A current transformer is a transformer, which produces in its secondary winding a current, which is proportional to the current flowing in its primary winding. The secondary current is usually smaller in magnitude than the primary current.
The principal function of a CT is to produce a proportional current at a level of magnitude, which is suitable for the operation of measuring or protective devices such as indicating or recording instruments and relays.
The rated secondary current is commonly 5A or 1A, though lower currents such as 0.5A are not uncommon. It flows in the rated secondary load, usually called the burden, when the rated primary current flows in the primary winding.
The primary winding can consist merely of the primary current conductor passing once through an aperture in the current transformer core or it may consist of two or more turns wound on the core together with the secondary winding.
These are two basic CT types. The first is commonly called a “ring” type CT as the core is usually annular, but in some cases it may be square or rectangular in shape. The second is usually known as a “wound primary” type CT.
The primary and secondary currents are expressed as a ratio such as 100/5. With a 100/5 ratio CT, 100A flowing in the primary winding will result in 5A flowing in the secondary winding, provided the correct rated burden is connected to the secondary winding. Similarly, for lesser primary currents, the secondary currents are proportionately lower.
It should be noted that a 100/5 CT would not fulfil the function of a 20/1 or a 10/0.5 CT as the ratio expresses the current rating of the CT, not merely the ratio of the primary to the secondary currents.
The extent to which the secondary current magnitude differs from the calculated value expected by virtue of the CT ratio is defined by the [accuracy] “Class” of the CT. The greater the number used to define the class, the greater the permissible “current error” [the deviation in the secondary current from the calculated value].
Except for the least accurate classes, the accuracy class also defines the permissible phase angle displacement between primary and secondary currents. This latter point is important with measuring instruments influenced both by magnitude of current and by the phase angle difference between the supply voltage and the load current, such as kWh meters, wattmeter’s, var meters and power factor meters.
Common burden ratings are 2.5, 5, 10, 15 and 30VA.
Current transformers are usually either “measuring” or “protective” types, these descriptions being indicative of their functions. The principal requirements of a measuring CT are that, for primary currents up to 120% or 125% of the rated current, its secondary current is proportional to its primary current to a degree of accuracy as defined by its “Class” and, in the case of the more accurate types, that a specified maximum phase angle displacement is not exceeded.
A desirable characteristic of a measuring CT is that it should “saturate” when the primary current exceeds the percentage of rated current specified as the upper limit to which the accuracy provisions apply. This means that at these higher levels of primary current the secondary current is less than proportionate. The effect of this is to reduce the extent to which any measuring device connected to the CT secondary is subjected to current overload.
On the other hand the reverse is required of the protective type CT, the principal purpose of which is to provide a secondary current proportional to the primary current when it is several, or many, times the rated primary current. The measure of this characteristic is known as the “Accuracy Limit Factor” (A.L.F.). A protection type CT with an A.L.F. of 10 will produce a proportional current in the secondary winding [subject to the allowable current error] with primary currents up to a maximum of 10 times the rated current.
No comments:
Post a Comment