Parametric Study on Unit Step Responses of Impulse Voltage Measuring Systems Based on FDTD Simulations

Unit step responses (USRs) of a USR-measuring system recommended by the International Electrotechnical Commission have been computed using the finite-difference time-domain (FDTD) method for various conditions of grounding and shield ring. The USR measuring system subject to analysis has a resistor divider of height 3.3 m and resistance 9 k
$\Omega$
. It follows from FDTD-computed results that USR parameters are not much influenced by either the laboratory-floor conductivity
$\sigma$
or its relative permittivity
$\varepsilon_{r}$
if
$\sigma$
is higher than or equal to 10 mS/m, and they are close to the USR parameters for
$\sigma=\infty$
. When
$\sigma$
is lower than about 0.1 mS/m, USR parameters are more influenced by
$\sigma$
and
$\varepsilon_{r}$
. Variations in the partial response time
$T_{\alpha}$
, the experimental response time
$T_{n}$
, the settling time
$T_{s}$
, and the overshoot
$\beta$
, due to the differences of
$\sigma$
and
$\varepsilon_{r}$
, are 8 ns, 14 ns, 150 ns, and 50%, respectively. Even if the width of a horizontal metal sheet is extended from
$w_{g}=$
2 to 3 m, these variations are not much reduced. The extension of the width of a vertical metal sheet from
$w_{v}=$
1 to 2 m does not influence the USR parameters much, either. The installing height of a shield ring
$h$
influences the USR parameters: with decreasing
$h$
,
$T_{\alpha}$
and
$T_{n}$
decrease while
$\beta$
and
$T_{s}$
increase. Note that the FDTD-computed USR for
$\sigma=$
0.1 S/m and
$\varepsilon_{r}=$
10 agrees well with the corresponding measured USR.