Research On The Master-slave Control Strategy Of MMC-MTDC System

At present, people have been focusing more on the development and utilizationof wind energy. Large-scale wind farms based on cluster mode which are far awayfrom the load centers continue to emerge. However, the wind power can’t becompletely consumed by the local load. It needs to be transmitted to anotherregional power grid by a centralized way. The best solution to this problem is themulti-terminal direct current system based on modular multilevel converter(MMC-MTDC system) with the advantages of both MMC and MTDC system. SinceMMC can withstand a high voltage and has a large capacity. MTDC system isflexible, reliable, and economical. Therefore, the control strategies of MMC-MTDCsystem are studied in this paper, so that it can better adapt to the randomness andintermittence of wind power.First, the basic principles of MMC are studied. Based on ignoring theswitching processes of sub-modules, the decoupled model between AC and DC sideof MMC is built. According to this model, the MMC is divided into AC-loop andDC-loop equivalently, both of which are decoupled from each other. The controllercomposition of MMC-HVDC is studied. According to the decoupled model betweenAC and DC side, the station-level controller is divided into AC-loop controller,circulating current suppressing controller and damping controller. Then, theAC-loop controller which is responsible for regulating the exchanged powerbetween MMC and AC system is designed based on the direct current control.Second, the control strategies of MMC-MTDC system are studied.Master-slave control, voltage margin control and multi-point DC voltage droopcontrol which are the most typical control strategies of MMC-MTDC system arestudied. Through comparative analysis, they are grouped into control strategies witha single master station and multiple master stations. Considering the DC voltagecontrol and active power control are strongly coupled, the control strategy withcompound master station is proposed. It makes the DC voltage converter stationfocus on the voltage control, and call other converter stations to compensatefluctuating power of the system. These converter stations constitute the compoundmaster stations, and share the control task of a traditional master station. Thiscontrol strategy can improve the DC voltage’s quality of the whole system.At last, the compound master station type MMC-MTDC system with windfarms is studied. By using PSCAD/EMTDC, a wind turbine model based on doublyfed induction generator is built. By using the equivalent parameter method, aequivalent wind farm model is built. Then, a four-terminal MMC-MTDC system model including wind farms using the control strategy with compound masterstation is built. Under wind speed changing and various fault conditions, theoperating characteristics of the system are simulated. According to the simulationresults, the corresponding measures are proposed to improve the stability of thesystem.