Schweitzer Engineering Laboratories SEL-387-0, Инструкция по эксплуатации

Страница: 120 / 568

3.64
SEL-387-0, -5, -6 Relay Instruction Manual
Date Code 20050919
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Overcurrent Element
The following example illustrates the equivalent operation on the primary
quantities entering the CTs. Assume:
CTR1 = 1000/5 = 200
CTR2 = 500/5 = 100
Pickup = 400 A (primary)
Then
Equation 3.52
Because CTR1>CTR2, the relay uses CTR1 as the common base.
Equation 3.53
The combined secondary current value is greater than the 51NC1P setting, so
51NC1 asserts and the time overcurrent curve begins timing.
As with the phase current element, four settings define the curve: 51NC1P is
the pickup setting, 51NC1C defines the curve equation, 51NC1TD defines the
time dial, and 51NC1RS determines how the curve resets.
Curve time-out causes the Relay Word bit 51NC1T to assert. When the current
drops below pickup, 51NC1 deasserts and the element resets according to the
setting 51NC1RS. At the completion of the reset, Relay Word bit 51NC1R
asserts. This bit normally will be at logic state 1, when the element is at rest
during normal system operation. You can verify the state of the bit using the
defines a vector quantity.
Primary Currents
IAW1 = 3000
∠
5° A
IAW2 = 1545
∠
3° A
IBW1 = 3010
∠
–123° A
IBW2 = 1480
∠
–118° A
ICW1 = 2950
∠
115° A
ICW2 = 1505
∠
124° A
Secondary Currents
IAW1 = 15
∠
5° A
IAW2 = 15.45
∠
3° A
IBW1 = 15.05
∠
–123° A
IBW2 = 14.80
∠
–118° A
ICW1 = 14.75
∠
115° A
ICW2 = 15.05
∠
124° A
XXXX
15 5
°
A
∠
15.05 123
°
A –
∠
14.75 115
°
A
∠
+ +
( )
+ 15.45 3
°
A 14.80 118
°
A – 15.05 124
°
A
∠
+
∠
+
∠ ( )
100
200
---------
•
2.1164 76
°
A
∠
0.11270 73
°
A
∠
+ = 2.230 76
°
A
∠
=
51NC1P
400 A
200
-------------- 2 A secondary = =