New Equivalent Circuit Model of UHVDC Converter Transformer Winding for the Calculation of Transient Potential Distribution

Power transformers especially UHVDC converter transformers, need not only to bear the working voltage, but also to have the ability to withstand a certain degree of overvoltage, such as lightning, operation overvoltage, and very fast transient overvoltage (VFTO) which is characterized by a fast rise time of several nanoseconds and an oscillating waveform caused by switch operation in Gas-insulated substation. In order to study the propagation of the transient overvoltage on the transformer windings, a model is needed which is able to simulate the transient potential distribution along the transformer winding. Obviously, an accurate computational model of transformer winding for potential distribution analysis under impulse voltage is very important for the design of transformer interturn insulation especially for large capacity transformers such as UHVDC converter transformers. Quite a lot of research have been done to seek an appropriate transient computational model for transformer windings. Two basic physical methods have been obtained. They are multiconductor transmission line models and equivalent circuit models. For the multiconductor transmission line model, the transformer winding is represented by distributed parameters, but the losses caused by hysteresis, eddy current and proximity effect cannot be modeled. On the contrary, the equivalent circuit model can take saturation and losses caused by the core into account, but always assumming that the influence of magnetic core is negligible at frequencies higher than 10kHz,which is not always valid in practice. Lightning impulse or VFTO waveforms usually contain abundant frequency components higher than 10kHz. To obtain a more accurate model and also to provide a wide-band frequency response, in this paper, a new equivalent circuit representation of UHVDC converter transformer winding is given taking into consideration of comprehensive frequency characteristic of core lamination stack.