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22 March 2019

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Selecting a Current Transformer (CT)

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:

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

 

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.

ANSI/IEEE IEC (5 A CTs)
C100 25VA, 5P20
C200 50VA, 5P20
C400 100VA, 5P20
C800 200VA, 5P20

 
Notes:
– C400 and C800 can be very large CTs
– Divide burden (VA) by 5 for 1 A CTs

 

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

 

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