GE P645

The CT requirements for low impedance REF protection are generally lower than those for differential protection.
As the line CT
s 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 current transformer knee-point voltage requirements are based on the following default settings for
transformer REF protection; IS1 = 27 A (primary value at 300/1 CT Ratio), IS2 = 270 A (primary value at 300/1 CT
ratio), K1 = 0%, K2 = 150%.
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.
5.2.6.1 CT REQUIREMENTS - ONE BREAKER APPLICATION
To achieve through-fault stability, the K dimensioning factor must comply with the following:
System Conditions K Kneepoint voltage (VK)
2I
n
< I
F
£ 64I
n
5 £ X/R £ 120
12
V
K
³ 12I
n
(R
CT
+ 2R
L
+ R
r
)
where:
● V
K
= kneepoint v
oltage
● K = CT dimensioning factor
● I
n
= rated current
● R
CT
= resistance of CT secondary winding
● R
L
= Resistance of a single lead from device to current transformer
● R
r
= resistance of any other protection devices sharing the current transformer
To ensure through-fault stability is achieved, the following ratios: should not exceed a maximum disparity of 7:1
V
K-HV
/ R
tot-HV
: V
K-TN1
/ R
tot-TN1
V
K-LV
/ R
tot-LV
: V
K-TN2
/ R
tot-TN2
V
K-TV
/ R
tot-TV
: V
K-TN3
/ R
tot-TN3
where:
● V
K-HV
= Kneepoint voltage of CT at HV side
● R
tot-HV
= Total burden connected to CT at HV side
● V
K-LV
= Kneepoint voltage of CT at LV side
● R
tot-LV
= Total burden connected to CT at LV side
● V
K-TV
= Kneepoint voltage of CT at TV side
● R
tot-TV
= Total burden connected to CT at TV side
● V
K-TN1
= Kneepoint voltage of TN1 CT
● R
tot-TN1
= Total burden connected to TN1 CT
● V
K-TN2
= Kneepoint voltage of TN2 CT
● R
tot-TN2
= Total burden connected to TN2 CT
● V
K-TN3
= Kneepoint voltage of TN3 CT
● R
tot-TN3
= Total burden connected to TN3 CT
This ensures that during a through-fault condition, the flux density in the CTs is not greatly different.
Chapter 8 - Restricted Earth Fault Protection P64x
182 P64x-TM-EN-1.3