Functions
6-46 7SJ62 Manual
C53000-G1140-C121-1
ground, phase-to-phase, and double phase-to-ground faults, sufficient voltage magni-
tude is available at the relay location to determine direction for all possible fault loca-
tions. For three-phase faults, stored voltage values are used to determine direction un-
less sufficient voltage magnitudes exists at the relay location. The stored voltage val-
ues correspond to the voltage magnitudes and angles during the last two cycles where
sufficient voltage magnitude was available to determine direction. When sufficient volt-
age magnitude is not available to determine direction, direction is locked until sufficient
voltage returns. When closing into a three-phase fault, if voltage magnitude is not suf-
ficient to determine direction and no stored voltage values exist in the buffer, the relay
element will trip without regard to fault direction.
For the directional ground fault elements, direction is determined by comparing V
0
with
I
0
. The current I
0
may be obtained from a current transformer connected in the residual
path or may be calculated by the device from the three phase currents. The voltage
V
0
may be calculated by the device from the three phase-to-ground voltages or the
voltage 3V
0
may be obtained by connecting the secondary windings of the voltage
transformers in a broken delta configuration. If the magnitude of V
0
or 3V
0
is not suf-
ficient to determine direction, then the directional ground elements will not initiate a trip
signal. If the current I
0
cannot be determined because only two current transformers
are utilized or the current transformers are connected in an open delta configuration,
then the directional ground elements will not be able to function.
Phase-to-ground faults are detected by the directional ground element, and may be
detected by the directional phase element associated with the faulted phase if the
magnitude of the fault current is sufficient. Phase-to-phase faults are detected by the
two directional phase elements associated with the faulted phases. Double phase-to-
ground faults are detected by the directional ground element, and may be detected by
the directional phase elements associated with the faulted phases if the magnitude of
the fault current flowing in the phase conductors is sufficient. Three-phase faults, of
course, are detected by all directional phase elements, but not by the directional
ground elements. As was stated earlier, in order for any ground fault to be detected by
a directional ground element, current transformers and voltage transformers must be
connected so as to supply sufficient magnitudes of zero sequence currents and volt-
age.
For a phase-to-ground fault, the voltage supplied to the directional phase element pro-
tecting the faulted phase is 90° out of phase with the phase-to-ground voltage existing
on the faulted phase at the relay location (see Figure 6-23). The device compensates
for this phase difference by adding 90° when the phase sequence is “abc” and sub-
tracting 90° when the phase sequence is “acb.” The resulting voltage is called the po-
larizing voltage. For phase-to-phase faults, the angles of the polarizing voltages asso-
ciated with the directional phase elements which protect the faulted phases can vary
based upon the location of the fault with respect to the device. The relationship be-
tween the polarizing voltage angle and the phase current angle for a phase-to-phase
fault is identical to the relationship between the polarizing voltage angle and the phase
current angle for a phase-to-ground fault only
when the fault location and relay loca-
tion are identical.
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