Long before most electrical apparatus fail, signs of trouble appear and can be detected by oil tests!
The condition of generation, transmission or distribution transformers can be determined by the analysis of electrical insulating oil. These fluids circulate as a dielectric and coolant and can be sampled, in most cases, while the equipment is energised. With outages minimised in modern times, this is a key attribute.
Oil testing can detect developing apparatus problems such as, local overheating at a loose connection or electrical discharge between turns, so problems can be managed and catastrophic failures prevented. Oils and other insulating materials degrade during their life as a result of heating, oxidation, and in more serious cases, from discharge activity. Accelerated or excessive degradation of the oil can be detected, but more important is to detect abnormal conditions or faults that can result in failure of the apparatus.
There are a variety of tests that can help detect problems with the insulating materials and the apparatus. Because diagnostics from oil data is so good today, condition-based maintenance is possible. With good knowledge of the condition of transformers, attention can be focused on problems so they are managed to minimise out of service time while reducing risk of a catastrophic failure. By understanding the true condition of transformers and how they age, proper maintenance can be used to extend the life of such important assets. To use oil tests effectively requires accurate data, background information as to where the sample was taken, nameplate information, and a good understanding of the diagnostics.
SOME TYPICAL TESTS
Oil Quality Testing
Colour (ASTM D 1500, ISO 2049): Insulating liquids darken with the presence of oxidation byproducts and foreign materials and are an indicator of ageing.
Dielectric Breakdown Voltage (ASTM D877 or 1816, IEC 60156): A low value indicates the presence of contaminants such as water, dirt or other conducting particles in the insulating liquid.
Interfacial Tension (ASTM D 971, ISO 6295): Monitors the progression of oxidation and detects contaminants such as soaps, paints, varnishes and byproducts of insulation ageing.
Acidity / Neutralization Number (ASTM D 974, IEC 62021-1): Monitors the progression of oxidation by detecting acidic compounds which accelerate deterioration of the solid insulation and are precursors to sludge formation.
Visual (ASTM D 1524, IEC 60296): Visual inspection identifies foreign material in the insulating liquid, which may lower its dielectric strength.
Power Factor or Dissipation Factor at 25°C (ASTM D 924, IEC 60247): High values indicate the presence of contaminants like carbon, polar compounds, metal soaps and byproducts of oxidation.
Water Content (ASTM D 1533, IEC 60814): Excessive moisture is one of the primary causes of low insulating liquid dielectric breakdown strength. High water content may be detrimental to the transformer under a variety of conditions. Reporting results in concentration (ppm) and percent relative saturation gives more effective interpretation of results
Specific Gravity or density (ASTM D 1298, IEC ISO 3675): Helps identify different types of insulating liquids.
Diagnostic Testing
Dissolved Gas Analysis (ASTM D 3612, IEC 60567): The single most important test you can perform to detect problems and head-off potential transformer failures. It monitors gas generation in transformers for advance notice of developing faults to properly manage risk. It's a good way to detect thermal and electrical problems and determine their severity.
Furanic Compounds (ASTM D 5837, IEC 61198): Since the paper is the most important dielectric component of the transformer, having the ability to assess its condition is a must. When the cellulose breaks down, furanic compounds are generated and can be used to detect accelerated ageing and localized problems.
Metals-In-Oil (Various methods): Dissolved and particulate metals such as copper, iron, zinc, and lead can be detected and can be indicators of incipient-fault conditions, potential bearing wear from pumps or other wear metals from vibration of components.
Keep up to Date
Corrosive Sulphur – There are sulphur compounds in oil that can be corrosive resulting in the formation of copper sulphide on conductors and in insulating paper. On conductors the copper sulphide is too resistive and causes overheating. In the paper copper sulphide is too conductive and can results in a dielectric failure. Copper sulphide particles can bridge insulation gaps resulting in dielectric failure in the oil.
Paper Quality Testing
Degree of Polymerization of Paper (ASTM D 4243, IEC 60450): This test provides a measure of paper ageing, and correlates with important physical properties like resistance to tearing and bursting. This is a critical factor in estimating the real ageing of the main transformer insulation. This test does require a paper sample so is used opportunistically when internal inspections are needed.
Doble Engineering Company
For accurate and reliable oil testing and professional diagnostics by a team of chemists and engineers come to Doble Engineering Company. We can help with creating a cost-effective test programme and diagnostic services. Specialised testing is available to analyse problems beyond the typical tests. When transformers develop problems Doble is there to help with you with the testing, assessment, and action plan.
Email: [email protected]
Tel: 01483 514120
