GE 339 Instruction Manual page 174

S3 PROTECTION
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0
6–40
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 6-16: Motor Derating Factor due to Unbalanced Voltage
1 2 3 4
PERCENT VOLTAGE UNBALANCE
NEMA
HOT/COLD BIASING
When the motor is running with a constant load below the overload level, the motor will
eventually reach a steady state temperature, which corresponds to a particular steady
state Thermal Capacity Used. As some thermal capacity is used, there is less thermal
capacity left in the motor to cover transient overloads than is available when the motor is
cold. Typically, the extent of this effect is calculated by taking the ratio of the motor's rated
Safe Stall Time Hot to its rated Safe Stall Time Cold. Safe Stall Time (also known as Locked
Rotor Time) is the time taken with the rotor not turning, for the motor to heat, at an
unacceptable rate, to a temperature beyond which motor damage occurs. "Cold" refers to
starting off with the motor at ambient temperature, "Hot" refers to starting off with the
motor at the temperature reached when running at rated load. The method used by the
thermal overload curve to account for the pre-overload state, is thus known as hot/cold
biasing.
The Hot/Cold Ratio setpoint is determined by the equation shown below:
where: HCR is the value of the Hot/Cold Ratio setpoint expressed as a fraction of 1.00.
The steady state Thermal Capacity Used is calculated according to the equation:
where: TCU is the steady state Thermal Capacity Used expressed as a percentage. I
ss
the equivalent motor heating current in per-unit on an FLA base, which was discussed in
the unbalance biasing section above.
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0 1
5
PERCENT VOLTAGE UNBALANCE
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS
2 3 4 5
GE Multilin
896815.CDR
Eq. 9
Eq. 10
eq
k=2
k=4
k=6
k=8
k=10
is