Abstract:
A transformer based power conditioner to provide high purity electrical power. Capacitor based low impedance filters may be connected in the transformer secondary for noise filtering. Surge suppressors may be connected from the start to the finish of the primary windings regardless of tap selection, to limit peak voltages due to transient electrical surges that reach the load. Filter performance can be improved as required by the end user, by cascading the filter circuits on the output lines.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transformer based power conditioner to provide high purity electrical power, preferably having capacitor based circuitry on the transformer secondary to provide noise filtering, and preferably having semiconductors on the transformer primary functioning as suppressors for transient electrical surges. 
     2. Description of the Related Art 
     High purity electrical power generally means that the power is substantially free from voltage spikes and sags with no significant neutral-to-ground voltage. A number of electronic devices require such high purity power. Among them are medical imaging systems such as X-rays, computer tomography, magnetic resonance imaging, and radiation treatment systems. All of these devices require a large amount of current but only for a short duration, that is, when the X-ray or magnetic generator is operational. The power during this exposure must be clean for good image quality. Additionally, the stand-by power between exposures must also be clean for the reliable operation of the computerized control and imaging processing subsystems, which operate between exposures. An example of a power conditioner to provide high purity electrical power is shown in U.S. Pat. No. 5,012,382. 
     For these types of systems, the voltage drop during the exposure period should be minimal, typically less than about 8%. This voltage drop is a result of the impedance of all upstream wiring, connections and transformers in the circuit. The reason for this limitation on voltage drop is that the exposure duration in the medical imaging systems is often calculated based on the magnitude of the line voltage present immediately before exposure. Significant changes in this voltage during exposure can result in unpredictable dosages. It is also important that operation of the generator does not produce voltage sags or spikes on the power lines which interfere with the reliable operation of other system components. 
     Other systems requiring high purity power include automated test equipment and telecommunications equipment. Automated test equipment (ATE) is used in a number of applications, one of which is the manufacture of semiconductors and printed circuit boards. For example, during the testing of semiconductors (like computer microprocessors) the ATE provides an array of inputs to the microprocessor and detects the response and response speed to those inputs. Impure power can cause the ATE to interpret the responses incorrectly, and as a result, eliminate good parts or retain bad parts. Printed circuit board (PCB) ATE provides an array of inputs to PCBs and determines if the traces have continuity to the desired areas of the PCB. Impure power can cause the ATE to give erroneous indications about PCB continuity. Manufacturers of such equipment continue to reduce the semiconductor and PCB test voltages to reduce the power requirements, allowing the ATE to be smaller and more cost effective for the end user. However, with lower operating test voltages, the susceptibility of the ATE to impure power becomes greater and the need for cleaner power increases. 
     Telecommunications equipment also requires clean power for similar reasons. Clean power allows the communications equipment to transmit with higher quality. This results in better sound and data quality and fewer dropped connections. Another advantage is that clean power ensures that telecommunications equipment will continue to operate without interruption for longer periods. 
     Surge suppression devices or L-C filters, or both, on the building wiring near the load provide another way to achieve the desired power quality. These devices shunt impulses above certain voltage or frequency levels from one wire to another. They typically are comprised of metal oxide varistors (MOVs), silicon avalanche diodes (SADs), gas discharge tubes, capacitors and inductors, and often incorporate resistors. An example of a transient voltage surge suppressor using MOVs and silicon surge suppression diodes is discussed in U.S. Pat. No. 4,802,055. Another example of MOVs used for electrical transient suppression is described in U.S. Pat. No. 5,038,245. Still another example of a way to suppress transients is discussed in U.S. Pat. No. 4,156,838, which does not employ magnetic coupling. 
     There are several limitations with many of these types of devices and filters. They shunt away voltage spikes or dips (“normal mode noise”), but in doing so increase the current in the neutral conductor, creating neutral-ground potentials (“common mode noise”) which can be even more damaging or disruptive than normal mode noise. Their effects are limited since they can only protect to a certain voltage or frequency level. For example, MOV&#39;s and avalanche diodes “wear out” with time and lose their effectiveness. 
     For this protection scheme to be as effective as possible, the inductance in series with the shunt elements (surge suppressors and capacitors in L-C filters) must be minimized. The wire length connecting the suppressors to the conductors makes a measurable difference in their effectiveness. Often, these devices are connected to the power lines by wires which are five to 50 feet in length due to physical placement constraints in the field or limited knowledge on the part of the installers, or both. The length of the wires between the power lines and the suppressors also limits the effectiveness of scaling the product for optimal performance. Surge suppressors are also frequently sized inappropriately to simplify installation at the expense of performance. 
     Also limiting the performance of shunt elements is the lack of ability to improve performance by cascading the filtering elements. Many of these filters employ single stage filtering that limits the effectiveness of the elements within the suppressors. 
     In an attempt to minimize lead length, maximize performance, and make site performance consistent, series power line filters have been used in these applications. Many of these devices have a series element (typically an inductor) that adds impedance to the line and increases cost significantly, but improves high frequency filtering performance greatly. The added series impedance has negative effects on power quality, particularly voltage regulation, during high current demand, as described earlier. It also limits the cost effectiveness of scaling these types of products. 
     Another alternative solution is to use a conventional shielded isolation transformer. The shield in the transformer increases the isolation of the output from the conducted ground (common mode) noise. A neutral-ground bond converts common mode noise to normal mode noise and allows a more effective use of filters and surge suppressors on the transformer as described above. 
     The extra impedance of the series power line filters (with inductors) or shielded isolation transformer methods results in lower power quality when the filters or transformers interact with computer loads. Modern computers have “switch mode” power supplies which draw their current in short bursts where the change in current with respect to time (di/dT) is fast, being the equivalent to 120 Hz or greater, instead of the usual 60 Hz for most conventional loads. Even at low load factors, these rapidly varying loads cause conventional transformers to produce outputs with a flat-topped voltage waveform and voltage spikes. Other switching transients within the system only add to the problem. 
     Often shielded isolation transformers and suppression/filtering devices are combined in the field in an attempt to provide the quality power desired. The limitations mentioned above also apply to this combination. 
     In contrast to the use of passive shunt element based solutions, other attempts to provide high purity power for these systems have utilized conventional voltage regulation schemes. Those attempts have generally not been successful. Electronic-controlled tap-switching voltage regulators are undesirable because the step voltage changes produced by tap changes during exposure often cannot be tolerated. Motorized variac solutions are typically not fast enough to compensate for most common power quality problems. Saturable-core ferroresonant transformers are also unacceptable as they have very high impedance and slow reaction time. They interact with the pulsed load by creating large voltage transients. None of these approaches can effectively produce the power quality required by the systems described earlier. 
     Thus the challenge in the prior art is to provide high quality power as previously defined to a load having relatively high peak pulsed power and moderate average power requirements, with a relatively small, economical power conditioner. 
     SUMMARY OF THE INVENTION 
     The present invention solves the problems stated by means of an improved power conditioner circuit which includes a special type of power line filter system having a low series impedance at both 60 Hz and at high frequencies when compared to conventional filters. This low impedance filter, coupled with a low impedance isolation transformer, provides superior power quality for loads with high peak pulsed power requirements. The power line filter system can be cascaded to increase performance in a cost-effective manner. The filter system may include suppressor elements in some embodiments. 
     This power line filter system incorporates several features not found in conventional filter systems. One feature of the invention is that the filter for each phase is connected to the secondary of the transformer before the phase-to-phase bond. The neutral conductors of each phase&#39;s filter are physically separated from one another to minimize coupling of high frequency components from input to output of the filter. A second feature is the specific way of internally wiring two or more separate shunt capacitors within the individual filter module. The shunt capacitors and their connections are physically separated from one another to minimize coupling of high frequency components from the input to the output of the filter module. Another feature is that the location of the filter inside the power conditioner enclosure delivers consistent, high performance of the system from site to site because the wiring is as short as possible, typically less than two inches and preferably approaching zero length. Also because of the compact nature of the power circuitry in the power conditioner enclosure, the wiring can be maintained in a production-controlled environment to ensure production consistency. 
     Another feature of the power line filter system of the invention when suppressor elements are included is the manner in which the suppression elements are connected to each primary winding. The suppressors are connected across the entire tapped winding, regardless of which tap is connected to the power line input. This connection scheme advantageously uses the magnetic coupling of the extended windings that are electrically connected to the input power line windings. Connecting the suppressor in this manner clamps transients more effectively than placing the suppressor directly across the power line input. 
     Transformers are often made with multiple primary sections for connection in series or in parallel to accommodate the different standard input voltages worldwide. A further feature of the invention is the use of an individual suppressor across each of the multiple primary winding sections in the transformer. The magnetic coupling between the primary sections causes the suppressors to share the current accompanying a high voltage transient, providing more reliable performance and a longer life for the suppressors. 
     One aspect of the present invention involves a transformer-based filtered power conditioning system, the system including a system ground or neutral. The system has a transformer having primary and secondary windings with the primary windings having input connections and the secondary windings having output connections. Input power lines are connected to the primary winding input connections and are adapted to be connected to a source of electrical power. In one embodiment, surge suppression elements are connected across the ends of each primary winding. Preferably, the surge suppression elements are connected to the start and finish points for each coil in the primary, regardless of tap selection for the input power. 
     In one embodiment, at least one filter circuit is provided, comprising at least one input shunt leg and at least one output shunt leg, each shunt leg having at least one resistor and at least one capacitor between two connections, thereby forming R-C circuits. Advantageously, a very low impedance element is connected between the connections of the input and output shunt legs, wherein a first connection of the input shunt leg is connected to one polarity of the secondary windings, a second connection of the input shunt leg is connected to the opposite polarity of the secondary windings; a first connection of the output shunt leg is one polarity of the output of the power conditioning system and is adapted to be connected to a load or to a first connection of an input shunt leg of a second filter circuit, and a second connection of the output shunt leg is the opposite polarity of the output of the power conditioning system and is adapted to be connected to a load and optionally connected to system neutral and ground, or to a second connection of an input shunt leg of a second filter circuit. 
     In one embodiment, the suppression elements are selected from the group consisting of metal oxide varistors (MOVs), silicon avalanche diodes (SADs), gas discharge tubes, and capacitor and inductor combinations, the combinations selectively including resistors. In one embodiment, the capacitors in each shunt leg are substantially equal in capacitance, and in another embodiment, the capacitors in each shunt leg are unequal in capacitance. 
     In one embodiment, the filter circuit further has an inductor. 
     Advantageously, the low impedance connecting the ends of the shunt legs comprises an electrically conductive element to which the shunt legs are mounted with the mountings being spaced apart on the electrically conductive element. The system is also preferably scalable by adding additional filter circuits to the secondary winding outputs. 
     In one embodiment, the filter circuits have an anti-parallel diode arrangement connected across each resistor in each shunt leg. In one embodiment, the filter circuit is connected between the secondary windings and the output neutral, and the connection to system ground is a phase-to-phase neutral bond 
     The power conditioning system is a three-phase system in one embodiment, but may also be a single-phase system in another embodiment, and is adaptable to any number of phases. 
     Other additional features of the invention which provide further improvements are described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, advantages and features of this invention will be more readily appreciated from the following detailed description, when read in conjunction with the accompanying drawings: 
     FIG. 1A is a perspective view of a typical load, and a power distribution unit enclosure which contains the power conditioner circuit of the invention; 
     FIG. 1B is a perspective view of the typical load, and a power distribution enclosure with any part or all of the power conditioning circuit in a separate external enclosure; 
     FIG. 2 is a schematic representation of an embodiment of a power conditioner circuit in accordance with the invention, showing how the transformer output is modified and how the power conditioner is connected to a sensitive electrical load; 
     FIG. 3 is a schematic circuit showing an alternative embodiment of the invention, with suppressors on the transformer input; 
     FIG. 4 is a schematic representation which combines the features of FIGS. 2 and 3; 
     FIG. 5 is a further alternative arrangement of the output of the transformer of FIG. 2 in accordance with the invention; 
     FIG. 6 is another alternative schematic arrangement of the output of the transformer of FIG. 2; 
     FIG. 7 shows a delta configuration of the output of the windings of the transformer of FIG. 2; 
     FIG. 8 is an alternative arrangement of the input side of the transformer of FIG. 2; 
     FIG. 9 shows a further alternative arrangement of the input side of the transformer of FIG. 2; 
     FIG. 10 is a further schematic arrangement of the output side of the transformer of FIG. 2; 
     FIG. 11 is an alternative schematic arrangement of the primary windings in a delta configuration; 
     FIG. 12 is another alternative delta configuration schematic arrangement of the primary windings; 
     FIG. 13 is a further alternative delta configuration schematic arrangement of the primary windings; 
     FIG. 14 is yet a further alternative schematic arrangement of the primary windings in a wye configuration; 
     FIG. 15 is a further alternative wye configuration of the primary windings; 
     FIG. 16 is a still further alternative wye configuration of the primary windings; and 
     FIGS. 17A-17D depict examples of single-phase transformers employing the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A depicts a cabinet  22  which houses a power conditioner transformer circuit of the invention. Auxiliary circuits may also be contained within the enclosure. Incoming power enters the system through line  27  and output line  26  supplies conditioned power to the equipment or load  31 . Although the power conditioner and the load are shown as separate cabinets, they could both be in a single enclosure. In addition, although FIG. 1A depicts the surge suppression circuitry  23  internal to cabinet  22 , such circuitry  23  could also be provided external to cabinet  22  as depicted in FIG.  1 B. 
     FIG. 2 illustrates a power conditioning circuit  35  comprised partially of isolation transformer  36  having primary  37 , secondary  38  and iron core  39 . The input to primary  37  includes input phase conductors  41 ,  42  and  43 , which are configured to be connected to an appropriate electrical power source. Line  51  is the input ground line and is preferably the legal minimum in size in accordance with NEC standards in the present embodiment. It is contemplated that the transformer will be equipped with conventional taps  45 ,  46  off the primary windings, enabling the transformer to correct for chronic low or high line voltage by connecting the input phase conductors  41 ,  42  and  43  to the appropriate respective taps. Note also that primary segments  80 ,  81  and  84 ,  85  are connected in series. By way of example, primary segments  82 ,  83  are shown connected in parallel. Either arrangement can work. 
     In one embodiment, the transformer  36  may be a shielded isolation transformer, as indicated by dashed line  61  representing a metal sheet positioned between the primary and secondary windings of the transformer. A zig-zag transformer, an auto-transformer, tap switchers, variacs, or a non-shielded transformer, among others are also appropriate for use with the present invention. 
     Each output line  62 ,  63  and  64  of the power conditioner includes filter circuits  55  that have at least capacitors connected to the transformer prior to the phase-to-phase neutral bond  60 . Preferably, a resistor  70  is connected in series with capacitor  71 . The resistor  70  may be either a resistor or merely a wire, with the small resistance of the wire providing resistance. In one embodiment, as shown in FIG. 2, the filter circuits also include inductors. For example, shown in finish line  52  of secondary coil  68  is inductance  66 . This can be a separate component or the leakage inductance of transformer  36 , or both. Similar inductance  67  is included on the other, or start line  53  of secondary winding  68 . This side of the winding leads to neutral  60  through line  69 . Connected between lines  52  and  53  is the series combination of resistor  70  and capacitor  71  in filter  55 . Resistor  70  can be either an actual resistor component or the resistance of the wire that connects the capacitor to the line. Connected between lines  62  and  69  is a similar series combination of resistor  72  and capacitor  73 . Between the resistor and capacitor combinations or shunt legs are very low impedance devices  74  and  75 , which can either be conductors, resistors, or inductors. A preferred arrangement is for impedance devices  74  and  75  to be a bar of conductive material, such as copper. Other highly conductive materials could be employed. By having each resistor/capacitor leg of filter  55  physically connected at spaced locations on each very low impedance device, a very small but measurable impedance will result between the two resistor/capacitor legs. “Very small” in this context is as low as one pico ohm. Filter performance was found to improve as impedances  74 ,  75  increased, and were tested to 1 mΩ. It is contemplated that filter performance will continue to increase as the impedance increases, but overall system performance will be degraded as the voltage drop through impedances  74 ,  75  becomes significant. It is preferred that capacitors  71  and  73  have equal capacitance but that is not a requirement. Those two capacitors may differ in capacitance in any practical manner which still meets the filter requirements. 
     The shunt legs of the filter  55  formed of the resistor capacitor series combination which are connected with a low impedance element  74 ,  75  could be described as having first, second, third and fourth quadrants: The first quadrant being the connection of the filter circuit  55  connected to one polarity of the secondary winding  68 , the second quadrant of the filter circuit  55  being considered the connection which is connected to the opposite polarity of the secondary winding  68 , the third quadrant of the filter circuit  55  being considered one polarity of the output of the power conditioning system and being adapted to be connected to the load via line  62  and the fourth quadrant of the filter circuit being considered the opposite polarity of the output of the conditioning signal and being adapted to be connected to a load or to a system neutral/ground  60 . 
     A similar filter or suppressor structure is connected in the other two transformer secondary output lines and need not be discussed in detail here. 
     Ground line  76  is provided for a supplemental ground if necessary, and as may be required in some instances for applications that require this ground, such as applications where each article requiring power also requires its own ground or its own grounding stake connection. 
     Sensitive electrical load  77  is the type of system discussed previously, which may be an X-ray machine, magnetic resonance imaging system or other system requiring high power for short time durations, or systems such as automated test equipment or telecommunications equipment that simply require very clean power. Output phase conductors  62 ,  63  and  64  are relatively large and preferably quite short, which provides higher quality power by minimizing impedance and pickup of radiated noise from other electrical lines or other interference sources. Lines  78  and  79  are derived neutral and ground respectively. 
     The resistor/capacitor filter legs in the output of winding  68  may have several different types of components to further optimize performance for a particular type of load. For example, these filter legs could incorporate MOVs, SADs, gas discharge tubes, or inductors, among others, either in place of or in addition to the elements shown. Other additional features will be discussed below. 
     In the embodiment of FIG. 3, the same isolation transformer is shown having the same output windings, but without the R-C filter structures. In this arrangement of the invention, each primary winding  80 ,  81 ,  82 ,  83 ,  84  and  85  of transformer primary  37  includes surge suppression elements  86 ,  87  connected between the start and the finish of each winding. Windings  80  and  81  are connected as shown to make up one phase of the primary of the transformer. Each suppression element should be rated to accommodate for the maximum voltage of its associated primary coil. The magnetic coupling of the windings  80  and  81  permit the suppression elements to clamp the input voltage in all primary wiring variations. Additionally, the magnetic coupling of  80  and  81  has the benefit of also forcing more equal energy distribution between suppression devices  86  and  87  than would occur by simply placing two suppression devices in parallel. This leads to less stress and longer life for the suppression devices as well as improved performance. 
     Surge suppressors  86  and  87  could be metal oxide varistors (MOVs) or silicon avalanche diodes (SADs), among others, which are connected in parallel with one or both primary windings  80  and  81  or any portion of either primary winding. A similar suppression structure is connected in parallel with windings  82 ,  83 ,  84  and  85  and need not be discussed in detail here. 
     It has been found that the basic objectives of the invention can be satisfied by this embodiment of the primary, without the enhancement of the secondary filters shown in FIG.  2 . 
     FIG. 4 is a schematic diagram of a preferred embodiment which includes the features of FIGS. 2 and 3 in a single combined structure. The combination provides for even better power purity because of the combined effects of the surge suppression devices on the input windings and the filter arrangements in the output windings of the transformer. There are many advantages to the power conditioning circuit of this invention, particularly the combination depicted in FIG.  4 . This power conditioner provides cleaner power than any known approach, regardless of the size or rating of the transformer. The circuit of FIG. 4 attenuates incoming or line-side disturbances with the magnetically coupled surge suppressors, the transformer leakage inductance, the transformer shield, and the L-R-C filters installed, preferably within inches of the output phase conductors. It is contemplated that the inductance in the L-R-C filter may be only the transformer leakage inductance, or an additional inductor may be employed. Note that, as previously stated, in many prior systems the transformer is located outside the room in which the load is positioned. Filters and surge suppressors, when used, have often previously been located between five and 50 feet from the output line. By having the filter suppression system in the same cabinet as the transformer, represented by enclosure cabinet  22  in FIG. 1 (and thereby all of the system elements, including breakers and surge suppressors, to name a few), the components can each be located with respect to one another at minimum distances for optimum filtering performance. As stated before, preferably those distances are minimized (as close to zero as practically possible). 
     Output, or load generated disturbances are also attenuated by the low power conditioner impedance and output filters. This helps decouple various connected loads. The low impedance neutral-ground bond eliminates incoming ground noise and attenuates load generated common mode noise. 
     The embodiment of FIG. 5 has added features which may be incorporated into the circuits of FIG. 2 or FIG.  4 . This embodiment adds additional R-C filter circuitry in cascade fashion to the transformer output windings to make the output scalable as may be required to optimize filter performance and cost. While two dual leg resistor-capacitor combinations are shown, additional filter modules could be added in the output windings of the transformer to improve performance further, as required. The structures and functions are the same as previously described. 
     Another set of features which may be incorporated into the invention of FIGS. 2,  4  or  5  is shown in FIG.  6 . Connected across each resistor  70 ,  72  in filter  56  is anti-parallel diode combination  91 ,  92 . These serve to further enhance the surge suppression and filtering effectiveness of the transformer output winding filter circuitry. 
     The discussion of the invention and its features to this point has considered a wye secondary transformer configuration. The invention works as well with a delta secondary configuration, as shown in FIG.  7 . All the previously discussed advantages, features and considerations apply to this transformer type so detailed discussion of FIG. 7 is not necessary. Filters  57  are connected between secondary coils  96  and phase-to-phase connections  97  of the secondaries, following the pattern of the FIGS. 2 and 4 embodiments. 
     That the transformer primary is not limited to the delta configuration of the previous figures and may also have a wye configuration is depicted in FIGS. 8 and 9. FIG. 8 shows the example of each primary winding  101  being a single segment with a suppressor device  102  connected across it. The wye configuration of FIG. 9 shows split primaries as discussed previously, each primary winding segment having a suppression device  105 ,  106  connected across it. Building neutral is represented by line  108  in each of these two figures. 
     A transformer output side is shown in FIG. 10, with the start of each secondary coil being connected to the load through the R-C filter. In this case the start  107  of each secondary coil is connected directly to system ground  60 . This is an alternative arrangement of the secondary coil connections. 
     An alternative arrangement of the primary windings and their connections to the input power lines is shown in FIG.  11 . Here input phase conductors  41 ,  42  and  43  connect to the winding taps. Suppressor elements  111 ,  112  and  113  are connected across the ends of primary windings  114 ,  115  and  116 , respectively. This is a delta configuration. 
     Another alternative arrangement of the primary windings in a delta configuration is shown in FIG.  12 . The windings are dual coils  121 ,  122 , each with suppressor elements  123  and  124 , respectively, connected across the coil segment ends. The primary coils are wired in series and, as in FIG. 11, the input voltage  41 ,  42 ,  43  is applied to the coil taps. Only one coil pair is specifically described but the others are connected the same way. 
     Still another arrangement of the input voltage and the primary coils is shown in FIG.  13 . Input line  41  is connected to the taps of coils  126 ,  127 , each of which has a suppressor element  128 ,  129  connected across the coil ends. The other two primary dual coils are similarly connected. In this embodiment the segments of each split primary are connected in parallel. This is also a delta configuration. 
     FIG. 14 shows a wye configuration of the primary windings. Suppressor  132  is connected across the ends of winding  131 , while input voltage  42  is connected to a tap of the winding. The same is true of the other windings shown. Neutral is designated by reference numeral  51 . 
     Another wye configuration is shown in FIG.  15 . Suppressor elements  133 ,  134  are connected across the ends of coil segments  135 ,  136 . Neutral line  51  is connected to common node  137 . The primary coils in each split pair are connected in series. 
     FIG. 16 shows still another wye configuration of the primary windings. This time the split primary segments are connected in parallel. The input voltage line  41  is connected to the taps of coil segments  141 ,  142 , each of which has a suppressor element  143 ,  144  across the ends. Neutral  51  is connected the same as before. 
     FIG. 17A depicts a single-phase transformer  200  incorporating both the suppressor and filter features of the present invention. The transformer  200  has a primary  202 , a secondary  204 , a suppressor element  206  and the filter  55 . FIG. 17A also depicts a load  210  coupled to the output of the filter  55 . As discussed above, additional output filter can be cascaded to add additional protection. The suppressor element  206  and the filter  55  are of any types described above. 
     FIG. 17B similarly depicts a single phase transformer  220  having a primary  222  and a secondary  224 . The primary and secondary each have first and second coils connected in a conventional fashion. However, as with previous embodiments in a three-phase transformer, the coils each have a suppressor element  206   a ,  206   b  coupled across the start and finish of each of the coils, regardless of whether the source is coupled to a tap of the transformer. As in FIG. 17A, the secondary coils each has a filter element  55   a ,  55   b  coupled to each coil at its output start and finish leads with the common neutral  226  connected at the output of the filters  55   a ,  55   b , similar to FIG.  17 A. FIG. 17B also depicts a load  228 . The various configurations illustrated for the three-phase transformers also apply to the single phase transformer. For example, transformers could be connected in parallel or series as described above, taps could be changed, filters could be cascaded, only suppressors or only filters could be provided. Accordingly, FIG. 17C depicts a single-phase transformer with no suppressor and FIG. 17D depicts a single phase transformer with no filter. 
     The invention can work equally well in a system with any number of phases. 
     While the present invention has been illustrated and described by means of a specific embodiment and additional features, numerous changes and modifications can be made to the embodiment and features shown without departing from the spirit and scope of the invention. The invention is defined by the claims and equivalents.