Insulating Oil Management
Sample Testing and Analysis
Regular sampling and testing of insulation oil as well as other synthetic insulants is a valuable technique in preventive maintenance of power transformers and HV equipment as it can highlight developing faults. Trend analysis can help determine transformer life and aid planning of refurbishment or repair work.
Test results are analysed to estimate the health condition of transformer. Rosh Engineering report on test results using in-house experts and provide advice and recommendations as necessary. We engage in dialogue with customers over the importance of a transformer in a network, the ease of gaining an outage or shutdown, the plant’s expected life, and availability of alternative transformers or power feed in determining recommendations. Telling you Carbon Dioxide levels are 2978ppm is not a great deal of help to many.
Rosh Engineering can also provide solutions to any issues highlighted from oil test results from replacement and flushing of PCB contaminated equipment to processing wet or degraded oil. However, Rosh are always mindful of reporting “wet” oil in a transformer as this is one of the few contaminants that can be introduced by human error such as by condensation in the bottle cap, to contamination in the valve opening. Trend analysis, improved sampling technique and repeating sampling can eliminate false reporting and unnecessary cost.
Standard Physical Property Tests carried out to include:
- Dielectric breakdown (kV) based on IEC60156
- Acidity (mgKOH/g) based on IEC62021-2 – Rate of increase of oil acidity in service is a good indicator of ageing.
- Water content (mg/kg) based on IEC60814
- Condition of Oil (Colour, Appearance and fibres per litre) DIHM MSUS109
Dissolved Gas Analysis (DGA) Tests to ASTM D3612 method C helps identify developing faults in a transformer:
- Hydrogen H2 (ppm),
- Oxygen O2 (ppm)
- Nitrogen N2 (ppm)
- Carbon Monoxide CO (ppm)
- Carbon Dioxide CO2 (ppm)
- Methane CH4 (ppm)
- Ethylene C2H4 (ppm)
- Ethane C2H6 (ppm)
- Acetylene C2H2 (ppm)
Information from DGA data over a period of time provide an advanced warning of developing faults and monitor the rate of fault development. It is possible to identify different faults such as partial discharge, corona, overheating and arcing.
Furfuraldehyde (mg/kg) Analysis (FFA) Test to BSEN61198:
The degree of polymerisation (DP) measurement of insulation paper is not practical in transformers in-service. Therefore, the most prominent component of paper decomposition 2-furaldehyde in oil — or the amount of cellulose that has broken down from the paper and entered the oil. It is considered to be directly related to DP of paper insulation. The level of 2-furaldehyde in oil is directly related to deterioration of insulating paper. However, it is important to track the trend of FFA in the oil over time, as this rate of change is key in determining life. Some transformers start off with a higher level than others so an absolute figure can be meaningless.
Polychlorinated Biphenyls (mg/kg) Analysis (PCB) Test to MSUS122:
Polychlorinated biphenyls (PCB) is synthetic chlorinated aromatic hydrocarbons which have good thermal, electrical properties and excellent chemical stability. The excellent chemical stability and resistance to biodegradation is a cause of concern for environmental pollution. The PCBs have found its way into mineral insulating oil is via cross contamination. The main reason for stopping further use is due to environmental risks of fire or explosion. At temperatures around 500⁰C, extremely toxic compounds of polychlorinated dibenzofurans are formed and the costs of decontamination is too high.
The samples are tested for Arochlor 1254, 1260 and 1242. If found positive, the transformers shall be treated with special care.
Interfacial Tension IFT (ASTM D971)
The interfacial tension between oil and water provides a means of detecting soluble polar contaminants and products of degradation. The degradation compounds are high in overloaded transformers. The decrease in interfacial tension IFT is a tool for detecting insulation deterioration in transformers.
Dielectric Dissipation Factor (DDF) and resistivity IEC 60247
The dielectric dissipation factor and resistivity values are used to monitor the condition of insulating oil. The high values of DDF and low values of resistivity affect the dielectric losses and/or insulation resistance of electrical equipment. The higher value of DDF may lead to thermal runaway leading to equipment failure. Since the DDF and resistivity values are affected by the presence of soluble contaminants and by products of aged insulation components, measurement of these values at regular intervals provide an early warning/indication to oil maintenance schedules.
Corrosive Sulphur content in ppm IEC 62535 and ASTM D1275 Method B
The sulphur in transformer insulating oil normally present as organo-sulphur, the amount depends on type of crude oil, refining process and degree of refining. The presence of elemental sulphur through contamination. At relatively high temperatures during switching operations, sulphur containing oil molecules may decompose and react with metal surfaces to form copper sulphide deposits in insulation paper. This leads to a reduction in electrical insulation properties and results in transformer failures.
Dibenzyl disulphide (DBDS) content in ppm
DBDS is potentially corrosive to copper surfaces at normal transformer operating temperatures and may form copper sulphide under certain condition. DBDS plays predominant role in the problem of corrosion.
Passivator content in ppm
Sometimes metal passivator is added to mineral insulating oil to minimize risk of corrosion, to improve oxidation stability and to suppress streaming electrification. It is essential to monitor this passivator content during service.
Inhibitor content in ppm IEC 60666
Inhibited oils have a different oxidation pattern to uninhibited oils. At the beginning of transformer service life, the synthetic inhibitor is added to transformer insulating oils to improve oxidation stability. This synthetic inhibitor is consumed with formation of oxidation products during service. The consumption of inhibitor is measured by the inhibitor concentration in oil. Therefore the inhibitor content shall be monitored at regular intervals and the frequency depend upon operation temperature and load level. This allows to schedule oil maintenance to top-up inhibitor.
The above service can be offered on yearly, 3 yearly or 5 yearly basis as required by customers.
Transformer Oil Handling services — Processing, Replacement, PCB Disposal and Midel Retro-filling
Rosh Engineering has over 35 years of oil handling experience. The service regime includes oil sampling and testing, oil processing, oil replacement and Midel 7131 retro filling works on site. This includes Life of transformer analysis following diagnostics tests which assess the condition of the transformer for continued use.
Various Oil Handling Services:
- Insulating Oil and Synthetic Oil supply
- Insulation liquid top-up
- Oil replacement in Transformers On Load Tap Changers & switchgear
- Oil disposal including PCB contaminated oil
- Refilling of transformers naturally or under vacuum
- Oil processing using Moble Processing Unit (MPU) – both direct transformer tank and external tankers
- Retro filling with Midel 7131 or Silicone fluids
- Oil regeneration service onsite or offsite
Advantages of regular oil maintenance:
- Improves Reliability by reducing the risk of unplanned outages and transformer down time
- Transformer life extension by reducing rate of paper or cellulose degradation
- Enhancing heat dissipation capability of transformer
- Removes moisture and improves oil breakdown strength
- Improves the insulation resistance of transformer
- Decontamination of PCB contaminated transformers according to PCB regulations