Study On Related Problems Of Neutral Point Potential Balance Of Diode Clamped Three-level Variable Frequency Drives

In the energy sources shortage times background, energy-saving has become a research-hotspot for researchers and a society universal attention question. By the year 2007, there are over 0.58 billion kW motor installed capacity accounting for 60 to 70 percent upon all power loads in our country and AC motors accounting for 90 percent upon all motors. Nowadays, the weighted average operation efficiency of varies motors is 3 to 5 percent lower than abroad, the motor driven system operation efficiency is 20 percent lower than abroad. The AC motor speed control problem is what researchers are devoting to solving along with the birth of it, especially the high voltage AC motor speed control issues.Medium Voltage Variable Frequency Drives (MVDs) can change the input frequency of AC motors so that to control the motors speed. At the same time, changing the main drive power system's voltage or frequency could improve the control performance, making sure the safety and steady operation of motors. The application of MVD could significantly increase the operation efficiency, decrease power energy consumption, realize motor's soft start, decrease the devices loss and prolonged motor's service life; besides, the MVD technology extension application field also includes High Voltage DC Transmission (HVDC), Active Power Filter (APF), Static Var Generator and so on.This thesis firstly introduces several widely used MVD topologies, studies their advantages and disadvantages. Then analyzes the neutral point clamped three-level MVD operation status, the emphasis is placed on DC-Link function and state transition. SVPWM algorithm is also studied, includes sector and triangle location of reference vector, the theory and realization of SVPWM pulse generation, puts forwards a simpler criterion. This thesis also investigates the neutral point voltage unbalance cause and proposes a new balance method by detecting the motor and neutral point current. Then a new control method based on hardware circuit is proposed and studied, and the parameter design is investigated. In the end this thesis analyzes the mechanism of MVD's common-mode voltage generation, points out that the balance of neutral point voltage is an effective way to suppress the CMV.