Functions
72
7UM62 Manual
C53000-G1176-C149-3
The function is also active above 70 Hz because at that frequency the protection is
again in operational condition 0.
2.11.1 Functional Description
Measuring
Principle
At frequencies below 10 Hz, the protection works in operating condition 0, with the
sampling frequency automatically set to nominal conditions (f
S
= 800 Hz for 50 Hz net-
works and 960 Hz for 60 Hz networks). From the sampled phase currents, a special
algorithm determines the peak values. These are converted into values proportional
to the r.m.s. values, and compared with the set threshold value.
Figure 2-25 shows the logic diagram.
Figure 2-25 Logic Diagram of the Startup Overcurrent Protection
2.11.2 Setting Hints
General
The startup overcurrent protection is only effective and accessible if it has been set to
at address
during configuration of the protection functions.
Address
O/C STARTUP
is used to switch the function ON or OFF, or to block
only the trip command (Block relay).
Pickup Value
Figure 2-24 shows that the currents during startup amount to approx. 20 % of the nom-
inal currents. This allows to set the protection routinely to less than nominal current.
As can be seen in Figure 2-25, the function is blocked when the operating condition
changes from 0 to 1. Additionally, blocking by binary input should be provided for.
Figure 2-26 shows an example of the estimated short-circuit currents at different fre-
quencies. As the short-circuit currents can amount to a multiple of the nominal current,
the pickup value could also be set to more than the nominal current, i.e. around the
usual values between 1.2 and 1.4 I/I
NG
.
>BLOCK O/C St
5571
5575
O/C Start L1 PU
5576
O/C Start L2 PU
5577
O/C Start L3 PU
5573
O/C Start BLK
5578
O/C Start TRIP
I
ST-L1
I
ST-L2
I
ST-L3
&
&
&
Operating state 1=1
OR
1802 STARTUP I>
1803 STARTUP T I>
Tripping
matrix
Tmin TRIP
CMD
