GE 489 Instruction Manual page 99

4 SETPOINT PROGRAMMING
Unbalanced phase currents will cause additional rotor heating that will not be accounted for by electromechanical relays
and may not be accounted for in some electronic protective relays. When the generator is running, the rotor will rotate in the
direction of the positive sequence current at near synchronous speed. Negative sequence current, which has a phase rota-
tion that is opposite to the positive sequence current, and hence, opposite to the rotor rotation, will generate a rotor voltage
that will produce a substantial rotor current. This induced current will have a frequency that is approximately twice the line
frequency, 100 Hz for a 50 Hz system or 120 Hz for a 60 Hz system. Skin effect in the rotor bars at this frequency will cause
a significant increase in rotor resistance and therefore, a significant increase in rotor heating. This extra heating is not
accounted for in the thermal limit curves supplied by the generator manufacturer as these curves assume positive
sequence currents only that come from a perfectly balanced supply and generator design.
The 489 measures the ratio of negative to positive sequence current. The thermal model may be biased to reflect the addi-
tional heating that is caused by negative sequence current when the machine is running. This biasing is done by creating
an equivalent heating current rather than simply using average current ( I
the equation shown below.
2 2
I = I + kI
eq 1 2
where: I = equivalent motor heating current in per unit (based on FLA)
eq
I = negative-sequence current in per unit (based on FLA)
2
I = positive-sequence current in per unit (based on FLA)
1
k = constant relating negative-sequence rotor resistance to positive-sequence rotor resistance, not to be confused
with the k indicating generator negative-sequence capability for an inverse time curve.
The figure below shows induction machine derating as a function of voltage unbalance as recommended by NEMA
(National Electrical Manufacturers Association). Assuming a typical inrush of 6 × FLA and a negative sequence impedance
of 0.167, voltage unbalances of 1, 2, 3, 4, and 5% equal current unbalances of 6, 12, 18, 24, and 30%, respectively. Based
on this assumption, the GE curve illustrates the amount of machine derating for different values of k entered for the
setpoint. Note that the curve created when k = 8 is almost identical to the NEMA derating curve.
ANCE BIAS K FACTOR
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0 1 2
PERCENT VOLTAGE UNBALANCE
NEMA
If a k value of 0 is entered, the unbalance biasing is defeated and the overload curve will time out against the measured per
unit motor current. k may be calculated conservatively as:
175
k (typical estimate); k
= --------- -
2
I
LR
GE Power Management
3 4 5
230
(conservative estimate), where I
= --------- -
2
I
LR
489 Generator Management Relay
4.10 S9 THERMAL MODEL
4.10.3 UNBALANCE BIAS
). This equivalent current is calculated using
per_unit
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0 1 2
PERCENT VOLTAGE UNBALANCE
GE POWER MANAGEMENT
is the per unit locked rotor current
LR
4
UNBAL-
k=2
k=4
k=6
k=8
k=10
3 4 5
4-61