In 1972, the Substations Committee of
the IEEE published Trans. Paper T72 131-6, which established
recommendations for minimum line-to-ground electrical clearances for
EHV substations based on switching-surge requirements. The
recommendations are based on a study of actual test data of the
switching-surge strength characteristics of air gaps with various
electrode configurations as reported by many investigators.
The results are shown in Table 17-5 and
include minimum line to- ground clearances for EHV system voltage
ratings of 345, 500, and 765 kV.
The clearances given in Table 17-4 are
considered adequate for both line-to-ground and phase-to-phase values
for the voltage classes up through 230 kV nominal system voltage
where air-gap distances are dictated by impulse (BIL) withstand
characteristics.
The National Electric Safety Code, IEEE
Standard C2-2002, also includes clearance requirements to the
substation fence.
The Substations Committee of the IEEE
has an ongoing effort to review phase-to-phase air clearances and is
currently balloting IEEE Standard P1427, Guide for Recommended
Electrical Clearances and Insulation Levels in Air Insulated Power
Substations.
Considerable information has been
published by CIGRE relative to establishing phase-to-phase air
clearances in EHV substations as required by switching surges. The
CIGRE method is based on nearly simultaneous and equal
opposite-polarity surge overvoltages in adjacent phases.
The phase to-ground surge overvoltage
is multiplied by a factor of up to 1.8 (the theoretical maximum phase
to-phase voltage would be twice the phase-to-ground surge
overvoltage). The estimated value of phase-to-phase overvoltage is
then compared with obtained clearances. Refer to an article in CIGRE,
Electra, no. 29, 1973, “Phase-to-Ground and Phase-to-Phase Air
Clearances in Substations,” by L. Paris and A. Taschini.
Suggested values of phase-to-phase
clearances for EHV substations based on the CIGRE method are shown in
Table 17-6. The table was formulated by choosing various
phase-to-ground transient voltage values such as are used in Table
17-5.
These values of phase-to-ground
overvoltage were multiplied by a factor of 1.8 to arrive at a value
of estimated phase-to-phase transient overvoltages.
An equivalent phase-to-phase critical
flashover value of voltage is next assumed by multiplying the
switching-surge phase-to-phase voltage by 1.21. Finally, this value
is compared with data in the CIGRE article prepared by Paris and
Taschini to arrive at air-clearance values based on switchingsurge
impulse voltages.
EHV substation bus phase spacing is
normally based on the clearance required for switching-surge impulse
values plus an allowance for energized equipment projections and
corona rings. This total distance may be further increased to
facilitate substation maintenance.
TABLE 17-4 Minimum Electrical
Clearances for Standard BIL Outdoor Alternating Current
TABLE 17-5 Minimum Electrical
Clearances for EHV Substations Based on Switching Surge and Lightning
Impulse Requirements (Line to ground)
Notes:
1. Minimum clearances should satisfy
either maximum switching-surge or BIL duty requirement, whichever
dictates the larger dimension.
2. For installations at altitudes in
excess of 3300 ft elevation, it is suggested that correction factors,
as provided in IEEE C37.30-1992, be applied to withstand voltages as
given above.
SS: switching surge
CFO: critical flashover
1 in # 25.4 mm.
TABLE 17-6 Suggested Electrical
Clearances for EHV Substations Based on Switching Surge Requirements
and Including U.S. Utility Practice (Phase to phase)
Note: 1 in # 25.4 mm; 1 ft # 0.3048 m.
∗The values of L-L switching-surge
clearances are based on the use of SS L-G crest voltages multiplied
by 1.8. This value of L-L SS voltage is then multiplied by 1.21 to
indicate an SS CFO value of voltage used to determine the clearances.
For a description of method used, refer
to CIGRE report by L. Paris and A. Taschini, Phase-to-Ground and
Phase-to-Phase Air Clearances in Substations, CIGRE, Electra, no. 29,
1973, pp. 29–44. L-G: line to-ground; L-L: line-to-line; SS:
switching surge; CFO: critical flashover.
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