GE 489 Instruction Manual page 123

4 SETPOINTS
The 489 can measure the demand of the generator for several parameters (current, MW, Mvar, MVA). The demand values
of generators may be of interest for energy management programs where processes may be altered or scheduled to
reduce overall demand on a feeder. The generator FLA is calculated as:
Generator FLA
Power quantities are programmed as per unit calculated from the rated MVA and rated power factor.
Demand is calculated in the following manner. Every minute, an average magnitude is calculated for current, +MW, +Mvar,
and MVA based on samples taken every 5 seconds. These values are stored in a FIFO (First In, First Out buffer).The size
of the buffer is dictated by the period that is selected for the setpoint. The average value of the buffer contents is calculated
and stored as the new demand value every minute. Demand for real and reactive power is only positive quantities (+MW
and +Mvar).
where: N = programmed Demand Period in minutes,
n = time in minutes
160
140
120
100
80
60
40
20
0
GE Multilin
Generator Rated MVA
--------------------------------------------------------------------------------------------------------------------- -
=
3 × Generator Rated Phase-Phase Voltage
N
1
∑
Demand --- -
=
N
n 1
=
t=0 t+10 t+20 t+30 t+40
Figure 4–21: ROLLING DEMAND (15 MINUTE WINDOW)
489 Generator Management Relay
Average
N
t+50 t+60 t+70 t+80 t+90 t+100
TIME
4.11 S10 MONITORING
(EQ 4.37)
(EQ 4.38)
808717A1.CDR
4-73
4