Abstract:
A photovoltaic system includes a plurality of photovoltaic modules and a DC motor connected to a three-phase generator driven by a shaft. The three-phase generator is connected to a power mains. The electric power supplied to the DC motor by the plurality of photovoltaic modules is repeatedly measured and adjusted, by changing an external excitation current of the DC motor, to the peak power attainable at the current ambient temperature and the current incident solar radiation intensity. The peak power is preferably determined by incrementally changing the excitation current in predetermined time intervals, until the supplied electric power produces a power level which can be regarded as the peak power.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application, Serial No. 10 2006 026 073.2-32, filed Jun. 3, 2006, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a photovoltaic system with one or more photovoltaic modules producing power that can be supplied to a power mains, and more particularly to a method for operating a photovoltaic system at peak power. 
     Nothing in the following discussion of the state of the art is to be construed as an admission of prior art. 
     In conventional photovoltaic systems, the DC current supplied by the photovoltaic modules is converted to an AC current with an electric converter or inverter, and the AC power is then fed to the power grid. Electric converters/inverters are presently commercially available for large systems designed for a total power output of up to 700 kW. However, these systems tend to be expensive. System with larger output power require several electric converters/inverters. For example, at least 9 electric converters/inverters are used today for a solar facility with an output power of 2.5 MW, with each converter/inverter having a power rating of 330 kW. 
     Although electric converters/inverters have a high power conversion efficiency, the capacitors in the system tend to produce a sluggish response to changes in the operating conditions. For example, the control unit of the converter/inverter may take between 20 seconds and 3 minutes before adapting to changes in the instantaneous incident solar radiation intensity. 
     The invention is based on the realization that in particular for larger systems, i.e., of 800 kW and above, the converter/inverter should be implemented as a combination of a single DC motor and a single AC generator. DC motors and AC generators with such high power ratings are commercially available. Such a motor/generator combination is much less susceptive to statistic failures due to the smaller number of components than a system using a plurality of electric converters/inverters. The combination also requires less maintenance. It is also advantageous that only a single unit has to be monitored during operation. 
     Use of a motor-generator combination for operating a solar system is disclosed, for example, in the German utility model application DE 20 2006 002 726 U1. However, this application only addresses the mechanical characteristic of the motor-generator combination and proposes to arrange several pole wheels on a shaft, with each pole wheel operating inside their own stator. This measure is intended to increase the conversion efficiency for regenerative energy generation. 
     There is still a need for extracting, preferably without losses, the maximum peak power from a photovoltaic system employing a DC motor-AC generator combination connected to a stationary power grid, independent of changes in the ambient temperature and in the incident solar radiation intensity. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a method for operating a photovoltaic system, which includes a DC motor electrically connected to a plurality of photovoltaic modules and a three-phase generator driven by the DC motor and electrically connected to a power mains, includes the steps of repeatedly measuring the electric power supplied by the photovoltaic modules to the DC motor, and changing an excitation field of the DC motor to adjust the supplied electric power to a peak power which the plurality of photovoltaic modules is capable of delivering at a current ambient temperature and at a current incident solar radiation intensity. 
     According to another aspect of the present invention, a photovoltaic system includes a plurality of photovoltaic modules, a DC motor with an input electrically connected to the photovoltaic modules and receiving from the photovoltaic modules a DC voltage and a DC current, and a three-phase generator driven by the DC motor via a shaft, with the generator producing an AC output voltage capable of being supplied to a power mains. The photovoltaic system further includes a first control unit having two inputs and an output, wherein a first of the inputs receives the DC voltage from the DC motor, and a computing unit having a first input receiving the DC voltage from the DC motor and an output producing an output signal. The output signal is iteratively computed and adjusted by the computing unit commensurate with a maximum permissible peak power for the current incident solar radiation intensity and the current ambient temperature. The output signal is then supplied to the second input of the first control unit, wherein in response to the signals received at the first and second input, the first control unit produces at the output an output signal which causes an excitation field of the DC motor to change, so that the DC motor generates the maximum permissible peak power. 
     According to another feature of the present invention, the supplied electric power may be determined by measuring the DC voltage received at the DC motor and the DC current supplied to the DC motor by the photovoltaic modules. 
     The peak power may be determined and adjusted in small steps, for example, by stepwise changing an excitation current of the DC motor in predetermined time intervals until the supplied electric power produces a power level which can be regarded as the peak power. 
     However, after determining the maximal value of the power level, the incident solar radiation and/or the temperature and/or the load may slightly change, requiring a slight adjustment in the peak power value. The optimal value of the excitation current that corresponds to the peak power must therefore be tracked. This may be achieved, for example, by identifying, after the peak power level has been determined, a change in the peak power level by incrementing or decrementing the excitation current in defined time intervals, and setting the peak power to the changed peak power level. 
     Increasing and decreasing the excitation current in small steps can be continued, resulting in an oscillatory behavior and a possible change in the peak power level that can subsequently also be measured and adjusted. 
     This “oscillatory behavior” should also be measured, for example, even when temperature and the incident solar radiation intensity are constant and the AC power mains is fixed, and no significant changes are expected. According to this operating mode, the determined peak power is maintained for approximately constant ambient temperature and approximately constant incident solar radiation intensity and fixed AC power mains by causing the excitation current to incrementally oscillate in predetermined time intervals about an optimum excitation current that produces the peak power. 
     Each defined time interval may less than 1 second, preferably less than half a second. The time required for starting up the photovoltaic system may be shortened by incrementally increasing the excitation current from a first predetermined experimental value to an optimum excitation current that produces the peak power, or by incrementally decreasing the excitation current from a second predetermined experimental value to the optimum excitation current that produces the peak power. 
     According to yet another feature of the Present invention, the actual value of the DC voltage of the photovoltaic module may be applied to the first input of the computing unit and the actual value of the DC current of the photovoltaic module may be applied to the second input of the computing unit, and the output signal may be iteratively computed and adjusted by also taking into account the actual value of the DC current. The first control unit may include a proportional-integral (PI) controller. 
     According to another feature of the present invention, the DC motor may be an externally excited DC motor, and the output signal of the first control unit may control an excitation current of the DC motor. In this way, the armature circuit and the excitation circuit of the DC motor are not connected with one another, but are only coupled magnetically. As a result, the corresponding specific peak power level for the instantaneous incident solar radiation intensity and the instantaneous temperature can be adjusted essentially loss-free. 
     According to another feature of the present invention, the system may include an excitation current measuring unit and a second control unit having two inputs and an output. The output signal of the first control unit is applied to a first input of the second control unit and an excitation current signal measured by the excitation current measuring unit is applied to the second input of the second control unit. The second control unit may also be a proportional-integral (PI) controller. 
     The DC motor may advantageously be excited by supplying the excitation current from a converter receiving electric power from the power mains. 
     The afore-described photovoltaic system has a long service life. 
     It has been confirmed through experimentation that the system achieves quickly, usually within several seconds, peak power (MPP). The system automatically adjusts to the new operating point in response to a change of external parameters, such as temperature, incident solar radiation intensity or changes in the AC power grid while retaining the aforementioned advantages: The system employs only a single electromechanical conversion device, consisting of the DC motor, the AC voltage generator, and a shaft connecting the motor and the generator. This reduces maintenance costs, and with appropriate care also increases the service life, compared to a system with a large number of electric converters/inverters. Converter units of this type are presently commercially available for a power of up to 2.5 MW. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
         FIG. 1  is a schematic diagram of the peak power MPP of a solar generator as a function of the time t of day from 6 a.m. until 6 p.m.; 
         FIG. 2  shows a typical current/voltage curve I(U) of a photovoltaic system; 
         FIG. 3  shows schematically an U-t curve; and 
         FIG. 4  shows a photovoltaic system according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Throughout all the Figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
     Turning now to the drawing, and in particular to  FIG. 1 , there is depicted as a continuous curve K the peak power MPP (peak power point) that a photovoltaic system can supply over the course of one day, indicated by time t, between 6 a.m. and 6 p.m. It will be assumed that there are no clouds or no large temperature changes occur. 
     According to the proposed exemplary method, the curve K is determined by measuring the power P in regular time intervals t, and by adjusting the determined peak power point MPP so as to obtain the maximum possible peak power MPP from the solar generator, which is in turn supplied to the energy converter implemented as an externally excited DC motor coupled to a three-phase AC generator (see  FIG. 4 ) which is in turn connected to a three-phase AC current grid. The oscillatory sampling with a computing unit and a connected proportional-integral (PI) controller is indicated by the jagged shape of the solid curve K. However, it should be noted that the sampling time intervals Δt are in the range of one second, preferably about half a second or less. As a result, the diagram MPP(t) over the course of the day and illustrated in  FIG. 1  is not to scale. 
     The continuous characteristic curve S 1  of  FIG. 2  shows a typical current/voltage characteristic I(U) of a photovoltaic system at a certain temperature and a certain incident solar radiation intensity. The characteristic curve S 1  has a peak power point MPP 1 . This point MPP 1  is defined so that the hatched area is a maximum, corresponding to the peak power P that can be supplied by the solar generator. 
     The aforementioned electromotive converter is controlled, as will be described in more detail below, to this power point by an iterative approximation. To this end, an upward and downward control operation is performed several times along the curve S 1  starting, for example, at the point P′ or the point P″ until the peak power point MPP 1  has been reached. The points P′ and P″ thereby correspond to experimental starting points E′ and E″, respectively, from where on the excitation current E is incrementally increased or decreased. 
     The iterative approximation will now be explained with reference to an arbitrarily selected example. 
     It will be assumed that P 1 , corresponding P′, is the starting point. This value P 1  then yields the first measurement of I and U from which the power P supplied by the photovoltaic modules to the DC motor can be determined (see  FIG. 3 ). After a time Δt=0.5 sec, the control unit (see  FIG. 3 ) changes the excitation current E slightly via the first control unit (see  FIG. 3 ). As a result, the DC voltage U decreases. The computing unit now determines from the new values of I and U a power value P 2 . The computing unit also determines that the power value P 2  has increased compared to the previous power value P 1 . 
     After an additional time Δt=0.5 sec, the computing unit again slightly changes the excitation current via the control unit, which causes another decrease in the DC voltage U. The computing unit now determines a power value P 3  and determines again that this power value P 3  has increased compared to the previous power value P 2 . 
     After a time Δt=0.5 sec, the excitation current is again slightly changed and an even greater power value P 4  is reached. It will be assumed that this is indeed the peak power value MPP 1 ; however, the computing unit is actually not able to ascertain this. 
     After another time Δt=0.5 sec, the computing unit decreases the voltage U again with the afore-described process by changing the excitation current. The computing unit now measures the value P 5  and determines that the power has decreased from P 4  to P 5 . At that time, the peak power value MPP 1  must therefore have been located somewhere between the values P 3  and P 5 . 
     After another time Δt=0.5 sec, the computer unit increases the DC voltage U which causes the power P at point P 4  to increase. To test this condition, the computer unit returns after a time Δt to the point P 3 , whereafter it then returns to the point P 4  and also tests point P 5  again. 
     The computing unit therefore continuously attempts to maintain the power point MPP 1  by oscillating about the power point P 4 , i.e., by increasing and decreasing the DC voltage U. 
       FIG. 3  shows a diagram of the voltage as a function of time U(t). The continuous curve U(MPP) thereby corresponds to the ideal voltage at the peak power point MPP, whereas the jagged curve corresponds to the incremental approximation to the corresponding ideal voltage. 
     It should be mentioned that the corresponding power P in  FIG. 2  is defined by the rectangles which bound the individual points. Of these rectangles, only the rectangle associated with P 4  is emphasized by hatching. 
     The characteristic curve S 1  in  FIG. 2  continuously changes depending on the incident solar radiation intensity and/or the temperature. If a change occurs, the dotted curve S 2  may be obtained. This produces a new peak power point, for example the value MPP 2 . With the afore-described control method, the DC voltage U is adjusted so that the solar generator is operated at the new peak power point MPP 2 . 
       FIG. 4  shows a photovoltaic system  1  with a solar generator  3  having a plurality of photovoltaic modules  5 . Each module  5  in turn includes a plurality of photovoltaic cells. The modules  5  are connected in a conventional manner in series and have terminals  7  at their respective ends at which the generated DC voltage U and the resulting DC current I can be obtained. Depending on the DC current I consumed by the load connected to terminals  7 , a DC voltage U corresponding, for example, to the voltage depicted in the curves S 1  and S 2  of  FIG. 2  can be supplied. 
     A DC motor  9  is connected to terminals  7 . The DC motor is implemented as an externally excited DC machine with an excitation winding  11 . The shaft  13  of the DC motor  9  drives a three-phase AC generator  15 , in particular a three-phase generator with a higher output voltage. The generator  15  is connected to a three-phase power grid  17 , supplying an AC voltage U W . In the exemplary embodiment, the three-phase power grid  17  is a public power grid operating at a constant voltage of, for example, 400 V and at constant frequency. The generator  15  operates in normal operation with a constant rotation speed (RPM) and is synchronized with the frequency of the three-phase power grid  17  in a conventional manner. 
     In addition, a computing unit  19  is provided for, among others, calculating the peak power point MPP of the solar generator  3 . The computer unit has a first input to which the DC voltage U at the DC motor  9  is applied. The second input of the computing unit  19  receives from a current measuring unit  21  the instantaneous value of the DC current I which is supplied by the solar generator  3  to the DC motor  9 . 
     The computing unit  19  generates an output signal for determining the maximal power point MPP of the solar generator  3  at the actual incident solar radiation intensity and the actual temperature. As discussed above with reference to  FIG. 2 , the output signal is provided every 0.5 sec and can be regarded as a new nominal value U* for the DC voltage U. 
     The new nominal value U* is supplied to the second input of the first control unit  23 , whereas the DC voltage U at the DC motor  9  is supplied to the first input. The control unit  23  is preferably a proportional-integral controller (PI-controller) whose output signal ΔU corresponds to the control deviation, which is then used to affect the excitation of the DC motor  9 , in particular the excitation current E. The field of the DC motor  9  is thereby weakened or strengthened, depending on the magnitude of the output signal ΔU. 
     To affect the motor field, the output signal ΔU is supplied to the first input of a second control unit  25 . This second control unit  25  is preferably a PI-controller and controls the excitation current E. The supplied output signal ΔU can therefore be viewed as a nominal excitation current signal E*. The actual value E of the excitation current is supplied from a measuring unit  27  that measures the excitation current in the excitation current circuit to the second input of the second control unit  25 . A comparison between the two signals ΔU=E* and E produces at the output of the second control unit  25  an output signal ΔE representing the control deviation, which is used for directly adjusting the excitation current E. 
     The excitation current E is supplied by a controllable line rectifier  29  which has an input connected to the three-phase power grid  17  and an output connected to the excitation winding  11 . It will be understood that another energy source may also be used. The power grid rectifier  29  supplies the required excitation current E to the excitation winding  11 . 
     It should be noted that an excitation current controller for the excitation current E in the excitation winding  11  is subordinate to the DC voltage control for the input voltage U of the DC motor  9 . 
     The peak power MPP is incrementally adjusted and measured, i.e., using small steps in the excitation current E, using the afore-described oscillatory control method. 
     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.