Study on transient voltage distribution characteristics of transformer windings
Presently, GIS is widely used in EHV/UHV transmission networks. When disconnector is switched on or off in GIS, the moving speed of disconnector’s moving contacts is relatively slow, which leads to multiple breakdown of SF6 gas gap between disconnectors. Each breakdown will cause the voltage of disconnector to drop within several nanoseconds, and produce very steep voltage traveling wave, which propagates on both sides from the disconnector and causes high frequency oscillation on GIS and adjacent devices. This is the formation of Very Fast Transient Overvoltage phenomena. The waveform of VFTO is steep and its amplitude is high, which could do great harm to the winding insulation of transformer and other equipment connected with it .
Very fast transient overvoltage with high amplitude and high waveform steepness can cause great harm to winding insulation of transformers and other equipment connected to it
Since the discovery of the VFTO phenomenon in GIS, whether large power transformers can withstand the impact of VFTO and how to eliminate the impact of VFTO, have been the concerns of power system.
Presently, in the design and manufacture of UHV transformer, the analysis of VFTO is mainly based on the theoretical analysis conclusions in the references and calculation with simplified transformer model. Therefore, the method of increasing margin is adopted in structural design and insulation measures, lacking the necessary data support for field measurement and research.
Due to the limitation of computing speed and storage capacity of the computer, the previous calculation is limited by the number of units for winding partitioning or can only analyse windings with fewer turns, and the inductance and capacitance parameters in the model are centralized and constant, without considering the frequency-varying parameters of the conductor due to skin effect. The model is relatively simple, and the calculation accuracy is poor.
In this paper, multi-conductor transmission line (MTL) model based on the representation of the winding by its individual turns is used to analyse the transient response of transformer windings under Very Fast Transient Overvoltages (VFTO). The unequal-length multiconductor transmission-line (MTL) model is adopted in considering the actual configuration of the transformer circular winding and the finite element method in time domain is used to calculate the voltage distributions along transformer windings by means of vector fitting and recursive convolution to dispose the frequency-dependent parameters .