GE STD15C - ADJUSTMENTS; TAP PLUG POSITIONING; UNBALANCE CURRENT MEASUREMENT

GEK—45307
ADJUSTMENTS
TAP
PLUG POSITIONING
Ratio Matching
Adjustment
To obtain a minimum
unbalance current in the
differential circuit,
means are
provided in
the STD relay
to
compensate for
unavoidable difference in
current—
transformer ratios. Taps on
the
relay transformer
primary windings
are rated 8.7
(1.74),
5.0
(1.0), 4.6 (0.92),
3.8
(0.76),
3.5 (0.7), 3.2 (0.64), and 2.9
(0.58)
amperes for each
line—current
transformer. The
tap plugs should
be moved
to
the
locations which most
nearly
match the expected CT currents for
the same
kVA
assumed
in each of the power transformer windings. The selection
of
taps
should
be guided
by
the method outlined
under CALCuLATIONS.
CAUTION
The
connection plug must
be
removed
from
the
relay
before
changing
tap
positions, in
order
to prevent open-circuiting
a CT secondary. A check
should be made after changing
taps, to
ensure that only
one plug Is
left
in any horizontal
row
of
tap holes. Inaccurate calibration and
overheating may result if more than one plug is connected
to
any
one
winding.
Unbalance Current Measurement
Unbalance
current measurement is useful in checking
the best tap setting when
matching
current-transformer ratios in
the
field.
It
is
also useful in
detecting
errors
or
faults
in the current
transformer
winding,
or small faults within
the
power
transformer
itself,
where the fault current is too
low
to operate
the relay.
The
type
STO relays have
a
special arrangement for measuring the unbalance
current
flowing in
the
differential circuit
without disturbing the relay
connections.
Provision is made for temporarily connecting a 5
V
high—resistance
AC voltmeter
(1000 or
more ohms per volt) across the secondary of the
differential current
transformer.
This
may be done by connecting the meter across terminals
8
and
9 (see
Figures 10 and
11).
When a perfect
match of relay currents
is obtained
by
the
ratio-matching taps,
the voltmeter will read
0,
indicating
no unbalance.
If the
voltmeter reads 1.5
volts
or less,
the unbalance current entering
or
leaving
a
given
tap
equals approximately 0.03 times the voltmeter reading times the tap rating. For
higher
voltmeter readings, the
approximate
unbalance
current
may be calculated by
substituting the voltage
reading and
tap
rating into the following
equation:
I
(unbalance) = (0.16 x
V
— 0.2) x Tap
The
unbalance percentage equals
100 times the unbalance
current divided by the
measured
tap current. For a
three—winding bank, this must
be checked with load on
at least
two pairs of
windings
In
order
to
ensure
that the connections are correct.
The curves
in Figure 17
show the approximate voltage across terminals
8
and 9
required to
operate the relay,
for various percent slope
tap
settings, and through—
currents
expressed as
percentage of tap.
To ensure
a
margin
of safety against false
operation,
the
unbalance
voltage should not exceed 75% of that required
to
operate
the relay
for
any
given through-current
and percent slope
tap setting. This extent
of
unbalance may
result from the relatively
high error currents
of low-ratio bushing
CTs
at
low multiples of tap
current.
22