Substation Earthing Compliance with High GPR and Transferred Potential
A 132/11 kV substation has a 70 m × 50 m earth grid of 150 mm² copper conductor at 0.8 m depth, with 12 meshes in each direction and four 3 m rods at the corners. Soil resistivity is 120 Ω·m to a measured depth of 20 m; a 100 mm layer of 3000 Ω·m gravel covers the switchyard. The 132 kV fault level is 25 kA, the earth-fault split factor is 0.60, and backup clearing time is 0.4 s. Commissioning gives a grid resistance of 1.8 Ω and a mesh touch voltage of 950 V at the worst location; the step voltage is 1800 V. The 11 kV cable sheaths and the telecom cable are bonded to the grid at the control building. The utility applies IEEE 80 and a 70 kg person. The relay upgrade that would reduce clearing time to 0.2 s is costed but not approved.
1. Calculate the GPR and the permissible touch and step voltages, and determine whether the installation complies.
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Grid current = 0.60 × 25 kA = 15 kA. GPR = 15,000 × 1.8 = 27 kV. For a 70 kg person, IEEE 80 touch voltage with a surface layer is E_touch = (1000 + 1.5 Cs ρs) × 0.116/√ts. With ρs = 3000 Ω·m, Cs ≈ 0.7 for 100 mm gravel, and ts = 0.4 s: (1000 + 1.5 × 0.7 × 3000) × 0.116/0.632 = 4150 × 0.1835 = 762 V. Step limit: (1000 + 6 × 0.7 × 3000) × 0.1835 = 13,600 × 0.1835 = 2496 V. The measured mesh touch voltage of 950 V exceeds 762 V, so the grid is non-compliant. The measured step voltage of 1800 V is below 2496 V, so step is acceptable. The GPR of 27 kV is high, but GPR alone is not the criterion; transferred potential is the main concern.
2. Evaluate the 0.2 s relay upgrade and one physical earth-grid improvement, with numbers, and recommend a cost-effective fix.
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The 0.2 s relay upgrade raises the touch limit to 762 × √(0.4/0.2) = 1078 V, which covers the measured 950 V with about 13% margin. That is the cheapest fix if the split factor and fault current are correct. But verify the split factor: if the actual split is 0.75, grid current is 18.75 kA and mesh voltage rises roughly in proportion to about 1188 V, which exceeds 1078 V. Also confirm that 0.4 s is the backup time; a breaker-failure condition can extend it. If margin is insufficient, add conductors or deep rods to reduce mesh voltage by 20–30%, or increase the gravel layer to 150 mm. A 150 mm layer raises Cs to about 0.8, so the touch limit becomes (1000 + 1.5 × 0.8 × 3000) × 0.1835 = 844 V at 0.4 s, or about 1194 V at 0.2 s. Combining the relay upgrade with 150 mm gravel gives a robust margin.
3. Discuss the transferred-potential risk to the 11 kV and telecom cables and state whether the relay upgrade alone is sufficient.
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The 11 kV cable sheaths and telecom cable are bonded at the control building, so a 27 kV GPR can appear on the telecom cable at its remote end, endangering personnel and equipment. Bond all incoming cables to the grid at the entry point, use insulating transformers or fibre for telecom, and isolate the fence with separate ground or insulating sections. The relay upgrade alone does not reduce GPR; it only shortens the permissible exposure time. At 0.2 s the touch criterion may pass, but transferred potential and step voltage at the fence still need checking. Also check conductor thermal sizing: 150 mm² copper is adequate for 15 kA for 0.4 s, so conductor replacement is not needed. A complete fix combines faster clearing, a thicker surface layer, and cable/fence bonding or isolation.