https://www.svri.nl/en/selecting-a-current-transformer-ct/
31 December 2019
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
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
FILED UNDER: GENERATORS, POWER SYSTEMS, PROTECTION
Copyright © 2019