Loading Effects on the Attenuation of the Transferred Transients Through Cable Winding Transformer (Dryformer)

Recently, dry power transformer (Dryformer) that performs one step transformation from transmission to distribution voltage levels, has been introduced [1]. The Dryformer (dry power transformer) combines the features of power transformer with that of station distribution transformer, by utilising XLPE insulated round conductor cable for winding construction. The parameters of the cable winding can be determined in the same way as for single core transmission cable, however, the influence of mutual inductance in the windings makes the transient propagation slower in the windings, than in corresponding transmission cable of the same size and length. Therefore it is important to have a better understanding of Dryformer behaviour with respect to transient surge voltage transfer under various loading conditions. The model for the Dryformer is derived earlier [2] based on the frequency characteristics of the transformer admittance functions measured (in frequency domain) at its terminals over the frequency bandwidth of up to 3MHz. The validity of formulated model has been verified in this paper by comparing the model predictions with experimentally obtained responses under resistive, capacitive, and inductive loads. The model is then applied to investigate the influence of the load characteristics on the transfer of surges due to lightning strike caused transient etc, across the Dryformer circuits. Surges that have various steepness, amplitude and duration characteristics are used in the simulations. Experimental and simulation results of the transferred transient voltage through unloaded Dryformer [3] have shown that there are considerable differences in the characteristics of the transferred surge voltages when compared to the transient transferred through the unloaded conventional transformers. Even though the types of loads that can be connected to the Dryformer terminals are naturally variable, attempt is made in this paper to evaluate resistive, inductive and capacitive elements, which represents certain major loading conditions encountered in practice. We have found that various loading conditions at Dryformer terminals influence the peak amplitude, shape of waveform and the duration of the transferred transient.