5.
THEORY
OF
OPERATION
5.1
RELAY BLOCK DIAGRAM
The simplified block diagram of Figure
5-1
shows the main features of the relay.
Further circuit details for troubleshooting at the component level are provided in
the Model 13g Relay Service Manual available from Multilin.
Each phase
is
isolated through a matching current transformer. The
AC
RMS
current
is
converted to the equivalent
DC
average voltage for each phase by
3 precision AC
to
DC converter/averager
circuits.
These three DC values are
then sampled at a rate of 3KHZ by
an
electronic switch. Each signal
is
amplified
according to the full load current control to normalize the signal
so
that
SOOmV
is
one full load regard
less
of
the actua I current this might represent. Other circuits
such
as
short
circuit,
rapid
trip
and
undercurrent
then
compare
the
actual
current
level
to
preset reference levels
to
determine
if
a
fault
exists.
The output of the 3 phase sampling multiplexer goes to a circuit which produces
the average
of
each of the phase RMS values. This average
is
alternately sub-
tracted from each of the 3 RMS phase values
in
the unbalance circuit. This
normalized value
is
then divided either by the full load current
or
the average
current during overloads to get the degree of unbalance. If this level exceeds the
unbalance control setting,
an
unbalance fault signal
is
generated.
Overload timing
is
determined by charging an integrator memory circuit at a
rate proportional
to
the average phase current squared. The output of the squar-
ing
circuit
is
placed
in
series
with
different
resistor
values
to
control
the integrator
charging current and
hence
the
appropriate run curve for any given level
of overload.
During startup mode,
as
determined by control circuitry, the integrator memory
charges at a rate determined by the start timer. This rate
is
either directly
in
terms of the start timer settings from a reference voltage or proportlonal to the
current squared when
using
the voltage compensation feature.
Only during idle operation
or
normal running will the memory integrator be given
a chance
to
slowly discharge. Emergency restart
or
no
memory
options
cause
the memory to discharge rapidly at these times. After the integrator reaches
the trip threshold, it must discharge to a much lower threshold before reset
is
possible producing the lockout characteristic. ·
A separate
relay
is
provided
for
alarm
functions
such
as
immediate overload
warning
and
undercurrent
both
of
which
are
based
on
phase
current level. RTD
and ground fault alarms are also both available.
A
constant
current
is
passed
through a remote RTD
sensor
in
the overtempera-
ture
option.
Three wires
and
a compensation circuit ensure
that
the temperature
sensed
is
not
affected
by
lead
resistance. RTD circuitry converts the
sensor
resistance value
to
a voltage of 10mV/°C. Separate adjustable comparators
are provided for detecting alarm and trip temperatures. A no sensor detection
circuit eliminates false tripping when no sensor
is
connected
or
the RTD
becomes damaged.
36
Courtesy of NationalSwitchgear.com
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