Transfer of transient surge voltage through XLPE cable winding transformer (Dryformer) circuits

Experiment and simulation results of conventional transformer design have shown that there are considerable differences in the characteristics of the surge voltages that are important in deciding the transferred voltages, depending on whether it is from primary to secondary or vice versa. Dryformer is a new technological advance of dry power transformer, which is constructed from XLPE insulated high voltage cables as its winding, and is designed to provide a direct link between ultimate customers and transmission systems in one step transformation [Leijon M, Andersson T. High and dry-dryformer, an innovative high-voltage transformer design that, eliminates the need for oil, promises a host of performance and environmental benefits. IEE Rev 2000:9-14. [1]; Leijon M, Dahlgren M, Walfridsson L, Ming L, Jaksts A. A recent development in the electrical insulation systems of generators and transformers. IEEE Electr Insular Mag 2001;17(3):10-15. [2]]. It is important therefore to have a better understanding of Dryformer behaviour in regard to transient surge voltage transfer between its circuits. This paper presents the application of a high frequency model for the study of transient surge voltage transfer between the Dryformer circuits due to surges having various steepness, amplitude and duration characteristics, which are expected in practise. The model is based on the creation of an RLCG network that gives the same admittance function as that obtained experimentally at the transformer terminals [Manyahi MJ, Leijon M, Thottappillil R. Transient response of transformer with XLPE insulation cable winding design (Dryformer). ELSEVIER Electr Power Energy Syst 2005;27(l):69-80.]. The validity of the formulated model has been verified by comparing the model predictions with experimentally obtained voltage transfer function. The model is successfully applied in estimating the threat levels posed to Dryformer winding and customer connected equipment due to transient overvoltages. (c) 2005 Elsevier Ltd. All rights reserved.