## 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.

**earth-fault protection**and

**earth-fault current calculations**.

### Measuring circuit and performance of measurement

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

**G**= supply generator_{1}**T**= transformer to be tested_{1}**T**= voltage transformer_{2}**T**= current transformer_{3}**P**= voltmeter_{2}**P**= ammeter_{3}**I**= test current

The zero-sequence impedance is dependent on the current flowing through the winding. Usually the value corresponding to **rated current I _{N}** is stated. This implies that the measurement is carried out with a

**test current of 3 x I**.

_{N}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