GE 489 - Manual (Page 78)

4-28 489 Generator Management Relay GE Multilin
4.6 S5 CURRENT ELEMENTS 4 SETPOINTS
4
defines the short time negative sequence capability of the generator (EQ 4.10)
where: K = constant from generator manufacturer depending on generator size and design
I
2
= negative sequence current as a percentage of generator rated FLA as measured at the output CTs
t = time in seconds when I
2
> pickup (minimum 250 ms, maximum defined by 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:
(EQ 4.11)
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 4–3: NEGATIVE SEQUENCE INVERSE TIME CURVES
K I
2
2
T =
Generator FLA
Generator Rated MVA
3 Rated Generator Phase-Phase Voltage ×
---------------------------------------------------------------------------------------------------------------------- =
NOTE
808791A2.CDR