Industrial Power Transformers -- Special features of transformers for particular purposes [part 9]

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Scott- and Le Blanc-connected transformers were once widely used as a means of interconnecting three-phase and two-phase systems. Nowadays the use of three-phase systems is so universal that the requirement for such connections is very rarely encountered. They can also be used to reduce the extent of phase unbalance when single-phase loads are supplied from three-phase supplies which means that the possibility exists that they might still occasionally be encountered in this mode of operation. Earlier editions of this work included a much more detailed treatment but the following brief descriptions are provided for completeness and to provide some coverage of all aspects of transformer design and operation.

FIG. 34 Phasor derivation of the Scott connection

The Scott connection

The Scott connection is one means of making the three-phase to two-phase transformation and utilizes two single-phase transformers connected to the three-phase system and to one another to achieve this. In FIG. 34, if A, B and C represent the three terminals of a three-phase system and N represents the neutral point, the primary windings of three single-phase transformers forming a delta-connected three-phase bank may be represented by the lines AB, BC and CA. If it is desired to arrange the primary windings in star, the corresponding lines on the diagram are AN, BN and CN. If, in the diagram, AN is continued to the point S, the line AS is perpendicular to the line BC, and it is evident that it would be possible to form a three-phase bank using only two single-phase transformers, their respective primary windings being represented in phasor terms by the lines AS and BC. With this connection it is possible to form a three- to three-phase bank consisting only of two single-phase transformers.

At the same time it is also possible, by giving each transformer a single secondary winding, to form a three- to two-phase bank. These secondary windings are represented in the diagram by the lines a1a2 and b1b2.

The simplest form of Scott group utilizes two single-phase transformers having primary turns in the ratio AS to BC. Both have the same number of secondary turns dictated by the required secondary phase voltage. The primary of the transformer having the larger number of turns, that is equivalent to BC, also has its primary winding center tapped and the connection brought out for connection to one primary pole of the other transformer.

The first transformer is known as the 'main' transformer and the other is known as the 'teaser,' and the ratio of primary turns on teaser to main transformer can be deduced from an examination of FIG. 34. ABC is an equilateral triangle for which the ratio of the length of perpendicular AS to side AB is equal to _/3/2:1, that is 0.866:1. Each secondary winding is simply a single phase winding, and the voltage across it and the current in it are precisely as would be expected for any single-phase transformer. On the three-phase side, if the line voltage is V, then:

Voltage across main transformer= V

and voltage across teaser transformer = 0.866V

Current in main transformer 1000 x kVA / _/3 V

Current in teaser transformer 1000 x kVA / _/3 V

where the required group output is stated in kVA.

By multiplying the voltage across each transformer by its current, the equivalent size of each is obtained. In the case of the main transformer, this is equal to 0.577 times the group output; and in the case of the teaser transformer, 0.5 times the group output. Therefore, in a Scott-connected group, the two-phase windings are equivalent to the windings of two ordinary single-phase windings of the same output, but on the three-phase side the winding of the main transformer is increased in size by 15.5 percent above what would be required in a single-phase transformer of the same output. Assuming that the primary and secondary windings of an ordinary single-phase transformer each occupies about the same space, then, for a Scott-connected group, the main transformer will need to be about 7.75 percent larger than a single-phase transformer pro viding the same output, but the teaser transformer size will not be increased.

FIG. 35 shows the arrangement of windings and connections for the Scott group for which the neutral point on the three-phase side is brought out for connection to ground if required. As will be apparent from examination of the geometry of the equilateral triangle ABC of FIG. 34, the position of the neutral divides the primary winding turns of the teaser transformer in the ratio of 2:1.

FIG. 35 Connections for non-interchangeable Scott group

Interchangeable groups

When the Scott connection was in common use it was often considered inconvenient that the pair of transformers constituting the Scott group were not interchangeable and because the cost of making them so was quite modest, this was commonly done. It is only necessary to provide each primary winding with the full number of turns with the center point of each brought out to an external terminal. Each primary must then have a tapping brought out at 86.6 percent of the total turns, and, if a neutral connection is required, a tap ping must be brought out at the appropriate position on each primary for this purpose. A diagram of connections for such a group is shown in FIG. 36.

Although it might appear that a large number of connections are required, it should be remembered that these transformer would normally only be used at 415 V or lower and with ratings of only a few kVA, so that the size of the leads and terminals, and consequently their cost, will not be great.

FIG. 36 Connections for interchangeable Scott group

Three phase to single phase

FIG. 37 Loadings of Scott-connected groups

In FIG. 37 the current distribution in a Scott group is shown for three different conditions. FIG. 37(a) shows the current distribution when the teaser transformer only is loaded; FIG. 37(b) shows the corresponding distribution when the secondary of the main transformer only is loaded; FIG. 37(c) is a phasor diagram of currents showing a combination of the conditions in the first two figures for the main transformer only.

Referring to FIG. 37(a) it can be seen that the current in the teaser windings on the three-phase side divides into two equal parts on passing to the main transformer, these two parts being in opposite directions. If the two halves of the primary winding on the main transformer are wound in such a way that there is a minimum magnetic leakage between them, these two cur rents will balance one another, and the main transformer will offer very little impedance to the flow of current even though its secondary is open circuit.

If, however, the coupling between these two halves is loose, the main transformer will appear as a choke to the current of the teaser transformer. It can be seen that the Scott connection will operate as a fairly effective means of supplying a single-phase load from a three-phase supply provided the main transformer is wound with its primary halves closely coupled. This is best achieved by winding them as two concentric windings on the same limb of the core. With this arrangement the single-phase load is distributed between the three phases of the supply equally in two phases with double the current in the third phase. When used in this way no load is applied to the secondary of the main transformer.

The Le Blanc connection

The alternative connection to the Scott for transforming from a three-phase to a two-phase supply is the Le Blanc connection. Although this latter connection has been accepted by engineers from the end of the nineteenth century it has not gained the same popularity as the Scott connection and is by no means so well known.

FIG. 38 shows the combined voltage phasor diagrams of the Scott and Le Blanc connections and it will be seen that the phase displacement obtained by both methods is identical and that the connections are interchangeable. It follows therefore that transformers having these connections will operate satisfactorily in parallel with each other if the normal requirements of voltage ratio and impedance are met.

FIG. 38 Phasor diagrams illustrating interchangeability of Scott and Le Blanc connections

The primary of the Le Blanc-connected transformer shown in FIG. 38 is connected in three-phase delta which is the normal interphase connection in the case of a step-down unit supplied from an HV source. Where the primary three-phase winding is connected in delta the inherent advantage of this winding for the suppression of third-harmonic voltages will be apparent. For fuller details of this aspect reference should be made to Section 2. Where the three phase side is the secondary, that is when the transformer is operating two to three phase it would be more convenient to use a star connection on the three phase side.

A core of the three-limb, three-phase design is employed for the construction of a Le Blanc-connected transformer compared with two single-phase cores for the Scott-connected transformer. In addition to a somewhat simpler standard core arrangement the Le Blanc transformer is less costly to manufacture due to the fact that for a given rating less active materials are required for its construction. The fact that a three-phase core, and hence a single tank, can be employed to house the Le Blanc transformer means that the unit is more economical in floor space than the Scott transformer, particularly if compared with the arrangement of two separate single-phase cores each in its own tank.

From the phasor and connection diagrams of FIG. 39, which is drawn to show the arrangement of windings for a three- to two-phase Le Blanc transformer, it will be seen that the HV primary is identical with that of any delta-connected winding and is constructed as such. The voltage of the output winding is established across the four two-phase terminals

a1 a2 and b1 b2 and the LV turns are so designed that the voltage phasor a1a2 is equal to b1b2. From the geometry of the phasor diagram the quadrature relationship between a1 a2 and b1b2 will immediately be apparent.

FIG. 39 Phasor and connection diagrams of a Le Blanc connected transformer

The phase relationship between the winding sections a and c which comprise one phase of the two-phase output is 120 apart so that each section a and c must have 57.7 percent of the number of turns required to develop the specified phase voltage a1 a2. Further, the winding sections a and c must have _3 times the number of turns of winding sections a’ and c’, resulting in winding sections a_ and c_ having 33.3 percent of the number of turns corresponding to the phase voltage b1 b2. It follows that winding section b must have 66.6 percent of the number of turns corresponding to the phase voltage b1 b2. These fixed relationships of number of turns between the winding sections a, a’, b, c and c’ follow from the basic voltage phasor diagram.

When transforming from a three-phase HV supply to an LV two-phase out put quite definite limitations are therefore imposed upon the design of the secondary winding of a Le Blanc-connected transformer due to the fact that only whole numbers can be employed for the winding turns, while at the same time certain fixed ratios of turns must be maintained between sections of windings.

These conditions are accentuated by an LV winding having comparatively few turns. In addition to these considerations of voltages of the various sections of the two-phase side, the ampere-turns of each phase of the primary winding are balanced by the phasor sum of the ampere-turns of the components of the secondary windings of the two-phase winding on the same phase.

The Le Blanc connection can be arranged for either two-phase three-wire or four-wire output windings, and will transform from three to two phase or vice versa with the three-phase side connected in either star or delta. The former is invariably employed for three-phase LV secondary windings and the latter for HV three-phase primary windings.

When supplying a balanced three-phase load from a star-connected secondary the regulation of the Le Blanc transformer will be comparable with that of a three-phase star/star-connected transformer and if it is required to load the transformer windings between line and neutral, and so cause appreciable unbalanced loading, a tertiary delta-connected winding should be provided.

The phasor and winding diagrams shown in FIG. 40 illustrate the modification necessary to the two-phase side of a Le Blanc transformer when the mid-points are required to be available on the two-phase winding. Compared with the arrangement of the windings of FIG. 39 it will be seen that each winding section a, a’, b, c and c’ of the diagram is subdivided into halves and interconnected to provide the mid-points at a2 and b2 of FIG. 40.

FIG. 40 Phasor and connection diagrams of a Le Blanc connected transformer when mid-points are required on the two-phase windings.

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