Current Transformers types are classified on the basis of the construction method. Based on the application requirements different types of CTs are available.
Current transformers used for metering and protection purposes are basically constructed in three different ways. Wound Current Transformers, Window Current Transformers, and Bar Current Transformers are the types of current transformers.
1. Wound Type Current Transformers:
In this method, there are two separate windings on a magnetic steel core. The primary winding consists of a few turns of heavy wire capable of carrying the full load current while the secondary winding consists of many turns of smaller wire with a current carrying capacity of between 5/20 amperes, dependent on the design. This is called the wound type due to its wound primary coil. In this wound-type CT, the primary winding is practically configured in series form with the conductor that controls the current.
Wire wound current transformer types can be employed to sense the currents in the range of 1A to 100A. Current transformers for indoor service are encapsulated in Epoxy resin-impregnated paper insulation.
- Support Type CT
It is a wound-type current transformer so arranged that it acts as a support for the conductor in the primary circuit. This current transformers types in addition to its primary function also act as busbar support.
APPLICATIONS: Medium voltage air-insulated primary distribution switchgear, Medium voltage air-insulated secondary distribution switchgear, Capacitor Banks.
2. Window Type Current Transformers:
A window or through type CT does not come with a primary conductor. In which the core has an opening through which the conductor carrying the primary load current is passed. This conductor constitutes the primary winding of the CT (one pass through the “window” represents a one-turn primary) and must be large enough in cross-section to carry the maximum current of the load.
The conductor over which the current transformer is mounted also serves as the primary winding. Thus the window-type current transformer has only the secondary winding.
This type of CT is easier to install, can withstand a greater thermal overcurrent, and reduces the failure rate. And this type of CT is much less costly than the bar type.
Types of Window Current Transformers:
- Bus Type CT/Support Window Type CT
A current transformer without a primary conductor, but with primary insulation, which can be fitted over a conductor or busbar.
APPLICATIONS: Suitable for round conductors, copper bars, mounting rails
- Wall Bushing Type CT
A current transformer without a primary conductor, but with primary insulation, can be used as a bushing. Toroidal model without a primary conductor for installation as a wall bushing.
Bushing-type current transformers are used wherever high currents are to be acquired and processed. They are directly placed on the primary conductor (bus bar or conductor) through the opening. The secondary side (usually a measuring device, energy meter, or display) is connected by front and rear connecting terminals.
Various mounting options are available such as wall mounting, cable mounting, and rail mounting.
APPLICATIONS: Designed for mounting around the internal bushings of a power transformer, circuit breaker, pothead, or in switchgear.
3. Bar Type Current Transformers:
A bar-type CT comes with a primary conductor. Ordinarily, A bar type CT is an in-line CT, much more costly, and is mounted in line with the bus bars (bolted on both sides). The bar-type CT is fully insulated to the switchgear voltage.
Types of Bar Type Current Transformers:
- Bar Primary Type CT
A variant of the window type is the bar type. The bar type has a fixed primary conductor mounted in the window which has standard terminal bar connections to attach cables.
APPLICATIONS: Designed for indoor service; suitable for operating meters and instruments, on both single-phase two-wire circuits and polyphase circuits.
- Bar Primary Bushing Type CT
A current transformer with a bar primary conductor is so constructed that can be used as a bushing. Toroidal type with a primary conductor to be installed as wall bushings.
APPLICATIONS: Distribution substations.