SCOTT AND LE BLANCCONNECTED TRANSFORMERS
Scott and Le Blancconnected transformers were once widely used as a means
of interconnecting threephase and twophase systems. Nowadays the use of threephase
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 singlephase loads are supplied from threephase 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 threephase to twophase transformation
and utilizes two singlephase transformers connected to the threephase system
and to one another to achieve this. In FIG. 34, if A, B and C represent the
three terminals of a threephase system and N represents the neutral point,
the primary windings of three singlephase transformers forming a deltaconnected
threephase 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 threephase bank using only two singlephase 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 threephase
bank consisting only of two singlephase transformers.
At the same time it is also possible, by giving each transformer a single
secondary winding, to form a three to twophase bank. These secondary windings
are represented in the diagram by the lines a1a2 and b1b2.
The simplest form of Scott group utilizes two singlephase 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 singlephase transformer. On the threephase 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 Scottconnected group, the twophase
windings are equivalent to the windings of two ordinary singlephase windings
of the same output, but on the threephase side the winding of the main transformer
is increased in size by 15.5 percent above what would be required in a singlephase
transformer of the same output. Assuming that the primary and secondary windings
of an ordinary singlephase transformer each occupies about the same space,
then, for a Scottconnected group, the main transformer will need to be about
7.75 percent larger than a singlephase 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 threephase 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 noninterchangeable 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 Scottconnected 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 threephase 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 singlephase load from a threephase 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 singlephase 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 threephase
to a twophase 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 Blancconnected transformer shown in FIG. 38 is connected
in threephase delta which is the normal interphase connection in the case
of a stepdown unit supplied from an HV source. Where the primary threephase
winding is connected in delta the inherent advantage of this winding for the
suppression of thirdharmonic 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 threelimb, threephase design is employed for the construction
of a Le Blancconnected transformer compared with two singlephase cores for
the Scottconnected 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 threephase 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 singlephase 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 twophase Le Blanc transformer,
it will be seen that the HV primary is identical with that of any deltaconnected
winding and is constructed as such. The voltage of the output winding is established
across the four twophase 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 twophase 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 threephase HV supply to an LV twophase out put
quite definite limitations are therefore imposed upon the design of the secondary
winding of a Le Blancconnected 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 twophase side, the ampereturns of each phase of the primary winding
are balanced by the phasor sum of the ampereturns of the components of the
secondary windings of the twophase winding on the same phase.
The Le Blanc connection can be arranged for either twophase threewire or
fourwire output windings, and will transform from three to two phase or vice
versa with the threephase side connected in either star or delta. The former
is invariably employed for threephase LV secondary windings and the latter
for HV threephase primary windings.
When supplying a balanced threephase load from a starconnected secondary
the regulation of the Le Blanc transformer will be comparable with that of
a threephase star/starconnected transformer and if it is required to load
the transformer windings between line and neutral, and so cause appreciable
unbalanced loading, a tertiary deltaconnected winding should be provided.
The phasor and winding diagrams shown in FIG. 40 illustrate the modification
necessary to the twophase side of a Le Blanc transformer when the midpoints
are required to be available on the twophase 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 midpoints at a2 and b2 of FIG. 40.
FIG. 40 Phasor and connection diagrams of a Le Blanc connected transformer
when midpoints are required on the twophase windings.
