Current Summation - Siemens 7UT51 Instruction Manual

7UT51 v3
The relay detects non-symmetric currents if the ratio of
the magnitude of I to the magnitude of I
min
smaller than a setting value, "SYM. Fact. I" (setting
2904):
I
min
I
max
When all the phase currents are small, they may be
non-symmetrical for reasons other than a faulty CT
circuit. Hence, no action is taken unless the value of
I is greater than a sensitivity threshold value,
max
"SYM.Ithres" (setting 2903):
I
max
If the ratio of I to I
min max
Figure 12.1, the relay will report that the currents are
non-symmetric for that particular winding (or bus
branch).
2903 SYM.Ithres
Current threshold above which at least one phase current
of the winding (or bus branch) must be before
-
current symmetry monitoring will react (see Figure 12.1).
–
1.00 In
Range: 0.10
Default: 0.50
2904 SYM.Fact.I
Ratio of minimum measured current to maximum
measured current below which current symmetry
monitoring will react (see Figure 12.1).
–
Range: 0.00 0.95
Default: 0.50
PRIM-2330C
is
max
< SYM.Fact.I
> SYM.Ithres
is in the BLOCK area of
1 0
(12.1)
0 5
0 0
(12.2)
Figure 12.1
12.4.2
If the relay is protecting a bus, an additional protection
self-monitoring feature is available. If the differential
current of any phase has a magnitude corresponding
to a load current, the relay assumes that a CT circuit is
faulty and blocks the differential protection. The details
and settings for this feature are described in
Section 6.6 on page 86.
Protection Self-Monitoring
OC Area
0 0 0 5
Magnitude of ma imum phase urrent,
( as a fra tion of I )
n
Current Symmetry Monitoring

Current Summation

12
1 0
I
121