Siemens SIPROTEC 7UM62 - Page 67

Functions
2.8 Definite-Time Overcurrent Protection (I>, ANSI 50/51) with Undervoltage Seal-In
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
67
Undervoltage Seal-In
The 1205 U< undervoltage stage (positive-sequence voltage) is set to a value below the lowest phase-to-phase
voltage admissible during operation, e.g. 80 V.
The seal-in time 1206 T-SEAL-IN limits the pickup seal-in introduced by the overcurrent/undervoltage. It must
be set to a value higher than the T I> time delay.
The dropout ratio r = I
DO
/I
PU
of the overcurrent pickup I> is specified at the address 1207 I> DOUT RATIO.
The recommended value is r = 0.95. For special applications, e.g. overload warning, it can be set to a higher
value (0.98).
Example:
The following secondary setting values result from this specification:
Pick-up threshold 1,4 · I
N Gen
Trip Time Delay 3 sec
Undervoltage Seal-In 0,8 · U
N Gen
Holding time of U< 4 sec
Dropout Ratio 0.95
Nominal current I
N Gen
483 A Nominal voltage U
N, Gen
6.3 kV
Nominal current I
N, CT, prim
500 A Nominal voltage U
N, VT, prim
6.3 kV
Nominal current I
N, sec
1 A Nominal voltage U
N, sec
100 V