Assume 10 identical substation bus support insulators in parallel located in a 500-kV substation located at sea level; this configuration can be described as an air gap, conductor to rod configuration at standard atmospheric conditions. Assume that an overall pfo for the 10 gaps of 0.5% is desired

Question

Assume 10 identical substation bus support insulators in parallel located in a 500-kV substation located at sea level; this configuration can be described as an air gap, conductor to rod configuration at standard atmospheric conditions. Assume that an overall pfo for the 10 gaps of 0.5% is desired when the configuration is exposed to a switching surge of 939 kV (2.3 pu on a 500-kV system). What is the required gap clearance in meters?

Accepted Answer

The desired pfo of one gap, p, then should be

[latex]pfo_{10gaps} =0.005=1-(1-p)^{10}[/latex]

and

[latex]p=1-(1-0.005)^{1/10}[/latex]

= 0.0005011

From tables or calculations for a normal or Gaussian distribution, a probability of 0.0005011 corresponds to 3.29 standard deviations below the mean [latex] (V_{50} )[/latex]. Therefore, the desired [latex] V_{50} [/latex] value is

[latex]939 kV=V_{50} (1-3.29*0.06) [/latex]

and

[latex]V_{50} =1170 kV[/latex]

A standard deviation of 6% is often used for the air gap for switching surge stresses.With the [latex] (V_{50} ) [/latex] of 1170 kV and noting that a conductor to rod gap has a k factor of 1.65 assumed to be reduced to 1.42 due to the presence of the insulator, the desired gap spacing value can be calculated by

[latex]1170 kV=1.42\frac{3400}{1+(8/d)}[/latex]

and

d = 2.56 m

Question 14.1: Assume 10 identical substation bus support insulators in par...