Research On The Lumped Parameter Modeling Of Transformer Windings Under VFTO

Switching operations in gas insulated substation (GIS) may generate very fast overvoltage (VFTO) which are dangerous for the insulation of the transformer. The experience shows that VFTO has a very short rise time (less than0.1μs) and its main oscillating frequency which depends on the length of GIS is between1MHz to40MHz. When VFTO invades into the transformer, an unbalance of voltage distribution in transformer windings can be caused. Another problem is the resonance in the transformer windings which may occurs when the frequency of the incoming VFTO matches some of the resonance frequencies of the transformer windings, and the interturn insulation is particularly vulnerable to high-frequency oscillation. Therefore, the study of the voltage distribution in transformer windings under VFTO is of essential interested for insulation design.This paper will use the lumped parameter model to model transformer windings, and propose a new model named the Piecewise Lumped Parameter (PLP) model. The PLP model divides each turn of transformer windings into proper sections and each section is composed of a new π-type equivalent circuit of lumped circuit model. Comparing with the results by the Multi-conductor Transmission Lines (MTL) model, this paper will summarize the reason for how to divide. Then use the PLP model to model the transformer windings, and form the circuit equations by the Modified Nodal Approach (MNA), and the method of PRIMA is used to reduce the orders. The facts has indicated the PLP model can calculate the overvoltage distribution under VFTO.