Study On Control Strategy Of MMC-HVDC For Offshore Wind Farm Interconnection

In recent years, wind power trends gradually shift from land to sea with the continuous development of wind power technology. Compared to integration via the traditional two-level or three-level voltage source converter based high-voltage direct current(VSC-HVDC), integration via modular multilevel converter based high-voltage direct current(MMC-HVDC) has the advantages of superior control, low output of the converter harmonic content and low switching loss. Therefore, this paper which focuses on the applications of MMC-HVDC in offshore wind power integration completes the following work:1、The operation principle of doubly fed induction generator(DFIG)is described in detail. The mathematical model in synchronous rotating coordinate system of the DFIG, the three-phase pulse-width modulation(PWM) converter and stator-side and rotor-side converter vector control strategies are deduced by the coordinate transformation.2 、 The topology and operation principle of modular multilevel converter are briefly introduced in the way of nearest level modulation. It points out that the output waveform contained large harmonic components when the modular multilevel converter has less sub-modular. This paper proposes an improved nearest level modulation by changing the traditional rounding function. The improvement strategy by changing the original rounding function, increasing the output of modular multilevel converter level number achieves the purpose of improving the quality of the output waveform.3、The switching device will have a high switching frequency which not only increases the switching losses but also affects the useful life under the traditional Modular multilevel converter sub-module capacitor voltagebalance control strategies. Thus an improved sub-module capacitor voltage equalizing strategy is proposed.By setting sub-module capacitor voltage maximum and minimum deviation range and the current sub-module open state to decide the next opening of sub-modules selection. This can reduce the frequency of sub-module switching.4、When grid side of MMC-HVDC failure, direct circuit(DC) system voltage rises rapidly, which will affect the normal operation of the system.This paper proposes an improved the grid side of MMC-HVDC fault ride through control strategy. When the DC system voltage rises, wind power grid voltage amplitude is reduced then the DFIG decreases output to achieve the goal of fault ride through.