GE STD16C - SETTINGS AND CALCULATIONS; CALCULATION OF SETTINGS

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