IEC 61869-2 requires the inter-turn insulation of a current transformer to withstand 4.5 kV peak. For PX and PXR CTs with a knee-point voltage above 450 V, the value is 10 times the r.m.s. knee-point voltage or 10 kV peak, whichever is lower. The routine test uses two procedures: A, with current in the primary and the secondaries open, and B, with voltage applied to the secondary and the primary open.
Short answer
The insulation between the turns of a secondary winding must withstand the overvoltages generated when the core saturates. IEC 61869-2 sets the inter-turn withstand voltage at 4.5 kV peak and defines a routine test with two alternative procedures, A and B (clauses 5.3.201 and 7.3.204). This guide explains the requirement, the procedures and what the result does not demonstrate.
The requirement
The rated withstand voltage for inter-turn insulation is 4.5 kV peak (5.3.201). The standard adds a special case for class PX and PXR CTs with a rated knee-point voltage Ek above 450 V. For them, the inter-turn withstand voltage is 10 times the r.m.s. value of Ek or 10 kV peak, whichever is lower.
Examples of the special case. A PX CT with Ek = 800 V has an inter-turn withstand voltage of 10 × 800 V = 8 kV peak, because that is lower than 10 kV. A PX CT with Ek = 1,200 V would give 12 kV, but the limit is 10 kV peak, the lower of the two. A PX CT with Ek = 400 V is not in the special case: its requirement remains 4.5 kV peak.
Two notes in the standard are important:
- Because of the test procedure, the waveform can be strongly distorted.
- With the test procedure (7.3.204), lower voltage values than those indicated may result.
It is a routine test
The inter-turn overvoltage test is one of the routine tests of IEC 61869-2 (7.3.204). It is made on every CT, on the whole winding, and complements the power-frequency voltage tests between primary and earth. The latter verify the insulation to the outside of the winding. The inter-turn test verifies the insulation between the turns of the same winding.
Procedure A: current in the primary
The secondaries are open, or connected to a high-impedance device that reads the peak value of the voltage. A nearly sinusoidal current flows through the primary, with a frequency between 40 and 60 Hz and an r.m.s. value equal to the rated primary current (or to the extended primary current, if specified). The duration is 60 s.
If the test voltage is reached before the rated primary current, the current is limited. If the voltage is not reached at maximum current, the voltage obtained is taken as the test voltage.
Procedure B: voltage on the secondary
With the primary open, the test voltage is applied for 60 s to the terminals of each secondary winding, at a suitable frequency. The r.m.s. value of the secondary current must not exceed the rated secondary current (or the extended one, if there is one).
The test frequency is chosen to reach the test voltage and must not exceed 400 Hz. If the voltage is not reached with the maximum secondary current and the maximum frequency, the voltage obtained is taken as the test voltage.
When the test frequency is more than twice the rated frequency, the duration is reduced with this relation:
t = 120 s × (rated frequency ÷ test frequency)
with a minimum of 15 s. For example, with a rated frequency of 50 Hz and a test frequency of 200 Hz, the duration is 120 × 50 ÷ 200 = 30 s.
Comparison of the two procedures
| Aspect | Procedure A | Procedure B |
|---|---|---|
| Primary | With current | Open |
| Secondary | Open or with a peak-reading device | With test voltage on each winding |
| Frequency | Between 40 and 60 Hz | Up to 400 Hz |
| Duration | 60 s | 60 s, reduced if the frequency exceeds twice the rated one |
| Limit | Rated or extended primary current | Rated or extended secondary current |
What it demonstrates and what it does not
A CT that passes this test demonstrates that its inter-turn insulation withstands the specified voltage for the duration of the test. It does not demonstrate that the CT can operate with the secondary open. The standard says so expressly in a note: current transformers must not operate with the secondary open, because of the dangerous overvoltages and heating that can occur.
Put another way, an open secondary in service is not justified by the test result.
What to ask for when receiving a CT
- The routine test report, with the procedure used (A or B), the voltage reached and the duration.
- For PX and PXR CTs with Ek above 450 V, the test voltage that corresponds to the specification (10 times r.m.s. Ek or 10 kV peak, whichever is lower).
- The test frequency and duration, if procedure B was used with a frequency more than twice the rated one.
These data make it possible to verify that the report complies with the standard and that the voltage reached was not far below the specified voltage.
What not to do
- Do not confuse this test with the power-frequency applied voltage test between windings and earth. They are different tests with different purposes.
- Do not accept a report without the voltage value reached: the standard allows the voltage obtained to be lower than the specified one, and that figure must be written down.
- Do not treat the result as permission to operate the CT with the secondary open.
Related topics
Inter-turn overvoltage is related to classes PX and PXR and to routine tests. The general view is in current transformer, terminals and secondaries, medium voltage and high voltage.
When to call us
At Tiza Group we review the factory reports of your current transformers and compare them with the specification, and we run acceptance tests on site. See our instrument transformer testing, in Colombia or elsewhere on request. If you are going to buy new CTs, engineering design and studies, remote and worldwide, includes the technical specification and the review of bids. Write to us for a quote.
Related guides
- Class PX and PXR CTs: Knee-Point Voltage and Kx
- Routine and Type Tests of an Instrument Transformer
- Current Transformer (CT): Parts, Functions and Tests
- CT Terminals, Secondaries and Wiring: P1, P2, S1, S2
- Medium-Voltage Current Transformer (CT): Classes
References
- IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, edition 1.0 (2012), clauses 5.3.201 and 7.3.204.
Frequently asked questions
Does the inter-turn overvoltage test prove the CT can operate with its secondary open?
No. The standard states this expressly: the test does not verify that the CT is suitable for operation with the secondary open. A CT must not operate with the secondary open, because dangerous overvoltages and heating can appear.
What happens if the test voltage is not reached?
If the voltage in the standard is not reached with the maximum primary current of procedure A, or with the maximum secondary current and frequency of procedure B, the voltage obtained is taken as the test voltage. The standard provides for this because the voltage depends on the core design.
Who chooses between procedure A and procedure B?
Unless otherwise agreed between manufacturer and buyer, the choice is the manufacturer's. Both procedures apply to the whole winding, and their result is valid to demonstrate compliance with the 4.5 kV peak requirement.