Transformer oil, also called insulating fluid, is essential for the functioning of transformers, capacitors, and various other high-voltage electrical equipment. Its main roles include providing electrical insulation, dissipating heat, and suppressing arcs.
However, insulating fluid can deteriorate over time, resulting in diminished performance and possible equipment failure. This blog will explore the scientific principles behind the degradation of insulating fluid, examining its causes, mechanisms, and consequences.
The Role of Insulating Fluid
Insulating oil in electrical equipment serves three primary functions:
- Insulation: Prevents electrical discharge between live parts and the grounded components.
- Cooling: Dissipates heat generated during operation.
- Arc Suppression: Minimizes electrical arcing within the equipment.
It prevents electrical discharge between live parts and grounded components, dissipates heat generated during operation, and minimizes electrical arcing.
The insulating fluid must possess good thermal conductivity and chemical stability at high temperatures for effective performance. The dielectric strength of the fluid, which indicates its maximum voltage capacity before breakdown, is measured in kilovolts per standard thickness. This is assessed by applying increased high voltage until the leakage current causes a spark between electrodes immersed in the fluid.
Types of Insulating Fluid
- Mineral Oil (Petroleum product)
- Naphthenic Oil
- Paraffinic Oil
- Synthetic Oil (Chemical product)
- Silicon Oil
- Ester Fluids
- Natural
- Synthetic
Each type of insulating fluid has its pros and cons. Reach out to the team at TJH2b with any questions!
Causes of Insulating Fluid Breakdown
Insulating fluid in high-voltage equipment experiences electrical, mechanical, and thermal stress during operation, leading to contamination. This contamination includes carbon deposits, sulfur, acids, various gases, and sludge—primarily oxidation products exacerbated by temperature and air exposure. Increased moisture and conductive impurities result in lower breakdown voltage in the transformer oil.
The degradation of insulating fluid can be attributed to several factors:
- Thermal Stress:
- Thermal stress occurs when high operating temperatures accelerate chemical reactions within the oil. This leads to the thermal decomposition of oil molecules, which results in the formation of gases and acids and reduces the oil’s insulating properties. Consequently, increased thermal stress can cause hot spots and overheating, potentially leading to equipment failure.
- Oxidation:
- Oxidation occurs when oil molecules are exposed to oxygen, leading to the formation of acidic compounds and sludge. This process impairs oil flow and insulation, reduces the dielectric strength of the oil, and can corrode metal components.
- Electrical Stress:
- Electrical stress occurs when high electric fields cause the oil to break down electrically. The mechanism behind this involves partial discharges within the oil, which create ionization and free radicals that break down oil molecules. The implications of electrical stress include the formation of conductive paths, which reduce insulation and increase the risk of short circuits.
- Contamination:
- Contamination involves the presence of foreign particles, moisture, and other impurities that can degrade the quality of oil. These contaminants create conductive pathways and trigger chemical reactions, leading to degradation. As a result, the contaminated oil experiences a reduction in dielectric strength, which can ultimately cause equipment failure.
Mechanisms of Insulating Fluid Breakdown
- Chemical Degradation:
- Chemical degradation involves chemical reactions within the oil that produce by-products like acids, aldehydes, and ketones. These by-products can lead to the formation of sludge and other deposits, which impair the oil’s insulating properties and flow characteristics.
- Thermal Decomposition:
- Thermal decomposition is a process in which elevated temperatures cause the breakdown of hydrocarbon chains in the oil. This results in the formation of gaseous products such as hydrogen and methane, as well as carbonaceous deposits.
- Electrochemical Degradation:
- Electrochemical degradation occurs when electrical discharges produce ionization and free radicals within the oil. These highly reactive species can then attack and break down oil molecules, resulting in reduced insulation and increased conductivity.
Significance of Transformer Oil Testing
Periodic insulation oil testing is crucial for maintaining the operational integrity of high-voltage equipment. Regular monitoring of the dielectric strength of insulating oil is essential to prevent internal arcing or total equipment failure. Unexpected breakdowns of key electrical apparatus during service can result in commercial losses, catastrophic human casualties, and decreased power system reliability.
Testing of Insulating Fluid
Standards such as ASTM D1816, ASTM D877, and IEC 60156 outline procedures and construction criteria for oil cells used to evaluate the dielectric strength or breakdown value of oil samples. A sample is extracted from the transformer’s drain valve, and its breakdown voltage is assessed. A test voltage is incrementally applied to electrodes immersed in the insulating oil, increasing at a constant, standard-compliant rate, typically 2kV/s. The test is repeated five to six times, with the average of these readings considered as the breakdown voltage of the sample under examination.
The dielectric strength or breakdown voltage of insulating oil must meet or exceed specified values. If the breakdown voltage approaches the minimum prescribed limit, additional diagnostic tests like oil resistivity and dielectric dissipation factor (tan δ) should be conducted. The standard practice recommends performing the BDV test biannually.
We give a more comprehensive breakdown in our blog: Interpreting Insulating Fluid Test Reports: What Do the Results Mean?
Monitoring and Managing Oil Breakdown
Effective monitoring and management of insulating oil are crucial for maintaining the performance and longevity of electrical equipment. Key strategies include (but are not limited to):
- Regular Testing
- Dissolved Gas Analysis (DGA): Detects gases produced by oil decomposition, indicating thermal and electrical stress.
- Acidity Testing: Measures the level of acidic compounds, indicating oxidation.
- Moisture Content: Determines the presence of water, which can reduce dielectric strength.
- Oil Reconditioning and Replacement
- Filtration: Removes particulates and sludge.
- Degassing: Eliminates dissolved gases.
- Reconditioning: Restores oil properties through chemical treatments.
- Replacement: Periodically replacing degraded oil with fresh oil.
- Preventive Maintenance
- Temperature Control: Maintaining optimal operating temperatures to reduce thermal stress.
- Sealing Systems: Preventing the ingress of moisture and contaminants.
- Electrical Inspections: Regular checks for partial discharges and electrical faults.
TJ’s Transformer Diagnostic Packages
Ensure the reliability and longevity of your high-voltage electrical equipment with TJ|H2b’s specialized diagnostic evaluations. Our industry-leading testing solutions help identify potential issues before they lead to costly failures:
- Transformer Condition Assessment (TCA®): Analyzes transformer oil to assess overall condition, detect faults, and estimate insulation degradation. Routine monitoring enhances long-term performance and reliability.
- Tap Changer Activity Signature Analysis (TASA®): Detects faults in on-load tap changers (LTCs), enabling proactive maintenance and reducing the risk of unexpected failures.
- Breaker Oil Analysis (BOA®): Evaluates oil-filled circuit breakers to identify maintenance needs, ensuring efficient servicing and preventing unnecessary downtime.
- Breaker Gas Analysis (BGA®): Tests SF6 gas in gas-insulated breakers to assess equipment condition and detect internal issues with a precise, high-integrity sampling method.
Proactive diagnostics lead to smarter maintenance, lower costs, and increased equipment reliability. Contact TJ|H2b today to schedule your diagnostic evaluation.
