Research On Modeling And Stability Control Of AC/DC Systems With VSCs

With the strategic readjustment of energy structure all around the world,voltage source based high voltage direct current(VSC-HVDC)systems,which are suitable for renewable energy integration and becoming more and more mature at the dc voltage level increase and power expansion technology,are gradually replacing line commutated converter based high voltage direct current(LCC-HVDC)systems as the mainstream in future dc transmission systems.But presently,owing that critical equipments of VSC-HVDC systems such as dc breakers and dc/dc converters fail to meet the engineering application requirements of high voltage bulk power transmission systems,the power capacity of commissioned VSC-HVDC systems takes a low share in ac/dc systems and it might take long time for the VSC-HVDC systems to thrive into large-scale dc systems.At this stage,in order to fully utilize VSC-HVDC systems in ac/dc systems,researchs on the electromagnetic trasnsient modeling of VSC-HVDC systems and the ac/dc system stability improvement based on the fast adjustable performances of VSCs at active and reactive power have a profound engineering significance.This thesis focuses on the modeling and the stability control of ac/dc systems with VSCs,and it is organized as follows,(1)Fast simulation modeling of modular multilevel converters(MMC),which is the widely adopted in VSC-HVDC systems is fully studied.Three fast simulation models of MMCs,which are respectively the fast simulation model based on simplifying IGBTs and their anti-parallel diodes,the dynamic averaged model based on switching functions and the detailed equivalent-circuit model based on nest fast and simultaneous solution(NFSS)are discussed in depth.And the preciseness of the NFSS based fast simulation model is verified though simulations in PSCAD/EMTDC.An improved fast model based on the NFSS based fast model is proposed.Compared with the existing fast model.The propsed fast model improves the accuracy of simulations during the dc blocking of MMCs.Comparision simulations are carried out to prove the preciseness and acceleration performance of the proposed improved model.(2)DC voltage control in VSC based multiterminal direct current(VSC-MTDC)systems is thoroughly studied.Master-slave control,dc voltage bias control and dc voltage droop control,which are the three most wiedly used dc voltage control strategies,are discussed about their applicable occasions and mechanisms.A novel control strategy,which combines the merits of both the dc voltage bais control strategy and the dc voltage droop control stategy,is proposed.Its adaptability to VSC-MTDC systems is demonstrated through simulation verifications.Estimate above four dc voltage control strategies by rating their properties in dynamic response characters,complexities of controllers,applicability and communication demands.(3)A hybrid HVDC system called the LCC-D-MMC hybrid HVDC system,which is configured with LCCs as rectifiers and MMCs as inverters,is proposed.High power diodes are installed in the overhead lines close to the MMC inverters to block the dc fault current paths in MMCs.Based on the equal area criterion,the dc line fault transient stability characteristics of ac/dc systems with three different MMC-HVDC configurations are analyzed.The first configuration is half bridge sub-module based MMC(HMMC)HVDC configuration,which clears dc line faults by tripping the ac circuit breakers.The clamp double sub-module(CDSM)based MMC(CMMC)HVDC configuration with dc line fault clearance ability constitutes the second configuration.The LCC-D-MMC system is the third configuration.To evaluate the power system transient stability characteristics in three MMC-HVDC configurations,an index,called critical ac transmitted power,is proposed.Excellent performance of the LCC-D-MMC configuration is demonstrated through comparison simulations in three test systems.Finally,the study is extended to a modified New England 39-bus system,and the simulation results also correspond with the theoretical analysis.(4)In a VSC-MTDC system which integrates wind farms with several asynchronous ac grids,an optimized dc power redistribution strategy in case of outage of a grid side VSC is proposed.Considering whether or not the transmitted power could be absorbed by the ac system where the VSC out of operation connects to,the parameters of the controllers in the grid side VSCs are reset.When the transmitted power could be absorbed by the ac system where the VSC out of operation connects to,the strategy guarantees the transmitted power would not be aborbed by asynchronous ac grids,meanwhile,the impacts of redistribution on frequency stability of the ac system are minimized.On the contray,when the ac system where the VSC out of operation connects to is unable to aborb the transmitted power,the strategy makes the remaining VSCs in the same ac system operate at maximum active power,and utilizes the pitch control and the virtual inertial control of the wind farms to absorb the left transmitted power.The feasibility of the proposed redistribution strategy is verified in a modified New England 39-bus system.(5)A siliding mode robust control based active power modulation(SMAPM)is presented for VSC-MTDC systems,which could enhance the stability of the onshore power systems subjected to ac disturances.During ac disturbances,the ac grid could be divided into two ares,and the center of the inertia(COI)reference frame is utilized to be the representative of the area's behavior.An index called effective total inertia constant,Heff is proposed to determine the modulation direction and intensity of each grid side VSC.The control law is derived based on sliding mode variable structure control method.Simulation verifications are run in a four-machine ac system and a modified New England 39-bus system.Besides,the robustness of the controller to signal latency,signal measurement noises and wind generation characteristics is also verified in the 39-bus system.