Research On Grid Connecting Control For Offshore Wind Farm Based On VSC-HVDC

Development and large-scale utilization of new energy represented bywind power is of great significance and strategic value in the aspects ofpromoting national economic development, environmental protection, solvingglobal energy crisis and so on, which has become one of the importantstrategic goals of the world to achieve sustainable development. In recentyears, Chinese wind power industry is to flourish and has gradually turned tooffshore from land-based wind farm. There are many advantages such asabundant wind energy reserves, high utilization of the wind generator, nottaking up land resources and so on. Therefore, it is one of the focuses ofindustry. As the technology of wind turbine control constantly improve,continues to mature, offshore wind farms have been expanding and distancewith the onshore grid is farther. So, offshore wind farm transmission problemsneed to be resolved. The technology of high voltage direct currenttransmission based on voltage source (VSC-HVDC), which is suitable forlong-distance transmission of offshore wind energy, has become one of thecurrent research focus. This has put forward higher requirements on theaspects of offshore converter voltage level, system dynamic performance, andnetwork power quality.This paper researches the related technologies of VSC-HVDC system foroffshore wind farm grid integration, mainly including two aspects of the maincircuit topology and control algorithm. The following problems are solved,including voltage balance and triggering method of high-power converterbased on the MMC structure, as well as a variety of advanced control algorithms application on the offshore wind farm VSC-HVDC system. Theresearches are outlined as follows.1. High voltage and high-power are the characteristics of theVSC-HVDC system applications. According to it, a new topology of modularmulti-level converter (MMC) was introduced, and its mathematical modelwas established. Based on these, for the control difficulty of unbalancedcapacitor voltage, the method was proved, that controlling the balancing ofcapacitor voltages between arms and within one, respectively. Two methodsof capacitor voltage sequencing and component added to reference signal forcirculating current suppression. Simulation studies were carried out for theMMC with DC output capacitance and the direct output one, using themulti-carrier PWM triggering technology. The simulation results show thatthe MMC modulation and voltage balancing control strategy designed in thispaper can significantly improve the quality of the output waveform andincrease the number of output level by nearly double compared toconventional methods.2. The method of MMC main circuit parameter selection is analyzed.The20MW VSC-HVDC system was researched based on the strategy ofdouble closed-loop vector control and the simulation model of whole systemwas established, including the capacitor voltage balancing control methodproposed. For the problems of too many PI parameters and difficult to adjust,the method of combination of particle swarm (PSO) searching algorithm andPID neural network was proposed. The traditional PI regulator was instead byPIDNN controller. The optimal weight of PIDNN is searched by PSOarithmetic, which made the dynamic tracking error to minimum, optimizingthe system control performance and avoiding the repeated adjustment of thePI parameters.3. Nonlinear control strategy for VSC-HVDC system was researched,including the virtual flux direct power control (VF-DPC), feedbacklinearization, passivity control, back-stepping method and deadbeat controland so on. For the VF-DPC, estimation method of flux was given when it was used in MMC. The capacitor voltage balancing control method andmulti-carrier trigger strategy was used to instead of the traditional look-uptable method, which enabled the VF-DPC could be applied to the MMCtopology. The VF-DPC strategy was compared with the traditional vectorcontrol method by simulation, the results show that it used in MMC caneffectively improve the dynamic performance of the large capacityVSC-HVDC system. For the feedback linearization control method, thenonlinear mathematical model of the converter was transformed intoequivalent linear model, and controller was designed based on exactlinearization model. For passivity control method, the controller was designedbased on system energy dissipation characteristics, and combined with theappropriate damping injection to make the system quickly converges to theequilibrium point. For the high-order and nonlinear characteristics of VSC,the back-stepping method is used for searching the Lyapunov function, andthen the control strategy of global asymptotic stability can be derived.Deadbeat control was used mainly to solve the system instability caused bythe delay of the sampling and calculating in actual system. It can predict thecontrol quantity of next-beat and compensate the impacts because of delaybased on recursive derivation of mathematical model. Then, all the nonlinearcontrol methods above mentioned were compared in the application ofVSC-HVDC, and the conclusion that the VF-DPC can improve systemperformance and ease for actual implementation.4. The experimental system was built, and then the system hardware andsoftware were designed. The double closed-loop vector control strategycommonly used in engineering was realized for the double-ended systemconnecting to grid running. The system power and voltage could track thegiven values, and the accuracy of steady-state is high. It verified thecorrectness of the control algorithm and effectiveness of the designedexperimental platform hardware. The experimental results lay a foundation ofhigh-power (20MW) offshore wind farm VSC-HVDC converter for practicalengineering application. The innovations are as follows:(1) For the problem of DC capacitor voltage balance of MMC, The bothmethods of capacitor voltage sorting and superimposing circulationrestraining components into reference signals were combinated. Andmulti-carrier modulation strategy was used to control the converter. This canimprove the quality and level number of MMC output wave.(2) Improved PID neural network controller was used for instead of thetraditional PI regulator, and combination of particle swarm search algorithmto optimize the neural network weights parameters. It can make systemdynamic tracking error minimum.(3) Direct power control method was applied to the MMC structure.Power and flux estimation methods for the MMC structure were approved.This can improve the dynamic response performance of the high-capacitysystem.