ABB Circuit Shield 87B User Manual

IB
7.6
.
1.7
-
7
Page
10
High
-
Impedance
Differential
Relays
As
an
approximation
,
note
that
the
relay
input
voltages
are
approaching
square
shape
and
currents
triangular
pulse
form
at
high
internal
faults
.
Thus
,
the
product
of
CT
secondary
RMS
current
(
max
.
internal
fault
,
symmetrical
)
and
squared
value
of
CT
Saturation
RMS
voltage
multiplied
by
a
factor
30
X
10
'
6
is
a
measure
of
wattsecond
energy
per
pulse
that
is
absorbed
by
the
relay
.
This
value
that
is
multiplied
by
the
maximum
expected
number
of
pulses
should
be
less
than
relay
Wsec
.
rating
.
Note
that
pulses
occur
every
half
cycle
and
their
number
depends
on
the
operating
time
of
the
lockout
relay
to
the
contact
closing
.
Therefore
,
fast
lockout
relays
are
preferred
.
Current
Sensitivity
-
Internal
Faults
Since
the
varistor
current
within
the
pickup
range
of
the
relay
is
negligible
,
minimum
fault
current
to
trip
is
controlled
only
by
the
relay
burden
current
and
the
magnetizing
currents
of
current
transformers
.
Obtain
relay
Current
from
Table
1
and
add
to
the
sum
of
magnetizing
currents
of
all
CT
'
s
in
the
circuit
at
selected
pickup
voltage
(
consult
CT
magnetizing
curves
)
.
If
all
CT
'
s
are
identical
,
one
could
also
check
the
approximate
number
of
circuits
or
breakers
,
that
can
be
connected
to
the
relay
at
a
desired
current
sensitivity
and
voltage
setting
.
To
obtain
the
number
,
subtract
the
relay
current
from
the
required
fault
current
sensitivity
and
divide
by
the
magnetizing
current
of
one
CT
.
Current
Transformers
-
Wiring
The
relay
is
designed
for
use
with
dedicated
CT
'
s
.
Toroidal
or
bushing
type
current
transformers
with
low
leakage
impedance
are
preferred
.
All
current
transformers
must
be
of
the
same
ratio
,
and
preferably
,
have
the
same
relaying
class
rating
(
saturation
voltage
)
.
Highest
available
CT
tap
is
recommended
to
assure
that
the
secondary
winding
is
fully
distributed
around
the
core
.
The
best
sensitivity
will
be
obtained
if
CT
lead
resistances
to
the
junction
point
are
minimized
and
the
junction
is
equidistant
in
lead
length
from
all
CT
'
s
.
There
should
be
only
one
ground
connection
in
the
secondary
connection
.
See
Figure
3
on
Page
5
.
CT
polarity
and
recommended
connections
should
be
observed
.
A
lockout
relay
should
be
used
as
shown
.
The
user
should
select
current
transformers
,
wiring
,
and
terminations
that
are
capable
of
withstanding
circuit
voltages
encountered
during
faults
.
If
CT
secondaries
are
additionally
protected
by
gaps
or
varistors
,
their
trigger
or
threshold
voltage
should
not
be
less
than
1600
-
2000
volts
.
Courtesy of NationalSwitchgear.com

ABB Circuit Shield 87B Specifications

General

Brand	ABB
Model	Circuit Shield 87B
Category	Relays
Language	English