The Research On Compensation Strategy Of Dynamic Voltage Restorer Based On Cascaded Multi-level Inverter

With the growing of power quality problems，the demand of voltage quality in powersupply system is becoming higher, voltage drop as the highest frequency of occurrence ofpower quality problems has get electricity scholars’ widespread concern．As one of the mosteffective power quality control devices, Dynamic Voltage Restorer (DVR) can eliminatevoltage sag in a few milliseconds, solve the phase voltage asymmetry and short-term powerinterruption.Low voltage switching devices can be applied in high voltage and high capacitysystem by using cascade multi-level technology. When the technology is used in DVR, itmakes DVR have unparalleled advantages in solving the voltage sag problem.In this paper, the existing energy compensation strategy of DVR is researched andimproved, the improved energy compensation is applied to the cascade multi-level DVRwhich has been designed, the main contents are as follows:(1) The advantages and disadvantages of full voltage compensation, in-phasecompensation and minimum energy compensation are analyzed and compared,currentconventional minimum energy compensation strategy is deeply studied, the compensationprocess of the minimum energy compensation strategy using system current as the referenceis derived,the compensation strategy insufficiency is proved in practical applications.(2) An improved minimum energy compensation strategy is proposed based on theinsufficiency of the minimum energy compensation strategy.At first,the compensation state ofDVR is determined by system voltage drop, and then the final location of the system currentand DVR output voltage are determined in accordance with the compensation state.Whiletaking into account the energy intrusion phenomenon of minimum energy compensationstrategy may occur, the control method is designed to suppress intrusion of energy. Simulationresults show that the method can compensate for the deficiencies of traditional minimumenergy compensation strategy, suppress energy intrusion problem effectively.(3) A cascade multi-level DVR simulation system is designed.According to thecharacteristics of multi-level cascade technology，comparison of modulation strategy based oncarrier phase-shift and carrier stacked, output voltage characteristics of unipolar SPWM andbipolar SPWM modulated strategy is analyzed. Simulation results show that the cascademulti-level inverter can improve device switching frequency, reduce the harmonic content ofthe output voltage, improve the harmonic characteristics,comparing two modulation strategiesat the same frequency modulation ratio, output voltage characteristics of unipolar SPWMmodulation strategy is more excellent, which is the basis of the system LC filter design.At thesame time,improved single-phase dq detection algorithm which has the advantage of a smallamount of calculation, fast and accurate detection as a voltage detection algorithm, thenegative feedback voltage control method which is simple and effective is applied in thesystem design.(4) The improved minimum energy compensation strategy is applied based on the abovedesign cascade multi-level DVR system and is verified by Matlab simulation.Simulationresults show that the improved minimum energy compensation strategy compared to theprevious conventional minimum energy compensation strategy is controlled simply,and makecurrent rms fluctuations is smaller.The study shows that the improved minimum energy compensation strategy is effective tocompensate voltage drop, reduce active power consumption of DVR, extend the effectivecompensation time of DVR.