INSULATION PROPERTIES OF TRANSFORMER OIL

What Is Transformer Oil ?

Transformer oil actually is Insulating oil which has excellent stable electrical insulation property at long time high temperature. Due to mass use this insulating oil in electrical oil-filled transformer, commonly it is called transformer oil. Not only transformer, it also uses in high-voltage capacitors, oil circuit reclosers, high-voltage switches & circuit breakers, florescent lamp ballast, etc..

Transformer Oil or Insulating Oil is also known as mineral insulating that normally obtained by fractional distillation and subsequent treatment of crude petroleum.

Chemical name of oil used in transformer

Transformer oil is called as mineral oil which is free from FATTY acids. And the Chemical name of Transformer oil is Hydrotreated Light Nepthanic Distillate.

Polychlorinated biphenyls (PCBs) were formerly used as transformer oil, since they have high dielectric strength and are not flammable. Unfortunately, they are also toxic, bioaccumulative, not at all biodegradable, and difficult to dispose of safely. When burned, they form even more toxic products, such as chlorinated dioxins and chlorinated dibenzofurans. 

Properties of Transformer Oil

Before use any substances in electrical equipment, it’s physical, chemical and electrical properties should be known to get maximum efficiency using at safe condition. Main properties of Transformer oil is as below-

It creates an acceptable level of insulation in combination with the insulating materials used in the conductors and coils; 

It also acts as a coolant to extract heat from the core and the wingdings. 

A good transformer oil must have the following Physical, chemical and electrical properties. 

Property Recommended value

Appearance Transparent, no impurities or suspended matter
Water-soluble acid (PH value) 5.4
Acid value (in KOH) (mg/g) 0.03
Flash point (closed) () 135
Water content (mg/L) 330kV~750kV 10
220kV 15
110kV and below voltage level 20
Interfacial tension (25) (mN/m) 40
Dielectric loss factor tanδ(%) 90
Before injection into electrical equipment0.5
After injection into electrical equipment0.7
Breakdown voltage (kV) 750kV 70
500kV 60
330kV 50
66kV~220kV 40
35kV and below voltage level 35
Volume resistivity (90) (Ω·m) 6*1010
Gas content in oil (%) (volume fraction) 330kV~750kV  1.0
Sludge and sediment (%) (mass fraction) 0.02
The particle size limit in transformer oil 500kV and above AC transformer  before operation (after hot oil circulation), the particle size larger than 5μm in 100mL oil is less than or equal to 2000

To learn more about insulation you can read the full separate article titled "GB50150 Standard"

What is BDV Test in Transformer Oil?

BDV in terms of transformer oil is the abbreviation of Break Down Voltage. BDV of Transformer Oil or Insulating Oil means the dielectric strength of oil. Break down voltage actually is measured by observing the voltage at sparking straits between two electrodes separated by specific gap which emerged in the testing insulating oil.

During long period of time operation of a transformer, it’s insulation oils will become degradation which results it becomes physically dirty and chemically in loosing function of the oil in transformer as insulation and cooling, in that case, transformer will face some serious problems.

Periodically BDV Testing is the main way to monitor and check the oil’s quality. When the testing result shows a very low value, the oil has to be replaced or purified.

The BDV value tested at gap 2.5 mm between two terminals of 50 kV and above is considered good for operation, while 60kV and above means the oil is in excellent condition.

What Is DGA test in Transformer Oil?

DGA is the shorten of Dissolved Gas Analysis in Transformer Oil. DGA of Insulating Oil is very useful to analysis to transformer health. There are two main causes of gas formation within an operating transformer- I) electrical disturbances and ii) thermal decomposition. Actually the breakdown of electrical insulating materials and related components inside a transformer generates gases within the transformer. The major gas produced is Hydrogen. The minor gas produced is Methane.

DGA usually consists of sampling the oil and sending the sample to a laboratory for analysis. Mobile DGA units can be transported and used on site as well; some units can be directly connected to a transformer.

International standard IEC 60599 is followed to DGA- Dissolved Gas Analysis for transformer oil testing.

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Post time: May-26-2022