GE GSY51A User manual

GEK
-
45441
C
Supersedes
GEK
-
45441
B
INSTRUCTIONS
MHO
AND
AUXILIARY
RELAY
for
OUT
-
OF
-
STEP
PROTECTION
TYPE
GSY
51
A
GENERAL
ELECTRIC
Courtesy of NationalSwitchgear.com

GEK
-
45441
DESCRIPTION
3
APPLICATION
OPERATION
OF
SCHEME
APPLICATION
CONSIDERATIONS
3
4
5
6
RATINGS
7
CURRENT
CIRCUIT
POTENTIAL
CIRCUIT
.
.
.
AMBIENT
TEMPERATURES
DC
RATINGS
TARGET
/
SEAL
-
IN
MHO
UNIT
CONTACTS
.
.
.
7
7
7
7
8
8
OPERATING
PRINCIPLES
9
CHARACTERISTICS
9
BURDENS
9
CURRENT
CIRCUITS
.
.
POTENTIAL
CIRCUITS
9
10
CALCULATION
OF
SETTINGS
GRAPHICAL
PROCEDURE
BLINDER
SETTINGS
MHO
UNIT
SETTING
VERIFICATION
OF
SETTINGS
10
10
10
11
12
CONSTRUCTION
13
GENERAL
13
DRAWOUT
RELAYS
.
.
.
POWER
REQUIREMENTS
13
13
ACCEPTANCE
TESTS
VISUAL
INSPECTION
MECHANICAL
INSPECTION
ELECTRICAL
TESTS
MHO
UNIT
-
ACCEPTANCE
TESTS
(
STANDARD
FACTORY
CALIBRATION
)
MHO
UNIT
-
ACCEPTANCE
TESTS
(
FACTORY
ADJUSTED
PER
CUSTOMER
'
S
SETTINGS
)
13
14
14
15
18
20
INSTALLATION
PROCEDURE
.
.
.
.
INSPECTION
LOCATION
MOUNTING
CONNECTIONS
MECHANICAL
INSPECTION
20
20
20
20
21
21
PERIODIC
CHECKS
AND
ROUTINE
MAINTENANCE
CONTACT
CLEANING
SERVICING
(
STANDARD
FACTORY
CALIBRATION
)
SERVICING
(
FACTORY
ADJUSTED
PER
CUSTOMER
'
S
SETTINGS
)
21
21
22
24
RENEWAL
PARTS
-
2
-
Courtesy of NationalSwitchgear.com

GEK
-
45441
MHO
AND
AUXILIARY
RELAY
FOR
OUT
-
OF
-
STEP
PROTECTION
TYPE
GSY
51
A
DESCRIPTION
The
Type
GSY
51
A
relay
is
designed
specifically
for
use
in
conjunction
with
the
Type
CEX
57
E
angle
-
impedance
relay
to
detect
an
out
-
of
-
step
condition
on
a
power
system
.
The
relay
includes
a
mho
-
type
distance
unit
with
provision
for
offsetting
its
characteristics
,
six
telephone
-
type
auxiliary
units
with
the
necessary
series
resistors
,
and
a
target
seal
-
in
unit
,
all
mounted
in
an
L
2
size
case
.
The
outline
and
panel
drilling
dimensions
are
shown
in
Figure
9
,
and
the
internal
connections
in
Figure
3
.
APPLICATION
The
usual
application
of
the
Type
GSY
51
A
relay
and
its
associated
Type
CEX
57
E
angle
-
impedance
relay
is
at
the
terminals
of
a
generator
to
provide
out
-
of
-
step
protection
of
the
machine
,
resulting
high
current
peaks
and
off
-
frequency
operation
may
cause
winding
stresses
,
pulsation
torques
and
mechanical
resonances
that
have
the
potential
of
damaging
the
turbine
-
generator
.
Therefore
,
to
minimize
the
possibility
of
damage
,
it
is
generally
accepted
that
the
machine
should
be
tripped
without
delay
,
preferably
during
the
first
half
-
slip
cycle
of
the
loss
of
synchronism
condition
.
When
a
generator
loses
synchronism
,
the
Formerly
the
generator
,
transformer
and
system
impedance
characteristics
were
such
that
the
electrical
center
during
loss
-
of
-
synchronism
conditions
generally
occurred
out
in
the
transmission
system
.
Hence
the
resulting
swing
impedance
locus
intersected
transmission
lines
and
would
be
detected
by
line
relaying
or
by
out
-
of
-
step
detection
schemes
located
at
line
terminals
,
and
in
most
instances
the
system
could
be
separated
without
the
need
for
tripping
generators
.
With
the
advent
of
EHV
systems
,
larger
generators
and
the
general
expansion
of
transmission
systems
,
the
impedances
involved
have
changed
significantly
.
Generator
and
step
-
up
transformer
impedances
have
increased
in
magnitude
,
while
system
impedances
have
decreased
.
As
a
result
,
on
many
systems
today
the
electrical
center
during
loss
-
of
-
synchronism
conditions
can
,
and
frequently
does
,
occur
in
the
generator
or
in
the
step
-
up
transformer
.
In
general
,
the
protection
normally
applied
in
the
generator
zone
,
such
as
differential
relaying
,
time
-
delay
system
back
-
up
,
etc
generator
during
a
loss
of
synchronism
.
will
not
protect
a
The
loss
of
excitation
relay
may
•
J
These
instructions
do
not
purport
to
cover
all
details
or
variations
in
equipment
nor
to
provide
for
every
possible
contingency
to
be
met
in
connection
with
installation
,
operation
or
maintenance
.
further
information
be
desired
or
should
particular
problems
arise
which
are
not
covered
sufficiently
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
,
IEEE
and
NEftA
standards
/
but
no
such
assurance
is
given
with
respect
to
local
codes
and
ordinances
because
they
vary
greatly
.
Should
-
3
-
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GEK
-
45441
provide
some
degree
of
protection
,
but
cannot
be
relied
on
to
detect
generator
loss
of
synchronism
under
all
system
conditions
.
Therefore
,
if
during
a
loss
of
synchronism
the
electrical
center
is
located
in
the
region
from
the
high
voltage
terminals
of
the
generator
step
-
up
transformer
down
into
the
generator
,
separate
out
-
of
-
step
relaying
should
be
considered
to
protect
the
machine
.
The
combination
of
the
GSY
51
A
and
CEX
57
E
relays
,
located
at
the
generator
terminals
,
is
intended
locus
passes
through
the
machine
or
step
-
up
transformer
impedance
,
scheme
utilizes
three
impedance
measuring
units
and
the
logic
circuitry
provided
by
the
auxiliary
units
in
the
GSY
to
evaluate
the
progressive
change
in
impedance
during
a
loss
of
synchronism
condition
,
and
to
initiate
tripping
when
the
angle
between
generator
and
system
voltages
is
900
or
less
,
typical
application
of
the
GS
51
A
and
CEX
57
E
relays
at
the
terminals
of
a
generator
is
covered
by
the
external
connection
diagram
in
Figure
10
A
.
contact
circuit
logic
is
shown
in
Figure
10
B
.
to
detect
an
out
-
of
-
step
condition
when
the
swing
This
This
The
Although
the
CEX
-
GSY
scheme
is
normally
applied
at
the
terminals
of
the
generator
,
there
are
some
applications
where
it
will
be
more
advantageous
to
apply
these
relays
at
the
high
voltage
terminals
of
the
step
-
up
transformer
.
This
is
discussed
further
in
the
section
on
CALCULATION
OF
SETTINGS
.
The
occurrence
of
a
single
-
phase
-
to
-
ground
fault
that
evolves
into
a
double
-
phase
-
to
-
ground
fault
may
,
under
certain
conditions
,
appear
as
an
impedance
swing
to
the
CEX
-
GSY
scheme
,
misoperation
during
the
clearing
of
such
a
fault
on
the
system
,
it
is
suggested
that
an
instantaneous
ground
overcurrent
relay
be
connected
to
supervise
the
CEX
-
GSY
contact
circuit
,
as
shown
in
Figure
10
B
.
To
avoid
this
remote
possibility
of
a
OPERATION
OF
THE
SCHEME
The
operating
principles
of
the
scheme
can
be
explained
with
the
aid
of
the
R
-
X
diagram
and
contact
circuit
in
Figure
10
B
.
This
diagram
shows
the
angle
-
impedance
,
or
blinder
,
unit
contacts
(
21
ST
/
A
1
,
21
ST
/
A
2
,
21
ST
/
B
1
and
21
ST
/
B
2
)
,
the
offset
mho
unit
contacts
(
21
M
/
a
and
21
M
/
b
)
,
and
the
contacts
of
the
six
auxiliary
units
(
X
,
XI
,
X
2
,
X
3
,
X
4
,
and
X
5
)
.
The
angle
impedance
units
and
their
associated
auxiliary
units
(
XI
,
X
2
,
X
4
,
and
X
5
)
operate
independently
to
determine
a
loss
of
synchronism
,
but
their
trip
output
unit
,
X
3
,
is
supervised
by
the
21
M
/
a
contact
via
21
M
/
X
.
This
assures
that
tripping
is
initiated
only
for
an
impedance
swing
that
traverses
the
mho
unit
characteristic
(
21
M
of
the
R
-
X
diagram
in
Figure
10
B
)
.
Final
tripping
is
permitted
by
21
M
/
b
when
the
impedance
swing
leaves
the
mho
characteristic
,
causing
the
mho
unit
to
reset
.
The
R
-
X
diagram
in
Figure
10
B
shows
the
are
of
closure
of
the
21
ST
/
A
and
21
ST
/
B
blinder
unit
contacts
A
1
,
A
2
,
Bl
,
and
B
2
.
impedance
phasor
terminates
to
the
right
of
blinder
B
,
21
ST
/
A
2
and
21
ST
/
B
2
will
be
closed
.
21
ST
/
B
1
will
be
closed
,
and
when
the
impedance
is
to
the
left
of
blinder
A
,
21
STA
1
and
21
ST
/
B
1
will
be
closed
,
steady
-
state
conditions
,
load
impedance
will
fall
somewhere
near
the
R
axis
,
For
example
,
when
an
When
the
impedance
is
between
the
blinders
,
21
ST
/
A
2
and
It
should
be
noted
that
for
normal
-
4
-
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GEK
-
45441
to
the
right
of
21
ST
/
B
and
outside
the
21
M
mho
characteristic
,
condition
21
ST
/
A
2
and
21
ST
/
B
2
will
be
closed
,
21
M
/
X
1
will
be
picked
up
,
21
M
/
b
will
be
closed
,
and
21
M
/
a
will
be
open
.
For
this
Referring
to
the
contact
circuitry
and
the
R
-
X
diagram
in
Figure
10
B
,
the
step
-
by
-
step
operation
of
the
scheme
during
an
impedance
swing
will
be
as
follows
:
Step
1
-
Assume
that
a
swing
impedance
locus
traverses
the
relay
characteristics
from
C
to
K
.
Before
the
swing
reaches
point
D
the
mho
unit
(
21
M
)
will
be
dropped
out
,
the
21
ST
/
A
2
and
21
ST
/
B
2
contacts
are
closed
,
and
the
two
21
M
/
X
1
contacts
are
closed
.
The
21
ST
/
A
1
and
21
ST
/
B
1
contacts
are
both
open
.
Step
2
-
When
the
swing
locus
enters
the
mho
unit
characteristic
at
point
D
,
the
21
M
/
b
contact
in
the
trip
circuit
opens
and
21
M
/
a
closes
,
picking
up
unit
21
M
/
X
,
which
closes
its
contact
in
the
X
3
coil
circuit
.
Step
3
-
The
impedance
swing
next
enters
the
area
,
G
,
between
the
two
blinder
characteristics
,
closing
21
ST
/
B
1
and
opening
21
ST
/
B
2
,
which
in
turn
de
-
energizes
the
XI
unit
.
Closure
of
the
B
1
contact
picks
up
X
2
through
the
A
2
contact
,
which
is
still
closed
,
and
the
XI
contact
,
which
has
a
12
-
cycle
time
-
delay
dropout
.
Unit
X
2
seals
in
around
the
XI
contact
.
Step
4
-
When
the
impedance
locus
enters
region
H
,
to
the
left
of
blinder
A
,
21
ST
/
A
2
opens
,
21
ST
/
A
1
closes
,
and
21
ST
/
B
1
remains
closed
,
opening
of
A
2
de
-
energizes
the
X
2
unit
and
the
closure
of
A
1
picks
up
unit
X
4
.
The
12
-
cycle
dropout
time
of
the
X
2
unit
provides
time
for
X
3
to
pick
up
through
the
X
2
and
X
4
contacts
,
and
the
X
unit
contact
which
has
previously
closed
.
these
contacts
,
and
the
X
3
contact
in
the
trip
circuit
closes
.
The
The
X
3
unit
seals
in
around
Step
5
-
When
the
impedance
locus
enters
region
K
,
the
mho
unit
resets
,
opening
21
M
/
a
and
closing
21
M
/
b
.
The
closure
of
21
M
/
b
completes
the
trip
coil
circuit
through
the
21
M
/
X
3
contact
which
closed
in
Step
4
.
The
previous
description
assumes
an
impedance
swing
from
right
to
left
(
C
to
K
)
,
which
will
be
the
case
for
generator
loss
of
synchronism
.
However
,
the
scheme
will
perform
correctly
for
swings
in
either
direction
,
and
hence
is
applicable
for
out
-
of
-
step
protection
out
on
the
transmission
system
as
well
as
at
the
generator
terminals
.
APPLICATION
CONSIDERATIONS
The
application
of
out
-
of
-
step
relaying
schemes
at
a
generator
,
or
out
in
the
system
,
is
not
a
simple
procedure
.
In
general
,
the
proper
application
of
such
schemes
requires
extensive
stability
studies
to
determine
the
following
:
Loss
-
of
-
synchronism
characteristics
(
i
.
e
.
,
impedance
loci
)
.
Maximum
expected
generator
slip
.
1
.
2
.
-
5
-
Courtesy of NationalSwitchgear.com

GEK
-
45441
Characteristics
of
expected
stable
swings
.
Expected
current
levels
.
The
relationship
between
this
information
and
the
application
of
the
CEX
-
GSY
scheme
is
covered
in
a
technical
paper
,
information
on
this
subject
,
he
should
request
it
from
the
nearest
General
Electric
Company
Sales
Office
.
3
.
4
.
If
the
user
desires
further
RATINGS
Refer
to
Figures
1
and
2
.
and
is
available
for
50
or
60
hertz
frequencies
.
The
GSY
51
A
relay
is
rated
for
120
volts
,
5
amperes
,
The
basic
minimum
reach
is
2
,
4
,
6
ohms
phase
-
to
-
neutral
at
90
o
lead
,
with
the
restraint
leads
connected
at
100
%
on
the
restraint
transformer
.
Each
basic
minimum
reach
can
be
selected
with
a
link
arrangement
located
behind
the
relay
.
There
are
two
sections
of
reach
on
one
tap
block
;
each
section
is
marked
A
-
B
and
2
,
0
,
4
.
By
connecting
both
sections
as
shown
in
Table
I
,
the
basic
minimum
ohmic
reach
of
the
relay
can
be
selected
.
TABLE
I
Minimum
Reach
Phase
-
to
-
Neutral
Links
"
A
"
"
B
"
2.0
ohms
4.0
ohms
6.0
ohms
2
0
0
4
4
2
The
basic
minimum
reach
can
be
extended
up
to
ten
times
the
basic
reach
by
reducing
the
restraint
circuit
setting
on
the
restraint
transformer
.
The
following
equation
can
be
used
for
increasing
the
basic
minimum
reach
of
the
relay
.
100
(
%
)
(
90
°
lead
)
Z
Relay
=
x
basic
minimum
reach
Transformer
setting
(
%
)
Ohmic
reach
settings
greater
than
the
basic
minimum
reach
can
be
set
within
1
%
increments
by
selecting
the
proper
taps
on
the
restraint
transformer
,
which
is
tapped
in
10
%
and
1
%
steps
.
The
relay
also
has
a
reverse
offset
transactor
with
a
phase
-
to
-
neutral
offset
reach
of
0
to
4
ohms
in
0.5
ohm
steps
.
The
taps
are
selected
with
the
"
H
"
and
"
L
"
leads
on
the
offset
tap
block
,
which
is
located
on
the
left
front
side
,
approximately
in
the
center
of
the
relay
.
The
transactor
angle
can
be
adjusted
with
the
R
63
rheostat
,
located
just
to
the
right
of
the
offset
tap
block
.
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6
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GEK
-
45441
CURRENT
CIRCUIT
The
current
circuit
is
continuously
rated
at
5
amperes
,
with
a
one
-
second
rating
of
250
amperes
.
Higher
magnitudes
of
current
above
5
amperes
can
be
applied
for
shorter
periods
of
time
,
according
to
the
following
equation
.
K
=
I
2
T
I
=
the
applied
current
in
amperes
T
=
time
in
seconds
K
=
Constant
=
(
2502
)
=
62500
t
seconds
=
—
where
:
Therefore
:
12
POTENTIAL
CIRCUIT
The
potential
circuit
is
continuously
rated
at
120
volts
at
rated
frequency
.
AMBIENT
TEMPERATURES
The
relays
are
designed
to
operate
continuously
at
rated
voltage
,
current
and
frequency
in
an
ambient
temperature
not
to
exceed
40
Oc
.
DC
RATINGS
The
DC
voltage
circuits
are
available
at
48
/
110
-
125
/
220
-
250
volts
DC
.
circuits
consist
of
telephone
relays
whose
contacts
,
when
closed
,
can
carry
30
amperes
for
one
second
,
with
a
continuous
rating
of
3
amperes
.
The
The
interrupting
rating
of
these
contacts
is
listed
in
Table
II
.
TABLE
II
Volts
Interrupting
Rating
in
Amperes
DC
Inductive
Non
-
Inductive
125
0.50
1.5
TARGET
/
SEAL
-
IN
The
combination
target
and
seal
-
in
unit
has
a
dual
rating
of
0.6
or
2.0
amperes
.
The
tap
setting
selected
on
the
target
/
seal
-
in
unit
is
determined
by
the
current
drawn
by
the
breaker
trip
coil
.
The
0.6
ampere
tap
is
used
with
trip
coils
which
operate
on
currents
ranging
from
0.6
ampere
to
2.0
amperes
at
the
maximum
control
voltage
.
The
0.6
ampere
tap
can
be
used
also
with
trip
coils
drawing
as
much
as
30
amperes
,
provided
that
the
voltage
drop
across
the
coil
with
trip
current
flowing
is
not
excessive
.
The
2.0
ampere
tap
can
be
used
with
all
trip
coils
that
draw
more
than
2.0
amperes
at
the
maximum
control
voltage
.
-
7
-
Courtesy of NationalSwitchgear.com

GEK
-
45441
The
ratings
of
the
target
/
seal
-
in
unit
are
shown
in
TABLE
III
.
TABLE
III
HI
-
G
TARGET
/
SEAL
-
IN
UNIT
2.0
Amp
Tap
0.6
Amp
Tap
2.0
amps
2.6
amps
3.5
secs
.
30
secs
.
0.18
ohm
0.65
ohm
0.54
ohm
Minimum
operating
Carry
continuously
Carry
30
amps
for
Carry
10
amps
for
DC
Resistance
60
cycle
impedance
50
cycle
impedance
0.6
amp
1.8
amps
0.5
sec
.
5
secs
0.78
ohm
6.2
ohms
5.1
ohms
MHO
UNIT
CONTACTS
The
mho
unit
contacts
,
when
closed
,
will
carry
30
amperes
momentarily
up
to
a
250
volt
DC
control
voltage
.
However
,
the
contacts
do
not
have
an
interrupting
rating
;
therefore
some
other
suitable
method
must
be
used
to
open
the
trip
circuit
after
a
trip
condition
.
OPERATING
PRINCIPLES
The
mho
unit
of
the
GSY
51
A
relay
is
of
the
four
-
pole
induction
cylinder
construction
,
where
the
torque
is
produced
by
the
interaction
between
the
polarizing
flux
and
the
fluxes
which
are
proportional
to
the
restraining
and
/
or
operating
quantities
.
The
torque
at
the
balance
point
of
the
mho
unit
can
be
expressed
by
the
following
equation
.
Torque
=
0
=
Ei
Cos
(
0
-
0
)
-
KE
2
-
Ks
Where
:
Phase
-
to
-
phase
voltage
Delta
currents
(
Il
-
l
2
)
Angle
of
maximum
torque
Power
factor
angle
of
the
fault
impedance
Design
constant
Ks
=
Control
spring
constant
E
I
9
0
K
To
prove
that
the
equation
defines
an
mho
characteristic
,
divide
both
sides
by
E
^
and
transpose
.
The
equation
reduces
to
:
^
Cos
(
0
-
0
)
=
K
Z
Y
Cos
(
0
-
0
)
=
K
Thus
,
the
unit
will
pick
up
at
a
constant
component
of
admittance
at
a
fixed
angle
,
depending
on
the
angle
of
maximum
torque
.
Hence
the
name
mho
unit
.
When
the
offset
is
used
,
the
transactor
is
energized
with
line
current
which
produces
a
voltage
proportional
to
the
current
and
added
to
the
line
-
to
-
line
voltage
in
the
unit
'
s
potential
circuit
.
or
This
voltage
will
offset
the
-
8
-
Courtesy of NationalSwitchgear.com

GEK
-
45441
circular
characteristic
of
the
mho
unit
as
shown
in
the
R
-
X
diagram
,
Figure
4
.
Although
not
shown
in
Figure
4
,
the
offset
can
be
set
between
0
and
4
ohms
in
0
,
5
ohm
steps
.
CHARACTERISTICS
The
mho
unit
produces
a
circular
impedance
characteristic
as
shown
in
Figure
4
.
The
diameter
of
the
characteriStic
can
be
increased
as
shown
in
Figure
5
,
by
reducing
the
restraint
tap
leads
on
the
restraint
transformer
,
recommended
that
the
restraint
leads
be
set
below
10
%
on
the
restraint
transformer
.
It
is
not
BURDENS
CURRENT
CIRCUITS
The
burden
imposed
on
each
current
transformer
at
5
amperes
by
the
current
circuits
of
the
GSY
51
A
relay
is
listed
in
Table
IV
for
each
basic
minimum
reach
tap
at
60
hertz
.
TABLE
IV
Basic
Reach
Tap
,
0
-
N
R
X
V
.
A
.
U
P
.
F
.
2.0
0.142
0.110
0.081
0.054
0.065
0.170
3.55
3.30
0.930
0.860
0.430
4.0
2.75
2.365
0.870
6.0
2.025
The
burden
at
50
hertz
will
be
slightly
lower
.
POTENTIAL
CIRCUITS
The
maximum
burden
imposed
on
each
potential
transformer
,
with
120
volts
60
hertz
applied
,
is
shown
in
Table
V
.
TABLE
V
FTT
:
Offset
Tap
,
0
-
N
R
X
W
V
7
A
7
0
705
451
12.53
12.84
13.10
13.06
12.93
14.87
14.82
14.66
14.28
13.67
0.840
0.866
0.893
0.914
0.945
0.5
725
417
1.0
752
377
2.5
785
348
4.0
855
297
The
50
hertz
burden
will
be
slightly
lower
.
-
9
-
Courtesy of NationalSwitchgear.com

GEK
-
45441
CALCULATION
OF
SETTINGS
The
determination
of
relay
settings
for
the
CEX
-
GSY
scheme
need
not
be
a
complicated
procedure
.
Preliminary
settings
of
the
blinder
and
mho
units
can
be
obtained
by
means
of
a
simplified
graphical
approach
on
an
R
-
X
diagram
,
and
then
the
validity
of
these
settings
can
be
checked
with
the
results
of
the
stability
study
.
GRAPHICAL
PROCEDURE
In
the
simplified
graphical
procedure
,
the
generator
is
represented
by
its
transient
reactance
(
X
1
d
)
.
reactance
(
Xj
)
and
the
system
impedance
(
Zs
)
is
plotted
to
scale
on
an
R
-
X
diagram
,
the
origin
of
which
is
at
the
terminals
of
the
generator
,
as
shown
in
Figure
18
.
If
the
system
impedance
is
variable
,
the
smallest
system
impedance
should
be
used
,
since
this
will
ensure
that
the
subsequent
blinder
settings
will
be
able
to
detect
the
smaller
swing
impedance
locus
associated
with
low
system
impedances
.
This
reactance
,
along
with
the
transformer
The
angle
of
The
total
impedance
line
is
then
drawn
between
points
C
and
D
.
this
line
with
respect
to
the
horizontal
axis
represents
the
system
angle
.
With
the
system
characteristic
thus
established
,
settings
for
the
blinders
can
now
be
determined
.
BLINDER
SETTINGS
Referring
to
Figure
18
,
the
distance
,
N
,
from
the
origin
to
the
blinder
,
and
angle
B
between
the
blinder
and
the
horizontal
axis
,
can
be
adjusted
separately
for
each
blinder
.
The
angle
,
B
,
is
normally
selected
so
that
both
blinders
are
approximately
parallel
to
the
total
impedance
line
,
CD
.
The
spacing
between
blinders
is
generally
selected
so
that
at
the
point
in
an
impedance
swing
where
the
blinders
operate
,
the
angular
separation
,
S
,
between
generator
and
system
is
1200
.
This
angular
separation
can
be
determined
by
drawing
construction
lines
at
points
C
and
D
that
are
30
°
from
the
total
impedance
line
,
CD
.
The
blinders
are
then
plotted
to
pass
through
the
120
°
points
E
and
F
at
an
angle
,
B
,
with
respect
to
the
R
axis
,
as
shown
in
Figure
18
.
It
should
be
noted
that
the
dashed
line
,
which
is
the
bisector
of
the
1200
angles
,
passes
through
the
impedance
center
of
the
system
and
would
represent
the
locus
of
the
impedance
swing
for
the
case
where
the
ratio
of
the
generator
internal
voltage
to
the
system
voltage
equals
one
(
1
)
.
MHO
UNIT
SETTING
The
mho
unit
is
set
so
that
it
will
permit
tripping
for
all
impedance
loci
that
will
appear
in
the
region
from
the
high
voltage
terminals
of
the
step
-
up
transformer
down
into
the
generator
,
generally
connected
with
its
forward
reach
looking
into
the
generator
and
with
its
offset
adjusted
to
encompass
the
transformer
reactance
with
some
margin
.
To
accomplish
this
,
the
mho
unit
is
The
forward
reach
of
the
unit
should
be
set
so
that
it
will
detect
all
impedance
loci
that
can
go
through
the
generator
but
not
operate
for
stable
transient
swings
.
A
forward
reach
setting
which
is
equal
to
2
to
3
times
generator
transient
reactance
(
X
'
^
)
would
meet
this
criterion
.
-
10
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