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GEK-26423
For convenience in field testing, the fault switch and tapped autotransforiner of Fig. 16
have been
arranged in a portable test box, Cat. No. 102L201, which is particularly adapted for testing
directicnal
and distance relays.
The box is provided with terminals to which the relay current and potential circuits
as well as the line and source impedances may be readily connected.
For a complete description of the
test box the user is referred to GEI-38977.
B.
T[STING TIlE MHO UNITS
To check the calibration
of
the who unit it is suggested that the test box, test reactor, and test
resistor be arranged with Type XLA test plugs as shown in Fig. 18. These connections are similar to the
schematic connections of Fig. 16, except that the XLA test plug connections are now included.
As noted
in the section on CHARACTERISTICS the mho unit provides an accurate distance measurement on ground faults
only for the special case where the zero sequence impedance is equal to the positive sequence impedance
to the fault.
The tests outlined below check the line-to—neutral ohms that the unit would measure under
these conditions.
After the who unit has been set for the desired reach, select a value of test impedance at 600, that
is RL
+
jXL /60°, which exceeds the reach setting of the unit by the smallest amount possible.
Then
using the test circuit of Fig. 18 (note that current limiting impedance X 5 and R 5 is omitted), turn the
test box fault switch SF to the 'ON" position and adjust the selector switches to obtain a balance point.
The percent tap setting of the test box autotransfornier, which should cause the starting unit to just
close its contacts, is given by equation (12).
Z cos (60
-
0)
% Tap
=
(100)
(12)
L
where:
ZMU
=
Ohmic reach of the mho unit (See Equation 5 in CHARACTERISTICS section).
ZL
=
Test impedance in ohms
0
=
Angle of test impedance
The portable test reactor (Cat. No. 6054975) and test resistor (Cat. No. 6158546) are normally sold
as a set identified by a calibration curve number shown on the nameplate.
The test resistor taps have
been set at the factory n conjunction with taps on the associated test reactor to provide a range of
impedances at 600 and 30
angles.
If one of the 60° impedance values thus obtained is used the angle 9
in the above equation will be 600.
If a resistor-reactor tap combination other than those covered by
the calibration sheet is used, or if the test reactor is used with some other non-inductive resistance
to approximate the 600 impedance, then the actual value of 0 should be used in equation 12.
The angles
of the test reactor at the various nominal tap settings are given in Table VI.
TABLE VI
TEST
REACTOR TAP
ANGLE
24
88°
12
87°
6
86°
3
85°
2
83°
1
81°
0.5
78°
As an illustration of the above assume that the 3 ohm basic minimum reach link setting is to be used
and that is has been decided to set the E tap on 45 percent.
This setting falls within the limits
of 10 and 67 percent determined in the example in the section SAMPLE CALCULATIONS OF SETTINGS.
Ohmic
reach of the starting unit at its 600 angle of maximum torque will then be 6.68 ohms, as determined from
equation 5 in the CHARACTERISTICS section.
Using a typical combination of test reactor and test resistor, the 600 impedance closest above this
reach setting is 14.4 ohms. The percent tap of the test box autotransfornier at which the who unit contacts
will just close can then be calculated as follows from equation (12):
Tap
6.68 c0
0
-
600)
(100)
=
46.3
13
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