Abstract:
A photovoltaic (PV) inverter system operates continuously in a buck converter mode to generate a sum of full wave rectified sine wave currents at a current node common to a plurality of buck converters in response to a plurality of full wave rectified sine wave currents generated via the plurality of buck converters. The PV inverter system increases the level of the voltage sourcing each buck converter when a corresponding DC power source voltage is lower than the instantaneous voltage of a utility grid connected to the PV inverter system.

Description:
BACKGROUND 
     The invention relates generally to electronic power conversion and more particularly to a very high conversion efficiency, grid connected, single phase, multi-source photovoltaic (PV) inverter. 
     Photovoltaic (PV) cells generate direct current (DC) power with the level of DC current being dependent on solar irradiation and the level of DC voltage dependent on temperature. When alternating current (AC) power is desired, an inverter is used to convert the DC energy into AC energy. Typical PV inverters employ two stages for power processing with the first stage configured for providing a constant DC voltage and the second stage configured for converting the constant DC voltage to AC current. Often, the first stage includes a boost converter, and the second stage includes a single-phase or three-phase inverter system. The efficiency of the two-stage inverter is an important parameter affecting PV system performance and is a multiple of the individual stage efficiencies with each stage typically causing one-half of the system losses. 
     Single phase photovoltaic inverters generally require a two-stage conversion power circuit to convert the varying DC voltage of a PV array to the fixed frequency AC voltage of the grid. Traditional PV inverters use a DC link as the intermediate energy storage step, which means that the converter first converts the unstable PV array voltage to a stable DC voltage and subsequently converts the stable voltage into a current that can be injected into the grid. 
     Traditional single phase PV inverters also undesirably control the power circuits with a fixed switching frequency using a plurality i.e. five, of switching devices that contribute to the overall switching losses. Switching losses are typically kept as low as possible when using traditional PV inverters by keeping the switching frequency low. 
     It would be both advantageous and beneficial to provide a residential photovoltaic inverter that employs fewer high frequency switching devices than that employed by a traditional PV inverter. It would be further advantageous if the PV inverter could employ adaptive digital control techniques to ensure the PV inverter is always operating at peak efficiency, even when drawing power from multiple input sources, including without limitation, PV arrays, batteries and fuel cells. 
     BRIEF DESCRIPTION 
     Briefly, in accordance with one embodiment, a photovoltaic (PV) inverter comprises:
         a bucking converter configured to generate a full wave rectified sine wave current at a current summing node common to a plurality of bucking converters in response to an available PV array power and a utility grid instantaneous voltage;   a boosting circuit configured to increase the level of the voltage sourcing the bucking converter when the PV array output voltage is lower than the instantaneous voltage of the utility grid; and   a current unfolding circuit comprising switching devices configured to switch synchronously with the utility grid so as to construct an AC current in response to the full wave rectified sine wave current.       

     According to another embodiment, a photovoltaic (PV) inverter system comprises:
         a plurality of bucking circuits, each bucking circuit associated with a corresponding DC power source; and   a single full bridge current unfolding circuit,   wherein each bucking circuit is configured to continuously operate in a buck mode to generate a corresponding full wave rectified sine wave current, and further wherein the plurality of bucking circuits are configured together to generate a resultant full wave rectified sine wave at a single common current node by summing the plurality of full wave rectified sine wave currents generated by the plurality of bucking circuits,   and further wherein the single full bridge current unfolding circuit generates a desired utility grid AC current in response to the resultant full wave rectified sine wave current.       

     According to yet another embodiment of the invention, a photovoltaic (PV) inverter system is configured to continuously operate in a buck converter mode to generate a sum of full wave rectified sine wave currents at a current node common to a plurality of buck converters in response to a plurality of full wave rectified sine wave currents generated via the plurality of buck converters, and is further configured to increase the level of the voltage sourcing each buck converter, via a corresponding boost converter, when a corresponding DC power source voltage is lower than the instantaneous voltage of a utility grid connected to the PV inverter system. 
     According to still another embodiment of the invention, a photovoltaic (PV) inverter system comprises:
         a plurality of soft switching bucking circuits, each soft switching bucking circuit associated with a corresponding DC power source and configured to substantially cancel a ripple current associated with a corresponding waveshaping inductor;   a boost converter corresponding to each soft switching bucking circuit, each boost converter configured such that it is not continuously boosting over the entire PV inverter switching cycle thereby minimizing the impact on PV inverter system efficiency for having to boost the corresponding DC power source voltage; and   a single full bridge current unfolding circuit configured to operate at near zero current and voltage levels during its switching period,   wherein each soft switching bucking circuit is configured to continuously operate in a buck mode to generate a corresponding full wave rectified sine wave current, and further wherein the plurality of soft switching bucking circuits are configured together to generate a resultant full wave rectified sine wave at a single common current node by summing the plurality of full wave rectified sine wave currents generated by the plurality of soft switching bucking circuits, and further wherein the single full bridge current unfolding circuit generates a desired utility grid AC current.       

    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is illustrates a photovoltaic inverter topology that is known in the art; 
         FIG. 2  illustrates a photovoltaic inverter topology according to one embodiment of the invention; 
         FIG. 3  is a graph illustrating a buck-boost switching scheme according to one embodiment of the invention; 
         FIG. 4  illustrates a photovoltaic inverter topology including a ripple current cancellation circuit according to still another embodiment of the invention; and 
         FIG. 5  illustrates a multiple source input photovoltaic inverter topology according to one embodiment of the invention. 
     
    
    
     While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
     DETAILED DESCRIPTION 
       FIG. 1  is illustrates a photovoltaic inverter  10  topology that is known in the art. Photovoltaic inverter  10  employs a two-stage power circuit to convert a varying DC voltage of a PV array  12  to a fixed frequency AC current for a power grid  14 . Photovoltaic inverter  10  uses a DC link capacitor  16  to implement the intermediate energy storage step. This means the PV inverter  10  first converts the unstable PV DC voltage  18  to a stable DC voltage  20  that is greater than the grid voltage via a boost converter, and subsequently converts the stable DC voltage  20  to a current  22  via a PWM circuit  24  that can then be injected into the grid  14 . Photovoltaic inverter  10  topology employs five switching devices  44 ,  46 ,  48 ,  50 ,  52  that are all switching at a high frequency and that undesirably contribute to the overall switching losses of the two-stage converter. 
       FIG. 2  illustrates a photovoltaic inverter  30  hard-switching topology according to one embodiment of the invention. Photovoltaic inverter  30  topology overcomes the necessity to employ a DC link to implement an intermediate energy storage step such as described above with reference to  FIG. 1 , because PV inverter  30  topology converts the PV array  12  voltage immediately into a current  32  that is the equivalent of a rectified grid current. This feature is implemented by stiffening each leg of the PV array  12  with a large capacitance  34 , effectively shifting the DC link to the PV array  12  thereby stabilizing the PV array output voltage during generation of the rectified grid current. 
     The subsequent inverter stage  36  merely needs to unfold the current  32  into the grid  14 , and does so without switching losses because the inverter stage switching devices  54 ,  56 ,  58 ,  60  switch only at the utility grid  14  zero voltage level and with zero current. The first stage  40  is thus the only stage that has switching losses from high frequency buck switching device  62  and high frequency boost switching device  64 , verses the traditional converter  10  that has five high frequency switching devices  44 ,  46 ,  48 ,  50 ,  52  such as depicted in  FIG. 1  that all contribute to the switching losses. 
     With continued reference to  FIG. 2 , photovoltaic inverter  30  includes a bucking circuit switch  62  that works in combination with a diode  66  and a wave shaping inductor  68 . PV inverter  30  also includes a boosting circuit switch  64  that works in combination with a diode  70  and a boost inductor  72 . 
     The bucking circuit comprising switch  62 , diode  66  and inductor  68  is operational at all times; while the boosting circuit comprising switch  64 , diode  70  and inductor  72  is operational only when the output voltage of the PV array  12  is lower than the instantaneous voltage of the utility grid  14 . The boosting circuit then pumps additional current from the PV array  12  stored in boosting inductor  72  into storage capacitor  74  whenever the output voltage of the PV array  12  is lower than the utility grid instantaneous voltage. The resultant combined voltage across capacitor  34  and capacitor  74  provides the voltage necessary to operate the bucking circuit that remains operational during the boost operating mode. 
     The above described buck and boost functions occur dynamically as depicted for one embodiment in  FIG. 3 . With reference now to  FIG. 3 , the boost function occurs whenever the utility grid instantaneous voltage  76  exceeds a desired PV array  12  output voltage of 220 volts. The voltage  80  provided by the boost capacitor  74  is added to the buck voltage  78  to allow proper bucking when the utility grid instantaneous voltage  76  exceeds 220 volts. This advantageously results in a boost converter that is not continuously boosting over the entire cycle thereby minimizing the impact on efficiency for having to boost the PV array voltage. 
     Traditional inverters such as described above with reference to  FIG. 1 , control the power circuits with a fixed switching frequency. The present inventors recognized that when the conversion efficiencies are very high, improvements can be gained by use of adaptive digital control techniques. An adaptive digital controller can thus be employed that adjusts the switching frequency to compensate for changes in the semiconductor devices  62 ,  64  and inductor  68 ,  72  performance for various operating conditions and temperatures so that the highest possible conversion efficiency is obtained. 
     Adaptive digital control techniques can include a boost circuit switch  64  control signal that is linked to the buck circuit switch  62  so that under certain conditions, boost switch  64  turn-on can be delayed, but such that boost switch  64  turn off can be delayed with respect to the buck switch  62  turn-off such that only one switch carries all of the losses while the other switch carries no losses. 
     In summary explanation, a photovoltaic inverter  30  topology advantageously functions with a significant reduction in the number of power electronic devices that will be switching at a high frequency at any point in time. This feature provides an additional benefit that is a result of lower conduction losses associated with slower devices that can be selected to complete the inverter system. 
     The photovoltaic array source  12  is stiffened via a large capacitance  34  such as described above to ensure a stable supply voltage source is provided for the bucking circuit. This large capacitance  34  advantageously does not compromise the safety aspects of the system as the PV source  12  is current limited. 
     Attached to the capacitor  34  is the first stage buck converter  40  that creates a full wave rectified sine current in the main inductor  68 . This current is then unfolded into the grid  14  by the full bridge inverter  36  connected to the output of the PV inverter  30 . 
     The PV inverter  30  topology was found to provide suitable working results so long as the PV source voltage remains higher than the instantaneous grid voltage. In cases where the PV source  12  voltage is less than the instantaneous grid  14  voltage, the operation of the PV inverter  30  is configured to ensure that the current in the bucking inductor  68  always flows from the PV source  12  to the grid  14 . This is achieved by turning on the boosting circuit to increase the input voltage to the bucking circuit to a value greater than the instantaneous grid voltage. 
     Because current boosting is employed only when necessary, i.e. when the PV array  12  voltage is lower than the instantaneous grid  14  voltage, inverter switching efficiency is increased above that achievable when compared to a conventional PV converter topology such as described above with reference to  FIG. 1 . 
     Photovoltaic inverter  30  can be just as easily implemented using a soft-switching topology according to another embodiment of the invention. The use of a soft switching topology allows slower devices having lower conduction losses to be selected for use in the buck converter portion of the PV inverter. PV inverter  30  utilizes a topology well suited to use of adaptive digital control methods, as stated above, for seeking the most efficient operating point for the system based on operating conditions such as, without limitation, temperature, input voltage and load power level. 
     Looking now at  FIG. 4 , a PV inverter  80  includes a ripple current cancellation circuit  82  that provides a means for reducing the main inductor  68  size without compromising the output ripple current requirements of the system. Ripple current cancellation circuit  82  allows use of a smaller inductor  68  having lower losses than that achievable with a larger inductor, and also allows the use of quasi resonant switching, significantly reducing switching losses. 
       FIG. 5  illustrates a multiple source input photovoltaic inverter topology  100  according to one embodiment of the invention. PV inverter topology  100  includes an output unfolding circuit  102  that functions solely to unfold a rectified current waveform produced by summing the rectified currents generated via a plurality of power sources including a first PV array  104 , a second PV array  106 , a battery bank  108  and a fuel cell  110 . As such, the unfolding circuit  102  never switches at high frequencies since the unfolding circuit switching devices  112 ,  114 ,  116 ,  118  only switch at twice the utility grid frequency. All of the power source currents  120 ,  122 ,  124 ,  126  are referenced to one common voltage to achieve the desired current summing function. 
     Each of the wave shaping inductors  128 ,  130 ,  132 ,  134  performs only a current wave shaping function, and therefore does not perform any type of current boosting function such as seen with known buck/boost converter designs. The present invention is not so limited however, and any number of many different types of power sources can be employed in similar fashion to implement a multiple source input PV inverter topology in accordance with the principles described herein. 
     Advantages provided by the PV inverter topologies  30 ,  100  include without limitation, buck and boost capabilities incorporated into a single DC to AC converter using a dual capacitor bank without employment of a conventional buck/boost topology. Other advantages include, without limitation, maximization of efficiency by minimizing the number of power semiconductor devices switched between the source and the load, the use of multiple technologies within a single PV inverter to enhance the high efficiency topology, such as the ripple current cancellation capabilities described above with reference to  FIG. 4 , the use of soft-switching techniques, a topology that is well suited to use of adaptive digital control methods for seeking the most efficient operating point for the system based on operating conditions such as, without limitation, temperature, input voltage and load power level, and optional step activation of the AC contactor, wherein after the contactor/relay is energized, the holding current is reduced just enough to maintain the holding state. 
     Maximizing efficiency by minimizing the number of series power semiconductors switched between the source and the load, and the selection of power semiconductors to achieve maximum efficiency provide further advantages over known PV inverters. 
     In summary explanation, the embodiments described above present a very high conversion efficiency grid connected residential photovoltaic inverter that can be used with multiple PV arrays and/or alternate energy sources. The inverter creates a sinusoidal current that is proportional to the grid voltage and exhibits a high power factor. The high efficiency is achieved by having only one power semiconductor device that will be switching at a high frequency, while all other devices will be switched at the grid frequency. The main switching device of the buck converter creates a full wave rectified sine wave current that is unfolded into the grid supply. This unfolding circuit also avoids switching losses as both the current and the voltage will be near zero across the devices during the switching period. For PV array voltages greater than the grid voltage, the circuit operates solely as a buck converter. For PV array voltages lower than the grid voltage, the boost circuit operates in synchronism with the buck converter. The configuration ensures that the buck converter voltage is always greater than the grid voltage. The output inverter full bridge is never switched at high frequencies. Further efficiency improvements can be realized by special selection of the switching devices, by digital control adjustments that ensure that the inverter is always operating at peak efficiency such as compensating the switching frequency for changes in input voltage, load current and system temperature, by implementation of the ripple current cancellation circuit so that a small lower loss inductor can be selected for the main circuit, and by adding a quasi resonant circuit across the main switching device that ensures soft switching, as described above. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.