ABB MMCO-2 - Page 6

I
.
L
.
41
-
121.2
1
.
Transformer
inrush
2
.
Capacitor
inrush
3
.
Faults
having
magnitudes
associated
with
loca
-
tions
at
or
beyond
the
next
protective
device
.
Consideration
must
be
given
to
the
added
effect
of
dc
offset
in
the
phenomenon
listed
.
In
general
,
a
setting
of
120
%
of
the
symmetrical
current
(
no
dc
offset
present
)
will
provide
adequate
security
.
4
.
SETTINGS
4.1
.
TIME
OVERCURRENT
FUNCTION
The
level
and
time
settings
are
dictated
by
those
nor
-
mal
system
phenomena
which
must
be
ignored
,
and
by
the
minimum
current
the
system
is
expected
to
deliver
to
a
fault
in
the
protected
zone
of
the
relay
.
For
the
phase
units
,
pick
up
and
time
settings
must
be
chosen
with
due
consideration
given
to
such
long
or
moderate
time
phenomena
as
:
4.3
.
INVERSE
TIME
CURVES
The
MMCO
relay
simulates
the
traditional
inverse
curve
sets
(
USA
or
IEC
version
)
,
which
are
perma
-
nently
stored
in
memory
.
1
.
Maximum
load
2
.
Cold
-
load
pickup
3
.
Motor
starting
Pickup
and
time
settings
above
any
of
these
influenc
-
ing
factors
must
be
chosen
.
Coordination
with
other
devices
must
also
be
achieved
.
A
time
of
0.3
sec
-
onds
plus
breaker
time
is
generally
recommended
as
a
coordinating
time
interval
(
CTI
)
between
adjacent
relays
being
coordinated
.
With
careful
evaluation
of
current
transformer
errors
,
setting
accuracy
and
worst
case
relay
errors
,
a
lower
CTI
may
be
used
.
4.3
.
1
.
USA
Version
/
CO
Curves
There
are
seven
families
of
CO
curves
:
1
.
MMCO
-
2
-
Short
Inverse
2
.
MMCO
-
5
-
Long
Inverse
3
.
MMCO
-
6
-
Definite
Inverse
4
.
MMCO
-
7
-
Moderate
Inverse
5
.
MMCO
-
8
-
Standard
Inverse
6
.
MMCO
-
9
-
Very
Inverse
7
.
MMCO
-
11
-
Extreme
Inverse
Where
a
ground
setting
is
chosen
,
attention
should
be
given
to
:
Each
family
contains
63
curves
determined
by
the
time
dial
setting
(
see
Figures
4
thru
10
)
.
The
expres
-
sions
for
the
CO
curves
are
shown
in
equation
(
1
)
for
l
/
IO
greater
than
1.5
per
unit
,
and
equation
(
2
)
for
l
/
IO
between
1.0
and
1.5
per
unit
.
1
.
Maximum
residual
load
unbalance
2
.
Loss
of
largest
single
-
phase
load
(
for
example
,
as
a
result
of
fuse
-
blowing
)
Pickup
for
ground
sensing
should
be
set
at
a
current
level
above
these
conditions
.
Time
delay
for
ground
tripping
should
be
set
to
coordinate
with
other
devices
.
T
=
[
To
+
K
/
(
l
/
IO
-
C
)
P
]
x
D
/
24
,
000
T
=
[
R
/
(
1
/
10
-
1.0
)
]
x
D
/
24
,
000
trip
time
in
seconds
definite
time
term
input
current
pickup
current
settings
scale
factor
for
the
basic
inverse
time
a
constant
a
constant
an
exponent
determining
the
inversions
time
dial
setting
from
1
to
63
(
1
)
(
2
)
where
T
To
I
Curve
shapes
may
be
chosen
independently
for
the
phase
and
ground
functions
.
A
single
setting
estab
-
lishes
the
curve
shape
for
the
2
(
or
3
)
phases
.
•
o
=
K
C
4.2
.
INSTANTANEOUS
OVERCURRENT
FUNC
-
TION
R
P
The
instantaneous
settings
must
be
chosen
to
be
above
:
D
6
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