Search

Premium Membership

Save 15% on Pro Premium Plan with discount code: UX226 (expires on July 31, 2021). Study specialized LV/MV/HV technical articles and papers.

Home / Technical Articles / Power transformers testing and commissioning at the site (instructions and precautions)

Commissioning of Power Transformers

Since I started the commissioning work 30 years back, transformer commissioning consisted of more tests, but nowadays, the number of tests has been reduced due to costs and time-saving. This article will go through complete testing that needs to be done at the site during the commissioning process of the transformer.

Power transformers testing and commissioning at the site
Power transformers testing and commissioning at the site (on photo: Transformer dynamic resistance measurements with the Omicron's CPC 100)

Table of contents:

Power transformers testing and commissioning at the site.

  1. Power transformer tests:
    1. Measurement of Winding Insulation Resistance
    2. Winding Resistance Test
    3. Voltage Ratio Test
    4. Polarity Test
    5. No-Load Losses Test
    6. Load Losses (Short Circuit) Test
    7. Vector Group Test
    8. Capacitance and tan δ Test
    9. Oil Breakdown Test
    10. Temperature Rise Test
    11. Frequency Response Analysis (FRA Test)
    12. Partial Discharge Test
  2. Transformer General Tests
  3. Transformer Primary Tests
  4. Transformer Secondary Tests
  5. Transformer Tripping Tests
  6. Transformer Load Tests

1. Power Transformer Tests

These tests include winding insulation resistance test, winding resistance test, vector group test, tan delta test, no-load test or iron losses, load test or copper losses, temperature rise test, partial discharge test, frequency response analysis test, and transformer oil breakdown test.

These tests are detailed as follows:


1.1 Measurement of Winding Insulation Resistance

The purpose of this test is to determine the insulation resistance of the high-voltage winding to the ground, low-voltage winding to the ground, and the high-voltage winding to the low voltage winding, also to assess the amount of moisture in transformer insulation.

This test is done using a DC Meger, 5000 V by applying this voltage -5000-volts DC for a period of one 30 Sec and if there is no failure in insulation, we extend the test to one minute. If the reading is not stable then we should extend the test to 5 minutes.

This test should be done between the windings as follows:

  1. High Voltage winding to the Earth as shown in the figure given below.
  2. Low Voltage winding to the Earth same as HV-Earth but the connection will be between LV-Earth.
  3. The high voltage winding to the low Voltage Windings as shown in the figure given below.

Figure 1 – Insulation resistance testing: HV-Earth and HV-LV

Insulation resistance testing: HV - Earth and HV - LV
Figure 1 – Insulation resistance testing: HV – Earth and HV – LV

We take the reading after 15 sec and after 60 sec the result will give the polarization index of transformer insulation, which indicates the transformer insulation condition.

Polarization Index (PI) =R60/R15 should be as follows:

  • More than 1.3 for the transformer with voltage up to 35kV
  • Between 1.5 to 1.7 for transformers with voltage range from 66 kV up to 500 kV
  • If PI reading is more than 3.1 this means the transformer insulation is in good condition.
  • As a guide for practical reading:
    • LV-Earth 1 G-Ohm.
    • HV-Earth 2.5 G-Ohm.
    • L.V to H.V 2.5 G-Ohm.

Conclusion:

Do not choose the megger test voltage as standard 5000 volts as some transformer’s manufacture has low insulation for big transformer neutral points, for example, 500MVA, 500kV Transformers and if you apply 5000 D.C voltage in the winding you will lose the transformer windings, so please refer to the specified D.C voltage to test the transformer windings as per manufacture manual instruction.

After the test, do not energize the transformer before earthing the windings to be sure that the windings are discharged.

If you find the insulation resistance is not satisfied, further investigation should be done in the transformer before energization.

For Megger, discharge the current to avoid shock. Switch off the power supply when connecting the circuit to avoid a spark.

Go back to the Contents Table ↑


1.2 Winding Resistance Test

The purpose of this test is to measure the D.C. resistance of the transformer windings, this test can be done by a voltmeter and ammeter method. For low reading values, we connect the voltmeter after the ammeter, but for high resistance values, we connect the voltmeter before the ammeter. A sensitive voltmeter and ammeter will be used with a 100 DC Amp. This test will give accurate results as shown below:

Figure 2 – DC winding resistance test (current-voltage method)

DC winding resistance test (current-voltage method)
Figure 2 – DC winding resistance test (current-voltage method)

This test can be done when a transformer is hot or cold, but the temperature of the winding and oil should be recorded during the test. If a transformer for example delta/star windings, then check the resistance as follows:

  1. HV winding resistance between phase R and phase S
  2. HV winding resistance between phase S and phase T
  3. HV winding resistance between phase T and phase R
  4. LV winding resistance between phase R and neutral N at Tap 9
  5. LV winding resistance between phase S and neutral N at Tap 9
  6. LV winding resistance between phase T and neutral N at Tap 9

Premium Membership Required

This technical article/guide requires a Premium Membership. You can choose an annually based Plus, Pro, or Enterprise membership plan. Subscribe and enjoy studying specialized technical articles, online video courses, electrical engineering guides, and papers. With EEP’s premium membership, you get additional essence that enhances your knowledge and experience in low- medium- and high-voltage engineering fields.

Check out each plan’s benefits and choose the membership plan that works best for you or your organization.

Save 15% – Get 15% discount on Pro Plan, apply discount code: UX226

Log In »Learn More »

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information
author-pic

Omar Salah

I have an Engineering Msc/Bsc Degree and more than 25 years of experience in commissioning activities, mainly at thermal power plant gas/oil-fired, combined cycle and petroleum power plants, and 11kV - 500kV substations. I worked with many international companies as an electrical commissioning manager and as an expert in setting protection relays in many different projects in Egypt.

Leave a Comment

Tell us what you're thinking... we care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

sixty two  −  fifty three  =  

EEP Academy Video Courses

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning.