TMS-811-1/AFJMAN
32-1080
tor. Additionally, lines serving large motors (above
transformers. A "C" classification means the ratio
error can be calculated, whereas a "T" classifica-
50 hp) require special consideration before apply-
tion is one which has to be derived by testing.
ing automatic reclosing. For these cases, improper
application of automatic reclosing can result in an
IEEE Std C57.13 permits either classification.
(2) CTs for circuit breakers. Table 4-4 lists
out-of-synchronism condition with catastrophic con-
sequences. Refer to REA Bulletin 65-1 for addi-
acceptable CT ratings for outdoor circuit breakers.
tional guidance.
For oil circuit breakers, short-circuiting type ter-
(b) Directional overcurrent or power relays.
minal blocks for CT leads will be located in the
Relays will be specified when required to protect
operating mechanism cabinet, and also in the
against the reverse flow of current or power when
metal-clad switchgear, if provided, or in the instru-
on site generation exists or is to be provided at the
ment and relay cabinet if metal-clad switchgear is
not provided.
installation. Similar protection is to be afforded
(3) CTs for protective relays. The following
when electrical power is to be provided over
protective relays, where used, will have three-
separate incoming lines owned by different utili-
phase-sets of current transformers exclusively ded-
ties and relaying is required to detect and correct
icated to their own use: bus differential relaying
abnormal conditions on the transmission or distri-
(87B), generator differential relaying (87G), line
bution lines that serve the installation.
(5) Protective devices coordination. A coordina-
differential relaying (87L), motor differential relay-
ing (87M), transformer differential relaying (87T).
tion study is necessary to determine settings of
Ground sensor (zero-sequence) type CTs will be
adjustable protective devices and ratings of associ-
connected only to the ground fault relay since
ated power fuses. Coordination studies will be
these CTs are unable to accurately serve any other
conducted in accordance with TM 5-811-14, IEEE
relaying or metering.
Std 242, and chapter 1 of this manual.
(4) CTs for metal-clad switchgear. For meter-
b. Instrument transformers. Instrument trans-
ing CTs, the designer will specify the CT ratio and
formers will be selected and applied in accordance
with the references listed below. Accuracy classes
accuracy class based upon the present and future
load current and the total connected burden. IEEE
are listed in IEEE Std C57.13. The designer will
Std C37.20.2 lists accuracy class ratings for metal-
check the burdens connected to determine the
clad switchgear. For protective relaying CTs, the
actual accuracy class required.
(1) Current transformers (CT) for power trans-
designer will specify the CT ratio and the relaying
formers. For power transformer bushing type CTs,
accuracy class based upon the present and future
full load current, the maximum short-circuit cur-
short-circuiting type terminal blocks will be lo-
rent available (including DC offset), interconnec-
cated in the transformer terminal cabinet and in
tion with other CTs (if required), and the total
the switchgear or instrument and relay cabinet, as
connected burden. Separate three-phase-sets of CTs
applicable. Where primary current sensing is nec-
will be used for protective relaying and for meter-
essary and neither oil circuit breakers nor primary
ing on mains, ties, and feeders. For a feeder, if the
switchgear are available, bushing-mounted CTs
only metering is an ammeter-and-switch, then
will be provided on power transformers. ANSI
both metering and relaying may be served by the
C57.12.10 permits a maximum of two CTs per
same set of CTs.
bushing for power transformers. The number of
(5) Voltage transformers (VT). VTs will be
CTs required is dependent upon whether differen-
specified in sets of two (V-V connection) for 3-wire
tial relaying is required, whether the burden
systems or three (Y-Y connection) for 4-wire sys-
rating of a single transformer is adequate, or
tems. Single VTs may be specified for use with
whether separate sets of current transformers are
synchronizing/synchro-check relays (device 25) or
required for primary and backup relaying. Since
under/over-voltage relays (devices 27, 59, or 27/59).
instruments and meters are provided on the sec-
Since VTs can be connected on either the source
ondary side of power transformers, metering class
side or the bus side of the main circuit breaker,
accuracy is not necessary for most applications,
consideration will be given to metering and relay-
unless devices specified in the project specifications
ing needs before specifying the connection location.
are used for revenue metering. Only relaying class
VTs will be connected to the source side of the
accuracy is available for multi-ratio units, so that
main circuit breaker (and to the generator-side of
when metering class accuracy is required, single-
generator breakers, if present); however, where
ratio units must be specified. The accuracy class
there is a double-ended bus or a second source
ratings of CTs at standard burdens are given in
IEEE Std C57.13. ANSI C57.12.10 requires a relay
connected to the bus, an additional set of VTs
(connected to the bus) may be needed. Where
accuracy class of C200 minimum for CTs in power
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