Modeling,Analysis And Control Of AC/DC Power Systems Containing VSC-HVDCs

Voltage source converter based HVDC systems(VSC-HVDC)have many technical advantages,such as immunity against commutation failure,dynamic reactive power support capability,self-commutated capability,low harmonic levels and easy extension to multi-terminal(MTDC)configurations,etc.Therefore,VSC-HVDCs are widely used in different kinds of transmission and distribution fields,such as power grid interconnection,connecting weak AC systems,supplying power to passive islands,renewable energy integration and supplying power to city grids,ect.In some special scenarios,problems on VSC-HVDC modeling,analysis and control still remain to be further studied.This paper takes the AC/DC power systems containing VSC-HVDCs as the research object,and focus on the related issues on modeling,analysis and control under three special scenarios of practical value.The main works are organized as follows.(1)Electromechanical transient modeling method of VSC-MTDC systems is studied.The main contents include the power flow calculation method for AC/DC systems containing MMC-MTDC systems,the equivalent method of the submodule capacitors of MMCs and the simulation method for the DC networks of VSC-MTDC systems.And the detailed electromechanical transient model of VSC-MTDC is developed.The limitation of the detailed model is also discussed.To increase the simulation step size which can be adopted by the electromechanical transient model of VSC-MTDC,the dynamic processes in the detailed model are analyzed quantitatively and simplified.And then,the simplified electromechanical transient model of VSC-MTDC is derived.Both the detailed model and the simplified model are implemented on PSS/E by the user-written function.They are compared with the electromagnetic transient VSC-MTDC models on PSCAD.The simulation results demonstrated that the steady-state power flow and the transient response characteristics of both the detailed and the simplified electromechanical transient models coincide well with those of the the electromagnetic transient VSC-MTDC models.Finally,based on a modified New England 39-bus system,the interaction between the AC systems and the VSC-MTDC systems during the transient is studied.(2)Stable operating region of VSC-HVDCs connected to weak ac systems is studied.Based on the steady-state power flow equations of the VSC AC side,the steady-state security and stability constraints of the VSC AC side system under two different VSC control modes are summarized,the variation law of the stability criterion with short-circuit ratios(SCR)is analyzed,the phenomenon that VSC cannot operate stably when it is connected to weak systems is explained,and the process of acquiring the critical short-circuit ratio(CSCR)is given.Based on the process,the influence of different operation factors(AC equivalent system voltage,AC equivalent system impedance,VSC operation mode,etc.)on the CSCR is studied,and the results are verified by PSCAD simulation.Finally,the main constraint factors of the CSCR are concluded.And the typical CSCR ranges for different VSC operation modes are given,which can provide references for planning and operation of VSC-HVDC projects.(3)A VSC-HVDC AC voltage-frequency coordination control strategy for improving AC system transient stability was proposed.In west China,numerous regional grids containing much small hydropower stations need to be connected to the backbone grid.In this scenary,the technical advantages of using the VSC-HVDC to connect asynchronously the weak regional grids to the backbone grid is analyzed.The necessity for the VSC-HVDC to participate in controlling the voltage and the frequency of the weak grids is pointed out.The priority order of the control objectives of the VSC-HVDC is arrange.And then,a VSC-HVDC AC voltage-frequency coordination control strategy for improving AC system transient stability is proposed.And the controller structure which can guarantee that the objectives are implemented in order of priority is designed.Finally,taking the northwest Yunnan province as an example,the VSC-HVDC connection scheme for the regional grids containing much small hydropower stations is designed.And the fault simulation of the VSC-HVDC AC voltage-frequency coordination control strategy and that of the typical control strategy are compared.The result demonstrates that the VSC-HVDC connection scheme can improve the transmission capability of the regional grids and the proposed strategy can effectively enhance the transient stability of the weak AC system.(4)Two VSC-HVDC switching schemes between grid-connected and passive islanding mode are designed.Firstly,the switch process and the instability mechanism between grid-connected mode and islanding mode of VSC-HVDC are analyzed.And then,a local information based VSC control mode switching scheme is proposed.Besides,a VSC droop control scheme is derived by modifying the controller for supplying power to passive network,which can seamlessly switch between the above two modes.The two switching schemes are tested and compared by simulation on PSCAD.Results show that VSC can switch steadily between grid-connected mode and islanding mode through either of the two schemes.These two schemes have both advantages and disadvantages.In practice,the VSC should choose proper schemes according to specific control objectives.