Basic study of possible waveforms generated in lightning impulse withstand voltage test on UHV equipment

In lightning impulse withstand voltage tests for substation equipment, waveforms regulated in the standard are sometimes difficult to generate on large equipment of the UHV class due to the level of capacitance of the test equipment "Ct" and the level of the residual inductance of the test circuit including the power supply "Ls". In the present study, the relationships between the capacitance of the test equipment, residual inductance of the circuit, the front time "Tf" of generated waveforms, and the overshoot rate "ß" were obtained using a simple equivalent circuit by assigning parameters systematically. Consequently, it has been found that, while Ct and Ls are small, there is a series resistance "Rs" range in which both the Tf and ß conditions are met and that, as Ct and Ls grow larger, there is an Rs range in which only one of the conditions is met but not both the conditions, and in the end even the Tf condition alone is not achieved. An increase either in ß or Tf increases the testable Ct upper limit or permissible Ls limit value, and an increase in Tf is found to be more effective.