The I.S.S. lists following classes of accuracy for protection core:

1) Accuracy class 5P  : The permissible composite error is limited to 5%.

2) Accuracy class 10P : The permissible composite error is limited to 10%.

3) Accuracy class 15P : The permissible composite error is limited to 15%.

Currently, majority of C.Ts are manufactured using toroidal cores where the magnetization characteristic is sufficiently sharp. Thus, for correctly designed C.Ts operating at a minimum of 200 Ampere turns, accuracy class 5P is automatically obtained and little is gained by specifying inferior accuracy for cores. It is therefore, suggested that the accuracy class 5P be generally specified. It may be noted, however, that for C.Ts operating at low value of Ampere turns, the magazine current, determined by the mean magnetic length and operating Ampere turns, may be high and it may not be practicable to obtain composite error within the specified limit of 5%. In such cases use of accuracy class 10P may be considered.

C.Ts cores proposed to be used for balance protection schemes are covered by accuracy class PS. When such cores are specified, information about the requirement of knee point Voltage and permissible magnetizing current at specified percentage of knee point voltage is also required to be furnished. It is recommended that this information may be furnished as follows:
Vk (min) = C (RCT + RL).......................equation 1 and
Imag (max) = PmA at Vk/F )

Where, a
Vk (min) - Minimum Knee point Voltage
C& - Constant governed by the system parameters and the relay characteristics.
Rct - Resistance of C.T. Secondary at 750C.
RL - One way lead resistance.
Factor usually specified by the relay manufacturers and may be 2 or 4 depending upon the applications.
PmA - Permissible magnetizing current in milliamps.
  • It is recommended that the purchaser should not assume the value of Rct. and calculate Vk but should leave the option of selecting Vk and Rct to the manufacture of Current Transformers.
  • Point by point matching of C.T. magnetization characteristic is usually neither possible nor it is required. It is considered sufficient to ensure that at the relay operating voltage, the difference in the magnetizing current drawn by C.T. cores on two sides of the relay is less than the relay operating current. This ensures through fault stability for the scheme of protection..
  • The constant ‘C’ in equation 1 above is usually specified by the relay manufacturers as 2 x IF where IF is the reflected fault current. It is suggested that this value of IF be determined in a realistic manner, and not merely by using the system fault MVA. This point is illustrated by following example.

Consider that a C.T. core for transformer differential protection is being specified. Consider also that transformer to be protected is a 50 MVA transformer with H.T. and L.T. voltages as 132 KV and 33 kV respectively, and the transformer percentage impedance is say 5%. The rated current on the H.T. and L.T side shall then be 218.69 Amps and 874.79 Amps respectively. The corresponding ratios of C.Ts on H.T. and L.T. side shall then be 250/1 Amp. and 1000/1 Amp respectively. Consider system fault level to be 3000 MVA on 33 kV side which corresponds to a system fault of 52.48 kA. Consider relay type DTH31 for which specified knee point voltage for differential core is

2 IF (RCT + 2 RL) to ensure through fault stability. If the value of IF is taken as

                52.48kA                 x C.T. Rated Secondary Current.

C.T. rated primary current

The requirement of Vk min shall be governed by the formula
2 x 52.48 x 103 (RCT + 2 RL) 103
i.e. 104.96 (RCT + 2 RL)
It is suggested, however, that the transformer impedance shall play an important part in determining the through fault current. As the percentage impedance of the transformer is 5%, the through fault current shall be limited to 20 times the rated current. Thus, the maximum through fault current shall be

20 x 874.79 = 17,495.8 Amps.

The value of factor 2 If shall be 35 instead of 104.96 as determined above. A similar calculation will apply while determining the minimum knee point voltage for 132 kV C.T. differential core.