GE STD15C Instructions Manual page 3

GEK-45 307
TRANSFORMER DIFFERENTIAL RELAYS
WITH PERCENTAGE AND HARMONIC RESTRAINT
STD15C and STD16C
INTRODUCTION
Relays of the
STD type are transfor mer
differen tial relays
provide d
with
the
features of percent age and harmon ic restrain t.
A static decision unit controls a
small telepho ne-type relay that provide s the contact output.
Percent age restrain t permits accurate discrim ination between internal and externa l
faults at high current, while harmon ic restrain t enables the relay to distingu ish,
by the differen ce
in wavefo rm, between
the differen tial
current caused
by an
internal fault, and that of transfor mer magnet izing inrush.
DES CR1 PTI ON
Each Type STD relay is a single-p hase unit.
The Type STD15C relay is designe d to be
used for
the protecti on of two-win ding
power transfor mers and has
two through -
current restrain t circuits and one differen tial—cu rrent circuit.
The Type STD16C relay is designe d for use with three— winding power transfor mers and
has three through -current restrain t circuits and one differen tial—cu rrent circuit.
It may also be used for four-cir cuit transfor mer protecti on (see Figure 1) when only
three circuits require through -current restrain t, while
the fourth circuit,
being
the weakes t, needs no through -current restrain t.
APPLICATION
The current transfor mer
ratios
and
relay taps should be selected
to obtain the
maximum sensitiv ity
without
risking thermal
overloa d of the relay or current
transfor mer
(CT), or the possibil ity
of misoper ation.
Therefo re, current
transfor mer ratios
in the various winding s
of the
power transfor mer should be
selected with the followin g points in mind:
1.
The lower the relay tap and the lower the CT ratio selected , the higher will be
the sensitiv ity.
Howeve r, the lowest CT ratio and the lowest relay tap may not
be compat ible with
some of the
followin g
restricti ons. Where a choice
is
availabl e of increasi ng either the CT ratio or the relay tap,
it is desirabl e
to increase the CT ratio in preferen ce to the relay tap.
Since the relay burden
is likely to be small
compar ed to the lead
burden,
increasi ng the CT ratio tends to improve the relative perform ance of the CT5,
as a result of reducin g the maximum seconda ry fault current and increasi ng the
accurac y of the CTs.
These 1nstrucron5 do not purport to cover i.11
details or variations in equipaent nez e provide for
every possible ctingency to be met
in connection with installation,
operation or IlntnCe . Should
further nforma ton be desi red or should parti cular probleme arise which are not covered ziWficiently 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, IEt andN standards,
but no such assurance is given with respect to local
codes
and ordinances because they varç greatly.
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