A current transformer is used to transform a primary current quantity in terms of its magnitude and phase to a secondary value such that in normal conditions the secondary value is substantially proportional to the primary value.

A current transformer is a transformer has its primary winding is connected in series with the power circuit with thick windings and few turns, usually one turn. And the secondary with thin windings and many turns.

A. Wound “Busbar” Type

The wound type has a separate primary and secondary winding mounted on a laminated iron 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.

The wound primary is used for the smaller currents, but it can only be applied on low fault level installations due to thermal limitations as well as structural requirements due to high magnetic forces.

This type of current transformer is designed so that the primary winding consists of one or more turns of heavy wire connected in series in the circuit and the secondary winding consists of a larger number of turns of relatively smaller wires and is connected to instruments or control devices.

B. Toroidal “Window” Type

If we enclose the current transformer in the bushing type in a molded case, we obtain another type of current transformer called the window type. Through the core and secondary winding, there is an insulated hole through which the user can place his own conductor.

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.

C. Bushing Type

A transformer bushing CT (BCT) is a window-type current transformer mounted around the bushing's flange. They may be inside the main tank (under the cover) or mounted externally. A bushing type of CT consists only of a toroidal-shaped core with a secondary winding

The CT core is made of Ferromagnetic material like Iron that shows saturation are composed of microscopic regions called magnetic domains that act like tiny permanent magnet. Before an external magnetic field is applied to the material, the domains are oriented in random directions.

Their tiny magnetic fields point in random directions and cancel each other out, so the material has no overall net magnetic field, when an external magnetizing field H is applied to the material and aligns parallel to the external field add together to create a large magnetic field which extends out from the material, this is called magnetization.

CT Error

Some of the primary current or the primary ampere-turns are utilized for magnetizing the core thus leaving less than the actual primary ampere turns to be transferred to the secondary ampere-turns. This naturally introduces an error in the transformation. The error is classified into; Ratio error and Phase error.

CT Polarity

This figure shows the polarity and circuit diagram for the conventional representation of a current transformer. The polarity marks X or ■ indicate the current flowing out at the polarity-marked terminal on the secondary side is substantially in phase with the current slowing in at the polarity-marked terminal on the primary side. There are 2 types of CT polarity: - subtractive and additive as shown in the below figure

Magnetization Curve

CT errors result from the excitation current, so in order to check the CT function it is essential to measure the excitation curve, The magnetizing current of a CT depends on the cross-section and the length of the magnetic circuit, the number of turns, and the magnetic circuit material.

This curve is the best method of determining a CT’s performance. It is a graph of the amount of magnetizing current required to generate an open-circuit voltage at the terminals of the unit.

At the knee point, if the voltage increased by 10%, the excitation current will increase by 50 %

CT response in the Saturation state

• If the load and the magnitude of the primary current are large enough to saturate the CT, neither the flux nor the induced voltage are sinusoidal. The excitation current can take large magnitudes and the resulting error will be large.

• The secondary current is no longer proportional to the primary current.

• The current error which corresponds to the magnetization current increases significantly

CT Secondary

If the load and the magnitude of the primary current are large enough to saturate the CT, neither the flux nor the induced voltage are sinusoidal.

Since CT normally has much more turns in secondary compared to the primary, the voltage generated on the open-circuited CT will be much more than the system volts, leading to flashovers. This peak value may be as low as a few hundred volts in a small measuring CT with a 5A secondary winding, but it might reach many kilovolts in the case of, say a 2000/1A protective CT with a large core section.

Open circuit voltage at CT is almost:

During normal operation of CT, the primary and secondary winding produces MMF which by Lenz’s law opposes each other. As the secondary MMF is slightly less than the primary MMF, the net MMF is small. This net MMF is the working/magnetizing MMF of the core of CT.

Now, in case the secondary winding is kept open, then the secondary current will be zero while the primary current of CT will remain the same. Therefore, the opposing MMF of secondary will no longer exist. Hence the net MMF is due to primary current only i.e. N1*I1 (N1 = No. of Turns in Primary, I1 = Primary Current) which is very large. This large MMF will produce a large flux in the core and will saturate the core.

Again, due to large flux in the core, the flux linkage of secondary winding will be large which in turn will produce a large voltage across the secondary terminals of the CT. This large voltage across the secondary terminals will be very dangerous and will lead to insulation failure and will lead to failure of CT.

The factor K is 20 in the case of 5P20 Class or 5 in the case of 0.2FS5 class. We can derive that the K factor is inversely proportional to the R_Burden So

• In the case of the Protection core, the connected burden shall not exceed the rated burden to avoid fast saturation for the CT

• In the case of the Metering core, the connected burden shall not be too large than the rated burden to avoid fast saturation for the CT at normal currents and shall not be too small than the rated burden to avoid high currents in the CT secondary circuit during fault conditions.

When the CT cannot provide sufficiently low SF [Security Factor] for various reasons, or a protection core CT is used with metering devices, Interposing CT is a 1:1 ratio current transformer that could be used in such cases that would limit the saturation level or converting the overall core performance from protection core into metering core.

If the current transformer has sets of three secondary winding, the individual winding should not be connected to the ground, the circuit formed with the sets of secondary winding of the current transformer must be connected at an only single point, at the neutral formation point of the CTs. The single grounding of current transformers eliminates the problems of the voltage generated at different ground points of the current transformer.

CT Single Earth Point

The instrument transformer secondary circuit should be connected to the station ground at only one point. This holds true regardless of the number of instrument transformer secondary windings connected to the circuit. The reasons for grounding at a single point are as follows:

1. The flow of fault current through the ground mat can cause potential differences at different points in the ground mat. If the instrument transformer secondary circuit is grounded at more than one location, these potential differences can result in the flow of current through the relay, instrument, and meter coils resulting in instrument inaccuracies and possible relay misoperation. Also, high neutral conductor currents resulting from multiple ground connections can cause thermal damage to the neutral conductor.

2. The use of a single grounding point facilitates the temporary removal and re-establishment of the ground connection when desired in order to test for insulation deterioration or accidental grounds in the instrument transformer secondary circuit

The CT Starpoint must be grounded

• It is an issue of safety for

a. Protecting Personnel against Electrical Shocks

b. Protecting Equipment against Surges

• During a secondary open circuit on one CT, a very high transient voltage would appear across all the three phase CTs. If the neutral terminal is grounded, the insulation needs to be done only for phase voltage and not for line voltage. This greatly reduces the size and insulation cost of the CT.

• CT Primary and secondary is linked through stray capacitance. If the CT secondary is not earthed, the primary voltage may induce high potential in the secondary winding.

An important tip for personnel safety and device protection is to always maintain a circuit on the secondary winding of any current transformer when current is flowing through the primary circuit. If the secondary circuit is opened when the primary current is flowing, then the voltage will go to a very high value, possibly causing electrical arcing and/or electrical shock to service personnel.

Due to that problem, a Metrosil is used to protect the CT in case of the circuit opened. The Metrosil is a nonlinear resistance which is acts as open circuit during the normal operation and acts as a short circuit if high voltage resulted from the open circuit.

IEC standard

The IEC standard describes the CT classes according to their performance as follows, The new IEC61869-2 standard has modified the TPX, TPY, and TPZ classes and canceled the TPS class.

Remanence Flux in Current Transformers

The phenomenon of remanence, also referred to as residual magnetism, describes the remaining magnetization of an iron core after the actual cause, an external magnetic field has been removed. In the case of a current transformer, this means that the core remains magnetized, although no more current flows.

Special Cases of Protections

In utility power system protection, we have a protection function called "breaker failure" protection. This protection is used to take appropriate action if/when a circuit breaker fails to open after a protective relay commanded it to. One of the methods used to determine if the breaker is still closed is a simple fault detector that monitors currents from current transformers (CTs) on the breaker.

In some cases, we need fast Breaker Failure Function action to support system stability. We not only may need this Breaker Failure Function to operate fast, but we also need it to be secure against incorrect operation. If it takes it a long time to recognize that the primary current is gone then it is subject to incorrect operation.

In CTs, after the high primary fault current has gone, it is typical to have a "subsidence current" flowing in the CT secondary (L-R circuit), this subsidence current is a simple decaying L-R transient, as shown below

After the primary current is gone, we need the current detector in the breaker failure relay to reset (to avoid accidentally timing out and tripping).