GE 745 TRANSFORMER MANAGEMENT RELAY, Instruction Manual

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GE Power Management 745 Transformer Management Relay 5-41
5 SETPOINTS 5.4 S2 SYSTEM SETUP
5
5.4.10 DEMAND METERING
This section assigns the demand setpoints for monitoring current demand on all three phases of each wind-
ings. Current demand on the 745 is performed one of three ways: Thermal, Rolling Demand or Block Interval.
y
DEMAND METERING
y
[ENTER] for more
This message indicates the start of the
DEMAND METERING
section. To
continue press or press to go to the next section.
CURRENT DEMAND METER
TYPE: Thermal
Range: Thermal, Block Interval, Rolling Demand (see table below).
Select the method to be used for the current demand metering.
THERMAL
Select ‘Thermal’ to emulate the action of an analog peak-recording thermal demand meter. The 745 measures
the current on each phase every second, and assumes the circuit quantity remains at this value until updated by
the next measurement. It calculates the "thermal demand equivalent" based on the following equation:
where d = demand after applying input for time t (in minutes)
D = input quantity (constant)
k = 2.3 / thermal 90% response time
The graph above shows the thermal response characteristic for a thermal 90% response time of 15 minutes. A
setpoint establishes the time to reach 90% of a steady-state value, just as the response time of an analog
instrument (a steady-state value applied for twice the response time will indicate 99% of the value).
BLOCK INTERVAL
Select ‘Block Interval’ to calculate a linear average of the current over the programmed demand TIME
INTERVAL, starting daily at 00:00:00 (i.e. 12 am). The 1440 minutes per day is divided into the number of blocks
as set by the programmed time interval. Each new value of demand becomes available at the end of each time
interval.
ROLLING DEMAND
Select ‘Rolling Demand’ to calculate a linear average of the current over the programmed demand TIME INTERVAL
(in the same way as Block Interval). The value is updated every minute and indicates the demand over the time
interval just preceding the time of update.
THERMAL 90% RESPONSE
TIME: 15 min.
Range: 5 min. / 10 min. / 15 min. / 20 min. / 30 min. / 60 min.
This message is displayed only when the CURRENT DEMAND METER TYPE is
set for Thermal . Enter the time required for a steady-state current to indicate
90% of actual value.
TIME INTERVAL:
20 min.
Range: 5 min. / 10 min. / 15 min. / 20 min. / 30 min. / 60 min.
This message is displayed only when the CURRENT DEMAND METER TYPE is
set for
Block Interval
or
Rolling Demand
. Enter the time period over which the
current demand calculation is performed.
E N T E R M E SSA G E
d t
( )
D 1 e
kt
–
–
( )
=
Time (min)
0
20
40
60
80
100
0 3 6 9 12 15 18 21 24 27 30
Demand
(%)