In a previous blog on Health Assessment of Transmission Towers, we underscored the importance of regular maintenance in order to ensure the optimal performance of any infrastructure asset.
Improper functioning of earthing and lightning protection system (LPS) would render critical infrastructure at electrical substations at grave risk. Therefore, it is crucial that routine testing and maintenance is undertaken and corrective measures implemented.
In this post, we will cover how SCOPE’s service company – ISOSCELES SALES & SERVICE PVT LTD (ISSPL) successfully carried out audit of electrical, instrument and lightning protection system of 220/110/33kV Substation for Goa Electricity Department (GED).
Audit Project Overview
GED, a department of the state government of Goa, is the only licensee operating in the state of Goa for Transmission and Distribution of Electricity. The organization is entrusted to undertake transmission, distribution and retail supply of electricity in the state of Goa. The department has around 6.4 Lakh consumers with an annual consumption of 3800 MU. Around 52% of its consumers are from industrial category.
ISOSCELES (ISSPL) won a contract with GED for auditing the quality of Electrical, Instrument Earthing and Lightning Protection System at one of its 220kV EHV substation. Based on audit, ISSPL also recommended corrective measures for ensuring network assets and personnel safety.
ISOSCELES conducted the following tests:
- Symptomatic check (visual check)
- Riser integrity test at all above-ground risers in the switch yard to check integrity of the risers with the ground grid
- Electrode resistance measurement test using fall-of-potential method as per IEEE 81-2012
- Tagg Slope Method, also as per provisions of IEEE 81-2012
- Step & Touch Measurement to analyze dangerous potentials
- Corrosion test to identify metal loss due to corrosion
- Soil Resistivity Tests
- Gravel resistivity measurement
- Lightning protection system
Audit Results and Recommendations
Symptomatic Check (Visual Check)
On completion of symptomatic check, it was found that existing earthing philosophies in a few areas were found to be critical and in need of urgent correction. In total, 14 important observations were noted, and corrective solutions implemented.
Riser Integrity Test
842 risers were found to be in healthy condition, 53 risers were found in critical condition and 67 risers were in open condition. Based on these results, the following recommendations were made:
- The earth risers must be cleaned of dust, rust/oil till virgin metal is exposed.
- Reconnect cleaned conductors to the equipment tightened up to 50 Nm torque
- Make a new connection from equipment using the conductor of size 75×10/75×12 mm
Individual Electrode Resistance Measurement
Resistance of 812 earth pits were measured. 126 earth pits were found to be in critical state. The following solutions were recommended:
- Earth pit rejuvenation: We rejuvenated existing earth pits (with resistance between 10 to 20 Ω) by mixing ground enhancement materials CUREC +: 3 KG and CURECON: 5KG compounds. This low-cost solution pulled down earth resistivity by 35%
- Installation of new earth pits: For earth pits with resistance greater than 20 Ω, 3 new parallel pits were installed as per specification of BS: 7430:1998 and IEEE 80:2013
- Integrity to earth grid: As per the provisions of BS EN 61000-5-2, we approved earth pit of all lightning arrestors and instrumentation systems to the main earth grid.
Tagg Slope Test
Tagg Slope Test revealed that the grid resistance of 220/110/33 kV substation was islanded. The lower & upper yard was islanded while middle yard was in critical state. The earth grid resistance was very high and required a new peripheral earth ring around the grid (1m away) and connect it to the existing grid at every 30 m interval. This was successfully done by utilising ground enhancement materials: CUREC and CURECON.
Step & Touch Voltage Measurement
Step & Touch Voltage measurement was made at 220/110/33 kV switchyard to analyze the dangerous potentials and corrective measures taken, as described in the table below:
|Measured touch voltage in location 1||Created 4 rods of 3m long around the CB structure and connected it to the earth grid to reduce touch voltage|
|Touch voltage at 1m away from grid||Created a surface layer of same resistivity gravel as inside the grid area till 1m away from the earth grid|
|Step voltage 20m away from earth grid||Step voltage reduced and maintained within the limits by provision of 3m long earth rods around the corner and periphery of the peripheral earth ring|
|Transfer potential||Insulated all metallic poles using paint till 2m from the ground level and lay gravel around the metallic pole for 2m|
Many equipment risers were in deteriorated state and in violation of cross-sectional area requirement as per IEEE 80:2013. In order to overcome the same, ISSPL ensured that the equipment riser was connected to the earth ring using 75×6 mm GI conductor.
Gravel Resistivity Measurement
ISSPL collected gravel samples from the switchyard for resistivity measurement and measured the same as per IEEE 81-2012. Resistivity was 60,065 Ωm.
Lightning Protection System
Anomaly found and corrective measures are given in the table below:
|Correction Type||Anomaly||Correction||Material||Quantity (nos)|
|Type – VIII||Direct Lightning Protection||Installation of 5 ESE system in the substation along with earth termination system||ESE – SAT 3||4|
|3 Earth Pits for each ESE system||12|
|17.2 mm Cu Coated steel rod of 3 m Long||12|
|75 x 6 mm GI Strip Interconnection of Pits- 3m each||36 m|
|Earth Enhancing Compound as per IEEE 80:2013 Clause 14.5 d (3 kg per bag)||48 bags|
|Earth Enhancing Compound as per IEEE 80:2013 Clause 14.5 b (5 kg per bag)||60 bags|
ISSPL successfully completed the audit of earthing and protection systems and implemented recommended measures as illustrated above.