ABB Circuit Shield 87B User manual

IB
7.6
.
1.7
-
7
Issue
C
A
llll
*
*
IM
»
INSTRUCTIONS
High
Impedance
Differential
Relay
©
CIRCUIT
0
SHIELD
Type
87
B
Catalog
Series
419
B
£
HIGH
-
IMPEDANCE
OIFPERENTIAI
RELAY
*
®
0
2
SQ
•
*
a
too
.
t
?
s
#
'
400
TR
#
VOLTS
AC
RMS
SET
TES
1
ABB
Power
TSD
Company
A
-
f n
.
own
P
.
,
USA
v
.
-
;
Courtesy of NationalSwitchgear.com

High
-
Impedance
Differential
Relays
IB
7.6
.
1.7
-
7
Page
2
TABLE
OF
CONTENTS
Page
2
Page
2
Page
3
Page
6
Page
11
Page
14
Introduction
Precautions
Placing
Relay
into
Service
Application
Data
Testing
.
.
.
.
Obsolete
Models
INTRODUCTION
These
instructions
contain
the
information
required
to
properly
install
,
operate
and
test
the
ABB
Type
87
B
high
-
impedance
single
phase
differential
relay
.
The
Type
87
B
relay
is
a
high
-
speed
or
instantaneous
differential
voltage
device
intended
primarily
for
bus
differential
protection
.
High
current
sensitivity
also
allows
the
relay
to
be
used
for
differential
protection
of
resistance
grounded
systems
and
machines
or
reactors
.
The
relay
contains
voltage
-
limiting
resistors
(
varistors
)
which
limit
voltage
across
its
input
terminals
to
a
safe
value
during
system
faults
.
The
relay
is
housed
in
a
case
suitable
for
conventional
semi
-
flush
panel
mounting
.
The
unit
offers
totally
drawout
construction
with
integral
test
facilities
.
Current
transformer
shorting
is
accomplished
by
a
direct
-
acting
spring
and
blade
assembly
upon
removal
of
the
relay
from
its
case
.
Sequenced
disconnects
eliminate
any
possibility
of
nuisance
tripping
during
withdrawal
or
insertion
of
the
relay
.
A
target
indicator
is
mounted
on
the
front
panel
.
The
target
is
reset
by
means
of
a
pushbutton
extending
through
the
relay
cover
.
Control
voltage
must
be
present
to
reset
the
target
.
Earlier
models
,
catalog
series
219
B
are
covered
in
IB
7.6
.
1.7
-
5
.
PRECAUTIONS
The
following
precautions
should
be
taken
when
applying
these
relays
:
1
.
Incorrect
wiring
may
result
in
damage
.
Be
sure
wiring
agrees
with
the
connection
diagram
for
the
particular
relay
before
the
relay
is
energized
.
Be
sure
control
power
is
applied
in
the
correct
polarity
before
applying
.
2
.
Apply
only
the
rated
control
voltage
marked
on
the
relay
front
panel
.
The
proper
polarity
must
be
observed
when
the
DC
control
power
connections
are
made
.
3
.
Do
not
apply
high
voltage
tests
to
solid
state
relays
.
If
a
control
wiring
insulation
test
is
required
,
withdraw
the
relay
from
the
case
before
applying
test
voltage
.
4
.
A
lockout
relay
(
e
.
g
.
type
LOR
,
HEA
,
or
WL
)
must
be
used
to
short
circuit
the
input
signal
side
of
relay
87
B
upon
operation
of
the
differential
circuit
due
to
faults
within
the
protected
zone
.
5
.
The
entire
circuit
assembly
of
the
relay
is
removable
.
The
unit
should
insert
smoothly
.
Do
not
use
excessive
force
.
6
.
Follow
test
instructions
to
verify
that
relay
is
in
proper
working
order
.
If
a
relay
is
found
to
be
inoperative
,
return
to
factory
for
repair
.
Immediate
replacement
of
the
relay
can
be
made
available
from
the
factory
;
identify
by
catalog
and
serial
numbers
.
We
suggest
that
a
complete
spare
relay
be
ordered
as
a
replacement
,
and
the
inoperative
unit
be
repaired
and
retained
as
a
spare
.
7
.
CAUTION
:
Since
troubleshooting
entails
working
with
energized
equipment
,
caution
should
be
taken
to
avoid
personal
shock
.
Only
competent
technicians
familiar
with
good
safety
practices
should
services
these
devices
.
Courtesy of NationalSwitchgear.com

High
-
Impedance
Differential
Relays
IB
7.6
.
1.7
-
7
Page
3
PLACING
THE
RELAY
INTO
SERVICE
L
RECEIVING
.
HANDLING
^
STORAGE
Upon
receipt
of
the
relay
(
when
not
included
as
part
of
a
switchboard
)
examine
for
shipping damage
.
If
damage
or
loss is
evident
,
file
a
claim
at
once
and
promptly
notify
Asea
Brown
Boveri
.
Use
normal
care
in
handling
to
avoid mechanical
damage
.
Keep
the
relay
clean and
dry
.
2
.
INSTALLATION
Mounting
:
The
outline
dimensions and
panel
drilling
and
cutout
information
is
given
in
Figure
1
.
Terminal
and
basic circuit identification
will be found
in
Figure
2
.
Connections
:
A
typical
external
connection
diagram
showing
(
3
)
Type
87
B
relays
used
for
bus differential
protection
is shown
in
Figure
3
.
Wiring
and interconnections should be
in
accordance
with instructions
.
Current
transformer
polarities
MUST
be
as
shown
.
The
relays
have metal
front
panels
which
are
connected
through
printed
circuit
board
runs
and
connector
wiring
to
a
terminal
at
the
rear
of
the
relay
case
.
The terminal is marked
“
G
”
.
In
all
applications
this terminal should be wired
to
ground
.
A
lockout
relay
must
be
wired
as
shown
in
Figure
3
.
DC
must
be
connected
in
the
proper
polarity
as
shown
in
the
diagram
.
To
obtain best sensitivities and facilitate
lower
pickup
settings
,
the
junction
points
of
CT
leads
should he made
equidistant
from
all
CT
'
s
,
or
leads
of
equal
resistance
provided
.
3
.
SETTINGS
VOLTAGE
PICKUP
:
75
-
400
Volts
AC RMS
,
settable
by
means
of
a
front
panel
switch
at
75
,
100
,
150
,
200
,
250
,
300
,
350
or
400
volts
.
TEST
FEATURE
:
A
built
-
in
test
push
button
is
provided
for
operational
trip
tests
and
mounted
behind the
removable
front
cover
.
When
using
the built
-
in
test
switch
,
release the button
as
soon as
the
87
B
operates
.
If
the button is held
in
,
the
relay
will
continually
repeat
its
trip
and
reset
functions
rapidly
,
and will
appear
to
chatter
.
RESET
:
The
relay
does
not
require
manual
reset
as
it
restores
itself
to
functional
status
upon
clearing
of
the faults and
resetting
of
the
lockout
relay
.
The
operation
indicator
is
of
the
memory
type
and
can
be
reset
after
a
fault
or
test
without
front
cover
removal
.
Courtesy of NationalSwitchgear.com

IB
7.6
.
1.7
-
7
Page
4
High
-
Impedance
Differential
Relays
TRIMMER
:
A
vernier
marked
pickup
inside
the
relay
on
the
upper
printed
circuit board allows fine
calibration
of
pickup
settings
at
selected switch
positions
.
4
.
Initial
Check
After
wiring
is
completed
and checked
,
apply
control
power
and
reset
the
operation
indicator
.
If
the
indicator
cannot
be
reset
,
recheck
connections
,
voltage
and
polarity
.
Do
not
attempt
any
other
tests
until
wiring
errors
are
corrected
.
Verify
ratio
of
current
transformers
.
Check
polarity
at
CT
'
s
and
connections
to
the
relays
.
Exercise
test
push
button
to
check
proper
operation
of
87
B
and lockout
relays
.
8.2
SO
209.33
1.197
6.562
166
-
69
6.873
174.63
30.16
t
500
12
-
7
RAMIL
C
.
2
.
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4.673
123.63
3
1
J
T
2
-
437
61.91
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a
O
SIOC
VIEW
FRONT
VICW
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-
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4
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ins
0
'
*
HOLts
f
—
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&
tr
93.66
0
PANEL
CUTOUT
I
T
0 0
0
0
0
0 0
0
iui
hSS
*
32.36
-
L
-
>
0
•
4
HUH
•
3
•
2
I
II
i
i
1
STUD
NUM
8
CPS
(
BACK
VlCW
)
3.167
80.96
6.625
<
66
-
26
NOTE
l
DIMENSIONS
ARE
Relay
Outline
and
Drilling
Figure
1
:
CT
‘
S
vA
STATION
GROUND
CONTROL
VOLTAGE
to
*
\
/
l
I
6
*
a
QS
-
O
^
-
O
4
(
-
>
(
+
)
G
o
„
o
„
OL
o
,
.
o
„
6
„
o
10
o
.
86
<
5
>
(
-
)
Figure
2
:
Basic
Relay
Connections
419
B
Series
Units
Courtesy of NationalSwitchgear.com

"
T
r
—
r
f
“
OBVICE
0
«
CONTACTS
FOR
TRIP
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LOCKOUT
*
ALARM
CIRCUITS
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BUS
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SOURCE
FEEDER
FEEDER FEEDER
TIE
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IDENTICAL
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RELAYING
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POWER
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±
:
B
6
88
"
C
cn
8
L
6
L
.
I
87
B
High
Impedance
Bus
Differential
Relay
,
Figure
3
s
Typical
External
Connections
,
Catalog
Series
419
B
,
for
Protection
of
Multiple
Bus
Sections
with
Bus
Ties
.
td
ON
:
p
:
00
CD
L
/
\
Courtesy of NationalSwitchgear.com

High
-
Impedance
Differential
Relays
IB
7.6
.
1.7
-
7
Page
6
SPECIFICATIONS
75
-
400
V
RMS
,
1500
Watt
-
seconds
Ratings
:
Input
Circuit
Table
1
;
Input
Impedance
and
Sensitivity
Current
Sensitivity
(
*
)
Primary
Amps
4000
:
5
2000
:
5
Pickup
Input
Resistance
(
burden
)
Ohms
±
10
%
Pickup
Current
A
.
RMS
±
10
%
Peak
Volts
(
Sine
)
Dial
1200
:
5
Setting
V
.
RMS
CT
CT
CT
20
12
106.1
1430
0.05
40
75
14
48
24
100
141.4
1600
0.06
19
64
32
150
212.1
1820
0.08
40
24
1950
0.10
80
200
282.8
48
29
96
250
353.6
2050
0.12
34
112
56
300
424.3
2120
0.14
38
128
64
495
2170
0.16
350
72
43
2210
144
400
565.7
0.18
(
*
)
System
current
values
are
listed
for
reference
only
and
represent
contribution
of
all
power
sources
.
Magnetizing
current
of
CT
'
s
is
not
included
.
Voltage
Pickup
:
±
5
%
of
setting
at
25
°
C
.
Tolerance
:
Operating
Time
:
Total
trip
time
:
23
-
40
milliseconds
(
includes
20
-
30
msec
,
time
of
the
lockout
relay
and
3
-
10
msec
,
operating
time
of
87
B
relay
)
.
Input
Withstand
:
150
Volts
RMS
continuous
400
Volts
RMS
for
5
minutes
1500
Wattsec
.
max
.
at
voltage
-
limiting
conditions
(
internal
system
faults
)
.
25
/
50
/
60
Hz
.
Frequency
:
Voltage
Limiting
:
1200
-
1500
Volts
instantaneous
at
input
currents
in
excess
of
0.5
-
1.0
ampere
.
Input
-
to
-
output
isolation
:
7500
V
peak
.
Dielectric
:
Catalog
Number
models
available
for
Control
Power
:
419
B
0002
419
B
0012
419
B
0022
419
B
0032
419
B
0042
419
B
0052
419
B
0082
419
B
0092
Drain
-
8
ma
(
stand
-
by
)
Drain
-
8
ma
(
stand
-
by
)
.
Drain
-
8.5
ma
(
stand
-
by
)
.
Drain
-
1
lma
(
stand
-
by
)
Drain
-
9.2
ma
(
stand
-
by
)
.
Drain
-
lOma
(
stand
-
by
)
.
Drain
-
12
ma
(
stand
-
by
)
.
Drain
-
18
ma
(
stand
-
by
)
.
+
20
%
-
50
%
+
20
%
-
50
%
+
20
%
-
50
%
+
20
%
-
50
%
+
20
%
-
50
%
+
20
%
-
50
%
+
20
%
-
50
%
+
20
%
-
50
%
110
Vdc
175
Vdc
220
Vdc
48
Vdc
125
Vdc
250
Vdc
32
Vdc
24
Vdc
Courtesy of NationalSwitchgear.com

High
-
Impedance
Differential
Relays
IB
7.6
.
1.7
-
7
Page
7
Output
Circuit
Contact
Rating
at
:
250
vdc
125
vdc
Tripping
Continuous
Break
30
Amperes
5
Amperes
0.3
Amperes
30
Amperes
5
Amperes
0.1
Amperes
More
than
3000
V
,
IMhz
.
bursts
at
60
Hz
repetition
rate
continuous
.
Transient
Immunity
;
Operating
Temperature
Range
:
Minus
-
30
to
+
70
degrees
C
.
UL
File
No
.
E
103204
UL
Recognized
:
PRINCIPLE OF
OPERATION
The
Type
87
B
differential
relay
contains
solid
-
state
circuits and
offers
high
-
speed
and
sensitive
protection
against
faults
within the
protected
zone
of
substations
or
equipment
.
Its
input
presents
a
high
impedance
burden
to current
transformers
(
see
table
1
)
for
secondary
differential
currents
below
0.5
amperes
and
corresponding
burden
voltages
below
1000
volts
peak
.
Leakage
current
through
the
relay
varistors
at
these
levels
is
negligible
.
The
voltage
pickup
setting
is
made
by
means
of
a
switch and trimmer
potentiometer
in
a
voltage
divider circuit
.
The
relay
responds
to
modified
instantaneous
voltage
increase
across
its
terminals above the
set
point
during
internal faults
.
The
pickup
voltage
should be
set
above the moderate
voltages
which
may
occur
under
external
or
through
-
fault conditions due
to
unequal
performance
or
saturation
of
CT
'
s
.
Filter and
delay
circuits
are
provided
against
system
transients
,
RFI
,
and
flux
exchange
between
current
transformers
.
The
relay
is
equipped
with
voltage
-
limiting
power
resistors
(
metal oxide
varistors
)
to
limit
the
input
voltage
to
safe
values
during
internal
system
faults
.
Input
burden
at
heavy
faults is
in
essence
,
a
relatively
high
dc
voltage
in
series with
a
resistance
of
a
few
ohms which
practically
does
not
impede
the build
up
of
CT
summation
currents
.
Hence
,
the
relay
absorbs
instantaneous
power
in
the order
of
kilowatts
while
responding
to
high
currents
and has
assigned
wattsecond
limits
.
The
energy
delivered
by
current
transformers
depends
on
their saturation
voltage
(
or
VA
rating
)
,
ratio
,
and fault
current
magnitude
.
The
Type
87
B
relay
is
primarily
designed
to
operate
a
fast
lockout
relay
which
,
in
turn
,
short circuits the
relay
inputs
and
trips
associated circuit
breakers
.
The
input
side
of
the
relay
is
isolated
from
the
output
circuits
by
an
optocoupler
.
The
output
side
consists
of
a
dry
contact
and
associated
clamping
,
delay
,
polarity
and
power
supply
circuits
.
A
target
is
provided
to
give
a
permanent
indication
of
relay
system
operation
and is
reset
by
means
of
the
front
panel
push
-
button
.
The built
-
in
test
feature
allows
operational
trip
test
of
the
Type
87
B
and
lockout
relay
.
When
a
differential
trip
occurs
,
the
output
contact
is
driven
closed
for
a
minimum
of
200
milliseconds
(
400
typical
)
then
automatically
resets
.
If
trip
level
input
is
maintained
,
the
relay
will continue
to
cycle
through
the
trip
and
reset
functions
.
Courtesy of NationalSwitchgear.com

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

High
-
Impedance
Differential
Relays
IB
7.6
.
1.7
-
7
Page
9
These
conservative
calculations
can
be
refined
to
produce
lower
settings
if
the
following
or
other
practical
conditions
are
considered
:
Maximum
fault
current
is
usually
lower
than
interrupting
ratings
.
Faulted
circuit
CT
'
s
of
standard
design
never
saturate
completely
.
Source
CT
'
s
may
enter
saturation
region
.
Phase
-
to
-
ground
external
fault
currents
in
resistance
grounded
systems
are
relatively
low
,
hence
3
-
phase
fault
currents
and
one
-
way
lead
resistance
become
the
controlling
factors
.
Junction
points
of
leads
may
not
be
equidistant
but
closer
to
CT
'
s
of
feeders
which
are
subject
to
external
faults
.
Withstand
-
Internal
Faults
The
thermal
withstand
of
the
relay
is
controlled
by
internal
dissipation
limits
.
Energy
depends
on
the
speed
of
the
differential
and
lockout
relay
operation
,
maximum
available
fault
current
,
and
CT
saturation
characteristics
.
In
general
,
the
wattsecond
rating
of
high
impedance
relays
of
any
type
should
be
checked
against
expected
energy
.
For
convenience
,
one
could
use
a
simplified
method
as
reflected
in
Table
2
that
is
based
on
limits
of
CT
saturation
voltages
with
respect
to
relay
currents
and
a
conservative
total
operating
time
of
40
milliseconds
(
from
inception
of
the
fault
to
closing
instant
of
the
lockout
relay
contacts
)
.
Table
2
Typical
Input
Current
and
Energy
Limits
Current
Transformers
Max
Current
Transformers
Max
system
fault
current
(
Symm
RMS
)
Max
.
relay
Current
(
Symm
RMS
)
Max
.
system
fault
current
(
Symm
RMS
)
Max
relay
Current
(
Symm
RMS
)
Current
Transformers
Max
system
fault
current
Symm
RMS
)
relay
Current
(
Symm
RMS
)
Minimum
Ratio
Max
Max
.
Satur
.
Voltage
Minimum
Ratio
Minimum
Ratio
Max
Satur
.
Voltage
Satur
..
Voltage
170
A
240
V
1500
:
5
1200
:
5
800
:
5
50
KA
37.5
KA
25
KA
100
A
2500
:
5
2000
:
5
1500
:
5
50
KA
37.5
KA
25
KA
350
V
300
A
170
V
1000
:
5
800
:
5
500
:
5
50
KA
37.5
KA
25
KA
150
A
260
V
50
KA
37.5
KA
25
KA
90
A
2000
:
5
1500
:
5
1000
:
5
360
V
3000
:
5
2500
:
5
1500
:
5
50
KA
37.5
KA
25
KA
250
A
200
V
1000
:
5
800
:
5
500
:
5
135
A
50
KA
37.5
KA
25
KA
280
V
2000
:
5
1500
:
5
1000
:
5
50
KA
37
.
5
KA
25
KA
75
A
4000
:
5
2500
:
5
2000
:
5
400
V
50
KA
37.5
KA
25
KA
220
A
210
V
1200
:
5
1000
:
5
600
:
5
50
KA
37.5
KA
25
KA
125
A
290
V
2000
:
5
1500
:
5
1000
:
5
50
KA
37.5
KA
25
KA
62
A
440
V
4000
:
5
3000
:
5
2000
:
5
50
KA
37.5
KA
25
KA
200
A
220
V
1500
:
5
1000
:
5
800
:
5
50
KA
37.5
KA
25
KA
310
V
110
A
2500
:
5
2000
:
5
1200
:
5
50
KA
37.5
KA
25
KA
40
A
600
V
(
*
)
50
KA
37.5
KA
25
KA
(
*
)
4000
:
5
(
*
)
CT
'
s
in
this
range
are
not
widely
available
due
to
limited
window
area
.
Consult
factory
.
If
the
maximum
fault
current
(
in
symmetrical
RMS
amperes
)
and
the
ratio
are
known
,
the
selected
saturation
voltage
of
any
CT
should
not
be
higher
than
indicated
on
the
corresponding
line
.
Conversely
,
the
CT
ratio
should
not
be
lower
than
one
shown
for
corresponding
values
of
the
system
fault
current
and
CT
saturation
voltage
.
Intermediate
values
can
be
obtained
by
interpolation
.
Alternatively
,
the
energy
can
be
accurately
calculated
from
the
instantaneous
product
of
secondary
'
current
and
voltage
pulses
.
One
should
bear
in
mind
,
however
,
that
the
total
circuit
is
not
linear
during
faults
.
Courtesy of NationalSwitchgear.com

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