Research On Control Strategy Of VSC-HVDC System Connected With Offshore Wind Farms

The interconnectio n of offshore wind farms wit h onshore gr id thro ugh vo ltage source converter based high vo ltage d irect current trans miss io n(VSC-HVDC)syste m has become a major developme nt trend in recent years.The DC system may wit hdraw fro m operation,the wind turbines ma y be d isconnected,and even the syste m ma y collapse,if there is a fa ult in the system.The contro l strategy of VSC-HVDC/MTDC transmiss io n syste m connected wit h offs hore wind far ms is stud ied all roud in t his paper,to improve t he system stab ilit y and ensure that t he wind far ms won't disconnected under any fault.First ly,the influence of offs hore wind far ms connect ing to the onshore gr id has been stud ied,inc lud ing t he influe nce on static vo ltage,trans ient vo ltage and frequenc y stabilit y of power system.A nd the operat iona l princ iple and trans ie nt characterist ics of VSC-HVDC,and the funct io n of each compone nt of the syste m are analised.The mat he mat ical models of var iable speed constant frequenc y doub ly fed wind ge nerator(DFIG)and VSC in different coordinates are designed.As to the aspect of controller design,the construct ion of PI controller and parameter tuning met hod are ana lyzed,and the double closed-loop active-reactive power decoupling control method of VSC is studied.Secondly,the simulat ion model of two-terminal HVDC transmiss ion s ystem wit h offs hore wind far ms is establis hed based on DIgSILENT/PowerFactory.And the transie nt characterist ics of VSC-HVDC system under the fa ult of o nshore gr id and wind farms are ana lyzed separately in detail,the n t he control strategies o f VSCHVDC system under different faults are studied.As to the fault of onshore gr id side,the influence of DC capacitor on DC voltage is ana lyzed and verified by simulat ion.Then,the unloading circuit is designed on the DC side of Receiver Converter Statio n(REC)to consume unbala nced power and help the system restore stabilit y.The simulat ion results s how that t his sefign can st ill limit DC volta ge and realize FRT under severe faults.Aiming at the fa ult of wind farm s ide,the strategy o f comb ined reactive power compensat ion contro l provided by send end converter(SEC)and DFIG dur ing the fa ult is proposed.The electromechanica l and electromagnet ic transie nt s imulat io ns of feeder and gr id-connected point faults are designed respective ly.The results show t hat the strategy can support AC voltage o f wind far m,help t he syste m restore stability a nd avoid d isconnect ion o f wind far m and DC system.Fina lly,based on the research o f VSC-HVDC syste m,the two-termina l fle xible HVDC system is extended to a four-termina l paralle l radia l struct ured VSC-MTDC system.Based on the study of its topolo gy and equiva lent circ uit,a coordinated contro l strategy is proposed to improve the fault r ide through(FRT)capabilit y of VSC-MTDC system.For REC,an improved prior it y reactive power co mpensatio n contro l strategy is introduced to support AC network volta ge during fa ult.For DFIG and SEC,power load shedding module and AC vo ltage sag contro l mo dule are introduced respective ly.Through t he coord inated control o f t he two modules,the unba lanced power is e liminated,the DC vo ltage amp lit ude is limited,and the FRT capab ility o f t he syste m is improved.It also solves t he prob lem t hat t he mec hanica l and electr ica l stresses may be too high,or the equipme nt may be damaged and the time liness may be insuffic ient when t he contro l of DFIG and SEC are used a lone.The effect iveness of t he coordinated control strategy is verified by s imulat ing d iffere nt voltage sags at grid-connected point.