This post analyzes the method statement for testing and commissioning of substation equipment.
It covers
- Current and voltage transformers,
- LVAC,
- Medium voltage and 132 kV gas insulated switchgear (GIS),
- MV and HV cables,
- Capacitor bank,
- Power transformers,
- DC systems and protection.
The common procedure is to verify readiness and healthiness of each equipment against approved drawings, nameplate data and factory (FAT) reports while following high voltage safety rules and company safety codes.
Instrument Transformers
Current transformer tests are insulation resistance, polarity, winding resistance, magnetization curve, ratio and burden, plus a one point earthing check.
Insulation is tested for 60 seconds:
- Primary to secondary at 500 V (LV) or 2500/5000 V (MV/HV/EHV) and
- Secondary to earth and
- Secondary to secondary at 500 V.
Polarity requires P1 and S1 to correspond and the CT Analyzer red leads go to P1/S1 and black leads to P2/S2.
The saturation point is where a 10% rise in secondary voltage causes a 50% rise in exciting current.
Burden is measured with a CT Analyzer or CPC 100 after isolating the secondary circuit.
For one point earthing, a 500 V test shows high insulation with the neutral link open and zero with it closed.
Voltage transformer tests are similar with an added primary to earth insulation test, polarity (A–N to a–n) using the CPC 100, winding resistance (primary by multimeter, secondary by milliohm meter or CPC 100) and ratio.
LVAC and Medium Voltage Switchgear
LVAC tests include insulation resistance, HV test, PLC scheme logic, indications, breaker travel and outgoing feeder identification.
The panel has three incomers and two bus couplers (five breakers), of which only three may close at once; paralleling of transformers must never occur.
On incomer failure the corresponding bus section closes; on restoration the bus section opens first and the incomer closes after a delay.
MV breaker tests comprise insulation resistance (5 kV, 60 seconds results in GΩ), contact resistance (IEC 62271-100), mechanical and electrical functional checks (IEC 60298), timing, busbar joint resistance, HV tests on busbar and breaker, and earth switch interlocks.
Electrical checks include spring charging time and current, anti-pumping and open–close–open (OCO) duty.
The earth switch may close only when the VCB is in the test position.
132 kV GIS
GIS tests include
- SF6 purity and dew point (IEC 62271-203),
- Density monitor calibration, SF6 alarms,
- Manual and electrical operation of disconnectors,
- Earth switches and breakers,
- Breaker timing (e.g. AREVA B165: closing 100–115 ms, opening 27–31 ms),
- Contact resistance,
- Leakage (maximum 0.5% per annum per compartment),
- HVAC withstand,
- PD sensor sensitivity (5 pC or less) and voltage selection/synchro close checks.
- Synchro limits are ±10% voltage, ±0.5 Hz and ±10° phase angle.
Before HVAC, erection and contact resistance should be complete and all devices operated at least five times.
Insulation resistance at 5000 V before and after must exceed 2000 MΩ.
MV and HV Cables
MV cable tests are phase identification, insulation resistance (1 kV DC, one minute, before and after HV), sheath integrity (10 kV DC, 60 seconds) and conductor HV test (25.4 kV DC, 15 minutes).
HV cable tests add a drum sheath test (10 kV, 1 minute), sectional phasing (50–150 V), sectional sheath test (5 kV, 1 minute), sheath voltage limiter (SVL) test, link box contact resistance, conductor resistance (limit 0.0283 Ω/km for 630 mm² corrected to 20 °C) and HVAC with partial discharge measurement.
HVAC uses a resonant set and the cable must withstand 132 kV for 1 hour, or 106 kV for old cable, with no PD detected at 76 kV and 114 kV.
Capacitor Bank
Tests under IEC 60871-1 and 4 include
- Capacitor insulation resistance (2500 V, above 100 MΩ),
- Capacitance against factory data,
- Damping reactor winding resistance (corrected to 75 °C),
- Reactor HV test (22.4 kv AC, 1 minute),
- Inductance and unbalance current.
Neutral current at 380 V must stay within the unbalance relay alarm setting and not exceed 20% of the trip setting.
Power Transformer
Transformer tests include
- Winding insulation resistance at 5 kV (readings at 15, 60 and 600 seconds; at least 2000 MΩ at 60 seconds),
- Polarization index (≤1.0 bad, ≥1.3 satisfactory, ≥1.5 good, ≥2.0 very good), core and clamp insulation (500 V; 2 kV for 60 seconds),
- Ratio (principal tap error ±0.5% or one tenth of impedance whichever is less),
- Vector group,
- OLTC continuity,
- Magnetizing current (400 V, before any DC test),
- Winding resistance (±3%, corrected to 75 °C),
- Tan delta (oil-filled winding not above 0.5%, condenser bushings 0.7% at 20 °C),
- Temperature indicator calibration (±2 °C),
- 48-hour oil leak test, oil BDV and
- Sweep frequency response analysis (20 Hz–2 MHz).
OLTC, AVR, parallel operation and cooling control checks complete the programme.
DC System and Protection
The 110 V charger has float, boost and commissioning voltages of 120.4 V, 137.6 V and 141.9 V; the 48 V charger has 53.2 V, 60.8 V and 62.7 V.
Testing covers load, alarms, diode voltage regulator stages, bus coupler transfer and battery charge/discharge (0.2C5A, to 1.1 V/cell over 5 hours, electrolyte ≤45 °C).
Protection testing includes CT primary injection, busbar sensitivity and stability and feeder and transformer differential relay checks.
Key Acceptance Criteria
| Test | Condition | Acceptance |
|---|---|---|
| LVAC insulation resistance | 1000 V tester | Above 200 MΩ |
| LVAC busbar HV test | 2 kV AC, 1 minute | Withstand 1 minute |
| MV breaker/busbar contact resistance | 100 A DC, micro-ohm meter | ±5% of factory value |
| MV busbar and breaker HV test | AC HV, 1 minute (busbar at 80% of FAT) | Withstand 60 seconds |
| GIS SF6 purity / moisture | Gas analyser, dew point meter | Above 95%; below 150 ppm (CB), 500 ppm (others) |
| GIS density monitor | Compared with standard gauge | Po = Pd ± 0.2 barg |
| GIS main circuit resistance | 100 A DC | ±20% of factory data |
| GIS HVAC test | 80% of factory voltage, 50 Hz, 1 minute | Withstand; PD below 10 pC |
| MV cable DC HV test | 25.4 kV DC, 15 minutes | No breakdown |
| 132 kV cable HVAC test | 132 kV, 1 hour, 200–300 Hz | No breakdown |
| Link box contact resistance | Micro-ohm meter | Below 20 µΩ |
| Capacitor IR / reactor HV | 2500 V; 22.4 kV AC 1 minute | Above 100 MΩ; withstand 1 minute |
| Transformer oil BDV | 2.5 mm sphere gap | Average above 70 kV, none below 60 kV |
Sheath test voltages differ by stage (10 kV after laying, 5 kV for the sectional test) and transformer clause numbering has gaps (12, 18, 21, 25, 27, 32–35).
Acceptance for several tests depends on factory reports or manufacturer data, which must be available on site.
