- 2082 Shrawan · 8 marks
Monitoring gaseous and liquid dielectrics for impurities
Short answer
Gaseous SF6 is monitored by density/pressure switches, dew-point meters, and SO2 sensors for decomposition. Liquid dielectrics are checked for breakdown voltage (BDV), moisture via Karl Fischer, dissolved gas analysis (DGA) for H2, CH4, C2H2 etc., tan delta, acidity, and furans. Rising impurities indicate internal faults.
How are gaseous and liquid dielectric materials monitored for impurities?
Gaseous and liquid insulation lose strength when impurities such as moisture, particles, air and decomposition products build up. They are therefore monitored periodically (and online) by sampling and by sensors fitted on the equipment. Rising impurity levels warn of internal faults (arcing, overheating, partial discharge) and of the need to filter, dry or replace the dielectric.
A. Gaseous dielectrics (mainly SF₆ in GIS and breakers)
| Impurity / parameter | Monitoring method |
|---|---|
| Gas density / pressure (leakage) | Temperature-compensated density monitors with alarm and lockout contacts |
| Moisture | Dew point meters (chilled mirror, capacitive hygrometer); limits per IEC 60480 |
| Purity (air, CF₄ content) | Purity analyser (sound velocity or thermal conductivity), gas chromatography |
| Decomposition products (SO₂, SOF₂, HF) | Detector tubes, electrochemical SO₂ sensors, gas chromatography |
| Particles and PD | UHF and acoustic PD sensors |
- Moisture is critical: it condenses on spacers at low temperature and causes surface flashover, and reacts with arc products to form corrosive HF.
- High SO₂/SOF₂ indicates arcing or partial discharge inside the compartment.
- Gas is reclaimed (filtered, dried) using gas-handling carts when limits are exceeded.
B. Liquid dielectrics (transformer and cable oil)
1. Breakdown voltage (BDV) test
- Oil sample is placed in a test cell with two electrodes 2.5 mm apart (IEC 60156); voltage is raised at 2 kV/s until breakdown; mean of 6 readings.
- Low BDV indicates moisture and particles. Typical good oil: above about 50–60 kV.
2. Moisture content – Karl Fischer titration (IEC 60814), in ppm; online moisture sensors.
3. Dissolved gas analysis (DGA) – gases dissolved in oil are extracted and measured by gas chromatography (IEC 60599). The gas pattern identifies the fault:
| Gas | Indicates |
|---|---|
| H₂ | Partial discharge |
| CH₄, C₂H₆, C₂H₄ | Overheating of oil |
| C₂H₂ (acetylene) | Arcing |
| CO, CO₂ | Overheating of paper insulation |
Ratio methods (Rogers, IEC ratio, Duval triangle) are used for diagnosis.
4. Dielectric loss () and resistivity – measured in a test cell at 90 °C; rise indicates polar contaminants and ageing products.
5. Acidity (neutralization number) and interfacial tension (IFT) – rising acidity and falling IFT show oxidation and sludge formation.
6. Furan analysis – furanic compounds (by HPLC) indicate ageing of paper insulation.
7. Others – particle count, colour, flash point, sludge content, and online sensors for hydrogen and moisture.
Oil found contaminated is treated by filtering, vacuum dehydration and degassing, or by regeneration with Fuller's earth.
✓ Answer checked by Claude Opus 5.5 (high effort)
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