Study Of DC Transmission System Based On Modular Multilevel Converter

Compared with the traditional DC transmission technology, Flexible DCtransmission technology has many incomparable advantages in supplying power to longdistance isolated loads and interconnecting asynchronous power grids. However,flexible HVDC system based on two level or three level has the disadvantages of lowvoltage grade, large loss and huge volume filter. And these disadvantages limit itsfurther application in HVDC project. DC transmission system based on modularmultilevel converter adapts modular design. Its output levels is high and is flexible to beadjusted to meet the demand of different voltage levels and power levels. It eliminatesthe high power bulky AC filters, reduces the switching frequency of the sub module,and is suitable for high power DC transmission.On the basis of introducing working principles of modular multilevel converter, themodulation strategies is expounded. An improved voltage balance control strategy isproposed. This control strategy is based on traditional voltage sort control and couldefficiently keep capacity balance and reduce switching frequency. By analyzing thederiving of circulating current, the circulating current suppression controller is designed.The simulation proves that the designed controller can effectively inhibit the circulatingcurrent.The mathematical model is created for MMC-HVDC in two phase synchronouscoordinates. The double closed-loop controller for converter level control is designed,combining the IGBT level control strategy, the controller can efficiently control the MMC-HVDCsystem. The MMC-HVDC system is proved superior by the simulation of output voltage andcurrent, active and reactive power, and SM’s capacitor voltage of the system.Because of the exist of negative sequence current component under three-phasevoltage fault conditions, paper has established the mathematical model of HVDCsystem. Considering the negative sequence component influence to PLL, designs thephase-locking and symmetrical components separating circuit. To restrain the negativesequence current, sequence voltage compensation control combined with doubleclosed-loop control is proposed. Through the simulation of the capacitance voltagefluctuating, internal circulation changing, the effectiveness of double closed-loopcontrol method and capacitor voltage balance strategy and circulating current inhibition strategy under three-phase fault conditions.