Characteristic of Surge Transfer Through Transformers : Study of Conventional Distribution Transformer and XLPE Cable Winding Transformer (Dryformer)

Abstract: Wide spread application and dependence on microelectronics based equipment in modern society has necessitated the study of power supply quality problems in low voltage power installations. In spite of improvements in surge protection, sensitive equipment connected to low-voltage side of distribution transformer has continued to fail due to transient voltages that are transferred through distribution transformer from high voltage circuits. Furthermore, the failures of distribution transformer primary winding due to lightning induced transients entering through unprotected low voltage (secondary) winding has been a subject of concern in recent years. Also the recent advent of XLPE cable winding transformer (Dryformer) that performs one step transformation from transmission to distribution voltage levels has introduced new challenges in our understanding of the transfer of transient surges between the transmission and distribution circuits. The knowledge of the signatures of transient surge voltages transferred between the distribution transformer and Dryformer circuits is essential for the evaluation of the transformer transient response, and hence, for devising appropriate protective methods. This thesis studies the transient response of distribution transformer and Dryformer, and provides an advanced understanding of transient overvoltage transfer between the circuits of these transformers.Transient response of distribution transformer and Dryformer are studied by laboratory experiments, with consideration of surges that have various steepness, amplitude and duration characteristics, replica of surges that are expected in practise. The differences between these transformer designs on transient responses are discussed in perspective of their basic difference in construction and winding design features. Simplified procedure for the formulation of transformer model suitable for simulating the transferred transient voltages is developed. The model is formulated based on terminal measurements of transfer and driving point short circuit admittance parameters, as elements of the nodal admittance matrix, obtained by either frequency domain or time domain measurements. The nodal admittance matrix elements are simultaneously approximated in the form of rational functions by vector fitting and admittance functions in the form of RLCG networks are realised. Ultimately, the high frequency transformer terminal model is created as a π-network consisting of the above RLCG networks for each of its branches, suitable for use in circuit simulation program in time domain. The realised model has an upper-frequency bandwidth of 3 MHz. The model validity is verified by comparing the model predictions with experimentally obtained responses, and it has been successfully applied in predicting the surge transfer between the transformer circuits due to various types of surges. Experiment and model prediction results shows that the characteristics of the transferred surge voltage depend both on the characteristics of the applied surge and on whether it is from primary to secondary or vice versa. The procedure for model formulation is general enough to be adapted for any two port devices that behave linearly in the frequency range of interest.