http://www.svri.nl/en/selecting-a-current-transformer-ct/

Selecting a Current Transformer (CT)

14 AUGUST 2014

If the current ratio, burden and class are selected, the current transformer (CT) has

been specified basically. Of course additional requirements like type, frequency,

isolation level, maximal short circuit current and environmental conditions should

also be specified.

 

Current ratio Ip/Is

The current ratio is the ratio between the primary and  secondary current.

For the primary current Ip you can select the first value that follows the highest

continuous current from the range:

1 – 1.25 – 1.5 – 2 – 2.5 – 3 – 4 – 5 – 6 – 7.5 A

Decades also belong to the range, like for example:

1000 - 1250 - 1500 - 2000 - 2500 - 3000 - 4000 - 5000 - 6000 - 7500 A

The secondary current Is is mostly 1A or 5A.

– 1A is my favorite, because the cable losses verliezen (VA) are 25x less compared

with 5A.

– 1A current transformer could be smaller at a required knee-point voltage.

– 5A was required when relays took their power supply from the sensing current.

– 5A is still commonly used in the US, unfortunately.

 

Accuracy

The accuracy of a current transformer depends among others of :

– burden

– class/saturation

– load

– frequency

– temperature

Accuracy based on burden and class (IEC)

For the burden you can select the first value that follows the actual load (including

cable losses) from the range:

2.5 – 5 – 10 – 15 - 30 VA

For the class you can select:

 

Sometimes the factor of

security (FS) is also

specified to protect metering

devices against high short

circuit currents. If FS = 5, the

composite error at 5 x Ip ≥ 10%. Standard values are:

FS 5 – FS 10

Accuracy based on knee-point voltage and Rct

For class X (BS), and class PX and PR (IEC) current transformer for protection relays

the accuray is not based on burden and class, but on:

– minimum knee-point voltage, with corresponding maximum magnetising current

– secondary winding resistance Rct (75°C)

- load

Note: IEC and ANSI/IEEE define knee-point voltage different

Accuracy based on ANSI/IEEE standards

For current transformers for metering you should select a burden in ohms and

an accuracy percentage. A B0.5 is a 0.5 ohm load, For example.

Burden: B0.1 – B0.2 – B0.5 – B0.9 – B1.8

Accuracy percentage: 0.3 – 0.6 – 0.9 – 1.2 – 2.4

For current transformers for protection there is a C-range, that is given in the

tabel with the IEC equivalent.

 

Notes:

- C400 and C800 can be very large CTs

– Divide burden (VA) by 5 for 1 A CTs

Application Class

Very accurate metering  0.1 - 0.2

Tariff (kWh) metering  0.5 – 0.5S – 0.2 – 0.2S

Instrument meters and control 1

Protection (P) 5P20 – 5P10 – 10P10

ANSI/IEEE IEC (5 A CTs)

C100 25VA, 5P20

C200 50VA, 5P20

C400 100VA, 5P20

C800 200VA, 5P20

 

Examples and tips

If the maximum continuous current is 1124 A (50 Hz), then the current

transformer specification for protection could be: 1250/1 A, 10 VA, 5P20

For kWh metering could be: 1250/1 A, 5 VA, cl. 0.2S

Or with a factor of security: 1250/1 A, 5 VA, cl. 0.2S FS 5

The accuracy class is only applicable if the total load including cable losses is

approximately equal to the current transformer burden.

If the burden of a current transformer for metering is much higher than the

load, instruments and devices could be damaged if short circuit occurs

somewhere.

A current transformer could comply with requirements of several

combinations, e.g. 30 VA, 5P10 and 15 VA, 5P20.

For metering the ANSI/IEEE specification of the current transformer could

be 500/5A, 0.3 B0.5 (, 60 Hz). In that case, the burden will be 0.5 x 5² = 12.5 VA.

For protection the ANSI/IEEE specification of the current transformer could be

500/5A C100 (, 60 Hz). The secondary current through a standaard 1 ohm load

can be 20 × 5 A = 100 A , with an inaccuracy less than 10%. The voltage over

the load will be 100 V.

 

See also

Wikipedia -   Current transformer

RS Isolsec -   Selecting a Current Transformer

 

References

IEC 61869-1 – Instrument transformers – Part 1: General requirements

IEC 61869-2 – Instrument transformers – Part 2: Additional requirements for

current transformers

IEC 60044-1 and IEC 185 are withdrawn current transformer standards

IEEE Std C57.13-2008 – IEEE Standard Requirements for Instrument

Transformers