Bus Protection - Siemens 7UT51 Instruction Manual

7UT51 v3
52
other
3
W 1 (HV)
W 2/W 3 (LV)
G
3
other
Protection of Unit Connected Generator and
Figure 3.8
Main Power Transformer with Overcurrent
Backup for Unit Auxiliary Transformer
PRIM-2330C
7U T 513
W 1
49-1
W 1
87
W 2
W 3
87
H S
W 2
49-2
50
51
other
3
52
Aux Bus
3.4

Bus Protection

Because of the very high fault currents associated with
bus faults, primary bus protection is normally
implemented with a high-impedance type differential
relays. However, when fault currents are of lower
magnitudes, as they tend to be with radial distribution
systems, the 7UT513 can be used in bus protection
schemes. Figure 3.9 shows a typical two-bus radial
distribution system with a tie breaker for greater
reliability. Because the CT ratios of the source
breakers, tie breaker, and feeder breakers are typically
different from one another, a 7UT513 is required since
it has three sets of phase-current inputs. The
supply-branch CTs are connected to one set of
phase-current inputs, the tie breaker CTs to another,
and the feeder breaker CTs (all of the same ratio and
tied together external to the relay) to the third. Backup
overcurrent protection (not shown in the figure) can
also be implemented internally by the 7UT513s.
Application Examples
3
35