Siemens siprotec 7SA6, Manual

Page: 375 / 876

Functions
6-185
7SA6 Manual
C53000-G1176-C156-2
Cable 1 3 km 7.5 A
Cable 2 5 km 12.5 A
Cable 3 2.6 km 6.5 A
Cable 4 5 km 12.5 A
Cable 5 3.4 km 8.5 A
Cable 6 3.4 km 8.5 A
Cable 7 2.6 km 6.5 A
Total 25.0 km 62.5 A
With an earth fault in cable 2, 62.5 A – 12.5 A = 50 A earth fault current will flow
through the measuring point, since 12.5 A flows directly from cable 2 into the fault.
Since that cable is amongst the longest, this is the most unfavourable case (smallest
earth fault current flows through the measuring point). On the secondary side, flows:
50 A/300 = 0.167 A.
The relay should be set at approximately half this value,
3I0>
=
0.080
A.
In resonant-earthed networks directional determination is made more difficult since
a much larger reactive current (capacitive or inductive) is superimposed on the critical
wattmetric (active) current. The total earth current available to the relay can therefore,
dependent upon the network configuration and location of the compensation coil, as-
sume very different values in magnitude and phase angle. The relay, however, must
evaluate only the active component of the earth fault current, that is, I
E
· cos
ϕ
. This
demands extremely high accuracy, particularly with regard to phase angle measure-
ment of all the instrument transformers. Also, the relay should not be set unnecessarily
sensitive. When used in compensated networks therefore, reliable directional determi-
nation is only expected when core balance or window-type transformers are used.
Here also, use the rule of thumb: setting at half the expected measured current,
whereby only the residual wattmetric current is applicable. This residual wattmetric
current is provided principally by the losses in the Petersen coil.
Example: The same network, as in the previous example, is considered to be compen-
sated by a Petersen coil. The coil is matched to the total network. The compensation
current is thus 62.5 A. The losses should be 4 %. For earth fault directional determi-
nation, core balance current transformers 60 A/1 A are fitted.
Since the residual wattmetric current is derived principally from the coil losses, it is,
independent of earth fault location, approximately the same:
4 % of 62.5 A = 2.5 A.
This active current is superimposed by a reactive current which can amount to up to
62.5 A for earth faults near the Petersen coil! On the secondary side we have
2.5 A/60 = 0.041 A.
As setting value
3I0>
=
0.020
A is selected.
If the earth fault protection is also to trip (Address
3001 Sens. Earth Flt
=
On
),
set in address
3008 TRIP Direction
, if for earth faults the signal is tripped
for-
ward
(normally in line direction),
reverse
(normally in direction of busbar) or
non-
directional.
This setting can only be modified with DIGSI
®
4 under “Additional
Settings ”.
Phase Angle
Compensation The high reactive current component in resonant-earthed networks and the unavoid-
able air gap of the core balance type current transformers require a phase angle com-
pensation of the current transformer. This is possible with addresses
3010
to
3013
.
For the actual connected burden the maximum angle phase displacement
CT Err.