Transformer oil testing
Transformer Oil Testing is a proven loss prevention technique that should be a part of any condition based predictive maintenance program. This early warning system can allow management to identify maintenance priorities, plan work assignment schedules, and order necessary parts and materials.
The transformer’s fluid not only serves as a heat transfer medium, it also part of the transformer's insulation system. It is therefore prudent to periodically perform tests on the oil to determine whether it is capable of fulfilling its role as an insulating media. Some of the most common tests for transformer oil are as follows: Dissolved Gas in Oil Analysis, Screen Tests, Moisture Content, Furan Compounds, and PCB content.
Dissolved gas-in-oil analysis, (DGA)
One important part of diagnostic testing is the dissolved gas analysis (DGA). The Dissolved Gas Analysis can give an early indication of abnormal conditions inside the transformer. The DGA test analyzes the gas concentrations that are dissolved in the transformer oil. Small samples of oil are taken for this test, using a clean, moisture-free container/syringe. It is very important that a DGA sample is kept airtight; otherwise the gases that are dissolved in the liquid can escape into the atmosphere, or atmospheric gases can enter the sample, which may change the test results and analysis.
The syringe is then sent to our laboratory for analysis. Gases are extracted from the oil then the gases are identified using gas chromatography equipment. The principal gases that are typically found are hydrogen, oxygen, nitrogen, carbon monoxide, carbon dioxide, methane, ethylene, ethane, and acetylene. Certain quantities and combination of these gases are indicative of insulation overheating/ overloading, liquid overheating, partial discharge (corona), or arcing in the transformer.
The paper insulation that is normally used to insulate the windings of a transformer is a cellulose product. The insulating paper normally found in transformers is about 2/3 of the insulating system and it will determine the useful life of a transformer. If a transformer becomes overloaded for any reason, the windings will generate more heat and deteriorate the cellulose insulation. A DGA test can identify an overloaded transformer by a test result showing high Carbon Monoxide, high Carbon Dioxide, and in extreme cases even elevated Methane and Ethylene. Furan analysis can be useful in determining the amount of life expended.
If a transformer is overloaded for a long period of time, the deteriorating condition of the cellulose will shorten the life of the transformer. A thumb rule of every 10°C rise in transformer temperature cuts the transformer life by half illustrates the temperature affects on cellulose. When the cellulose insulation breaks down to the point where arcing starts to occur, the transformer must be taken out of service.
Insulation liquid overheating
Overheating of the liquid insulation is a slightly different problem in transformers. A DGA test will indicate high thermal gases (Methane, Ethane, and Ethylene) as a result of overheating of the liquid. These gases are formed from a breakdown of the liquid caused by heat.
Heating may be caused by poor contacts on a tap changer, or loose connections on a bushing or a grounding strap, or circulating currents in the core due to an unintended core ground. Actions that can be taken once a thermal gas problem is detected would depend on the severity of the problem. If conditions are not severe the transformer should be monitored closely. If the conditions get worse, and thermal combustibles elevate, then the transformer will need to be taken out of service. If the combustibles are stable and remain present, then the transformer should be inspected at the next outage or a scheduling of downtime should result.
Corona is considered to be partial discharge and occurs at areas of high electrical stress such as at a sharp point or edge along an electrical path. Partial discharge is commonly explained as being intermittent unsustained arcs, which are shot off of the conducting material, likes a stream of electrons. If these arcs contact solid insulating material, they can cause cumulative damage.
Corona is detected in a DGA by indications of elevated hydrogen. If corona is detected by a DGA test, other methods of pinpointing the exact location of the problem can be used --a partial discharge detector can be used to detect the popping noise that corona is making.
Arcing is the most severe condition in a transformer because it indicates a breakdown of the insulation. The presence of acetylene is an indicator of arcing; and even low levels of this gas should cause concern. Normally, arcing occurs only after other problems surface, which is identified through DGA testing. If the arc occurs in the area of cellulose-insulation, carbon dioxide and carbon monoxide will also be elevated.
Arcing can be generated in many areas of a transformer. Insulation breakdown in the windings, from coil to coil or coil to ground will result in arcing. A portion of the insulation may become deteriorated to the point that it can no longer contain the stress of the electrical conductor. If a winding shorts from turn to turn, or phase to phase, or phase to ground, arcing will occur and the transformer will have failed. When arcing occurs in the area of the windings, the usual result is de-tanking of the transformer, and a rewind conducted. A loose connection may also cause arcing but of greater significance would be arcing due to insulation breakdown.
Acetylene is a dangerous and volatile gas that can explode if exposed to the atmosphere like when the transformer is opened for inspection. Extreme care should be taken when opening a tank containing insulating fluid.
A power factor (dissipation factor) test measures the dielectric losses in the liquid and hence the amount of energy dissipated as heat. By testing the oil at a standard temperature, the test results can be compared to standard values.
This screen test result can help reveal the quality and the integrity of the insulation liquid, and can form the basis for judgment as to suitability for continued service. The Screen test can be used as a maintenance test for determining when a filtering or change of the transformer liquid is in need. TOGA will normally perform this test at 20 degrees C (68 degrees F), but an additional test at 100 degrees C (212 degrees F), boiling may be required.