Claims:We Claim:

1. A multiphase transformer rectifier unit for converting a three-phase alternating current (AC) supplied from a power distribution system to direct current (DC) supplied to at least one load , comprising:
a transformer comprising:
an input operative to receive three phase AC electrical currents; and
a plurality of multiphase outputs, each individual output including a set of at least five output terminals and being operative to provide prime numbered phase staggered voltage AC output currents at the output terminals, with the AC output currents of each multiphase output being at a non-zero phase angle relative to all other multiphase outputs;
and
a multiphase rectifier unit operatively coupled with the multiphase outputs to convert the corresponding AC output currents to DC electrical power.
2. The multiphase transformer rectifier unit of claim 1, wherein the transformer is a non-isolated autotransformer.
3. The multiphase transformer rectifier unit of claim1, wherein said rectifier circuit for AC/DC conversion as a Prime numbered diode bridge converter’s (PNDBC); wherein PNDBC be 1, 2 or 3 followed by a transformer.
4. The multiphase transformer rectifier unit of claim 3, wherein the total number of output pulses created by the PNDBC is twice the prime numbered phase; wherein for 5 phase -2 PNDBC connected has 20 number of step-pulses and for 5phase-3 PNDBC has 30 number of step-pulses.
5. The multiphase transformer rectifier unit of claim 3, wherein transformer is non-isolated as both the primary and secondary winding set of the transformer are wounded on the same core, wherein transformer is an autotransformer, creating virtual neutral by connecting the inner zigzag windings at an appropriate point of phases to fashion outer phases, wherein phase difference between inner and outer phases is 60 degrees and 120 degrees between inner phases.
6. The multiphase transformer rectifier unit of claim 1, wherein transformer has 24 windings with mutual delta and zig-zag configuration.
7. The multiphase transformer rectifier as claimed in claim1, wherein the plurality of magnetic cores include a core having an EI, UI and UU core configuration and material used for the core is CRGO (Cold Rolled Grain Oriented) silicon steel.
8. The multiphase transformer rectifier unit of claim 1, wherein prime numbered phase staggered voltage AC output phases be 5-phase, 7-phase, 11-phase, 13-phase etc. not multiple factor of 3.
9. A method for converting a three-phase alternating current (AC) supplied from a power distribution system to direct current (DC) supplied to at least one load, comprising:
an input circuit for connection to a three phase AC source;
connecting the three phase AC source as input to windings of a transformer;
transforming the three phase AC electrical currents to create a N (two or more) sets of prime numbered phases AC output currents by the transformer, N being greater than 1, each set of prime numbered AC output currents being at a non-zero phase angle relative to other set;
rectifying each individual set of prime numbered AC output currents through M prime numbered diode bridge converter's (PNDBC) to DC load, M being greater than 1; and
inverting DC load to provide AC output power to AC motor load.

10. The method of claim 9, wherein transforming and rectifying further comprise of passing the N( being two) sets of prime numbered 5-phase supply to 2 PNDBC (P=10), M (being 2) i.e. two 10 pulses which are phase staggered by ±? degree, wherein each PNDBC A and B are fed by two 5-prime numbered sets of phase-staggered voltages A1, A2, A3, A4, A5 and B1, B2, B3, B4, B5; the 10 phase-staggered voltages are at an angle of ±? degree, the phase angle between A1 & B1 is ±(?/2)° with supply voltage VA as reference, the phase angle for other phase staggered voltages between A2 & B2, A3 & B3, A4 & B4 and A5 & B5 is ±(?)°, wherein the PNDBC A and PNDBC B are fed through floating winding phasors of the transformer and floating winding phasor are displaced by 18 degrees.
11. The method of claim 10, to fashion the phase staggered voltage ±(?)° further comprising:
creating virtual neutral by connecting the inner windings at appropriate point of phases to shape outer phases of the transformer;
estimating the turns value for the various windings of the transformer for the three phase AC source; and
calculating the voltage value across each windings of the transformer through the turns value three phase- prime numbered five phase voltage AC source.
12. The method of claim 10, wherein phase difference between inner and outer phases is 60 degrees and 120 degrees between inner phases.

13. The method of claim 6, wherein multiphase transformer rectifier unit having mutual delta and zig-zag configuration, the multiphase transformer rectifier unit, magnetic rating is reduced by completely eliminating ZSBT and IPT. , Description:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
(See section 10, rule 13)
“A MULTIPHASE TRANSFORMER RECTIFIER UNIT AND A METHOD THEREOF”
By
NATIONAL INSTITUTE OF TECHNOLOGY KARNATAKA, SURATHKAL
Indian National of
National Institute of Technology Karnataka Surathkal, Mangalore 575025, Karnataka, India

The following specification particularly describes the invention and the manner in which it is to be performed.

Field of the invention
The present invention mainly relates to the field of converters i.e. alternating current to direct current (AC/DC) converters. More particularly to an AC/DC converter including an asymmetric multiphase staggering auto-configured transformer.

Background of the invention
The conventional power converters (phase-controlled rectifiers) accepting AC line input voltage and providing a DC output voltage introduce harmonic and reactive currents into the power generation and delivery system. Harmonic and reactive currents do not contribute power to the load, and therefore serve only to cause interference in power and communication systems and generally increase the ratings of the power generation and delivery equipment required for a given amount of power.
The Rectifiers are used to rectify AC voltages and generate DC voltages across DC buses. A typical rectifier includes a switch based bridge including two switches for each AC voltage phase which are each linked to the DC buses. The switches are alternately opened and closed in a timed fashion, which, as the name implies, causes rectification of the AC voltage. As well known in the energy industry the global standard for AC power distribution is three phase and therefore three phase rectifier bridges are relatively common.
When designing a rectifier configuration there are three main considerations including cost, AC line current harmonics and DC bus ripple. With respect to AC current harmonics, when an AC phase is linked to a rectifier and rectifier switches are switched, the switching action is known to cause harmonics on the AC lines. AC line harmonics caused by one rectifier distort the AC voltages provided to other commonly linked loads and therefore should generally be limited to the extent possible. In fact, specific applications may require that large rectifier equipment be restricted in the AC harmonics that the equipment produces.
With respect to DC link ripple, rectifier switching typically generates ripple on the DC bus. As with most hardware intensive configurations cost can be minimized by using a reduced number of system components and using relatively inexpensive components where possible.
It is well known in AC to DC rectification that AC current harmonics and DC ripple may be improved by increasing the number of AC phases, which are rectified by the rectifier. These AC phases are phase-shifted from each other. For example, by rectifying nine-phase AC current instead of three-phase, harmonics and ripple are reduced appreciably. Where AC harmonic restrictions are placed on rectifier systems such restrictions are often satisfied by employing an 18-pulse rectifier that requires a nine-phase source of AC power. As the global standard for AC power distribution is three phase, 18-pulse rectifiers require three-to-nine phase power converters between utility supply lines and rectifier switches.
The isolation transformers for converting three-phase AC power to nine-phase AC power are known in the art but have several shortcomings. First isolation transformers must be rated for the full power required. Second, isolation transformers are typically relatively large as separate primary and secondary windings are required for isolation purposes.
The three phase AC-DC converters are used in the industrial applications have total harmonic distortion of more than 25%, which will cause poor power factor, voltage distortion, overloading of capacitor bank, heating of equipment etc., connected to the system. One of method to reduce harmonic distortion is by using phase shift transformer based multi-pulse AC-DC converter. Present industrial application uses 6 or 12 pulse phase shift transformer based multi-pulse AC-DC converter which are capable of reducing total harmonic distortion lesser than 10 %. These, phase shift transformer based multi-pulse AC-DC converters are either isolated or non-isolated with symmetric in configuration. In general, symmetric auto-connected transformers produce harmonic content greater than 10% of the fundamental component. In general, the complexity of the system increases with increases in pulse number. Also, higher the pulse number lowers the harmonic content with increased magnetic rating. So, a trade-off should be drawn between pulse number and system complexity. For achieving higher pulse number, pulse doubling methods have been used in present technology which requires zero sequence blocking transformer (ZSBT). Due to this ZSBT the magnetic rating of the overall system is increased by 10-12 % of load rating
The drawback present in the existing techniques is higher harmonic distortion with increased magnetic configuration of the transformer used. And also, in the existing symmetrical transformer configuration the distortion level is high with low power factor. With the single transformer configuration, it has nowhere achieved the advantages like neutral point creation, path to third harmonic current circulation, less magnetic rating, less system configuration and complexity.
So, there is a need in the art with improved design to and model an asymmetric auto-configured transformer with less harmonic content less than the symmetric auto-configured transformer with reduced magnetic rating.

Objective of the invention
The main objective of the invention is to fashion an asymmetric non-isolated multiphase staggering auto-configured transformer rectifier unit for front-end AC-DC converter applications.
Another objective of the invention is to develop an AC-DC multiphase transformer rectifier unit with reduced magnetic components.
Yet another objective of the invention is to reduce the input supply current and voltage harmonics with unity power factor.
Further objective of the invention is to develop an AC-DC transformer rectifier unit with inherent power factor correction.
Yet a further objective of the invention is to minimize the zero sequence current components of the system by incorporating suitable modification in the auto-configured transformer.
Yet a further objective of the invention is to minimize the neutral current in case of any unbalance in the three-phase supply system by zigzag transformer.
Next objective of the invention is to create a path for third harmonic current circulation in auto-configured transformer under fault condition.
Yet another objective of the invention is to achieve an act equivalent to higher pulse numbered AC- DC rectifier from auto-configured transformer.

Summary of the Invention
Various aspects of the present disclosure are now summarized to facilitate a basic understanding of the disclosure, wherein this summary is not an extensive overview of the disclosure, and is intended neither to identify certain elements of the disclosure, nor to delineate the scope thereof. Rather, the primary purpose of this summary is to present some concepts of the disclosure in a simplified form prior to the more detailed description that is presented hereinafter.
Accordingly, in one aspect of the present invention is a multiphase transformer rectifier circuit/unit for converting a three phase alternating current (AC) supplied from a power distribution system to direct current (DC) supplied to at least one load. The multiphase transformer rectifier includes a transformer and a multiphase rectifier unit. The transformer is non-isolated as both the primary and secondary winding set of the transformer are wounded on the same core. The transformer is an autotransformer via connecting the inner zigzag windings at an appropriate point of phases to fashion outer phases. The phase difference between inner and outer phases is 60 degrees and 120 degrees between inner phases. The transformer has 24 windings with combined delta and zig-zag configuration. The core of the transformer having an EI, UI and UU core configuration and material used for the core is CRGO (Cold Rolled Grain Oriented) silicon steel.
The transformer includes an input operative to receive three phase AC electrical currents and a plurality of multiphase outputs. The each individual output including a set of at least five output terminals to provide prime numbered phase staggered voltage AC output currents at the output terminals. The multiphase rectifier circuit operatively coupled with the multiphase outputs to convert the corresponding AC output currents to DC electrical power. The rectifier circuit for AC/DC conversion as a Prime numbered diode bridge converter’s (PNDBC). The total number of output pulses created by the PNDBC is twice the prime numbered phase.
Accordingly, in another aspect of the present invention a method for converting a three-phase alternating current (AC) supplied from a power distribution system to direct current (DC) supplied to at least one load. The method includes an input circuit for connection to a three phase AC source and connecting the three phase AC source as input to windings of a transformer. The method further includes transforming the three phase AC electrical currents to create a N (two or more) sets of prime numbered phases AC output currents by the transformer, N being greater than 1. Then rectifying each individual set of prime numbered AC output currents through M prime numbered diode bridge converter's (PNDBC) to DC load, M being greater than 1.
The transforming and rectifying further includes passing the N (being two) sets of prime numbered 5-phase supply to 2 PNDBC (P=10), M (being 2). The each PNDBC A and B are fed by two 5-prime numbered sets of phase-staggered voltages A1, A2, A3, A4, A5 and B1, B2, B3, B4, B5. The 10 phase-staggered voltages are at an angle of ±? degree and the phase angle between A1 & B1 is ±(?/2)° with supply voltage VA as reference. The phase angle for other phase staggered voltages between A2 & B2, A3 & B3, A4 & B4 and A5 & B5 is ± (?) °. The PNDBC A and PNDBC B are fed through floating winding phasors of the transformer and floating winding phasor are displaced by 18 degrees. Then, finally inverting DC load to provide AC output power to AC motor load.
Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention

Brief description of the drawings
The following description and drawings set forth certain illustrative implementations of the disclosure in detail, which are indicative of several exemplary embodiments of the present invention in which the various principles of the disclosure may be carried out. The illustrated examples, however, are not exhaustive of the many possible embodiments of the disclosure. Other objects, advantages and novel features of the invention will be set forth in the following detailed description when considered in conjunction with the drawings, in which:
Figure 1 illustrates multiphase transformer rectifier unit for converting a three-phase alternating current (AC) supplied from a power distribution system into prime numbered phase voltages to direct current (DC) supplied to a single load, according to one embodiment of the invention.
Figure 2 illustrates a winding diagram of an asymmetric auto-configured transformer, according to one embodiment of the invention.
Figure 3 illustrates the phasor diagram representing the phase staggered angle positions of an asymmetric auto-configured transformer, according to one embodiment of the invention.
Figure 4 illustrates the simulation test done on the asymmetric non-isolated multiphase staggered auto-configured transformer for constant dc load, under varying load conditions, according to one embodiment of the invention.
Figure 5 illustrates the % input source current Total Harmonic Distortion (THD) and Power Factor (PF) variation under different load levels for both the primitive 6-pulse configuration and developed asymmetric auto-configured transformer, according to one embodiment of the invention.
Figure 6 illustrates the winding illustration of asymmetric auto-configured transformer with key numbers, according to one embodiment of the invention.
Figure 7 illustrates the Phasor displacement diagram of asymmetric auto-configured transformer with key numbers, according to one embodiment of the invention.
Figure 8 illustrates the winding connection illustration of asymmetric auto-configured transformer with key numbers, according to one embodiment of the invention.
Figure 9 illustrates the phasor diagram representation of resultant phasors of zig-zag transformer and resultant phasors of floating phases, according to one embodiment of the invention.
Figure 10 illustrates the different core size used for asymmetric auto-configured transformer with key numbers, according to one embodiment of the invention.
Detailed description of the invention
Referring now to the figures, several embodiments or implementations of the present invention are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale. The various embodiments shown and described below include motor drive-type power converters that include passive diode-bridge type rectifiers, although any form or type of power converter may be used for driving any form of load and that includes active or passive rectifier receiving multiphase AC output currents from a phase-shifting transformer. The illustrated embodiments, moreover, are shown using non-isolating autotransformers, although this is not a strict limitation of the disclosure, and isolating-type phase-shifting transformers may be employed or autotransformers may be used in combination with separate isolating transformers.
It is understood that the use of specific component, device and/or parameter names (such as those of the executing utility/logic described herein) are for example only and not meant to imply any limitations on the invention.
The invention may thus be implemented with different nomenclature/terminology utilized to describe the components/devices/ parameters herein, without limitation. Each term utilized herein is to be given its broadest interpretation given the context in which that terms is utilized. Specifically, the term "auto-configured transformer" includes, but is not limited to, any transformer device within which the inner windings include a zig-zag windings and exhibits functional characteristics (with respect to power and reduction in higher order harmonics by blocking the flow of zero sequence currents that are associated with multiple-pulse rectifiers) similar to those described herein when describing the zig-zag auto- transformers. The term transformer, auto-transformer, autoconfigured transformer, asymmetric non-isolated autotransformer shall have the same meaning and reference to any one of the above terms shall include the same meaning terms.
FIGURE 1, an exemplary multiphase transformer rectifier unit is illustrated, which includes a transformer receiving multiphase AC power from an AC power source via delta zig-zag configuration that provides a neutral grounded via a fault current limiting resistor. The transformer is an asymmetric non-isolated autotransformer formed by connecting three numbers of single phase multi winding transformer or single three phase multi winding transformer. The asymmetric auto configured transformer has inherent characteristics of a passive filter to reduce the line current harmonics at the input AC source end and does not require any external passive filter connection, thereby improving the quality of power. The transformer is non-isolated as both the primary and secondary winding set of the transformer are wounded on the same core. This reduces the size of the transformer, thereby reducing the cost of the overall system. The plurality of magnetic cores include a core having an EI, UI and UU core configuration and the material used for the core is CRGO (Cold Rolled Grain Oriented) silicon steel. The transformer has 24 windings with mutual delta and zig-zag configuration. The inbuilt delta configuration provide path to third harmonic current circulation. As the transformer is asymmetric, aids in magnetic reduction and also maintaining less harmonic distortion. The autoconfigured transformer has immanent zig-zag configuration formed by impedances to suppress the zero sequence current components and also to provide return path to fault currents. The inherent property of ZSBT (zero sequence blocking transformer) helps in reducing the magnetic rating of the overall system by greater extent. The transformer is competent to achieve reduced Total Harmonic Distortion (THD) of 3.36% by completely eliminating the ZSBT and IPT (Inter Phase Transformer).
The multiphase transformer rectifier unit (200) for converting a three-phase alternating current (AC) (43, 44, and 45) supplied from a power distribution system to direct current (DC) (52) supplied to at least one load, including a transformer (49) and a multiphase rectifier circuit (210). The autotransformer (49) is fashioned by creating virtual neutral by connecting the inner zigzag windings at an appropriate point of phases to fashion outer phases, to reduce the magnetic rating within the transformer itself. The zig-zag windings KVA rating is only small fraction of autoconfigured transformer and the 1/3rd part of winding in the phases of the autoconfigured transformer. So, by simplicity accomplish neutral point as well as 1/3rd of phase voltage in the autoconfigured transformer. The magnetic rating of autoconfigured transformer is only 30% of the load rating. The phase difference between inner and outer phases is 60 degrees and 120 degrees between inner phases. The transformer (49) includes an input operative to receive three phase AC (43, 44, and 45) electrical currents and a plurality of multiphase outputs. The each individual output including a set of at least five output terminals and being operative to provide prime numbered phase staggered voltage AC output currents (A1, A2, A3, A4, A5 and B1, B2, B3, B4, B5) at the output terminals. The prime numbered phase staggered voltage AC output phases be 5-phase, 7-phase, 11-phase, 13-phase etc. which is not multiple factor of 3. The AC output currents of each multiphase output being at a non-zero phase angle relative to all other multiphase outputs.
The multiphase rectifier circuit (210) operatively coupled with the multiphase outputs (A1, A2, A3, A4, A5 and B1, B2, B3, B4, B5) of the transformer to convert the three phase (43, 44, and 45) AC electrical currents as input to DC (52) electrical power. The rectifier circuit (210) for AC/DC conversion is a Prime numbered diode bridge converter’s (PNDBC) (50, 51) carries only lower magnitude of input ac source voltage. The PNDBC be 1, 2 or 3 followed by a transformer (49). The rectifier circuit (210) may include many different forms of rectifier circuits. The total number of output pulses created by the PNDBC is twice the prime numbered phase, i.e for 5-phase, the number of output pulses generated are 10- pulse which can be generalized as, Total number of output pulses from a PNDBC (P) = 2 x Number of prime numbered output phases (N). Example, for 5 phase- 2 PNDBC connected has 20 numbers of step-pulses and for 5phase-3 PNDBC has 30 numbers of step-pulses. The transformer has inherent property of the smoothing means (L and C) and doesn’t require any additional devices to create a multi- pulse/multiphase at the input. So, the asymmetric non-isolated multiphase staggered autoconfigured transformer, higher number of step-pulses for the input source current is accomplished by lower order of prime numbered phases. By increasing the number of step-pulses for the ac input current the harmonics are reduced and the power factor is well improved.
In a multiphase staggered autoconfigured transformer the required phase angle between two adjacent phase is f degree which can be obtained as,
Adjacent phase displacement angle (f) = 360º / Total output prime numbered phases.
In phase-staggered approach, the required phase staggering angle among the PNDBC is ? degree, which can be obtained as,
Phase-staggered angle (?) = 360°/ Total number of output pulses
In accordance with another aspect of the invention, a method for converting a three-phase alternating current (AC) (43, 44, and 45) supplied from a power distribution system to direct current (DC) (52) supplied to at least one load is provided. The method includes an input circuit for connection to a three phase AC (43, 44, and 45) source to windings of a transformer. For the input AC source, the auto-configured transformer (49) intended for current harmonic reduction is a hybrid (combination or cross) of multipulse/multiphase and phase-staggering approach with inherent power factor correction. The transformer (49) transforms the three phase AC electrical currents (43, 44, and 45) to create an N (two or more) set ( A1, A2, A3, A4, A5 and B1, B2, B3, B4, B5) of prime numbered phases AC output currents to DC electrical power used to drive a corresponding load (52), N being greater than 1. These set of prime numbered phases can be either 5-phase, 7-phase, 11-phase, 13-phase and so on. The each set of prime numbered AC output currents being at a non-zero phase angle relative to other set. Further, rectifying each individual set of prime numbered AC output currents through M prime numbered diode bridge converter's (PNDBC) (210) to DC load (52), M being greater than 1 and finally inverting the DC load (52) to provide AC output power to AC motor load.
The method of transforming and rectifying further comprise of passing the N (being two) sets of prime numbered 5-phase supply to M (being 2) 2 PNDBC (P=10), i.e. two 10 pulses which are phase staggered by ±? degree. The each PNDBC A and B are fed by two 5-prime numbered sets of phase-staggered voltages A1, A2, A3, A4, A5 and B1, B2, B3, B4, B5. The 10 phase-staggered voltages are at an angle of ±? degree, the phase angle between A1 & B1 is ±(?/2)° with supply voltage VA as reference. The phase angle for other phase staggered voltages between A2 & B2, A3 & B3, A4 & B4 and A5 & B5 is ±(?)°. The rectification moreover may be performed using the rectifier for each of the resultant multiphase outputs. The PNDBC A and PNDBC B are fed through floating winding phasors of the transformer.
The number of tapings of different phase windings is equal to the number of output phases. The phase difference between inner and outer phases is 60 degrees and 120 degrees between inner phases. The phase staggered voltage ±(?)° further includes, creating the virtual neutral by connecting the inner
windings at appropriate point of phases to shape outer phases of the transformer. Then estimating turns value for the various windings of the transformer for the three phase AC and calculating the voltage value across each windings of the transformer through the turns value three phase- prime numbered five phase voltage AC source. The direction of current flow in the windings of the auto-configured transformer is by dot convention to fashion required phase shift between the phases. The natural air cooled system and proper insulation system make convenient operation of the autoconfigured transformer.
Figure 2 & 3 illustrates a winding and phasor diagram of an asymmetric auto-configured transformer, according to one embodiment of the invention. The constants K1-K19 decide the winding turns value. A typical value of three- phase and prime numbered five-phase voltages of ac source are chosen to determine the turns value for various windings.

Thus the values obtained from above calculations are summarized as follows. K1=0.3295, K2=0.0903, K3=0.1042, K4=0.2928, K5=0.1985, K6=0.3117, K7=0.5593, K8=0.2645, K9=0.0848, K10=0.119, K11=0.2296, K12=0.1986, K13=0.0269, K14=0.3289, K15=0.0006, K16=0.0208, K17=0.3678, K18=0.445, K19=0.0207. Considering one volt per turn, the number of turns or voltage value across each winding of the proposed asymmetric autoconfigured transformer is decided by the winding turns value K1-K19 which is only a portion of input source voltage.
Figure 4 illustrates the extensive simulation test has been performed on the asymmetric non-isolated multiphase staggered autoconfigured transformer for constant dc load, under varying load conditions. The set of simulated waveforms consist of three phase input source-end voltage, three phase input source-end current, input voltage to PNDBC A, input voltage to PNDBC B, output constant dc voltage from PNDBC A , and output constant dc voltage from PNDBC B are shown in Fig. 4.
Table-1 shows the simulation results of the asymmetric non-isolated phase staggered autoconfigured transformer on power quality indices with different load levels. It is realized that the power quality index of input source-end current (i) and voltage (VTHD) are 3.36% and 1.24% respectively, when the leakage impedance of the asymmetric autoconfigured transformer winding is kept as 5%. The asymmetric multiphase staggered autoconfigured transformer has resulted in reduced input source-end current harmonics of less than 5% for different load levels and power factor of 0.99 (close to unity) is inside IEEE standard requirements. This hybrid method of combining multipulse and multiphase, results in notable upgrading in power quality at the input ac source current.
Load level Input
Source Current Iia(A) Input Source
Current iTHD
(%) Input source
Voltage
VA(V) Input source
Voltage THD VTHD (%) Power
Factor
(PF)
Light load (20%) 5.47 5.75 218.24 3.36 0.9914
Half load (50%) 12.87 4.33 219.49 2.46 0.9923
Full load (100%) 25.46 3.36 220.43 1.24 0.99
Table-1
A comparative study of performance and effectiveness of a primitive 6-pulse configuration and developed autoconfigured transformer. The % input source current Total Harmonic Distortion (THD) and Power Factor (PF) variation under different load levels for both the primitive 6-pulse configuration and developed asymmetric autoconfigured transformer is shown in Figure 5.
Figure 6 illustrates the winding diagram of asymmetric autoconfigured transformer with key numbers. The key numbers 1, 2 and 3 is the inner zig-zag transformer phase A, B and C main windings, 4&5 is the outer zig-zag transformer phase A main windings, 6 is the outer zig-zag transformer phase B main winding and 7&8 is the outer zig-zag transformer phase C main windings. The neutral point created by this zig-zag transformer provides return path to the fault currents. The phase difference between inner and outer phases of zig-zag transformer is 60 degrees. Also, phase difference between inner phases is 120 degrees. The key numbers 10-13 is the part of A phase of auto-configured transformer main windings, 14-16 is the part of B phase of auto-configured transformer main windings and 17&18 is the part of C phase auto-configured transformer main windings.
The key numbers 20&21, 22&23, 24&25, 26&27, 28&29 are the floating windings of the auto-configured transformer. The key numbers 20&21 are the floating windings which are taken form phase B of the auto-configured transformer. The key numbers 22&23 are the floating windings which are taken form phase B of the auto-configured transformer. The key numbers 24&25 are the floating windings which are taken form phase C of the auto-configured transformer. The key numbers 26&27 are the floating windings which are taken form phase A of the auto-configured transformer. The key numbers 28&29 are the floating windings which are taken form phase B of the auto-configured transformer. These floating windings are given to the input of PNDBC-A and PNDBC-B. The PNDBC-A is fed from five phase winding tapings which are denoted by the corresponding windings key numbers 20,22,24,26,28 and the PNDBC-B is also fed from other five phase winding tapings which are denoted by the corresponding windings key numbers 21,23,25,27,29.
Figure 7 illustrates the phasor displacement between the floating phasor and main phasor of asymmetric auto-configured transformer with key numbers. The key numbers 30, 31 and 32 are represents the main supply input voltage phasors or the input phases of auto-configured transformer. The key numbers 33&34 represents the floating winding VA1 and VB1 phasors which are displaced by 18 degrees. Similarly, the key numbers 35&36,37&38, 39&40 and 41&42 represents the floating windings VA2 & VB2, VA3 & VB3, VA4 & VB4 and VA5 & VB5 phasors respectively which are also displaced by 18 degrees.
Figure 8 illustrates the winding connection illustration of asymmetric auto-configured transformer with key numbers. The key numbers 53-60 represents the phase A winding, key numbers 61-73 represents the phase B winding and key numbers 74-81 represents the phase C of the auto-connected transformer. The internal connections between the phases of the auto-connected transformer are given in the Table-II. The sequence of connections are stated from phase A, phase B and phase C as per in Table-II.
Phase A Phase B Phase C

Remarks
Transformer
winding taken from Transformer
winding connected to Transform
er winding taken from Transformer
winding connected to Transform
er winding taken from Transformer
winding connected to
A1s B1s B1s C1s C1s A1s Neutral Point
A1e A5e or A6s B1e B4e or B5s C1e C4e or C5s
A2s C6e B2s Already Done C2s Already Done Input Phase VA
w.r.to Ph A Conn.
A2e A3s B2e B3s C2e C3s -
A3e A4s B3e B4s C3e C4s -
A4e A5s B4e B5s C4e C5s -
A5e A6s B5e B6s C5e C6s -
A6e B2s B6e B7s C6e Already Done Input Phase VB
w.r.to Ph A Conn.
A7s B5e or B6s B7e C2s C7s Floating
Terminal VA3 Phase VC w.r.to
Ph B Conn.
A7e Floating
Terminal VA4 B8s C2e or C3s C7e B2e or B3s -
A8s B6e or B7s B8e Floating
Terminal VA5 C8s Floating
Terminal VB3 -
A8e Floating
Terminal VB4 B9s C3e or C4s C8e B3e or B4s -
- - B9e Floating
Terminal VB5 - - -
- - B10s C5e or C6s - - -
- - B10e Floating
Terminal VA1 - - -
- - B11s Floating - - -
- - B11e A2e or A3s - - -
- - B12s Floating
Terminal VA2 - - -
- - B12e A3e or A4s - - -
- - B13s Floating
Terminal VB2 - - -
- - B13e A4e or A5s - - -
Table-II
Figure 9 illustrates the phasor diagram representation of resultant phasors of zig-zag transformer and resultant phasors of floating phases, according to one embodiment of the invention. The key numbers 82-84 represents the resultant phasors of zig-zag transformer, key numbers 85-94 represents the resultant phasors of floating phases VA1, VB1, VA2, VB2, VA3, VB3, VA4, VB4, VA5, VB5 respectively.
Figure 10 illustrates the different core size used for asymmetric autoconfigured transformer with key numbers, according to one embodiment of the invention. The key numbers 95&96, 97&98, and 99&100 represents the EI, UI, and UU core of the autoconnected transformer respectively. The suitable core is selected for the hardware implementation. The material used for the core is CRGO (Cold Rolled Grain Oriented) silicon steel. The triplex or milinex type lamination sheets are used for this dry type auto-configured transformer.

Advantages of the invention
An asymmetric autoconfigured transformer have less magnetic rating, no need of passive filters, eliminates ZSBT and IPT.
The inherent zigzag transformer to reduce zero sequence current, provide path to third harmonic current circulation, carries lower magnitude of input ac source voltage, maintains less harmonic distortion even with distorted input supply voltages.
In case of any one mains phase failure or interruption of one mains phase, the system continues to operate at reduced power with unchanged sinusoidal current/voltage shape. With the reduced number of phases (as in case of mains phase failure) is able to achieve least value of Total Harmonic Distortion (THD) with reduced number of components (number of diodes).
The asymmetric autoconfigured transformer has special winding configuration, the operation of system has stable characteristics even under single phasing, ie, the system remains stable condition even under fault in any one of the input phases.
Example: 12 pulse autoconnected transformer for high voltage direct current (HVDC) application in ABB.
Applications: Switched Mode Power Supplies (SMPS), Adjustable speed drives, HVDC etc.
Figure 1-10 is merely representational and is not drawn to scale. Certain portions thereof may be exaggerated, while others may be minimized. FIGS.1-10 illustrates various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.
The above examples are merely illustrative of several possible embodiments of various aspects of the present invention, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, systems, circuits, and the like), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component, such as hardware, software, or combinations thereof, which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the illustrated implementations of the invention. In addition, although a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.

We Claim:

1. A multiphase transformer rectifier unit for converting a three-phase alternating current (AC) supplied from a power distribution system to direct current (DC) supplied to at least one load , comprising:
a transformer comprising:
an input operative to receive three phase AC electrical currents; and
a plurality of multiphase outputs, each individual output including a set of at least five output terminals and being operative to provide prime numbered phase staggered voltage AC output currents at the output terminals, with the AC output currents of each multiphase output being at a non-zero phase angle relative to all other multiphase outputs;
and
a multiphase rectifier unit operatively coupled with the multiphase outputs to convert the corresponding AC output currents to DC electrical power.
2. The multiphase transformer rectifier unit of claim 1, wherein the transformer is a non-isolated autotransformer.
3. The multiphase transformer rectifier unit of claim1, wherein said rectifier circuit for AC/DC conversion as a Prime numbered diode bridge converter’s (PNDBC); wherein PNDBC be 1, 2 or 3 followed by a transformer.
4. The multiphase transformer rectifier unit of claim 3, wherein the total number of output pulses created by the PNDBC is twice the prime numbered phase; wherein for 5 phase -2 PNDBC connected has 20 number of step-pulses and for 5phase-3 PNDBC has 30 number of step-pulses.
5. The multiphase transformer rectifier unit of claim 3, wherein transformer is non-isolated as both the primary and secondary winding set of the transformer are wounded on the same core, wherein transformer is an autotransformer, creating virtual neutral by connecting the inner zigzag windings at an appropriate point of phases to fashion outer phases, wherein phase difference between inner and outer phases is 60 degrees and 120 degrees between inner phases.
6. The multiphase transformer rectifier unit of claim 1, wherein transformer has 24 windings with mutual delta and zig-zag configuration.
7. The multiphase transformer rectifier as claimed in claim1, wherein the plurality of magnetic cores include a core having an EI, UI and UU core configuration and material used for the core is CRGO (Cold Rolled Grain Oriented) silicon steel.
8. The multiphase transformer rectifier unit of claim 1, wherein prime numbered phase staggered voltage AC output phases be 5-phase, 7-phase, 11-phase, 13-phase etc. not multiple factor of 3.
9. A method for converting a three-phase alternating current (AC) supplied from a power distribution system to direct current (DC) supplied to at least one load, comprising:
an input circuit for connection to a three phase AC source;
connecting the three phase AC source as input to windings of a transformer;
transforming the three phase AC electrical currents to create a N (two or more) sets of prime numbered phases AC output currents by the transformer, N being greater than 1, each set of prime numbered AC output currents being at a non-zero phase angle relative to other set;
rectifying each individual set of prime numbered AC output currents through M prime numbered diode bridge converter's (PNDBC) to DC load, M being greater than 1; and
inverting DC load to provide AC output power to AC motor load.

10. The method of claim 9, wherein transforming and rectifying further comprise of passing the N( being two) sets of prime numbered 5-phase supply to 2 PNDBC (P=10), M (being 2) i.e. two 10 pulses which are phase staggered by ±? degree, wherein each PNDBC A and B are fed by two 5-prime numbered sets of phase-staggered voltages A1, A2, A3, A4, A5 and B1, B2, B3, B4, B5; the 10 phase-staggered voltages are at an angle of ±? degree, the phase angle between A1 & B1 is ±(?/2)° with supply voltage VA as reference, the phase angle for other phase staggered voltages between A2 & B2, A3 & B3, A4 & B4 and A5 & B5 is ±(?)°, wherein the PNDBC A and PNDBC B are fed through floating winding phasors of the transformer and floating winding phasor are displaced by 18 degrees.
11. The method of claim 10, to fashion the phase staggered voltage ±(?)° further comprising:
creating virtual neutral by connecting the inner windings at appropriate point of phases to shape outer phases of the transformer;
estimating the turns value for the various windings of the transformer for the three phase AC source; and
calculating the voltage value across each windings of the transformer through the turns value three phase- prime numbered five phase voltage AC source.
12. The method of claim 10, wherein phase difference between inner and outer phases is 60 degrees and 120 degrees between inner phases.

13. The method of claim 6, wherein multiphase transformer rectifier unit having mutual delta and zig-zag configuration, the multiphase transformer rectifier unit, magnetic rating is reduced by completely eliminating ZSBT and IPT.

Abstract
A Multiphase Transformer Rectifier unit and a method thereof
A multiphase transformer rectifier unit for converting a three phase alternating current supplied from a power distribution system to direct current supplied to at least one load. The multiphase transformer rectifier includes a transformer and a multiphase rectifier unit. The transformer includes an input operative to receive three phase AC electrical currents and a plurality of multiphase outputs. The each individual output including a set of at least five output terminals to provide prime numbered phase staggered voltage AC output currents at the output terminals. The multiphase rectifier unit operatively coupled with the multiphase outputs to convert the corresponding AC output currents to DC electrical power.
Figure 1 (for publication)