Electronics practitioners versed in both audio and radio frequency techniques are familiar with impedance-matching applications of transformers.
Generally, the objective is to convert a resistance to a higher value in a step-up transformer or to convert a resistance to a lower value in a step down transformer. In the first case, the converted value of resistance will be greater than that associated with the primary by the square of the secondary to primary turns ratio. In the second case the converted resistance will be less than that associated with the primary by the ratio of the secondary to primary turns, squared.
More generally, impedances are also convertable in this manner with transformers. Moreover, the conversion can be accomplished with autotransformers as well as with conventional two winding transformers. A familiar impedance transformation takes place in the output transformer used to match the output impedance of an audio output stage to the inordinately low voice-coil impedance of a dynamic speaker via a step-down transformer.
An interesting application of impedance transformation is encountered with single-phase a.c. induction motors in which a large capacitor is needed to split the phase of the applied line voltage so that the motor starts essentially as a two-phase motor. Unless something of this nature is done, the motor will not develop starting torque. Once under acceleration, the capacitor is cut out of the circuit by a centrifugal switch. A practical problem arises because of the large capacitance required; because of cost and physical size, such a starting capacitor usually has to be an electrolytic type.
These can be marginally satisfactory, but tend to have adverse ageing characteristics, high leakage current, sloppy capacitance values, limited temperature tolerance and other manifestations of unreliability. Avoiding the need for the electrolytic starting capacitor is a worthwhile design objective.
An isolation transformer is designed to specifically address the problems associated with referencing its internal shields to ground. It is constructed with two isolated Faraday shields between the primary and secondary windings. When properly installed, the shield, which is closest to the primary winding, is connected to the common power supply ground and the shield closest to the secondary winding is connected to the shield of the circuit to be isolated. The use of two shields in the construction of the isolation transformer diverts high frequency noise, which would normally be coupled across the transformer to the grounds of the circuit in which they occur. The two shields provide more effective isolation of the primary and secondary circuits by also isolating their grounds. The isolation transformer adds a third capacitance between the two Farady shields, which may allow coupling of high frequency noise between the system grounds. However, increasing the separation between the two Faraday shields normally minimizes this third capacitance. Additionally, the dielectric effect of the shields plus the increased separation of the windings significantly reduce the inter-capacitance between the windings. An equivalent circuit for an isolation transformer is presented in Figure below.
R1 = Resistance in Primary Windings
R 2 = Resistance in Secondary Windings
L1 = Primary Inductance Which Creates Leakage Flux
L2 = Secondary Inductance Which Creates Leakage Flux
M = Mutual Transformer Inductance
C1 = Capacitance Between Primary Windings and Primary Shield
C2 = Capacitance Between Secondary Winding and Secondary Shield
C12 = Capacitance Between Primary and Secondary Shields
Transformer with zig-zag connection:
The interconnected-wye-wye connections have the advantages of the star–delta connections with the additional advantage of the neutral. The interconnected-wye or zigzag connection allows unbalanced phase load currents without creating severe neutral voltages. This connection also provides a path for third-harmonic currents created by the nonlinearity of the magnetic core material. As a result, interconnected -wye neutral voltages are essentially eliminated. However, the zero-sequence impedance of interconnected-
wye windings is often so low that high third-harmonic and zero-sequence currents will result when the neutral is directly grounded. These currents can be limited to an acceptable level by connecting a reactor between the neutral and ground. The interconnected-wye-wye connection has the disadvantage that it requires 8% additional internal kVA capacity. This and the additional complexity of the leads make this type of transformer connection more costly than the other common types discussed above.
The stable neutral inherent in the interconnected-wye or zigzag connection has made its use possible as a grounding transformer for systems that would be isolated otherwise.
One of current-regulating transformer is shown in Fig. below. Here, one winding, often the secondary, is free to move up and down. As a manifestation of Lenz's law, there is a force of repulsion between the windings that increases with load current. The actual physical displacement of the moving winding is a balance between load current and weight. The greater the gap between primary and secondary, the greater is the leakage inductance associated with these windings. Current regulation takes place because higher leakage inductance results in lower current availability from the secondary.
A dashpot dampens the tendency for undesirable mechanical oscillation.
A nice feature of this scheme is that various levels of constant current can be conveniently selected by changing the weight. Load current is directly stabilized against load resistance and indirectly stabilized against line voltage. Good constancy of load current results. This type of current-regulating transformer has found its greatest use with street lighting systems. When the lamps used in such networks require d.c, a rectifier can be used without affecting the basic operation of the current-regulating transformer. Although this constant current technique is straightforward and reliable, it has an inherent shortcoming. Because of the purposefully high leakage inductance, the power factor is inordinately low. In large systems, this leads to high installation and operating costs; it can also upset line voltages.
It is well known that harmonic currents circulating in lines paralleling telephone wires or through the earth where a telephone earth return is adopted produce disturbances in the telephone circuit. This is only of practical importance in transmission or distribution lines of some length (as distinct from short connections to load), and then as a rule it only occurs with the star connection using a fourth wire, which may be one of the cable cores or the earth.
Similar interference may take place in the pilot cores of discriminative protective gear systems, and unless special precautions are taken relays may operate incorrectly.
The remedy consists either of using a delta-connected transformer winding or omitting the fourth wire and earthing at one point of the circuit only.
Also you can refer to ANSI/IEEE C57.105 ( Guide for application of transformer connections in three phase distribution systems) for additional informations.