Hybrid Modulation Strategy With Neutral Point Potential Balance of T-Type Three-Level Converter Under Switch Open-Circuit Fault

With the loss of a large number of space vectors under switch open-circuit faults, the neutral point potential (NPP) control of T-type three-level converters is faced with a huge challenge. It becomes harsh to balance the NPP and maintain the symmetry of line voltages under switch open-circuit faults simultaneously. In order to improve the NPP control performance and stable operation of T-type three-level converters, this article presents a hybrid modulation strategy that can enhance the NPP control ability even under switch open-circuit faults. Through a carrier-based analysis tool, two candidate modulation strategies are first evaluated from the perspective of the adjustment range of the neutral point current. The first one maintains the conventional three-level modulation in two nonfault phase arms. The second one degrades the mode of modulation from three-level modulation to two-level modulation in two nonfault phase arms. By making full use of the NPP oscillation characteristics of the two candidate methods, a hybrid modulation strategy based on transition duty-cycle and enabled bridge-arm adjustment is put forward to achieve a comprehensive optimal control performance. Numerical analysis and experiments are carried on to prove the correctness and feasibility of the proposed method.