CT for Overcurrent Protection: Which Class to Use

Short answer

In overcurrent protection, CT saturation can delay the trip, and the effect depends on the measuring principle and the time characteristic. IEC TR 61869-100 recommends that the accuracy limit factor exceed the highest feasible setting; the 5P and 10P classes of IEC 61869-2 are defined with symmetrical AC current only.

What happens to the relay when the CT saturates

Overcurrent relays are used mainly in radial and industrial networks. In their simplest form they measure the current and compare it with the pickup value. They operate instantaneously or with a delay, with definite time or inverse time. Selectivity is achieved by time, by current or by both (IEC TR 61869-100, clause 11.3.1).

When the current transformer (CT) saturates, the secondary current that reaches the relay is lower than the real one. The technical report points out that the effect on protection depends on two things: the measuring principle used (RMS value, peak value) and the time characteristic chosen. It also depends on the relay technology: electromechanical, static or numerical.

In general, saturation produces an operating delay. With a high DC component, saturation can leave the secondary current temporarily below the pickup. With an inverse-time characteristic this is easy to see: a lower current increases the operating time. If there is an instantaneous or time-delayed trip, the operating time is lengthened.

Saturation also distorts the phase angle. In directional functions, such as directional overcurrent protection 67, the phase error can give a wrong direction decision and, in the worst case, a non-selective trip.

What defines a class P

Class P is defined for a symmetrical steady-state short-circuit current (IEC 61869-2, 2012 edition, Table 204). It is designated with the maximum permitted composite error and the accuracy limit factor (ALF):

Standard classes: 5P and 10P (clause 5.6.202.2.3).
Standard accuracy limit factors (ALF): 5, 10, 15, 20 and 30 (clause 5.6.202.2.1).
Composite error at the limit: 5 % for 5P and 10 % for 10P (Table 205).
Standard rated output: 2.5, 5, 10, 15 and 30 VA, with higher values at choice (clause 5.5.201).
Burden: at a power factor of 0.8 lagging, except if the output is below 5 VA, in which case 1.0 is used (clause 5.6.202.2.4).

For example, a 10 VA 5P20 CT must keep a composite error of 5 % at most when 20 times the rated primary current flows with the rated burden.

The ALF is related to a voltage: the secondary limiting e.m.f. is EALF = ALF × Isr × √((Rct + Rb)² + Xb²), with the secondary resistance Rct and the rated burden (clause 3.4.209). If the real burden is lower than the rated one, the real limit factor is higher than the nameplate one. If it is higher, the factor is lower.

How to read the designation

The designation of a class P CT is made up of the maximum composite error in percent, the letter P and the accuracy limit factor (clause 5.6.202.2.2). The low-remanence one uses PR instead of P (clause 5.6.202.3.2).

Designation	Composite error at the limit	Primary current at the limit
5P10	5 %	10 times the rated current
10P20	10 %	20 times the rated current
5PR20	5 %, with remanence limited to 10 %	20 times the rated current

In all three classes, the ratio error at rated current is ±1 % for class 5 and ±3 % for class 10, and the phase displacement does not exceed 60 minutes in class 5 (Table 205). For class 10 the standard does not limit the phase displacement.

What the technical report recommends

Clause 11.3.2 of the report gives two criteria for the accuracy limit factor (or the symmetrical short-circuit current factor Kssc):

Use a typical limit factor, which the report justifies because it represents the typical maximum current of an inverse curve.
Verify that it is greater than the highest prospective, feasible setting of the protection. The relay setting is used, referred to the rated primary current of the CT.

The report’s example starts from a short-circuit current, a maximum protection setting and the real resistive burden, and concludes with a 5P class whose limit factor and output cover that setting. With other data, the conclusion changes.

Does your CT cover the relay's setting range?

Is the highest pickup setting below ALF × Ipr with the real burden?
- Yes: Does the relay trip in less than one cycle with a high DC component?
  - Yes: Ask the relay manufacturer for the transient requirement (Ktd)
  - No: Class P covers the application
- No: Raise the accuracy limit factor or reduce the real burden

When a class P is not enough

The technical report (clause 11.2.2) explains how the context changed. Previously, electromechanical relays had high burdens and operated at long times, when the DC component had already decayed. Only the symmetrical AC component mattered, and a high-output 5P20 CT was enough in most cases.

Today networks can have larger primary time constants. Modern relays measure and operate in one cycle or less. The DC component now has a significant influence on core saturation.

With a high rated output, a core can end up very “unloaded” when working with a low real burden, and its accuracy can be worse than that of a core designed for the operating burden. This must be checked when electromechanical relays are replaced with digital ones that have a much lower burden.

What not to do

Do not choose the accuracy limit factor out of habit. Compare it with the highest pickup setting that can be used.
Do not use the nameplate ALF with a real burden different from the rated one.
Do not reuse an old CT with a new relay without calculating the limiting voltage with the real burden.
Do not assume a 5P covers the DC component: it is defined with symmetrical AC current.

The class of an overcurrent CT is compared in the equivalences and calculated with the Ktd. The secondary current changes the burden, as explained in 1 A or 5 A. The relay that uses this signal is described in overcurrent protection.

When to call us

At Tiza Group we verify CT saturation with the real wiring and relay burden, and we define the class your protection needs in protection coordination studies, worldwide, remotely. We also carry out current and voltage transformer tests in Colombia, or elsewhere on request. You can estimate your case with the free CT burden and saturation tool. Write to us and we will quote it.

References

IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, 2012 edition, clauses 3.4.209, 5.5.201, 5.6.202.2, Table 204 and Table 205.
IEC TR 61869-100, application guide for current transformers in protection systems, 2017 edition, clauses 11.2.2 and 11.3.

Frequently asked questions

What does a saturated CT do to an overcurrent relay?

It temporarily reduces the secondary current the relay sees. If the current falls below the pickup, the relay does not operate or takes longer. With an inverse-time characteristic, the delay is clearly visible. The exact effect depends on the measuring principle (RMS value, peak value) and on the relay technology (IEC TR 61869-100, clause 11.3.1).

Why is a 5P20 class still the most used in overcurrent protection?

Because it was defined for high-burden electromechanical relays, which operated at long times when the DC component had already decayed, and because in many applications the high rated burden compensates for the low limit factor. The technical report states that this model is still used in most applications, but that it is not recommended for new substations with new cores (clause 11.2.2).

What accuracy limit factor is needed?

It must be greater than the highest prospective, feasible pickup setting of the protection, referred to the rated primary current, and it must hold with the real connected burden. The standard values are 5, 10, 15, 20 and 30 (IEC 61869-2, clause 5.6.202.2.1).