Current Transformer

Current Transformer

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 transformer are connected in ac power circuits to feed the current coils of indicating and metering instruments (ammeters, wattmeters, watt-hour meters) and protective relays.The current transformer basically consists of an iron core on which are wound a primary and one or two secondary windings. The primary winding of the CT is connected in series with the load and carries the actual power system current while secondary is connected to the measuring circuit or the relay.The primary winding is normally single turn winding and the number of turns on the secondary windings depend upon the current to be carried by the power circuit.The larger the current to be carried by the power circuit more the number of turns on the secondary.

Operation

A CT functions with the same basic working principle of electrical power transformer, but here is some difference.
In a general purpose transformer, primary current varies with load or secondary current. In case of CT, primary current is the system current and this primary current or system current transforms to the CT secondary. Hence secondary current or burden current depends upon primary current of the current transformer.The primary current of the CT does not depend upon whether the load or burden is connected to the secondary or not or what is the impedance value of burden.Generally CT has very few turns in primary where as number of turns in secondary is large in number. Due to this type of arrangement, the current transformer is often referred to 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.

Burden

The product of voltage and current on the secondary side when the CT is supplying the instrument or relay with its maximum rated value of current is known as rated burden.Burden is measured in volt-amperes (VA).Generally the VA ratings ranges from 5 to 150 VA.The volt-ampere rating of CTs is low(5-150 VA) as compared with that of power transformers(a few KVA to several MVA).

 

                   fig:CT(current transformer)

CT ratio

• Most current transformers have a standard secondary rating of 5 amps with the primary and secondary currents being expressed as a ratio such as 100/5.It means when 100 amps is flowing in the primary conductor it will result in 5 amps flowing in the secondary winding Or one of 500/5 will produce 5 amps in the secondary for 500 amps in the primary conductor, etc.

• By increasing the number of secondary windings, the secondary current can be made much smaller than the current in the primary circuit being measured.
• 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.

CT ratio – Example

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.

• 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 due to the very large increase in magnetizing flux in the secondary core.
• This results in a high voltage being induced in the secondary winding equal to the ratio of: Vp(Ns/Np) being developed across the secondary winding.

Secondary Current:

Voltage across Ammeter:

Assume our current transformer from above is connected to a 480 volt three-phase power line.

This 76.8kV is why a current transformer should never be left open-circuited or operated with no-load attached
when the main primary current is flowing through it. If the ammeter is to be removed, a short-circuit should be
placed across the secondary terminals first to eliminate the risk of shock.

Types of CT

There are three basic types of current transformers:

Bar type

Pic source:Electrical4u

• 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.

Wound type

pic sources:circuit globe

• Wound CTs have a primary and secondary winding like a normal transformer.
• 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.

Window type

• They are constructed with no primary winding and are installed around the primary conductor.

• Window CTs can be of solid or split core construc-tion.

• The primary conductor must be disconnected when installing solid window CTs.
• However, split core CTs can be installed around the primary conductor without disconnecting the primary conductor.

Handheld current transformer

• Known as “clamp meters” also
• Clamp meters can measure current without disconnecting or opening the circuit.
• These are available for measuring currents from 100 up to 5000 amps, with sizes from 1′′ to over 12′′ (25-to-300mm).

Standard ratios

• The most common CT secondary full-load current is 5 amps
• indicating devices, metering equipments, and protective relays are connected to CT secondary
• CT’s with a 1 amp full-load value and matching instruments with a 1 amp full-range value are also available.
• Many new protective relays are programmable for either value.
• The secondary current of 0.1 amp is also used in some cases for static relays.
• Primary current rating ranges from 10 amp to 3000 amp or more.
• CT ratios are expressed as a ratio of the rated primary current to the rated secondary current.

• A 300:5 CT will produce 5 amps of secondary current when 300 amps flow through the primary.
• As the primary current changes, the secondary current will vary accordingly.
• With 150 amps through the 300 amp rated primary, the secondary current will be 2.5 amps (150:300 = 2.5:5).
• When the rated primary amps is exceeded, which is usually the case when a fault occurs on the system, the
amount of secondary current will increase but, depending on the magnetic saturation in the CT, the output may not be exactly proportional.

Application of CT

  • Current transformers are used extensively
  • for measuring current
  • For monitoring the operation of the power grid.
  • Multiple CTs are installed for various uses.
  • For example, protection devices and metering may use separate CTs to provide isolation between metering and protection circuits.
  • Separate CTs used in metering and protection may have different characteristics (accuracy, overload performance) to be used for the devices.

Knee Point Voltage of Current Transformer

• This is the significance of saturation level of a CT core mainly used for protection purposes.
• The sinusoidal voltage of rated frequency applied to the secondary terminals of current transformer, with other winding being open circuited, which when increased by 10% cause the exiting current to increase 50%.
• The EMF induced in the CT secondarywindings is
                 E2 = 4.44φfT2

Where,

f is the system frequency,
φ is the maximum magnetic flux in Wb.
T2 is the number of turns of the secondary winding.
The flux in the core, is produced by excitation current Ie
.

We have a non-liner relationship between excitation current and magnetizing flux. After certain value of excitation current, flux will not further increase so rapidly with increase in excitation
current. This non-liner relation curve is also called B – H curve.

• Again from the equation above, it is found that, secondary voltage of a current transformer is directly proportional to flux φ. Hence one typical curve can be drawn from this relation between secondary voltage and excitation current as shown below.
• It is clear from the curve that, linear relation between V & Ie is maintained from point A & K. The point ′A′ is known as ′ankle point′ and point ′K′ is known as ′Knee Point′.

Accuracy class and errors

phasor diagram

Is- Secondary current.
Es- Secondary induced emf.
Ip- Primary current.
Ep- Primary induced emf.
KT- Turns ratio = Numbers of secondary turns/number of primary turns.

I0- Excitation current.
Im – Magnetizing component of I0

Iw- Core loss component of I0

Φm – Main flux.

Accuracy class is determined by the value of errors. There are two types of errors in a CT
(i) Current ratio error and
(ii) Phase angle error

Current ratio error

• From above phasor diagram it is clear that primary current Ip is not exactly equal to the secondary current multiplied by turns ratio, i.e. KTIs

• This difference is due to the primary current is contributed by the core excitation current.Current ratio error is mainly due to energy component of excitation current.

• The error in current transformer introduced due to this difference is called current error of CT or sometimes ratio
error in current transformer.

Where; IP = Primary current; KT = Turn ratio; IS = Secondary current

Phase angle error

• For an ideal CT the angle between the primary and reversed secondary current vector is zero.

• But for an actual CT there is always a difference in phase between two due to the fact that primary current has to supply the component of the exiting current.
• The angle between the above two phases is termed as phase angle error in current transformer or CT.
• Here in the phasor diagram it is β. The phase angle error is usually expressed in minutes.

Limits of error for measuring CTs

A.For measuring CTs

B.For protective CTs

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