Siemens siprotec 7SA6 User Manual page 332

Functions
6-150
approximately 10 %, on transformers and motors approximately 20 % above the
maximum expected (over-)load current.
When using a personal computer and DIGSI
optionally entered as primary or secondary values. When applying the setting
parameters as secondary values, the primary currents must be converted to the
secondary side of the current transformer.
Calculation example:
110 kV overhead line 150 mm
maximum transmittable power
P = 120 MVA corresponds to
max
I = 630 A
max
current transformers 600 A/5 A
security margin
When applying settings with primary values, the following setting results:
Set value I> = 1.1 · 630 A = 693 A
When applying settings with secondary values, the following setting results:
Setting value I> 1.1
=
The earth current stage ,! (address ), must still be able to detect the smallest
earth fault current that may be present. For very small earth currents the earth fault
protection is most suited (refer to Section 6.5).
The time delay 7 ,SK! (address ) which has to be set is derived from the grading
plan of the system. If implemented as emergency overcurrent protection, shorter
tripping time delays (one grading time margin longer than the fast tripping stage) are
advisable, as this function is only activated when the local measured voltage fails.
The time 7 ,! (address ) can usually be set with a smaller time delay
according to a separate earth fault grading plan.
The set times of the definite time stages are pure additional time delays which do not
include the operating (measuring) time of the protection. If only the phase currents of
a particular stage should be monitored, the pickup value of the earth current stage is
set to ∞.
The setting parameter ,! 7HOHS%, (address ) determines if it is possible to
use the binary input "!2& ,QVW75,3" to bypass the trip delay times 7 ,SK!
(address ) and 7 ,! (address ). The binary input (if assigned) is
common to all stages of the overcurrent protection. With ,! 7HOHS%, = <HV it is
therefore determined that the I>–stages trip without time delay following pick-up, if an
operate signal is present at the binary input; if the setting is ,! 7HOHS%, = 1R the
set trip time delays always come into force.
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 Sub-section
"General"), the parameter ,! 627) (address ) is set to Yes. We recommend,
however, not to choose the sensitive stage for the switch on to fault function as
energizing of the line onto a fault usually causes a large fault current. It is important to
avoid that the selected stage picks up in a transient way when energizing the line.
®
2
as in the example in Sub-section 6.2.4.2:
1.1
630 A
⋅ ⋅
--------------- - 5 A 5.8 A
=
600 A
4 to apply the settings, these can be
C53000-G1176-C133-1
7SA6 Manual