Sel 751A - Table 4.6 Residual Overcurrent Settings; Figure 4.2 Ground Fault Protection Using Core-Balance CT

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 Defau lt
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
ABC
1 A
10:1
Window CT
Feeder
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