IEC 61869-1 requires partial discharge measurement on instrument transformers with Um of 7.2 kV or more. The level, in picocoulombs, must not exceed the limit of Table 3, which depends on the insulation type, the neutral earthing and the transformer type. The measurement follows IEC 60270 and is made after a preconditioning step.
Short answer
A partial discharge is a localized electrical discharge in the insulation that does not completely bridge it. IEC 61869-1, 2007 edition, sets a maximum permitted limit in clause 5.3.3.1 and the way to measure it in 7.3.2. The general principle of the test is in partial discharge test.
Who it applies to
The partial discharge measurement is a routine test according to Table 10 of the standard: it is made on every transformer manufactured.
The partial discharge requirement applies to instrument transformers with Um greater than or equal to 7.2 kV. Um is the highest voltage for equipment. It covers current transformers (CT) and voltage transformers (VT) at medium, high and extra-high voltage.
What limit must be met
Table 3 of IEC 61869-1 gives the maximum permitted partial discharge level, in picocoulombs (pC). The limit depends on three items:
- Insulation type: liquid- or gas-immersed, or solid.
- System neutral earthing: earthed neutral with an earth fault factor less than or equal to 1.4, or isolated neutral or not effectively earthed, with a factor greater than 1.4.
- Transformer type: CT and VT with one earthed terminal, or VT with no earthed terminals.
The test voltages are expressed as a function of Um, r.m.s. value. The table shows three references: Um, 1.2·Um/√3 and 1.2·Um. For each combination of insulation type, neutral and transformer, the table states at what voltage to measure and what level must not be exceeded. That is why there is no single picocoulomb value valid for all equipment: before comparing a result, confirm which row of the table corresponds to your transformer.
Three notes in the table help in applying it:
- If the neutral system is not defined, the limits for isolated or not effectively earthed neutral apply.
- The maximum permitted level also applies at frequencies other than the rated one.
- CT means current transformer and VT voltage transformer.
For VTs whose rated voltage is much lower than Um, IEC 61869-3 allows the manufacturer and purchaser to agree on lower preconditioning and measuring voltages. In that case the measuring voltage drops in the same proportion as the preconditioning voltage.
How the measurement is made
The circuit and instruments follow IEC 60270. The standard shows three example circuits: the basic one, an alternative one and a balanced one, which helps suppress external noise.
The instrument measures apparent charge in pC and is calibrated within the test circuit. The requirements of the standard are these:
| Item | Requirement |
|---|---|
| Wide-band instrument | Bandwidth of at least 100 kHz and upper cut-off of 1.2 MHz at most |
| Narrow-band instrument | Resonance frequency between 0.15 and 2 MHz |
| Narrow band, preferred values | From 0.5 to 2 MHz, or the frequency of highest sensitivity if possible |
| Sensitivity | Must detect 5 pC |
The background noise must be clearly lower than the sensitivity. Pulses known to come from external disturbances may be disregarded. If electronic processing is used to reduce noise, it must be shown that it still detects repeating pulses.
Preconditioning and measuring time
Clause 7.3.2.2 requires a preconditioning step before measuring. The voltage is then brought to the Table 3 values and the partial discharge level is measured in a time of 30 s at most. The standard offers two procedures:
- Procedure A: the measuring voltages are reached by lowering the voltage after the power-frequency withstand voltage test.
- Procedure B: the measurement is made after the power-frequency test. The voltage rises to 80% of the power-frequency withstand voltage, is held for at least 60 s and drops without interruption to the measuring voltages.
Unless otherwise agreed, the manufacturer chooses the procedure. The test report must state which one was used.
For a VT with no earthed terminals, IEC 61869-3 requires two tests. The voltage is applied to each high-voltage terminal in turn, and the other terminal is connected to a low-voltage terminal, to the case and to the frame.
- Power-frequency withstand voltage test
- Preconditioning. Procedure A (lowering the voltage) or B (80% for 60 s or more)
- Measurement at the Table 3 voltages. In 30 s at most
- Does the level exceed the limit? No: continue; Yes: Test not passed.
- Transformer compliant
What result is accepted
The measured level must not exceed the Table 3 limit (clause 7.3.2.2). If it does, the transformer does not meet the requirement. The principle of the test and its field methods are in partial discharge test. Measuring equipment according to IEC 60270 is described in IEC 60270 partial discharge meter.
What not to do
- Do not apply the requirement to equipment with Um below 7.2 kV.
- Do not compare the measured level with the limit of another insulation type or another neutral earthing.
- Do not accept a measurement whose background noise is comparable to 5 pC.
- Do not accept a report that does not state the preconditioning procedure used.
Related topics
The partial discharge limit is set together with the impulse tests and the insulation level. It is measured as part of the routine tests. In the field, see the partial discharge test and the high-voltage CT.
When to call us
At Tiza Group we support the receiving and commissioning of your instrument transformers, including the review of factory test reports. Learn about our CT and VT testing service, in Colombia or elsewhere on request. Write to us and we will quote it.
Related guides
- Impulse tests on instrument transformers
- Um and insulation level of an instrument transformer
- Routine and type tests of an instrument transformer
- Partial discharge test: what it is and where it applies
- OMICRON MPD 600: partial discharge measurement
References
- IEC 61869-1, Instrument transformers - Part 1: General requirements, 2007 edition, clauses 5.3.3.1 (Table 3), 7.3.2.1 and 7.3.2.2.
- IEC 61869-3, clause 7.3.2.2, note 301 and the test of VTs with no earthed terminals.
- IEC 60270, partial discharge measurements.
Frequently asked questions
Which instrument transformers get a partial discharge measurement?
Those with a highest voltage for equipment (Um) equal to or above 7.2 kV. That is the scope of clause 5.3.3.1 of IEC 61869-1. Lower-voltage equipment has no such requirement in that standard.
What sensitivity must the measurement have?
Clause 7.3.2.1 requires the measuring system to be able to detect a partial discharge level of 5 pC. The background noise must be clearly lower than that sensitivity.
Is the partial discharge limit the same for a CT and a VT?
No. Table 3 of IEC 61869-1 gives different limits depending on three items: the insulation type (liquid or gas, and solid), the system neutral earthing and the transformer type: CT, VT with one earthed terminal, or VT with no earthed terminals.