Current Transformer Basics
The Current Transformer ( C.T. ), is a type of "instrument
transformer" that is designed to produce an alternating current in its secondary winding which is proportional to
the current being measured in its primary. Current transformers reduce high voltage currents to a much lower
value and provide a convenient way of safely monitoring the actual electrical current flowing in an AC transmission line
using a standard ammeter. The principal of operation of a current transformer is no different from that of an ordinary transformer.
Typical Current Transformer
Unlike the voltage or power transformer looked at previously, the current transformer consists of only
one or very few turns as its primary winding. This primary winding can be of either a single flat turn, a coil of heavy duty
wire wrapped around the core or just a conductor or bus bar placed through a central hole as shown.
Due to this type of arrangement, the current transformer is often referred too as a "series transformer"
as the primary winding, which never has more than a very few turns, is in series with the current carrying conductor.
The secondary winding may have a large number of coil turns wound on a laminated core of low-loss magnetic
material which has a large cross-sectional area so that the magnetic flux density is low using much smaller cross-sectional
area wire, depending upon how much the current must be stepped down. This secondary winding is usually rated at a standard
1 Ampere or 5 Amperes.
There are three basic types of current transformers: "wound", "toroidal" and "bar".
- • Wound current transformers The
transformers primary winding is physically connected in series with the conductor that carries the measured current flowing
in the circuit. The magnitude of the secondary current is dependent on the turns ratio of the transformer.
- • Toroidal current transformers These
do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a
window or hole in the toroidal transformer. Some current transformers have a "split core" which allows it to be opened,
installed, and closed, without disconnecting the circuit to which they are attached.
- • Bar-type current transformers This
type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is
equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted
to the current carrying device.
Current transformers can reduce or "step-down" current levels from thousands of amperes
down to a standard output of a known ratio to either 5 Amps or 1 Amp for normal operation. Thus, small and accurate
instruments and control devices can be used with CT's because they are insulated away from any high-voltage power lines.
There are a variety of metering applications and uses for current transformers such as with wattmeter's, power factor meters,
watt-hour meters, protective relays, or as trip coils in magnetic circuit breakers, or MCB's.
Generally current transformers and ammeters are used together as a matched pair in which the design of
the current transformer is such as to provide a maximum secondary current corresponding to a full-scale deflection on the
ammeter. In most current transformers an approximate inverse turns ratio exists between the two currents in the primary and
secondary windings. This is why calibration of the CT is generally for a specific type of ammeter.
For most current transformers the primary and secondary currents are expressed as a ratio such as 100/5.
This means that when 100 Amps is flowing in the primary winding it will result in 5 Amps flowing in the secondary winding.
By increasing the number of secondary windings, N2, the secondary current can be made much
smaller than the current in the primary circuit being measured. In other words, as N2 increases,
I2 goes down by a proportional amount.
We know from our tutorial on double wound transformers that its turns ratio is equal to:
from which we get:
As the primary usually consists of one or two turns whilst the secondary can have several hundred
turns, the ratio between the primary and secondary can be quite large. For example, assume that the current rating of the
primary winding is 100A. The secondary winding has the standard rating of 5A. Then the ratio between the primary and the
secondary currents is 100A-to-5A, or 20:1. In other words, the primary current is 20 times greater than the secondary current.
It should be noted however, that a current transformer rated as 100/5 is not the same as one rated as
20/1 or subdivisions of 100/5. This is because the ratio of 100/5 expresses the "input/output current rating" and not the
actual ratio of the primary to the secondary currents. Also note that the number of turns and the current in the primary
and secondary windings are related by an inverse proportion.
But relatively large changes in a current transformers turns ratio can be achieved by modifying the
primary turns through the CT's window where one primary turn is equal to one pass and more than one pass through the window
results in the electrical ratio beng modified.
So for example, a current transformer with a relationship of say, 300/5A can be converted to another of
150/5A or even 100/5A by passing the main primary conductor through its interior window two or three times as shown. This
allows a higher value current transformer to provide the maximum output current for the ammeter when used on smaller primary
Current Transformer Primary Turns Ratio
A bar-type current transformer which has 1 turn on its primary and 160 turns on its secondary is to be
used with a standard range of ammeters that have an internal resistance of 0.2Ω's. The ammeter is required to give a
full scale deflection when the primary current is 800 Amps. Calculate the maximum secondary current and secondary voltage
across the ammeter.
Voltage across Ammeter:
We can see above that since the secondary of the current transformer is connected across the
ammeter, which has a very small resistance, the voltage drop across the secondary winding is only 1.0 volts at full primary
current. If the ammeter is removed, the secondary winding becomes open-circuited and the transformer acts as a step-up
transformer resulting in a very high voltage equal to the ratio of: Vp(Ns/Np) being
developed across the secondary winding.
So for example, assume our current transformer from above is connected to a 480 volt three-phase
power line. Therefore:
This is why a current transformer should never be open-circuited or operated with no-load attached
when the main primary current is flowing. If the ammeter is to be removed, a short-circuit should be placed across the
secondary terminals first. This is because when the secondary is open-circuited the iron core of the transformer operates at
a high degree of saturation, which produces an abnormally large secondary voltage, and in our simple example above, this was
calculated at 76.8kV!. This high secondary voltage could damage the insulation or cause electric shock if the CT's terminals
are accidentally touched.
Handheld Current Transformers
There are many specialized types of current transformers now available. A popular and portable type which
can be used to measure circuit loading are called "clamp meters" as shown. Clamp meters open and close around a current
carrying conductor and measure its current by determining the magnetic field around it, providing a quick measurement
reading usually on a digital display without disconnecting or opening the circuit.
As well as the handheld clamp type CT, split core current transformers are available which has one end
removable so that the load conductor or bus bar does not have to be disconnected to install it. These are available for
measuring currents from 100 up to 5000 amps, with square window sizes from 1" to over 12" (25-to-300mm).
Then to summarise, the Current Transformer, (CT) is a type of instrument transformer
used to convert a primary current into a secondary current through a magnetic medium. Its secondary winding then provides
a much reduced current which can be used for detecting overcurrent, undercurrent, peak current, or average current conditions.
A current transformers primary coil is always connected in series with the main conductor giving rise to
it also being referred to as a series transformer. The nominal secondary current is rated at 1A or 5A for ease of measurement.
Construction can be one single primary turn as in Toroidal, Donut, or Bar types, or a few wound primary turns, usually for
low current ratios.
Current transformers are intended to be used as proportional current devices. Therefore a current transformers
secondary winding should never be operated into an open circuit, just as a voltage transformer should never be operated into
a short circuit. Very high voltages will result from open circuiting the secondary circuit of an energized CT so their
terminals must be short-circuited if the ammeter is to be removed or when a CT is not in use before powering up the system.
In the next tutorial about Transformers we will look at what happens when we
connect together three individual transformers in a star or delta configuration to produce a larger power transformer called
a Three Phase Transformer
used to supply 3-phase supplies.