Current transformer protection class (5P/10P) and knee-point voltage — why a metering CT is unsuitable for protection

The guide on current transformer measurement class and burden for kWh metering covers classes 0.2/0.5/1, which describe the permitted measurement error at normal operating current. This article covers the other main category of current transformer: the one intended for protection, where the requirement is essentially the opposite.

Why a metering CT is unsuitable for protection

A metering CT is optimised to be very accurate at normal operating current (around rated level, up to perhaps 1.2× rated). To protect the connected metering equipment (kWh meter, ammeter) against the very high current during a fault, a metering CT is moreover deliberately designed to saturate quickly at a multiple of the rated current — the core becomes magnetically saturated and the secondary current then no longer rises proportionally with the (much higher) primary fault current. For metering purposes, that is desirable behaviour. For protection, however, it is a problem: a protection relay needs precisely during the fault, at a strongly elevated primary current, a reliable proportional secondary signal in order to assess the fault correctly.

The protection class: composite error limit and accuracy limit factor

A current transformer for protection purposes is therefore specified according to a protection class per IEC 61869-2 (formerly IEC 60044-1), denoted for example as 5P10 or 10P20:

The first number (5 or 10) is the maximum composite error in percent at the accuracy limit current — i.e. how accurate the CT must still be at the extreme end of its specified range.
The letter P stands for "protection".
The second number (10 or 20 in the examples) is the accuracy limit factor (ALF): the multiple of the rated primary current up to which the CT must remain within the specified composite error. A 5P10 CT with a rated primary current of 100 A must therefore remain within 5% composite error up to 1000 A (10× rated), instead of saturating well below that as a metering CT would.

Knee-point voltage: where saturation sets in

In addition to the protection class, a protection CT — particularly for applications such as high-impedance earth-fault protection or differential protection (see the guide on transformer differential protection) — is also often characterised via the knee-point voltage (Vk): the point on the CT's excitation curve at which a 10% increase in voltage causes a 50% increase in excitation current (the definition per IEC 61869-2/IEC 61869-11 for class PX CTs). Below the knee-point voltage, the CT behaves nearly linearly; well above the knee-point voltage, strong saturation occurs. A higher knee-point voltage (achieved via a larger core or fewer secondary turns) means the CT remains linear up to a higher secondary burden and a higher primary fault current before saturation occurs.

Why CT saturation can mislead a differential relay

For a heavy, but for the protected zone external (through-going) short circuit, one of the CTs around a differential zone can saturate while the other does not — for example due to a difference in residual (remanent) magnetisation of the cores, a difference in secondary burden, or a difference in the time constant of the fault current on the two sides. This unequal saturation produces an apparent differential current, even though there is no internal fault at all. This is one of the reasons why, as covered in the guide on differential protection, the restraint characteristic is made less sensitive at high through-going current: that partly compensates for the increasing risk of CT saturation errors during heavy external faults.

Note: the exact choice of protection class, ALF and knee-point voltage for a specific application (differential, earth-fault or overcurrent protection) follows from the relay manufacturer's application guidelines and the system study; this article covers the concepts, not a ready-made selection table for every application.

Practical relevance

When specifying or checking current transformers for a protection application, it is important to recognise that a CT with only a measurement-class designation (0.2/0.5/1) is not suitable for protection purposes, and that choosing a protection CT with too low an accuracy limit factor or knee-point voltage for the specified secondary burden can lead to unreliable relay operation during heavy faults.

Common mistakes

Applying a metering CT (class 0.2/0.5/1) for a protection function — it is deliberately designed to saturate at high fault current, which is precisely what a protection relay cannot afford.
Ignoring the ALF (accuracy limit factor) of a protection CT when determining whether it is suitable for the expected fault current — a 5P10 CT loaded with 15× rated current during a fault falls well outside its specified accuracy range.
Not checking a CT's knee-point voltage for a high-impedance earth-fault or differential application — too low a knee-point voltage relative to the secondary burden and the expected fault current increases the risk of saturation and incorrect or missed relay operation.
Assuming CT saturation can only lead to a missed trip — unequal saturation between CTs around a differential zone can also lead to a false trip on an external fault.