As the oil quality directly affects the performance and lifespan of the transformer. Therefore, various test standards at national and international level have been developed to ensure its reliability and exact condition. But still the various power and utility sectors are in doubt which test method for particular parameter is to be followed. So, this article provides detailed review and comparison of various test methods described in IS, IEC, ASTM, and IEEE.

The various parameters indicate oil condition and health assessment are categorized into: (a) Functional Parameters, (b) Refining/Stability parameters, (c) Performance Parameters, and (d) Health, Safety & Environmental Parameters.

1. FUNCTIONAL PARAMETERS

a. Kinematic Viscosity

Kinematic Viscosity (mm2/s) is the measure of the flow of resistance of oil under the influence of gravity indicates its ability to circulate effectively in the transformer. Generally, transformer oil should have a low viscosity to facilitate effective cooling within the transformer. However, several factors such as; temperature, age, etc. have significant affects on it as viscosity decreases with increase of temperature (therefore generally measures at 40 and 100 deg C) and increases with the aging of transformer oil.

b. Pour Point

The lowest temperature in degrees Celsius (°C) at which oil can sustain its fluid nature and flow generally expressed as a pour point. It becomes very crucial to ensure proper transformer operation, particularly in cold climates, as oil solidification can impede the oil's ability to circulate within the transformer, hindering its cooling capabilities. The pour point is influenced by the type of oil and the amount of wax content within it. Paraffin-based oils, for example, tend to have higher pour points due to their higher wax content compared to naphthenic-based oils.

c. Water Content

Transformer oil can absorb moisture from the surrounding environment, especially if the transformer's sealing system is compromised. Additionally, water can be a by-product of the aging process of both the oil and the paper insulation within the transformer. Water in transformer oil can significantly reduce its dielectric strength, which is its ability to resist electrical breakdown. Besides reducing dielectric strength, high water content can accelerate the aging of the insulating materials (both oil and paper) within the transformer, leading to a shorter lifespan. A highly accurate and precise method for determining water content (mg/kg) in transformer oil is the Karl Fischer titration method.

d. Breakdown Voltage (BDV)

Breakdown voltage also known as Dielectric Strength generally represents in kilovolt (kV) is a measure of voltage at which transformer oil loses its insulating ability and allows an electrical discharge to pass through. Since transformer oil acts as an insulator, preventing electrical current from flowing between different parts of the transformer and therefore should be higher as it can be. The BDV is measured by applying a voltage to the oil and observing at which voltage a spark occurs between two electrodes.

e. Density

Density is an important property of transformer oil as it influences its ability to circulate and cool the transformer effectively. The density of transformer oil typically ranges from 0.84 to 0.89 g/ml (or 840 to 890 kg/m³) at 20°C. This value can vary slightly depending on the specific oil type and manufacturer, but it should not exceed 0.9 g/ml (900 kg/m³) at 20°C.

f. Relative Density/Specific Gravity

Density is an important property of transformer oil as it influences its ability to circulate and cool the transformer effectively. The density of transformer oil typically ranges from 0.84 to 0.89 g/ml (or 840 to 890 kg/m³) at 20°C. This value can vary slightly depending on the specific oil type and manufacturer, but it should not exceed 0.9 g/ml (900 kg/m³) at 20°C.

g. Dielectric Dissipation Factor (DDF) / Tan Delta

Tan delta, also known as the dissipation factor, is a measure of the dielectric loss in transformer oil. It reflects the oil's ability to withstand electrical stress and is crucial for assessing the overall health of a transformer's insulation system. When an AC voltage is applied to transformer oil, the current flows through it, but the phase difference between the current and the voltage is not exactly 90 degrees. Tangent of this non-ideal phase difference (δ) angle known as tan-delta and represents dielectric loss, indicating energy dissipated as heat. Lower tan-delta values indicate better insulation properties and lower dielectric losses, meaning the oil is more effective at insulating and preventing leakage currents.

h. Resistivity

Resistivity, also known as specific resistance (GΩm), is a measure of an insulating oil is the ability to resist the flow of electric current. In transformers, oil acts as an insulator and coolant and therefore, high resistivity is crucial for preventing electrical breakdowns and ensuring efficient operation. Resistivity is temperature dependent and generally decreases with increase in temperature. This is why resistivity is measured at both 27°C and 90°C to ensure adequate insulation under different operating conditions.

i. Particle Content

Transformer oil is a light yellowish liquid composed primarily of alkanes, naphthenic saturated hydrocarbons, and aromatic unsaturated hydrocarbons. During operation particles like; (a) cellulose (originate from the degradation of insulating paper), (b) metals especially iron and copper (generated by wear and tear or corrosion of transformer components), (c) impurities (Impurities can enter the oil during maintenance or from the environment) and (d) water (a significant component can be present in dissolved or droplet form) can be generated or introduced as contaminants.

These particles, especially cellulose and metal particles, can decrease the oil's dielectric strength and increase the risk of discharge and breakdown. Moreover, excessive cellulose particles can accumulate under electric fields, forming bridges that lower the oil's resistivity. Particle contamination accelerates the aging of the transformer's insulating paper.

2. REFINING / STABILITY PARAMETERS

a. Colour

Fresh transformer oil is typically a clear OR pale-yellow colour. As the oil ages and degrades, it can change to a yellow or orange tint, and eventually turn brown or black. A brown or black colour indicates that the oil's insulating properties are likely compromised. Other unusual colours like green or milky white can also appear in certain situations. In summary, the colour of transformer oil is a good indicator of its condition and can help identify potential problems before they become serious.

b. Appearance

Colour and Appearance both are the separate quantities and therefore should not be consumed with each. Where the colour indicates particularly the colour of oil ranging from pale yellow to dark brown, there the appearance signifies how an oil appears means either it looks like clear and free from sediments or suspended matter looks in it. The appearance of transformer oil is the easiest way to assess its quality and potential issues.

c. Acidic Value

Acidity (mg/KOH) in transformer oil refers to the presence of acidic by-products formed during the oil's oxidation process or from external contamination. Acidity is measured by the "acid value" or "neutralization number," which indicates the amount of potassium hydroxide (KOH) needed to neutralize the acids present in a gram of oil, expressed as mg KOH/g. High acidity levels can lead to insulation degradation, corrosion, and sludge formation, ultimately shortening the transformer's lifespan.

c. Acidic Value

Acidity (mg/KOH) in transformer oil refers to the presence of acidic by-products formed during the oil's oxidation process or from external contamination. Acidity is measured by the "acid value" or "neutralization number," which indicates the amount of potassium hydroxide (KOH) needed to neutralize the acids present in a gram of oil, expressed as mg KOH/g. High acidity levels can lead to insulation degradation, corrosion, and sludge formation, ultimately shortening the transformer's lifespan.

d. Interfacial Tension (IFT)

Interfacial tension (IFT) in transformer oil refers to the tension at the interface between the oil and another immiscible liquid, typically water. It's measured in mN/m. IFT is a crucial indicator of transformer oil quality, reflecting the presence of polar contaminants and products of oxidation. A high IFT indicates a healthy oil, while a low IFT suggests the presence of contaminants like moisture, oxidation products, or other polar compounds.

e. Corrosive Sulphur

The corrosive sulphur test in transformer oil aims to detect the presence of sulphur compounds that can react with copper and other metals in the transformer, forming copper sulfides that can deposit on insulation and reduce its dielectric strength, potentially leading to transformer failure. If corrosive sulphur is detected, mitigation strategies like using metal passivators, adsorbent filters, or oil change-out can be employed to reduce the concentration of corrosive sulphur compounds.

f. Antioxidants

Antioxidant tests in transformer oil are crucial for evaluating the oil's oxidation stability and remaining useful life. These tests help determine the concentration of antioxidants, which are added to transformer oil to prevent oxidation and degradation. Common tests include those for specific antioxidant types like DBPC, DBP, Phenols, Amines, and DBDS. Antioxidants in transformer oil help mitigate the negative effects of oxidation, which can lead to the formation of sludge, acids, and other by-products that degrade the oil's insulating properties and reduce the lifespan of the transformer.

g. Metal Passivators

Metal passivators are additives used in transformer oil to protect copper components from corrosion caused by corrosive sulphur compounds. They work by forming a protective film on the metal surfaces, preventing direct contact with corrosive agents and thus reducing the rate of corrosion.

h. Furan Content Analysis

Furanic compounds are formed as by-products when the cellulose insulation paper inside a transformer degrades due to factors like heat, moisture, and oxygen. These compounds then dissolve into the transformer oil, making their analysis a valuable indicator of the paper's condition. Therefore, furan content analysis becomes crucial test in transformer oil analysis that helps assess the extent of degradation of the solid insulation (typically cellulose-based paper) within a transformer. Measuring the concentration of furanic compounds, especially 2-furaldehyde, in the oil provides insights into the paper's aging and remaining lifespan.

3. PERFORMANCE PARAMETERS

a. Oxidation Stability

Oxidation stability in transformer oil refers to its resistance to degradation caused by oxidation, which is the reaction of oil with oxygen. This degradation leads to the formation of sludge and acids, which can negatively impact the transformer's performance and lifespan. High oxidation stability is crucial for ensuring long-term reliability and optimal heat transfer within the transformer. Understanding the factors that influence oxidation, the consequences of poor stability, and methods for assessing it are crucial for ensuring the reliable operation of transformers.

b. Gassing Tendency

Gassing tendency in transformer oil refers to its ability to either absorb or release gases under electrical stress. Oils with a positive gassing tendency release gas, while those with a negative gassing tendency absorb gases. This characteristic is crucial for transformer performance and longevity, as the type and amount of gases produced can indicate potential issues.

Oils with a positive gassing tendency release gas like hydrogen (H2), methane (CH4), ethane (C2H6), and acetylene (C2H2) when subjected to electrical stress, particularly under partial discharge conditions. This can be caused by factors like the oil's composition (e.g., aromatic content) or the presence of impurities.

Oils with a negative gassing tendency absorb gases, which can be beneficial in minimizing the build-up of gases like hydrogen in the transformer. This is often linked to the oil's ability to chemically react with certain gases, effectively removing them from the system.

c. Electrostatic Charging Tendency (ECT)

Transformer oil can exhibit electrostatic charging, a phenomenon where charge separation occurs during the flow of the oil, particularly in forced-oil-cooled transformers. This electrostatic charging, known as electrostatic charging tendency (ECT), can lead to the formation of a charge cloud, potentially damaging the insulation and leading to failures. Several factors influence the ECT, including the type of oil, flow rate, and the materials it comes into contact with.

The charge cloud generated by electrostatic charging can cause partial discharges within the insulation, leading to its degradation and eventual failure. In severe cases, electrostatic charging can contribute to catastrophic failures of power transformers.

d. Stray Gassing

Stray gassing (SG) is the formation of gases in electrical insulating oils when they are heated to relatively low temperatures. It's primarily caused by the chemical instability of oil molecules after refining procedures, which may involve hydrogen treatment to remove impurities. These treatments can lead to oversaturation of hydrocarbon chains with hydrogen, which is then released as H2 gas, along with other hydrocarbons like CH4 and C2H6, when the oil is heated. The gases produced by stray gassing can be similar to those produced by faults in transformers, such as partial discharges (PD) or overheating. This can lead to misinterpretations of Dissolved Gas Analysis (DGA) results, potentially triggering false fault alarms.

It's crucial to differentiate stray gassing from fault-related gassing. Stray gassing tends to produce relatively small amounts of hydrogen and methane, and the gas production rate is often constant, unlike in fault conditions where the rate tends to increase.

e. Dissolved Gas Analysis (DGA)

Dissolved Gas Analysis (DGA) of transformer oil is a diagnostic tool that helps determine the internal condition of a transformer by analysing the gases dissolved within the oil. The results of the gas analysis are interpreted based on the identified gas ratios and trends to determine the type and severity of any potential faults including but not limited to thermal faults, electrical faults, and partial discharges. DGA can detect faults in their early stages, allowing for timely repair and preventing catastrophic failures.

f. Compatibility

Transformer oil compatibility refers to the ability of a transformer oil to coexist and function properly with the various materials within a transformer, including solid insulation, seals, and other components. Compatibility is crucial for maintaining the long-term reliability and safety of transformers.

Transformer oils can interact with different materials in various ways, potentially leading to degradation, swelling, or embrittlement of insulation or seals. Some transformer fluids, like silicone oil, are not miscible with others, meaning they won't mix properly and can create phase separations, potentially leading to arcing and failure. Compatibility testing is often done to assess how different oils react with materials like rubber seals, insulating paper, and varnishes, according to Power Systems Technology.

g. Sediments and Sludge

Sediments in transformer oil are undesirable deposits that can negatively impact the transformer's performance and longevity. These sediments can hinder heat transfer, reduce insulation resistance, and potentially lead to equipment failure. They can consist of insoluble materials (sediment) and/or sludge (sticky, fibrous contamination), both of which can interfere with the oil's ability to cool and insulate the transformer.

4. HEALTH, SAFETY & ENVIRONMENTAL PARAMETERS

a. Flash Point

The flash point is the lowest temperature in deg C at which a liquid's vapor will ignite momentarily when exposed to a flame or spark. It's a critical safety parameter for transformer oil, as it indicates the risk of fire or explosion if the oil is heated above this temperature. Transformer oil derived from mineral oil typically has a flash point around 140°C.

b. Fire Point

The fire point of transformer oil, also known as the ignition point, is the temperature in deg C at which the oil vapor, when mixed with air, will ignite and continue to burn for at least 5 seconds. A high fire point is desirable in transformer oil to minimize the risk of fire and explosion. While the flash point is the lowest temperature where a liquid can ignite briefly, the fire point is the temperature where the flame will continue burning for a longer duration.

c. Polycyclic Aromatic Compounds (PCA)

PCAs are a class of aromatic hydrocarbons containing multiple fused benzene rings, and can also include related sulphur and nitrogen compounds. High PCA content, particularly in extender oils, is linked to carcinogenicity. PCAs can affect the oil's properties, potentially impacting its insulating and cooling capabilities. The IP 346 test method is widely used to assess the PCA content, measuring the amount of oil soluble in dimethyl sulfoxide (DMSO).

d. Polychlorinated Biphenyls (PCB)

PCBs (Polychlorinated Biphenyls) were once widely used in transformer oil due to their excellent insulating properties and non-flammability. However, they are now known to be persistent in the environment and can cause health issues, leading to regulations and bans. Transformer oils containing PCBs are now classified as hazardous waste. Transformer oils are tested for PCB content, and oils with high concentrations (e.g., above 500 ppm) are often removed from service. Methods for removing PCBs from transformer oil include chemical treatment, adsorption, and other techniques.