An Equivalent Operation Test Method for Low-voltage Arm of T-type DC-DC Converter Used in HVDC Grids

T-type DC-DC converter used in HVDC grids have gained increasing attention because it can and achieve DC voltage conversion and transmit power bidirectionally under high voltage and current conditions. To ensure the operational reliability of T-type DC-DC converter, an operation test shall be carried out before leaving the factory. Taking the cost factor into consideration, equivalent operation test method whose test object are half-bridge or full-bridge valve modules from arms of T-type DC-DC converter including high-voltage, multiplexing together with low-voltage arms becomes a popular and feasible scheme. Having a simple topology and convenience for control, traditional back-to-back power test platform based on half-bridge sub modules can deal with equivalent operation tests for half-bridge valve modules of VSC valves as well as T-type DC-DC converters. However, consisting of full-bridge sub modules, valve module in low-voltage arm of T-type DC-DC converter requires unidirectional testing current and bipolar voltage stress, which cannot be satisfied by half-bridge back-to-back platform. In order to solve this problem, an equivalent operation test method based on back-to-back platform using full-bridge is proposed in this paper with its PSCAD simulation results. Furthermore, prototype of the platform has been built based on which equivalent operation test for full-bridge valve module from LV arms of 320kV T-type DC-DC converter has been done, achieving 923.35A DC component with 109.59A AC component <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\delta$</tex> (RMS) in testing current.