A present transformer is a gadget for the change of current from a higher incentive to lower esteem, or for the change of current at a high voltage into a proportionate current at a low voltage as for the earth potential. Current transformers (CTs) are utilized related to air conditioning instruments, meters or control-mechanical assembly where the current to be estimated is of such extent that the meter or instrument curl can’t helpfully be made of adequate current-conveying limit. Current transformers are additionally utilized where hv current is to be metered because of the trouble of giving satisfactory protection in the meter itself. In meter practice, CTs are utilized when the current to be estimated is more than 100 A.
CONSTRUCTION OF CURRENT TRANSFORMER
The centers of CTs are normally developed with overlays of silicon steel. High-Permeability nickel steel, for example, Mumetal or Permalloy is utilized for cores where a high level of exactness is wanted. The essential winding conveys the current to be estimated and is associated with the primary circuit. The optional winding conveys a present relative to the current to be estimated and the second terminals are associated with the present windings of the meter or the instrument. Both the windings are protected from the center and from one another. The primary circuit of a CT (Fig. 2.46) is commonly a solitary turn winding B (called a bar primary)and conveys full-load current. The optional winding S has countless.
Current transformer ratios are generally specified in terms of full-load primary and secondary currents. Usually, the secondary windings are designed for rated values of 5 A, although 1 A and 2 A ranges are also used. For example, 1000/5 A current transformer may be úšed with a 5-A ammeter to measure currents up to 1000 A. Fig. 2.46 shows the connections of a current transformer.
Current Transformer Symbol
BURDEN OF A CURRENT TRANSFORMER
The burden of a CT is the value of the load connected across the secondary terminals. It is expressed as the output in voltampere (VA), The rated burden is the value of the burden marked on the nameplate of the CT.
EFFECT OF OPEN SECONDARY WINDING OF A CURRENT TRANSFORMER
Under ordinary working conditions, the optional twisting of a CT is its weight and the auxiliary is constantly shut. At the point when the current flows through the essential winding a current additionally moves through the optional winding and the ampere-turns (mmf) of each winding are considerably equivalent and inverse dynamic, the auxiliary ampere tums will be really 1% to 2% not exactly essential ampere-turns, the distinction is used in charging the center. In this way, if the auxiliary twisting of a CT is opened, while the current is moving through the essential twisting, there will be no demagnetizing motion due to the secondary current. Because of the nonattendance of the counter amperetums of the optional, the unopposed essential mmf will set up an unusually high motion in the center. This motion will create over the top center misfortunes with resulting warming and a high voltage will be instigated over the optional terminals. This high voltage might be adequate to make a risk life and breakdown of the protection. Likewise, loss of exactness in the future may happen, in light of the fact that the extreme mmf leaves remaining attraction in the center. In this way, the auxiliary of a CT ought to never be open when the essential is conveying current.