Table of Contents
Advertisement
To prove that the equation defines a mho characteristic divide both side� by E
equation reduces to:
Thus, the unit will pick up at a constant component of admittance at a fixed angle depending on the
angle of maximum torque.
MHO UNIT
1.
Impedance C haracteristic
The impedance characteristic of a mho unit is shown in Fig. 6 for the one ohm basic minimum reach
setting at a maximum torque angle of 60 degrees. This circle can be expanded by means of the mho taps on
the autotransformer tap block providing a range of up to 4/1, or by changing the basic minimum reach of the
unit by means of the links on the rear providing a total range of up to 12/1. The circle will always pass
through the origin and have a diameter along �he 60 degree impedance line equal to the ohmic reach of the
unit as expressed by the following:
where:
The angle of maximum torque of the mho unit can be adjusted up to 75 degrees ( see SERVICING) with
resulting increase in reach to approximately 120 percent of its reach at 60 degrees for the same tap setting.
This is shown by the dotted characteristic in Fig.6.
Directional Action
2.
The mho unit is carefully adjusted to have correct directional action under steady-state , low voltage
and low current conditions.
zero volts between 0 and 60 amperes.
tacts between the current limits in Table IV for the three basic minimum reach settings at the voltage
shown. This adjustment is a function of the core ( inner stator) position and should any adjustment be
necessary see SERVICING.
BASIC MIN
REACH TAP
0. 5
n
1.0 n
1. 5
n
2
n
3
n
*The unit is set at the factory on the middle tap ( 1! 1 for 0.5/l/1.5 unit and 2n
for 1/2/3 ohm unit) for correct directional action over the indicated current
range. A variation of �10 percent can be expected on the values listed.
For performance during transient low-voltage conditions, where the voltage was normal at 120 volts prior
to the fault, refer to the paragraph below on memory action.
GEK-45495
z
1
cos
or:
Y cos
Hence, the name mho unit.
C HARACTERISTICS
Ohmic Reach
Z min
basic min. phase-to-neutral ohmic reach of the unit.
=
For faults in the non-tripping direction, the contacts will remain open at
For faults in the tripping direction , the unit will close its con
*VOLTS
2. 0 volts
2 .0 volts
2 .0 volts
2.0 volts
2 . 0 volts
(0
9)
K
-
(0
- 9)
K
=
( 100) z
=
tap setting (%)
TABL E IV
5
2
and transpo�e.
CURRENT RANGE FOR
CORRECT-DIRECTIONAL
ACTION
12 - 60 amps
6 - 60 amps
4.5 - 60 amps
3 - 60 amps
2 - 60 amps
The
Advertisement
Table of Contents
