2.11 Overcurrent protection
217
7SA522 Manual
C53000-G1176-C155-3
Overcurrent Stages
I
ph
>, 3I
0
>
(O/C with DT)
For the setting of the current pickup value, ,SK! (address ), the maximum oper-
ating current is most decisive. Pickup due to overload should never occur, since the
device in this operating mode operates as fault protection with correspondingly short
tripping times and not as overload protection. For this reason, a pickup value of about
10 % above the expected peak load is recommended for line protection, and a setting
of about 20 % above the expected peak load is recommended for transformers and
motors.
When using a personal computer and DIGSI
®
to apply the settings, these can be op-
tionally entered as primary or secondary values. If secondary quantities are used, all
currents must be converted to the secondary side of the current transformers.
Calculation Example
:
110 kV overhead line 150 mm
2
maximum transmittable power
P
max
= 120 MVA
corresponding to
I
max
= 630 A
Current Transformer 600 A / 5 A
Safety factor 1.1
With settings in primary quantities the following setting value is calculated:
Setting value I> = 1.1 · 630 A = 693 A
With settings in secondary quantities the following setting value is calculated:
The earth current stage ,! (address ) should be set to detect the smallest
earth fault current to be expected. For very small earth currents the earth fault protec-
tion is most suited (refer to Section 2.7).
The time delay 7,SK! (address ) results from the time grading schedule de-
signed for the network. If implemented as emergency overcurrent protection, shorter
tripping times are advisable (one grading time step above the fast tripping stage), as
this function is only activated in the case of the loss of the local
measured voltage.
The time 7,! (address ) can normally be set shorter, according to a separate
time grading schedule for earth currents.
The set times are mere additional delays for the independent stages, which do not
include the inherent operating time of the protection. If only the phase currents are to
be monitored, set the pickup value of the earth fault stage to ∞.
The parameter ,!7HOHS%, (address ) defines whether the time delays 7
,SK!! (address ) and 7,!! (address ) can be bypassed by the binary
input ´!2&,QVW75,3µ (FNo 7110). The binary input (if allocated) is applied to all
stages of the time-overcurrent protection. With ,!7HOHS%, = <(6 you can set the
I> stages to trip immediately after the pickup, only if the binary input is activated. Set
time delays for ,!7HOHS%, = 12 are always activated.
If the I> stage is to trip when switching the line on to a fault with or without a short delay,
627)7LPH'(/$< (address , see above and refer to Subsection “General”),
the parameter ,!627) (address ) is set to <(6. We recommend, however, not
to choose the sensitive setting for the switch on to a fault function as energizing of the
To Next Page
Loading...
Need help?
General
