You might have come across while dealing with transformers with the word “Partial Discharge”. Sometimes people ask “have you carried out partial discharge test?”. So let us understand what partial discharge is about in simple words.

What are Partial Discharges? 

Electrical discharges in insulating media that only partially bridges the insulation between conductors are partial discharges. According to ASTM, Partial Discharge is a localized discharge resulting from transient gaseous ionization of an insulation system when the voltage stress exceeds a critical value.

Affects of Partial Discharges on transformers

So in what way the partial discharge affect a transformer? It has a distinct affect on the life of the transformer. During the service of a transformer, the partial discharges deteriorate the insulation system of the transformer thereby reducing the life of the transformer. Some of the conditions during which partial discharges may occur are:

• Improper drying of insulation
• Providing improper insulation without relating with the voltage class of the transformers thereby resulting in severe stress
• Sharp edges in windings, leads and any other parts
• Insufficient oil insulation

Any/all of the above may cause ionization resulting in gas bubbles. This will gradually reduce the strength of the insulation and hence impacts the life of the transformer.

Partial Discharge Test

So is it necessary to carryout a Partial Discharge test on all transformers? It depends on the supplier and the customer up to certain level of transformers. Partial Discharge (PD) test is not a routine test for transformers (oil type) below 300kV class. For dry type transformers partial discharge measurement is a routine test and shall be made according to IEC 60270. This is for all windings having voltages >= 3.6kV
When a transformer is subjected to PD testing, it may be considered as a diagnostic test on insulation system. This is unlike the impulse test where the transformer is tested for its insulation levels and if failed results in destruction. In other words partial discharge testing is a non destructive testing and is like a diagnostic test the results of which give an idea about the insulation strength. 
One more advantage of this test is that it gives a fair idea about the complete insulation strength of the transformer. In an impulse test only the insulation strength of line end is known and it will not give much idea about the insulation strength in other areas.
Conclusion 
In simple words we can sum it up as “Partial Discharge test is a non destructive, diagnostic way of identifying the potential areas of premature insulation breakdown. By carrying out this test the weak areas in insulation of a transformer can by identified which may lead to breakdown of transformer over a period of time in operation much ahead than its intended life time.”