Abstract:
An apparatus and method of control for converting DC (direct current) power from a solar photovoltaic source to AC (alternating current) power. A novel DC-to-AC power converter topology and a novel control method are disclosed. This combination of topology and control are very well suited for photovoltaic microinverter applications. Also, a novel variant of this control method is illustrated with a number of known photovoltaic DC-to-AC power converter topologies. The primary function of both control methods is to seek the maximum power point (MPP) of the photovoltaic source with novel, iterative, perturb and observe control algorithms. The control portion of this invention discloses two related control methods, both an improvement over prior art by having greatly improved stability, dynamic response and accuracy.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 11/161,639, filed on Aug. 10, 2005, entitled “Photovoltaic DC-to-AC Power Converter and Control Method”. 

   FIELD OF THE INVENTION 
   The present invention relates to an electrical power converter used with a solar photovoltaic source to condition and couple the DC energy of the solar photovoltaic source to the AC lines of an electric utility. 
   BACKGROUND OF THE INVENTION 
   Most of today&#39;s solar photovoltaic (PV) power sources are utility connected. About 75% of these installations are residential rooftop systems with less than 2 kW capability. These systems typically comprise a number of PV modules arranged in series configuration to supply a power converter, commonly called an inverter, which changes the direct current (DC) from the modules to alternating current (AC) to match the local electrical utility supply. 
   There is a difficulty with small solar power systems on residential rooftops. Gables and multiple roof angles make it difficult on some houses to obtain enough area having the same exposure angle to the sun for a system of 2 kW. A similar problem arises where trees or gables shadow one portion of an array, but not another. In these cases the DC output of the series string of modules is reduced to the lowest current available from any cell in the entire string. This occurs because the PV array is a constant current source unlike the electric utility, which is a constant voltage source. 
   An inverter that economically links each PV module to the utility grid can solve these problems as the current limitation will then exist only on the module that is shaded, or at a less efficient angle and does not spread to other fully illuminated modules. This arrangement can increase total array output by as much as two times for some configurations. Such a combination of a single module and a microinverter is referred to as a PV AC module. The AC output of the microinverter will be a constant-current AC source that permits additional units to be added in parallel. 
   PV AC modules now available suffer poor reliability owing to early failure of the electrolytic capacitors that are used to store the solar cell energy before it is converted to AC. The capacitor aging is a direct consequence of the high temperature inherent in rooftop installations. 
   The electrolytic capacitors in the power circuit perform two functions. First, the capacitors hold the output voltage of the PV modules close to the maximum power point (MPP) output despite variations in sunlight, temperature or power line conditions and second, the capacitors store energy at the input and even out the DC voltage variations at the power-line frequency that result from changing the DC to AC. These functions place an additional stress on the capacitor causing internal heating that adds to the effects of high external temperature. 
   The high temperature reached by PV system inverters is a natural consequence of their outdoor mounting. This requires a rainproof enclosure that complicates the heat removal process. The coincidence of maximum power throughput and losses at exactly the time of maximum heating by the sun on both the enclosure and the ambient air exacerbates the condition. 
   Existing inverter topologies have made the electrolytic capacitor an integral part of the inverter circuit because of the high capacitance value required to store energy from the PV module. If high capacitance is required, the only economic choice is the electrolytic capacitor. Plastic film capacitors are recognized as superior in aging characteristics, but are much more expensive for the same capacitance. Thus, a means to avoid use of electrolytic capacitors can contribute to the reliability of PV power sources. 
     FIG. 2  illustrates the control system for a conventional photovoltaic (PV) DC-to-AC power converter. This power converter has a pulse width modulated, voltage regulating boost stage and a pulse width modulated, current regulating buck stage. Sinusoidal reference  62  follows AC line  90  voltage and frequency. AC line current reference  63  is generated by multiplying sinusoidal reference  62  by scaling factor  66 . Actual AC line current  64  is compared to AC line current reference  63  to create error signal  65 . Error signal  65  drives buck stage  60  as part of this servo loop. Current  41  and voltage  42  of PV source  10  are sensed and multiplied to provide  43 , a measure of PV source  10  output power. Scaling factor  66  is periodically adjusted to determine the amount of energy sourced onto AC line  90 . A control means is used to periodically perturb ( 45 ) scaling factor  66  and observe ( 44 ) the effect on PV output power  43 . If an increase in scaling factor  66  results in an increase in PV power  43 , scaling factor  66  is incrementally increased every perturb cycle until an increase in scaling factor  66  results in a decrease in PV power  43 . This is how the maximum power point (MPP) of PV source  10  is established. Boost stage  40  is transparent to this perturb and observe function and serves as a typical voltage regulator to maintain the voltage at energy storage capacitor  50  at a regulated voltage higher that the peak voltage of the AC line. Fixed reference voltage  48  is compared to feedback voltage  49  creating error signal  47  to drive boost stage  40 . In some inverters designed to work with PV voltages higher than the peak AC line voltages, boost stage  40  is not required. 
   The problem with this prior art control method is instability and poor dynamic response. If current reference  63  requests a current and therefore power to be delivered into the AC line that PV source  10  cannot supply, the control loop becomes unstable, PV source  10  voltage collapses and cannot be recovered without restarting the power converter and the perturb and observe process. This prior art control method is unstable when operating on the lower-voltage side of the PV source maximum power point. The maximum power point of a photovoltaic source usually changes slowly but moving cloud cover, wind gusts and partial, momentary PV source shadowing can abruptly push the maximum power point into an unstable region for this control method. 
   SUMMARY OF THE INVENTION 
   The invention is a novel DC-to-AC power converter topology and a novel control method that makes this combination of topology and control very well suited for photovoltaic microinverter applications. Also, a novel variant of this control method is disclosed for application with a number of known photovoltaic DC-to-AC power converter topologies. The primary function of both control methods is to seek the maximum power point (MPP) of the photovoltaic source with iterative, perturb and observe algorithms. The control portion of this invention discloses two related control methods, both an improvement over prior art by virtue of having greatly improved stability, dynamic response and accuracy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the preferred embodiment of the power converter control method. 
       FIG. 2  shows the prior art power converter control method. 
       FIG. 3  shows an alternate embodiment of the power converter control method. 
       FIG. 4  shows the preferred embodiment of the power converter electrical circuit topology to be used with the control method illustrated in  FIG. 1 . 
       FIG. 5  shows a bi-polar boost, prior art inverter electrical topology to be used with the control method illustrated in  FIG. 1 . 
       FIG. 6  shows a polyphase, prior art inverter electrical topology to be used with the control method illustrated in  FIG. 3 . 
       FIG. 7  shows an H-bridge with transformer, prior art inverter electrical topology to be used with the control method illustrated in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates the preferred embodiment of the control arrangement for the DC-to-AC power converter. This power converter has a pulse width modulated, voltage regulating boost stage and a pulse width modulated, current regulating buck stage. Sinusoidal reference  62  follows the AC line voltage and frequency. AC line current reference  63  is generated by multiplying sinusoidal reference  62  by scaling factor  61 . Scaling factor  61  is a measure of the voltage on energy storage capacitor  50 . Actual AC line current  64  is compared to AC line current reference  63  to create error signal  65 . Error signal  65  drives buck stage  60  as part of this servo loop. The net effect is the voltage on energy storage capacitor  50  is regulated by the amount (the amplitude) of current  64  sourced into AC line  90 . Current  41  and voltage  42  of PV source  10  are sensed and multiplied to provide  43 , a measure of PV source  10  output power. PV reference voltage  46  is the desired operating point of PV source  10  and is compared to actual PV source  10  voltage  42  in a servo loop where error voltage  47  drives boost stage  40 . A control means is used to periodically perturb ( 45 ) PV reference voltage  46  and observe ( 44 ) the effect on PV output power  43 . If an increase in PV reference voltage  46  results in an increase in PV power  43 , PV reference voltage  46  is incrementally increased every perturb cycle until an increase in PV reference voltage  46  results in a decrease in PV power  43 . This is how the maximum power point (MPP) of PV source  10  is established. If the irradiance on and the temperature of photovoltaic source  10  are substantially stable. PV source voltage  42  will dither a small amount about the MPP of PV source  10 . This control method provides stable operation at any PV source operating point. As conditions change the control circuit will track and seek a new MPP. 
     FIG. 3  illustrates an alternate embodiment of the control arrangement for the DC-to-AC power converter. This power converter has a pulse width modulated, current regulating buck stage and no boost stage. Sinusoidal reference  62  follows the AC line voltage and frequency. AC line current reference  63  is generated by multiplying sinusoidal reference  62  by scaling factor  67 . Scaling factor  67  is the error signal or difference between the actual operating voltage  42  of PV source  10  and PV reference voltage  46 , the desired operating voltage of PV source  10 . A fixed offset voltage  49  is also added scaling factor  67 . Actual AC line current  64  is compared to AC line current reference  63  to create error signal  65 . Error signal  65  drives buck stage  60  as part of this servo loop. Current  41  and voltage  42  of PV source  10  are sensed and multiplied to provide  43 , a measure of PV source  10  output power. A control means is used to periodically perturb ( 45 ) PV reference voltage  46  and observe ( 44 ) the effect on PV output power  43 . If an increase in PV reference voltage  46  results in an increase in PV power  43 , PV reference voltage  46  is incrementally increased every perturb cycle until an increase in PV reference voltage  46  results in a decrease in PV power  43 . This is how the maximum power point (MPP) of PV source  10  is established. If the irradiance on and the temperature of photovoltaic source  10  are substantially stable. PV source voltage  42  will dither a small amount about the MPP of PV source  10 . This control method provides stable operation at any PV source operating point. As conditions change the control circuit will track and seek a new MPP. 
     FIG. 4  illustrates the preferred embodiment of the DC to AC power converter topology. Photovoltaic source  10  is connected at power converter input terminals  20  and  21 . Capacitor  30  holds the photovoltaic source  10  voltage substantially constant during the high frequency switching cycle of boost circuit  40 . Boost circuit  40  is a conventional pulse width modulated boost circuit comprising inductor  41 , semiconductor power switch  43  and diode  45 . Boost circuit  40  converts the voltage on capacitor  30  to a voltage greater than the peak voltages of AC line  90 . Buck circuit  60  is a conventional pulse width modulated buck circuit comprising inductor  61 , semiconductor power switch  62  and diode  63 . Boost circuit  40  is a voltage regulator. Buck circuit  60  is a current regulator regulating half sinewaves of current, synchronized with the voltage of AC line  90 . Unfolder circuit  70  is novel. Switch  71  is closed when AC line  91  is positive with respect to neutral terminal  80  and switch  72  is closed when AC line  92  is positive with respect to neutral terminal  80 . Diodes  73  and  74  provide protection against the simultaneous closure of switches  71  and  72 . It may appear that this arrangement permits DC to flow into the power line, which is limited by present interconnection standards. However, the true limitation is to avoid DC flux in the core of the power line transformer. This is accomplished by the alternate pulsating current in the two lines. The effect on the transformer is no different from that of a full-wave rectifier. A considerable advantage of this preferred embodiment is that negative pole  20  of the photovoltaic source  10  is maintained at the potential of AC line neutral  80 . This greatly improves safety and reduces electrical noise emitted from the PV source  10 , minimizing interference with nearby residential electronic equipment. The configurations for boost circuit  40  or buck circuit  60  can be any circuit that accomplishes the described results and are not limited to the circuit configurations shown in  FIG. 4 . 
   The topology illustrated in  FIG. 4  when used with the control method illustrated in  FIG. 1  enables two substantial improvements over the prior art. In the invention, boost stage  40  is allowed to run with its own independent feedback loop controlled solely for holding the maximum power point (MPP) of PV source  10 . This control is such that there are very little 60 Hz or 120 Hz current components in capacitor  30 . Capacitor  30  must store only enough energy to cover one high frequency switching period of boost stage  40 . This switching period is roughly 1/300th as long as the rectified 60 Hz period. Thus capacitor  30  can be much smaller than in a conventional inverter. The second change is to allow the voltage on energy storage element, capacitor  50 , to go much higher than the peak voltage of AC line  90 . Since stored energy is proportional to the square of voltage, any voltage increase exponentially reduces the capacitance value required of capacitor  50 . The value of capacitor  50  can now be in the order of 100 nanofarads per watt converted. Therefore, both capacitors can be low enough in capacitance value to be economic plastic film units. Also, operation at high boost ratios also requires some means to constrain the voltage on capacitor  50  to levels that are safe for semiconductor devices  43 ,  45 ,  62  and  63 . The present invention controls the voltage on capacitor  50  by adjusting the current out of the inverter into AC line  90  while maintaining its sinusoidal quality. This topology enables the use of a control method with two independent control loops that do not interfere with each other in the presence of rapid changes in the amount of power available from PV source  10  or rapid changes in AC line  90  voltages. 
     FIG. 5A  shows an alternate embodiment of the invention where a bipolar boost circuit and an H-bridge buck circuit are used with the control method illustrated in  FIG. 1 . Photovoltaic source  10  is connected at power converter input terminals  21  and  22 . Capacitors  31  and  32  hold the photovoltaic source  10  voltage substantially constant during the high frequency switching cycles of boost circuit  40 . Boost stage  40  is a pulse width modulated bipolar boost circuit comprising inductor  41 , semiconductor power switch  43  and diode  45  for the positive monopole and inductor  42 , semiconductor power switch  44  and diode  46  for the negative monopole. Boost circuit  40  converts photovoltaic source  10  voltage to a positive voltage on capacitor  51  and a negative voltage on capacitor  52 , both with respect to AC line neutral  80  and both substantially greater than the respective positive and negative peak voltages of AC line  90 . Energy storage element  50 , comprising capacitors  51  and  52 , stores energy to limit the voltage excursions across capacitors  51  and  52  as energy is drawn from energy storage element  50  at twice the AC line frequency. Buck stage  60  comprises of two half-bridge circuits. Power semiconductor devices  64  and  65  and inductor  61  are the boost circuit components feeding AC line  91 . Power semiconductor devices  66  and  67  and inductor  62  are the boost circuit components feeding AC line  92 . Boost circuit  40  is a voltage regulator. Buck circuit  60  is a current regulator regulating sinewave currents into AC lines  91  and  92 . AC line  90  is a typical residential 120/240V, split-phase utility service. This arrangement of any conventional, prior art, boost and buck converters is obvious. The amalgamation of this topology and the control method illustrated in  FIG. 1  is claimed as useful, novel and an improvement over prior art. 
     FIG. 5B  shows a variation of the topology illustrated in  FIG. 5A  where photovoltaic source  10  is bipolar with a positive source  11  connected across capacitor  31  and a negative source  12  connected across capacitor  32  and where all said polarity references are with respect to photovoltaic common connection point  20  and AC line neutral  80 . 
     FIG. 5C  shows a variation of the topology illustrated in  FIG. 5A  where buck stage  60  uses only one half-bridge circuit comprising semiconductor power devices  64  and  65  and inductor  61 . This topology variation is intended for use with single phase AC line  90  rather than the split-phase configuration shown in  FIG. 5A . 
     FIG. 6A  illustrates a conventional, known DC to poly-phase AC power converter topology where photovoltaic source  10  is connected to input terminals  21  and  22  across the energy storage element, capacitor  50 . Buck circuit  60  comprises semiconductor power devices  64  through  69  and inductors  61 ,  62  and  63  feeding AC lines  91 ,  92  and  93  respectively. This topology of any conventional, prior art, poly-phase buck converter is known. The amalgamation of this topology and the control method illustrated in  FIG. 3  is claimed as useful, novel and an improvement over the prior art. 
     FIG. 6B  shows a variation of the topology illustrated in  FIG. 6A  where photovoltaic source  10  is bipolar with a positive source  11  connected across capacitor  31  and a negative source  12  connected across capacitor  32  and where all said polarity references are with respect to photovoltaic common connection point  20  and AC line neutral  80 . This configuration enables the power converter to be connected to a 4-wire, wye configured AC line  90 . 
     FIG. 7  illustrates a conventional DC to AC power converter topology where photovoltaic source  10  is connected to input terminals  21  and  22  across the energy storage element, capacitor  50 . Buck circuit  60  is configured as a full or H-bridge and comprises semiconductor power devices  64 ,  65 ,  66 ,  67  and inductor  61 . Buck regulator  60  is a current regulator regulating sinewave current synchronized with AC line voltage  90  into primary winding  71  of transformer  70 . Transformer  70  provides voltage isolation and steps up the voltage on primary winding  71  to a higher voltage on secondary winding  72 . This topology of any conventional buck converter and a step-up, line-frequency transformer is known. The amalgamation of this topology and the control method illustrated in  FIG. 3  is claimed as useful, novel and an improvement over prior art.