# Testing of Power Transformer – Measurement of Zero-Sequence Impedance

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## Earth-fault protection

### Purpose of the measurement

The zero-sequence impedance is usually measured for all star-connected windings of the transformer. The measurement is carried out by supplying a current of rated frequency between the parallell connected phase terminals and the neutral terminal.

The zero-sequence impedance per phase is three times the impedance measured in this way. The zero-sequence is needed for earth-fault protection and earth-fault current calculations.

### Measuring circuit and performance of measurement

Circuit for zero-sequence impedance measurement in shown below, where:

• G1 = supply generator
• T1 = transformer to be tested
• T2 = voltage transformer
• T3 = current transformer
• P2 = voltmeter
• P3 = ammeter
• I = test current

The zero-sequence impedance is dependent on the current flowing through the winding. Usually the value corresponding to rated current IN is stated. This implies that the measurement is carried out with a test current of 3 x IN.

However, this is not always possible in practice since the current must be limited to avoid excessive temperature of metallic constructional parts.

The zero-sequence impedance is measured as function of test current, and when necessary the final result is obtained by extrapolation.

### Test Result

The zero-sequence impedance is usually given as a percentage of the rated phase impedance.

When the transformer has a three-limb core and no delta-connecter windings, the zero-sequence impedance is about 30…60 %. When the transformer has a delta-connected winding, the zero-sequence impedance is 0.8…1.0 times the corresponding short-circuit impedance.

In the test report the zero-sequence impedance values at the principal and extreme tappings are stated.

Reference: Testing power transformers – ABB

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#### Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

1. Niaz
May 23, 2017

Helo,

..i have noticed that, in our 333MVA unit main transformer, YND11 , 400/15 KV, neutral point is solidly earthed.. do you know why? Cause, 15KV generator is high impedance grounded.. but 400 KV transformer also should have impedance grounded.. why it is solidly grounded?

2. Azizul islam
Jan 25, 2017

good

3. smisencontrols
Jul 10, 2015

Top Manufacturer of 3 phase control transformers in Maharashtra
I’m new on this community,
and this is my very first post.
I really appreciate your hard work .
Thank you for this
Information.

4. smisencontrols
Jul 10, 2015

Control Power Transformers Mumbai India
I’m new on this community,
and this is my very first post.
I really appreciate your hard work .
Thank you for this
Information.

5. smisencontrols
Mar 27, 2015

Top Manufacturer of Single phase control transformers in Mumbai
This is something I really wanted to read-up on, plus you gave some extra links for further reading.

Thanks for the resources!

6. smisencontrols
Mar 17, 2015

Top Manufacturer of 3 phase control transformers in Mumbai
thank you for this information.

Aug 19, 2014

Electrical and Electronic engineer in thirst of learning about power systems

8. Venkat Iyer
Aug 06, 2014

Great …. What is the mehodology of earthing to be adopted when substations are located on different floors in a high rise building? If earthing system is TT, what are earthing conductors can be grouped like HT panel body, Transformer body, LV panel body, neutral side of transformer etc.

Would be grateful if you could throw some lights with some supporting documents.

I am located in Mumbai, India.

Thanks and Regards,
Venkat Iyer

9. KarSho
Aug 05, 2014

Simply Thanks…

10. pavan
Jul 28, 2014

Explain with motor traction