Transient-state analysis of MV instrument transformers

This study illustrates how selected instrument voltage transformers (VTs-I) behave in a transitory state. Some laboratory tests are conducted on medium voltage (MV) inductive voltage types to evaluate the voltage transferred from primary to secondary windings when a fast voltage impulse (1.2/50 μs) excites three machines constructively in different ways. Possible effects of metal parts present in the laboratory on the VTs-I behavior are analyzed, and other tests are conducted to determine a suitable shielding for the VTs-I. In order to interpret the experimental results, a theoretical study is performed using a simplified model of the instrument transformer. Finally, characteristic parameters are assigned values that were derived from experimental tests.