6–76 345 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
S3 PROTECTION CHAPTER 6: SETPOINTS
Negative sequence timed overcurrent (51_2)
The SR3 relay has two Negative Sequence Overcurrent elements per setpoint group. The
negative sequence overcurrent protection responds to negative sequence |I
2
|current,
where it is calculated as .
The negative sequence overcurrent elements are uniquely suited to detect phase-phase
faults and are not sensitive to balanced loads. While negative sequence elements do not
respond to a balanced load, they do detect the negative sequence current present in an
unbalanced load. For this reason, select an element pickup setting above the maximum
expected I
2
current due to load unbalance.
The Negative Sequence TOC1(2) Trip (Alarm) Pickup flag is asserted when the negative
sequence current is above the PKP value. The Negative Sequence TOC1(2) Trip (Alarm)
operate flag is asserted if the element stays picked-up for the time defined by the selected
inverse curve and the magnitude of the current. The element drops from pickup without
operation if the measured current drops below 97-99% of the pickup value, before the time
for operation is reached. When Definite Time is selected, the time for Negative Sequence
TOC operation is defined only by the TDM setting. The selection of Definite Time has a base
time delay of 0.1s, multiplied by the selected TD multiplier. For example, the operating time
for TOC set to Definite Time and a TDM set to 5 will result in 5*0.1 = 0.5s.
PATH:
SETPOINTS > S3 PROTECTION > SETPOINT GROUP 1(2) > NEG SEQ TOC1(2)
NEG SEQ TOC1(2) FUNC
Range: Disabled, Latched Alarm, Alarm, Trip
Default: Disabled
For details see Common setpoints
.
If the Trip setting is selected, the IOC menu allows the user to select the operation
ofeither one of the two trip output relays “RLY1 TRIP W1 BKR”, “RLY2 TRIP W2 BKR”, or
both.
PHASE CT INPUTS
Range: CT(W1), CT(W2)
Default: CT(W1)
The Negative Sequence TOC menu provides selection for either CT(W1) input currents or
CT(W2) currents. If CT(W1) is selected, the NEG SEQ TOC element will use CT ratings
associated with transformer winding 1. If the CT(W2) setting is selected, the relay will use
the CT ratings associated with transformer winding 2.
NEG SEQ TOC1(2) PKP
Range: 0.05 to 20.00 x CT in steps of 0.01 x CT
Default: 1.00 x CT
This setting defines the negative sequence TOC pickup level associated with the ratings
of the CTs selected under NEG SEQ CT INPUT.
For example if CT(W1) = 100:5, and the CT(W2)=500:5, a pickup of 0.5 xCT with NEG SEQ
CT INPUT = CT(W1) will be equivalent to a pickup of 0.5x100 = 50Amps negative
sequence current. The same pickup setting with NEG SEQ CT INPUT selected as CT(W2)
will lead to a pickup threshold of 250 Amps neg. sequence current
NEG SEQ TOC1(2) CURVE
Range: - ANSI Extremely/Very/Moderately/Normally Inverse- Definite Time- IEC Curve A/
B/C and Short Inverse- IAC Extremely/Very/Inverse/Short
Default: Ext Inverse
This setting defines the shape of the selected overcurrent inverse curve.
To Next Page
Loading...
