Research On Control Strategy Of VSC-HVDC Connecting Offshore Wind Farm

With the rapid development of electronic technology and renewable energy generation, there emerges flexible DC transmission system based on voltage sourced converters (VSCs) which is called VSC-HVDC. VSC-HVDC system is a new type of HVDC technology, including voltage sourced converter (VSC) and insulated gate bipolar transistor (IGBT). Up to now, the hardware implementation of VSC-HVDC control strategy and transmission capacity becomes the bottleneck. The main work in this paper is that proper control strategy is applied to improve the system transmission capacity and simplify the hardware implementation as well.The topology and working principle of two-level VSC-HVDC are illustrated in the paper. Compared with the space vector algorithm, direct power control coordinately control active power and reactive power respectively. And from the algorithm complexity, direct power control is easier to implement compared with space vector algorithm. To improve the transmission capacity, the topology and working principle of three-level VSC-HVDC are illustrated. Applying direct power control algorithm, three hysteresis control devices control ideally active power, reactive power and mid-point voltage.Two-level VSC-HVDC system and three-level VSC-HVDC system based on direct power control are simulated. The sampling values of bilateral-side converter of VSC-HVDC including AC current, AC voltage and DC voltage are adopted to directly control the transmission power. For two-level system, simulation results indicate that VSC-HVDC system can maintain the stability of DC voltage, coordinately control active power and reactive power and satisfy different reactive demands of the system by regulating reference value of reactive power on system side. For three-level system, mid-point voltage, active power and reactive power are controlled ideally. Compared with two-level system, three-level system could achieve greater transmission capacity and better power quality.For the application of VSC-HVDC, VSC-HVDC system based on voltage source converter connecting large-scale offshore wind farm is illustrated in the paper. The simulation system is built on the basis of power control algorithm, implementing the independent control of bilateral-side converters. On wind turbine side, there are multiple wind generators with respective converter, adopting the power control method based on hysteresis comparison and the converter on system side adopts the virtual flux direct power control. The results of the simulation prove that the VSC-HVDC system connecting the large-scale offshore wind farm and power system can operate under a normal and stable situation. Every wind generator can be controlled to work under the ideal state. And this system is able to control the active power and reactive power coordinately to satisfy different reactive demands of the power system. Connecting three-level VSC-HVDC with offshore wind farm, problems of transmission capacity and power quality are solved.