Instrument Transformers

1.What is an Instrument Transformer?

• It could be a electrical device that’s utilized in conjunction with any measuring device (i.e.,   Ammeter,Voltmeter, Wattmeter, Watt-hour-meter …etc.) or protecting instrumentality (i.e., Relays).

• It utilizes the current-transformation and voltage transformation properties to live high ac current and voltage.

2.Types of instrument transformers:

This instrument transformers area unit of 2 types:-

1. Current transformers

2. Potential transformers

3.Applications of Instrument Transformers:

•  For measuring of high ac current, its usual to use low vary meter with appropriate shunt.

• For measuring of high ac voltage, low vary ac voltmeters area unit used with high resistances connected serial.

• For measuring of terribly high ac current and voltage, we tend to cannot use these strategies. Instead, we tend to use specially made HV instrument transformers to insulate the high voltage circuit from the measuring circuit so as to shield the measuring instruments from burning.

•Application of Instrumentation Transformers In dc circuits for current and voltage measuring, we tend to use low vary dc ammeters and voltmeters with rectifiers connected in their secondary circuits.

•The large primary application of Instrument Transformers is for the protection and management of installation and power instrumentality of high and extremely high ratings.

•The operating of that instrument transformers area unit almost like those of the normal transformers. Basic Construction and Magnetic Circuit of Instrument Transformers Polarity of Instrument Transformers Common connections of instrument transformers

4.What is current Transformer (CT)?

• A current electrical device could be a electrical device, that produces in its secondary coil low current, that is proportional to the high current flowing in its coil.

• The secondary current is sometimes abundant smaller in magnitude than the first current.

• The look of CT depends on which sort of instrument is connected to its secondary coil. measuring device OR protecting instrument.-Measuring instrument CT is anticipated to administer correct results up to a most of a hundred and twenty fifth of its traditional full-load rated current.-Protective instrument CT is anticipated to be correct for up to twenty times of its traditional full-load rated current (about 2000% of its full-load rated current!!..??).

• Based on the sort of kit that the Ct is employed for, its saturation can vary. At identical time it’s expected to be linear within the entire operating vary.

5.Construction of C.T.

• C.T. features a coil of 1 or a lot of turns made from thick wire connected serial with the road whose current is to be measured.

• The secondary consists of an oversized variety of turns made from fine wire associate degree is connected across an meter or a relay’s terminals.

6.Function of CT

• The principal operate of a CT is to provide a proportional current at grade of magnitude, that is appropriate for  the operation of low range measuring or protection or protecting devices like indicating or recording instruments and relay

• The primary and secondary currents area unit expressed as a magnitude relation like 100/5 or 1000/5

• With a 100/5 magnitude relation CT, 100A flowing within the coil can end in 5A flowing within the secondary coil, only if the proper rated burden is connected to the secondary coil.

“Class” of a CT

• The extent to that the particular secondary current magnitude differs from the calculated worth, expected by the virtue of the CT magnitude relation, is outlined because the accuracy “Class” of the CT

• The larger the quantity wont to outline the category, the larger the permissible “current error” [the deviation within the actual secondary current from the calculated value].

7.Specifications of CT:

CTs ought to be specified as follow:

RATIO: Input / output current magnitude relation VA:

Total burden (rating) together with pilot wires.

Common burden ratings area unit a pair of.5, 5, 10, fifteen and thirty VA. For example

• Moving iron meter is 1-2 VA

• Moving coil rectifier meter is 1-2.5 VA

• Electro-dynamic instrument is a pair of.5-5 VA

• Maximum demand meter is 3-6 VA

• Recording meter or electrical device is 1-2.5 VA

CLASS: The accuracy needed for the operation

DIMENSIONS: most & minimum limits. For example:

• 0.1 Or 0.2 for preciseness measurements.

• 0.5 for top grade Board of Trade unit meters and industrial grade Board of Trade unit meters.

• 3 for general industrial measurements

• 3 or five for approximate measurements. BURDEN (OHMIC): (Depending on pilot lead length) Current

8.Transformers Stepping:

• These area unit used with low vary ammeters to live current in high voltage alternating circuits wherever it’s not sensible to attach instrument and meters on to lines

• They area unit increase transformers (voltage ratio) as a result of after we increase the voltage this decreases.

• The current could be a reduction during a best-known magnitude relation referred to as this magnitude relation.

9.Multi-ratio CT:

•As indicated within the previous Figure, current transformers having middle tapped secondary area unit noted as a twin magnitude relation CT.

• Dual magnitude relation CT area unit utilized in applications wherever it’s necessary to possess accessible 2 ratios of primary to secondary current from identical secondary coil of the CT.

• This could also be accomplished by adding a faucet within the secondary coil to induce a second magnitude relation.

•The magnitude relation obtained by the faucet is sometimes common fraction the magnitude relation obtained by the complete secondary coil.

• A schematic example is previously shown with two hundred amperes flowing within the primary, a association X2 – X3 can manufacture five amperes out of the secondary. Because the load grows to four hundred amperes, the secondary circuit are reconnected to X1 – X3 to still manufacture five amperes within the secondary circuit.

10.Working (Measurement)

• If a current electrical device has primary to secondary current magnitude relation of 100:5 then it steps up the voltage twenty times and step down this 1/20 times of its actual worth.

• If we all know this magnitude relation and also the reading of associate degree a.c. ammeter, the first current will be calculated as

 Primary Current = CT ratio × ammeter reading

Why CT secondary should never be open?

  • Ammeter resistance is very low; the current transformer normally works as a short-circuited instrument.
  • If for any reason the ammeter is taken out of secondary winding then the secondary winding must be short-circuited with the help of a short-circuit switch.
  • If this is not done, then a high m.m.f. (Ampere-turns IT) will set up a high flux in the magnetic core and it will produce excessive core losses which produce heat and high voltage across the secondary terminals.
  • The high voltage can damage any electronic components in secondary side.
  • Hence the secondary of any current transformer should never be left open.

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