SUBSTATION ELECTRICAL BUS AND PARTS CLEARANCES REQUIREMENTS BASIC INFORMATION AND TUTORIALS



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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