Types of Current Transformers and Their Construction from freeamfva's blog

Types of Current Transformers and Their Construction

The total power in a transformer is the same on the primary and secondary sides. The only way to step down the current is by stepping up the voltage. Therefore, a current transformer is a modified step-up voltage transformer.Get more news about Current transformer core,you can vist our website!

Current transformers are unique because they usually have only one winding (see Figure 1). The primary is connected to the line load in series. When the primary has a high current rating, the primary winding may be a straight conductor that runs through the magnetic circuit's center. This straight conductor indicates a one-turn winding.

When the primary has a low current rating, the primary winding may consist of several turns wrapped around the core. This provides the required flux on low-current applications or to compensate for line drop to a power meter.

The secondary consists of many turns of wire wrapped around a core. The number of turns is determined by the desired turns ratio of the current transformer. The primary current of a current transformer is not controlled by the secondary, as it would be in a two-winding potential transformer. The secondary of a current transformer cannot affect the current in the primary, as the load on the feeder determines the primary current.

When the primary circuit is energized, the secondary of a current transformer must never be left open. When the circuit is operational, the load on the secondary maintains low magnetizing currents and thus low turn-to-turn potentials. When the secondary becomes an open circuit, the magnetizing currents rise, and the current transformer acts as a step-up potential transformer. The voltage can rise to a destructive level and cause a short between the turns as the result of the degradation of the insulation. Therefore, a current transformer should always have its secondary shorted when not connected to an external load (see Figure 2.)All transformers have losses in power transfer from resistance, magnetizing current, hysteresis, and other factors. These factors must be compensated for in the design of the transformer in order to ensure an accurate measurement.

A current transformer is constructed of high-permeability steel at the flux density at which the transformer operates. The flux density is kept to a low value so that the magnetizing current is low. The circular coil of high-silicon steel provides the low-reluctance magnetic circuit needed to provide the necessary field strength for the secondary winding. The three types of current transformers in general use are the window, bar, and wound.A window current transformer is a transformer that consists of a secondary winding wrapped around a core and the primary sent through the opening in the core. After the secondary is wound around the core, the assembly is placed into a mold, and an insulating material is injected around the transformer. Taps are brought out from the winding (see Figure 3). A power line is passed through the window and acts as the primary. This completed assembly is referred to as a window current transformer.

A bar current transformer is a special type of window current transformer with a solid bar placed permanently through the window. A bar current transformer can withstand the stresses of heavy overcurrent. To avoid magnetic stresses that could destroy the bus and damage the transformer, care must be taken to properly mount these transformers with respect to adjacent conductors. This type of transformer is typically found on installations where the potential is 25kV or less (see Figure 4).

A wound current transformer is a transformer with separate primary and secondary windings wrapped around a laminated core. A wound current transformer is designed so that the primary winding consists of one or more turns of large cross-section wire connected in series with the circuit to be measured. This kind of current transformer is located on the high-voltage side of substations and contains a primary conductor that carries the current and a wound current transformer for the output current (see Figure 5).