Sel 751A, Instruction Manual

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4.8
SEL-751A Relay Instruction Manual Date Code 20100129
Protection and Logic Functions
Basic Protection
When a core-balance CT is connected to the relay IN input, as in Figure 2.24 ,
use the neutral overcurrent element to detect the ground faults. Calculate the
trip level settings based on the available ground fault current and the core-
balance CT ratio.
EXAMPLE 4.3 Ground Fault Core-Balance CT Application
A resistance-grounded power system limits the ground fault currents.
The resistor is sized to limit the current to 10 A primary. The three
feeder leads are passed through the window of a 10:1 core-balance CT.
The CT secondary is connected to the SEL-751A
IN
current input
(terminals Z07, Z08), as shown in Figure 4.2 . Setting the Neutral OC
CT Ratio (CTRN, see Figure 4.2 ) equal to 10 and Neutral Trip Lvl
(50N1P) equal to 0.5 A or lower with 0.10-second time delay ensures
that the element will quickly detect and trip for feeder ground faults.
Figure 4.2 Ground Fault Protection Using Core-Balance CT
Table 4.6 Residual Overcurrent Settings (Sheet 1 of 2)
Setting Prompt Setting Range
Setting Name :=
Factory Default
RES OC TRIP LVL OFF, 0.50–100.00 A
a
,
0.10–20.00 A
b
50G1P := OFF
RES OC TRIP DLY 0.00–5.00 sec 50G1D := 0.50
RES OC TRQ CON SEL
OGIC
50G1TC := 1
RES OC TRIP LVL OFF, 0.50–100.00 A
a
,
0.10–20.00 A
b
50G2P := OFF
RES OC TRIP DLY 0.00–5.00 sec 50G2D := 0.50
RES OC TRQ CON SEL
OGIC
50G2TC := 1
RES OC TRIP LVL OFF, 0.50–100.00 A
a
,
0.10–20.00 A
b
50G3P := OFF
RES OC TRIP DLY 0.00–5.00 sec 50G3D := 0.50
RES OC TRQ CON SEL
OGIC
50G3TC := 1
RES OC TRIP LVL OFF, 0.50–100.00 A
a
,
0.10–20.00 A
b
50G4P := OFF
Z07 Z08
10 A
C-Ground Fault
10 A
A B C
1 A
10:1
Window CT
Feeder 52