Abstract:
The photovoltaic system includes a plurality of photovoltaic modules which are connected to form a string or several strings connected in parallel, thereby forming a photovoltaic generator having a positive terminal and negative terminal. A DC constant voltage source connected to the photovoltaic generator to raise the potential of the positive terminal relative to ground potential. This reduces the flow of electrons out of the TCO layer of the modules, thereby reducing or completely eliminating cathode discharges which damage the modules.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application, Serial No. 10 2007 050 554.1, filed Oct. 23, 2007, 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 and more particularly to a photovoltaic system with a plurality of electrically connected photovoltaic modules. 
     Photovoltaic systems are generally known. These systems typically include a number of photovoltaic modules connected in series to form a so-called string. Each photovoltaic module in turn includes about 100 photovoltaic cells, which are also electrically connected in series. A single photovoltaic cell generates a voltage of about 0.5 V when illuminated by sunlight. As a result, each string has under load a voltage across the string of about 500 V depending on the specific application of the system. This voltage is also referred to as string voltage. In the following example, a string voltage of about 500V under load and of about 800 V under open-circuit conditions (no load) is assumed. Several strings, e.g. 10 strings, may be connected in parallel, with the generated energy then transmitted via a bus for further use. 
     The generated electrical energy is provided in form of a DC voltage which is converted into an AC voltage by an inverter. Typical conventional exemplary circuit diagrams are shown in  FIGS. 1 and 2 , where identical components are designated with identical reference symbols 
     As shown in  FIG. 1 , the photovoltaic system includes a plurality of photovoltaic cells  3  connected in series and forming, in the illustrated example, two strings  5  which are connected in parallel. The photovoltaic generator  6  formed in this manner has a first string terminal  7  at a negative potential and a second string terminal  9  at a positive potential. The first string terminal is the negative terminal of the photovoltaic generator at a first (lower) potential P 1 , and the second string terminal is the positive terminal of the photovoltaic generator at a second (higher) potential P 2 . An inverter  11  is connected to the string terminals  7  and  9 . The voltage between the two string ends  7 ,  9  under load is, as mentioned above, about 500 V. 
     As illustrated in  FIG. 1 , the photovoltaic system is operated at a fixed potential reference potential, i.e., the negative potential P 1  is connected to ground  13 , and the positive potential P 2  is, commensurate with the number of serially connected photovoltaic cells of the two strings  5 , about 500 V under load. 
     The basic disadvantage of the circuit of  FIG. 1  is its high affinity to “attract” lightning due to the connection of the negative potential to ground. Accordingly, wide-ranging precautions must be taken to prevent lightning strikes which could destroy the inverter, causing a loss of several 100.000             in larger systems. Alternatively, complex overvoltage protection devices would have to be installed, raising the overall price tag of the system  1 . The components carrying high voltages must also be protected against accidental contact. There is a danger of an electric shock for a person standing on the ground who touches lines or conductor parts at, for example, the maximum line voltage U 0 . All bare components installed in the system must therefore be grounded.
     It has been observed that a very small, but measurable current can flow from the individual modules  3  to ground  13 , but that the modules  3  are not damaged even after prolonged operation under normal conditions. 
     The second circuit diagram shown in  FIG. 2  includes, to simplify the drawing, in the photovoltaic generator  6  only a single string  5  constructed of serially connected modules  3 . This photovoltaic system  1  reduces the risk for lightning strikes and eliminates the danger of an electric shock for bystanders, but has another disadvantage. The magnitude of the voltage at each of the two string terminals  7 ,  9  is about the same with reference to ground  13  in the illustrated so-called potential-free operation of the photovoltaic system  1 . The positive potential P 2  to ground  13  under open circuit conditions (U 0 =800 V) is in this example about +400 V, while the negative potential P 1  to ground  13  is also about −400 V. These voltages to ground are caused, in spite of the potential-free operation, by a non-negligible relatively small conductance (=the inverse of the ohmic resistance) of the relative the long connecting lines between the modules  3  (wiring of the system  1 ) and the cables to the inverter  11 . The small conductance is symbolized in the schematic circuit diagram by a resistor  14 , which is connected approximately between the center of the series connection  5  of the modules  3  and ground  13 . Parasitic discharges to ground  13  then become finite and the potential with reference to ground  13  is distributed as mentioned above, i.e., +400 V and −400 V, which represents the most advantageous energy distribution for the overall system  1 . 
     It has been observed that a discharge from anodes, i.e., a discharge from the part of the photovoltaic system  1  with a positive potential to ground, does not damage the affected photovoltaic modules  3 . Conversely, a discharge from cathodes causes damage to the photovoltaic modules over an extended period of time, damaging (eroding) the edge region of the TCO layer of the photovoltaic modules  3  and causing a premature permanent power drop. The TCO layer is typically referred to as the semiconductor layer in a module  3  which is disposed between two glass panes. Several exemplary discharges are depicted in  FIG. 2  by the arrows  15   a ,  15   b ,  17   a ,  17   b.    
     It should be mentioned that electrons flow from the top modules  3   a  at a positive potential to the bottom modules  3   b  at a negative potential, as shown by arrows  15   a . This is referred to as “electron absorption.” The arrows  15   a  extending from module to module indicate a “cathode discharge.” A small “anode discharge” may also occur as indicated by the module to module arrows  15   b  pointing in the opposite direction. As already mentioned, “electron absorption” can damage the modules  3   b  during their service life. 
     Other “cathode discharges” are symbolized by arrows  17   a . In this case, the voltage division by resistor  14  causes electrons to flow from the bottom modules  3   b  to ground  13 . A small “anode discharge” is also present, as indicated by arrows  17   b . These “cathode discharges” (arrows  17   a ) should also be prevented if possible. 
     In case of an electrical fault, conventional equipment with potentially exposed electrical high-voltage components puts service personnel at risk. A check can only be performed with a voltage tester and by systematically contacting all conductors. This may take several weeks in large high-power photovoltaic systems and is therefore not practical. 
     In addition, it is also not possible to detect when one or more parallel-connected strings  5  of photovoltaic modules  3  are disconnected, either because these modules  3  became defective or due to theft. 
     Accordingly, there is a need to keep cathode discharges on modules as small as possible or to eliminate them entirely, and to provide the system against lightning strikes. There is further a need to protect personnel from accidental electric shock and to detect theft. 
     SUMMARY OF THE INVENTION 
     The disclosed system according to the invention essentially eliminates “cathode discharges” and thereby prevent damage in the edge regions of the solar modules. 
     According to one aspect of the present invention, a photovoltaic system includes a photovoltaic generator with a plurality of photovoltaic modules electrically connected in form of at least one string. A first terminal of the photovoltaic generator has a first potential of negative polarity and a second terminal of the photovoltaic generator has a second potential of positive polarity. A device which includes a DC voltage source is connected to one of the first or second terminal of the photovoltaic generator for raising the first or second potential relative to ground. 
     With this measure, the negative potential is shifted up, preferably to a positive value with respect to ground potential, which reduces or completely eliminates “cathode discharges”, i.e., flow of electron into the various modules. 
     According to another advantageous feature of the invention, the photovoltaic system may also include a potential-free inverter having a DC input terminals connected to the terminals of the photovoltaic generator in one-to-one correspondence. This inverter generates an AC output for supplying power mains. 
     According to another advantageous feature of the invention, the DC voltage source may be a constant voltage source having a predetermined output voltage, for example, between 150V and 1500V. 
     According to another advantageous feature of the invention, the photovoltaic may include a switch detecting a leakage current between the photovoltaic generator and ground potential, wherein the switch disconnects the DC voltage source from the photovoltaic system and/or the ground potential, if the leakage current is greater than a predetermined limit current. 
     According to another advantageous feature of the invention, a current sensor may be connected between ground potential and the negative terminal of the DC voltage source, measuring a leakage current. A comparator receives at first and second inputs the predetermined limit current and the leakage current and outputs a disconnect signal if the detected leakage current is greater than a predetermined limit current. The disconnect signal actuates the switch which in response disconnects the DC voltage source from the photovoltaic system and/or the ground potential. Instead or in addition to the disconnect signal, the comparator may also output an alarm signal form transmission to an alarm system. 
    
    
     
       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 a conventional grounded photovoltaic system; 
         FIG. 2  is a schematic diagram of a conventional potential-free photovoltaic system; 
         FIG. 3  is a schematic diagram of a first embodiment of a potential-free photovoltaic system with an elevated potential; and 
         FIG. 4  is a schematic diagram of a second embodiment of a potential-free photovoltaic system with an elevated potential. 
     
    
    
     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  FIGS. 1 and 2 , there are shown schematic diagrams of a conventional grounded and a conventional potential-free photovoltaic system  1 , respectively, which have already been described above. The photovoltaic system  1  in  FIG. 1  is grounded, i.e. the negative potential P 1  at the first string terminal  7  is connected to ground potential  13 . The open circuit voltage of this arrangement is, as mentioned above, approximately 800 V. Conversely, the photovoltaic system  1  in  FIG. 2  is potential-free, i.e., each string  7 ,  9  has about the same voltage, with P 2  about +400 V against ground  13  and P 2  about −400 V against ground  13 . Because air is not an ideal insulator, electrons exit from the modules  3   a  having a positive voltage with respect to ground  13  and enter the modules  3   b  having a negative voltage with respect to ground  13 . The electrons entering the TCO layer of the photovoltaic modules  3   b  damage (erode) the edge region of the layer, which can be prevented by employing the measures shown in  FIGS. 3 and 4 . 
     In a first embodiment according to the invention illustrated in  FIG. 3 , a DC voltage source  23  with a positive terminal  27  and a negative terminal  25  supplies a constant voltage of, for example, U z =1000 V. When the positive terminal  27  is connected to the second string terminal  9 , the voltage at the second string terminal  9  has a positive potential P 2  of +1000 V, whereas the negative potential P 1  is floating. If the voltage U 0 =800 V across the string  5  is subtracted from U z =1000 V, then the potential P 1  at the first string terminal  7  becomes +200V under open circuit conditions, and +500V under load. The potential P 1  is therefore positive which is quite important. I.e., all the photovoltaic modules  3  have a positive potential with respect to ground  13 , and the electrons are discharged from each positive module  3  to ground. The disadvantages resulting from electrons exiting the negative TCO layer of the modules is thereby eliminated. 
     Regulations from regulatory agencies as well as limited technical options may limit the voltage to which the potential P 1  can be raised. However, even this limitation represents an improvement by significantly reducing electron absorption  17 . 
     The discharge path is indicated by arrows  29 . The discharge from modules  3 , which are at a positive potential, is referred to as “anode discharge.” The discharges indicated by reference symbol  29  are therefore “anode discharges” which are harmless for the modules  3 . 
     In addition, even the bottom module  3  is at a positive potential relative to ground  13  which reduces the risk of a lightning strike. 
     It was mentioned above that the device  23  for raising the potential is a constant voltage DC source. It should be mentioned that the higher the additional voltage U z  of the constant DC voltage source  23 , the greater is the protection against a lightning strike. It should also be mentioned that even an additional voltage U z  of “only” about 600V significantly reduces harmful “cathode discharges” and hence also reduces the risk of damage to the modules  3 . 
     The same effects are attained if the device  23  is a constant DC current source. 
     The system  1  illustrated in  FIG. 3  also includes means for protecting personnel in form of a safety switch  31  or an alternative circuit breaker  31 A. This arrangement protects a person who accidentally touches a part of the system  1 . The switch  31  or  31 A is configured so that the constant voltage source  23  is disconnected when a predetermined limit current i* is reached. In other words, current i does then no longer flow through the body of the endangered person. The current value i* for the maximum allowable leakage current i can be, for example, 20 mA. It will be understood that in addition, an alarm signal “a” can be generated for sounding an alarm. 
     The same concept applies to a system  1  illustrated in  FIG. 4 . The terminal P 1  is here connected with the positive terminal  27  of the DC constant voltage source  23 . The negative terminal  25  is connected via the safety switch  21  to ground  13 , resulting in the arrows  29  already described with reference to  FIG. 2  for anode discharges which are harmless for the modules  3 . 
     In other words: to prevent endangerment of personnel, a safety switch  31  or alternatively a circuit breaker  31 A are employed which are used to disconnect at a suitable location the connection between the ground  13  and terminal  27  of the DC voltage source  23 . This switch  31 ,  31 A has the function of a FI protective switch. Instead of a leakage current switch  31 ,  31 A, a sensor or current measurement device  33  may be connected in-line, which measures the current i through the sensor  33  and supplies a corresponding measurement value to a comparator  35 , which also receives a predetermined current limit value i*. If the value of the current i exceeds the limit value i*, then a disconnect signal p is transmitted which causes the switch  31  or  31 A to switch off, thereby disconnecting one or both terminals of the DC voltage source  23  from the photovoltaic generator. This disconnect signal p can also operate as alarm signal a to indicate the dangerous state of the system  1  or the endangerment of a person. 
     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.