4.77
Date Code 20100129
Instruction Manual
SEL-751A Relay
Protection and Logic Functions
Demand Metering
Figure 4.40
Demand Current Logic Outputs
For example, when residual ground demand current IGD goes above
corresponding demand pickup GNDEMP, Relay Word bit GNDEM asserts to
logical 1. Use these demand current logic outputs (PHDEM, GNDEM, and
3I2DEM) to alarm for high loading or unbalance conditions.
The demand values are updated approximately once a second. The relay stores
peak demand values to nonvolatile storage every six hours (it overwrites the
previous stored value if it is exceeded). Should the relay lose control power, it
will restore the peak demand values saved by the relay.
Demand metering peak recording is momentarily suspended when SEL
OGIC
control equation setting FAULT is asserted (= logical 1). The differences
between thermal and rolling demand metering are explained in the following
discussion.
Comparison of
Thermal and Rolling
Demand Meters
The example in
shows the response of thermal and rolling demand
meters to a step current input. The current input is at a magnitude of zero and
then suddenly goes to an instantaneous level of 1.0 per unit (a “step”).
PHDEM
Relay
Word
Bits
Demand Current
Pickup Settings
(A Secondary)
IBD
IAD
IA
ICD
Analog
Quantities
Demand
Currents
Instantaneous
Currents
Serial Port
Command
Demand Function
Maximum
Current
Logic
Thermal (EDEM = THM)
or
Rolling (EDEM = ROL)
Reset
Demand
PHDEMP
IGD
Reset
Demand
Thermal (EDEM = THM)
or
Rolling (EDEM = ROL)
GNDEM
GNDEMP
IG
(residual)
Demand Function
3I2D
Reset
Demand
Thermal (EDEM = THM)
or
Rolling (EDEM = ROL)
3I2DEM
3I2DEMP
3I2
MET RD
Demand Function
Summary of Contents for 751A
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