Siemens 7UT51

7UT51 v3
Bus Differential Protection (87B)
6
PRIM-2330C 81
6.3 Describing the CT
Scheme
Unlike electromechanical relays, numerical relays do
not require a complicated CT scheme to match
measured currents. Instead, information about the
protected object and the CTs is entered into the relay’s
memory. Then, using the measured currents, the relay
performs any necessary calculations to match the
currents. The following CT information is required for
input into the relay:
• The nominal
primary rating of the CTs
• The nominal
secondary rating of the CTs
• The orientation of the CTs
This section describes these choices and settings.
6.3.1 Nominal Primary Rating
of CTs
Specify the primary rated current for each CT set:
6.3.2 Nominal Secondary Rating
of CTs
The relay is manufactured with either 1A or 5A current
inputs. The choice should match the rated nominal
secondary current of the current CTs monitoring the
protected bus. The relay is shipped with the same
rating for all current inputs (except for the tank leakage
current input, if ordered), but a hardware modification
can be made to have different rated currents (1A or
5A) for different current inputs (see Chapter 17.9). If
any current input will be rated 5A, order the relay with
5A current inputs.
6.3.3 Orientation of CTs
The orientation of the CTs monitoring each branch of
the protected bus must be specified in order to ensure
that the correct polarity is used when calculating the
differential currents:
For example, if terminal 3A2 (non-polarity, phase A
input of Winding 1) is connected to the end of the CT
closest to the busbar, Address 1303 should be set to
“
Towards Busbar
.”
1302 In CT 1 (for branch 1)
1304 In CT 2 (for branch 2)
1306 In CT 3 (for branch 3, 7UT513 only)
Rated primary current of the CTs fitted to each
bus branch.
Range: 1
–
100,000 A
Default: 2000
1303 CT1 STARPT (for branch 1)
1305 CT2 STARPT (for branch 2)
1307 CT3 STARPT (for branch 3, 7UT513 only)
Orientation of which end of each winding’s CTs are
connected to the corresponding non
-
polarity terminal of the
relay’s phase current inputs. For wye connected CTs, this is
the non
-
polarity ends of the CTs, which are usually
connected to a common point (or star
-
point)
Option Description
TOWARDS BUSBAR Towards the busbar
TOWARDS LINE Towards the line.