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It
is not possible to use line current directly for measurement
and relaying purpose due to its high value and insulation
problem. CURRENT TRANSFORMER which is widely known as 'CT'
is a piece of electrical equipment which converts line
current values which are suitable for the devices to which
is connected e.g. measuring instruments, meters, protection
relays.
BASIC
FUNCTIONS OF CTs ARE :
- To
reduce line current to a value which is suitable for
measuring
instruments, relays etc.
- To
isolate the measuring instruments, meters, relays etc.
from line
voltage of an installation.
- To
protect measuring instruments against short circuit
currents
- To
sense abnormalities in current and give current signals
to protective
relays to isolate the defective system.
CURRENT
TRANSFORMERS ARE BROADLY CLASSIFIED AS :
- RING
TYPE CTs : A CT which has an opening in the center to
accommodate to primary conductor (busbar or cable)
through it.
- BAR
PRIMARY CTs : A CT in which the primary winding
consists of bar of suitable size and material forming an
integral part of the CT.
- WOUND
TYPE CTs : A CT having a primary winding of more than
one full turn wound on the core.
FOLLOWING
ARE FEW GENERAL TYPES OF CTs.
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1)INTERPOSING
CTs :
These
CTs are used in conjunction with main CTs to alter the ratio
of main CT or to provide isolation to meters or relays from
main CTs secondary circuit. Primary current of these CTs are
generally lower than 10 amp. Due to which they are always
wound primary types. |
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2) SUMMATION CTs :
When current flowing in more than one feeder is required to
be metered, summation current transformer are used. These
CTs are provided with more than one primary winding and one
common secondary. The standard primary & secondary
currents are 5 or 1 amp. Summation current transformers are
generally manufacture confirming to ISS 6949. |
3) BUSHING TYPE OR BUS
DUCT
CTs : These CTs are Ring type construction & can be
mounted on Bus duct or Bushing turret of power transformer.
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Core
Balance Current Transformer
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4) CORE BALANCE CTs :
These
are ring type CTs & suitable for the measurement of the
sum of three phase currents in a 3 phase cable. Under normal
operating conditions this sum is zero. In the event of an
earth-fault the sum of the current. These CTs are generally
used with English Electric make CTUM 15 static relay. It is
necessary to specify leakage current to be detected along
with minimum setting of the relay and size of cable at the
time of ordering CT. |
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Furnace Current
Transformers
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5) FURNACE TRANSFORMERS : These
transformers are generally split core type which can be
easily mounted on the bus of furnace transformer. Primary
current of these transformers are of value 10,000 Amp &
above. Special precaution is necessary to reduce the heat
developed in the transformers due to eddy current during
design and manufacture. |
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6) PRECISION GRADE CURRENT
TRANSFORMERS :
These CTs are of accuracy of 0.1, 0.2 or
0.5 and used as a standard current transformers to check
accuracies of other transformers. These current transformers
are either wound primary or ring type manufactured in
teakwood cases. |
FOLLOWING
ARE GUIDE LINE FOR ORDERING CTR.
- Primary
current.
- Secondary
current.
- VA
Burden.
- Accuracy
class.
- Instrument
security factor.
- Accuracy
limit factor.
- Environmental
condition.
- Application.
- Fault
current of the system.
- Voltage
of the system.
- Any
special features required.
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