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Date Code 20130726 Instruction Manual SEL-710 R

Protection and Logic Functions

Basic Motor Protection

EXAMPLE 4.8 Ground Fault Core-Balance CT Application

A resistance-grounded transformer limits the current for motor orcable ground faults. The resistor is sized to limit the current to 10 Aprimary. The three motor leads are passed through the window of a 10:1

core-balance CT. The CT secondary is connected to the SEL-710 IN

current input (terminals Z07, Z08), as shown in Figure 4.5. Setting theNeutral OC CT Ratio (CTRNsee Group Settings (SET Command) on page 4.3)equal to 10 and Neutral Trip Lvl (50N1P) equal to 5 A with a 0.10-secondtime delay ensures that the element will quickly detect and trip formotor ground faults, but prevent misoperation resulting from unequalbreaker or contactor pole closing times.

Figure 4.5 Ground Fault Protection Using Core-Balance CT

When a core-balance CT is not available, use the 50G residual overcurrent

elements. Set the residual overcurrent trip level (50G1P) between one-half and

one-fifth of the full-load phase current, and set the residual overcurrent trip

delay (50G1D) equal to 0.2 seconds. Set the warn level more sensitively, but

with a longer time delay. The long time delay allows the sensitive warn

element to ride through the false residual current that can be caused by phase

CT saturation during motor starting.

.

Table 4.11 Residual Overcurrent Settings

Setting Prompt Setting Range Setting Name := Factory Default

RES OC TRIP LVL OFF, 0.1020.00 xFLA 50G1P := OFF

RES OC TRIP DLAY 0.005.00 s 50G1D := 0.50

RES OC WARN LVL OFF, 0.1020.00 xFLA 50G2P := OFF

RES OC WARN DLAY 0.0120.0 s 50G2D := 10.0

Table 4.12 Negative-Sequence Overcurrent Settings

Setting Prompt Setting Range Setting Name := Factory Default

NSEQ OC TRIP LVL OFF, 0.1020.00 xFLA 50Q1P := 3.00

NSEQ OC TRIP DLY 0.1120.0 s 50Q1D := 0.1

NSEQ OC WARN LVL OFF, 0.1020.00 xFLA 50Q2P := 0.30

NSEQ OC WARN DLY 0.1120.0 s 50Q2D := 0.2

Z07

Z08

10 A

C-Ground Fault10 A

A B C

1 A

10:1

Window CT

Motor

NOTE: Phase CT ratios are typicallyhigher than core-balance (CB) CTratios. For this reason, the relaysensitivity to motor ground faults isless when the residual overcurrentelement is used instead of the CBelement. A separate ground faultdetection method should be used if aCB CT is not available in applicationswhere resistance grounding reducesthe available ground fault current.

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