Operating Principles; Characteristics; Mho Unit - GE CEY51A Instructions Manual

GEK-I2lS
OPERATING PRINCIPLES
The mho units of the CEY51 relay are of the four-pole induction-cylinder
construction in which torque is produced by the interaction between a polarizing
flux and fluxes proportional to the restraining or operating quantities.
The schematic connections of the Ml unit are shown in Figure 4. The two side
poles, energized by phase-to-phase voltage, produce the polarizing flux. The flux
from the front and rear pol es, energi zed by the di fference between the secondary
vol tage of transactor TR-01-2 and a percentage of the same phase-to-phase voltage,
interacts with the polarizing flux to produce torque. The torque equation can be
written as follows:
Torque
=
KE( IZn - TE) cos
B
where:
=
phase-to-phase voltage (EI2)
=
delta current (II - 12)
=
transfer impedance of transactor
=
angle between
E
and (IZTl - E)
design constant
autotransformer tap setting
(1)
TR-01-2 (design constant)
That this equation (1) defines a mho characteristic can be shown graphically by
means of Figure 5. The vector IZT13 at an angle
Q
determines the basic minimum
reach of the unit for a particular tap setting of the
trans~ctor
TR-01-2
prima~y.
Assuming finite values of E and
(lzn -
TE), the balance pOlnt, torque
=
0, wlll
occur where cos
=
0, that is, where the angle
B
is 900. The locus of the terminus
vector TE (Point A in Figure 5) that will cause the angle
B
to be always 900 in a
circle passing through the origin and with the vector IZTl as a diameter.
Considering further the diagram in Figure 5, we note that the angle
B
is less
than 90 0 for an internal
fault (point C) and net torque will be in the closing
direction (cos
S
is positive); and that the angle
B is greater than 90 0 for an
external fault (point D) and the net torque will be in the opening direction (cos
B
is negative).
See Figure 5, Graphical Representation of Mho Operating Principle.
CHARACTERISTICS
MHO UNIT
Impedance Characteristic
The impedance characteristic of the mho unit is shown in Figure 6 for the 0.75-
ohm basic-minimum-reach setting at a maximum-torque angle of 600. This
characteristic is obtained with the terminal voltage of the relay supplied directly
to the restraint circuit of the mho unit, that is, with the mho taps on the auto-
transformer tap block set at 100%. This circular impedance characteristic can
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