CVT Transient Response: Classes T1, T2 and T3

Short answer

When a short circuit occurs on the line, the primary voltage falls almost to zero. An inductive voltage transformer follows it almost instantly. A capacitor voltage transformer (CVT) stores energy in its capacitors and in its reactor. That is why its secondary voltage takes time to disappear, and the way it does so is called the transient response. IEC 61869-5 classifies it as T1, T2 and T3 and checks it with a type test.

What the standard defines

Clause 6.503 describes the behavior with a short circuit on the primary. A short circuit is applied between the high-voltage terminal A and the low-voltage terminal N, which is earthed. Afterwards, the secondary voltage of the CVT must decay, within a specified time Ts, to a given percentage of its peak value before the short circuit.

The characteristic is therefore a ratio: the secondary voltage Us(t) at time Ts, divided by the peak value of the secondary voltage before the short circuit. The standard expresses it as a percentage. A small value means the signal disappeared quickly.

The shape of the decay also counts. Figure 507 of the standard shows that the secondary voltage can decay in two ways.

• Aperiodic damping. The voltage falls continuously toward zero, without crossing the axis.
• Periodic damping. The voltage oscillates around zero with decreasing amplitude.

For a given class, the standard accepts either one, and also the use of a reliable damping device (note 1 of Table 507).

Classes T1, T2 and T3

Table 507 defines three transient response classes. Each one sets, for several instants Ts after the short circuit, the maximum percentage of the previous peak voltage that may remain.

They are written after the protection accuracy class (5.6.502.1). With the two protection classes, 3P and 6P, six designations result.

A designation such as 3PT1 includes the performance of accuracy class 3P and transient response T1. The percentage and time values of each class are taken from Table 507 when specifying. According to note 2 of that table, classes 3PT3 and 6PT3 require a damping device.

Other rules in Table 507:

• Manufacturer and purchaser may agree on other ratio and Ts values (note 3).
• The choice of class depends on the characteristics of the specified protection relays (note 4).
• If a damping device is used, proof of its reliability must be part of the agreement between the parties.

Another CVT transient: trapped charges

Informative annex 5B of IEC 61869-5 describes a phenomenon different from the short circuit, specific to a CVT whose high-voltage sensor is a purely capacitive divider. When a line is disconnected, charges can remain trapped on it. If the line is not earthed or discharged with a low-impedance device, those charges can remain for days. The worst condition occurs when the disconnection happens at the peak of the voltage.

In that case, the high-voltage capacitor C1 stays charged, while capacitor C2 discharges through the electromagnetic unit. When the line is re-energized, C2 charges again. The resulting voltage decays exponentially, with the time constant of the electromagnetic unit, and is superimposed on the sinusoidal signal. The standard states that this produces a very significant error.

How it is tested

Test 7.2.504 is carried out only on protection CVTs. It can be done on the complete CVT or on an equivalent circuit with the real capacitors.

• Source short circuit. The high-voltage source is short-circuited at the actual primary voltage. In the equivalent circuit, the primary voltage multiplied by C1/(C1 + C2) is used.
• Burdens. It is tested with 100 % of the rated burden and with 25 % or 0 %. The burden is a pure resistance or a series combination of resistance and inductive reactance at power factor 0.8 (range II). The nature of the burden affects the result.
• Other windings. Metering or other windings are loaded as in practice, without exceeding 100 % of the specified burden.
• Short-circuit angle. It is done twice at the peak of the primary voltage and twice at the zero crossing, with a tolerance of ±20° around those instants.
• Test voltages. In continuous operation, 1.0 and 1.2 times rated voltage. In short-duration overvoltages, 1.5 or 1.9 times.
• Equivalent circuit. Figure 509 shows the arrangement: intermediate voltage terminal, compensating inductance, intermediate transformer, burden, recording of the primary and secondary voltages, short-circuiting device and source.
• Recording. The primary and secondary voltages are recorded with an oscilloscope and the records form part of the report.

The series burden impedance is calculated from the rated secondary voltage and the rated burden. It is split between resistance and reactance to give power factor 0.8. The tolerance is less than ±5 % on the magnitude and ±0.03 on the power factor. The reactance must be of the linear type, for example an air-core coil.

The standard notes that modern microprocessor-based protection systems have unity power factor. Manufacturer and purchaser may agree to test with the burden as it is connected in practice.

What it means for your project

• The transient response class is a requirement inherited from the relays. Define it together with the protection engineer, not only with the equipment buyer.
• Confirm the actual burden you will connect to the CVT, because the test result depends on the nature of the burden.
• If your CVT is metering only, the transient response is not tested. The standard limits the test to protection CVTs.

What not to do

• Do not accept “T1” or “T3” without reading in the test report the burden and the short-circuit angle with which it was tested.
• Do not accept a 3PT3 or 6PT3 CVT without a damping device or without proof of its reliability agreed with the manufacturer.
• Do not copy the class from another project without reviewing the relays of this one.

Related topics

Transient response relates to the CVT accuracy class and to ferroresonance. It is a data point reviewed in distance protection. The overview is in capacitor voltage transformer, and secondary supervision in fuse failure.

When to call us

At Tiza Group we verify the CVT at commissioning and check that its burden and wiring match what was tested. If you need to define the transient response class together with the relays, we do it with you as part of our service of testing and commissioning of current and voltage transformers, available in Colombia, or elsewhere on request. For the settings of the relays that use that signal, learn about our remote relay configuration, which we provide worldwide. Write to us and we will send you a quote.

Related guides

• Accuracy classes of a CVT and its frequency range
• Ferroresonance in a capacitor voltage transformer
• Power line carrier accessories in a capacitor voltage transformer
• Capacitor voltage transformer (CVT) in HV and EHV
• Distance protection 21 (PDIS): what it is and how it works

References

• IEC 61869-5, Instrument transformers - Part 5: Additional requirements for capacitor voltage transformers, 2011 edition, clauses 5.6.502.1, 6.503 (with Table 507 and Figure 507) and 7.2.504.

Frequently asked questions