Research On Control Strategies Of VSC-HVDC Transmission System

In1990, the idea of High Voltage Direct Current Transimission, which is basedon Voltage Source Converter, was first proposed by Boon-Teck Ooi et al, who camefrom McGill University of Canada. Based on Voltage Source Converter and PulseWidth Modulation technology, VSC-HVDC (High Voltage Direct Current Based onVoltage Source Converter) technology has good controllability and flexibility. In thefields of wind power grid, island power supply, AC system interconnection, buildingMulti-terminal HVDC transmission systems and expanding the capacity of urbanpower distribution networks, VSC-HVDC has good application prospects. In addition,VSC-HVDC has a smaller impact on the environment compared to conventionalHVDC transmission. Therefore, VSC-HVDC has become an important direction forsmart power technology development in the future.Using new control strategies and designing new controllers are importantresearch contents in the field of the VSC-HVDC.At present, the conventional control strategies are: fixed DC voltage control,fixed AC voltage control, fixed active voltage control and fixed reactive powercontrol. To maintain the level of the DC voltage and ensure the transfer of the activepower, fixed DC voltage control is adopted at one side, at the same time, fixed activevoltage control is adopted at another side. The majority of controllers are consists ofthe outer part and the inner part. The output of the outer loop of controller provides areference signal for the inner loop. Following works are done.Mathematical model of the VSC-HVDC is established. Based on the topologystructure of dual-side VSC-HVDC linking active AC system, the mathematical modelunder the abc coordinate is derived. Further more, the mathematical model under thedq coordinate is got by Park transformation. Applying feedforward compensationtechnique to decouple the d-axis component and the q-axis component, and further itmakes the active and reactive power independently controlled become a reality.Finally, the current decoupling controller is designed based on PI control method.Realize feedback linearization of the mathematical model of the VSC-HVDC. Onthe basis of the state space expression under the dq coordinate of VSC-HVDCsystem’s mathematical model, the VSC-HVDC system is decoupled through suitablenonlinear feedbacks and coordinate transformations. It is referred to the feedback linearization method of multiple-input multiple-output system in nonlinear controltheory.Compare and make some analyses to the simulations of the VSC-HVDC systemusing two control strategies. In connection with the VSC-HVDC system models, acontroller is designed by conventional PI control method; and another one is designedbased on nonlinear feedback linearization method. Finally, simulations inPSCAD/EMTDC illustrates that, the use of non-linear feedback linearization controlstrategy can decouple the d-axis component and the q-axis component better. At thesame time, the nonlinear controller can implove the flexibility of the power systemand enhance its stability, compared with conventional linear decoupling controlstrategy based on PI mothed...