instrument transformer,application of instrument transformer and construction_types of instrument transformer_type of CT transformer,VT transformer

instrument transformer,application of instrument transformer and construction,types of instrument transformer,type of CT transformer,VT transformer

what is instrument transformer?

• instrument transformer is used in conjunction with any measuring instrument (i.e,ammeter , voltmeter , wattmeter ,watt hour-meter atc)or protective equipment(i.e relays) .
• it utilize the current transformation and voltage transformation properties to measures high ac current. 

Application of instrument transformer:-

• for measurement of high ac current , it is usual to use low range ac ammeter with suitable shunt .
• for measurement  of high ac voltage ,low range ac voltmeters are used with high resistance connected in series.
• for measurement of very high ac current and voltage ,we cannot use these methods . instead , we use specially constructed  HV instrument transformer to insulate the high voltage circuit from the measuring circuit in order to protect the measuring instrument from burning.
• Application of instrument transformer in dc circuits for current and voltage measurement ,we use low range dc ammeter and voltmeter with rectifiers connected in their secondary circuits.
• The vast primary application of instrument transformers is for the protection and control of power system and power equipment of high and very high ratings .
• The working of these instrument transformers are similar to those of the ordinary transformers.

There are two types of transformer .

1. current transformer(ct)
2. potential transformer(vt)

current transformer(CT)

• A current transformer is a transformer ,which produces in its secondary  winding low current , which is proportional to the high current following in its primary winding .
• The secondary current is usually much smaller in magnitude than the primary current.
• The design of CT depends on which type of instrument is connected to its secondary measuring instrument or protective instrument .

• measuring instrument CT is expected to give accurate result up to a maximum of 125% of its normal full-load rated current. 

•  protective instrument CT is expected to be accuratr for up to 20 times of its normal full-load rated current (about 2000%of full load rated current).      

Construction of CT

• ct has a primary coil of one or more turns made of thick wire connected in series with the line whose current   is to be measure .
• The secondary consists of a large number of turns made of fine wire and s connected across and is connected across an ammeter or a relay's terminals .
• According to construction there are two type of  CT transformer.

1. window type 
2. bar type 

Function of CT

• The principle of a CT is to produce a proportional current at a level of magnitude ,which is suitable for the operation of low range measuring or protective devices such as indicating or recording instruments and relays.
• The primary and secondary current are expressed as a ratio such as 100/5 or 1000/5.with 100/5 ratio CT, 100 Ampere flowing in the primary winding will result in 5 A  flowing in the secondary winding , provided that the correct rated burden is connected to the secondary winding . 

 Potential transformer(PT or VT)

•  PT is a step down transformer having many primary turn but a few secondary turns.
• In a step down transformer the voltage decreases and the current increases , thus voltage can be easily measured by using a low- range voltmeter instrument. 
• the voltage is stepped-down in a known ratio called the voltage ratio.

   construction of PT

• PT is a step down transformer having many primary winding turn but a few secondary winding turns that makes it a step-down transformer.
• A voltmeter is connected to the secondary winding is usually a voltmeter of 150 V.