Schweitzer Engineering Laboratories SEL-311A, Instruction Manual

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3-2 Distance and Overcurrent Elements Date Code 20011205
SEL-311A Instruction Manual
Table 3.1: Phase Distance Calculations
Positive-Sequence Polarized
Mho Element Compensator-Distance Mho Element
Distance
Calculation
in a Digital
Relay
) mem V • I • Z 1 Re(
) mem V • V Re(
*
AB AB
*
AB AB
Z
Ð
=
Phase A-B
) mem V • I • Z 1 Re(
) mem V • V Re(
*
BC BC
*
BC BC
Z
Ð
=
Phase B-C
) mem V • I • Z 1 Re(
) mem V • V Re(
*
CA CA
*
CA CA
Z
Ð
=
Phase C-A
Z = Impedance measurement at
the
line angle
( ) ( )
[ ]
* I • Z V • I • Z V Im mPP
BC BC AB AB
- - =
Phase-to-Phase Element
( ) ( )
[ ]
* mem V • 25 . 0 jV • I • Z V Im mABC
C AB AB AB
- - - =
Three-Phase Element
mPP = Phase-to-phase torque calculation. Positive
torque restrains, negative torque operates.
mABC = Three-phase torque calculation. Positive
torque restrains, negative torque operates.
Z = Replica line impedance at operating or
balance point.
As mentioned previously, a digital relay mho-element tests the angle between a line drop-
compensated voltage and a polarizing (reference) voltage. Figure 3.1 through Figure 3.3 show
the operating voltages “inside” positive-sequence polarized mho elements and compensator-
distance mho elements. Note that V1mem is the polarizing voltage for the positive-sequence
polarized mho element and (Z • I - V) is the line drop-compensated voltage. In the compensator
distance phase-to-phase element, the polarizing voltage is the unfaulted phase-to-phase voltage,
and the line drop-compensated voltage is the faulted phase-to-phase voltage. In the compensator
distance three-phase element, the polarizing voltage is (-jV
AB
- 0.25 • V
C
mem) and the line drop
compensated voltage is (V
AB
- Z • I
AB
).