Determination of Parasitic Inductance of Surge Protective Devices

This work investigates the effectiveness of various state-of-the-art techniques for determining the parasitic inductance of surge protective devices (SPDs) across a wide frequency range. This is critical, as the intrinsic inductive characteristics of SPDs influence their maximum residual voltage, which directly affects their protection efficiency against fast-front transients with high current derivatives. To provide an innovative solution to the problem of parasitic inductance determination, the parasitic inductance of single-phase and three-phase Deutsche Institut für Normung (DIN) rail SPDs is estimated by replacing the voltage-limiting components with copper blocks. Three methods are employed to estimate the parasitic inductance: 1) impedance spectroscopy measurements, 2) impulse current tests, and 3) finite element analysis (FEAs) simulations. The results are analyzed to evaluate the accuracy of the proposed methods and address key challenges encountered in determining the parasitic inductance. It is found that the parasitic inductance of the protection mode exhibiting the longer surge current path is up to 1.6 times greater than that of the shortest surge current path for the three-phase ( $3+1$ ) SPD, and consequently the maximum residual voltage increases by ~10% when subjected to its nominal discharge current of 20 kA with an 8/ $20~\mu $ s waveform for the same protection modes.