Transformer oil is used as an insulating medium in power and instrument transformers, oil-filled bushings and switches.
The operating conditions of oil in electrical equipment (heating with operating current, the action of a burning arc, contamination with particles of solid fibrous insulation, moisture from contact with the environment, etc.) impose rather stringent requirements on it.
Fresh transformer oil must be tested in accordance with the requirements before being poured into the equipment. Operational transformer oil is tested in accordance with the requirements.
For testing, a sample of transformer oil that arrived from the manufacturer or is in electrical equipment is taken from the bottom of the tank or equipment tank, after flushing the drain hole with oil. The container in which the oil sample is taken must be clean and well dried.
The minimum breakdown voltage of oil is determined on apparatuses of the OLS-90 UIM oil BDV tester type in an oil-punching vessel with a standard spark gap, which consists of two flat brass electrodes 8 mm thick with rounded edges and 25 mm in diameter with a distance between the electrodes of 2.5 mm.
Before testing, the jar or bottle with the oil sample is slowly turned upside down several times, ensuring that there are no air bubbles in the oil. The porcelain vessel in which the oil is tested is rinsed three times with the oil of their samples together with the electrodes. Oil is poured onto the walls of the vessel and the electrodes in a thin stream so that air bubbles do not form. After each rinse, the oil is completely drained.
The level of filled oil in the vessel should be 15 mm above the top edge of the electrode. The protective oil in the vessel must be allowed to settle for 15-20 minutes. to remove air bubbles. The voltage increases until breakdown is performed smoothly at a rate of 1–2 kV/s. After a breakdown, which is marked by a spark between the electrodes, the voltage is reduced to zero and increased again until the next breakdown. In total, six breakdowns are made with intervals between them of 5-10 minutes. After each breakdown, carbonized oil particles are removed by stirring from the gap between the electrodes with clean glass or metal rods. The liquids are then allowed to stand for 10 minutes.
The voltage at which the first breakdown occurs is not taken into account. The breakdown voltage of transformer oil is determined as the arithmetic average of five subsequent breakdowns.
Norms of acceptance tests.
Scope of acceptance tests of transformer oil.
In accordance with the requirements, transformer oil is tested at the installation site of electrical equipment in the following scope:
1. Analysis of oil before pouring into equipment.
2. Oil analysis before turning on the equipment.
3. Testing the oil from the apparatus for stability when it is mixed.
Analysis of oil before pouring into equipment.
Each batch of transformer oil received from the plant, before being poured into the equipment, must be subjected to one-time tests for all indicators.
Oils manufactured according to specifications should be tested for the same indicators, but the test standards should be taken in accordance with the specifications for these oils.
Oil analysis before turning on the equipment.
The oil refilled into the equipment must be subjected to a stored analysis before it is switched on after installation. The abbreviated oil analysis includes: determination of the minimum breakdown voltage, qualitative determination of the presence of mechanical impurities and suspended carbon, determination of the acid number, elucidation of the water extract reaction or quantitative determination of water-soluble acids and determination of the flash point.
Testing oil from apparatuses for stability when mixing it.
When pouring fresh standard oils of different grades into electrical equipment, the mixture is checked for stability in mixing proportions, and the stability of the mixture should not be worse than the stability of one of the mixed oils, which has less stability.
Carrying out periodic checks, measurements and transformer oil in operation
During operation, the quality of transformer oil must comply with the standard.
The scope and frequency of operating oil tests depends on the specific type of equipment or apparatus.
For power transformers, autotransformers and oil reactors, transformer oil is tested in the scope and terms, in accordance with the standards.
For oil circuit breakers, transformer oil is tested in the scope and terms, in accordance with the standards.
For instrument transformers, the transformer oil has been tested in the amount and timing, according to the standards.
For oil-filled bushings, transformer oil is tested in the scope and terms, in accordance with the standards.