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This field-validated case study on “Special Pit-Based Earthing in Transmission Lines Older Than 35 Years” presents practical techniques for mitigating lightning-induced incidents that result in back flashover (BFO), tripping, and insulation breakdown in transmission lines. Such events are often momentary and, in some severe cases, causes prolonged supply interruptions affecting system reliability.
MPPTCL maintains approximately 43,500 circuit kilometers of transmission lines, of which a significant proportion are more than 35 years old. Ageing infrastructure combined with high lightning exposure in cross country terrain and varying soil resistivity conditions increases the vulnerability of these lines to surge- related disturbances. The study focuses on identifying localized grounding deficiencies and implementing special pit earthing solutions to reduce Tower Footing Impedance (TFZ) and fault clearing time to improve overall transmission system reliability. The effect of inductance has been considered during the design and implementation of pit based earthing.
This case study paves the path of rapid discharge of high frequency lightening current within the first cycle of 20 mS to avoid the insulation failure of the affected tower as well as adjacent towers or equipment. Since transmission lines are exposed and susceptible to lightning incidence, status of discharge path including status of foot earthing, contact resistance at joints and interfaces is uncertain (I.e. shield wire, tower body, and grounding system). This uncertainty of effectiveness of existing earthing system poses significant challenges. Identification of ineffective earthing , detailed investigation and implementation of corrective action are often time consuming and resource intensive. Therefore conventional mitigation measures applied uniformly across long transmission lines are uneconomical and ineffective.
