Some times adding of air-gap in transformers core may be a benefit design technique for transformers performance improvement and can be applicable in electrical systems. For example we can mention following items:
- Shunt reactors
- Constant-voltage transformer
- Air-gapped current transformers
- Linear current transformers
- TPY and TPZ current transformers
Shunt reactors are available in two design configurations: coreless and iron core.
Coreless oil-immersed shunt-reactor designs utilize a magnetic circuit or shield, which surrounds the coil to contain the flux within the reactor tank. The steel core that normally provides a magnetic flux path through the primary and secondary windings of a power transformer is replaced by insulating support structures, resulting in an inductor that is nearly linear with respect to applied voltage. Conversely, the magnetic circuit of an oil-immersed iron-core shunt reactor is constructed in a manner similar to that used for power transformers, with the exception that an air gap or distributed air gap is introduced to provide the desired reluctance. Because of the very high permeability of the core material, the reluctance of the magnetic circuit is dominated by the air gap, where magnetic energy is primarily stored. Inductance is less dependent on core permeability, and core saturation does not occur in the normal steady-state-current operating range, resulting in a linear inductance. A distributed air gap is employed to minimize fringing flux effects, to reduce winding eddy losses (adjacent to the gaps), and improve ampere-turns efficiency.
A well-known solution for electrical “noise” in industrial plants has been the constant-voltage transformer, or CVT .
The magnetic shunt on the central core has the following effects on the core’s reluctance. It reduces the reluctance of the core. This can be thought of as introducing more resistance in parallel to an existing resistance. The magnetic shunt in the CVT design allows the portion of the core below the magnetic shunt to become saturated while the upper portion of the core remains unsaturated. This condition occurs because of the presence of the air-gap between the magnetic shunt and the core limbs. Air has a much higher reluctance than the iron core.
Therefore, most of the flux passes through the lower portion of the core .In terms of an electrical analogy, this configuration can be thought of as two resistances of unequal values in parallel. The smaller resistance carries the larger current, and the larger resistance carries the smaller current.
The CVT is designed such that:
• The lower portion of the central limb is saturated under normal operating conditions, and the secondary and the resonating windings operate in the nonlinear portion of the flux-current curve.
• Because of saturation in the central limb, the voltage in the secondary winding is not linearly related to the voltage in the primary winding.
There is ferroresonance between the resonating winding on the saturated core and the capacitor. This arrangement acts as a tank circuit, drawing power from the primary. This results in sustained, regulated oscillations at the secondary with the applied line frequency.
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 unit protection schemes in which the outputs into multiple secondary circuits must remain linear for and proportioned to the widest practical range of input currents.
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 behavior 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.
TPY and TPZ current transformers
CT'S with an air gap in the magnetic path have the type designations TPY and TPZ. The introduction of an air gap in the iron core reduces the main inductance and therefore also the ct time constant. In consequence, a much smaller over sizing factor is permissible, which means that the cross section of the iron is also greatly reduced in comparison with the class TPX.
Class TPY CT'S, which are also referred to as anti-remanence air gap ct's have only a few (usually two) small air gaps to limit the residual flux after a fault has be tripped. The residual flux factor is about 0.1and the c.t time constant about 0.1 to 1 s. If after a fault has been tripped the load current is not restored, a c.t with a closed iron core can maintain its residual flux for hours or even days and depending on the half-cycle when the line or plant is reenergised can immediately saturate.