Sidebar 13-3. Maximum Restraint Operating Characteristic – Continued
Under test and in-service conditions, the following equations determine whether the relay operates
along the flat minimum pickup or slope of the operating characteristic. If:
I
restraint at balance
> minpu x
1
100
/
slope
1
in per unit, use Equation 1, or if
I
restraint at balance
< minpu x
1
100
/
slope
1
in per unit, use Equation 2.
Example (Increasing One Input from Balance):
Note: For simplicity, this discussion is based on a two input differential application. The principals
extend to three and four winding applications.
TAP 1 = 2.00 Minpu = 0.30
TAP 2 = 3.80 Slope = 15%
Input 1 current at balance = 3 per unit (6 amperes)
Input 2 current at balance = 3 per unit (11.40 amperes)
Input 1 is being increased relative to a fixed input 2 current. Thus, the maximum restraint used by the
relay is Input 1, the larger of the two currents in per unit.
I
restraint at balance
= 3 > 0.3
1
15
.
0
1
= 1.70 per unit
9365200990 Rev F
BE1-CDS240 Testing and Maintenance
13-39
Then,
I
1trip
= I
1balance
/(1-(
100
slope
))
= 3/(1-0.85) = 3.53 per unit (see the following figure.)
1
2
3
4
5
MAXIMUM RESTRAINT CURRENT (IN MULTIPLES OF TAP)
OPERA
T
IN
G
CURR
E
N
T
(
IN
M
U
LT
IP
LE
S
OF
TA
P)
123
Slope = I
OP
*100/I
RESTR
= 15%
minimum pickup
= 0.30 x tap
Differential current
3.53
Maximum Restraint Characteristic Example
Summary of Contents for BE1-CDS240
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