Basler Electric BE1-87T - Manual (Page 75)

9171300990 Rev V BE1-87T Installation 4-33
Setting the BE1-87T
The following setting procedures include two examples:
1. Using the MVA rating of the highest-rated winding for all the other windings when making the
calculations.
2. Using the top kVA rating of the transformer.
Each procedure can be used as a means to understand the principles involved, and by replacing the
variables of the example, can become a procedure of general application. Variable abbreviations and
definitions are provided in Table 4-1, List of Variables.
Method
Both procedures determine:
1. The matching tap and slope settings required to implement the restrained function, and
2. The unrestrained pickup setting as a multiple of the BE1-87T tap setting (i.e., the
INPUT
switches).
The matching tap procedure is conventional, providing tap values proportional to the normal currents as
seen by the relay. An exception occurs with multiple-winding banks where zero-balance current is
assumed in each pair of windings, successively.
NOTE
The dc component of the input current is effectively blocked by the gapped
cores of the input CTs. Therefore, for offset fault currents or magnetic inrush,
the dc component of the waveform can be ignored in fault current calculations.
Procedure One
Refer to Figure 4-35 for a one-line drawing of this example. Refer to Figure 4-34 for the three-line
representation of this transformer.
Step 1. Determine the primary current (I
P
) of each winding:
Tap and Phase Shift Settings
Use the MVA rating of the highest-rated winding for all the other windings when making the
calculations. (This procedure assures that the taps follow the voltage ratios. Refer to Appendix
A, Setting Note 1.)
HIGH TERTIARY LOW
I
I
P
P
=
=
250,000
13.2 3
10,935
Step 2. Determine the CT secondary current (I
S
)of each winding:
HIGH TERTIARY LOW
IP =
(MVA rating of transformer) (X 1,000)
V 3
LINE - LINE
( ) ( )
IP
IP =
=
250,000
345
418
3
I
I
P
P
=
=
250,000
138 3
1,046
I
I
S
P
=
CT ratio
I
I
S
S
=
=
418
120
349 .
I
I
S
S
=
=
10 935
600
1822
,
.
I
I
S
S
=
=
1046
240
4 36 .