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7.6
SEL-400 Series Relays
Instruction Manual
Date Code 20171006
Metering
Demand Metering
FAULT SEL
OGIC
Control Equation
The relay suspends updating maximum/minimum metering when SEL
OGIC
con-
trol equation FAULT asserts to logical 1. If there is a fault, the elements pro-
grammed in FAULT pick up and assert Relay Word bit DFAULT (Delayed
FAULT Suspend). This Relay Word bit remains asserted for one minute after
SEL
OGIC
control equation FAULT deasserts. While DFAULT is asserted, the
relay does not record maximum/minimum data.
In addition, the relay also suspends demand metering during the time that Relay
Word bit DFAULT is asserted.
Demand Metering
Economic operation of the power system involves the proper allocation of the
load demand among the available generating units. By knowing the demand
requirements at different points in the system and at different times of the day
you can optimize your system generation resources or your consumption of elec-
tric power. The relay provides you this demand information and enables you to
operate your power system with an effective economic strategy.
NOTE:
Not all SEL-400 series relays
support demand metering.
The relay uses longer-term accumulations of the metering quantities for reliable
demand data.
Thermal Demand and Rolling Demand
Two methods exist for measuring power system current and power demand.
These methods are thermal demand metering and rolling demand metering.
illustrate the step input response of the two demand
measuring methods with setting DMTC (demand meter time constant) at 15 min-
utes.
Thermal Demand
Thermal demand is a continuous exponentially increasing or decreasing accumu-
lation of metered quantities. Thermal demand measurement is similar to parallel
RC network integration. Thermal demand metering response is at 90 percent
(0.9 per unit) of the full applied value after a period equal to the DMTC setting
(15 minutes in
).
Summary of Contents for SEL-421-4
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