Proper sizing of counterpoise conductors is critical for ensuring grounding efficiency and lightning/fault protection in transmission lines. This article provides a detailed, step-by-step explanation of sample calculations for determining the required size of Copper-Clad Steel (CCS) counterpoise conductors, based on the Transmission Engineering Standard TES-P-122.10.
What is a Counterpoise Conductor?
A counterpoise conductor is a buried grounding conductor used in transmission lines to:
- Reduce footing resistance
- Divert lightning and fault currents safely into the earth
- Improve reliability and reduce tower surge impedance
- Enhance overall grounding performance
Copper-clad steel conductors are preferred due to:
- High tensile strength
- Adequate conductivity
- Corrosion resistance
- Cost efficiency
1. Case A: 230 kV and 380 kV Transmission Lines
Given Data
- Fault Current (If) = 63 kA
- Current in faulted structure near substation = 50% of If
⇒ I = 31.5 kA - Current in faulted structure away from substation = 30% of If
⇒ I = 18.9 kA - Fault clearing time (tc) = 0.5 sec (30 cycles)
- Maximum allowable temperature (Tm) = 300°C (average)
- Ambient temperature (Ta) = 40°C
- Conductor conductivity = 40%
General Formula Used
The required conductor size in circular mils (Acmils) is determined using the standard thermal capacity equation:
Acmils = 1973.52 × I × √ tc ×
ln [ (Tm + Tc + αr × (Tm – Ta)) / (Ta + αr × (Tm – Ta)) ]
the constants and thermal properties are already grouped, resulting in:
Acmils = 1973.52 × I ×
ln [ (Tm + 0.00378 × 4.397 × 10^4) / (Ta + 0.00378 × (Tm – Ta)) ]
Which is simplified further in the sample calculation as:
Acmils = 1973.52 × I ×
ln [216077 / (40 + (285 – 40)) ]
The thermal constant 285 is derived from material properties and conductor characteristics.
1(a). Fault Near Substation
Current (I) = 31.5 kA
Formula applied:
Acmils = 1973.52 × 31.5 ×
ln (216077 / 260)
Result
Acmils = 358,897 cmil
Recommended Standard Size
➡ 396 kcmil
or
➡ 19 strands of No. 7 conductor
1(b). Fault Away from Substation
Current (I) = 18.9 kA
Formula applied:
Acmils = 1973.52 × 18.9 ×
ln (216077 / 260)
Result
Acmils = 215,338 cmil
Recommended Standard Size
➡ 232 kcmil
or
➡ 7 strands of No. 5 conductor
2. Case B: 69 kV, 110 kV, 115 kV, and 132 kV Transmission Lines
Given Data
- Fault Current = 40 kA
- Current near substation = 20 kA
- Current away from substation = 12 kA
Using the same formula, the results are:
2(a). Fault Current = 20 kA (Near Substation)
A = 227,871 cmil
Recommended Size
➡ 232 kcmil
or
➡ 7 No. 5 strands
2(b). Fault Current = 12 kA (Away from Substation)
A = 136,723 cmil
Recommended Size
➡ 146 kcmil
or
➡ 7 No. 7 strands
Summary Table – Recommended Counterpoise Sizes
| Line Voltage | Fault Location | Current (kA) | Calculated Area (cmil) | Recommended Size |
|---|---|---|---|---|
| 230kV / 380kV | Near S/S | 31.5 | 358,897 | 396 kcmil / 19 No.7 |
| 230kV / 380kV | Away from S/S | 18.9 | 215,338 | 232 kcmil / 7 No.5 |
| 69–132 kV | Near S/S | 20 | 227,871 | 232 kcmil / 7 No.5 |
| 69–132 kV | Away from S/S | 12 | 136,723 | 146 kcmil / 7 No.7 |
Conclusion
Selecting the correct size of copper-clad steel counterpoise conductors is essential for ensuring:
- Adequate current-carrying capability
- Safe dissipation of lightning and fault currents
- Long-term reliability of transmission line grounding systems
This article provided complete examples of sample calculations for multiple voltage levels using industry standards. You may directly apply the same procedure using local system parameters to determine conductor sizes for actual project requirements.
