IEEE
-
IXPS
PANEL
DISCUSSION
Power System Protection
-
Power Monitoring
-
Data Logging
-
Remote Interrogation System
BRUCE
G.
BAILEY,
MEMBER
IEEE
Siemens Energy
&
Automation, Inc.
P.O.
Box 29503
Raleigh, North Carolina 27626
The power monitoring system described uses
microprocessor technology to provide protection,
real-time status displays and event logging for
industrial
AC
power distribution and generation
systems. The system consists of a display and
monitoring unit connected by a single twisted-pair
EIA RS-485 communications interface to as many
as 32 microprocessor-control led power meters and/or
trip
units
on low voltage power circuit breakers.
Both metering
and
protection functions use
RMS
sensing to properly account for harmonics
that
distort waveforms.
HARDWARE
The power
meter
measures voltage
and
current for
each phase plus the averages,
kVA,
kW,
kW
demand,
kVAR,
Amps demand, power factor, frequency,
kWHR,
and
kVARHR.
It receives
its
signals from standard
voltage and current transformers
and
functionally
replaces
up
to 12 individual meters and transducers
in one package. It
is
normally not provided
with
a display,
but
communicates all data to a central
display and monitoring unit. It is mounted in
an
auxiliary compartment of the switchgear.
In addition to the metering functions, it provides
four discrete status inputs to monitor user-selected
contacts such as circuit breaker position,
transformer fan status and the like. One of the
the discrete inputs also acts as a counter, allowing
pulsed outputs from other devices to be read into
the system.
A
typical application is reading utility
meter
kWHR
pulses. Control relay outputs can be
added as
an
option and are Programmable to open
or close on any measured parameter for demand or
load control, or to provide protective function
contacts for under/over voltage, voltage unbalance,
phase reversal and current unbalance. Nonvolatile
memory stores all set-up and historical data.
All
real-time measured values, setup, and historical
data can be communicated to the display and moni-
toring
unit.
The optional local display has large, easily read
characters to simultaneously show volts
,
amps and
a selectable power function. Sealed membrane function
keys are easy to use to scroll through the phases
and power functions. Accuracy
is
0.25% for volts
and amps, and
0.5%
for power functions, which is
an
improvement over traditional instruments.
The trip unit
is
the familiar low voltage power
circuit breaker protective system which mounts
integrally on the circuit breaker and derives its
current signal and tripping energy from current
sensors mounted on the circuit breaker. The
unit
is
available
in
several versions covering the entire
range of requirements, including:
-
When
unit
long time delay with pickup
LED
selectable thermal memory (for motor over-
load protection)
instantaneous tripping
microprocessor watchdog
LCD
target
t;o
indicate trip-out due to
"overload", short circuit", or "ground
faul ti' and microprocessor watchdog error
detection to indicate "disabled"
short time delay tripping with field
selectable I2t ramp, pickup
LED
and
LCD
target stating "short circuit"
ground fault tripping with I2t
ramp,
pickup
LED
and
LCD
target stating "ground fault"
zone interlocking for both short time and
ground fault trip
used in the power monitoring system, the
trip
is enhanced with communication caDabilitv
so
that
RMS
phase and ground current val'ues can
be read. It can also signal long time delay pick-up,
tripping on overload, short circuit, ground fault,
or watchdog self-check operation, and provide
programmable current unbalance
tripping.
The
interconnections for zone interlocking are done
separately from the communications bus.
The trip device protective functions are independent
of the communications link. Tripping of the circuit
breaker will not be compromised by failure
of
the
communications
bus.
The display and monitoring unit has
a
9-inch
monochrome screen for data and graphics display,
large sealed membrane keypad,
and
is
normally mounted
on the switchgear in
a
central location. It provides
a
variety of information display screens and user
programmable features, and maintains a 64-entry
event log
in
its
nonvolatile memory.
Screens include
single-line diagrams of selected portions of the
power system, large character displays of summary
data, and tabular displays of detailed data from
the power meter and
trip
units
connected
to
it.
The display and monitoring unit is the user's access
to the system. The user can view data acquired
from the devices, and can configure the
unit
to
respond to data and events as needed by a particular
installation. System configuration capability
is
password protected and is done at the factory
before shipment.
Detailed instructions are provided
explaining how to further personalize the system
to the needs of the application.
A
packet-based
communications protocol with error checking provides
the necessary integrity of transmitted data.
The display and monitoring unit can also be provided
with a second communication port for transmitting
information to a remote
PC
host.
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