HKB SINGLE
RELAY CURRENT
Fig !-Vectors for Normal Balanced Conditions and for a
Phase-A-to-Ground Fault.
essentially
120°
out-of-phase with the respec
tive zero sequence phase B and C components.
For
phase faults not
involving ground, the
type
HQS
relay
does not
operate as the zero
sequence currents are zero.
The type
HZM-3
relay is connected to receive
s�ar current and the delta voltage which leads
this
current.
With
this
connection
and by
proper
setting, the
relay
can
be
made
to
operate
on phase
faults
only.
For close-iD
line
to
ground faults, the
star current and
the
leading delta
voltage are approximately,
120°
apart as shown in Figure
1.
This
figure
shows
Va,
Vb, Vc
for normal
conJitions.
When
a
phase
a
line-to-ground
fault
occurs,
Va collapses
to
Va, and
the
fault
current
Ia lags
this
voltage
by the
system
angle
shown
60°
in the figure.
The
delta voltage
Vab is reduced slightly and now
leads the relay current by approximately
120°.
On an
R
and
X
impedance diagram as shown in
Figure
2,
the
type
HZM-3
relay sees
ground
faults
in
the
shaded
area
in
the
second
quadiant.
By off setting and shifting the ohm
circle
so
that
the
relay
response
in the
region of
90
to
180°
lag
is very limited, the
relay
can be
made
inoperative
on
ground
faults.
It should
be
remembered
that
the scalar
impedance
seen
by
the
distance
relay
is
inherently
higher for
ground faults than for
2
-X
Fig. 2-0perating Characteristics of the Type HZM-3 Relay
on Single-Line-to-Ground Faults plotted on an
a-X
Diagram.
phase faults.
On
phase faults the delta relay
voltage
is
almost
completely
collapsed
as
compared
to
a collapse
of not more than
57%
for
ground
faults.
Thus
for heavy close-in
ground faults
the impedance seen by the relay
will be outside the relay setting.
The
three poles
of the breaker are tripped
as follows for the various faults.
For inter
nal single line-to-ground the type
HKB
and
HQS
relays operate to trip only the faulted phase.
In
other words for
phase A to ground faults,
the
HKB
relay
contacts
R
close, and the type
HQS
phase
selector relay contacts
S
A close to
trip
phase A
pole.
For
a phase B-to-ground
fault, contact
R
and
&s
close to trip phase B
pole, and
for a phase-C-to-ground fault, con
tacts
R
and
3c
close
to
trip
phase C pole.
For these
faults
contacts
ZA, AB, and Zc of
the type
HZM-3
relay should not operate.
For
any
internal
three-phase,
phase-to
phase, or
two-phase to-ground fault contact
R
and
one or more
of the type
HZM-3
relay con
tacts ZA, ZB, or Zc close and energize CSl and
CS2
contactor
switches.
These
in turn trip
all three poles of the breaker.
On
two phase
to...,ground
faults
the phase
selector contact
associated with the unfaulted phase will oper
ate
and initiate tripping of
this phase, but
simultaneously CSl and CS2 will also trip this
phase as well as the other two.
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