Three-Phase Transformers

Three-phase transformers mostly come in core-type designs. An arrangement of the magnetic circuit of a three-phase core-type trans­former appears in Fig. 6.5a. Each of the three identical single-phase transformers carries on one leg of its core both the primary and the secondary, with the second leg left free. Let us arrange the three transformers so that the three free legs lie adjacent to one another and form a common leg 0 (Fig. 6.56). The common leg 0 provides the magnetic path for the three magnetic fluxes which are equal in amplitude and shifted in phase by one-third of a period (120°). But the sum of these fluxes is zero at any instant Φ_А+Φ_B+Φ_с=0. Clearly, there is no need for the common leg because the magnetic fluxes do not thread it. The three legs clamped together with yokes at the top and bottom and arranged in one plane from design con­siderations, as illustrated in Fig. 6.5c, are quite sufficient to form the magnetic circuit (three-legged core). Each leg carries the high-voltage and the low-voltage phase winding.

The magnetic lines of the flux through the central leg are shorter than those of the fluxes in the extreme legs, so the flux through the central leg meets a lower reluctance to its flow than the fluxes in the extreme legs. That is why the phase winding arranged on the central leg carries a smaller magnetizing current than the phase windings on the extreme legs.

The windings of three-phase transformers are similar in design to single-phase windings. The adopted notation of the phases is the following: the capital letters A, B, and С denote the starts of the phases of the high-voltage windings, capital letters X, Y, and Z denote the finishes of these phases, and digit 0 stands for the common junction point, called the neutral point. The lower case letters (a, b, с and x, y, z) identify the starts and finishes of phases on the low-voltage side and the digit 0 identifies the neutral point.

The primary and secondary windings of a three-phase transformer can be connected to the primary and secondary three-phase networks in star Υ;and delta Δ;. In the star connection, the starts of the three phases are connected to a common neutral point and the finishes to the corresponding wires of the three-phase network. In the delta connection, the start of the first phase is connected to the finish of the second, the start of the second to the finish of the third, and the finish of the third to the start of the first. The points of connection serve as terminals for the three-phase network.

The symbol Υ; is used to identify the star-connected three-phase winding with the neutral brought out from the neutral point.

In the Soviet Union, the standards specify the three connection groups for the windings of three phase transformers:

Υ/ Υ-0; Υ/ Δ-11; Υ/ Δ-11

Here, the symbol ahead of the slant line denotes the type of connection of the high-voltage (HV) winding, the symbol behind the slant line identifies the type of connection of the low-voltage (LV) winding, and the digit (digits) after the hyphen denotes the angle between the phasors of line emfs in HV and LV windings. This angle designates the phase displace­ment in angular units that are multiples of 30°. In other words, an angle of 30° is taken as an angular unit. Thus, the symbols of the first connection group identify a star-star connection with the neutral on the LV side and digit 0 denotes the zero shift, namely, 0 X 30° = 0°. In the designation of the second connection group, the figure 11 indicates the shift by 11 angular units: 11 X 30° = 330°.

The phase emfs induced in HV and LV windings depend on the type of winding connection, terminal marking on the HV and LV sides, and direction of wind of coils on the legs. If the HV and LV coils are wound in the same direction, the emfs induced in these coils will be in phase. If one coil is wound in the direction opposite to that of the other coil, the emfs induced will be out of phase by 180°.

In practical circuits, preference is given to the star connection on the HV side because the phase voltage is then a factor of lower than the line voltage, with the result that the problem of winding insulation becomes simpler. LV windings are more often connected in delta since this connection makes the transformer less sensitive to unbalanced loads. In practical use is also the star-connected three-phase secondary network with a neutral. Such a four-wire three-phase circuit can supply power at the phase and the line voltage, for example, at 127 and 220 V and at 220 and 380 V. To increase the power of transformer substations and also to simplify the prob­lem of redundancy, the transformers are made to operate in parallel, the obligatory condition being that they belong to the same con­nection group.