BI-DIRECTIONAL THREE-LEVEL FRONTEND CONVERTER FOR POWER QUALITY IMPROVEMENT

Multi-level converters are intensively used in various high-power applications like induction motor drive, wind farm integration, HVDC transmission etc. This paper presents three-phase three-level flying capacitor converter as front-end topology with current error space phasor based hysteresis controller applied to it. The controller is self-adaptive in nature, and detect the vector, region and sector based on the position of reference voltage vector. This keeps current error space phasor within the prescribed hexagonal boundary. During the emergencies, proposed controller takes the converter in overmodulation mode to meet the load demand and once the need is satisfied, controller brings back the converter in normal operating range. Simulation results are presented to validate behaviour of controller to meet the said contingencies. By assuring the switching of only adjacent voltage vectors, the proposed controller is able to eliminate the random switching as observed in case of conventional hysteresis controller. Capacitor voltage unbalance is the major limitation of flying capacitor converters. To overcome the same, capacitor voltage balancing scheme for three-level flying capacitor front-end converter is also designed and simulated under various dynamic conditions. Results of unity power factor and low total harmonic distortion (THD) in grid current are presented under various steady state and dynamic conditions to validate the performance of the proposed controller.