Substation Equipment Testing
Routine and functional testing of transformers, circuit breakers, CTs, VTs and other primary equipment.
Primary substation equipment (transformers, breakers, CT/VTs, disconnectors, arresters and earthing) is the main asset of a substation, and each item is assessed with specific tests. The goal is not just pass or fail; it is to record baseline values so later maintenance can show trends and decisions can be made before a failure.
Power and distribution transformer tests
| Test | Purpose | Practical note |
|---|---|---|
| Insulation resistance and PI/DAR | Condition of insulation between windings and tank | 5 or 10 kV; record oil temperature for correction |
| Turns ratio (TTR) | Winding and tap-changer integrity | All taps; typical tolerance ±0.5% |
| Winding resistance (DC) | Internal connections and tap-changer contacts | Compare phases and factory values |
| Tan δ and capacitance | Moisture and ageing of insulation, bushings | Bushing test in UST mode |
| Excitation current | Core faults and inter-turn faults | Compare the three phases |
| Short-circuit impedance | Mechanical deformation of windings | Compare with nameplate and previous test |
| SFRA (IEC 60076-18) | Winding and core displacement | Especially after transport or a severe through-fault |
| Dissolved gas analysis (DGA) | Internal faults such as arcing, hot spots, partial discharge | Standard sampling per IEC 60599 |
| Oil tests (breakdown voltage, moisture, acidity) | Insulating oil quality | Before energization and periodically |
CTs and VTs
- Ratio and polarity by primary or secondary injection
- Excitation curve (knee point) to confirm no saturation at fault current
- Secondary winding resistance and burden to check accuracy class
- Insulation resistance and single-point secondary earthing
- For VTs: ratio, polarity, burden, fuse and secondary circuit condition
Circuit breakers and disconnectors
| Test | Application |
|---|---|
| Contact resistance (micro-ohm) | Main contacts; typically tens of micro-ohms |
| Timing | Open and close times, pole discrepancy, trip and close |
| Travel curve | Mechanism condition and stroke |
| Insulation and withstand | Pole insulation in open and closed positions |
| SF6 pressure and quality | Leakage, moisture and purity |
| Minimum coil voltage and current | Operation at 70% and 85% of rated voltage |
Other equipment
- Surge arresters: insulation resistance and leakage current; resistive tests
- Earthing system: earth resistance (fall-of-potential, IEEE 81), touch and step voltage, conductor continuity
- MV switchgear: insulation resistance, AC or VLF withstand, busbar contact resistance, mechanical interlocks
- Battery and DC charger: capacity, cell voltages, discharge test, charger and DC earth-fault relay
- Thermography: find hot spots in connections under load
Standards and reporting
Tests follow IEC 60076, IEC 62271, IEC 61869 and IEEE C57. The report includes nameplate data, ambient conditions, calibrated instruments, measured values, comparison with limits and a pass or fail result, and is kept as a reference for later maintenance.
Scope of work
- Complete transformer testing: TTR, winding resistance, tan δ, SFRA
- Oil sampling and DGA interpretation
- CT/VT ratio, polarity and excitation curve tests
- Breaker contact resistance, timing and travel curve
- Earth resistance, touch and step voltage measurement
- Battery, charger tests and thermography
- Baseline reference report for later maintenance