Current Transformers and Voltage Transformers

hi..

simple CT- current transformers transform the current levels which are inversely proportional to the ratio of the primary and secondary windings of the transformer.

i.e C1/ C2= N2/N1

PT/VT- voltage transformer transforms voltage level directly proportional to the turns in windings

here V1/V2 = N1/N2

so if u know the values of primary and secondary turns and any one level of current/ voltage .. you can calculate the other level..

also..

there is no energy conversion taking place here...

only transformation takes place.. higher to lower level or vice-versa .

accordingly they are called as step up or step down transformers.

A CT the current in its secondary winding is proportional to the alternating current flowing in its primary.

also, current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.

testing o n transformers basically involves ROUTINE tests and IMPLUSE VOLTAGE tests.

If too much current goes through a coil or winding, the winding heats up and can either open up completely, or the insulation between turns of wire can break down, causing the coil to be partly or completely shorted.

hope this link will help..

http://ezinearticles.com/?Testing-Transformer---3-Proven-And-Simple-Methods-To-Test-Transformers&id=475191

CT's and VT's work as described above to convert primary quantities in the order of kV's and 100's of Amperes to smaller magnitudes normally voltage to 110V and current to 1A or 5A for use in protection relays, meters, transducers.

Important tests carried out include insulation resistance and ratio tests to confirm the transformation ratio. Particular to CT's are polarity tests to acertain/confirm the winding sense of the primary and secondary terminals i.e. flow direction of current in primary to be correctly reflected in secondary. Another important test for CT's is the knee point/ saturation curve determination to know the point at which the CT loses it's linearity of transformation and hence its useful range of operation.

Tests carried out on CTs & VTs, from an electrical panel builder point of view :

1/ When you purchase CTs and PTs from a supplier, they are supposed to match the suppliers characteristics, and should already have gone through the suppliers control tests at the end of their manufacturing (at least the routine tests from the standards, if the supplier claims compliance).
2/ Some panel builders decide anyway to control these CTs/PTs when they receive them, before installing them in the switchboards (specific test equipment exists).
3/ In all cases, the installation of these CTs and PTs into the switchboard should be done in an appropriate way :
* to maintain and not deteriorate the assembly required characteristics : insulation distance for example
* to match the CTs/PTs characteristics : for example, with a window-type (also said "toroid") CT, the primary circuit going-through should be properly centered to ensure precise measurement
4/ CTs & PTs might then be tested when doing some functional tests of the switchboard : in fact, it's the complete chain of measurement which is tested in this case.
5/ When checking the dielectric withstand of an assembly with the industrial frequency values, the VTs shall be disconnected as otherwise they would saturate their magnetic core and appear as a low impedance with possible internal damage (dielectric withstand tests of instrument transformers are performed at higher frequencies to prevent such risk).

Very important safety reminder for CTs : never leave open the secondary circuit of a current transformer when fed on primary. High voltage may surge at the secondary circuit terminals, and might be dangerous for people, and lead to the destruction of the current transformer

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