Basler BE1-CDS240, Instruction Manual

Page: 557 / 580

NOTE
Unmonitored capacitor banks should be avoided if possible or they must be treated very
carefully to avoid false tripping due to inrush currents.
Step 1. Calculate desired minimum pickup setting in times tap. Choose the desired minimum pickup in
the factor X calculated
using the following equation. If the ideal tap (TAPn
I
) is equal to the actual tap (TAPn
A
), the factor
will be 1. For this example, this is the case, so the minimum pickup of 0.25 per unit will equal a
minimum pickup of 0.25 times tap.
per unit on the transformer full load base (i.e., 0.25). Multiply this times
CTRn KVn TAPn
TAPn
A
A
* * 3 *
COMPn MVA TAPn
X
I
* 1000 *
 
Step 2. Calculate the unmonitored load in times tap. Use the following equation:
9365200990 Rev F BE1-CDS240 Settings Calculations D-7
CTRn TAPn
COMPn pri Iunmon
Iunmon
*
*

0 Iunmon

Step 3. Calculate the Minpu setting. Add the two to get the minpu setting.
s.
As a reality check, convert the minimum sensitivity to high side and low side primary amperes. The
ickup settings in times tap can be related to primary amperes by this equation:
Step 4. Convert pickup setting to primary ampere
p
COMPn
CTRn TAPn Minpu
Ipri
* *

A Ipri 176
1
160 * 39 . 4 * 25 . 0
 
@ 230 kV
A Ipri 252
300 * 69 . 4 * 25 . 0
 
@ 115 kV
1
A 2930
1
800 6 14 25 0
Ipri
 
* . * .
@ 3.8 kV
Choose Unrestrained Pickup Setting
This two-step process selects the unrestraint element setting to provide security for both inrush and
external fault conditions.
Step 1. Calculate the expected inrush current based upon the self-cooled rating of the transformer.
Asymmetrical CT saturation during a heavy internal fault generates 2
nd
harmonics that can inhibit the
sensitive percentage restrained unit until the CT recovers. The unrestrained unit provides fast clearing for
these internal faults. Thus, the unrestrained unit should be set as sensitively as possible without
operating on inrush.
Typical numbers used in the industry for inrush have been eight to twelve times the self-cooled (bottom)
MUA rating of the transformer. Due to the operating characteristics of the BE1-CDS240 unrestrained
differential element, a setting of six times the self-cooled rating provides security for inrush. We can use a
lower
URO pickup setting and maintain security for unrestrained tripping because inrush current typically
25 0
0 1 25 0 Minpu
.
.

  