Basler BE1-CDS220, Instruction Manual

Page: 464 / 476

D-6 Settings Calculations BE1-CDS220
Step 4: Convert pickup setting to primary amps.
As a reality check, convert the minimum sensitivity to high side and low side primary amperes. The pickup
settings in times tap can be related to primary amperes by Equation E-8.
COMPn
CTRn TAPn Mpu
Ipri
∗ ∗
=
kV 69 @ A 41
1
60 79 . 2 25 . 0
=
∗ ∗
=
Ipri
kV 12.47 @ A 231
1
160 79 . 5 25 . 0
=
∗ ∗
=
Ipri
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 sensitive as possible without operating
on inrush.
Typical numbers used in the industry for inrush have been eight to twelve times the rating of the
transformer. Due to the operating characteristics of the BE1-CDS220 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 has a high peak
that is non-sinusoidal. The relay extracts the fundamental component of the operate current for use by the
unrestrained element to greatly reduce its sensitivity to this non-sinusoidal inrush current. Also, the high
initial peak generally decays quickly to a lower value.
Since the tap values are based upon the forced cooled rating and adjusted if necessary by the factor
X calculated in Calculate Minpu, Step 1, Equation D-6, the pickup can be calculated by Equation D-9.
cooled forced MVA
cooled self MVA
X URO
∗ ∗ =
6
6 . 3
20
12
1 6
= ∗ ∗ =
URO
Equation D-9
This URO value (3.6 rounded up to 4.0) is the minimum setting to avoid tripping during inrush. The next
step will determine the value required to avoid tripping for the maximum external fault under worst case
CT saturation.
Calculate Maximum External Fault
Step 1: Calculate the maximum external fault (IE) in multiples of tap. Use Equation D-10 or D-11.