| Switching operations in Gas Insulated Substations (GIS) may generate Very Fast Transient Overvoltage (VFTO) which is a kind of instantaneous overvoltage lasting for a few milliseconds. Its rise-time is less than 0.1 milliseconds and its oscillating frequency is of several megahertz. VFTO is very dangerous for the power system, especially for the transformer which is directly connected to GIS. So it's needed to establish a reasonable model to study the influence of transformer windings under VFTO. In this paper, multi-conductor transmission line (MTL) model and lumped parameters model are used to analyze the transient response of transformer windings under VFTO. Using MTL model, a system of large-scale equations will be obtained. Many kinds of Model Order Reduction (MOR) methods are studied. However, the direct calculation in frequency-domain is extremely time and memory consuming. To reduce the computational expense, a fast algorithm is presented in this paper based on the integrated congruence transform(ICT) for the MTL model of transformer windings. The presented algorithm can be directly applied to the partial differential equations of a distributed line without any discretization and generate a passive finite order system as its model. In addition, this paper presents a lumped parameters model of the testing transformer windings and forms the circuit equations by Modified Nodal Approach. Then the method of PRIMA is used to reduce the orders. The passivity of reduced system is also guaranteed and the calculated results are satisfactory. |
