Investigating the Practical Applications of the Frequency Response of the Transformers Extracted Using the Lightning Impulse Test Results

Abstract

The Lightning Impulse (LI) test is performed on newly manufactured power transformers as a routine Factory Acceptance Test (FAT). A well-known Marx Impulse Generator (MIG) is utilized in this test. The setting of the MIG can be changed to obtain standard LI wave shape. Since various power transformers may have windings with dissimilar designs, different MIG settings may be required for each transformer. The accurate computer simulation of the LI test circuit can give help in finding the optimum setting of the MIG. The Frequency Response (FR) of the power transformer impedance is required in such simulations. Similarly, the transformer FR is required in calculating the Transient Recovery Voltage (TRV) across the contacts of the Circuit Breakers (CB) in the case of the Transformer Limited Fault (TLF). The accurate calculation of such TRVs has a great importance in selecting the proper rating for CBs. The FR of the transformer can be measured directly with network analyzers or some other conventional test instruments. However, performing an additional test to obtain the transformer FR imposes extra cost and efforts. Alternatively, it can be achieved by using the routine LI test results which is readily available. Fortunately in both mentioned applications similar connections are required for transformer terminals. In this paper, the procedure to extract the transformer FR using the LI test results is presented. Then, the validity of such extracted FRs is investigated by comparing them with the ones measured by conventional test instruments. As an innovation, the extracted transformer FRs are used in the LI test circuit simulation and the accuracy of the method is examined by experimental works. Moreover, the application of the extracted FR in TLF TRV calculation is investigated as well. The validity of the all presented theoretical concepts are evaluated using the experimental test results on a real large power transformer.