High Impedance Ref Protection Application; High Impedance Ref Operating Modes - GE P642 Technical Manual

P64x
5.2.6.2 CT REQUIREMENTS - ONE-AND-A-HALF BREAKER APPLICATION
According to the test results, 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 £ 20
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 4:1
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.
5.3

HIGH IMPEDANCE REF PROTECTION APPLICATION

5.3.1

HIGH IMPEDANCE REF OPERATING MODES

In the examples below, the respective Line CTS and measurement CTs must have the same CT ratios and similar
magnetising characteristics.
P64x-TM-EN-1.3
K
V
27
/ R
K-TN1 tot-TN1
/ R
K-TN2 tot-TN2
/ R
K-TN3 tot-TN3
Chapter 8 - Restricted Earth Fault Protection
Kneepoint voltage (VK)
³ 27I
(R + 2R + R )
K n CT L r
183