Siemens SIPROTEC 4 7SJ61 Manual page 83

[ueb-einph-anordnung-020926-rei, 1, en_US]
Figure 2-27
The voltage across R is then
V = Ι · ( 2R + R
R 1 a2
It is assumed that the pickup value of the 7SJ61 corresponds to half the knee-point voltage of the current
transformers. In the balanced case results
V = V / 2
R S
This results in a stability limit Ι
stable:
[ueb-einph-stabilitaetslimit-021026-rei, 1, en_US]
Calculation Example:
For the 5 A CT as above with V
longest CT connection lead 22 m (24.06 yd) with 4 mm
[ueb-einph-stabilitaetslimit-5a-021026-rei, 1, en_US]
that is 15 × rated current or 12 kA primary.
For the 1 A CT as above with V
longest CT connection lead 107 m (117.02 yd) with 2,5 mm
[ueb-einph-stabilitaetslimit-1a-021026-rei, 1, en_US]
that is 27 × rated current or 21.6 kA primary.
Sensitivity with High-impedance Protection
The voltage present at the CT set is forwarded to the protective relay across a series resistor R as proportional
current for evaluation. The following considerations are relevant for dimensioning the resistor:
As already mentioned, it is desired that the high-impedance protection should pick up at half the knee-point
voltage of the CT's. The resistor R can calculated on this basis.
Since the device measures the current flowing through the resistor, resistor and measuring input of the device
must be connected in series. Since, furthermore, the resistance shall be high-resistance (condition: R >> 2R
R , as mentioned above), the inherent resistance of the measuring input can be neglected. The resistance is
i2
then calculated from the pickup current Ι
SIPROTEC 4, 7SJ61, Manual
C53000-G1140-C210-6, Edition 05.2016
Simplified equivalent circuit of a circulating current system for high-impedance protection
)
i2
, i.e. the maximum through-fault current below which the scheme remains
SL
= 75 V and R
S
= 350 V and R
S
pu
= 0.3 Ω
i
2 cross-section, this corresponds to R
= 5 Ω
i
2 cross-section, this corresponds R
and half the knee-point voltage:
2.4 Single-Phase Overcurrent Protection
= 0.1 Ω
a
= 0.75 Ω
a
Functions
+
a2
83