Instrument Transformer and Power Management (P1) Course
Chapter (6) : Current Transformers
6.1 Introduction :
Direct measurement of heavy currents would involve large and expensive instruments and relays in a wide variety of designs.
The instrument current transformer provides a means of changing the magnitude and quantity of current being measured so that relatively small and less expensive instruments of standard design can be used.
They also protect personnel, the measuring devices and the control equipment from the danger of high voltage by providing insulation between the primary and secondary circuits, these resulting in increased safety.
Protective relays in a.c. power system are connected in the secondary circuits of current transformers and voltage transformers.
The design and use of these transformers is quite different from that of power transformers.
In current transformer, primary current is not controlled by condition of the secondary circuit.
6.2 Current transformer must be classified into tow groups :
- Protective current transformer, used in association with relays, trip coils, pilot wires, etc. It uses the letter "P" in their rating, to distinguish them from the measuring current transformer. A protective current transformer would be specified using rated output, accuracy class and accuracy limit factor.
- Measuring current transformer, used in conjunction with ammeter, wattmeter, etc. Measurement current transformers are specified by rated output and accuracy class.
6.3 Current transformer theory
6.3.1 Current transformers-stead1 : state behavior :
Current transformers are among the most commonly used items of electrical apparatus and yet, surprisingly, there seems to be general lack of even the most elementary knowledge concerning their characteristics, performance and limitations among those engineers who are continually using them.
The importance of current transformers in the transmission and distribution of electrical energy can not be over emphasized because it is upon the efficiency of current transformers, and the associated voltage transformers, that the accurate metering and effective protection of those distribution circuits and palnt depend.
Current and voltage transformers insulate the secondary (relay, instrument meter ) circuits from the primary (power) circuits and provide quantities in the secondary which are proportional to those in the primary.
The role of a transformer in protective relaying is not as readily definite as that for metering and instrumentation. Whereas the essential role of a measuring transformer is to deliver from its secondary winding a quantity accurately representative of that which is applied to the primary side, a protective transformer varies in its role according to the type of protective gear it serves.
Failure of a protective systems to perform its function correctly is often due to incorrect selection of the associated current transformers. Hence, current and voltage transformers must be regarded as constituting par of the protective system and carefully matched with the relays to fulfill the essential requirements of the protection system.
There is no great difference between a protective voltage transformer and a measuring voltage transformer, the difference being only in the nature of the voltage transformed. Normally the same transformer can serve both purpose.
If voltage, perform transformers reasonably accurately its duty will have been fulfilled. This can not be said for current transformers as the requirement for protective current transformer are often radically different from those of metering.
It is true that in some cases the same transformer may serve both proposes but in modern practice this is the exception rather than the role, the primary difference is that the measuring current transformer is required to retain a specified accuracy over the normal range of load current, whereas the protective current transformer must be capable of providing and adequate output over a wide range of fault conditions, from a fraction of full load to many times full load.
6.3.2 Current transformer theory :
A current transformer consists essentially of an iron core with tow windings, one winding is connected in the circuit whose current is to be measured and is called the primary and the other winding is connected to the burden and called the secondary.
The flow of current in the primary winding produces an alternating flux in the core and this flux induces an e.m.f. in the secondary winding which results in the flow of secondary current when this winding is connected to an external closed circuit.
The magnetic effect of the secondary current, in accordance with fundamental principles, is in opposition to that the primary and the value of the secondary current automatically adjusts itself to such a value, that the resultant magnetic effect of the primary and secondary current, produces a flux required to induce the e.m.f. necessary to drive the secondary current through the impedance of the secondary.
In an ideal transformer, the primary ampere-turns are always exactly equal to the secondary ampere-turns and the secondary current is, therefore, always proportional to the primary current.
In an actual current transformer, however, this is never the class. All core material, so far discovered required a certain number of ampere turns to induce a magnetic flux required to induced the necessary voltage.
This can be illustrated as shown in the diagram, EP and ES are the voltage in the primary and secondary windings, respectively, which must be in anti-phase or 180° apart.
In order to maintain the flux the primary current must supply a current Ic in phase with the voltage, to overcome the iron loss (made up of hysteresis and eddy currents) and a current Im at right angle to the voltage and in phase with the flux to magnetize the core.
These two current combine to form the exciting current Ie, if a burden is connected to the secondary and draws a current Is lagging behind the voltage by an angle θ , a corresponding current Is\ must flow in the primary. The total primary current Ip is therefore the sum of Ie and Is.
If the primary current reduces, the secondary current will also automatically reduce, and since the secondary impedance is fixed, the secondary voltage and flux in the core will be reduced in the same proportion.
However, due to the non linearity of the magnetizing curve of the core iron, the exciting current Ie decreases in a different ratio. This results in the current error and phase error not being directly proportional to the current level. Indeed the errors rise significantly at lower primary current levels.
That is, with a given impedance in the secondary circuit, the exciting ampere-turns forms a larger proportion of the total at small primary currents than at large, and it is the exciting current required to magnetize the core which causes the errors.
The most accurate current transformer is one in which the exciting ampere turns are least in proportion to the secondary ampere-turns. Exciting ampere-turns may be reduced in three principal ways :
1- By improving the quality of the magnetic material.
Cold rolled grain oriented silicon steel (C.R.O.S.S) has a magnetization characteristic with a knee point at 1.6 teals.
Nickel steel (proprietary name mutual) has a knee point of 0.7 teals
2- By decreasing the main magnetic path of the core.
3- By reducing the flux density in the core.
Over any one period of time the quality of the available magnetic material (1) is fixed.
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