(AP) 6 Application Notes
4.5
Current Transformer Requirements
Low Impedance REF
For accuracy, class X or class 5P Current Transformers (CTs) are strongly
recommended.
The CT requirements for low impedance REF protection are generally lower than those
for differential protection. As the line CTs for low impedance REF protection are the same
as those used for differential protection the differential CT requirements cover both
differential and low impedance REF applications.
The CT knee-point voltage requirements are based on the following settings for
transformer REF protection; IS1 = 27, IS2 = 270, K1 = 0%, K2 = 150%.
A series of internal and external faults were performed to determine the CT requirements
for the REF function. These tests were performed under different X/R ratios, CT burdens,
fault currents, fault types and point on wave.
The K dimensioning factor for the REF function is smaller than that for the transformer
differential protection. Since the highest K factor must be considered, the CT
requirements for transformer differential must be considered.
4.5.1
One Breaker Application
According to the CT requirements test results, to achieve through fault stability the K
dimensioning factor must comply with the following expression:
System conditions
2In < IF
64In
5
120
X/R
K (CT dimensioning factor)
K = 12
Knee point voltage
VK
12
In
(RCT + 2RL + Rr)
Where:
VK
=
Minimum current transformer knee-point voltage
K = CT dimensioning factor
Rct
=
Resistance of current transformer secondary winding ( )
RL
=
Resistance of a single lead from relay to current transformer ( )
Rr
=
Resistance of any other protective relays sharing the current transformer ( )
In
=
CT secondary nominal current (either 1A or 5)
IF
=
maximum external single phase fault
Table 21 - CT dimensioning factor
To ensure that the quoted operating times and through fault stability limits are met the
following ratios should not exceed a maximum disparity ratio of 7:1:
Vk-HV / Rtot-HV : Vk-TN1 / Rtot-TN1
Vk-LV / Rtot-LV : Vk-TN2 / Rtot-TN2
Vk-TV / Rtot-TV : Vk-TN3 / Rtot-TN3
This ensures that during a through fault condition the flux density in the current
transformers is not greatly different.
Where:
Vk-HV = Knee point voltage of CT at HV side
Rtot-HV = Total burden connected to CT at HV side = (R CT + 2Rl + Rr)
Vk-LV = Knee point voltage of CT at LV side
Rtot-LV = Total burden connected to CT at LV side = (RCT+ 2Rl + Rr)
Vk-TV = Knee point voltage of CT at TV side
Rtot-TV = Total burden connected to CT at TV side = (RCT+ 2Rl + Rr)
Vk-TN1 = Knee point voltage of TN1 CT
Rtot-TN1 = Total burden connected to TN1 CT = (RCT + 2Rl + Rr)
Vk-TN2 = Knee point voltage of TN2 CT
Rtot-TN2 = Total burden connected to TN2 CT = (RCT + 2Rl + Rr)
Page (AP) 6-146
P64x/EN AP/A62
(AP) 6 Application Notes
Current Transformer Requirements
Vk-TN3 = Knee point voltage of TN3 CT
Rtot-TN3 = Total burden connected to TN3 CT = (RCT + 2Rl + Rr)
4.5.2
One and a Half Breaker Application and Autotransformer Application
According to the CT requirements test results, to achieve through fault stability the K
dimensioning factor must comply with the following expression:
System conditions
2In < IF
64In
5
120
X/R
Where:
VK
=
K
=
Rct
=
RL
=
Rr
=
In
=
IF
=
K (CT dimensioning factor)
K = 27
Knee point voltage
VK
27
In
(RCT + 2RL + Rr)
Minimum CT knee-point voltage
CT dimensioning factor
Resistance of CT secondary winding ( )
Resistance of a single lead from relay to CT ( )
Resistance of any other protective relays sharing the CT ( )
CT secondary nominal current (either 1A or 5)
maximum external single phase fault
Table 22 - CT dimensioning factor
To ensure that the quoted operating times and through fault stability limits are met the
following ratios should not exceed a maximum disparity ratio of 4:1:
Vk-HV / Rtot-HV : Vk-TN1 / Rtot-TN1
Vk-LV / Rtot-LV : Vk-TN2 / Rtot-TN2
Vk-TV / Rtot-TV : Vk-TN3 / Rtot-TN3
This ensures that during a through fault condition the flux density in the current
transformers is not greatly different.
Where:
Vk-HV = Knee point voltage of CT at HV side
Rtot-HV = Total burden connected to CT at HV side = (RCT + 2Rl + Rr)
Vk-LV = Knee point voltage of CT at LV side
Rtot-LV = Total burden connected to CT at LV side = (RCT+ 2Rl + Rr)
Vk-TV = Knee point voltage of CT at TV side
Rtot-TV = Total burden connected to CT at TV side = (RCT+ 2Rl + Rr)
Vk-TN1 = Knee point voltage of TN1 CT
Rtot-TN1 = Total burden connected to TN1 CT = (RCT + 2Rl + Rr)
Vk-TN2 = Knee point voltage of TN2 CT
Rtot-TN2 = Total burden connected to TN2 CT = (RCT + 2Rl + Rr)
Vk-TN3 = Knee point voltage of TN3 CT
Rtot-TN3 = Total burden connected to TN3 CT = (RCT + 2Rl + Rr)
P64x/EN AP/A62
Page (AP) 6-147