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| CURRENT TRANSFORMER BASIC |
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
WORKING OF CT
The primary and secondary currents are expressed as a
ratio such as100/5. 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.
BURDEN RATING
Common burden ratings are 2.5, 5, 10, 15 and 30VA. Currenttransformers 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% or125% 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 les 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.
Preferred primary and secondary current ratings [and
therefore ratios], classes, burdens and accuracy limit factors are defined in
BS3938 and other comparable national standards, together with other minimum
performance requirements, physical construction requirements, etc.
It should be remembered when using a CT that where there
are two or more devices to be operated by the secondary winding, they must be connected
in series across the winding. This is exactly the opposite of the method used
to connect two or more loads to be supplied by a voltage or power transformer
where the devices are paralleled across the secondary winding.
With a CT, an increase in the burden will result in an
increase in the CT secondary output voltage. This is automatic and necessary to
maintain the current to the correct magnitude. Conversely, a reduction in the
burden will result in a reduction in the CT secondary output voltage.
This rise in secondary
voltage output with an increase in burden means that, theoretically, with
infinite burden as is the case with the secondary load open circuit, an
infinitely high voltage appears across the secondary terminals. For practical
reasons this voltage is not infinitely high, but can be high enough to cause a
breakdown in the insulation between primary and secondary windings or between
either or both windings and the core. For this reason, primary current should
never be allowed to flow with no load or with a high resistance load connected
across the secondary winding.
CTs should be specified as follows:
RATIO: input / output current ratio
VA: total burden including pilot wires.
CLASS: Accuracy required for operation
DIMENSIONS: maximum & minimum limits
Metering CTs
In general, the following applies:
CLASS
• 0.1 or 0.2 for precision measurements
• 0.5 for high grade kilowatt hour meters for commercial
grade kilowatt hour meters
• 3 for general industrial measurements
• 3 or 5 for approximate measurements
Protection CTs
In addition to the general specification required for CT
design, protection CT’s require an Accuracy Limit Factor (ALF). This is the
multiple of rated current up to which the CT will operate while complying with
the accuracy class requirements.
In general the following applies:
• Instantaneous overcurrent relays & trip coils -
2.5VA Class 10P5
• Thermal inverse time relays - 7.5VA Class 10P10
• Low consumption Relay - 2.5VA Class 10P10
• Inverse definite min. time relays (IDMT) overcurrent -
15VA Class10P10/15
• IDMT Earth fault relays with approximate time grading -
15VA Class10P10
• IDMT Earth fault relays with phase fault stability or
accurate time grading required - 15VA Class 5P10
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