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How Electrical Testing Leads To Solid Transformer Performance

How Electrical Testing Leads To Solid Transformer Performance

Testing is vital to ensure a transformer runs properly and efficiently. And while the cheapest and most common testing for transformers is generally oil testing, their results are often questionable. In such cases, verifying the original test results with a second oil sample can suffice. However, to make the most of costly transformer maintenance procedures, conducting a series of electrical testing (ET) methods on your transformer can better guide your decision-making.

Electrical testing for transformers: why and when to do it

Compared to annual oil testing, routine electric transformer testing is performed on an as-needed basis to verify unsatisfactory oil tests or on a three to five-year rotation, the latter being more common for utility companies. An important thing to note about this type of testing is that since it involves complicated procedures, the transformer must be isolated and de-energized first. The following are a few of the primary reasons why performing electrical testing may be necessary:

1. Verify unacceptable oil test results – ET can provide more in-depth information on which repairs are necessary whenever an oil test produces questionable results.

2. Validate repair work – Check the completed repairs for potential issues.

3. After the transformer trips offline – If a transformer trips offline, it is vital to check if it suffered any damage during the incident.

4. Acceptance testing – There is a risk of damage to the new or rewound transformer during the shipping and receiving process. Doing your own electrical testing and comparing the results with the manufacturer’s final ET results determines whether the transformer sustained any damage during the trip.

5. Moving the transformer – Similar to the previous reason, ET checks whether relocating the transformer caused damage to the unit.

6. Bringing the transformer out of storage – ET shows if a transformer was stored properly and can be energized and become operational.

Electrical testing transformer bushings

Performing an ET on the transformer bushings examines whether their insulation properties remain intact. Make sure to clean out any dirt and contaminants from the de-energized bushings prior to testing, as they can skew test results. Once done, three tests provide different key information about the unit, namely:

C1 Bushing Test

Also known as an Ungrounded Specimen Test (UST), the C1 test checks the core insulation starting at the centre conductor to the potential tap.

C2 Bushing Test

On the other hand, the C2 test or Guard Test Mode (GST) checks the insulation’s dielectric integrity from the tap to the ground sleeve. It is still considered best practice to do this test even if the bushing nameplate offers no value for a baseline.

Hot Collar Test

This test is regularly conducted for solid-type bushings without any taps. It works well at uncovering contamination, deterioration, low liquid or compound levels, and any potential gaps in the compound. Moreover, it can also work as a supplementary test to the previous two tests for capacitance-graded bushings with taps.

Electrical Testing Transformers

The following are the most common routine ETs to perform on the transformer unit itself.

Power factor series testing

This type of testing is made up of the following tests: Insulation Power Factor, Single Phase Excitation, Capacitance, and Tip-up (if necessary). The Insulation Power Factor series specifically measures several key aspects of the transformer, including:

  • CH – this covers all the insulation found between the grounded parts and high-voltage windings, including the windings, bushings, oil, structural insulating components, and insulation.
  • CL – covers all the parts between the grounded parts and low-voltage windings.
  • CHL – all the barriers, winding insulation, and oil between high and low-voltage windings.

When it comes to getting data about gradual insulation deterioration in the transformer, the insulation power factor beats every other test, like the DC winding insulation resistance test. Furthermore, it tests values like excitation and capacitance and yields key information about damage and movement unachievable through insulation resistance tests.

Other electrical tests

Should you require additional information regarding damage or movement in your transformer, the following tests can provide you with what you need:

  • Winding Resistance
  • Core ground Insulation Resistance
  • Transformer Turns Ratio (10kV)
  • Excitation
  • Leakage Reactance
  • Swept Frequency Response Analysis


ET is vital for various situations and provides more accurate and reliable data that helps to determine which repairs may or may not be needed. Upon determining all the repair work that must be done, it is best to engage with companies providing mechanical and electrical engineering services to facilitate these repairs and get the transformer back online. These companies also have the capabilities to carry out other services, such as electric motor rewinds, to ensure all your equipment runs smoothly.