High frequency modeling and experimental verification of distribution transformers using transfer function approach

Abstract

High frequency transformer model represents a mandatory requirement to study transient performance of distribution systems under lightning strokes. In Finland, the distribution systems are frequently subjected to these strokes. In this paper, an accurate and simple model is introduced for the distribution transformers concerning two-port network implemented using transfer functions. Compromising between simplicity and accuracy facilitates the high frequency modeling of transformer for complicated networks. The study is done using two distribution transformers used in Finland, DYN5 (100 kVA) and DYN11 (500 kVA). The proposed model parameters of the two-port network are experimentally extracted for each transformer under the impulse tests. System identification of the impedance parameter is done using the denoised experimental signals for primary and secondary voltages and currents. The model is verified using experimental results under unloaded and loaded conditions. The evaluation is accomplished concerning time domain and frequency responses. Also, the proposed model is compared with published accurate model for loaded, unloaded, and medium voltage spark-gap (MVSG) operation of the first transformer, 100 kVA. The proposed model achieves simple implementation and accurate results for unloaded and loaded transformers as well as under MVSG operation. The simulated results are carried out using the MATLAB and ATPDraw programs.