What is a current transformer and types of current transformer

What is a current transformer

Current transformers are devices that transform current from one level to another, the current transformer is used for the measurement of electrical currents. The combination of current and voltage transformers is known as instrument transformers. Sometimes the current in a circuit will be too high to directly apply to measure instruments so in that case, a current transformer will be useful, this transformer will create a reduced current which is proportional to the current in the circuit and it can be connected to the instruments. A current transformer also isolates the measuring instruments from the high voltage in the monitored circuit. These transformers are used in the electrical power industry for metering and protective relays.

Why do we need a current transformer

The current transformer steps down the current of a circuit to a lower value and is used in the same types of equipment as a potential transformer. This is done by constructing the secondary coil consisting of many turns of wire, around the primary coil, which contains only a few turns of wires, so the high values of current can be measured in this way.

How does a current transformer work

A current transformer works on the principle of variable flux. The secondary current will be equal and opposite to the primary current in an ideal current transformer. When an alternating current flows in the primary winding that current will create a magneto-motive force which results in an alternating flux in the core, which in turn induces an EMF in the primary winding and in any other winding, wound on or linked with the core. Current transformers are used in a power system to step down the current to a lower level so that the small rating ammeters can be used to measure it. The error in reproduction will appear in both the amplitude and phase. The error in amplitude is called the current or ratio error and the error in the phase is called phase error. If the secondary current is high then the current error is positive and if the secondary current leads the primary then the phase error is positive.

What is the burden of a current transformer and what is meant by CT burden

The external load such as transducers or relay, which is connected to the secondary of the current transformer is called the burden. The burden is always expressed in volts, amperes or in ohms. The total burden can be considered as the sum of the burden of the device and the burden of interconnecting leads and the internal burden of CT windings, this burden is so small and it can be ignored.

What are the types of current transformers

Window/ring-type current transformer

In this type, no primary windings are installed around a primary conductor and these are the most commonly used type of current transformer. The electric field created by current flowing through the conductor interacts with the current transformer core to transform the current to the appropriate secondary output. The primary conductor needs to be disconnected when installing solid window CTs.

Bar type current transformer

This type of current transformer is placed on the panel board to measure the current of bus bars. The bus bars whose current is to be measured is made to pass through the current transformer. The secondary winding is placed on the circular type and the ammeter is connected to the secondary windings. These current transformers operate on the same principle of window ring-type CTs but they have a permanent bar installed as a primary conductor.

Clamp on/ tong tester type current transformer

This type of CTs can be used with a single conductor, the core of the CT can be split with the help of a trigger switch and therefore, in order to measure the current the core can be clamped around a live conductor. Primary is the single conductor and the secondary is wound on the core of CT.

Bushing type current transformer

This type of current transformer is specially constructed to fit around a bushing. Mostly they can’t be accessed and their nameplates can be found on the transformer or circuit breaker cabinets. An example of this type of transformers are CTs in bulk oil circuit breaker.

Wound type current transformer

This type of current transformer has one primary winding and one secondary winding like a normal transformer. These CTs are rare and they are used at very low ratios and currents. Typically in current transformer secondary circuits to compensate for low currents, to match different CT ratios in summing applications or to isolate different CT circuits.

How to select a current transformer

Current transformers can be selected by choosing the highest equipment voltage are also be selected by the rated transformer ratio such as the secondary rating is chosen where the lead burden is insignificant. A high primary current rating is also preferred. These transformers are also selected by the rated burden, it is the property of the circuit connected to the secondary winding that determines the active and reactive power at secondary terminals. Selection is also be done according to the accuracy, measuring CTs are need to be accurate over the normal working range of current, while the protective CTs must be able to maintain its accuracy several times to the rated current.

How current transformer is connected

These transformers have only one primary winding and they could have one or more secondary windings that are wound on the core of magnetic material. Metering and protection device are connected to the secondaries of CT. The primary winding is connected in series and transforms the line current to the standard 1A or 5A which is suitable for the meter or relay.

What are the applications of current transformer

  • Current transformers are used for control circuits and for instrumentation in high current power applications
  • Current transformers can be used to perform the roles for safety protection and current limiting.
  • These transformers can be used to detect single phase on a three-phase power system
  • They can be used for the current monitoring of the power systems when the current reaches a specific level