Learn How To Specify Current Transformers

Instrument and protection CTs

Current transformers are used to supply information to the protective relays and/or current, power and energy metering “instruments”. For this purpose they must supply a secondary current proportional to the primary current flowing through them and must be adapted to network characteristics: voltage, frequency and current.

They are defined by their ratio, power and accuracy class. Their class (accuracy as a function of CT load and of overcurrent) is chosen according to the application.

A “protection” current transformer (CT) must saturate sufficiently high to allow a relatively accurate measurement of the fault current by the protection whose operating threshold can be very high. Current transformers are thus expected to have an Accuracy Limit Factor (ALF) that is usually fairly high. Note that the associated “relay” must be able to withstand high overcurrents.

An “instrument”current transformer (CT) requires good accuracy around the nominal current value. The metering instruments do not need to withstand currents as high as the protection relays. This is why the “instrument” CTs, unlike the “protection” CTs, have the lowest possible Safety Factor (SF) in order to protect these instruments through earlier saturation.

The matching of CTs with protection relays calls for a thorough knowledge of current transformers. The following section gives a few reminders of CTs corresponding to this use.

Characterisation of CTs

An example of a protection CT //

• Rated primary current: 200 A,
• Rated secondary current: 5 A.

Its accuracy load: Pn = 15 VA
Its accuracy limit factor is ALF = 10

For I = ALF. In, its accuracy is 5% (5P), (see figure 1)

To simplify, for the protection CT given in example, the ratio error is less than 5% at 10 In , if the real load consumes 15 VA at In. However these data are not sufficient. Also, it is useful to know the standard values.

12 definitions related to current transformers //

• Rated (nominal) primary current I1
• Rated (nominal) secondary current I2
• Ratio (I1 / I2)
• Accuracy load
• Rated (nominal) accuracy power Pn
• Real power Pr
• Accuracy class
• Special accuracy class
• Real accuracy factor (Fp or Kr)
• Accuracy limit factor (ALF or Kn)
• Short time withstand current
• CT rated voltage

≡ Rated (nominal) primary current I₁

Defined by standards, it is chosen from the discrete values: 10 – 12.5 – 15 – 20 – 25 – 30 – 40 – 50 – 60 – 75 A and their decimal multiples.

≡ Rated (nominal) secondary current I₂

Equals 1A or 5 A.

≡ Ratio (I₁ / I₂)

The primary and secondary currents are standard, thus these values are discrete. (Learn more about ratios of magnetic HV instrument current transformers – Here)

≡ Accuracy load

Load value on which the accuracy conditions are based.

≡ Rated (nominal) accuracy power P_n

Expressed in VA, it is the apparent power supplied to the secondary circuit for the nominal (rated) secondary current and the accuracy load. The standard values are: 1 – 2.5 – 5 – 10 – 15 – 30 VA.

≡ Real power P_r

In this technical article, it is the power corresponding to the real load consumption of the CT at I_n.

≡ Accuracy class

This class defines the error limits guaranteed on the ratio and on the phase shift in specified power and current conditions. For the nominal 5P and 10P classes, the table in figure 6 defines these limits.

Figure 2 // Errors on the module and the phase at nominal current
(according to standard IEC 60044-1)

≡ Special accuracy class

Class X is a class defined by British standard BS 3938. It must also be defined in the future standard IEC 60044-1 under the name of class PX. This class specifies the minimum value of the knee point voltage Vk of the CT.

It also imposes a maximum value of Rct (CT secondary winding resistance). Sometimes, it specifies the maximum value of the magnetising current Io at knee point voltage.

If we consider the magnetising curve V(Io) of the CT, the knee point voltage Vk is defined as the point on this curve from which a 10% increase in voltage causes a 50% increase in the magnetising current Io. Class X corresponds to a better metering accuracy than classes 5P and even more so 10P (see figure 3).

It is always possible to find an equivalence between a CT defined in class X and a 5P CT or in some cases even a 10P CT.

≡ Real accuracy factor (F_p or K_r)

This is the ratio between the overcurrent corresponding to the nominal error and the rated current of the CT when the real load is different from the nominal load.

≡ Accuracy limit factor (ALF or Kn)

This is the ratio between the nominal overcurrent (e.g. 10 In) and the rated current (In).

≡ Short time withstand current

Expressed in kA, this is the maximum current Ith that can be withstood for one second (when the secondary is short-circuited). It represents the thermal withstand of the CT to overcurrents (the standard values are given by the standards mentioned in the appendix).

≡ CT rated voltage

This is the rated voltage to which the CT primary is subjected. It is important to remember that the primary is at HV potential and that one of the terminals of the secondary (which must never be opened) is normally earthed.

Just as for any devices, a maximum withstand voltage for one minute at power frequency and a maximum impulse voltage withstand are also defined. Their values are defined by the standards.

For example: for a rated voltage of 24 kV, the CT must withstand 50 kV for 1 minute at 50 Hz and 125 kV at the impulse voltage.

CT with several secondaries

Some current transformers may have several secondaries dedicated to protection or to metering. The most typical cases are CTs with 2 secondaries, more rarely with 3 secondaries. Physically, these CTs group in the same mould the equivalent of 2 or 3 separate CTs that can have different classes and ratios (see figure 4 below).

Current transformers – VIDEO sessions

What are CTs and why use them?

CT Polarity

CTR

Wye connected CTs

Delta connected CTs

Current transformer model

Reference // Cahier Technique Schneider Electric no. 194 – Current transformers: how to specify them by Schneider Electric