Second-Harmonic Restraint - Siemens 7UT51 Instruction Manual

7UT51 v3
4.6 Second
Restraint
When a transformer is energized, the resulting inrush
current usually causes a transient differential current.
This inrush current, which can be several times
greater than the transformer's rated nominal current, is
characterized by a considerable second harmonic
content that is practically absent from a short-circuit
current. Hence, the presence of a relatively large
second harmonic current is often used to block the
differential protection during inrush. The second
harmonic restraint is set On or Off in Address 1610.
1610 2nd HARMN RSTR
Blocking the differential protection for inrush
(second harmonic) current exceeding the threshold
setting is chosen in Address 1611. Inrush restraint
operates independently for each phase, unless
crossblocking is set in Address 1612.
Option Description
ON Use inrush restraint (default).
OFF Do not use inrush restraint.
1611 %2nd HARMN
Minimum amount of second harmonic differential
current (as a percentage of the fundamental
component of the differential current) that will block the
differential protection (if Address 1610 is set to ON).
–
Range: 10 80%
Default: 15%
In most applications, inrush restraint should operate
independently for each phase since it is possible for a
fault to occur on only one phase during inrush. If this
happens, the differential current of that phase will not
contain the high percentage of second-harmonic
current characteristic of inrush, and so the differential
protection of that phase will not be blocked. However,
PRIM-2330C
Harmonic
-
Transformer Differential Protection (87T and 87HS)
if desired, the relay can be configured to have the
inrush restraint of any phase "crossblock" the
differential protection of all three phases for a specified
duration. The choice and duration are set in
Address 1612.
-
1612 X BLK 2HRM
Duration (in cycles) after pickup of the differential
protection to block differential protection on all
phases (crossblock) if inrush restraint is blocking it
on at least one phase.
Range: 0
Default: 0 (no crossblocking)
Specify (in cycles) the duration of crossblocking.
Enter 0 for no crossblocking (each phase blocked
independently). Enter ∞ for crossblocking to continue
for as long as the differential protection of any phase
is blocked by inrush
The effect of second-harmonic restraint is illustrated in
Figure 4.10. Note that if the differential current
becomes larger than the 87HS trip threshold, a trip will
occur (see Section 4.4.2 on page 56). This will result
in a trip even if a large DC component, in a large fault
current causing CT saturation, is being interpreted as
a large second-harmonic component.
See Section 4.10 on page 62 for the events that
control and annunciate the start and end of harmonic
restraint.
1 0.0
5 .0
2 .0
1 .0
0 .5
0 .2
0 .1
Figure 4.10 Inrush (2nd Harmonic) Restraint
Characteristic
1000 cycles, or ∞
–
-
current restraint.
8 7H S R M S
A d d re ss 16 0 4
B lock
Trip
A rea
A rea
8 7T P U
A d dre ss
1 60 3
1 5
0 1 0 2 0 3 0 4 0
R a tio o f 2 nd H a rm on ic C urre nt to
Fu nd a m e ntal C o m p on e nt C urren t
4
5 0
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