Siemens 7SJ82 - Page 539

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Figure 6-107 Simplified Connection Diagram of a Layout for High-Impedance Restricted Ground-Fault
Protection
The voltage at R is, then,
V
R
= I
1
· (2R
a2
+ R
i2
)
A further assumption is that the pickup value of the SIPROTEC 5 device corresponds to half of the knee-point
voltage of the current transformers. In the edge case,
V
R
= V
KP
/2
The stability limit I
SL
results, which means the through fault current up to which the arrangement remains
stable:
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EXAMPLE
For the 5 A transformer as above with V
KP
= 75 V and R
i
= 0.3 Ω
Longest connection line = 22 m with 4 mm
2
cross-section; that corresponds to R
a
= 0.1 Ω
[foisl5aw-310113-01.tif, 1, en_US]
In the example, the stability limit is 15 × rated current or 12 kA primary.
For the 1 A transformer as above with V
KP
= 350 V and R
i
= 5 Ω
Longest connection line = 107 m with 2.5 mm
2
cross-section; that corresponds to R
a
= 0.75 Ω
[foisl1aw-310113-01.tif, 1, en_US]
In the example, the stability limit is 27 × rated current or 21.6 kA primary.
Sensitivity Consideration for High-Impedance Restricted Ground-Fault Protection
The voltage present at the set of current transformers is supplied to the protection device via a series resistor R
as proportional current for evaluation. For dimensioning of the resistor, the following must be taken into
account:
The high-impedance restricted ground-fault protection should pick up at approximately half of the knee-point
voltage of the current transformers. From this, you can calculate the resistor R.
Since the device measures the current through the resistor, the resistor and measuring input of the device
must be connected in series. Since the resistor still should be high-impedance (aforementioned condition R >>
2R
a2
+ R
i2
), the inherent resistance of the measuring input can be neglected. The resistance results thus from
the pickup current I
pick
and half of the knee-point voltage:
Protection and Automation Functions
6.12 Overcurrent Protection, 1-Phase
SIPROTEC 5, Overcurrent Protection, Manual 539
C53000-G5040-C017-8, Edition 07.2017