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Home / Technical Articles / The Essentials Of Current Transformers In Power Circuits (Theory and Practice)

Current & voltage in power circuits

If the voltage or current in a power circuit are too high to connect measuring instruments or relays directly, coupling is made through transformers. Such measuring transformers are required to produce a scaled down replica of the input quantity to the accuracy expected for the particular measurement.

Current Transformers In a Nutshell
Current Transformers In a Nutshell

This is made possible by the high efficiency of the transformer. During and following large instantaneous changes in the input quantity, the waveform may no longer be sinusoidal, therefore the performance of measuring transformers is important.

Many protection systems are required to operate during the transient disturbance in the output of the measuring transformers following a system fault. The errors in transformer output may delay the operation of the protection or cause unnecessary operations.

Therefore the functioning of such transformers must be examined analytically.


  1. Simple equivalent circuit of current transformer
  2. Current transformer connections (primary/secondary)
    1. Errors
      1. Current or Ratio Error
      2. Phase Error
    2. Composite Error
    3. Accuracy Limit Current of Protection Current Transformers
    4. Class PX Current Transformers
    5. CT Winding Arrangements
      1. Wound primary type
      2. Bushing or bar primary type
      3. Core-Balance Current Transformers
      4. Summation Current Transformers
      5. Air-gapped current transformers
    6. Winding Arrangements
      1. Over-Dimensioned CTs
      2. Anti-Remanence CTs
      3. Linear Current Transformers
    7. Secondary Winding Impedance
    8. Secondary Current Rating
    9. Rated Short-Time Current
    10. Transient Response of a Current Transformer
      1. Primary Current Transient
      2. Practical Conditions
    11. Harmonics During the Transient Period
    12. Test Windings

1. Simple equivalent circuit of current transformer

The transformer can be represented by the equivalent circuit of Figure 1, where all quantities are referred to the secondary side.

Equivalent circuit of transformer
Figure 1 – Equivalent circuit of transformer

When the transformer is not 1/1 ratio, this condition can be represented by energizing the equivalent circuit with an ideal transformer of the given ratio but having no losses.

Voltage and current transformers for low primary voltage or current ratings are not readily distinguishable. For higher ratings, dissimilarities of construction are usual. Nevertheless the main differences between these devices are the way they are connected into the power circuit.

Voltage transformers are much like small power transformers, differing only in details of design that control ratio accuracy over the specified range of output. Current transformers have their primary windings connected in series with the power circuit, and so also in series with the system impedance.

The response of the transformer is radically different in these two modes of operation.

This technical article will explain all important aspects of current transformers in MV and HV measurement and protection applications.

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2. Current transformer connections (primary/secondary)

The primary winding of a current transformer is connected in series with the power circuit and the impedance is negligible compared with that of the power circuit.

The power system impedance governs the current passing through the primary winding of the current transformer. This condition can be represented by inserting the load impedance, referred through the turns ratio, in the input connection of Figure 1 above.

This approach is developed in Figure 2, taking the numerical example of a 300/5A CT applied to an 11kV power system. The system is considered to be carrying rated current (300A) and the CT is feeding a burden of 10VA.

Derivation of equivalent circuit of a current transformer
Figure 2 – Derivation of equivalent circuit of a current transformer

A study of the final equivalent circuit of Figure 2(c), taking note of the typical component values, reveals all the properties of a current transformer.

It can be seen that:

  1. The secondary current is not affected by change of the burden impedance over a considerable range.
  2. The secondary circuit must not be interrupted while the primary winding is energized. The induced secondary e.m.f. under these circumstances is high enough to present a danger to life and insulation.
  3. The ratio and phase angle errors can be calculated easily if the magnetizing characteristics and the burden impedance are known.

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2.1 Errors

The general vector diagram applicable for voltage transformer (click to see it) can be simplified by omitting details that are not of interest in current measurement. Take a look at the Figure 3.

Errors arise because of the shunting of the burden by the exciting impedance. This uses a small portion of the input current for exciting the core, reducing the amount passed to the burden.

So, Is = Ip − Ie

where Ie is dependent on Ze, the exciting impedance and the secondary e.m.f. Es, given by the equation:

E= I(Z+ Zb),


  • Zs = the self-impedance of the secondary winding, which can generally be taken as the resistive component Rs only
  • Zb = the impedance of the burden
Vector diagram for current transformer (referred to secondary)
Figure 3 – Vector diagram for current transformer (referred to secondary)

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2.1.1 Current or Ratio Error

This is the difference in magnitude between Ip and Is and is equal to Ir, the component of Ie which is in phase with Is.

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2.1.2 Phase Error

This is represented by Iq, the component of Ie in quadrature with Is and results in the phase error Φ .

The values of the current error and phase error depend on the phase displacement between Is and Ie, but neither current nor phase error can exceed the vectorial error Ie. With a moderately inductive burden, resulting in Is and Ie approximately in phase, there is little phase error and the exciting component results almost entirely in ratio error.

A reduction of the secondary winding by one or two turns is often used to compensate for this.

For example, in the CT corresponding to Figure 2, the worst error due to the use of an inductive burden of rated value would be about 1.2%. If the nominal turns ratio is 2:120, removal of one secondary turn would raise the output by 0.83% leaving the overall current error as -0.37%.

For lower value burden or a different burden power factor, the error would change in the positive direction to a maximum of +0.7% at zero burden; the leakage reactance of the secondary winding is assumed to be negligible.

No corresponding correction can be made for phase error, but it should be noted that the phase error is small for moderately reactive burdens.

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2.2 Composite Error

This is defined in IEC 61869 1 and 2 as the r.m.s. value of the difference between the ideal secondary current and the actual secondary current. It includes current and phase errors and the effects of harmonics in the exciting current.

The accuracy class of measuring current transformers is shown in Table 1 and Table 2.

Table 1 – Limits of CT error for accuracy classes 0.1 to 1.0

Accuracy Class% current+/- Percentage current
(ratio) error
+/- Phase displacement

Table 2 – Limits of CT error for accuracy classes 3 and 5

Accuracy Class% current+/- current (ratio) error, %

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2.3 Accuracy Limit Current of Protection Current Transformers

Protection equipment is intended to respond to fault conditions, and is for this reason required to function at current values above the normal rating.

Protection class current transformers must retain a reasonable accuracy up to the largest relevant current. This value is known as the ‘accuracy limit current’ and may be expressed in primary or equivalent secondary terms.

The ratio of the accuracy limit current to the rated current is known as the ‘accuracy limit factor‘. The accuracy class of protection current transformers is shown in Table 3.

Table 3 – Protection CT error limits for classes 5P and 10P

ClassCurrent error at
rated primary
current (%)
Phase displacement at
rated current
Composite error at
rated accuracy limit
primary current (%)
Standard accuracy limit factors are 5, 10, 15, 20, and 30

Even though the burden of a protection CT is only a few VA at rated current, the output required from the CT may be considerable if the accuracy limit factor is high. For example, with an accuracy limit factor of 30 and a burden of 10VA, the CT may have to supply 9000VA to the secondary circuit.

Alternatively, the same CT may be subjected to a high burden.

For overcurrent and earth fault protection, with elements of similar VA consumption at setting, the earth fault element of an electromechanical relay set at 10% would have 100 times the impedance of the overcurrent elements set at 100%.

Although saturation of the relay elements somewhat modifies this aspect of the matter, the earth fault element is a severe burden, and the CT is likely to have a considerable ratio error in this case. Therefore it is not much use applying turns compensation to such current transformers.

It is generally simpler to wind the CT with turns corresponding to the nominal ratio.

Current transformers are often used for the dual duty of measurement and protection. They then need to be rated according to a class selected from Table 1, Table 2 and Table 3.

The applied burden is the total of instrument and relay burdens. Turns compensation may well be needed to achieve the measurement performance. Measurement ratings are expressed in terms of rated burden and class, for example 15VA Class 0.5.

Protection ratings are expressed in terms of rated burden, class, and accuracy limit factor, for example 10VA Class 10P10.

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2.4 Class PX Current Transformers

The classification of Table 3 is only used for overcurrent protection. Class PX is the definition in IEC 61869 for the quasi-transient current transformers formerly covered by Class X of BS 7626, commonly used with unit protection schemes.

Guidance was given in the specifications to the application of current transformers to earth fault protection, but for this and for the majority of other protection applications it is better to refer directly to the maximum useful e.m.f. that can be obtained from the CT.

In this context, the ‘knee-point’ of the excitation curve is defined as ‘that point at which a further increase of 10% of secondary e.m.f. would require an increment of exciting current of 50%’.

See Figure 4 below.

Definition of knee-point of excitation curve
Figure 4 – Definition of knee-point of excitation curve

Design requirements for current transformers for general protection purposes are frequently laid out in terms of  knee-point e.m.f., exciting current at the knee-point (or some other specified point) and secondary winding resistance.

Such current transformers are designated Class PX.

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2.5 CT Winding Arrangements

Several CT winding arrangements are used. These are described in the following sections.

2.5.1 Wound primary type

This type of CT has conventional windings formed of copper wire wound round a core. It is used for auxiliary current transformers and for many low or moderate ratio current transformers used in switchgear of up to 11kV rating.

Wound Primary Current Transformer
Figure 5 – Wound Primary Current Transformer (Low ratio, wound primary current transformer, suitable for primary currents from 2.5A-100A with 5A outputs. To be used with ammeters, watt meters, or transducers.)

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2.5.2 Bushing or bar primary type

Many current transformers have a ring-shaped core, sometimes built up from annular stampings, but often consisting of a single length of strip tightly wound to form a close-turned spiral. The distributed secondary winding forms a toroid which should occupy the whole perimeter of the core, a small gap being left between start and finish leads for insulation.

Such current transformers normally have a single concentrically placed primary conductor, sometimes permanently built into the CT and provided with the necessary primary insulation.

In other cases, the bushing of a circuit breaker or power transformer is used for this purpose.

At low primary current ratings it may be difficult to obtain sufficient output at the desired accuracy. This is because a large core section is needed to provide enough flux to induce the secondary e.m.f. in the small number of turns, and because the exciting ampere-turns form a large proportion of the primary ampere-turns available.

The effect is particularly pronounced when the core diameter has been made large to fit over large EHV bushings.

Bar Primary Type Current Transformer
Figure 6 – Bar Primary Type Current Transformer (CT having Single Primary turn with the Primary Conductor embedded)

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2.5.3 Core-Balance Current Transformers

Ring type is the most usual type of core-balance current transformer (CBCT). Cable is passing through the centre of CT and in that way forms the primary winding.

Core Balance Current Transformers (CBCT’s) are employed for providing earth leakage protection in a power system. They are different from normal protective and metering current transformers due to their performance requirement.

In insulated or compensated neutral networks, earth leakage currents are small and are, generally, much smaller than nominal load currents.

Such earth leakage currents should not be allowed to exist for long periods of time, since they eventually cause insulation failure on healthy phases and subsequently develop into phase-to-phase or double phase to earth faults.

To protect cable circuits and overhead transmission lines with cable terminations, core balance current transformers are commonly employed. Generally, it is sufficient to incorporate insulation monitoring only to indicate appearance of earth leakage, and not for disconnection. The operating staff in such
cases will be able to take measures to switch the load over to other feeders and switch out the faulted circuits for repairs.

Exception to this rule are circuits which supply power to peat pits, ore mines, and similar loads where, in view of safety considerations, the protection system is designed to switch out the circuit in the event of occurrence of earth leakage.

CBCT’s are manufactured with one core and one secondary winding. The number of secondary turns does not need to be related to the cable/feeder rated current because no secondary current would flow under normal balanced conditions.

This allows the number of secondary turns to be chosen such as to optimize the effective primary pick up current. The choice of ratio should therefore be left to the manufacture to obtain the best possible results.

CBCT’s are used with suitable relays for earth leakage protection purposes. A CBCT encircles 3 Phases, 3-core cable or 3 single core cables.

Core-balance CT wiring
Figure 7 – Core-balance CT wiring: The core balance transformer allows the direct measure of residual currents (earth leakage current). The presence of a homopolar component is significant to an earth default. This component is measured via a toroidal transformer placed around the 3 phases + neutral or phase + neutral (following the network type ), or on the connexion to the earth. The vector sum of this currents is null when there is no fault. This balance is broken when a earth leakage current exist. At this time, the vector sum of currents is not longer null and this difference is measured by the toroid.

During healthy conditions i.e. when there is no earth leakage current, the secondary of CBCT does not carry any current, as there is no net magnetic flux in the core.

In case an Earth Leakage occurs, a net unbalanced current sets up flux in the core of the CBCT and current flows through the secondary winding, causing the relay to operate. If more than one 3-phase cable are used in parallel, it is recommended that a separate CBCT on each cable be connected across a common relay or a separate relay be provided for each CBCT.

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2.5.4 Summation Current Transformers

The summation arrangement is a winding arrangement used in a measuring relay or on an auxiliary current transformer to give a single-phase output signal having a specific relationship to the three-phase current input.

Summation current transformer is used when the currents in a number of feeders don’t need to be individually metered but summated to a single meter or instrument. The summation CT consists of two or more primary windings which are connected to the feeders to be summated, and a single secondary winding, which feeds a current proportional to the summated primary current.

A typical ratio would be 5+5+5/ 5A, which means that three primary feeders of 5 are to be summated to a single 5A meter.

Summation Current Transformer
Figure 8 – Summation Current Transformer

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2.5.5 Air-gapped current transformers

These are auxiliary current transformers in which a small air gap is included in the core to produce a secondary voltage output proportional in magnitude to current in the primary winding.

Sometimes termed ‘transactors’ and ‘quadrature current transformers’, this form of current transformer has been used as an auxiliary component of traditional pilot-wire unit protection schemes in which the outputs into multiple secondary circuits must remain linear for and proportional to the widest practical range of input currents.

Air-gapped current transformer
Figure 9 – Air-gapped current transformer

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2.6 CT Winding Arrangements

CTs for measuring line currents fall into one of three types.

2.6.1 Over-Dimensioned CTs

Over-dimensioned CTs are capable of transforming fully offset fault currents without distortion. In consequence, they are very large. They are prone to errors due to remanent flux arising, for instance, from the interruption of heavy fault currents.

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2.6.2 Anti-Remanence CTs

This is a variation of the over-dimensioned current transformer and has small gap(s) in the core magnetic circuit, thus reducing the possible remanent flux from approximately 90% of saturation value to approximately 10%. These gap(s) are quite small, for example 0.12 mm total, and so the excitation characteristic is not significantly changed by their presence.

However, the resulting decrease in possible remanent core flux confines any subsequent d.c. flux excursion, resulting from primary current asymmetry, to within the core saturation limits.

Errors in current transformation are therefore significantly reduced when compared with those with the gapless type of core. Transient protection Current

Transformers are included in IEC 61869-2 as types TPX, TPY and TPZ and this specification gives good guidance to their application and use.

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2.6.3 Linear Current Transformers

The ‘linear’ current transformer constitutes an even more radical departure from the normal solid core CT in that it incorporates an appreciable air gap, for example 7.5-10mm.

As its name implies the magnetic behaviour tends to linearization by the inclusion of this gap in the magnetic circuit.

However, the purpose of introducing more reluctance into the magnetic circuit is to reduce the value of magnetizing reactance. This in turn reduces the secondary time-constant of the CT, thereby reducing the over-dimensioning factor necessary for faithful transformation.

Figure 10 shows a CT for use on HV systems.

Alstom OSKF 72.5kV to 765kV high voltage current transformer
Figure 10 – Alstom OSKF 72.5kV to 765kV high voltage current transformer

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2.7 Secondary Winding Impedance

As a protection CT may be required to deliver high values of secondary current, the secondary winding resistance must be made as low as practicable. Secondary leakage reactance also occurs, particularly in wound primary current transformers, although its precise measurement is difficult.

The non-linear nature of the CT magnetic circuit makes it difficult to assess the definite ohmic value representing secondary leakage reactance.

It is however, normally accepted that a current transformer is of the low reactance type provided that the following 4 conditions prevail:

  1. The core is of the jointless ring type (including spirally wound cores).
  2. The secondary turns are substantially evenly distributed along the whole length of the magnetic circuit.
  3. The primary conductor(s) passes through the approximate centre of the core aperture or, if wound, is approximately evenly distributed along the whole length of the magnetic circuit.
  4. Flux equalizing windings, where fitted to the requirements of the design, consist of at least four parallel-connected coils, evenly distributed along the whole length of the magnetic circuit, each coil occupying one quadrant.

Alternatively, when a current transformer does not comply with all of the above requirements, it may be proved to be of low-reactance. In this case the composite error, as measured in the accepted way, does not exceed by a factor of 1.3 that error obtained directly from the V-I excitation characteristic of the secondary winding.

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2.8 Secondary Current Rating

The choice of secondary current rating is determined largely by the secondary winding burden and the standard practice of the user. Standard CT secondary current ratings are 5A and 1A.

The burden at rated current imposed by digital or numerical relays or instruments is largely independent of the rated value of current. This is because the winding of the device has to develop a given number of ampere-turns at rated current, so that the actual number of turns is inversely proportional to the current, and the impedance of the winding varies inversely with the square of the current rating.

However, electromechanical or static earth-fault relays may have a burden that varies with the current tapping used.

Interconnection leads do not share this property, however, being commonly of standard cross-section regardless of rating. Where the leads are long, their resistance may be appreciable, and the resultant burden varies with the square of the current rating.

For example a CT lead run of the order of 200 metres, a typical distance for outdoor EHV switchgear, could have a loop resistance of approximately 3 ohms. The CT lead VA burden if a 5A CT is used would be 75VA, to which must be added the relay burden (up to of perhaps 10VA for an electromechanical relay, but less than 1VA for a numerical relay), making a total of 85VA.

Such a burden would require the CT to be very large and expensive, particularly if a high accuracy limit factor were also applicable.

With a 1A CT secondary rating, the lead burden is reduced to 3VA, so that with the same relay burden the total becomes a maximum of 13VA. This can be provided by a CT of normal dimensions, resulting in a saving in size, weight and cost.

Hence modern CTs tend to have secondary windings of 1A rating. However, where the primary rating is high, say above 2000A, a CT of higher secondary rating may be used, to limit the number of secondary turns. In such a situation secondary ratings of 2A, 5A or, in extreme cases, 20A, might be used.

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2.9 Rated Short-Time Current

A current transformer is overloaded while system short-circuit currents are flowing and is short-time rated. Standard times for which the CT must be able to carry rated short-time current (STC) are 0.25, 0.5, 1.0, 2.0 or 3.0 seconds.

A CT with a particular short-time current/ time rating carries a lower current for a longer time in inverse proportion to the square of the ratio of current values.

The converse, however, cannot be assumed, and larger current values than the STC rating are not permissible for any duration unless justified by a new rating test to prove the dynamic capability.

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2.10 Transient Response of a Current Transformer

When accuracy of response during very short intervals is being studied, it is necessary to examine what happens when the primary current is suddenly changed.

The effects are most important, and were first observed in connection with balanced forms of protection, which were liable to operate unnecessarily when short-circuit currents were suddenly established.

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2.10.1 Primary Current Transient

The power system, neglecting load circuits, is mostly inductive, so that when a short circuit occurs, the fault current that flows is given by:

Fault current


  • Ep = peak system e.m.f.
  • R = system resistance
  • L = system inductance
  • β = initial phase angle governed by instant of fault occurrence
  • α = system power factor angle = tan−1 ωL/R

The first term of Equation 6.1 represents the steady state alternating current, while the second is a transient quantity responsible for displacing the waveform asymmetrically.

ip is the steady state peak current:

Steady state peak current

The maximum transient occurs when sin (α − β) and no other condition need be examined. So:

Steady state peak current formulae

When the current is passed through the primary winding of a current transformer, the response can be examined by replacing the CT with an equivalent circuit as shown in Figure 2(b).

As the ‘ideal’ CT has no losses, it transfers the entire function, and all further analysis can be carried out in terms of equivalent secondary quantities (is and Is). A simplified solution is obtainable by neglecting the exciting current of the CT.

The flux developed in an inductance is obtained by integrating the applied e.m.f. through a time interval:

Flux developed in an inductance

For the CT equivalent circuit, the voltage is the drop on the burden resistance Rb. Integrating for each component in turn, the steady state peak flux is given by:

Steady state peak flux

The transient flux is given by:

Transient flux

Hence, the ratio of the transient flux to the steady state value is:

Ratio of the transient flux to the steady state value

where X and R are the primary system reactance and resistance values. The CT core has to carry both fluxes, so that:

The CT core has to carry both fluxes

The term (1+X/R) has been called the ‘transient factor’ (TF), the core flux being increased by this factor during the transient asymmetric current period. From this it can be seen that the ratio of reactance to resistance of the power system is an important feature in the study of the behaviour of protection relays.

Alternatively, L/R is the primary system time constant T, so that the transient factor TF can be written:

Transient factor TF formulae

Again, fT is the time constant expressed in cycles of the a.c. quantity T’ so that:

Transient factor TF formulae

This latter expression is particularly useful when assessing a recording of a fault current, because the time constant in cycles can be easily estimated and leads directly to the transient factor.

For example, a system time constant of three cycles results in a transient factor of (1+6π), or 19.85. That is, the CT would be required to handle almost twenty times the maximum flux produced under steady state conditions.

The above theory is sufficient to give a general view of the problem.

In this simplified treatment, no reverse voltage is applied to demagnetize the CT, so that the flux would build up as shown in Figure 11.

Response of a CT of infinite shunt impedance to transient asymmetric primary current
Figure 11 – Response of a CT of infinite shunt impedance to transient asymmetric primary current

Since a CT requires a finite exciting current to maintain a flux, it does not remain magnetized (neglecting hysteresis), and for this reason a complete representation of the effects can only be obtained by including the finite inductance of the CT in the calculation.

The response of a current transformer to a transient asymmetric current is shown in Figure 12 below.

Response of a current transformer to a transient asymmetric current
Figure 12 – Response of a current transformer to a transient asymmetric current


  • is = the nominal secondary current
  • i’s = the actual secondary output current
  • ie = the exciting current


is = ie + i’s


Exciting current relation to actual secondary output current


Exciting current relation to actual secondary output current

which gives for the transient term:

Exciting current


  • T = primary system time constant L/R
  • T1 = CT secondary circuit time constant Le/Rb
  • I1 = prospective peak secondary current

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2.10.2 Practical Conditions

Practical conditions differ from theory for the following reasons:

Reason #1

No account has been taken of secondary leakage or burden inductance. This is usually small compared with Le so has little effect on the maximum transient flux.

Reason #2

Iron loss has not been considered. This has the effect of reducing the secondary time constant, but the value of the equivalent resistance is variable, depending upon both the sine and exponential terms.

Consequently, it cannot be included in any linear theory and is too complicated for a satisfactory treatment to be evolved.

Reason #3

The theory is based upon a linear excitation characteristic. This is only approximately true up to the knee-point of the excitation curve.

A precise solution allowing for non-linearity is not practicable.

Solutions have been sought by replacing the excitation curve with several chords. A linear analysis can then be made for the extent of each chord. The above theory is sufficient to give a good insight into the problem and to allow most practical issues to be decided.

Reason #4

The effect of hysteresis, apart from loss as discussed under (Reason #2) above, is not included. Hysteresis makes the inductance different for flux build up and decay, so that the secondary time constant is variable.

Moreover, the ability of the core to retain a ‘remanent’ flux means that the value of ΦB developed in Equation above has to be regarded as an increment of flux from any possible remanent value positive or negative. The formula would then be reasonable provided the applied current transient did not produce saturation.

A precise calculation of the flux and excitation current is not feasible. The value of the study is to explain the observed phenomena.

The asymmetric (or d.c.) component can be regarded as building up the mean flux over a period corresponding to several cycles of the sinusoidal component, during which period the latter component produces a flux swing about the varying ‘mean level’ established by the former. The asymmetric flux ceases to increase when the
exciting current is equal to the total asymmetric input current, since beyond this point the output current, and hence the voltage drop across the burden resistance, is negative.

Saturation makes the point of equality between the excitation current and the input occur at a flux level lower than would be expected from linear theory.

When the exponential component drives the CT into saturation, the magnetizing inductance decreases, causing a large increase in the alternating component ie.

The total exciting current during the transient period is of the form shown in Figure 13 and the corresponding resultant distortion in the secondary current output, due to saturation, is shown in Figure 14.

Typical exciting current of CT during transient asymmetric input current
Figure 13 – Typical exciting current of CT during transient asymmetric input current
Distortion in secondary current due to saturation
Figure 14 – Distortion in secondary current due to saturation

The presence of residual flux varies the starting point of the transient flux excursion on the excitation characteristic.

Remanence of like polarity to the transient reduces the value of symmetric current of given time constant which the CT can transform without severe saturation. Conversely, reverse remanence greatly increases the ability of a CT to transform transient current.

If the CT were the linear non-saturable device considered in the analysis, the sine current would be transformed without loss of accuracy.

In practice the variation in excitation inductance caused by transferring the centre of the flux swing to other points on the excitation curve causes an error that may be very large. The effect on measurement is of little consequence, but for protection equipment that is required to function during fault conditions, the effect is more serious.

The output current is reduced during transient saturation, which may prevent the relays from operating if the conditions are near to the relay setting. This must not be confused with the increased r.m.s. value of the primary current due to the asymmetric transient, a feature which sometimes offsets the increase ratio error.

In the case of balanced protection, during through faults the errors of the several current transformers may differ and produce an out-of-balance quantity, causing
unwanted operation.

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2.11 Harmonics During the Transient Period

When a CT is required to develop a high secondary e.m.f. under steady state conditions, the non-linearity of the excitation impedance causes some distortion of the output waveform. In addition to the fundamental current, such a waveform contains odd harmonics only.

However, when the CT is saturated unidirectionally while being simultaneously subjected to a small a.c. quantity, as in the transient condition discussed above, the output contains both odd and even harmonics.

Usually the lower numbered harmonics are of greatest amplitude and the second and third harmonic components may be of considerable value. This may affect relays that are sensitive to harmonics.

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2.12 Test Windings

On-site conjunctive testing of current transformers and the apparatus that they energize is often required. It may be difficult, however, to pass a suitable value of current through the primary windings, because of the scale of such current and in many cases because access to the primary conductors is difficult.

Additional windings can be provided to make such tests easier and these windings are usually rated at 10A.

The test winding inevitably occupies appreciable space and the CT costs more. This should be weighed against the convenience achieved and often the tests can be replaced by alternative procedures.

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Sources //

  1. Network protection and automation guide by (ex) Alstom Grid, now General Electric
  2. Core-balance current transformer by Loreme
  3. Core-balance current transformer by Gilber and Maxwell transformers
  4. Electric Power Substations Engineering By James C. Burke

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Janardhan Namde
    Apr 08, 2022

    What happens if the Protection CT selected are too high/large in Ratio. The Feeder current in actual is 12 Amps at 33kV. However the Vendor ABB have provided the CT’s with Ration of 200/1 for REF615 Protection Relay.

  2. Jim Beam
    Nov 15, 2020

    In Figure 2 (c) It says referred to Secondary side but I think it looks like it is actually referred to the Primary side?
    If you were referring to the Secondary side, wouldn’t you be dividing Zr by the turns ratio a^2 (i.e. divide by 60) and not multiply by 60?

  3. adam bump
    Oct 02, 2020

    i really likethis program it is intersting becuase i am a 14 year old who likes electrical engineering

  4. BYJU
    Feb 16, 2020

    In fig 2 a and 2b it is not clear how 6350 V appears across the primary terminal of the current transformer. Could you pls elaborate as I understand the primary winding is in series with voltage source and by virtue of the low impedance of the primary winding, voltage across it is expected to be low as per voltage divider rule?
    Thank you for the excellent article.

  5. Mahdi Jamshidi
    Dec 16, 2019

    HI Thank you for your content
    I know little

    please if may send content from high-voltage power stations such as tests

  6. RatnaKumar
    Aug 04, 2019

    Excellent topic on CT’s

  7. G.M.Fateh Kayser
    Jul 16, 2019

    Dear Edvard Csanyi
    can you describe VCB panel and ACB Panel for 0.5 MVA Sub-station , I mean proper equipment and their ideal ratings.

  8. V Nyoni
    Jun 28, 2019

    Great article,very informative.

  9. Akhilesh
    Jun 28, 2019

    CT ratio 100/5 explore like capicter. Its possible

    Apr 22, 2019

    dear sir i am manufacturer of handmade ct,s so i want to buy a book on ct please suggest me how i can purchase book in india with different ct,s

  11. ASHOKA
    Mar 26, 2019

    Excellent topic about CTs.

  12. omara
    Dec 04, 2018

    This subject is good & also the demonstration but no figures appeared which make little difficulties to get full understanding.
    Please attach the figures

    thanks you

    • Edvard
      Dec 04, 2018

      It’s fixed, should be fine now.

  13. Mokhoele Mahao
    Dec 03, 2018

    Very nice article, thanx

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