IEC 61869-1 requires a lightning impulse type test on the primary terminals of instrument transformers. With Um below 300 kV it is 15 impulses per polarity, and from 300 kV it is 3. The chopped, multiple chopped and switching impulse tests complete the series, each with its own acceptance criterion.
Short answer
An impulse is a very fast voltage peak that imitates a lightning strike or a grid switching operation. IEC 61869-1, 2007 edition, uses it in three type tests and two special tests, all applied between the primary terminals and earth. They verify that the insulation withstands the overvoltages of its rated level according to Table 2 of the standard.
Which impulse tests there are
Table 10 of the standard classifies them as follows:
| Test | Class | Clause |
|---|---|---|
| Lightning impulse on primary terminals | Type test | 7.2.3.2 |
| Switching impulse on primary terminals | Type test | 7.2.3.3 |
| Wet test for outdoor equipment | Type test | 7.2.4 |
| Chopped impulse on primary terminals | Special test | 7.4.1 |
| Multiple chopped impulse on primary terminals | Special test | 7.4.2 |
A type test shows that all equipment made to the same specification complies. A special test is one the manufacturer and purchaser agree on in addition.
Common rules (clause 7.2.3.1)
- The test follows IEC 60060-1 and the specific standards for each type of transformer.
- The case, the core if earthed, and all secondary terminals are connected to earth.
- Impulses are applied at a reference level and at the rated level. The reference level is between 50% and 75% of the rated impulse withstand voltage.
- The peak value and waveform are recorded. A change in the waveform, at the reference level or at the rated level, is evidence of insulation failure.
- Recording the earth current as well improves fault detection.
- On a voltage transformer, the voltage is applied between each primary line terminal and earth. If the transformer has no earthed terminal, the other terminal is connected to earth (IEC 61869-3).
Lightning impulse (clause 7.2.3.2)
It is applied with positive polarity and with negative polarity. The impulses are not corrected for atmospheric conditions. The number depends on Um:
| Um | Impulses per polarity |
|---|---|
| Below 300 kV | 15 consecutive |
| 300 kV or more | 3 consecutive |
With Um below 300 kV, the preferred procedure is procedure B of IEC 60060-1, adapted to equipment with self-restoring and non-self-restoring insulation. The transformer passes if each series has at least 15 impulses, there are no disruptive discharges in the non-self-restoring insulation and the disruptive discharges do not exceed two per series. It is confirmed with 5 consecutive impulses withstood after the last discharge. That gives a maximum of 25 impulses per series.
If there are discharges and it cannot be shown during the test that they occurred in self-restoring insulation, the equipment is dismantled and inspected at the end. If the non-self-restoring insulation is damaged, the test is not passed.
With Um from 300 kV, the transformer passes if there is no disruptive discharge or other evidence of failure, such as a change in the waveform.
Switching impulse (clause 7.2.3.3)
It is applied with positive polarity. It is 15 consecutive impulses, corrected for atmospheric conditions. Outdoor equipment is tested under rain, with the wetting procedure of IEC 60060-1. The criterion is the same as for lightning impulse with Um below 300 kV: at least 15 impulses, no discharges in non-self-restoring insulation, a maximum of two disruptive discharges and a maximum of 25 impulses.
Wet test of outdoor equipment (clause 7.2.4)
- With Um below 300 kV, power-frequency voltage is applied at the value in Table 2, corrected for atmospheric conditions.
- With Um from 300 kV, a positive-polarity switching impulse is applied at the value in Table 2.
Chopped impulse (clauses 5.3.3.2 and 7.4.1)
It is only performed if specified, and it does not apply to equipment for GIS. The peak value is 115% of the rated lightning impulse withstand voltage. The impulse is the standard one of IEC 60060-1, chopped between 2 µs and 5 µs, with negative polarity only and combined with the negative lightning impulse test. The sequence is:
- Um below 300 kV: one full impulse, two chopped (four on a voltage transformer with no earthed terminal) and fourteen full.
- Um from 300 kV: one full impulse, two chopped and two full.
A difference between the waveform of the full impulses before and after the chopped ones indicates an internal fault. On capacitor voltage transformers (CVT) and on capacitor dividers and units, this test is mandatory as a type test, to check the internal series connections of the capacitor elements (IEC 61869-5).
Multiple chopped impulse (clauses 6.8 and 7.4.2)
If specified, oil-immersed transformers with Um from 300 kV must withstand multiple chopped impulses. They test the behavior of the internal screens and connections under high-frequency transient currents, caused mainly by disconnector switching.
- Negative-polarity impulses, chopped near the peak, at 70% of the rated lightning impulse withstand voltage.
- 600 consecutive impulses are applied, about one per minute. By agreement they may be reduced to 100.
- The waveform is recorded at the start, at the end and at least every 100 impulses.
The result is evaluated against four criteria:
- No change in the waveform attributable to internal discharges.
- Partial discharge within the limits of Table 3.
- Capacitance and dissipation factor equal before and 24 h after the test.
- Increase of dissolved gases in the oil, measured 72 h later, of 20 µl/l of hydrogen, 5 µl/l of methane and 1 µl/l of acetylene at most.
If any one is not met, the test is not passed.
What not to do
- Do not compare the number of impulses of one unit with that of another with a different Um.
- Do not accept a test as passed without reviewing the waveform record.
- Do not require chopped impulse on equipment for GIS, and do not omit it on a CVT.
- Do not confuse these type tests, made at the factory, with those made in the field, such as the on-site withstand voltage test.
Related topics
The impulse level comes from the insulation level. Partial discharge and the routine and type tests complete the insulation verification. For a CVT, see capacitor voltage transformer and the surge arrester.
When to call us
At Tiza Group we review the impulse test reports and requirements of your instrument transformers with you before you accept them at the substation. Learn about our CT and VT testing service, in Colombia or elsewhere on request. Write to us and we will quote it.
Related guides
- Um and insulation level of an instrument transformer
- Partial discharge in instrument transformers
- Routine and type tests of an instrument transformer
- Capacitor voltage transformer (CVT) at HV and EHV
- High-voltage current transformer (CT): types
References
- IEC 61869-1, Instrument transformers - Part 1: General requirements, 2007 edition, clauses 5.3.3.2, 6.8, 7.1.2 (Table 10), 7.2.3, 7.2.4, 7.4.1 and 7.4.2.
- IEC 61869-3, clause 7.2.3, for inductive voltage transformers.
- IEC 61869-5, clause 5.3.3.2, for capacitor voltage transformers.
- IEC 60060-1, high-voltage test techniques.
Frequently asked questions
How many lightning impulses are applied to an instrument transformer?
It depends on Um. With Um below 300 kV, 15 consecutive impulses of each polarity. With Um from 300 kV, 3 impulses of each polarity. In the first case the standard allows up to two disruptive discharges per series, which leads to a maximum of 25 impulses per series.
Is the chopped impulse test mandatory?
Only if it is specified, and it does not apply to equipment for GIS. Its peak value is 115% of the rated lightning impulse withstand voltage. For capacitor voltage transformers (CVT) it is a mandatory type test, according to IEC 61869-5.
How do you know the insulation failed during the test?
By a disruptive discharge in non-self-restoring insulation, or by a change in the recorded waveform, which indicates an internal fault. The recorded earth current helps detect the fault.