Research On Control Stategies Of VSC-HVDC

With a variety of large-scale renewable energy feeding into the grid, Traditionalpower equipment, construction and operation of technology is harder to adapt thiscondition. So it’s necessary to use new technology to meet the demands and theVSC-HVDC technology can solve this problem perfectly. So the voltage source inverteris deeply studied in the dissertation.First the VSI and grid-connected parallel VSI’s mathematical model insynchronous coordinate system were build, then VSI’s current and voltage controlstrategies were designed. Then using the PSCAD simulates the two level VSC-HVDCand MMCVSC-HVDC at the fault time.A hybrid control strategy that composed by quasi-resonant proportion andrepetitive control is proposed in the dissertation, it aims at improving the grid currentdistortion when the power grid contains harmonics or in unbalanced condition. Theproposed strategy can solve not only quasi-resonant proportion’s phase error but alsorepetitive control’s inaccurately following the power current. Then designed a phaselocked loop based on the second-order generalized integral. The simulation andexperiment’s indicated hybrid control strategy can improve the grid current effectively,reduce the grid current’s THD. And the proposed PLL can detect the positive sequencevoltage and phase at the power grid unbalanced faults quickly and accurately. As notusing the notch filter to separate the positive and negative sequence voltage, it can lockthe phase faster. The grid current can be better controlled by using this PLL.Parallel voltage source inverters can improve system’s power and reliability,reduce the cost. But it can generate the circulation current, increase system’s loss,reduce system’s efficiency. To solve the circulating current of grid-connected N modules,the deadbeat control and virtual VSI are proposed in the dissertation, restraining thecirculating current by regulating the zero vectors. The simulation and experiment’sindicated regardless of the given currents the net conductors or are different or not, thestrategy can make the zero current equally, reach every module’s averaged flow,improve the dynamic response, suppress the circulating current perfectly.At last, a30kW HVDC laboratory was built, the platform is mainly composed ofsix voltage source inverters. Firstly, designing all the control circuit dependently,debugging the system’s hardware, modify the primary loop, and based on this,programming the software, given vector voltage control, hybrid current control strategyand circulating current control experiments have been carried out on this platform.