Abstract
Three-level neutral-point-clamped converters are usually chosen for high-power applications as motor drive. Comparing with two-level converts, three-level neutral-point-clamped converters have half the voltage stress on switching device for the same DC-link voltage. The three-level neutral-point-clamped converters are able to output 3 level-step-shaped phase voltage and 5 level-step-shaped line-to-line voltage. So it can reduce harmonics in the output voltage and current. The performance of a three-level neutral-point-clamped converters depends on its PWM modulation. In the two classic PWM : sine-triangle modulation and space-vector modulation, this thesis chooses phase disposition PWM of sine-triangle modulation in terms of simple and easy implementation by adding zero-sequence injection voltage for different control. The neutral point potential fluctuation is an important problem for three-level neutral-point-clamped converters. Under certain conditions like unbalanced load or non-uniform switching, the neutral point potential can fluctuate or continuously drift to uncontrollable levels. The output performance will become lower and the switching device may fail due to overvoltage stress. In this thesis the neutral current is analyzed through redundant switching states of space vector PWM. Then a voltage balancing control technique is presented by adding a zero-sequence voltage to carrier based modulation. This method utilizes suitable redundant switching states to charge or discharge the capacitors, and controls the neutral point potential. The proposed method is validated by simulation and experimental results.