Transformer Testing Triad | Commission, Routine, and Diagnostic Testing

18 October 2019


Last week, we brought you with us on a road trip to the beautiful sunny beaches of California where things took an unexpected turn for the worst at a local Oktoberfest celebration.  If you were too busy to join us last week, it’s okay. We know it’s hard to focus on a blog in the month of October when you are surrounded by all things pumpkin-spice. Don’t fret. You can find all of the festival fiasco details right here, while you are sipping that pumpkin spice latte and munching on some pumpkin bread.

We are going to break transformer testing down into three categories – commissioning, routine screening, and diagnostic testing situations. For each of these categories, we will explore (and define) the recommended tests involved. Please note that many of the tests fall into multiple categories. Gasp. We are only going to explain a test once though, so you’ll have to refer to a previous section if you want a refresher. Make sense? If not, you will quickly see what we are talking about in the first section. Let’s get started.


When you are installing a new transformer in the field (a.k.a. commissioning), testing will help expose any equipment stress or damage that may have occurred during its transport. Travel is stressful, and transformers are particularly vulnerable to moisture ingress during the installation process. During commissioning, you will also want to test to make sure the transformer is performing at the same levels, as it was in the manufacturer’s plant. Finally, you’ll want to establish a baseline from which to compare future test results to. Without this, you will struggle to diagnose future problems effectively, since you won’t know what a “healthy” transformer looks like.

You might freak out when you see the list of recommended tests. It’s quite extensive. But you only do this one time! So, make it count. Anyways, here’s the list.

Transformer turns ratio: The TTR (see what we did there?) test will confirm that your transformer has the correct ratio of primary turns to secondary turns. Wait a second, what’s a turn? Very important question. Check out the graphic below for a very simple visual of a transformer. It’s fairly easy to understand. Basically, there are two wires (primary and secondary) looped around an iron core. This creates a coil. By the way, a “turn” is just one of the loops around the core. In the first (primary) coil, a voltage is introduced, which generates a current in the coil, magnetizing the iron core. This magnetic flux then introduces a current in the secondary coil. Still with us? Here comes the most important part. The transformer’s ability to step voltage up or down depends directly on the ratio of turns between the primary and secondary coil. A TTR test can help you identify shorted turns, open windings, incorrect winding connections, or other faults.


Power Factor: In this form of AC testing, we apply a known voltage and measure the resulting leakage or loss of current (from the electrical insulation). If you’re already familiar with insulation resistant testing, then you probably already understand what we are saying. A power factor test will help you evaluate the integrity of your transformer’s electrical insulation. In an ideal situation, your power factor would be zero. However, insulation is never perfect. A small amount of leakage current is always present, so you should expect values slightly above zero. If you are a physics and math fiend, then you may be interested in the actual definition of power factor. It’s the cosine of phase angle between voltage and current. Unfortunately, we don’t have time to dive in to that in depth, at the moment, but we always want to give you all the facts.

Variable Frequency Power Factor and Dielectric Frequency Response (DFR): Using the power factor test alone, may not give you a comprehensive view of your transformer’s insulation. That’s where these guys come in. The variable frequency power factor is a great side-kick to your tradition power factor test, since it allows you to identify your system’s individual temperature correction (ITC) factor. If you are working with multiple transformers, certain testing instruments can automatically identify the ITC, giving you a temperature corrected power factor result that you can be confident in. You gotta love that. Don’t worry, we didn’t forget about the DFR test. The dielectric frequency response test will give you an accurate and reliable moisture assessment of the insulation and conductivity of the oil in your transformer. Remember, that oil insulation thing we mentioned last week? Yeah that’s back. High moisture content limits the loading capability and accelerates the aging process of your transformer. You know what they say, “nobody likes an aging transformer”. Just kidding, no one has ever said that. But you get the severity of moisture ingress, we hope.

Surge arrester loss (watts) and current: Okay, a surge arrester is a grounded component, installed on the transformer to protect it from over-voltage transients. The arrester behaves likes an insulator, so measuring the losses (in watts) and current with a power factor test will usually give you a good indication of potential contamination or deterioration in your arrester. Although this test does not measure the arrester’s fault to ground capabilities, it is likely that if its insulation becomes compromised, its grounding capacity is failing too. Ugh, double whammy.

Winding resistance: This test can identify faulted (open or shorted) windings, as well as verify the integrity of various switches and welded or mechanical connections. When it comes to your transformer, poor design, assembly, handling and transportation, the environment, and overloading can all lead to problems or faults. Thanks to the winding resistance test, you can sleep well at night, knowing your test instrument has ensured that your connections and insulation resistance are good to go.

Sweep frequency response analysis (SFRA): We could spend a whole blog teaching you about SFRA, but we unfortunately, do not have the time today. If you’re still with us though, and you want to gain a deeper understanding, check out this webinar. In the meantime, we will try to explain it in a sentence or two. With SFRA you can see a BUNCH of results, including: core movements, faulty core grounds, winding deformations and displacements, partial winding collapse, shorted turns, and more, but we are out of breath. This test is particularly handy, since it can detect core, mechanical, and electrical problems all at once. Wow. When you are commissioning your transformer, the SFRA will give you a “fingerprint”, which you can use as a baseline of comparison for future testing. Every transformer will have a unique fingerprint. Does it get any more special than that? Nope.

Okay, brief interjection for some new vocabulary. Tap changer. These things regulate the output voltage of a transformer by changing the number of turns in a winding, thus altering the transformer’s turn ratio. Got it? Good, you need to know that moving forward. You should also note that not all transformers have tap changers. So, if you’re commissioning a transformer without one, you’re probably going to want to skip the tap changer specific tests. Sorry, was that too obvious?

Exciting Current Test: If you were searching for a test that will help you identify defects in your transformer’s core and coils, then you’ve found it. With this test, individual phase excitation current values are measured and recorded, and various tap changer problems (loose contacts, improper wiring, open or short-circuited turns, etc..) can be identified. To minimize the amount of current, these measurements are typically made on the high voltage winding only. If you are working with a three-phase transformer, then please, only make measurements on one phase at a time.

Dynamic winding resistance: This test measures the DC current and resistance, as a function of time as the on-load tap changer changes the tap position. In general, this test will assess the integrity of all components which make, break, or carry current while the tap changer is operating.

Core ground: If you find yourself researching transformers in your free time (we don’t judge), this also may be referred to as DC insulation resistance. This test will check for unintended core grounds, as well as identify any potential problems with the core ground’s insulation. Low insulation resistance values between the core and ground might indicate shifting of the core’s lamination or other contamination of the core-to-ground insulation.

Dielectric breakdown on oil sample: A safe, efficient, and happy transformer is one that has clean insulating oil. Moisture, gases, and other sediment or solids can build-up in the oil, which will negatively alter the insulation’s quality. This test will take a sample of your transformer’s oil, apply a test voltage between two electrodes, and increase the voltage until an electrical breakdown occurs. The results are then compared to the recommended standards, so you’ll know exactly where your transformer’s oil is scoring comparatively.

Bushing CT ratio: Just to keep you in the loop, a bushing is the object that is mounted on top of the transformer, kind of like an alien’s antenna, that brings current through a grounded barrier into the transformer. See the picture below. It can also be located under a cover inside the main tank, but that looks less cool. If you are dealing with a specific type of transformer (bushing current transformer), then this test will identify the ratio of primary to secondary turns, as we discussed previously.

Leakage reactance: Finally, almost done! Magnetic flux, which we mentioned way back when, is not perfect. Sometimes, small amounts of flux will link a winding to itself, rather than between the primary and secondary winding. This will cause a self-reactance (or leakage reactance) in that winding under question, which can lead to voltage drops in either winding – not good.

Okay, that was a lot. Don’t worry though, the rest of this blog will be a breeze now that we’ve defined all of those. Hang on, please.

Routine Screening

We mentioned this last week, but when it comes to routine or preventative maintenance, you’re not going to run every test possible. That is a terrible use of your precious resources. Instead, you will look at your current situation, and identify which tests would be best suited for today’s agenda. Is there a specific problem? Or are you looking for a screening tool to assess a problem? Once you figure that out, here is the full list of tests we recommend.

  • Transformer turns ratio
  • Bushing CT ratio
  • Power factor
  • Variable frequency power factor
  • Surge arrester loss and current
  • Exciting current
  • Winding resistance 
  • Dielectric breakdown on oil sample
  • Dielectric frequency response (DFR)

Notice anything? Um, yes. We’ve already talked about all of these! Great. Moving right along.


Ah, the moment you’ve all been waiting for. The end of this blog. The longest transformer blog in the history of the world. Sorry. There’s a lot of information to cover here.

If you are trying to diagnose a transformer, then any electrical test is fair game. This does NOT, under any circumstance, mean you should be performing every test available to you. Again, critical thinking skills here, people. Use the systems of your transformer, operating and test history, and other available information to choose a diagnostic test (or tests).

In your arsenal of potential tests, you will find every test we’ve mentioned thus far. No, we are not listing those again. Enough is enough. However, we have just one (okay two) more things to add.

Frequency response of stray losses: Certain instruments will give you the ability to identify short circuit conditions that may exist between individual strands within a conductor bundle. This test can also help identify structural components that may contribute to leakage flux.

Partial discharge: If there is a gap or air bubble in your insulation, then a partial discharge may occur. Although they are weak in energy, these discharges release thermal energy that can cause premature aging, tearing, or damage to your transformer’s insulation. Ouch.

Oh wow! That’s it. You made it. We made it. If you never want to read another word about a transformer again, we understand.


- Meredith Kenton, Digital Marketing Assistant