TYPE KAB HIGH IMP EDAN C E
B U S DIFFE R E NTIAL R E L AY
*
O P E R AT I ON
The relay is connected as shown in external
connection figure
5.
In normal operation the voltage
at the relay terminals is approximately zero. In
the case of an external fault, the ct in the faulted
feeder will produce a secondary voltage necessary
to force the total secondary fault current through
its windings. Neglecting the effect of load current,
the unfaulted ct's will produce no secondary voltage.
If both the source ct's and the faulted feeder ct's
are not saturated, the voltage at the relay remains
approximately zero.
However, during severe external faults the feeder
ct may saturate. The source ct's would then, have
to produce enough voltage to force their currents,
through their own windings and through the winding
and leads of the faulted ct. In this case a voltage
would appear across the junction point equal to
the total secondary currents of the source ct's
multiplied by the resistance of the faulted ct
secondary winding plus its leads to the junction
point. The relay is usually set to operate at some
voltage higher than the anticipated voltage expected
for this condition. This faint voltage will be lower
than the calculated voltage since the fault ct will
always produce some voltage and also the source
ct will tend to saturate. The de offset component
of this voltage does not affect the relay operation
by reason of the series tuned circuit as well as the
insensitivity of the voltage unit to de.
In the case of an internal fault, the feeder ct's
have in effect open circuited primaries and therefore
secondary impedances equal to the magnetizing
impedance which is high. The source ct 's will
produce high voltages to drive the fault current
through
the
fe eder
ct secondaries.
This high
voltage will appear at the relay terminals and will
be well above the pickup setting.
During severe internal faults the source ct
will saturate to limit the RMS value of secondary
voltage. However, the peak voltages of the wave
form could be quite high and overstress the insula
tion. A varistor is connected internally to reduce
this voltage. Figure 2 shows the electrical char
acteristics of this device.
The overvoltage unit is a high speed device
and will operate in
2
cycles at twice pickup using
a sine wave test voltage. Under fault condition, the
crest voltage is rather high for the first few cycles,
the operating speed of the V-unit would be faster,
Figure
6
shows the operating speed of 1.
5
cycles
I.L.
41-337.4A
from the typical staged faults. However, an over
current unit is provided which may be connected
in series with the varistor and will operate on
current flow during severe internal faults when the
varistor conducts current to limit the secondary
voltage. The application of overcurrent unit has no
effect on the operating speed of the voltage unit.
C HARAC T E R I S T I C S
Overvoltage Unit (V)
The range of pickup of the overvoltage unit is
adjustable from
75
to 300 volts. The pickup is obtain
ed by means of adjusting the spring windup.
*
Speed of operation is 2 cycles at twice pickup.
(sine wave test voltage).
Over current Unit (IT)
The range of the overcurrent unit is 3 to
48
am
peres . A tap is used to obtain this by use of two
settings (Hi and Lo ). The Lo setting permits the
core screw to be adjusted over a 3 to
12
ampere
range. The Hi setting permits a 12 to
48
ampere
range of adjustment. The scale plate is calibrated
in multiples of minimum pickup. The pickup should
be within
±
10% of the setting.
Indicating Contactor Switch (ICS)
The indicating contactor switch has two taps
that provide a pickup setting of 0.2 or
2
amperes. To
change taps requires connecting the lead located in
front of the tap block to the desired setting by means
of a screw connection.
Trip Circuit Constants
Contactor Switch-
0. 2 ampere tap
6 . 5
ohms d-e· resistance
2.0
ampere tap
0.15
ohms d-e re sistance
* S ET T I NG C A L C U LAT I O N S
1 .
SETTING VOLTAGE UNIT
The setting of the voltage unit can be expressed
as follows:
IF
VR
=
K
CRs
+
RL) -
N
where VR =pick up setting of the V-unit, (RMS,
volts.)
Rs
=
de resistance of ct secondary wind
ing, including internal leads to bush
ing terminals.
RL
=
resistance of lead from junction
points to the most distance ct. (One-
3
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