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The ground unit can be selected to be either zero sequence voltage polarized or negative sequence voltage polarized.
Negative sequence polarization is recommended for parallel lines with mutual coupling.
Zero sequence polarization utilizes the zero sequence components of the currents and voltages into the relay, and the
unit operates when 3I
leads 3V
0
required that 3I
> 0.5 A and 3V
0
Negative sequence polarization utilizes the negative sequence components of the currents and voltages into the relay
and the unit has its maximum torque line when I
Fault Locator
The REL 356 fault locator feature computes the magnitude and phase angle of the fault impedance and the distance to
the fault in both miles and kilometers. The fault impedance is calculated from the voltage and current phasors of the
faulted phase(s). Thus, proper faulted phase selection is essential for good fault locator results. The distance to the fault
is computed by multiplying the imaginary part of the fault impedance times the voltage and current transformer ratios
(VTR/CTR) and dividing by the distance multiplier setting (XPUD). The impedance calculations for the various fault
types are:
V
=
XG
Z
+
⋅
XG
I
k
I
X
0
for single line to ground faults,
−
V
V
=
XG
YG
Z
−
XY
I
I
X
Y
for line to line faults, and
V
=
A
Z
ABC
I
A
for three phase faults
where X, Y = any A, B, C phase.
Faulted Phase Selector
The REL 356 includes a phase selector for indication of the faulted phase(s). The phase selector is current-only based
why connection of voltages is not needed for correct operation. As the REL 356 is intended for three pole trip only, the
phase selector is not used in the trip logic but for indication and for fault location calculation.
Note that the phase selector algorithm is based on comparing pre-fault and fault currents why, during secondary injection
testing, a sudden change between the pre-fault and fault condition is required for correct indication.
Measuring Elements and Operational Logic
by more than 30° or lags by more than 150°. For the operation of these units, it is
0
> 1 V.
0
leads V2 by 98
2
ABB REL 356 Current Differential Protection
°
with the current 3I
> 0.5 A and 3V
> 3 V.
2
2
4-7
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