Verification of withstand capability for very fast transients of a 200 MVA, 500 kV GSU transformer by modelling and testing

Abstract

Transformers in the network are often exposed to voltage stresses that are different from the impulses during the factory assessment tests (FAT). In many cases transformers can withstand these voltages. In combination with gas-insulated switch gears (GIS), it is typical that these voltages are characterized by a very high steepness in combination with a short duration time called very fast transients (VFT). Since for VFT voltages the difference to the FAT is remarkable, customers ask for an extended proof of the withstand capability. It is also clear that customers are conducting their own system studies, on the one hand, to get more detailed knowledge about voltage stresses to be expected at switching events and fault events. On the other hand, it is important to consider these results when doing the specification for transformers to connect to GIS. To cover these requirements, a step-by-step approach was followed starting with providing simplified transformer models and calculating the frequency response based on the manufacturer’s white-box model. These models are delivered to the customer. Resonance frequencies are derived, and the safety margins as well a proof for acceptable resonant voltages, e.g., the HV terminal, is given. Next, to prove the withstand capability for voltage stresses at energisation, inductive opening, reignitions and disconnecting events are performed. Due to the content of very high frequencies in the signals – going up to 5 MHz – it is necessary to use proper models during the design process of the transformer. Turn-wise models of the transformer windings provide a sufficient resolution for the required frequency range (>10 MHz). The use of turn-wise models of the transformer’s windings is mandatory. It is a challenge that a transformer can reliably withstand the VFT voltages. The withstand capability is proven by numerical simulation as well as by a special test. In the test sequence a very steep chopping, by applying a special SF6 chopping device with customer-specified voltage levels, finally successfully proved the transformer’s withstand capability for VFT.