Research On The Control Strategy Of VSC-HVDC And VSC-MTDC Systems For Wind Farm-Grid Connection

The conventional energy sources such as oil, natural gas, coal suffer from resourceshortages and environment pollution. Alternatively, as a relatively mature technology, theexploitation of wind energy has becoming a keystone policy of renewable energydevelopment in many countries. With the promotion of national energy policy, wind powerhas developed rapidly worldwide with its capacity increasing continually in power grids.Today’s wind farms are developed along the trend of large-scale and the capacity of a singlewind farm may be hundreds of MW or even thousands of MW. The large-scale wind farmsare mostly far away from the load center and need to be connected to power grid vialong-distance transmission lines. Nevertheless, wind power is of strong fluctuations andintermittency, if it is integrated to the grid via ac transmission lines, the stability andreliability of the transmission will be restricted by the disturbances and the voltage supportingability of the power grid. The long-distance transmission of wind power has been became abottleneck of the wind power development.The voltage source converter based HVDC (VSC-HVDC) is a promising technology forconnecting large wind farms to ac girds stably due to its advantages such as the independentcontrol of active and reactive power and good flexibility. Nonetheless, conventionalVSC-HVDC adopts constant power or constant current control strategy which can not adaptto the power fluctuation and frequency-following features of the wind power. In order toachieve high efficiency and high reliability grid-connection of wind farms, innovation on theVSC-HVDC control strategy is necessary and important.Firstly, in this dissertation, the detailed electromagnetic transient models of two types oftypical wind turbine generator (WTG) including squirrel cage induction generator (SCIG) anddoubly-fed induction generator (DFIG) are established, where the models are validated viasimulations in PSCAD/EMTDC and the operation characteristics of WTGs are obtained. Thenthe methods to establish the single-equivalent generator model of wind farms are presentedbased on the research requirement of VSC-HVDC transmission with wind farms integrated.In chapter3, the operation principle of VSC-HVDC is presented and its detailedmathematic model in abc coordinates is established, then a mathematic model in dq0rotating coordinates is deduced. On this basis, modeling and simulation studies on the diode-clampedneutral-point-clamped converter based VSC-HVDC and the modular multilevel converter(MMC) based VSC-HVDC are carried out.Two kinds of nonlinearity decoupling control methods are proposed according to thenonlinear characteristic of VSC-HVDC. The first strategy is a nonlinearity decoupling controlof VSC-HVDC based on EL (Euler-Lagrange) model. Based on the energy passivity of VSC,an energy storage function is set up according to the state error and the error storage functionis constructed as the Lyapunov function. The VSC-HVDC is astringed to desired stableequilibrium point rapidly by damping assigning. The passivity controller is designedaccording to the astringency condition of error storage function, and decoupling control ofeach variable is achieved. The other control strategy is the PCHD based robust control ofVSC-HVDC. Based on PCHD (Port-controlled Hamiltonian with Dissipation) equation, thePCHD model of voltage source converter is achieved and its strict passivity is proved.Desired energy function is constructed and used as Lyapunov function by assigning linkmatrix and damping matrix. Influence of VSC dc equivalent resistance on system iseliminated by assigning damping matrix. The IDA-PB (Interconnection and DampingAssignment Passivity-based) controller is designed according to desired equilibrium point,state variable and IDA-PB control theory. Both control methods are simulated undertakendifferent operation conditions in PSCAD/EMTDC, the results show that the proposedmethods have good performance and strong robustness either in dynamic state or static state.For converter-level control, a constant active power and frequency auxiliary controlstrategy which is applicable for grid-connection of wind farms with both ac and dc lines isproposed based on the voltage vector orientation and current direct control. Forgrid-connection of wind farms with only VSC-HVDC lines, a new control strategy isproposed where the ac bus voltage of the wind-farm-side VSC is modulated to simulate aninfinity source node with constant amplitude, constant frequency and constant phase angle.The control strategies can adapt to the VSC-MTDC systems. A type of star topology is usedfor the VSC-MTDC connecting multiple wind farms. In the star topology, one of the VSC dcbus is assigned as the confluence node and all the wind power share a common dctransmission corridor. The simulation results undertaken different operation conditions provethe validation and feasibility of the presented strategies and topology.A more flexible novel dual-mode control strategy for VSC-HVDC transmission isproposed in chapter6which can improve the performance of the ac/dc hybrid transmissionsystems integrated wind farms. In the control scheme, a novel ac voltage and load angle control approach is designed for the wind farm side VSC. The proposed control strategy canadapt to both ac-dc parallel and simplex dc interconnection modes dispensing with switchingof the control approach. In the ac/dc hybrid transmission mode, the active power transmittedvia VSC-HVDC can be regulated by adjusting the load angle between the voltage of windfarm ac bus and the output voltage of wind farm side VSC. In the simplex HVDCinterconnection mode, the voltage of converter ac bus is automatically modulated to beconstant amplitude and adaptive frequency to achieve the synchronous transmission offluctuating wind power. Moreover, damping to resonances in the system is improved byadditional high-pass filter. A new direct current vector control approach is applied to the gridside VSC to regulate the dc voltage to reference value. The ac/dc hybrid transmission systemsconnected wind farms equipped with SCIGs and DFIGs respectively are modeled andsimulated by PSCAD/EMTDC. The simulations considering a series of operation conditionsprove the validity and feasibility of the proposed approaches.Finally, the proposed dual-mode control strategy is applied to the practical project of theNanao island wind farm-grid interconnection via ac and VSC-MTDC hybrid transmission,where simulation studies prove the validity of the proposed strategy.Studies of the dissertation are instructive to the designing of the control system ofVSC-HVDC and VSC-MTDC for wind farm-grid interconnection.