ABB Circuit Shield 87B - APPLICATION; Voltage Settings - External Faults

High
-
Impedance
Differential
Relays
IB
7.6
.
1.7
-
7
Page
8
APPLICATION
The
Type
87
B
is
a
single
-
phase
relay
of
the
high
-
impedance
high
-
speed
type
.
It
is
used
in
well
known differential circuits
with standard
current
transformers
.
Typically
,
three
type
87
B
relays
and
a
lockout
relay
are
required
in
standard
configurations
for
bus differential
protection
against
phase
-
to
-
phase
and
phase
-
to
-
ground
faults
.
A
properly
set
relay
will
discriminate between external
(
through
feed
)
and internal
system
faults
.
The
relay
is
a
voltage
actuated device but its
pick
-
up
sensitivity
is
proportional
to current
magnitudes during
internal
faults
.
The
voltage
across
the
relay
input
is
practically
zero
in
symmetrical
circuits
during
normal
operation
,
hence the continuous
input
rating
is
of
less
importance
.
Moderate
voltage
may
appear
in
the
secondary
circuits
due
to
unequal
performance
or
saturation
of
some
CT
'
s
.
However
,
these
voltages
will result
in
smaller differential
currents
(
in
percent
)
than
in
schemes with
low
impedance
differential
relays
.
Thus
,
high
differential
impedance
(
usually
several orders
of
magnitude higher
than
the
shunt
impedance
of
saturated
CT
'
s
)
leaves
current
transformers
force
-
coupled
through
their secondaries
and allows better
discrimination between
faults
.
Internal faults lead
to
summation
of
all
infeed
secondary
currents
in
the
relay
with
some
shunting
effect of
idle
CT
secondaries
.
Relay voltage
and
current
rise
rapidly during heavy
internal faults
.
The
signal
,
while
being voltage
-
limited
by
varistors
,
causes
the
relays
to
operate
and self
-
protect
the
input
of
87
B
relay
until circuit breakers
trip
.
All
differential
relays
of
high
-
impedance
type
have limited
input
current
or
energy
ratings
.
The described
protection
can
be
conveniently
expanded
to
more
CT
'
s
if
breakers
are
added
to
the
existing
scheme
.
The standard
Type
87
B
relay
does
not
contain
restraining
circuits
.
Applications
which involve either
lightning
arresters
(
directly
connected
to
the
bus
)
or
non
-
relayed
capacitors
with
grounded
mid
-
points
or
other
equipment
subject
to
high
magnetizing
inrush
or non
-
canceled
infeed should
be referred
to
the
factory
.
Application
of
the
Type
87
B
relay
and calculation
of
proper
settings
require
consideration
of
the
above
described
properties
of
the
scheme
.
In
general
and
to
provide optimum
performance
,
attention
is recommended
to
:
proper
selection
of
current
transformers
and
wiring
,
voltage settings
above
voltages
and transients
expected
under external
fault
or
inrush conditions
,
sensitivity
to
internal
faults
,
and
relay
withstand
at
maximum
fault
magnitudes
.
For
example
,
the
following
selection
procedures
can
be
adapted
for
a
given
bus
differential
protection
scheme
:
Voltage
Settings
-
External
Faults
The
relay
should be
set
above
maximum
secondary
circuit
voltage
which will
appear
during
external
faults
.
Assume
that
the
CT
of
a
faulted
feeder
or
load breaker
is
completely
saturated
and
one
or
more source
CT
'
s
are
forcing
current
through
its
secondary winding
.
1
.
Identify
ratio and location
of
CT
'
s
.
Obtain
or
calculate
DC
resistance
of
the
secondary
winding
of
the
CT
and
two
-
way
lead
length
from
the
junction
point
to
the farthest
CT
.
Correct
resistance
values
to
the
highest
ambient
or
operating
temperature
.
2
.
Obtain
interrupting rating
of
the
largest
circuit breaker
(
symm
.
A
RMS
)
referred
to
secondary
side
of
CT
'
s
.
3
.
Product
of
this
current
and resistance
sum
obtained above
is
symmetrical
AC RMS
voltage
.
4
.
To
obtain minimum
relay
setting
,
multiply
this
value
by
a
factor
(
e
.
g
.
2.5
)
which
will
account
for safe
margins
,
DC
offset
,
resistance tolerances and
current
transfer
between
CT
’
s
.
5
.
The
voltage
thus
calculated
represents
the
minimum
safe
setting
of
the
relay
.
If
the value is between
set
points
,
use
the
next
higher
pickup
setting
.
Still
higher settings
will
provide
additional
safety
margins
with
some
increase
in
detectable internal
fault
magnitude
.
In
most
cases
,
the
voltage
pickup
of
the
87
B
relay
can
be
set
above
the saturation
voltage rating
(
knee
point
)
of
CT
’
s
.
Courtesy of NationalSwitchgear.com