Single-Phase Differential Protection For Busbars - Siemens SIPROTEC 7UT613 series Manual

2 Functions
Differential Current
Monitoring
Feeder Current
Guard
2.2.6

Single-phase Differential Protection for Busbars

124
Figure 2-41 Definition of current direction at busbar with 4 feeders
The differential protection feature of the 7UT613/63x refers all currents to the rated
current of the protected object. The device is informed during setting about the rated
current of the protected object (in this case the busbar or line), and about the primary
rated CT currents. Measured value matching is therefore reduced to magnitude fac-
tors. The basis for current comparison is the rated busbar current (address 371 I
PRIMARY OP.). If the feeders or ends have different rated currents, the largest of the
three rated currents is used as the basis for the current comparison, and all other cur-
rents are converted accordingly. As a rule, no external matching devices are neces-
sary.
Whereas a high sensitivity of the differential protection is normally required for trans-
formers, reactors, and rotating machines in order to detect even small fault currents,
high fault currents are expected in case of faults on a busbar or a short line so that a
higher pickup threshold (above rated current) is conceded here. This allows for a con-
tinuous monitoring of the differential currents on a low level. A small differential current
in the range of operational currents indicates a fault in the secondary circuit of the
current transformers.
This monitor operates phase-selectively. When, during normal load conditions, a dif-
ferential current is detected in the order of the load current of a feeder, this indicates
a missing secondary current, i.e. a fault in the secondary current leads (short-circuit or
open-circuit). This condition is annunciated with time delay. The differential protection
is blocked in the associated phase at the same time.
With busbars and short lines, a release of the trip command can be set if a threshold
is exceeded by one of the incoming currents. The three phase currents at each mea-
suring location of the protected object are monitored for over-shooting of a set value.
Trip command is allowed only when at least one of these currents exceeds a certain
(settable) threshold.
7UT613/63x Depending on the ordered model, it provides 9 or 12 current inputs of
equal design. This allows for a single-phase differential current busbar protection for
up to 9 or 12 feeders.
There are two connection possibilities:
• One 7UT613/63x is used for each phase. Each phase of all busbar feeders is con-
nected to one phase dedicated device 7UT613/63x.
• The three phase currents of each feeder are summarised into a single-phase sum-
mation current. These currents are fed to one device per feeder.
7UT613/63x Manual
C53000-G1176-C160-2