Connections; Unbalance Current Measurement - GE STD15C Instructions Manual

Internal connection diagrams are shown in Figs. 10 and 11.
Figs. 7. 8 and g for differential applications.
Of course, any through current
provided the taps are properly chosen.
the relay is mounted on an insulating panel, one of the
cntly grounded by a conductor not less than No. 12 AUG gage copper wire
Every circuit in the
the connecting plug should engage first.
make sure the auxiliary brushes are bent high enough to engage the connecting plug or test plug before the
main brushes; otherwise. the CT secondary circuit may be opened where one brush touches the shorting
before the circuit is
, RATIO
To obtain
to compensate for unavoidable differences in current transformer ratios. Taps on the relay
are rated 8.7, 5.0. 4.6. 3.8. 3.5, 3.2 and 2.9 amperes for each line current
primary
The tap plugs should be
same KVA assumed in each of
method outlined under CALCULATIONS.
tap positions in order to prevent open-circuiting a CT secondary.
to insure that only one plug is left in any horizontal row of tap holes.
overheating may result if more than one plug is connected to any one uinding.
UNBALANCE CURRENT
Unbalance current
former ratios in the field.
winding, or small faults within the power transformer itself where the fault current is too low to operate
the relay.
The type STD relays have a special arrangement for
differential circuit without disturbing the relay connection.
a 5 volt high-resistance A.C. voltmeter (1000 or more
current
a perfect match of relay currents is obtained by the ratio matching taps, the voltmeter
and 11).
will read zero. indicating no unbalance.
entering or leaving a given tap
For higher voltmeter readings, the approximate unbalance current may be calculated
voltage reading
(Unbalance)
The unbalance percentage
For a three winding bank. this
insure that the connections are correct.
The curves in Fig. 16 show the approximate voltage across terminals 8 and g
ielay for various percent slope tap settings and through currents expressed as percentage of tap. To
insure a margin of safety against false operation, the unbalance voltage should not exceed 75 percent of
that required to operate the relay for any given through current and percent slope tap setting. This
of unbalance may result
of tap current.
Courtesy of NationalSwitchgear.com
case has an auxiliary brush.
On every current circuit or other circuit witn shorting bars,
the plug to the other main brush.
unbalance current in the'differential circuit. means
to the locations which most nearly match the expected CT currents for the
power transformer windings.
The connections plug must be removed
is useful in checking the best tap setting when matching current
It is also useful in detecting errors or faults in the
This may be done by connecting the meter across
If the voltmeter reads 1.5 volts or less. the unbalance current
approximately 0.03 times the voltmeter reading times the tap
tap rating into the following equation:
0.2) Tap'
100 times the unbalance current divided by the measured tap
checked with load on at least two pairs of
the relatively high error currents of low ratio bushing CT's at low

CONNECTIONS

winding may used for any
This is the shorter brush in the case which
ADJUSTMENTS
TAP PLUG POSITIONING
The selection of
Provision is made for temporarily connecting
per volt) across the secondary of the
Typical wiring diagrams are given in
supporting studs should be
its quivalent.
should be guided by the
the relay before changing
A check should be made after changing
Inaccurate calibration
the unbalance current flowing in the
8 and g (see Figs. 10
in the relay
transformer
the
In order to
to operate the