GE STD15C Instructions Manual page 22

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.
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