GE STD15C Instructions Manual page 4

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.
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