Transformer Oil Testing or BDV Test

Transformer Oil (or insulating oil) is a special type of oil which has excellent electrical insulating properties and is stable even at high temperatures. Transformer oil acts as an insulation as well as coolant (dissipating heat of the transformer).

 

Source: electrical4u.com

Transformer oil is also used to preserve the transformer’s core and windings-as they are fully immersed inside the oil. Transformer oil prevents oxidation of the cellulose-made paper insulation and acts as a barrier between the atmospheric oxygen and the cellulose-avoiding direct contact and hence minimizing oxidation. The level of transformer oil is typically measured using a MOG (Magnetic Oil Level Guage).

There are two main types of transformer oil used in transformers viz., i) Paraffin based, and ii) Naphtha based.

Tho’ Paraffin based is highly available, Naphtha oil is mostly preferred over the former due to the following reason:

·      Naphtha oil is more oxidized and forms ‘sludge’ but the ‘sludge’ from Naphtha is more soluble than the one formed from Paraffin. The sludge does not precipitate at the bottom of the transformer and thus, does not obstruct the circulation of the oil (cooling of transformer).

·      Paraffin oil-based leads to a high pour point due to the dissolved waxes inside it.

The properties/parameters of transformer are:

·      Electrical properties such as Dielectric strength, resistance, dielectric dissipation factor.

·      Chemical properties such as water content, acidity, sludge content.

·      Physical properties such as interfacial tension, viscosity, flash point, pour point.

In the following section, only one of the electrical properties i.e., Dielectric Strength will be explained.

Dielectric Strength-also known as breakdown voltage (BDV) is measured by applying a voltage to the insulating oil (with two electrodes immersed in the oil). As the voltage increases, the current through the oil increases sharply causing the oil to lose the insulating properties and become a conductor. This phenomenon is known as the ‘breakdown of the insulating oil’. The critical voltage value at which the oil breaks down is known as ‘breakdown voltage’ or ‘dielectric breakdown’. The measurement is taken 3-6 times in the sample of oil and the average of the values are considered. The minimum breakdown voltage at which the oil can safely be used in the transformer is considered as 30 KV.  

Let’s consider the following scenario.

Oil Sample No.	Sample Test No.	BDV Value (KV)	Avg BDV	Oil BDV Value
1	1	52	(52+42+47+40+43+40)/6	44
	2	42
	3	47
	4	40
	5	43
	6	40

In the above example, the BDV test on average from the 6 samples is 44 KV. Thus, the insulating oil value of 44 (which is above 30KV) is considered good.