For an inductive measuring voltage transformer, IEC 61869-3 defines classes 0.1, 0.2, 0.5, 1.0 and 3.0. The class is the maximum voltage error, in percent, between 80 and 120 % of rated voltage. The burden must be in range I (1 to 10 VA, power factor 1) or in range II (10 to 100 VA, power factor 0.8 lagging). For the capacitor type, IEC 61869-5 allows 0.2, 0.5, 1.0 and 3.0.
Short answer
The accuracy class of a measuring voltage transformer is the maximum voltage error, in percent, that the manufacturer guarantees between 80 and 120 % of rated voltage, with the burden within a declared range. IEC 61869-3 defines classes 0.1, 0.2, 0.5, 1.0 and 3.0 for the inductive transformer. IEC 61869-5 defines 0.2, 0.5, 1.0 and 3.0 for the capacitor voltage transformer (CVT). In addition to the voltage error, each class limits the phase displacement. The class is valid only with a burden within the range declared on the rating plate.
What a class measures
The voltage transformer does not reproduce the primary voltage perfectly. IEC 61869-1 defines two errors (clauses 3.4.3 and 3.4.4):
- Ratio error or voltage error (ε). It appears because the actual transformation ratio is not equal to the rated ratio. It is expressed in percent.
- Phase displacement (Δφ). It is the phase difference between the primary voltage phasor and the secondary voltage phasor, so that the angle is zero in an ideal transformer. It is positive when the secondary voltage leads the primary. It is expressed in minutes or in centiradians.
The accuracy class is the designation of a transformer whose ratio error and phase displacement remain within specified limits, under prescribed conditions of use. In a measuring voltage transformer, the class is designated by the highest permissible percentage voltage error at rated voltage and with rated burden (IEC 61869-3, clause 5.6.301.1).
Standard classes and limits
| Class | Voltage error (±%) | Phase displacement (±minutes) | Phase displacement (±centiradians) |
|---|---|---|---|
| 0.1 | 0.1 | 5 | 0.15 |
| 0.2 | 0.2 | 10 | 0.3 |
| 0.5 | 0.5 | 20 | 0.6 |
| 1.0 | 1.0 | 40 | 1.2 |
| 3.0 | 3.0 | No limit | No limit |
This is Table 301 of IEC 61869-3. It applies to the single-phase inductive transformer. Class 3.0 does not limit the phase displacement. The capacitor transformer uses the same limits for classes 0.2, 0.5, 1.0 and 3.0 (Table 501 of IEC 61869-5), and does not define class 0.1.
The standard warns that guidance on choosing the class for each application will be included in a future annex. For that reason the choice of class is left to the project specification.
Where the class must be met
The limits apply at any voltage between 80 and 120 % of rated voltage, at rated frequency, and with a burden within one of the two ranges of the standard (clause 5.6.301.3):
| Range | Preferred rated outputs | Power factor | Where the class is met |
|---|---|---|---|
| I | 1.0 – 2.5 – 5.0 – 10 VA | 1 | With any burden from 0 VA to 100 % of rated |
| II | 10 – 25 – 50 – 100 VA | 0.8 lagging | With burdens between 25 and 100 % of rated |
For a transformer with secondary taps, the accuracy requirements refer to the highest transformation ratio, unless stated otherwise. Errors are determined at the transformer terminals and include the effect of fuses or resistors that are an integral part of the unit.
On the rating plate, a range I transformer shows it before the burden, for example 0 VA – 10 VA class 0.2. A range II transformer shows the output followed by the class, for example 100 VA class 0.5.
For the CVT, the standard adds that the limits are met at any value of temperature and frequency within the reference ranges (IEC 61869-5, clause 5.6.501.3).
With two secondary windings
The two windings interact: the burden on one affects the error of the other. The standard asks for an output range to be specified for each winding under test, and each must meet its class within its range while the other carries any burden between zero and its rated value. If the ranges are not given, 25 to 100 % of the rated output of each winding is taken.
If a winding is loaded only occasionally and for a short time, or is used only as a residual voltage winding, its effect on the others can be neglected.
How it is verified
IEC 61869-3 requires accuracy type tests at 80, 100 and 120 % of rated voltage, at rated frequency, with the outputs of Table 305 (clause 7.2.6.301):
| Range | Preferred rated outputs | Test output (% of rated) |
|---|---|---|
| I | 1.0 – 2.5 – 5 – 10 VA | 0 and 100 |
| II | 10 – 25 – 50 – 100 VA | 25 and 100 |
In the routine test (clause 7.3.5.301), the test is in principle the same. A reduced number of voltages or burdens is allowed if a type test on a similar transformer demonstrated that the reduced number is enough to verify compliance.
What it means for a project
- Revenue metering and power quality. The class is chosen in the specification according to the accuracy the application requires. The standard does not choose it for you.
- Actual burden. Add up what the meters, converters and relays connected to the secondary consume, and compare it with the declared range. A burden outside the range invalidates the class.
- Cables. The burden is the impedance of the entire secondary circuit. Include the cable in the sum, along with the connected equipment.
- One winding for metering and another for protection. A protection transformer carries a measuring class and a protection class. This is explained in the guide on 3P and 6P classes.
What not to do
- Do not assume the class is met with any burden: it is valid only within the declared range.
- Do not connect protection or other loads to the metering winding without checking the total output.
- Do not compare classes between transformers without also comparing the burden range.
- Do not use class 3.0 for applications that depend on the phase angle: it has no phase displacement limit.
Related topics
The metering classes of the VT are complemented by the protection 3P and 6P classes and by the voltage factor. Secondary supervision is covered in VT fuse failure. The overview is in voltage transformer and inductive transformer.
When to contact us
At Tiza Group we verify the accuracy of measuring voltage transformers in the field and check that the connected burdens are within the plate range. See our testing and commissioning service, available in Colombia, or elsewhere on request. If you need to define the class and burden for a new project, see our engineering design and studies service, which we provide worldwide, remotely. Write to us and we will send you a quote.
Related guides
- 3P and 6P classes of a protection voltage transformer
- Rated voltage factor of a voltage transformer
- Accuracy classes of a CVT and its frequency range
- Inductive and capacitive voltage transformer: tests
- Inductive voltage transformer (VT) for MV and HV
References
- IEC 61869-1, Instrument transformers - Part 1: General requirements, 2007 edition: clauses 3.4.3 to 3.4.8.
- IEC 61869-3, Instrument transformers - Part 3: Additional requirements for inductive voltage transformers, 2011 edition: clauses 5.5.301, 5.6.301, 7.2.6.301, 7.3.5.301, Table 301 and Table 305.
- IEC 61869-5, Instrument transformers - Part 5: Additional requirements for capacitor voltage transformers, 2011 edition with the 2015 correction: clause 5.6.501 and Table 501.
Frequently asked questions
What does class 0.2 mean on a voltage transformer?
It means the voltage error does not exceed ±0.2 % and the phase displacement does not exceed ±10 minutes (0.3 centiradians), at any voltage between 80 and 120 % of rated voltage and with a burden within the declared range. The limits come from Table 301 of IEC 61869-3.
What is the difference between burden ranges I and II?
Range I is 1.0 to 10 VA at power factor 1, and the class must be met with any burden from 0 to 100 % of rated burden. Range II is 10 to 100 VA at power factor 0.8 lagging, and the class is met between 25 and 100 % of rated burden.
What happens if the transformer has two secondary windings?
Each winding must meet its class within its output range while the other carries any burden between zero and the maximum of its range. If the ranges are not specified, the standard uses 25 to 100 % of the rated output of each winding.