GE Multilin 489 Instruction Manual page 147

CHAPTER 5: SETPOINTS
Note
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
Courtesy of NationalSwitchgear.com
NEG. SEQUENCE O/C
MESSAGE
RESET RATE: 227.0 s
Rotor heating in generators due to negative sequence current is a well known
phenomenon. Generators have very specific capability limits where unbalanced current is
concerned (see ANSI C50.13). A generator should have a rating for both continuous and
also short time operation when negative sequence current components are present.
The short time negative-sequence capability of the generator is defined as follows:
where: K = constant from generator manufacturer depending on size and design;
I = negative sequence current as a percentage of generator rated FLA as
2
measured at the output CTs;
t = time in seconds when I
setpoint).
The 489 has a definite time alarm and inverse time overcurrent curve trip to protect the
generator rotor from overheating due to the presence of negative sequence currents.
Pickup values are negative sequence current as a percent of generator rated full load
current. The generator FLA is calculated as:
Generator FLA ------------------------------------------------------------------------------------------------------------- -
=
3
Negative sequence overcurrent maximum time defines the maximum time that any value
of negative sequence current in excess of the pickup value will be allowed to persist before
a trip is issued. The reset rate provides a thermal memory of previous unbalance
conditions. It is the linear reset time from the threshold of trip.
Unusually high negative sequence current levels may be caused by incorrect phase CT
wiring.
FIGURE 5–3: Negative-Sequence Inverse Time Curves
Range: 0.0 to 999.9 s in steps of 0.01
2
K I T
=
2
> pickup (minimum 250 ms, maximum defined by
2
Generator Rated MVA
× Rated Generator Phase-Phase Voltage
(EQ 5.10)
(EQ 5.11)
808791A2.CDR
5–37