Functions
2.20 Out-of-Step Protection (ANSI 78)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
173
Transformation ratios:
Current Transformer CT
Ratio
= 500 A/1 A
This gives the secondary transient reactance of the generator:
Z
b
≈ X
d
' thus determines the setting of address 3505 Zb.
The secondary short circuit reactance of the unit transformer is derived by considering the transformation
ratios:
If characteristic 1 covers 85 % of the transformer reactance, this results in the setting of Z
c
≈ 0.85 · 4.2 Ω ≈ 3.6
Ω.
Assuming that the impedance of the additional line section to be monitored together with the transformer short-
circuit impedance amounts to about 10 Ω (for .../1 A transformers), the resulting setting value is 3507 Zd -
Zc = 6.4 Ω.
The width Za of the polygon is determined by the total impedance Z
tot
. In this calculation example, the total
impedance Z
tot
is that of characteristic 1 (the sum of generator reactance and one portion of the unit transformer
reactance; that is the sum of the setting values for Z
b
and Z
c
= 12 Ω + 3.6 Ω = 15.6 Ω):
Z
a
≈ 0.289 · 15.6 Ω ≈ 4.5 Ω.
Number of Power Swings
Address 3509 REP. CHAR. 1 determines the number of out-of-step cycles which after transgression of char-
acteristic 1 lead to tripping. If no special calculations are available, setting 1 (or 2) is recommended, since
power swings within the power station area should not be tolerated too long because the power swing frequen-
cy tends to increase causing greater machine stress. On the other hand, for power swings with the electrical
centre being in the network itself a higher number of crossings can be tolerated, so that address 3510 REP.
CHAR. 2 can usually be set to 4.
Each time characteristic 1 or 2 is passed through, a holding time set at address 3511T-HOLDING) is started.
After expiry of the hold time a pickup retracted by resetting counter n
1
or n
2
to zero, i.e. a power swing is again
"forgotten". This time should be set higher than the longest expected out-of-step cycle period (i.e. for lowest
power swing frequency). Settings between 20 s and 30 s are usual.
Each time one of the counters n
1
or n
2
is incremented, the holding time is restarted, and an annunciation "Out-
of-step characteristic 1" or "Out-of-step characteristic 2" is issued. These annunciations disappear after the
time set at address 3512 T-SIGNAL. If this time is set higher that the time between two power swings, the
annunciation "Out-of-step characteristic 1(2)" begins on the first out-of-step detection and ends after the last
out-of-step detection, after the set time T-SIGNAL.
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