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GEK-26437
Torcue
KE (IZT1
-
TE)
cos
p
(1)
where
E
=
phase—to-phase voltage (F
12
)
I
delta current ('11
-
12)
ZT
1
=
transfer impedance of transactor TR-l (design constant)
p
=
angle between E and (1Z
11
-
E)
K
=
design constant
T
=
auto-transformer tap setting
That this equation (1) defines a mho characteristic can be shown graphically by means of Fig. 5.
The
vector IZT1 at an angle 9 determines the basic minimum reach of the unit for a particular tap setting of the
transactor TR-l primary.
Assuming finite values of E and (IZT1
-
TE), the balance point, torque
=
0, will
occur where cos P
=
0, that is where the angle P is 90 degrees.
The locus of the terminus of vector TE
(point A in Fig. 5) which will cause the angle p to always be 90 degrees is a circle passing through the
origin and with the vector IZT1 as a diameter.
Considering further the diagram in Fig. 4, we note that the angle P is less than 90 degrees for an
internal fault (point C) and the net torque will be in the closing direction (cos p is positive); and that
the angle p is greater than 90 degrees for an external fault (point 0) and the net torque will be in the
.pning direction (cos P is negative).
M 2 Unit
The schematic connections of the M 2 unit are shown in Fig. 3.
The two side poles, energized by phase—
to-phase voltage, produce the polarizing flux.
The flux in the front pole, which is energized by a percent
age of the same phase—to-phase voltage, interacts with the polarizing flux to produce restraint torque.
The
flux
in
the rear poTe, which is energized by the two line currents associated with the same phase-to—phase
voltage, interacts with
the
polarizing flux to produce operating torque.
The torque at the balance point of the unit can therefore be expressed by the following equation:
Torque
0
=
El cos
(0
-
9)
-
KE
2
(2)
where:
E
=
phase-to—phase voltage (F
12
)
I
=
delta current (11—12)
9
angle of maximum torque of the unit
0
=
power factor angle of fault impedance
K
=
design constant.
To prove that equation (2) defines a mho characteristic divide both sides by F 2 and transpose.
The
equation reduces to:
3-cos
(0
-
0)
=
K
or:
Y cos
(0
-
9)
=
K
(3)
Thus, the unit will pick up at a constant component of admittance at a fixed angle depending on the
angle of maximum torque.
Hence the name mho unit.
CHARACTERISTICS
'
mit
pedance Characteristic
Th
impedance characteristic of the
unit is shown in Fig. 5 for the 0.75 ohm basic minimun reach
settinG at a maxiMum torque angle of -75 degrees.
This characteristic is obtained with the terminal voltage
o tne relay supplied directly to the restraint circ.,t of the M 1 unit, that is with the M 1 taps on the
ajtc-transforrer
tao
block set at 100 percent.
This circular impedance characteristic can be enlarged,
tmit
i unit redh increased, by up to 10/1 by reducing the percentage of the terminal voltage supolied
Er
the
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