Startup Overcurrent Protection (ANSI 51)
73
7UM62 Manual
C53000-G1176-C149-3
Figure 2-26 Short-Circuit Currents in the Generator during the Startup (Generator: 300 MVA,
15.75 kV, 50 Hz)
Delay Times
Since the generator circuit breaker is open during startup, there is no need to coordi-
nate the delay time with the network. Wherever possible, no delay time should be ef-
fective at all since the operating time of the protection function is extended proportion-
ally to the lower frequency (see Chapter 4, Technical Data).
Where a sensitive setting is selected, a delay time may be useful to avoid overfunc-
tioning. This delay time should be based on the lowest detectable frequency of 2 Hz,
and set to 0.5 s.
Short-Circuit
Coordination
Figure 2-27 shows the interaction between the short-circuit protection functions, such
as:
−
Startup overcurrent function
−
Differential protection
−
I>> stage as back-up stage for 10 Hz and higher
The pickup thresholds in this figure are for orientation only.
The differential protection Idiff and the overcurrent protection I>> are effective from ap-
prox. 10 - 11 Hz.
The startup overcurrent protection I-ANF provides additional protection in the lower
frequency range.
The result is a short-circuit protection concept in which the functions complement one
another.
0
0.2
0.4
0.6
0.8
1
1.2
0
1
2
3
4
5
6
f/fn
I/InG
