GE P642 Technical Manual page 206

Chapter 8 - Restricted Earth Fault Protection
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 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
£ 64I
2I < I
n F n
5 £ X/R £ 120
where:
V = kneepoint voltage
●
K
K = CT dimensioning factor
●
I = rated current
●
n
R = resistance of CT secondary winding
●
CT
R = Resistance of a single lead from device to current transformer
●
L
R = resistance of any other protection devices sharing the current transformer
●
r
To ensure through-fault stability is achieved, the following ratios: should not exceed a maximum disparity of 7:1
V / R : V
K-HV tot-HV
V / R : V
K-LV tot-LV
V / R : V
K-TV tot-TV
where:
● V = Kneepoint voltage of CT at HV side
K-HV
R = Total burden connected to CT at HV side
●
tot-HV
● V = Kneepoint voltage of CT at LV side
K-LV
R = Total burden connected to CT at LV side
●
tot-LV
V = Kneepoint voltage of CT at TV side
●
K-TV
R = Total burden connected to CT at TV side
●
tot-TV
V = Kneepoint voltage of TN1 CT
●
K-TN1
R = Total burden connected to TN1 CT
●
tot-TN1
V = Kneepoint voltage of TN2 CT
●
K-TN2
R = Total burden connected to TN2 CT
●
tot-TN2
V = Kneepoint voltage of TN3 CT
●
K-TN3
● R = Total burden connected to TN3 CT
tot-TN3
This ensures that during a through-fault condition, the flux density in the CTs is not greatly different.
182
K
V
12
/ R
K-TN1 tot-TN1
/ R
K-TN2 tot-TN2
/ R
K-TN3 tot-TN3
Kneepoint voltage (VK)
³ 12I
(R + 2R + R )
K n CT L r
P64x
P64x-TM-EN-1.3