Phase shifting transformer or autotransformer

A phase shifting polygonal transformer or autotransformer for a three phase electrical distribution system is provided, in which the transformer output winding or autotransformer winding is composed solely of three main coils and three auxiliary coils, alternately interconnected in series. The outputs for the transformer/autotransformer are connected to taps in the coils offset from the connections between coils, which eliminates the need for multiple auxiliary coils in each phase. The transformer/autotransformer of the invention is thus much simpler and less expensive to manufacture than a conventional phase shifting transformer or autotransformer, but reduces harmonics just as effectively.

FIELD OF INVENTION 
This invention relates to electrical transformers. In particular, this 
invention relates to three phase, phase shifting polygonal transformers or 
autotransformers for supplying power to multiple loads, in which the 
transformer outputs have the same voltage levels but are phase shifted 
relative to each other at selected phase angles. 
BACKGROUND OF THE INVENTION 
Phase shifting polygonal transformers and autotransformers are commonly 
used in three phase electrical networks supplying multiple non-linear 
loads. Non-linear loads, such as electronic equipment and equipment using 
various kinds of arc processes, produce undesirable harmonic currents 
within the network. Such harmonic-producing loads are becoming an 
increasingly large portion of the electrical load in many electrical 
networks, and can result in an unexpectedly high harmonic content in the 
electrical distribution system. This can lead to a number of problems 
which are well known to those skilled in the art. 
Several techniques have been developed for reducing levels of different 
types of harmonics, including: 
1. Different kinds of L-C filters tuned to different harmonic frequencies; 
2. Specialized filters such as zero phase sequence filters of various types 
for three phase, four wire systems with single phase loads; and 
3. Different kinds of phase shifters that allow for the creation of a 
quasi-multipulse system, and thus reduce harmonic levels for selected 
harmonics. 
All of these techniques are well known and have been in widespread use for 
many years. 
For three phase, three wire systems, polygonal phase shifting transformers 
and autotransformers are frequently used to reduce harmonics. However, in 
any case where more than two loads must be phase shifted, the use of 
conventional polygonal phase shifters has been problematic. It is 
difficult to keep the voltage levels of the various outputs within 
acceptable limits while obtaining the desired phase shift between outputs. 
This requirement considerably complicates the configuration of a 
conventional polygonal phase shifting transformer, as for example in the 
transformer described in U.S. Pat. No. 5,063,487 issued Nov. 5, 1991 for a 
Main and Auxiliary Transformer Rectifier System for Minimizing Line 
Harmonics. 
SUMMARY OF THE INVENTION 
This invention provides a family of economical, passive electromagnetic 
phase shifting transformers and autotransformers which allow for the 
construction of quasi-multiphase systems that reduce harmonic distortion 
created by non-linear loads in three phase systems, while involving 
substantially less cost and complexity than conventional polygonal 
transformers. 
The transformers/autotransformers of the invention utilize specially 
configured windings in which the output connections are tapped into each 
coil at a position offset from the coil connections, to obtain the desired 
phase shift between outputs and yet maintain a consistent voltage level 
for all outputs. 
The invention thus provides a phase shifting transformer output winding or 
autotransformer winding for a three phase electrical distribution system, 
comprising three auxiliary coils each alternately connected in series to 
three main coils at coil connections, and a plurality of outputs for each 
of the three phases, wherein all outputs are connected to the main and 
auxiliary coils at positions offset from the coil connections. 
The invention further provides a phase shifting autotransformer for a three 
phase electrical distribution system comprising a core having three legs, 
a winding disposed on the core comprising three main coils alternately 
connected in series to three auxiliary coils at coil connections, an input 
connection for each phase, and a plurality of outputs for each phase, 
whereby the outputs comprise taps in the main coils or the auxiliary coils 
or both at positions offset from the coil connections. 
The invention further provides a phase shifting transformer for a three 
phase electrical distribution system comprising a core having three legs, 
an input winding disposed on the core, an output winding disposed on the 
core comprising three main coils alternately connected in series to three 
auxiliary coils at coil connections, and a plurality of outputs for each 
phase, whereby the outputs comprise taps in the main coils or the 
auxiliary coils or both at positions offset from the coil connections.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates an output winding of a conventional hexagonal phase 
shifting transformer for a three phase, three wire system having four 
outputs phase shifted 15.degree.. The winding consists of a total of 15 
coils interconnected so that the outputs are formed at the coil 
connections. In addition to the three main coils, total of 15 auxiliary 
coils are required, five for each phase, to maintain a consistent voltage 
level as between the two middle outputs and the two outside outputs of 
each phase. This allows for the desired phase shift between outputs, while 
maintaining consistent voltage levels for all four outputs. 
This is a complex configuration which requires at least eighteen separate 
coils for the hexagonal transformer shown, and eighteen separate 
connections to be made between coils on different core legs, which is both 
costly and highly labour intensive. 
A four output hexagonal phase shifting autotransformer of the invention is 
illustrated in FIGS. 2 and 3 in which adjacent outputs are phase shifted 
15.degree., as in the conventional transformer output winding of FIG. 1. 
In the autotransformer illustrated in FIGS. 2 and 3 the main coils 30, 40, 
50 are alternately interconnected to the auxiliary coils 60, 70, 80 in 
series, as in a conventional polygonal transformer, and the main and 
auxiliary coils 30, 40, 50 and 60, 70, 80 are conventionally disposed 
about the three legs of the core (not shown). However, each auxiliary coil 
60, 70, 80 is wound with a greater number of turns than required to 
achieve the required phase shift between the middle outputs a2-c2 and 
a3-c3, which is 15.degree. in the embodiment illustrated, and each main 
coil 30, 40, 50 is similarly wound with a greater number of turns, 
providing a simple hexagonal configuration. The correct voltage level and 
phase angle for each output a1-a4, b1-b4 and c1-c4 is produced by 
providing the main and auxiliary coils 30, 40, 50 and 60, 70, 80 with taps 
at suitable positions along each coil. 
In effect, the coil lengths "over-extend" the output positions. In the 
transformer of the invention, all outputs a1-a4, b1-b4 and c1-c4 are 
tapped into the main coils 30, 40, 50 and auxiliary coils 60, 70, 80 at 
the desired phase angle. No output connections are made at the connections 
between the auxiliary and main coils; the output connections are all 
offset from the respective coil connections between the main and auxiliary 
coils 30, 40, 50 and 60, 70, 80. 
Thus, the two outside outputs a1, a4 are connected to taps 34, 42 in the 
main coils 30, 40, respectively; outputs b1, b4 are respectively connected 
to taps 44, 52 in main coils 40, 50; and outputs cl, c4 are respectively 
connected to taps 54, 32 in main coils 50, 30. The two middle outputs are 
tapped into the auxiliary coils, so that outputs a2, a3 are connected to 
taps 62, 64, respectively; outputs b2, b3 are connected to taps 72, 74, 
respectively; and outputs c2, c3 are respectively connected to taps 82, 
84. 
According to this arrangement, a phase shift of 15.degree. between adjacent 
outputs of each phase is achieved in the transformer or autotransformer of 
the invention, while the output voltage level remains consistent for all 
four outputs, without the need for additional auxiliary coils. The 
line-to-line voltage and phase shift angle are determined by the location 
of the tap for each specific output. Thus, the effect of this arrangement 
is to produce in a simple hexagonal winding a plurality of phase shifted 
outputs, each offset from the connections between the main and auxiliary 
coils and each having the same voltage level and, within any particular 
phase, the same phase shift angle. 
Although the phase shifting transformer/autotransformer of the invention 
reduces harmonics several times over ordinary transformers and 
autotransformers, the manufacture of a transformer or autotransformer of 
the invention is considerably simpler and less expensive than conventional 
designs, and far less labour intensive. The six coil connections between 
the main and auxiliary coils 30, 40, 50 and 60, 70, 80 are the only 
connections required in the transformer/autotransformer of the invention. 
The various parts of the distribution system are connected directly to the 
taps in the main and auxiliary coils. The trigonometric calculations for 
determining the size of each winding and the locations of the taps for the 
various outputs are well known to those skilled in the art. 
It will be apparent that the invention is equally applicable to both 
transformers and autotransformers. For example, FIG. 4 illustrates a 
transformer output winding for the same three phase, three wire system as 
the autotransformer of FIGS. 2 and 3, but is distinguishable by the 
absence of the inputs A, B and C (the primary winding for the transformer 
of FIG. 4 is of conventional design and is not shown). 
The transformer/autotransformer of the invention is effective for any 
distribution system supplying up to five loads, whether or not a neutral 
conductor is present. If a neutral point is required, the simple addition 
of a conventional "Y" winding, of full or reduced capacity, will suffice 
to achieve this. For example, FIG. 5 illustrates a transformer according 
to the invention similar to that illustrated in FIGS. 2 and 3 but having a 
neutral conductor X.sub.o. The three coils 90, 92, 94 of the neutral 
winding are tapped into the auxiliary coils 60, 70, 80. 
A three output embodiment of the invention is illustrated in FIG. 7. In 
this embodiment the phase shift between adjacent outputs is 20.degree., to 
reduce conventional harmonics of the 5th, 7th, 11th, 13th etc. orders. The 
design and operation of this transformer/autotransformer is exactly the 
same as the four output embodiment described above, except that the first 
output (a1-c1 in FIG. 2) has been omitted. 
It should be noted that in the four and three output embodiments of FIGS. 
2-6 the main and auxiliary coils 30, 40, 50 and 60, 70, 80 should be 
designed so that the nominal output voltage is slightly larger than the 
desired output voltage, to compensate for the voltage drop in the output 
winding conductors (usually about 2% of the input voltage). 
A five output embodiment of the invention is illustrated in FIG. 7. In this 
embodiment the outputs are phase shifted 12.degree., to reduce harmonics 
of the 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th etc. orders. The 
voltage level of the centre output a3, b3, c3 is slightly lower than that 
of the other outputs, but since the centre output a3, b3, c3 is connected 
directly to the input winding it experiences no voltage drop. Thus, the 
actual output voltage level at for example outputs a3 will be comparable 
to the output voltage level of the other four outputs a1, a2, a4 and a5. 
As noted above, the invention will operate effectively with up to five 
outputs, phase shifted as closely as 12.degree.. As is known, it is 
possible to reduce higher harmonic orders in a distribution system 
involving multiple transformers or autotransformers by connecting the 
input conductors at different positions in the different transformers. The 
phase shift between different transformers can be as small as required to 
suppress the higher harmonics. 
The invention having been described with reference to a preferred 
embodiment, it will be apparent to those skilled in the art that 
modifications and adaptations may be made to the invention without 
departing from the scope of the invention. All such variants are intended 
to fall within the scope of the invention as delimited by the appended 
claims.