Computation of Inter-turn Voltages in Transformer Windings with Interconnected Distribution Cable

The paper deals with the use of the hybrid model to compute inter-turn voltages in transformer windings when a distribution cable is connected to the transformer. This model uses a combination of a single-transmission line model (STLM) and a multiconductor transmission line model (MTLM). It is shown that the hybrid model can be used with full success for computation of inter-turn voltage distribution in layer-type distribution transformers. Measured voltages at specific taps are compared with the computed voltages when a pulse with a short rise time is applied at the transformer terminal. In this work, the influence of the cable on the voltage distribution in transformer windings will be analyzed. OMPUTATION of inter-turn voltages in transformers is of great importance for the design of transformer insulation. During fast surges like lightning and switching the transformers, very fast transients take place which result in an increase of the inter-turn voltages. Switching operations in a gas insulated substation (GIS) are also known to produce very fast transient overvoltages (VFTO's) which are dangerous for the transformer insulation. Also, in medium voltage systems where vacuum circuit breakers are used (1,2), high-frequency oscillations occur which can be dangerous because of their short rise time. Another problem is the external resonance which occurs when the natural frequency of the cable matches the natural frequency of the transformer (3). Most of the time, the greatest problem is the internal resonance which might occur when the frequency of the input surge is equal to some of the resonance frequencies of the transformer. The experience shows that VFTO's within GIS can be expected to have even a rise time of 0,1 μs and an amplitude of 2.5 p.u. (4). The inter-turn insulation is particularly vulnerable to high- frequency oscillation, and therefore, the study of the distribution of inter-turn overvoltages is of essential interest. In this work, an accurate computation of voltage transients is done by making use of the hybrid model based on simplified Telegrapher's equations. It is verified that this way of modeling can also be used for layer type transformers with full success. First we consider the situation when an approximated