Confidential UK DNO / ICP — 132kV Primary Substation, UK
132kV Circuit Breaker Maintenance & SF₆ Gas Analysis
Northern HV carried out planned maintenance on a 132kV live tank SF₆ circuit breaker at a UK primary substation. Full hydraulic system service, timing tests, and gas quality analysis — returned to service with zero gas loss.
Client
Confidential UK DNO / ICP
Location
132kV Primary Substation, UK
Contractor
Northern HV Ltd
Project Overview
Northern HV was commissioned to carry out planned preventative maintenance on a 132kV live tank hydraulic-operated SF₆ circuit breaker at a UK primary substation. The breaker formed part of a critical incomer circuit supplying mixed industrial and domestic load — so getting this right mattered.
The scope covered full mechanical and electrical maintenance, hydraulic system inspection, and comprehensive SF₆ gas quality analysis. All work was carried out in line with OEM recommendations and BS EN / IEC standards.
We delivered the works safely, on programme, and with zero SF₆ gas loss.
Scope of Works
- Safety & Isolation
- Review of client switching schedule and RAMS
- Application of safety documents under DNO procedures
- Isolation, proving dead, and application of earths
- Environmental controls for SF₆ handling
- Full compliance with Electricity at Work Regulations 1989 and F-Gas Regulations
Mechanical & Hydraulic System Maintenance
- Visual inspection of porcelain insulators and grading rings
- Inspection of hydraulic pipework and seals
- Nitrogen pre-charge pressure checks
- Hydraulic pump motor operation verification
- Oil sampling and condition assessment
- Inspection of operating pistons and linkages
- Functional checks of anti-pumping and trip circuits
Contact & Timing Tests
- Open and close timing tests using calibrated primary injection equipment
- Contact synchronisation checks (pole discrepancy)
- Travel curve analysis
- Coil current signature analysis
SF₆ Gas Analysis
- SF₆ purity testing
- Moisture content measurement (ppmV / °C dew point)
- SO₂ concentration testing
- Decomposition product analysis
- Gas pressure and temperature compensation
Technical Challenges
Hydraulic System Condition
The 132kV breaker used a hydraulic operating mechanism with nitrogen accumulators. During inspection, we identified minor pressure drop in one accumulator — a common early-stage fault that, left unchecked, leads to slow operation or failure to trip.
We rectified this through controlled recharging with calibrated equipment. All hydraulic pressures were verified against OEM tolerances before returning to service.
SF₆ Gas Quality Verification
SF₆ insulation gas requires careful analysis to confirm dielectric integrity and detect any signs of internal arcing or seal degradation. We used a calibrated multi-parameter gas analyser to test purity, moisture content, SO₂ levels, and dew point.
Results came back excellent — 99.6% purity, moisture at 120 ppmV (limit is 200), dew point at -42°C, and SO₂ below 1 ppm. No gas reclaiming or filtering required.
Environmental Compliance
SF₆ is a potent greenhouse gas, so handling it properly isn't optional. All our engineers are F-Gas certified, and we used a closed-loop recovery system throughout. Zero gas loss recorded, no top-up required, and full gas handling logs provided for the client's environmental records.
Safety Performance
Works were completed under DNO safety procedures with full isolation, proving dead, and application of earths before any maintenance began.
All SF₆ handling complied with F-Gas Regulations. Our engineers maintained environmental controls throughout, with zero gas release to atmosphere.
Project Outcome
The 132kV circuit breaker was returned to service without issue. The maintenance intervention confirmed the breaker's mechanical condition as good, hydraulic system fully operational (following accumulator adjustment), SF₆ gas quality excellent, and electrical operation within OEM tolerances.
Our recommendations to the client:
- Continue annual visual inspections
- Repeat full SF₆ analysis at next major maintenance interval
- Monitor accumulator pressure quarterly
The work reduced risk of hydraulic failure, confirmed dielectric integrity, ensured continued regulatory compliance, and extended asset service life.
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