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DFR: The New Standard in Transformer Testing

Written by Megger

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We all know the single most effective way to prevent power failures and shortened equipment lifespan is by keeping moisture to a minimum, along with heat and oxygen. It's fair to say that most equipment failure and subsequent losses (time, resources, etc) could be mitigated, through routine insulation testing. 

When it comes to improving testing standards, the process is often long and expensive. Because of this, many industry players focus on improving test and measure equipment, which means we have very advanced tools that can often be let down by outdated regulations.

It also means when we do see updates to international standards, it's a very good thing and often welcome. One example is the adoption of DFR (Dielectric Frequency Testing) over traditional PF (Power Factor) and DF (Dissipation Factor), over 25 years after it was first introduced.

Let's look at the old methods and compare them to DFR.


Testing the integrity of the insulation - until recently - has been the standard testing system for power stations. The problem there is that if the insulation is compromised, by the time you catch it, the damage is often done. A positive result then requires more testing. We do enough testing in our industry, we should make sure we're not overdoing it.


An AC low voltage maintenance test, focused on loss factor. It's often included in acceptance testing stage. Similar to PF testing, sharing some protocols, DF testing is a good tool to have in your maintenance arsenal. Saying that, it again only offers a piece of the puzzle, which is why you'll see it used alongside other relative testing methods.


DFR is an advanced technique that tests the moisture level of oil-impregnated transformers, rather than the insulation or other hardware. It is such a huge time, maintenance and - therefore - money saving test that the IEEE has (after around 25 years) created standards for DFR testing, the IEEE C57.161-2018 – “Guide for Dielectric Frequency Response Test”. It is used in the field to determine the water content in the solid cellulose insulation of oilpaper insulation, insulation failures, and contamination of insulation. 


In short, DFR testing gives a clearer, more detailed picture on the state of any fluid or oil-impregnated power apparatus. By determining the current level of moisture, you can get an understanding of the actual physical condition of your test subject. On the other hand, using methods that only test insulation or dissipation - while useful in plenty of circumstances - don't give a good understand of why these things are happening. That requires more tests to be done to diagnose the cause, and still more to diagnose the condition.

Is it time to include DFR into your power station maintenance routines?

To make sure you have the right equipment for your transformer testing needs, always choose Megger. They have been at the forefront of oil testing since their conception, and proudly keep ahead of the industry with their testing equipment. You can find our latest power transformer catalogue here.

For case studies on DRF testing done by Megger in the field, click here.

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