Functions
2.13 Startup Overcurrent Protection (ANSI 51)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
100
Figure 2-28 Short-circuit currents in the generator during startup (generator: 300 MVA, 15.75 kV, 50 Hz)
Delay
Since the generator circuit breaker is open during startup, there is no need to coordinate the delay time with
the network. Wherever possible, no delay time should be effective at all since the operating time of the protec-
tion function is extended proportionally to the lower frequency (see Chapter Technical Data).
If a sensitive setting is selected, a delay time may be useful to avoid overfunctioning. This delay time should
be based on the lowest detectable frequency of 2 Hz, and set to 0.5 s.
Short-Circuit Protection Coordination
The figure below shows the interaction between the short-circuit protection functions, such as:
• Startup overcurrent protection
• Differential Protection
• I>> stage as back-up stage for 10 Hz and higher
The pickup thresholds here are orientation values.
The differential protection Idiff and the overcurrent protection I>> are effective from approx. 10 - 11 Hz. Addi-
tionally the startup overcurrent protection I-ANF operates. It provides protection in the lower frequency range.
The result is a short-circuit protection concept where the functions complement one another.
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