Text 15. Current transformers

Why Current Transformers are Used. — A current transformer is an instrument transformer for the transformation of current from one value to another, usually a lower one, or for the transformation of current at a high voltage into a proportionate current at a low voltage with respect to earth potential. Current transformers are used in con­junction with alternating-current meters or instruments where the current to be measured is of such magnitude that the meter or instru­ment current coil cannot conveniently be made of sufficient carrying capacity. They are also used wherever high-voltage current has to be metered, because of the difficulty of providing adequate insulation in the meter itself. In this connection supply voltages exceeding 660 volts are considered to be high voltage. In meter practice current trans­formers are used wherever the current to be metered exceeds 100 am­peres, and in some instances a lower value than this is regarded as the desirable maximum for direct measurement.

Construction of Current Transformers. — A current transform­er comprises a magnetic circuit, usually in the form of iron stamp­ings assembled together to form a core, on which are wound two electric circuits called the primary winding and secondary winding respectively. The primary winding carries the current to be measured and is connected in the main circuit. The secondary winding carries a current proportional to the current to be measured and the secondary terminals are connected to the current winding of the meter or in­strument. Both windings are insulated from the core and from each other. The secondary insulation is arranged to withstand a test pres­sure of 2,000 volts applied between the winding and the core for one minute. The insulation of the primary is arranged to withstand for one minute a test pressure applied between the primary and seconda­ry windings approximately equal to four times the voltage existing under working conditions. During this test the core and the seconda­ry winding are connected together.

The primary circuit of a current transformer may consist of a sin­gle conductor in the form of a bar or cable instead of a winding, when the current to be measured is of the order of 600 amperes or more. In low-voltage circuits the current to be measured may be so heavy that it is not convenient to provide a primary integral with the transformer and the latter then consists of an iron core of appropriate shape with a secondary winding thereon. The nominal full-load current of a trans­former is termed the "rated primary current" and is the value in am­peres of the primary current marked on the rating plate.

The secondary winding of a current transformer is usually con­structed to deliver five amperes to the meter or instrument when rated primary current flows in the main circuit. This is referred to as the "rated secondary current" and five amperes is the standard value adopted in most countries.

Notes

proportionate - пропорційний

magnitude - величина

capacity [ke'paesiti] - об’єм

insulation [insju'leifn] - відособленість

to exceed [ik'siid] - перевищувати

to comprise [kam'praiz] - містити; охоплювати

to apply [a'plai] - вживати; використовувати

core [ко:] - сердечник; стрижень

to withstand [wid'staend] - вистояти

Comprehension questions:

1. Why current transformers are used?

2. What is the construction of current transformers?

3. What is the importance of current transformers in industry?