An Improved Topology for Multipulse AC/DC Converters Within HVDC and VFD Systems: Operation in Degraded Modes

An improved topology for a multipulse ac/dc converter based on six-pulse rectifier modules operating in a degraded mode is described. Its main application is HVDC and variable-frequency-drive systems, where reliability, availability, and maintainability are the most critical requirements. A new concept, called “three-phase electronic Z/z transformers,” which provides separated voltages with reconfigurable phase-shifting angles and voltage ratios, is discussed. The number of acceptable faulty rectifier modules is defined at the design stage. Such features enable ac/dc rectifier systems to be modular and reconfigurable. In contrast with conventional topologies, the proposed structure can operate optimally as an 18-pulse, 12-pulse, or six-pulse rectifier in faulty conditions of local transformers or rectifier modules. An implementation method for a degraded 24-pulse rectifier with its control architecture including the Z/z transformers' command unit is discussed. A 1.5-kV, 18-kVA, 18-pulse ac/dc converter associated with a real-time system control is designed and tested. Promising results validate the new concept.