Basler Electric BE1-87T - Manual (Page 89)

9171300990 Rev V BE1-87T Installation 4-47
• For delta-connected CTs, or with ∆ 1 or ∆ 2 jumpers on Analog Board #2, shown in Figure
2-4, (and based on phase-to-phase fault): (See Setting Note 3.)
I
I
F3
T
E =
3 ( )
HIGH LOW
I
E
=
=
32
2 0
15.96
.
I
E =
=
66 3
7 2
15.9
( )
.
Step 18. Determine the unrestrained pickup level in multiples of tap (X TAP): (See Setting Note 4.)
HIGH LOW
X TAP =
=
(0.7)15.96
11.17
I
E
not maximum
This calculation assumes that the CTs carrying the maximum fault saturate severely, yielding only 30% of
the expected ratio current. This leaves 70% of the fault current as a false differential current.
NOTE
The restrained element will not operate due to the large 2nd harmonic
component present in the highly distorted current.
Step 19. Using the results of Step 18, set the UNRESTRAINED PICKUP LEVEL control.
Referring to the table on the BE1-87T front panel, select the tap position ( X TAP) that is higher
than the result obtained in Step 18. Therefore, for this example, select SET position G (=12 X
TAP) which is higher than the above result of 11.17 X TAP.
If this value exceeds 21 (max setting), raise the tap settings toward the upper end of the tap
range. If after the highest tap has been reached the unrestrained trip settings still exceeds 21,
security is affected. The user should remember that the 70% saturation is conservative. A close
look at the system L/R and CT performance is recommended. Chances are that the risk will be
tolerable.
The slope equation determines the slope setting required to maintain a margin of about 12% of I
OP
at the
breakpoint of the slope characteristic. This margin varies slightly with the actual taps but remains secure
over the tap range.
Slope Setting
Step 20. Determine the multiples of self-cooled current (I
T
): Refer to Appendix A, Setting Note 5.
Where:
I
R
= relay current (from Step 4)
T = the input tap (from Step 9)
MVA
SELF COOLED
and MVA
FORCE COOLED
are given in Figure 4-35.
( ) = 0.7 X TAP x I Max
E
( .)
I
I
T
T
R
=
MVA
MVA
SELF-COOLED
FORCED-COOLED
( )
( )