Fast-processing modulation strategy for the neutral-point-clamped converter with total elimination of the low-frequency voltage oscillations in the neutral point

This paper presents a novel modulation strategy for the neutral-point-clamped converter. This strategy overcomes one of the main problems of this converter, which is the low-frequency voltage oscillation that appears in the neutral point under some operating conditions. The proposed modulation strategy can completely remove this oscillation for all the operating points and for any kind of loads, even unbalanced and nonlinear loads. The algorithm is based on a carrier-based PWM. Nevertheless, it can generate the maximum output voltage amplitudes attainable under linear modulation, such as space-vector modulation. Furthermore, this technique can be implemented with a very simple algorithm, and hence can be processed very quickly. A control loop for balancing the voltages on the DC-link capacitors is also proposed. This balancing strategy is designed so that it does not increase the switching frequencies of the devices when it is applied to the converter. The proposed modulation technique is verified by simulation and experiment