GE STD15C User manual

GE
K-45307K
INSTRUCTIONS
TRANSFORMER
DIFFERENTIAL
RELAYS WITH
PERCENTAGE
AN!)
HARMONIC
RESTRAINT
TYPES
STD15C
and
STD16C
GE
Protection
and
Control
205
Great
Valley
Parkway
Malvern,
PA
19355-1337

GEK-45307
DESCRIPTION
3
APPLICATION
3
CALCULATION
OF
SETTINGS
5
Method
5
Current
Transformer Connections
5
Determination
of
CT
Turns
and
Type
STD
Relay
Tap
Setting
5
Current Transformer
Ratio
Error
6
Percent
Slope
Setting
8
I
Determination of
CT
Turns
and
STD
Relay
Tap
Setting
8
II Percent
Ratio
Error
9
IA
Repeat
-
CT
Turns
and
Relay
Tap
Setting
9
hA
Repeat
-
Percent
Ratio
Error
10
III
Percent
Slope
Setting
10
RATINGS
10
Models
12STD15B
and
12STD16B
10
Auxiliary
Relay
Control
Circuit
11
CHARACTERISTICS
ii
Pickup
and
Operating
Time
11
Overcurrent
Unit
Pickup
11
Percentage
Differential Characteristics
12
Harmonic
Restraint
Characteristics
12
BURDENS
13
CONSTRUCTION
14
Current
Transformers
14
Through—Current
Restraint Circuit
14
Differential-Current
Circuit
15
Overcurrent
Unit
15
Main
Operating
Unit
16
Target
and
Seal-in
Unit
16
Case
16
RECEIVING,
HANDLING
AND
STORAGE
17
ACCEPTANCE
TESTS
17
Visual
Inspection
17
Mechanical
Inspection
17
Electrical
Tests
17
Test
Equipment
18
INSTALLATION
PROCEDURE
18
Tests
18
Pickup
19
Harmonic
Current
Restraint
19
Through-Current
Restraint
20
Instantaneous
Overcurrent
Unit
21
Dropout
of
Main
Unit
21
Location
21
Mounting
21
Connections
21
ADJUSTMENTS
21
Tap
Plug Positioning
21
Percent
Slope
Setting
23
OPERATION
23
Targets
23
Disabling
of
Type
STD
Relay
23
MAINTENANCE
23
Contact Cleaning
23
PERIODIC
TESTS
24
Pickup
24
Harmonic
Current
Restraint
24
Through-Current
Restraint
25
SERVICING
25
RENEWAL
PARTS
26
2

GEK-45
307
TRANSFORMER
DIFFERENTIAL
RELAYS
WITH
PERCENTAGE
AND
HARMONIC RESTRAINT
STD15C
and
STD16C
INTRODUCTION
Relays
of
the
STD
type
are
transformer
differential
relays
provided
with
the
features
of percentage
and
harmonic restraint.
A
static decision
unit
controls
a
small
telephone-type
relay
that
provides
the
contact output.
Percentage
restraint
permits
accurate
discrimination
between
internal
and
external
faults
at
high
current,
while
harmonic
restraint
enables
the
relay
to
distinguish,
by
the
difference
in
waveform,
between
the
differential
current
caused
by
an
internal
fault,
and
that
of
transformer
magnetizing
inrush.
DES
CR1
PTI
ON
Each Type
STD
relay
is
a
single-phase
unit.
The
Type
STD15C
relay is designed
to
be
used
for
the
protection
of
two-winding
power
transformers
and
has
two
through-
current
restraint
circuits
and
one
differential—current
circuit.
The
Type
STD16C
relay
is designed
for
use
with
three—winding
power
transformers
and
has
three
through-current
restraint
circuits
and
one
differential—current
circuit.
It
may
also
be
used
for
four-circuit
transformer
protection
(see
Figure
1)
when
only
three
circuits
require
through-current
restraint,
while the
fourth circuit,
being
the
weakest,
needs
no
through-current
restraint.
APPLICATION
The
current
transformer
ratios
and
relay
taps
should be
selected
to
obtain
the
maximum
sensitivity
without
risking
thermal
overload
of
the
relay
or
current
transformer
(CT),
or
the
possibility
of
misoperation.
Therefore,
current
transformer
ratios
in
the
various windings
of
the
power
transformer
should
be
selected
with the
following
points
in
mind:
1.
The
lower
the
relay
tap
and
the
lower
the
CT
ratio
selected, the
higher
will
be
the
sensitivity.
However, the
lowest
CT
ratio
and
the
lowest
relay
tap
may
not
be
compatible
with
some
of
the
following
restrictions.
Where
a
choice
is
available
of
increasing
either
the
CT
ratio
or
the
relay
tap,
it
is
desirable
to
increase
the
CT
ratio
in
preference
to
the
relay
tap.
Since
the
relay
burden is
likely
to
be small
compared
to
the lead
burden,
increasing
the
CT
ratio
tends
to
improve
the
relative
performance
of
the
CT5,
as
a
result
of
reducing
the
maximum
secondary
fault
current and
increasing
the
accuracy
of
the
CTs.
These
1nstrucron5
do
not
purport
to cover
i.11
details
or
variations
in
equipaent
nez
e provide
for
every
possible
ctingency
to
be
met in
connection
with
installation,
operation
or
IlntnCe. Should
further
nforma
ton
be
desi red
or
should
parti
cular
probleme arise
which
are
not
covered
ziWficiently
for
the
purchaser’s
purposes,
the
matter
should
be
referred
to the
General
Electric
Company.
To
the
extent
required
the
products
described herein
meet
applicable
ANSI,
IEt
andN
standards,
but
no
such
assurance
is
given
with
respect
to
local
codes
and
ordinances because
they
varç greatly.
3

GEK—45307
2.
The
CT
secondary current
should not
exceed
the continuous
thermal
rating
of
the
CT secondary
winding.
3.
The
relay
current
corresponding
to
maximum kVA (on
a
forced-cooled
basis)
should not
exceed twice
tap
value, which
is
the
thermal rating of
the relay.
4.
The CT
ratios
should
be
high
enough that the
secondary
currents will
not damage
the relay
under maximum
internal
fault conditions
(refer to RATINGS).
5.
The
relay
current corresponding
to rated
kVA
of
the
power
transformer
(on
self-
cooled
basis) should
not
exceed the
relay tap value
selected
(magnetizing
inrush
night
operate
the
instantaneous overcurrent
unit). If the
transformer
under
consideration
does
not
have
a
self—cooled
rating,
the
transformer
manufacturer should
be
consulted for
the
“equivalent
self—cooled
rating”;
that
is the rating
of
a
self-cooled
transformer
that would
have the
same
magnetizing
inrush
characteristics
as the transformer
being considered.
6. The
current
transformer
tap chosen must
be
able
to
supply
the
relay
with
8
times rated
relay
tap
current,
with
an
error
of less than 20%
of
the total
current.
If
the
current
transformers produce
an error
of
greater
than
20%
at
less
than
8
times
tap
value,
the
harmonic
content of
the
secondary
current
may
be
sufficient
to cause
false
restraint on
internal faults.
7. The CT ratios
should
be
selected
to
provide balanced secondary
current
on
external
faults. Since
it
is rarely
possible to
match the secondary
currents
exactly
by
selection of
current
transformer ratios,
ratio—matching
taps
are
provided
on
the
relay by
means
of which the
currents may usually
be
matched
within
5%.
When
the
protected
transformer
is
equipped
with
load-ratio
control
it
is
obvious
that a close match
cannot be obtained
at
all points
of the ratio-
changing
range. In this case,
the secondary
currents are
matched
at
the middle
of the
range and the
percentage_differential
characteristic
of
the relay
is
relied
upon to prevent
relay operation
on the unbalanced
current
which
flows
when the
load-ratio
control
is
at the ends
of the range.
8.
In
some applications,
the
power
transformer will
be
connected
to the
high
voltage
or low voltage
system through four
breakers
(as shown in
Figure
1)
as
for example
in
a
ring—bus
arrangement.
In
this case,
the
CT
ratios
must
be
selected
so
that
the secondary
windings
will not
be
thermally
overloaded
on
load current
flowing around
the ring in
addition to
the
transformer
load
current.
It
is recommended that
CTs
on
each of the
two low
voltage
(or
high
voltage)
breakers
be
connected
to a
separate restraining
winding
to assure
restraint
on heavy
through-fault
current
flowing
around the
ring bus.
It is not
desirable
to
protect
two
parallel transformer
banks with
one
set
of
differential
protection,
since
the
sensitivity of the
protection
would
be
reduced.
In addition,
if
the
banks can be
switched separately,
there is
a
possibility
of
false
operation on magnetizing
inrush
to
one transformer
bank, causing
a
“sympathetic
inrush”
into the bank
already energized.
In
this
case,
the
harmonics
tend to
flow
between
the banks,
with the
possibility
that there will
be
insufficient
harmonics
in
the
relay
current
to
restrain the
relay.
Typical
elementary
diagrams
for the
STD15C and
STD16C are
illustrated
in Figures
2
and 3.
4

GEK—45307
CALCULATION
OF SETTINGS
METHOD
The
calculations
required for determining
the proper relay
and
CT
taps are
outlined
below.
A
sample calculation, for the
transformer shown in
Figure
4,
is
then
given.
CURRENT
TRANSFORMER CONNECTIONS
Power
Transformer Connections
Current
Transformer
Connections
Delta-Wye
Wye-Delta
Wye-Del ta
Del ta-Wye
Delta-Delta
Wye—Wye
Wye-Wye
Delta-Delta
Delta-Zigzag
with
O
phase
Delta—Delta
shift
between primary
and
secondary
DETERMINATION
OF CT
TURNS
AND
TYPE STD RELAY TAP SETTING
1.
Determine
the maximum line currents
(Max.
I)
on the basis
that each power
transformer
winding may carry the maximum
forced—cooled rated kVA
of
the
transformer.
— Maximum Transformer kVA
Max. -
(Line kV)
2. Determine
the full—load rated
line
currents (100%
I)
on
the basis that
each
power transformer winding may carry the
full self-cooled rated kVA of
the
transformer,
or
the “equivalent”
self-cooled ratings.
— 100%
Transformer kVA
100%
‘P -
(Line
kV)
Actually,
this calculation
does not mean that all
windings
will necessarily
carry
these maximum
load currents continuously. This is
only a convenient way
of
calculating
the
currents in the other
windings in
proportion to
their
voltage ratings.
This
is the requirement for selecting the
relay tap
setting
so that the relay will
not
operate
for
any external
fault.
3.
Select
CT
ratios so that the secondary current
corresponding
to maximum
Ip does
not exceed the CT
secondary
thermal
rating
(5
amperes).
In the case
where a
transformer is connected to
a
ring
bus, for
example,
the CT ratio
should be
selected so that the CT
thermal rating will not
be
exceeded by the maximum
load
current
in
either breaker. Also, select CT ratios so
that the
relay currents
can
be properly matched by means
of the relay taps. (Highest
current not more
than 3
times
lowest
current).
For
Wye-connected
CTs
Tap Current
= 100%
I
5

GEK-45307
For Delta-connected
CTs
Tap
Current
=
LnL
N
where
N is
the
number
of CT secondary
turns.
4. Check
the
matching
of
relay
currents
to
relay
taps,
to keep the
mismatch
error
as low as possible.
Calculate
the
percentage
of
mismatch
as
follows: on
two—winding
transformers,
determine
the
ratio of
the two relay currents
and the
tap values
selected.
The
differences
between
these ratios, divided
by
the
smaller
ratio, is
the percent
of mismatch.
The
mismatch normally
should not
exceed
5%.
For
three-winding
transformers,
the percent of
mismatch error
should be
checked
for
all combinations
of currents or
taps.
If taps cannot
be selected
to keep this
percentage
error within
allowable
limits,
it will
be
necessary to
choose a
different
CT ratio on
one or
more
lines,
to obtain a better
match between
relay currents
and
relay taps.
5.
Check to see
that the sum
of
the
relay currents
that
will
be
applied to
the
relay for
a
fault
at
the
terminals of the power
transformer
is
less than 220
amperes
RMS for
1
second.
If the
period
during
which a fault current
flows
in
the
relay can
be
definitely limited
to a shorter
time,
a
higher
current can be
accommodated
in accordance with
the
relation:
(Amperes)?
x
seconds
48,400
Also
check that the
suni
of
the
multiples of
tap current
on an
internal
or
external
fault
do
not
exceed
150.
CURRENT
TRANSFORMER
RATIO
ERROR
The
CT ratio
error must be
less
than 20% at
8
times relay rated
tap
current.
This
is based on
the
instantaneous
unit
being
set at its
normal setting,
which
is
8
times
tap
rating.
If
the
instantaneous
unit
pickup
is
raised
above
this value,
the
20%
figure
must
be
reduced,
as
described
in the CHARACTERISTICS
section.
The
calculations
listed below
are for the
worst—fault condition,
as
far as CT
performance
is concerned,
which is an internal
ground
fault
between
the
CT
and
the
transformer
winding, with none
of
the
fault
current
supplied through
the neutral of
the
protected
transformer.
1.
Determine
the
burden
on each CT, using
the
following
expressions:
a. For
Wye-connected
CTs
=
+ Ne+2f
+ 2R
Ohms
(Equation
1)
b. For Delta-connected
CTs
Z
= 2B
+ Ne
+
2f
+ 2R Ohms
(Equation
2)
1000
6

GEK-45307
where
B
STD
relay
total
burden
(see
Table
I)
N
= number
of
turns
in
bushing
CT
e
bushing
CT
resistance
per turn,
milliohms
(at
maximum
expected
tempera
ture)
f
busing
CT
resistance
per
lead,
milliohms
(at
maximum
expected
temperature)
R
one-way
control
cable
lead
resistance
(at
maximum
expected
temperature)
TABLE
I
Total
Burden
for
60 Cycle
Relays
STD
TAPS
AMPS
8
X
TAP
AMPS
BURDEN
OHMS
(B)
MIN
P.U.
AMPS
2.9
23.2
0.180 0.87
3.2
25.6
0.156
0.96
3.5
28.0 0.140
1.05
3.8
30.4
0.120
1.14
4.2
33.6
0.112
1.26
4.6
36.8
0.096 1.38
5.0
40.0
0.088
1.50
8.7
69.6
0.048
2.61
2.
Determine
CT
secondary
current
for
8
times
tap
setting.
=
8
x
STD relay
tap
rating
(Note:
For
the
location
of
fault
assumed,
all
the fault
current
is
supplied
by
one CT,
so
that
CT
current
and
relay
current
are the
same,
regardless of
whether
the
CTs
are
connected
in
wye
or
delta.)
3.
Determine
secondary
CT
voltage
required
at
8
times
tap
setting.
Esec
=
IZ
4.
From excitation
curve
of
particular
tap
of
current
transformer
being
used,
determine
excitation
current
IE.
corresponding to
this
secondary
voltage,
Esec.
5.
Determine
the
percent
error
in
each
CT
by the
expression:
%
error
=
1 E
X
100
Is
This
should
not
exceed
20%
of any
set
of CTs.
If
it
does,
it
will be
necessary
to
choose
a
higher
tap
on that set
of CTs,
and repeat
the
calculations
on selection
of
relay
taps,
mismatch
error,
and percent
ratio
error.
7

GE:K-45307
PERCENT
SLOPE
SETTING
The proper
percent
slope
required
is
determined
by the
sum
of:
a.
The
maximum
range
of manual
taps
and
the
load—ratio-control,
or automatic
tap
changing
means,
in percent.
b.
The
maximum
percent
of
mismatch
of
the
relay
taps.
Set
the desired
percent
slope
by
means
of
R3
(See
Figure
6A).
The
percentage
slope
setting
selected
should
be
greater
than
the
ratio
of
maximum
total
error
current
to
the
smaller
of the
through
currents.
In general,
if the
total
error
current
does not
exceed
20%,
the
25%
setting
is
used.
If
it
exceeds
20%,
but
not
35%,
the
40%
setting
is
used.
If the
movable
lead
is
used
(as
in
Figure
1,
for
example)
the
percent
slope
setting
should
be
chosen
about
twice
as high,
since
the movable
lead
provides
no restraint.
EXAMPLE
(REFER
TO FIGURE
4)
I.
Determination
of
CT
Turns
and
STD Relay
Tap
Settings
1.
Transformer
and
Line
A
BC
2.
Maximum
I
= 3750/
J3
(Line
kV)
19.7
49.5
157
3.
100%
Ip
= 3000/
J
(Line
kV)
15.7
39.6
125
4.
Assume
CT
turns
(N)
20 20
60
5.
Maximum
I
secondary
(less
than
5a)
0.98
2.47
2.62
6.
100%
I
secondary
0.79
1.98
2.08
7.
CT connections
Delta
Wye
Delta
8.
Relay
Current
for
100%
I
Sec.
1.37
1.98
3.60
Select
a relay
tap for
one
of the
line
currents
and
calculate
what
the
currents
in
other
lines
would
be if
they
were
increased
in
the
same
ratio.
If
any
current
is
greater
than
V3
times
any other,
the
8.7
tap
should
be
chosen
for it,
and
new
ideal
relay
taps
calculated
for
the
other
lines.
9.
Ideal
Relay
Taps
(Set
C
8.7)
3.31
4.78 8.7
10.
Try
Relay
Taps
3.2
4.6 8.7
11.
Check
Mismatch
Error
Ratio
of
Taps
on
Lines
B-A
=
1.43
Ratio
of
Sec. Lines
Currents
1.98
-
1.37
—
1.44
Mismatch
1.44
- 43
=
0.7%
Ratio
of Taps
on
LinesC-B
-!.j-
=
1.89
Ratio
of
Sec.
Line
Currents
=
1.82
1.89
-
1.82
Mismatch
1
8
=
3.8%
8

GE
K—45
307
Ratio
of
Taps
on Lines
C-A
2.72
Ratio of
Sec.Line Currents
= 2.63
Mismatch
2.72 -2.63
=
(All
are
less than
5%; therefore 1
mismatch error
is
not
excessive)
12. Check
that
the sum
of the
maximum relay currents
is
less than 220 amps
for
1
second, and therefore,
short-time rating
of relay
is
not exceeded.
II. Percent Ratio
Error
ASSUME (all
measured at
their maximum
One-way
CONTROL
CABLE RESISTANCE
Bushing
A CT
resistance
per turn
B
II
C
II II
Bushing A CT
resistance
per lead
B
II II
II II
Ii
II II
expected
temperatures)
(R)
= 0.284
ohms
Ce) =
4 rnilliohms
(e)
2.5
Ce)
= 2.3
(f)
= 75
niilliohms
(f)=525
(f)
= 18.6
1.
Burdens
on
CTs,
using
Equation
1
or Equation
2 from page
6.
a.
Line
A, Z
= 2
(0.156)
+ (20x4)
+
(2.0 x 75)
1000
= 0.312
+ 0.205
+ 0.568
=
1.085
+ 2 (.284)
b. Line
3,
Z = 0.096
+ (20
x
2.5)
+ (2.0 x 52.5)
1000
A
T.o85
25.6
27.8
1
.00
3.4%
+ 0.568
B
0.8
36.8
29.4
50
136%
C
0.833
69.6
58.0
0.5
0.8%
Exciting
current
on line
B is too high;
should
improve CT
performance.
IA - Repeat
— CT
Turns
and
Relay
Tap
Setting
try
higher tap
on CT
to
1. 100%
2.
Try
CT
turns
(necessary to change
C
also
for proper matching)
3. 100% 1 secondary
4.
Relay Current
5. Ideal
Relay
Taps (Set
C
8.7)
6.
Use Relay
Taps
7.
Mismatch
Error is
less
than
5%
= 0.096 + 0.138
+ 0.568
0.80
c.
Line C,
Z 2 (0.048)
+ (60
x
2.3)+(2
x 18.6)
+ 0.568
= 0.096
+ 0.180
+
0.568 0.833
2. Impedance,
ohms
3.
8
times
tap,
amperes
4. E 5
CT voltage require
(IZ)
5.
JE
required,
from excitation
curve
6. % Ratio Error
15.7 39.6
125
20
40 80
0.79 0.99 1.56
1.37
0.99
2.70
4.40
3.19
8.7
4.6
3.2 8.7
9

GEK-45307
hA
Repeat - Percent Ratio
Error
1.
Burden
on
CTs
Line
A,
Z
=
0.192
+
0.205
+ 0.568
= 0.965
Line
B,
Z
0.156
+
0.188
+
0.568
=
0.912
Line
C,
7
= 0.096
+
0.226
+
0.568
= 0.890
2.
Impedance
Ohms
0.965
0.912
0.890
3.
8
times
Tap,
Amperes
36.8 25.6 69.6
4.
CT voltage
required
(17)
35.6 23.4
61.9
5.
‘C
required, from
excitation
curve
1.1 0.25
0.17
5.
%
of
Ratio
Error
3.1%
1.0%
0.3%
Percent
error
is
less
than
20%,
so
CT
taps
and
relay
taps
are
satisfactory.
III
Percent
Slope
Setting
1.
Assume
load
ratio
control
maximum
range
10.0%
2.
Relay
tap
mismatch,
from
IA above (Lines
A-B)
4.6%
Use
25%
setting
14.6%
RATI
NGS
MODELS
12STD15C AND
12STD16C
Continuous
Rating
The
through-current
transformer
and
differential-current
transformer
will
stand
twice
tap value
for
any
combination
of
taps,
or they
will
stand
twice
tap
value
if
all
but
one
of
the
restraint
windings
carry 0 current,
and
the
full restraint
current
(equal
to
twice
tap value)
flows
through
the
differential—current
transformer.
Short
Time
Rating
(Thermal)
220
amperes
for
1
second,
measured
in
the primary
of
any
transformer
of
the
type
STD
relay.
Higher
currents
may
be
applied
for
shorter
lengths
of
time
in
accordance
with
the
following
equation:
= 48,400
where
=
current
amperes
t
=
time
in
seconds.
Short
Time
(Electrical
For
both
the
STD15C
and
STD16C the
sum
of
the
multiples
of
tap
current fed
to
the
relay
from
the
several
sets
of
current
transformers
should not
exceed
150.
These
multiples
should
be
calculated
on the
basis of
RMS
symmetrical
fault
current.
This
limitation
is
a
result
of
the
voltage
rating
of
the
rectifiers
in
the
through—
current
restraint
circuit.
Note
that
in
Figure
1
external
fault
current
can
flow
through
circuit
breakers
52—1
and
52—2
without
being
limited
by
the
transformer
i
in
peda
n
ce.

GEK—45
307
TABLE
II
TARGET
AND
SEAL-IN
UNIT
2.0
Amp
Tap
0.6
Amp
Tap
0.2
Amp
Tap
DC
Resistance
0.13
Ohms
0.6
Ohms
7
ohms
Carry
Continuously
0.5
Amps
1.5
Amps
0.25
Amps
Carry
30
Amps
for
Secs.
0.5
Secs.
Carry
10
Amps
for
30
Secs
4
Secs.
0.2
Secs.
AUXILIARY
RELAY
CONTROL
CIRCUIT
The
STD15C
and
STD16C
relays
are
available for
use
with
4S, 125,
and 250
DC
or
48,
110
and
220
DC
control
voltage,
depending
upon
the
relay
model.
A
plate
with
small
links
located
on the
front
of
the
relay
enables
the
selection
of
one
of
these
voltages.
The
STD
relay
is
provided with
two
open
contacts connected to
a
common
output
circuit.
The
current—closing
rating
of
the
contact
is
30
amps
for
voltages
not
exceeding
250
volts. If
more
than
one
circuit
breaker
is
to be
tripped,
or
if
the
tripping
current
exceeds
30
amperes,
an auxiliary
relay
must
be
used
with the
STD
relay.
After
the
breaker
trips, it
is
necessary
that
the
tripping
circuit
of
these
relays
(STD
and
auxiliary)
be
de-energized
by
an
auxiliary
switch
on
the
circuit
breaker
or by
other
automatic
provisions.
A manual
reset
relay
is
recommended and
normally
used.
CHARACTERISTI
Cs
PICKUP
AND OPERATING
TIME
The
operating
characteristic
is
shown
in
Figure 7.
The
curve
for various percentage
slopes
shows
the
percent
slope
versus
the
throughcurrent
flowing
in
the
transformer.
The
percentage slope
is
a
figure
given
to
a particular
slope
tap
setting,
and
indicates
an
approximate
slope
characteristic.
Pickup
at
zero
restraint
is
approximately
30%
of
tap
value
(see
Table
III).
The
dropout time,
when
the
operating
current
is reduced
to zero
from
any
value
above
pickup,
is
less
than
25
milliseconds.
Curves
of the
operating
time
of
the
main
unit
and
of
the
instantaneous
unit
are
shown
in
Figure
5,
plotted
against
differential
current.
The
main
unit
operating
time
includes
auxiliary
unit
operating
time.
OVERCURRENT
UNIT
PICKUP
The
overcurrent
unit
is
adjusted
to pick
up
when
the
differential
current
transformer
ampere-turns
are
8
times
the
ampere
turns
produced
by
rated
tap
current
flowing
in
that
tap.
For
example:
When
only
one
CT
supplies
current,
and
the
tap
plug
for
the
CT
is
in
the
5
ampere
tap,
40
amperes
are
required
for
pickup.
This
pickup
value
is based
on
the
AC
component
of
current
transformer
output only, since
the
differential
current
transformer
in
the
relay
produces
only
a
half
cycle
of
any
DC
(offset)
component
present.
11

GEK—45
307
If
ratio
matching
taps
are
chosen
so
that rated
CT
current
is
not
greater
than
the
tap
rating
on
a
self-cooled
basis,
the overcurrent
unit will
not
pick
up
on
magnetizing
inrush.
If
CT
currents
are
greater
than
tap
rating,
there
is
danger
that
the
unit
may
pick
up,
especially
on
small
transformer
banks.
If
this
happens,
it
is
recommended
that
the
CT
ratio
or
relay
tap
setting
be
increased,
rather
than
increasing
the
pickup
of the
overcurrent
unit.
If
the
overcurrent
setting must
be
raised,
the
requirements
on
CT
error
will
be
more
stringent,
in
accordance
with
the
following
equation:
E
20
- (2.5)
(P-8)
where
E
= CT
error
current
in
percent,
at
pickup
of
the
overcurrent
unit
P
=
Pickup
of
overcurrent
in
multiples
of
tap
setting.
PERCENTAGE
DIFFERENTIAL
CHARACTERISTICS
The
percentage
differential characteristics
are
provided
by
through—current
restraint
circuits.
In
addition
to
the
operating
circuit,
which is
energized
by
the
differential
current
of
the
line
current
transformers,
the
relay is
equipped
with
a
restraining
circuit
that
is
indirectly
energized
by
the
transformer
secondary
currents.
For
the
relay
to
operate,
the
current
transformer
secondary
currents
must
be
unbalanced
by
a
certain
minimum
percentage,
determined
by
the
relay
slope setting
(as
shown
in
Figure
7).
This
characteristic
is
necessary
to
prevent
false
operation
on
through-fault
currents.
High
currents
saturate
the
cores
of
the
current
transformers
and
cause
their
ratios
to
change, with
the
result
that
the
secondary
currents
become
unbalanced.
Percentage
restraint
is
also
required
to
prevent
operation
by
the
unbalanced
currents
caused
by
imperfect
matching
of
the
secondary
currents,
as
previously
described
under
Determination
of
CT
Turns
and
STD
Relay
Tap
Settings.
HARMONIC
RESTRAINT
CHARACTERISTICS
At
the
time
a
power transformer
is
energized,
current
is supplied
to
the
primary
that
establishes
the
required
flux
in
the
core.
This
current
is
called
magnetizing
inrush,
and
in
the
primary
winding
flows
only
through
the
current
transformers.
This
causes
an
unbalance
current
to
flow in
the
differential
relay, which
would
cause false
operation
if
means
were
not provided
to
prevent
it.
Power
system
fault
currents
are
of
a
nearly
pure
sine
waveform,
plus
a
DC
transient
component.
The
sine
waveform
results
from
sinusoidal
voltage
generation
and
nearly
constant
circuit
impedance.
The
DC
component
depends
on
the
time
in
the
voltage
cycle
at
which
fault
occurs,
and
upon
circuit
impedance
magnitude
and
angle.
Transformer
magnetizing-inrush
currents
vary
according
to the
extremely
variable
exciting
impedance
resulting
from
core
saturation.
They are
often
of
high
magnitude,
occasionally
having
an
RMS
value
with
100% offset,
approaching
16
times
full-load
current
for
worst
conditions
of
power
transformer
residual
flux
and
point-
of-circuit
closure
on
the
voltage
wave.
They
have
a very
distorted
waveform
made
up
of
sharply
peaked
half-cycle
loops
of
current
on
one
side
of
the zero
axis,
and
practically
no
current
during
the
opposite
half
cycles.
The
two
current waves
are
illustrated
in
Figure
8.
12

GEK—45307
Any
current of
distorted,
nonsinusoidal
waveform
may be
considered
as being
composed
of
a
DC
component
plus
a
number
of
sine-wave
components
of different
frequencies;
one
of the
fundamental system frequency,
and the
others,
called
“harmonics,”
having
frequencies
which
are
2, 3,
4, 5,
etc.,
times
the fundamental
frequency.
The
relative
magnitudes
and
phase
positions
of the
harmonics
with reference
to
the
fundamental
determine
the
waveform.
When analyzed
in this
manner,
the
typical
fault-current
wave
is
found
to contain
only
a very
small
percentage
of harmonics,
while
the
typical
magnetizing-inrush—current
wave
contains
a
considerable
amount.
The high
percentage
of harmonic
currents
in the magnetizing-inrush—current
wave
afford
an excellent
means
of
distinguishing
it electrically
from the
fault-current
wave.
In the
Type
STD
relays,
the
harmonic components
are
separated
from
the
fundamental
component
by
suitable
electric
filters.
The harmonic
current
components
are
passed
through
the restraining
circuit
of
the
relay,
while
the fundamental
component
is
passed through
the operating
circuit.
The
DC component
present
in both
the
magnetizing-inrush—
and offset-fault—current
waves
is
largely
blocked
by the
auxiliary
differential-current
transformer
inside
the relay,
and
produces
only
a
slight
momentary
restraining
effect.
Relay
operation
occurs
on differential-current
waves
in which
the
ratio
of harmonics
to fundamental
is lower
than a
given
predetermined
value,
for
which
the
relay
is
set (e.g.
an
internal
fault—current
wave),
and
is restrained
on
differential—current
waves
in
which the
ratio
exceeds
this value
(e.g.
magnetizing-inrush—current
wave).
BURDENS
Burdens
are shown
in
Table
III
and IV.
Burdens
and minimum
pickup
values
are
substantially
independent
of
the
percent
slope settings,
and
are
all approximately
100%
power factor.
Figures
given
are
burdens
imposed
on
each
current
transformer
at
5.0
amperes.
TABLE
III
ZERO
OPERATING
CIRCUIT
*
RESTRAINT
CIRCUIT
TA
G
RESTRAINT
60
CYCLE RELAYS
60
CYCLE
RELAYS
SETTIN
PICKUP
BURDEN
IMPEDANCE BURDEN
IMPEDANCE
AMPS
AMPS
VA
OHMS
VA
OHMS
2.9
0.87 3.2 0.128
1.3 0.052
—
3.2
0.96
2.7 0.108
1.2
0.048
3.5 1.05
2.4 0.096
1.1
0.044
3.8
1.14
2.0
0.080
1.0 0.040
4.2
1.26
1.9
0.076
0.9
0.036
4.6
1.38
1.6
0.064 0.8
0.032
5.0
1.50
1.5
0.060
0.7
0.028
8.7
2.61 0.7
0.028 0.5 0.020
*
Burden of operating coil
is
0
under normal
conditions.
Burden
of 50-cycle
relay is
the same
or slightly
lower.
TABLE
IV
DC
CONTROL CIRCUIT
BURDEN
RATED VOLTS
48 125 250
48
110
220
MILLIANPS
140
105
88
140
96
80
13

GEK—45307
CONSTRUCTION
Figure
6
shows
the
internal
arrangement
of
the
components
of
the
STD15C
relay.
Refer
also to
the
internal
connection diagrams,
Figures
10
and 11,
which
will
identify
the
parts
more
completely.
CURRENT
TRANSFORMERS
In
the
Type
STD15C
relay,
the
through-current
transformer
has
two
primary
windings,
one
for
each
line-current-transformer
circuit.
Winding
No.
1
terminates
at
stud 6
and
winding
No.
2
terminates
at
stud
4.
In
the
Type STD16C
relay,
there
are
three
separate
through—current
transformers,
each
with
only one
primary
winding,
and
each
terminating
at a
separate stud,
windings
No.
I,
No.
2
and
No.
3
corresponding
to
studs
6,
4 and 3
in
that
order.
In
both
relays
there
is
a
differential—current
transformer
with
one
primary
lead
brought
out
to
stud 5.
The
primary
circuit
of
each of
these
transformers
is
completed
through
a
special
tap
block
arrangement.
Two
or
three
horizontal
rows
of
tap
positions
are
provided
(depending
on
whether
the
relay
is
a
Type
STD15C
or
STD16C),
one
row
for
each
through-current
transformer
winding. A
tap
on
the differential—current
transformer
is
connected
to
a
corresponding
tap of
the
through—current
restraint
windings
by
inserting
tap
plugs
in
the
tap blocks.
When the
STD16C
relay
is
used
on
four-circuit
applications,
as
shown
in
Figure
1,
the
fourth
circuit
CT
is
connected
to
stud
7,
and
the
jumper
normally
connected
between
terminals 6
and
7
at
the
rear of
the
relay
cradle
should
be
disconnected
at
the
terminal 6
end
and reconnected
to
the
upper
row
in
the tap
block
(above
the row
marked
winding
1),
which
connects
it
directly
to
the
differential-current
transformer
in
the
STD
relay.
The
terminal
on
the
movable
lead
should be
placed
under the tap
screw
that
gives
the
best
current
match for
the
current
in
the
movable
lead.
The
taps
permit
matching
of
unequal
line—current
transformer
secondary
currents.
The
tap
connections
are so
arranged
that
in
matching
the secondary
currents,
when a
tap
plug
is
moved
from
one
position
to
another
in
a
horizontal
row,
corresponding
taps
on
both
the differential—current
transformer
winding
and one
of
the
through—
current
transformer
windings
are
simultaneously
selected
so
that
the
percent
through-current
restraint
remains
constant.
It
should
be
recognized
that
pickup
current
flows
not
only
through
differential-
current
transformer
but
also
through
one
of
the
primary
windings
of
the through-
current
transformer,
producing
some
restraint.
However, compared
to
the
operating
energy,
this
quantity
of
restraint
is
so
small
that
it
may
be
assumed
to
be
zero.
THROUGH-CURRENT
RESTRAINT
CIRCUIT
A
full
wave
bridge
rectifier
receives
the
output
of
the
secondary
of
each
through
current
restraint
transformer.
In
the
STD16C
relay,
the
DC
outputs
of
all
three
units
are
connected
in
parallel.
The
total output
is
directed
to
the
percent
slope
14

GEK—45307
rheostat
(R3)
located
on
the
front
of
the
relay.
By
means
of
adjusting
the
rheostat,
the
percent
slope
may
be
varied
from
15%
to
40%.
The
output
is
put
through
an
isolating
transformer,
rectified,
and
directed
to
the
sensitive
solid
state
amplifier
that
controls
the
telephone-type
relay.
DIFFERENTIAL-CURRENT
CIRCUIT
The
differential-current
transformer
secondary
supplies
1)
the
instantaneous
unit
directly;
2)
the
operating
(tripping)
signal
to
the
solid—state
amplifier
through
a
series-tuned
circuit;
and
3)
the
harmonic
restraint
isolating
transformer
through
a
parallel
resonant
filter.
The
operating
and
restraint
currents
are
each
rectified
by
a
full—wave
bridge
prior
to being
supplied
to
the
sensitive
sense
amplifier.
The
series
resonant
circuit
is
made
up
of
a
5
microfarad
capacitor
(Cl)
and
a
reactor
(Li)
that
are
tuned
to
pass
currents
of
the
fundamental
system
frequency
and
to
offer
high
impedance
to
currents
of
other
frequencies. Resistor
Ri
is
connected
in
parallel
on
the
AC
side
of
the
operate
rectifier,
and
can
be
adjusted
to give
the
desired
amount
of
operate
current.
The
output
of
the
rectifier
is
applied
to
the
operating
circuit
of
the
sense
amplifier.
The
parallel
resonant
trap
is
made
of
a 15
microfarad
capacitor
(C2)
and
a
reactor
(L2)
that
are
tuned
to
block
fundamental
frequency
currents
while
allowing
currents
of
harmonic
frequencies
to
pass with
relatively
little
impedance.
Resistor
R2
is
connected in
parallel
on
the
AC
side
of
the
harmonic
restraint
rectifier,
and
can
be adjusted
to
give
the
desired
amount
of
harmonic
restraint.
The
output
of
the
rectifier
is paralleled
with
the
through-current
restraint
currents
and
applied
to
the
restraint
circuit
of
the
sense
amplifier.
It
will
be
evident
that
if
the
differential
current
applied
to
the
relay
is
sinusoidal
and
of
system
frequency,
it
will
flow
mostly
in
the
operating
circuit
and
hence
cause
the
relay
to
yield
an
output.
If,
however,
the
differential
circuit
contains
more
than
a
certain
percentage
of
harmonics,
the
relay
will
be
restrained
from
operating
by the
harmonic
currents
flowing
in
the
restraint
circuit.
A
ThyriteR
resistor
connected
across
the
secondary
of
the
differential-current
transformer
limits
any
momentary
high—voltage
peaks
which
may
occur,
thus
protecting
the
rectifiers
and
capacitors
from
damage,
without
materially
affecting
the
characteristics
of
the
relay.
OVERCURRENT
UNIT
The
instantaneous
unit
is
a hinged—armature
relay
with
a
self—contained
target
indicator.
On
extremely
heavy
internal
fault
currents,
this
unit
will
pick
up
and
complete
the
trip
circuit.
The
instantaneous
unit
target
will
be exposed,
to
indicated
that
tripping
was
through
the
instantaneous
unit.
Because
of
saturation
of
the
CTs
and
relay
transformers
at
high
fault
currents,
it
is
possible
that
less
operating
currents
will
be
provided
from
the
differential—
current
transformer
than
the percentage
slope
tap
would
imply,
and
more
harmonic
restraint
will
be
provided than
the
actual
harmonic
content
of
the
fault
current
would
supply.
As
a
result,
under
conditions
of
a
high
internal
fault
current,
the
main
unit
may
be
falsely
restrained.
Tripping
is
assured,
however,
by
the
15

GEK—45307
overcurrent
unit
operation.
Pickup
is
set above
the level
of
differential
current
produced
by
maximum
magnetizing
inrush
current.
Figure
5
shows
the
relative
levels
of
pickup
and
speed
of operation
of
the main
unit
and
the
overcurrent
unit.
MAIN
OPERATING UNIT
The
primary
functioning
unit
of
the
STD
relay
is
a
solid-state amplifier,
whose
output
controls
a
simple
telephone
relay.
The sense
amplifier
is
shown
in
Figures
10
and
11
as
a large
rectangle. The amplifier
consists
of
many
electronic
components
mounted
on
a printed
circuit
card
in
the
top
half
of
the relay.
This
printed
circuit
card
is
installed
in
a
ten-prong
printed
card
design
socket.
A
schematic
of
this card
is
shown
in
Figure
9.
This
component
is adjusted
prior
to
leaving
the
factory,
and
should
require
no
further
attention.
The
telephone-type relay
is
mounted
vertically
in
the
mid-section
of the
relay.
It,
too,
has
been
carefully
adjusted
at
the
factory,
and
should
require
no
further
attention.
If
this
small
relay
has
been
disturbed,
refer
to the
section
under
ADJUSTMENTS.
TARGET
AND
SEAL-IN
UNIT
There
is
a
target
and seal-in
unit
mounted
on
the top
left
of the
relay.
This
unit
has
its
coil
in series
and
its contacts
in parallel
with
the main
contacts
of
the
telephone—type relay.
When
the
telephone—type
relay contacts
close,
the
seal-in
unit
operates, raising
its
target
into
view
and
sealing
around
the
telephone-type
contacts.
The
target
of
this
unit
will
remain
exposed
until
released
by
pushing
a
button
beneath the
lower
left corner
of
the
cover
of
the relay
case.
CASE
The
case
is
suitable
for
surface
or
semi-flush
panel
mounting,
and
an
assortment
of
hardware
is provided
for
either
method.
The
cover
attaches
to
the case,
and carries
the
target reset
mechanism
for the
trip
indicator
and
instantaneous unit.
Each
cover
screw
has
provision
for
a
sealing
wire.
The
case
has
studs
or
screw
connections
at
the
bottom
for
the
external
connections.
The
electrical
connections
between
the
relay
unit
and
the case
studs
are
made
through
spring—backed
contact
fingers
mounted
in
stationary
molded
inner
and
outer
blocks,
between
which
rests
a
removable
connecting
plug that
completes
the
circuit.
The
outer
block,
attached
to
the case,
holds
the
studs
for
the
external
connections,
and
the inner
block
has
terminals for
the internal
connections.
The
relay
mechanism
is
mounted
in
a
steel
framework
called
the
cradle,
and
is
a
complete
unit,
with
all leads
terminating
at
the
inner
block.
This
cradle
is held
firmly
in
the
case
by a latch
at
the
top
and bottom
and
a
guide
pin
at
the
back of
the
case.
The
case
and
cradle
are
so constructed
that
the
relay
cannot
be inserted
in
the
case
upside
down.
The
connecting plug,
besides
making
the
electrical
connection
between
the
blocks
of
the
cradle
and
case, also
locks the
latch
in
place.
The
cover,
which
is
fastened
to
the
case
by thumbscrews,
holds
the connecting
plug
in
place.
To
draw
out the
relay
unit,
the
cover
is
removed
and
the
plug
is
drawn
out.
Shorting
bars
are provided
in the case
to
short
the
current-transformer
circuits
16

GEK-45307
(see
Figure
12).
The
latches
are
then
released
and
the
relay
unit can
be
easily
drawn
out.
A
separate
testing
plug
can
be
inserted
in
place
of
the
connecting
plug
to
test
the
relay
in
place
on
the
panel, either
from
its
own
source
of
current,
or
from
other
sources.
Or,
the
relay
unit
can
be
drawn
out
and
replaced
by
another
which
has
been
tested in
the
laboratory.
RECEIVING.
HANDLING
AND
STORAGE
These
relays,
when
not
included
as
part
of
a
control
panel,
will be
shipped
in
cartons
designed
to
protect
them
against
damage.
Immediately
upon
receipt
of
a
relay,
examine
it
for
any damage
sustained
in
transit.
If
injury
or
damage
resulting
from rough
handling is
evident,
file
a
damage
claim
at
once with
the
transportation
company
and
promptly
notify
the
nearest
General
Electric
Sales
Office.
Reasonable
care
should
be
exercised
in
unpacking
the relay,
in
order
that
none
of
the
parts
may
be
injured
or
the
adjustments
disturbed.
If
the
relays
are
not
to
be
installed
immediately,
they
should
be
stored in
their
original
cartons in
a
place
that
is
free
from
moisture,
dust
and
metallic
chips.
Foreign matter
collected
on
the
outside
of
the
case
may
find
its
way
inside when
the
cover
is
removed,
and
cause
trouble
in
the
operation
of the
relay.
ACCEPTANCE
TESTS
Immediately
upon
receipt
of
the
relay,
an
inspection
and
acceptance
test
should
be
made
to
make
sure
that
the
relay
has
not
been
damaged
in
shipment
and
that
the
relay
calibrations
are
unchanged.
VISUAL INSPECTION
Check
the nameplate
stamping
to
make
sure
that
the
model number,
rating and
calibration
range
of
the
relay received
agree with the
requisition.
Remove
the
relay
from
its
case
and
check
by
visual inspection
that
there
are
no
broken
or cracked
molded
parts,
or
other
signs
of
physical
damage,
and
that
all
screws
are
tight.
frIECHANICAL
INSPECTION
Check
the
operation
of
the
telephone-type
relay
and
instantaneous
overcurrent
unit
manually,
to
see
that
they
operate
smoothly,
without
noticeable
friction
or
binds.
Check
the
contact
gap
and
wipe
of
these
units,
which
should
agree with
values
given
in
the
section
on
SERVICING.
ELECTRICAL
TESTS
It
is
recommended
that
the
following
electrical
tests be
made immediately
upon
receipt
of
the
relay.
17

GEK—45307
1.
Minimum
pickup
of
main
operating
unit.
2.
Minimum
pickup
of
the
instantaneous
overcurrent
unit.
3.
A
single
check
point
on
the
harmonic
restraint
characteristic.
4.
A
single
check
point
on
the
slope
characteristic
curve,
for
the
approximate
slope
expected
to
be
used.
Test
Equipment
In
order
to facilitate
tests,
the
following
test
equipment
is
recommended:
1.
Two
load
boxes
for regulating
the
test
currents
2.
Three
ammeters
(two
AC and
one DC)
for
measuring
the
test
currents
3.
A
test
rectifier
for
checking
the
relays
response
to
the
second
harmonic.
(See
Figure
13.)
4.
One
indicating
lamp
5.
Two
single-pole
double—throw
switch
selectors
6.
A
double-pole
single-throw
line
switch.
Check
the
pickup of
the main
unit,
using
the
connections
shown
in
Figure
14.
During
this
test,
the selector
switches
(S2
and S4)
are
open,
and
current
passes
through
one
restraint
winding
and
the
operate
winding
only.
For
example,
with
a
relay
set
with
a
25%
slope
and
at
the
2.9 ampere
ratio-matching
taps,
the
main
unit
should
pick
up at
30%
of
tap
rating,
1O%,
or
the
pickup
should
be between
0.78
and
0.96
amperes.
A
source
of
DC
power
at
rated
voltage
should
be
connected
as
shown
in
Figure
14;
the
indicating
lamp
will
provide
a
signal,
showing
that
the
main
unit
has
operated.
For
an
additional
pickup
test,
the
pickup
should
be
1.5
amperes,
with
current
flowing
in
terminals
5
and
6
and
the tap
plugs
in
the
5
ampere
tap
and
the
25%
slope
tap
position.
If
the
pickup
is
between
1.35
and
1.65
amperes,
no
adjustment
should
be made.
The
pickup
of
the
relay
has
wider
permissible
variations
than
most
protective
relays,
but
due
to
the
relay
design
and
application,
the
relay
accuracy
is
entirely
adequate
under
all
conditions,
even
during
transformer
magnetizing
inrush
or
severe
fault
conditions.
With
the
selector
switch,
ST,
in
the
A
position,
check
the
harmonic
current
restraint,
as
described
in
the section
on
INSTALlATION
PROCEDURE.
The
instantaneous
overcurrent
unit
should
be
checked
by passing
a
high
current
through
the
5-6
terminals.
The
pickup
should
be
about
eight
times
tap
rating.
Check
through—current
restraint,
as
described
in
the
section
on
INSTALLATION
PROCEDURE.
INSTALLATION
PROCEDURE
TESTS
Before
placing
the
relays
in
service,
check
the relay
calibration
to
be
used
in
its
final
location,
to
ensure
that
it
is correct.
The following
test
procedure
is
outlined for
this
purpose.
18

GEK—45307
CAUTION
The
Relay calibration
is
accomplished
by
adjusting
resistors
Ri,
R2
and
R3.
Changes
made
in
any
one
of
these
resistors
will
affect
the
other
two
settings.
In
the
event
one setting
is
changed,
the
pickup,
harmonic
restraint,
and
through—current
restraint
adjustment
procedures
should
be
repeated
until
no
further
deviation
from
proper
calibration
is
noted.
Best
results
can
be
obtained
if
the
through-current
restraint
adjustment
is
made
only
after
the
other
two
settings
are
correct.
PICKUP
The test
circuit
for
pickup
is
shown in
Figure
14,
with
S?
open.
The
first
values
given
are
for
a
5
ampere
rated relay.
Those
in
(
)
are for
a
1
ampere
rated
relay.
Pickup
should
be
1.5
(0.3) amperes
with
current
flowing
in
terminals
5
and
6,
and
the
tap
plugs in
the
5
(1.0)
ampere
position,
and
25%
slope
tap
setting.
The
pickup
operation
should
be
repeated
several times,
until
two
successive
readings
agree
within
0.01 ampere
with the
total
pickup
current
being
interrupted
between
successive
checks.
If
pickup
is
found
to
be
from
1.35
to 1.65
(0.27
to
0.33),
the
setting
should
not
be
disturbed.
With
DC
control
voltage applied
to
the proper
terminals
of
the
relay,
the
pickup
of
the
telephone-type
relay
can
be
used
as
an
indication
of
operation
of
the
amplifier.
This
voltage
may
be
applied
as
shown in
Figure
15,
and
the
indicating
lamp
will
verify
that
the
amplifier
has
produced
an
output
signal.
Before
Ri
is
adjusted
for pickup,
put
a
DC
voltmeter
(one
volt
scale)
on
pin
2
(-)
and
pin
8
(+)
of
the
sense
amplifier
card.
The
pins
are
counted
from
right
to
left
when
viewed
from
the
card
socket wiring
connections.
Apply pickup
current
and
adjust
Ri
for
a
voltage
input
to
the
sense
amplifier
card
of
0.430 to
0.470
volts.
If
the
pickup
is
not
in
limits
with
this
setting,
then adjust
P1
on
the
sense
amplifier
card
to
obtain
the
proper
pickup
value.
HARMONIC
CURRENT
RESTRAINT
The
harmonic
restraint
is
adjusted
by
means
of
a
Test
Rectifier
used
in
conjunction
with
suitable
ammeters
and
load
boxes.
The
test
circuit
is
as
shown
in
Figure
15,
with
S2
closed
to
position
P.
Tests
should
be
made
on
the
5.0
(1.0) ampere
and
25%
slope
taps.
The
analysis
of
a
single-phase
half-wave
rectified
current
shows
the
presence
of
fixed
percentages
of
DC,
fundamental,
and
second
harmonic
components,
as
well
as
negligible
percentages
of
all
higher
even
harmonics.
This
closely
approximates
a
typical
transformer
inrush
current,
as seen
at
the
relay
terminals,
inasmuch
as
its
principal
components
are
DC,
fundamental,
and
second
harmonic.
Although
the
percent
second harmonic
is
fixed,
the
overall
percentage
may
be
varied
by
providing
a
path
for
a
controlled
amount of
by-passed
current
of
fundamental
frequency.
The
by
passed current
is
added
in
phase
with
the
fundamental
component
of
the
half-wave
rectified
current,
and
thus
provides
a
means of
varying the
ratio
of
second
harmonic
current
to
fundamental
current.
19

GE
K-45
307
The
following
expression
shows
the
relationship
between
the
percent
second
harmonic,
the
DC
component, and
the
by-pass
current.
%
Second
Harmonic
=
0.212
‘DC
100
0.4511
+ 0.51Dc
Figure
16
is
derived
from
the
above
expression.
It
shows
the
percent
second
harmonic
corresponding
to
various
values of
by—pass
current
(ii)
for
a
constant
DC
set
at
4.0
(0.8)
amperes.
The
relay
is
calibrated
with
a
composite
RMS
current of
two
times
tap
value. When
properly
set,
the
relay
will
restrain with
greater
than
20%
second
harmonic, but
will
operate
with
second harmonic
equal
to
20%
or
lower.
With
the
DC
ammeter
(12)
set
at
4.0
(0.8) amperes,
the
auxiliary
relay
should
just
begin
to
close its
contacts
with
gradually
increasing
bypass
current
(‘i)
at
a
value
of
4.5—5.5
(0.9—
1.1)
amperes.
This
corresponds
to
19-21%
second harmonic
(see
Figure
16),
providing
a
2%
tolerance
at the
set
point
to
compensate
for
normal
fluctuations
in
pickup.
It
should
be
noted
that
the
current
magnitude
in
the
rectifier
branch
(12)
is
slightly
influenced
by
the
application
of
by-pass
current
(Ii)
and
should
be
checked
to
make
sure
it
is
maintained
at
its
proper
value.
If
harmonic
restraint
is
found
to
be
out
of
adjustment,
it may
be
corrected
by
adjusting
rheostat
R2.
(See
CAUTION
at
beginning
of
INSTALLATION
PROCEDURES
section.)
THROUGH-CURRENT
RESTRAINT
The
through-current
restraint,
which
gives
the
relay
the
percentage
differential
or
percent
slope
characteristics,
is
shown in
Figure
7.
It
may
be
checked
and
adjusted
using
the
circuit
illustrated
in
Figure
15,
with
S2
closed
to
position
B.
Ammeter
I
reads the
differential
current
and
13
reads
the
smaller
of
the
two
through—
currents.
r-
CAUTION
These
currents
should
be
permitted
to
flow
for
only
a
few
seconds
at
a
time,
with
cooling periods
between
tests;
otherwise,
the
coils
will
be
overheated
-
NOTE:
The
percent
slope
tolerance
is
10%
of
nominal,
all
in
the
plus
direction.
This
is
to
ensure
that
the
slope
characteristic
never
falls
below
set
point
value.
In
testing
STD16C
relays,
the
setting
should
be
checked
with switch
S4
first
in one
position
and
then
the
other,
thus
checking
all
the
restraint coils. With
the
current
tap
plugs
in
5.0
(1.0)
ampere
position
and
the
percent slope
set
in
the
40%
position,
the
relay
should
just
pick
up
for
values
of
the
1j
and
13
currents
indicated
in
Table
V
(VA).
Repeat
with
the percent
slope tap
set
in
the
25%
and
15%
positions.
If
any
one
of
these
set
points
is
found
to
be
other
than
as
prescribed,
the
adjustment
may
be
made made by
adjusting
R3.
It
should
be
noted
that
the
current
magnitude
in
the
through-current
branch
(13)
is
slightly
influenced by
the
application
of
differential
current
(Ii)
and
should
be
checked
to
make
sure
that
it
is
maintained
at
its
proper
value.
TABLE
V
TABLE
VA
%
SLOPE
AMPERES
—-
-
-
SETTING
13
40
30
12.0-13.2
40.0—44.0
25
30
7
.5-
8.3
25.0—27.5
15
30
4.5-
5.0
15
.0—16.5
1RUE
SLOPE
SLOPE
AIPERES
TRUE
SLOPE
(11/13
x 100)
SETTING
13
I
(1/3
X
100)
40
6
2.4-2.64
40-44
25
6
1
.5—1
.66
25—27.5
15
6
0
.9—1
.00
15—16.5
20