Siemens SIPROTEC 7SS60 Manual page 54

Functions
Basic circuit,
supervision
5-4
The simple circuit shown in Figure 5-2 is sufficient to derive the sum of all feeder
currents in one phase, provided that the current transformers of all feeders have the
same transformation ratio. In most cases, however, the current transformers of the
feeders in a power system will have different transformation ratios because the
incoming and outgoing lines carry different rated currents and the transformers are
matched to these currents. To provide the uniform transformation ratio required by the
busbar protection, matching CTs are installed upstream of the protection system input;
these transformers are designed to convert the input from the main current
transformers into an identical transformation ratio for all feeders. In the 7SS60 system
the matchting CTs are also responsible for matching to the nominal current of the
measuring inputs. The output current of the matching CTs is 100 mA with a primary
nominal current.
To the single-phase protection circuit shown in Figure 5-2, matching CTs would be
added, as shown in Figure 5-3.
Main CT
Matching CT
Figure 5-3 Differential protection of a busbar, with matching CTs
The 7SS601 measuring system includes a busbar protection function B and a
supervision function S, which monitors the connection circuit of the protection. The
supervision is set to pick up at much lower currents than the protection function, which
is designed for short-circuit currents, so that it can respond to such a low current in the
summation path even at normal load currents.
The supervision function detects faults and interruptions in the current transformer
circuits.
If now, in this error condition, a short-circuit occurs outside the protected area, e.g. in
the feeder, the current would not be recognized by the measuring system and
therefore appear as differential current. If no countermeasures were taken, the
protection would initiate a TRIP command. As a result, a busbar would be spuriously
tripped in the critical period following the tripping of the faulted line, when the healthy
station components are of particular importance to maintain the supply of the power
system.
This is prevented by the supervision function, which detects a fault current flowing
even at load current conditions. After a specified time delay, which is necessary for
the system to settle down after a tripped short-circuit, an alarm is raised if the current
persists, and the tripping can be blocked according to the setting. In this way, the
supervision prevents the tripping of the busbar, even if due to an error in the
connection circuit all criteria for the output of a TRIP command would be fulfilled.
In addition, the alarm output by the supervision informs the user of the circuit error.
7SS60
B
S
E50417-G1176-C132-A2
7SS60 Manual