CVT Accuracy Classes and Frequency Range (IEC 61869-5)

Short answer

The accuracy class of a capacitor voltage transformer (CVT) is not defined by voltage alone. IEC 61869-5 also ties it to a frequency range, a burden range and temperature. A class 0.2 metering CVT meets 0.2 % voltage error at any temperature and at any frequency between 99 % and 101 % of rated. A class 3P protection CVT does so between 96 % and 102 %.

Why the CVT has conditions of its own

In an inductive transformer, the error depends mostly on the core and the burden. A CVT also has a capacitive divider and a compensating reactance. The capacitance changes slightly with temperature. The tuning of the circuit depends on frequency. That is why the standard requires errors to be met over the whole reference range of both variables, and not at a single point.

The scope of IEC 61869-5 is single-phase CVTs connected between line and earth, for highest system voltages Um from 72.5 kV and frequencies from 15 to 100 Hz.

Reference frequency range

Clause 5.4 adds two requirements to the range in IEC 61869-1.

On a 60 Hz system, 96 % to 102 % is 57.6 Hz to 61.2 Hz. The protection range is the wider one.

Metering classes

The metering class is designated by the maximum permissible percentage voltage error at rated voltage and with rated burden (clause 5.6.501.1). The standard classes for single-phase metering CVTs are 0.2, 0.5, 1.0 and 3.0 (5.6.501.2).

Voltage and phase displacement errors must not exceed the values in Table 501, for the corresponding class, at any temperature and frequency within the reference range. The maximum phase displacement is 10, 20 and 40 minutes in classes 0.2, 0.5 and 1.0. Class 3.0 has no phase displacement limit.

Two details of the standard change how a CVT report is read.

• Errors are determined at the CVT terminals and include the effect of fuses or resistors that form an integral part of the unit (5.6.501.3).
• If the secondary winding has taps, the accuracy requirements refer to the highest transformation ratio unless stated otherwise.

Protection classes

The protection class is designated by the maximum permissible percentage voltage error from 5 % of rated voltage up to the voltage corresponding to the voltage factor, followed by the letter P (5.6.502.1). The standard classes are 3P and 6P (5.6.502.2).

Table 502 sets the limits at 2 %, 5 % and 100 % of rated voltage and at rated voltage multiplied by the voltage factor (1.2, 1.5 or 1.9).

If a transformer has error limits different from those at 5 % of voltage and at the upper limit, the standard calls for an agreement between manufacturer and purchaser (note 502 of 5.6.502.3). The transient response classes T1, T2 and T3 are written after the accuracy class, for example 3PT1.

Burden ranges

The error limits are guaranteed within a burden range. IEC 61869-5 (5.5.501) defines two.

The standard allows other burdens to be declared, even if not standard, when they are associated with other standard classes.

The rated thermal limiting output (5.5.502) is the highest power the CVT can deliver without overheating, at unity power factor. The preferred values are 25, 50 and 100 VA and their decimal multiples. At that burden the error limits may be exceeded. It is specified and tested on a single winding with the others open. So, if you use the thermal limiting output, you must agree the simultaneous use of several windings with the manufacturer.

Residual voltage winding

A winding intended to be connected in open delta with identical ones to produce residual voltage has a rated output chosen from the values in 5.5.501 (5.5.503). Its class is 3P or 6P (5.6.502.4). Its thermal limiting output refers to a duration of 8 hours at rated voltage multiplied by the voltage factor (5.5.504), because in normal operation these windings deliver almost no power and are loaded only during a fault.

If the CVT has two or more secondary windings and one of them is loaded only occasionally or is used only as a residual winding, its effect on the others can be neglected (note 503 of 5.6.501.3).

How to use this data when specifying

1. Write the class of each winding, the burden range and the rated burden in VA.
2. State the voltage factor that corresponds to the system earthing.
3. Ask for the transient response class if the relays require it. Its choice depends on the characteristics of those relays.
4. Check that the actual connected burden, cables included, falls within the burden range of the class.

What not to do

• Do not assume the metering class of the CVT holds at any frequency. It holds only within 99 % to 101 %.
• Do not use a metering winding for protection: its error limits and frequency range are different.
• Do not load a winding above its rated burden expecting the class to hold. At the rated thermal limiting output, the error limits may be exceeded.

Related topics

The accuracy class of a CVT depends on temperature and frequency, and is complemented by the transient response. The divider capacitance and ferroresonance are the other two topics specific to the CVT.

When to call us

At Tiza Group we verify the ratio, polarity and divider condition of a CVT before energizing it, and we check that the connected burden falls within the range of the class. If you need to define classes and burdens for a project, we do it as part of our engineering design and studies service, which we provide worldwide, remotely. We also offer testing and commissioning of current and voltage transformers in Colombia, or elsewhere on request. Write to us and we will send you a quote.

Related guides

• Transient response of a CVT: classes T1, T2 and T3
• CVT capacitance and tan δ
• Ferroresonance of the capacitor voltage transformer
• Capacitor voltage transformer (CVT) in HV and EHV
• Inductive and capacitor voltage transformer: tests

References

• IEC 61869-5, Instrument transformers - Part 5: Additional requirements for capacitor voltage transformers, 2011 edition, clauses 1, 5.4, 5.5.501 to 5.5.504, 5.6.501 and 5.6.502 with Tables 501 and 502.

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