The Research Of Modular Multilevel Converter Applied In Medium Voltage Drive

Nowadays, multilevel technology has become the mainstream of the high voltage driver.Under the existing technology, people have been able to be relatively effective in theimplementation of high voltage driver. But there are still many problems, the diode clamp andfly across the capacitor multilevel converter is mostly flat only three level five products andare difficult to meet the domestic6KV and10KV even higher voltage level requirements.Although,Cascade H-bridge voltage inverter with a certain number of unit modules cascadedcan achieve arbitrary voltage grade, there is no common dc bus for a two-way flow ofenergy.So it is difficult to be applied to the operation that need four quadrant operation in thehigh performance of high voltage system. Though the power cell of the cascade type voltageconverter can be replaced by a controlled PWM rectifier, it greatly increases the complexityof the system, at the same time, still can’t avoid using a transformer to down the grid voltageto an appropriate level. Based on the above reasons, this paper puts forward a new kind ofhigh voltage driver system with high performance based on modular multilevel converter(MMC).Modular multilevel converter is new kind of multilevel structure that attracts a lotintention this years. This new structure is similar to the cascade multilevel inverter structure,also with is certain modules cascaded, but they different in the power flowing. Modularmultilevel converter gets power from the dc bus, it can be formed as a system able tooperating in four quadrant with a back-to-back structure. This article introduce the new typeof high voltage driver system from the following several aspects:1) In this paper the modular multilevel converter topology and basic working principle areintroduced, which include half bridge modular multilevel structure and the whole bridgemodular multilevel structure. And all kinds of working state of the whole bridge structure andhalf bridge are discussed in detail. The modulation strategies for modular multilevel converterare also discussed.2) In this paper the electrical model of MMC is introduced. And the mathematical model isalso established based on the modular multilevel electric model. After the establishment ofthis mathematical model, a dc bus voltage balance strategy is proposed for the modularmultilevel inverter.3) the PWM rectifier algorithm and vector control algorithm of induction motor areapplied to modular multilevel converter in order to obtain a high performance system of highvoltage driver.4) Based on this new type high voltage driver, a MATLAB simulation model is set up to verify the basic modulation algorithm of the modular multilevel converter, whichinclude the voltage balance strategy, PWM rectifier control strategy and the control strategyof induction motor. Simulation results show the feasibility of the scheme in theory.