๐ Lesson 17
D5
TGPR Mitigation Strategies in GIS and AIS Substations
TGPR is the sudden, dangerous voltage spike in the ground near electrical equipment when lightning or a fault dumps huge current into the earth.
๐ฏ Learning Objectives
- โ Calculate TGPR magnitude using grounding impedance and fault current parameters
- โ Design a low-impedance grounding grid for GIS/AIS substations to limit TGPR below IEEE Std 80 safe thresholds
- โ Analyze touch and step voltage hazards from TGPR using CIGRE/IEEE methods
- โ Apply equipotential bonding strategies between GIS enclosures, control buildings, and grounding systems to suppress potential differences
- โ Explain how soil resistivity modeling and grid depth affect TGPR mitigation effectiveness
๐ Why This Matters
In GIS and AIS substations, a single lightning strike or ground fault can inject >30 kA into the grounding system โ causing local ground voltage to surge hundreds of kilovolts in microseconds. Without proper TGPR mitigation, this can arc across insulators, destroy IEDs, electrocute personnel, and trigger cascading outages. Real-world incidents at 500-kV substations in Brazil and South Africa confirm TGPR as a top cause of unexplained GIS failures โ making it not just theoretical, but mission-critical for reliability and safety.
๐ Core Principles
TGPR arises from Ohmโs Law applied to grounding: V = I ร Z_g, where Z_g is the frequency-dependent impedance of the grounding system to remote earth. Unlike steady-state grounding, TGPR involves high-frequency components (1โ10 MHz), so inductance and soil ionization dominate over resistance. In GIS, the metal enclosure acts as a Faraday cage but couples capacitively to internal conductors โ requiring careful bonding to prevent internal flashovers. In AIS, elevated structures increase exposure but simplify grounding geometry; however, interconnected equipment (CTs, PTs, surge arresters) creates complex potential gradients. Effective mitigation hinges on minimizing Z_g (via grid density, depth, and counterpoise), equalizing potentials (bonding), and diverting energy (arresters + shield wires).
๐ TGPR Magnitude Estimation
The peak TGPR is approximated using the effective grounding impedance at dominant frequency (typically ~100 kHz for lightning). For preliminary design, the low-frequency resistive approximation suffices when validated against soil ionization effects.
๐ก Worked Example
Problem: A 400-kV AIS substation has a symmetrical grounding grid (60 m ร 60 m, 10ร10 mesh, 12 mm copper conductor, buried at 0.8 m depth). Soil resistivity is 150 ฮฉยทm. Maximum asymmetrical fault current is 42 kA (3-cycle RMS, 1.9ร peak factor). Calculate peak TGPR assuming grid resistance dominates impedance.
1.
Step 1: Compute grid resistance R_g using IEEE Std 80-2013 Eq. (27): R_g โ ฯ / (2L_total / A_grid^(1/2)) โ simplified to R_g โ 0.658 ร ฯ / โ(A) ร [1 + (2h/โA)] for square grids, where A = 3600 mยฒ, h = 0.8 m โ R_g โ 0.658 ร 150 / 60 ร [1 + (1.6/60)] โ 1.65 ฮฉ.
2.
Step 2: Apply peak fault current: I_peak = 42 kA ร โ2 ร 1.9 โ 42 ร 1.414 ร 1.9 โ 112.5 kA.
3.
Step 3: Calculate TGPR = I_peak ร R_g = 112.5 kA ร 1.65 ฮฉ = 185.6 kV.
Answer:
The estimated peak TGPR is 185.6 kV, exceeding IEEE Std 80 recommended limits for touch voltage (<1.5 kV for 100 ms duration). Grid optimization (deeper burial, added rods, or ring electrode) is required.
๐๏ธ Real-World Application
At the 345-kV Tres Rรญos GIS substation (Costa Rica, 2021), repeated CT failures were traced to TGPR-induced flashover between secondary cables and grounded GIS frames. Post-fault analysis revealed a 220-kV TGPR during nearby lightning strikes due to high soil resistivity (280 ฮฉยทm) and insufficient bonding between GIS enclosure and control building ground. Remediation included installing a 30-m radial counterpoise, upgrading interconnecting bonds to 95-mmยฒ Cu, and adding 12 driven rods (3.0 m depth) tied to the main grid โ reducing measured TGPR to <25 kV and eliminating failures over 3 years of operation.
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