Schweitzer Engineering SEL-387-0, Instruction Manual

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3.55
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Overcurrent Element
51PC1 and 51NC1 Element
Figure 3.26 shows the logic for the inverse-time combined overcurrent
elements, 51PC1 and 51NC1.
Figure 3.26 51PC1 and 51NC1 Combined Inverse-Time O/C Elements
The relay determines whether to assert Relay Word bit 51PC1 by selecting the
greater of the two CT ratios, using this ratio as a common base in calculating
the combined secondary current, and then comparing this combined secondary
current against the 51PC1P pickup setting. This pickup setting is a secondary
quantity that the relay calculates by dividing the primary current pickup by the
greater of the two CT ratios.
For CTR1<CTR2, the relay performs the following operation on the
secondary quantities it receives from the CTs:
Equation 3.34
where n = Phase A, Phase B, or Phase C.
The following example illustrates the equivalent operation on the primary
quantities entering the CTs. Assume:
CTR1 = 600/5 = 120
CTR2 = 2000/5 = 400
I n W1 = 2000 A (primary)
I n W2 = 1000 A (primary)
Pickup = 8000 A (primary)
where n = Phase A, Phase B, or Phase C.
_
+
51PC1
Pickup
Curve Type
Time Dial
51PC1 Wdg 1 and Wdg 2
Phase Inverse-Time
Overcurrent Element Curve
Timing and Reset Timing
Settings:
51PC1P
51PC1C
51PC1TD
51PC1RS E/M Reset? (Y/N)
Pickup
Curve Type
Time Dial
51NC1 Wdg 1 and Wdg 2
Residual Inverse-Time
Overcurrent Element Curve
Timing and Reset Timing
Settings:
51NC1P
51NC1C
51NC1TD
51NC1RS E/M Reset? (Y/N)
51PC1T
51PC1R
IAW1
+
51PC1P
_
+
51NC1
51NC1T
51NC1R
_
+
51NC1P
IAW2
IBW1
IBW2
ICW1
ICW2
IRW1
IRW2
Scalar
Multiply
∑
MAG
∑
MAG
∑
MAG
∑
MAG
Scalar
Multiply
Scalar
Multiply
Scalar
Multiply
Scalar
Multiply
Scalar
Multiply
Scalar
Multiply
Scalar
Multiply
MAX
InW1
CTR1
CTR2
--------------- •
 
 
InW2
+