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OIL IMMERSED CURRENT TRANSFORMERS

 

Oil Immersed Current Transformers
 
Treatment Plant


 

APPLICATION
Direct measurement of current in High Voltage System is Not possible because of insulation problem of measuring instruments. It is also not possible to use current flowing through the system directly for protection purpose due to its high insulation problem.

BASIC FUNCTIONS OF CURRENT TRANSFORMERS ARE :

CURRENT TRANSFORMERS

 1. To reduce the line current to a value
     which is suitable for standard measuring
     instruments relays etc.
 2. To isolate the measuring instruments, meters,
    relays etc. from high voltage side an installation.
 3. To protect measuring instruments against short
     circuit currents.
4.  To sense abnormalities in current and give
     current signals to protective relays
     to isolate the defective system.

    There are four main factors which determine the capability of current
     transformer i.e.
    * Insulation Level (Service Voltage)
    * Rated primary current
    * Short time withstand current
    * Burden and Accuracy
THE CURRENT TRANSFORMERS MUST :
1. Withstand operational voltage and over voltage in the network
2. Withstand rated primary current in continuous operation without exceeding 
    maximum allowed temperature rise.
3. Be capable o sustain thermal and mechanical stresses developed due to
    system falut current
4. Feed current to external circuit with specified accuracy at specified primary
   currents

 

Treatment Plant

CONSTRUCTION :
Core & Secondary Winding
Primary Winding
Insulation
Procelain Insulators
Tanks & Bases
Testing
Packaging & Transportation
Maintenance

HOW TO SELECT THE C.T.
    It is impotant to specify correct parameters of CT while ordering for optimum
design. Following are main factors for selecting current transformer.

1. SERVICE VOLTAGE : System voltage in which CT is to be installed e.g.
    11kV, 22kV, 33kV ETC.
2. INSTALLATION : Whether OUTDOOR or INDOOR
3. ATMOSPHERIC CONDITIONS : Such as condition of Pollution, Altitude
    Ambient Temperature etc.
4. INSULATION LEVEL : If insulation level other than associated with service
    voltage is required, it should be specifically mentioned.
5. RATED PRIMARY CURRENT : Specify rated primary current / currents (if
    required more than one value). Also indicate if different primary current
    is required for different cores.
6. CONTINUOUS PRIMARY CURRENT : Max. primary current that can be withstood
    continuously by current transformer e.g. 120% of primary current
7. RATED SECONDARY CURRENT : Whether 1 Amp. or 5 Amps.
8. SHORT TIME CURRENT & ITS DURATION : Specify fault current of the system
    in which CT is to be installed along with its duration. it is most important to
    specify realistic value of S.T.C. as at lower primary current, higher S.T.C.
    value necessitates bulky & costlier design. Also specify dynamic current if
    other than 2.5 times S.T.C. is required.
9. NO.OF CORES THEIR BURDENS, ACCURACY : Basis of application, No. of cores,
    their burdens and accuracy class should be specified. It is advisable to specify
    minimum required Burden for metering core as unnecessary high burden will
    necessitate bulky and costlier design. Specified accuracy is guaranteed for
    100 % to 25% of rated burden only. Current transformer offers minimum error
    if 75% to 60% burden  is connected to secondary, Therefore, ideally rated
    burden higher than 1.5 time actual burden should be specified. Also, it is
    important to specify correct burden in context of instrument Security Factor
    (I.S.F.). The I.S.F. indicates the over current as multiple of rated current at
    which the metering core will saturate, thus limiting the secondary current
    flowing through meter and protect it from damage. If actual burden connected
    is half of the rated burden, the I.S.F. will increase two-fold of its rated value.
10.
KNEE POINT VOLTAGE, SECONDARY RESISTANCE AND
     EXCITATION CURRENT : For differential protection, R.E.F. Protection, Bus Bar
     Protection, C.T. with accuracy class PS is required. The Knee Point Voltage,
     Secondary resistance and excitation current should be for this core. It is
     always better to specify Formula for Knee Point Voltage related to relay used
     for the protection. This will help designer to optimise the design.

 

 



| OIL IMMERSED INSTRUMENT TRANSFORMERS | | Resin Cast  Insulated INSTRUMENT Transformers |
| Current Transformers
| | Voltage Transformers | | HIGH VOLTAGE TESTERS |
| LOW TENSION CURRENT TRANSFORMERS
|

All Instrument Transformers are made to IS2705, BS3938, BS7626, IEC 60044-I, AS1675, ANSIC57.13, CAN3-C-13-M83, IEC 60044-2, IS 3156, AS AMMENDED

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