Current Transformer | Potential Transformers | Instrument transformers

Current transformer

A current transformer (CT) may be a sort of electrical device that’s accustomed live electrical energy (AC). It produces a current in its secondary that is proportional to the present in its primary. Current transformers, along side voltage or potential transformers, are instrument transformers. Instrument transformers scale the big values of voltage or current to little, standardized values that square measure simple to handle for instruments and protecting relays. The instrument transformers isolate activity or protection circuits from the high voltage of the first system. A current electrical device provides a secondary current that’s accurately proportional to the present

flowing in its primary. the present electrical device presents a negligible load to the first circuit. Current transformers are the current-sensing units of the facility system and square measure used at generating stations, electrical substations, and in industrial and business power distribution.A current electrical device incorporates a primary coil, a core(mostly air core) and a secondary. The electrical energy within the primary produces associate degree alternating magnetic flux within the core, that then induces associate degree electrical energy within the secondary. correct current transformers want shut coupling between the first and secondary to confirm that the secondary current is proportional to the first current over a large current vary. this within the secondary is that the current within the primary (assuming one flip primary) divided by the amount of turns of the secondary. Current transformers usually incorporates a element steel ring core wound with several turns of copper wire. The conductor carrying the first current is responded to the ring. The CT’s primary, therefore, consists of one ‘turn’.

CTs are nominal by their current quantitative relation from primary to secondary. The rated secondary currrent

rent is often standardized at one or five amperes. for instance, a 4000:5 CT coil can provide an output current of five amperes once the first winding current is 4000 amperes. This quantitative relation also can be wont to realize the resistivity or voltage on one aspect of the electrical device, given the acceptable price at the opposite aspect. For the 4000:5 CT, the secondary resistivity will be found as ZS = NZP = 800ZP, and therefore the secondary voltage will be found as VS = NVP = 800VP. In some cases, the secondary resistivity is stated the first aspect, and is found as ZS′ = N2ZP. Referring the resistivity is finished just by multiplying initial secondary resistivity price by the present quantitative relation. The coil of a CT will have faucets to supply a spread of ratios, 5 taps being common.

Current electrical device shapes and sizes vary depending on the top user or switch gear manufacturer. low-tension single quantitative relation metering current transformers ar either a hoop kind or plastic wrought case.
Split-core current transformers either have a two-part core or a core with a removable section. this permits the electrical device to be placed around a conductor while not having to disconnect it 1st. Split-core current transformers ar usually employed in low current measure instruments, typically transportable, battery-operated, and hand-held (see illustration lower right).


The accuracy of a CT is affected by a number of factors including:

  • Burden
  • Burden class/saturation class
  • Rating factor
  • Load
  • External electromagnetic fields
  • Temperature
  • Physical configuration
  • The selected tap, for multi-ratio CTs
  • Phase change
  • Capacitive coupling between primary and secondary
  • Resistance of primary and secondary
  • Core magnetizing current

Accuracy categories for varied styles of measurement and at normal loads within the secondary circuit (burdens) ar outlined in IEC 61869-1 as categories zero.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3. the category designation is Associate in Nursing approximate live of the CT’s accuracy. The magnitude relation (primary to secondary current) error of a category one CT is eighteen at rated current; the magnitude relation error of a category zero.5 CT is 0.5% or less. Errors in section also are necessary, particularly in power measurement circuits. every category has an allowable most section error for a such as load resistivity.
Current transformers used for protecting relaying even have accuracy needs at overload currents in way over the conventional rating to confirm correct performance of relays throughout glitches. A CT with a rating of two.5L400 specifies with an output from its secondary of twenty times its rated secondary current (usually five A × twenty = a hundred A) and four hundred V (IZ drop) its output accuracy are going to be inside a pair of 2.5 percent.


Based on the function performed by the current transformer, it can be classified is follows:

Measuring current transformers: These current transformers are used together with the activity devices for the activity of current, energy, and power.

Protective current transformers: These current transformers are used together with the protection equipments such as trip coils, relays, etc.

Based on the function construction, it can also be classified as follows:

Bar Type: This type consists of a bar of appropriate size and material forming an integral a part of the electrical device.

Wound Type: This type includes a primary coil of ore than one full flip wound over the core.

Window Type: This type has no primary. The secondary wind of the CT is placed round the current flowing conductor. The magnetic field of force created by current flowing through the conductor induces current within the secondary, that is employed for measuring.

Phase shift

Ideally, the first and secondary currents of a current electrical device ought to be in part. In follow, this is often not possible, but, at traditional power frequencies, part shifts of some tenths of a degree are possible, whereas less complicated CTs might have part shifts up to 6 degrees.[3] For current measurement, part shift is immaterial as ammeters solely show the magnitude of this. However, in wattmeters, energy meters, and power issue meters, part shift produces errors. For power and energy measurement, the errors area unit thought of to be negligible at unity power issue however become a lot of important because the power issue approaches zero. At zero power-factor, any indicated power is entirely because of this transformer’s part error.[3] The introduction of electronic power and energy meters has allowed current part error to be mark out.


The ideal current transformer could also be outlined together during which any primary condition is reproduced within the secondary circuit within the actual quantitative relation and section relationship.

For an ideal transformer:

ITp = Is Ts

Ip / Is = Ts / Tp

Therefore the quantitative relation of primary and secondary currents up to the turns quantitative relation. conjointly the first and secondary currents area unit specifically 1800 in part.
In an actual electrical device, the windings have resistance and electrical phenomenon and conjointly the electrical device has magnetizing and loss part of current to keep up the flux (see Figure 2). Therefore, in an actual electrical device the ratio of current isn’t up to the turns quantitative relation and conjointly there’s a part distinction between the first current and therefore the secondary currents mirrored back on the first aspect and consequently we’ve got ratio error and point in time error.

Kn = turns ratio

= number of secondary winding turns / number of primary winding turns,

rs, xs = resistance and reactance respectively of the secondary winding,

rp, xp = resistance and reactance respectively of the primary winding,

Ep, Es = primary and secondary induced voltages respectively,

Tp, Ts = number of primary winding and secondary winding turns respectively,

Ip, Is = primary and secondary winding currents respectively,

θ = phase angle of the transformer

Φm = working flux of the transformer

δ = angle between secondary induced voltage and secondary current,

I= exciting current,

Im = magnetizing component of exciting current

Il = loss component of exciting current,

α = angle between Io and Φm

Actual transformation ratio R = Ip / Is

= Kn + (Il cos δ + Isin δ)/ KnIs

Phase angle θ = 180/ π (Il cos δ + Isin δ)/ KnIs

Ratio error = (KnIs – Ip)/ Ip x 100%

= (Kn – R) / R x 100 %

Equations related to CT:

  1. Turns Ratio (TR) = n =Vp/Vs = Np/Ns = Is/Ip
  2. Secondary Current (Is) = Ip(Np/Ns)
  3. Vs = Vp(Ns/Np)

Leave a comment