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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
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