dual-infeed HVDC systemFirstly,an equivalent single-infeed VSC-HVDC(LCC-HVDC)model of the hybrid dual-infeed HVDC system is deduced.Based on the equivalent-single infeed VSC-HVDC(LCC-HVDC)model,an equivalent single input single output(SISO)transfer function model of VSC-HVDC(or LCC-HVDC)is build,and its accuracy is verified by detailed electromagnetic transient simulation model in PSACD/EMTDC.Then,based on the equivalent SISO control model,the interaction effects between VSC-HVDC subsystem and LCC-HVDC subsystem is analyzed,which shows that i)the stability margin and control robustness of the hybrid dual-infeed HVDC system will be reduced by reducing the LCC-HVDC(or VSC-HVDC)AC system strength,ii)and could be improved by appropriately increasing active power controller,PLL and inner current controller bandwidth,or by appropriately reducing reactive power controller bandwidth of VSC inverter station,iii)or by appropriately reducing extinction angle controller and PLL bandwidth of LCC inverter station.Finally,an optimization tuning algorithm is presented to systematically optimize the controller parameters,which could effectively improve the small-signal stability of the hybrid dual-infeed HVDC system under a relatively weak AC grid.