Abstract:
Diodes and switching means as well as an autotransformer connect multiple AC sources of variable frequency and voltage to the utility grid, without first rectifying and thereby transferring the AC input energy to a common DC bus. Instead, the individual positive and negative AC input half waves or parts of those half waves, if and when they occur during a particular time interval, are injected into the positive or negative half wave of the utility AC power directly and respectively. The amount of energy transferred may be controlled by a microprocessor. Assuming the prime mover that generates AC voltages in multiple coils is a windmill, then the required circuitry to accomplish this energy transfer will be physically small enough to allow that circuitry to reside in the windmill itself, and thereby this energy conversion method significantly simplifies the installation of a windmill that uses this energy conversion method.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to transferring electric power generated by multiple sources of AC power to a utility grid without first rectifying and thereby first transferring the electric power of all AC inputs into a common two wire DC bus. 
     2. Detailed Background of the Invention 
     Two patents, U.S. Pat. Nos. 8,049,351 and 8,178,987, for horizontal axis windmill designs, by Mahawili describe an electrical generating structure with two major components that are shown in  FIG. 9 . First, there are rotating permanent magnets attached to the periphery of a windmill rotor and second there is a stator constructed from a multiplicity of coil pairs, which are located in a stationary ring structure surrounding the periphery of the rotor of the windmill. (The word “stationary” means stationary with respect to the rotor, the windmill itself still turns around its vertical axis for the purpose of facing into the wind.) 
     As shown in a cross-sectional view in  FIG. 10 , a pair of coils  97  in the stator form a U-shaped channel within which the permanent magnets  98  move. When viewing  FIG. 10  the reader should assume that the movement of the magnet is perpendicular to the plane of the paper, i.e. the movement of the magnet is either into or out of the paper. The movement of these magnets generate voltages in the coil pairs by Faraday&#39;s law, which thus convert the mechanical energy of the windmill into electrical energy. 
     This windmill design has been implemented by the firm Windtronics and the design consists of 6 permanent magnets rotating within a U-shaped channel of 68 coil pairs. The fact that the number of permanent magnets is much smaller than the number of coil pairs in the stator leads to an unusual property. As a permanent magnet flies by a coil pair, that coil pair generates one complete AC voltage wave (or cycle) consisting of one positive half wave and one negative half wave. Another AC voltage wave is not generated in a particular coil pair until the next permanent magnet flies by. So each coil pair generates an AC voltage wave only some of the time, but looking at all coil pairs together, there are always some coil pairs that are generating AC voltage waves at any moment in time. Also, the fact that the number of stator coil pairs is not divisible by 3, leads to the property that the voltage waves generated by one permanent magnet pair has a three phase relationship with the voltage waves generated by the other two permanent magnet pairs. It should be mentioned that the six permanent magnets are equally distributed around the periphery of the rotor as shown in  FIG. 9 ; therefore, the angular distance between adjacent magnets is 60 degrees. A permanent magnet pair is defined as two magnets that are located at opposite sides of the periphery. 
     For the current implementation by Windtronics, in order for the electric power generated by this windmill to be transferred to the utility grid, the AC voltage waves generated by each of the 68 coil pairs are rectified into DC by 68 separate full wave rectifiers consisting of 4 diodes each physically located next to each coil pair. The DC output of each of these full wave rectifiers is connected to a two wire DC bus also located around the periphery, which is connected through slip rings and cabling to a conventional commercially available DC to AC inverter for connection to the utility grid, which is typically located in a building located near the windmill. 
     It is the object of this invention to improve on the circuits that transfer the AC power generated by the windmill, such that the rectifying step, which currently transfers all AC power through full wave rectifiers to a common DC bus, is eliminated and all input AC half waves are separately and directly injected into the utility grid. The elimination of the rectifying step into a common DC bus increases the efficiency of the conversion process by eliminating some of the diodes that would otherwise be used and therefore eliminates the losses that these diodes would incur. It is an additional object of this invention to make this converter small enough, so that all the required components fit into the windmill itself, such that the windmill can be connected directly to the utility grid; which is a desirable feature, because all the work and expense of providing a separate inverter will be eliminated. 
     3. Description of the Prior Art 
     U.S. Pat. No. 6,122,182 by Mihail Moisin describes an AC to AC scheme for powering gas discharge lamps. The cited patents: U.S. Pat. No. 4,468,725 by Colin Schauder, U.S. Pat. No. 4,896,091 by Andre Kislovski, U.S. Pat. No. 5,005,115 by Colin Schauder and U.S. Pat. No. 5,010,471 by J. Ben Klaassens et. al. are AC to AC converter systems of various kinds; but none of them deal with the task of injecting variable frequency alternating current pulses from a multiplicity of sources into the constant frequency utility grid. 
     This invention is also related to U.S. Pat. No. 5,892,664 by this inventor (Dietrich Vedder), which also deals with the task of transferring variable frequency AC power into a utility grid, but uses complete rectification of the input AC power. 
     SUMMARY OF THE INVENTION 
     The present invention is an AC to AC converter that transfers electrical energy from multiple AC power sources of variable voltage and frequency to a provider of AC power of constant voltage and constant frequency for co-generation purposes. The provider of electric power may be an electric utility or a stand-alone producer of electric power that produces substantially larger amounts of power than is co-generated. 
     The multiple sources of AC electric power are connected via switching means to one of several taps of an autotransformer that is directly connected to the utility power source of constant voltage and frequency. Now assume that the maximum output voltage of the input AC is higher than the maximum voltage present at a particular tap of the autotransformer; then the switching means will connect those AC sources to that particular tap of the autotransformer, such that current will flow directly into the utility grid. The switching means are controlled, such that a current path is only open when the voltage wave of the utility power source is either in the middle of the positive voltage wave for the purpose of accepting positive current pulses from the AC power sources, or in the middle of the negative voltage wave for the purpose of accepting negative current pulses from the AC power sources. The middle of the positive or negative voltage wave of utility power should be understood to mean about 60 to 120 electrical degrees in the middle of the 180 degree width of the positive or negative half wave. The frequency of the AC power sources may be less than the frequency of the utility power, equal to the frequency of utility power or greater or significantly greater than the frequency of utility power. A positive half wave or part of a positive half wave of any AC input source will only conduct current into the utility grid, if it lines up with the middle of the positive voltage half wave of utility power. A negative half wave or part of a negative half wave of any AC input source will only conduct current into the utility grid, if it lines up with the middle of the negative voltage half wave of utility power. For this reason a little less than half of any of the voltage half waves generated by the multiplicity of AC input sources will cause current to flow into the utility grid. However, there will always be current flow into each utility AC half wave, because there always will be some input AC half waves that will line up with them. 
     When comparing the method disclosed by this invention against the conventional method of injecting the energy generated by the windmill into the utility grid using a common DC bus and a DC to AC inverter, then one realizes that one of the main functions of the inverter that chops the DC into AC for the purpose of injecting those newly created AC pulses into the utility grid has been replaced in this invention by the naturally produced AC pulses produced by the windmill generation system itself; thus a considerable simplification of the energy transfer apparatus from windmill to utility grid has been achieved. 
     The switching means are controlled using a microprocessor, such that the power transferred into the utility grid is optimized. The optimization of transferred power is done by the well-known method of periodically testing whether the optimum power transfer point is above or below the current operating point. If the current operating point is not at the optimum point, then the control mechanism moves the operating point towards the optimum point. 
     The electric power transfer is controlled, i.e. is increased or decreased, by using two control methods as appropriate. One control method increases or decreases the width of each conduction period within each utility half wave; the other control method switches between different taps of the autotransformer, such that the operating voltage is either increased or decreased. For example, if the power generating windmill increases its rotational speed, then the AC input voltages will increase and the conductive path may need to be switched to an autotransformer tap with a higher voltage. Decreasing wind speed may lead to switching to a lower voltage tap of the autotransformer. 
     Injecting current pulses into the utility grid in the discussed manner generates some harmonics that need to be filtered out. This is done with a capacitor and perhaps an inductor at the utility side of the autotransformer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of one embodiment of the invention. Three pairs of Insulated Gate Bipolar Transistors (IGBTs) are shown; each pair being responsible for switching currents at a particular voltage level. One IGBT in a pair is responsible for positive half wave switching and the other IGBT is responsible for negative half wave switching. Diodes are used to isolate the IGBTs from each other, the diodes are also used to isolate the voltage generation coils from one another. No control circuits for the IGBTs are shown in this figure. 
         FIG. 2  gives a schematic of the control structure showing the inputs used. On the basis of these inputs, control decisions are made. In addition, six control outputs are provided, one each for the six IGBTs shown in  FIG. 1 . 
         FIG. 3  shows a drive circuit for an individual IGBT that ensures voltage isolation for that IGBT due to employment of an optical coupler. 
         FIG. 4  shows the relationship between the input AC voltage waves and the output (utility) voltage wave. The cross hatched areas show when conduction of current takes place. Notice that the input AC voltage waves are shown to reflect a three phase relationship among one another. 
         FIG. 5  shows another embodiment of the invention, which applies to the case where the frequency of the input AC is considerably higher than the frequency of the utility power. As a result Silicon Controlled Rectifiers (SCRs) are used to do the switching. 
         FIG. 6  shows an optically coupled transistor to drive each SCR shown in  FIG. 5 . 
         FIG. 7  shows the relationship between the input AC voltage waves and the output (utility) voltage wave for the embodiment shown in  FIG. 5 . Again, the cross hatched areas show when conduction takes place. The SCR trigger interval is not centered in the middle of the utility voltage wave, it is moved to the front of the voltage wave to avoid causing short circuits at the tail end of the utility voltage wave. Again, the input AC voltage waves are shown to reflect a three phase relationship among one another. 
         FIG. 8  shows a third embodiment of the invention that also uses SCRs to do the switching, but in this embodiment all the diodes of the previous embodiments are replaced by SCRs, which leads to the reduction of one diode in each current path, and therefore, this embodiment does not incur the losses of those diodes. 
         FIG. 9  illustrates prior art, it displays the generator structure that was discussed in the background material and how that structure relates to the windmill rotor. There are two stationary rings of coils  97 , which form a U-shaped channel within which permanent magnets  98  move. The permanent magnets are attached to the periphery of the windmill rotor. 
         FIG. 10  also illustrates prior art, it shows a more detailed cross-section of the U-shaped channel. The coils  97  are attached to mounting base  100 , and the magnets  98  are attached to magnet holders  99 , which are themselves attached to the rim  101  of the windmill rotor. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Description of the First Embodiment 
       FIG. 1  shows one embodiment of an electrical energy transfer circuit in accordance with the present invention. Three voltage sources labeled  1 ,  2  and  3  are shown and represent a number of voltage sources that may reach as high as 100 or more. The prime mover that generates a voltage in those voltage sources may be a windmill or other prime mover and is not shown. One end of each coil (voltage source)  1 ,  2  and  3  is connected to ground  33  and to the autotransformer  39 . The other end of coil  1  is connected to the six diodes  4 ,  5 ,  6 ,  7 ,  8  and  9 . Each of those diodes is connected to its own switching IGBT as follows: Diode  4  connects with IGBT  22 , diode  5  connects with IGBT  23 , diode  6  connects with IGBT  24 , diode  7  connects with IGBT  25 , diode  8  connects with IGBT  26  and diode  9  connects with IGBT  27  as shown. The six IGBTs are divided into three pairs. IGBT pair  22  and  23  is connected to and supports the lowest voltage tap  31  of the autotransformer, IGBT pair  24  and  25  is connected to and supports the middle voltage tap  30  of the autotransformer and IGBT pair  26  and  27  is connected to and supports the high voltage tap  29  of the autotransformer. Within each pair of IGBTs, one of them supports the positive voltage half wave and the other one supports the negative voltage half wave of utility power. So IGBTs  23 ,  25  and  27  with their connected diodes support the positive voltage half wave of utility power and IGBTs  22 ,  24  and  26  with their connected diodes support the negative voltage half wave of utility power. As just described, the six diodes  10 ,  11 ,  12 ,  13 ,  14  and  15  support coil  2  and diodes  16 ,  17 ,  18 ,  19 ,  20  and  21  support coil  3 . All IGBTs are protected, when turning off, against inductive over-voltages by a resistor and a capacitor labeled  28 . A current transformer  41  that is center-tapped delivers pulses that indicate the size of the current flowing to or from the autotransformer and the utility connection. The magnitude of these current pulses indicates the amount of energy transferred into the utility grid. 
       FIG. 2  illustrates the control structure for the circuit in  FIG. 1 . This structure consists of a microprocessor and memory  55  with input interface  42  and an output interface  43 . Five separate measurement interfaces (inputs) are provided. Interface  44  will be connected to point  29  on the autotransformer to find the zero crossings of the utility voltage wave, which establishes the timing of the switching operation of the circuit. Interface  45  will be connected to point  40  of coil  1 ; through this interface the timing and frequency of the input voltage wave forms are established. Interface  46  is also connected to point  40  of coil  1  for the purpose of measuring the magnitude of the generated voltage in coil  1 . Interface  47  connects with point  36  of the current transformer to measure the size of the positive current half wave and interface  48  connects with point  37  to measure the size of the negative current half wave. The control outputs  49  and  50  control the operation of the high voltage IGBTs  27  and  26  respectively; control output  49  is for the positive voltage half wave and control output  50  is for the negative voltage half wave. Similarly, control outputs  51  and  52  control the medium voltage IGBTs and control outputs  53  and  54  control the low voltage IGBTs. 
     The control outputs do not connect directly with each IGBT; an optical coupler circuit shown in  FIG. 3  ensures that each IGBT is electrically isolated from the control circuits. There is an optical coupler circuit for each IGBT. This circuit consists of an optical coupler  56  with input  59 . This input  59  connects with one of the control outputs shown in  FIG. 2 . The optical coupler has its own power supply  57  and provides control output  58  for its assigned IGBT. 
       FIG. 4  shows the voltage waveforms in relation to each other. Waveform  60  represents the voltage waveform provided by the utility. Voltage waveforms  61  represent the input voltage waveforms, all three phases of the input voltage waveforms are shown. The conduction interval  62  is positioned in the center of the utility voltage waveform on both the positive and the negative side. Conduction, and therefore transfer of electrical energy, only occurs in that interval. The cross hatched areas  63  show the times when conduction takes place. The actual current flow is limited by the inductance and the resistance of the voltage generating coils themselves. 
     Operation of the First Embodiment 
     When the circuits of the first embodiment are turned on by connecting the autotransformer  39  to the AC power provided by the utility, then the first thing the program in the microprocessor  55  ( FIG. 2 ) does is to make a number of measurements through terminal  44  ( FIG. 2 ) of the zero crossings of the alternating voltage provided by the utility. By establishing the time differences between those zero crossing measurements, the program establishes whether it is dealing with 60 cycle power, which is associated with a zero crossing interval of 8.333 milliseconds, or with 50 cycle power, which is associated with a zero crossing interval of 10 milliseconds or with electric power using some other frequency. In addition to repeatedly measuring the zero crossing intervals, it compares those multiple measurements to make certain that the frequency provided by the utility is stable, i.e., the zero crossing intervals are identical in duration. The program then tests the voltage of the representative sample of the input ac voltage through terminal  46 , it also checks that the measurements of the current (through terminals  47  &amp;  48 ), into or out of the utility grid are still zero, because no switching has yet been done to connect the input to the utility power connection. This checking of measured values is continued for perhaps 10 or 20 seconds to ensure that all the transient events that occur as a result of turning on the utility power have settled down. Then the program in the microprocessor compares the voltage of the input sample through terminal  46  and compares it against the voltage of all three of the autotransformer taps. (The utility voltages are preset in the program.) It will then select the autotransformer tap, the voltage of which is a few volts below the voltage value of the input. 
     The program is now ready to transfer energy to the utility. Assuming that the low voltage tap  31  ( FIG. 1 ) of the autotransformer was selected, it will now work with the IGBTs  22  and  23  that are associated with that autotransformer tap, by turning on IGBT  23  during the peak period of the positive half wave and by turning on IGBT  22  during the peak period of the negative half wave. The initial period when the respective IGBTs are on will be something like 4 milliseconds for the 60 cycle case, centered at the peak of the half wave. Current measurements are then made during each conduction cycle, i.e. for both the positive and the negative half wave. Input voltage measurements are made as well. If the current and voltage measurements increase for successive half waves, then the width of the conduction cycle will be increased to allow more current to flow. If the current and input voltage measurements decrease, then the conduction cycle is narrowed, because not enough energy is produced by the windmill to sustain the input voltage. 
     Switching between individual taps of the autotransformer is initiated by the program in the microprocessor  55  ( FIG. 2 ), if one of two events happens. Assume first that we started with the middle voltage tap  30  of the autotransformer. If the input voltage increases to the point where it exceeds the value of the voltage of the high voltage tap  29 , then the program, will simply switch for subsequent half cycles to the two IGBTs  26  &amp;  27  that are associated with the high voltage tap, and after making that change, the algorithm followed is the same as explained above. If the input voltage decreases to the point where it is no longer larger by several volts than the voltage of the middle voltage tap of the autotransformer, then the program will switch to the two IGBTs that connect to the low voltage tap  31 . Once that switch is made for subsequent half waves, then the algorithm will make the conduction interval for each half wave wider or narrower as the measurements suggest and as previously explained. 
     There are two cases left to discuss: What happens when the input voltage falls below the value of the low voltage tap  31  of the autotransformer? In that case, no switching during the conduction interval will take place and the program will continue to take measurements until the input voltage increases, so that energy transfer can again take place. What happens when the input voltage exceeds some voltage boundary value well above the voltage of the high voltage tap  29  of the autotransformer? At that point the program will have to shut the windmill down, because we must have an extremely high wind condition and the output of the windmill needs to be connected to energy dump resistors provided for the task of slowing the windmill down. 
     Description of the Second Embodiment 
       FIG. 5  illustrates the second embodiment. In this embodiment it is assumed that the frequency of the AC input voltage waves are considerably higher than the frequency of the utility voltage wave. This assumption allows the replacement of the IGBTs, shown in  FIG. 1 , by SCRs. This change has the consequence of changing the treatment of the conduction interval, which will be explained in detail below when  FIG. 7  is discussed. 
       FIG. 6  shows a simple optical coupler  70  with input  71  and output  72  for driving the inputs of the SCRs. The diode  73  is needed to protect the optical coupler from the periodic negative voltages that will appear at the anode of the SCR. The resistor  74  is dimensioned to limit the trigger current into the SCR. There must be one optical coupler per SCR. The inputs of these optical couplers must be connected to one of the six control output ports  49 - 54  shown in  FIG. 2 . 
       FIG. 7  illustrates the waveform relationships for the embodiment illustrated in  FIG. 5 . Again waveform  60  is the utility waveform and waveforms  75  are the AC input waveforms. Again all three phases of the input are shown. The frequency of the input waveform is higher; therefore, the input waves are narrower than shown in  FIG. 4 . The SCR trigger intervals  76 , shown for both the positive and negative waveform, are not symmetrical with respect to the utility waveform; they extend only a little bit beyond the peak of the utility waveform. Also notice that cross hatched areas  78  extend beyond the trigger interval, because SCRs have the property of staying on until the main current through them is zero, which does not happen until the input voltage is below the utility voltage. Since the chosen devices are SCRs that cannot be turned off by a control signal, we must limit this circuit to the cases where the input voltage waveforms are narrow enough with respect to the utility waveform, such that the input voltage is guaranteed to dip below the utility voltage before the zero crossing of the utility voltage. 
     Operation of Second Embodiment 
     The operation of the second embodiment is identical to the operation of the first embodiment except for this difference. Due to the fact that Silicon Controlled Rectifiers (SCRs) cannot be turned off by the control signal, the triggering interval  76  ( FIG. 7 ) for the SCRs is asymmetrical with respect to the peak of each half wave of utility power. So, for the purpose of determining timing positions within each half wave, assume that each half wave starts at zero electrical degrees, has its peak at 90 electrical degrees and is back to zero at 180 electrical degrees. The interval during which SCR triggering takes place starts at something like 45 degrees and is pulled back to something like 30 degrees to increase current and is pushed forward to something like 50 or 60 degrees to decrease current. But the termination of the triggering interval needs to take place at about 100 degrees, so as to make sure that all input voltage waves cross the voltage wave of the currently used autotransformer tap before its zero crossing at 180 degrees. If that does not happen, then we are injecting a positive current during a negative voltage wave or injecting a negative current during a positive voltage wave, either scenario causes a potential short circuit that needs to be avoided. The exact degree value of the end of the SCR triggering interval may be varied somewhat by taking into account the width of the input pulses. If the input voltage pulses are quite narrow, compared to the utility power half wave, i.e. the input frequency is quite high, then we can push it a bit beyond 100 degrees. 
     Even though the SCR triggering interval is not symmetrical with respect to the peak of the half wave of utility power, the actual conduction interval that results from this asymmetric triggering interval is approximately symmetric with respect to the peak of the half wave of utility power as  FIG. 7  illustrates. 
     Description of the Third Embodiment 
       FIG. 8  illustrates the third embodiment of the invention. In this embodiment it is again assumed that the frequency of the AC input voltage waves are considerably higher than the frequency of the utility voltage wave. This assumption also allows the use of SCRs in a different manner than they were used in the second embodiment. Compared with the second embodiment, in the third embodiment all six diodes per coil are replaced by SCRs. Given that change, coil  1  is then associated with SCRs  79 ,  80 ,  81 ,  82 ,  83  and  84 . SCRs  79  and  80  are connected directly with the low voltage connection  31  on the autotransformer  39 , SCRs  81  and  82  are connected directly with the middle voltage connection  30  on the autotransformer and SCRs  83  and  84  are connected directly with the high voltage connection  29  on the autotransformer. Within each pair of SCRs one supports the positive voltage wave and the other supports the negative voltage wave. Coils  2  and  3  have identical sets of six SCRs associated with them as shown. 
     The optical coupler  70  shown in  FIG. 6  is also used with this embodiment. However, since the number of SCRs, and therefore the number of optical couplers, has greatly increased, each of those control output ports ( 49 - 54  in  FIG. 2 ) must enable as many optical couplers as there are coils, a control signal amplification circuit may be necessary to generate enough control signal power to drive them all. 
     The discussion of the waveforms and the triggering intervals described with the aid of  FIG. 7  also applies to this embodiment. 
     Operation of the Third Embodiment 
     The operation of the third embodiment is identical to the operation of the second embodiment except for this difference. While only a single SCR needs to be turned on per conduction interval in the second embodiment, in the third embodiment many SCRs need to receive a triggering signal for each conduction interval at the same time. Therefore, as previously mentioned, control signal amplification may need to be provided to ensure that all SCR associated optical couplers receive enough control signal power to operate properly. 
     Possible Modifications for All Three Embodiments 
     Two modifications may be made in all three embodiments. First, the autotransformer  39  may be replaced by a regular transformer, if separation of the circuit ground from the ground of utility power is desired; however, an autotransformer may be considerably smaller than a regular transformer for a given power rating, because for an autotransformer a large percentage of the energy transferred is by conduction and not by transformer action. Second, the number of taps on the autotransformer, or on a regular transformer if present, may be larger or smaller than three. In fact for ac input sources that have a voltage that only varies within a narrow range, a single tap may suffice; in that case the sole control on the amount of energy transferred will be by variation of the conductance interval. 
     Microprocessor Peripherals 
     Additional peripheral devices may be attached to the microprocessor  55  (in  FIG. 2 ) such as digital display devices to show the amount of power being transferred, or the values of measurements obtained while operating. Also, communication devices may be attached to the microprocessor for the purpose of monitoring and operating the current invention remotely. 
     It is understood that the foregoing detailed description is given merely by way of illustration and many modifications can be made therein without departing from the spirit or scope of the present invention.