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CHAPTER 6: SETPOINTS
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
a significant increase in rotor resistance and therefore significant additional rotor heating.
This extra heating is not accounted for in the thermal limit curves supplied by the motor
manufacturer, as these curves assume only positive sequence currents from a perfectly
balanced supply and balanced motor construction.
To account for this additional heating, the relay allows for the thermal overload curve to be
biased with negative sequence current. This biasing is accomplished by using an
equivalent motor heating current rather than the simple motor terminal current (I
equivalent current is calculated according to the equation:
where:
I
= equivalent motor heating current in per-unit on an FLA base
eq
I
= average of each motor terminal's RMS current in per-unit on an FLA base I
avg
negative sequence to positive sequence current ratio k = value of the Unbalance K Factor
setpoint, which is used to adjust the degree of unbalance biasing.
k may be estimated as:
where I
is the locked rotor current in per-unit on an FLA base.
LR
If a k value of 0 is entered, the unbalance biasing is defeated and the overload curve will
time out against the average per-unit motor current.
The figure below shows the recommended motor derating as a function of voltage
unbalance recommended by NEMA (the National Electrical Manufacturers Association). To
illustrate this relay's unbalance biasing, assume a typical induction motor with an inrush of
6 x FLA and a negative sequence impedance of 0.167. With this impedance, voltage
unbalances of 1, 2, 3, 4, and 5% on the motor terminals will result in current unbalances of
6, 12, 18, 24, and 30% respectively. Based on these assumptions, the derating resulting
from this relay's unbalance biasing for different values of k is as illustrated in the GE Multilin
curve below. Note that the curve for k = 8 is almost identical to the NEMA derating curve.
Figure 15: Motor Derating Factor due to Unbalanced Voltage
1
2
3
4
PERCENT VOLTAGE UNBALANCE
NEMA
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0
1
5
PERCENT VOLTAGE UNBALANCE
S3 PROTECTION
). This
avg
Eq. 7
/ I
=
2
1
Eq. 8
2
3
4
5
GE Multilin
896815.CDR
6–63
k=2
k=4
k=6
k=8
k=10
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