Patent Document

This nonprovisional application claims priority under 35 U.S.C. §119(a) to German Patent Application No. DE 10 2010 009 120.0, which was filed in Germany on Feb. 24, 2010, and which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photovoltaic generator with an array of multiple parallel-connected strings of series-connected photovoltaic modules, wherein a portion of the photovoltaic modules of a string can be short-circuited by means of a shorting switch, the activation of which takes place when a predefined voltage value across the string is exceeded. 
     2. Description of the Background Art 
     Photovoltaic systems of this nature are extremely well known. As a general rule, these systems are constructed such that a plurality of strings are connected in parallel. In these designs, the maximum number of strings is based on the output of the inverter to which the strings are connected. Modern inverters can be designed for a DC input voltage of up to approx. 900 volts. 
     At the present time, it is customary to construct each string in the system from eight photovoltaic modules, each of which has 60 photovoltaic cells. Thus a total of 480 cells are connected in series with one another. In the open-circuit case, a voltage of 1.5 volts is present at each cell, resulting in a string voltage of 720 volts, which is considerably below the maximum voltage of 1000 volts specified by the manufacturers of the modules. If a higher voltage is present, this can lead to destruction of the modules and the entire system. 
     During operation of the system, the open-circuit voltage of the cells drops to an operating voltage of approximately 1 to 1.1 volts, so that a voltage between 480 volts and 510 volts is present between the ends of the conventional strings. In the example shown in the figures that follow, an operating voltage of 1 volt per cell is assumed for the sake of simplicity, hence a voltage of 60 volts across a single voltaic module with 60 cells. In the event that the operator of the grid to which the photovoltaic system is connected should disconnect it from the grid for any reason (e.g. a short circuit in the supply cable), the voltage jumps to the aforementioned 720 volts, which is not critical for the modules or the system. 
     On the other hand, it would be desirable to operate the photovoltaic modules and also the inverter with a voltage higher than 480-510 volts in normal operation, ideally at the maximum permissible voltage of 1000 volts. This is not possible, however, since a voltage of approximately 1500 volts in the open-circuit case would lead to the destruction of the photovoltaic modules, the inverter, and the system. 
     For operating the photovoltaic system at a higher operating voltage, it is known from DE 3041078 to employ a shorting switch that short-circuits a portion of the modules in the event that an overvoltage arises. 
     For a large system with hundreds of arrays, however, this measure entails high expenditures for wiring and switches. Some of the arrays are located hundreds of meters apart from one another, and there is an additional need for several kilometers of cable that must be laid and connected. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to protect a large-scale photovoltaic system from overvoltage in the absence of an AC feed, with a low cabling cost. 
     This object is attained in accordance with the invention in that, in the case of a large-scale system with a plurality of parallel-connected arrays, the shorting switch is only provided on some of the arrays. Hence, only the arrays that are located near the inverter and the control unit need to be provided with the shorting switch, which results in a significant cable savings. As a result of the reduction of the voltage at one or more arrays, the voltage at the other arrays in the parallel circuit is pulled down to a voltage value that is tolerable for the inverter. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIG. 1  a schematic representation of a conventional photovoltaic generator with eight modules per string; 
         FIG. 1   a  illustrates a conventional module with 60 photovoltaic cells; 
         FIG. 2  illustrates a schematic representation of a string with 16 modules; 
         FIG. 3  illustrates a diagram of the method of operation of an MPP regulator; 
         FIG. 4  illustrates a diagram of the morning startup behavior of a photovoltaic generator according to the invention; 
         FIG. 5  illustrates a circuit arrangement for simultaneous switching of multiple interconnected strings; and 
         FIG. 6  illustrates a photovoltaic generator with multiple arrays according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Shown in  FIG. 1  is an ordinary commercial photovoltaic generator  1 , which includes a number of parallel-connected strings S, which in turn include a number—eight in the exemplary embodiment shown—of series-connected photovoltaic modules M. Each photovoltaic module M has series-connected photovoltaic cells  7 , as is evident from  FIG. 1 . For example, it is typical for a photovoltaic module M to use 60 cells with 1.5 volts open-circuit voltage each, or else 130 cells of approx. 0.69 volts each. In both cases, a voltage of approximately 90 volts arises across the module M at open circuit, which is to say approximately 720 volts for eight modules. During operation, this voltage drops to approximately 60 to 65 volts, so that a string voltage Ust of 480 to 510 volts results. 
     The ends of the parallel-connected strings S are connected to the input  9  of an inverter  11 , the output  13  of which feeds the generated electricity into a grid, for example. 
     The open-circuit voltage of 720 volts is significantly below the currently permissible limit of 1000 volts, which the manufacturers of photovoltaic modules specify as the upper limit for their product. In operation, a correspondingly larger safety margin is achieved relative to the 1000 volts. In the known systems of this type, it would be desirable to fully utilize the maximum permissible voltage of 1000 volts so that the cross-sections of the cables that are to be laid can be kept small. 
     This purpose is served by the photovoltaic system  1  shown in  FIG. 2 . Shown there is a single string S, this time with 16 photovoltaic modules M, which is connected in parallel with other strings that are not shown and is routed to the input  9  of the inverter  11 . In the example embodiment shown, the string S has double the number of modules M, hence  16 , each of which is constructed as shown in  FIG. 1   a . This results in an impermissibly high open-circuit voltage of 1440 volts across the string S, but a permissibly high operating voltage of 960 to 1020 volts, which are present at one of the modules M and at the input  9  of the inverter  11 . Exceeding the permissible level by 20 volts is considered tolerable here. 
     In order to prevent destruction of the inverter  11  and module M in the event of a disconnection from the grid, a shorting switch  15  is provided. The switch  15  is positioned such that it short-circuits between one tenth and one half, in particular between one quarter and one half, of the modules M. The switch  15  is controlled by a threshold detector (not shown), which detects when the voltage across the string S exceeds the predefinable value, 1000 volts in the example here. 
     Another important advantage in daily operation of the photovoltaic generator  1  according to the invention is explained below with reference to  FIGS. 3 and 4 .  FIG. 3  shows the curve of the generated current I as a function of the associated voltage U of a typical photovoltaic system  1 . By means of an MPP (Maximum Power Point) regulator, this current/voltage curve  17  is held at a point at which maximum output is present. This maximum output is the product of IMPP and UMPP, and corresponds to the cross-hatched region, which in this case occupies a maximum area. The MPP regulator regulates on the curve  17  along the double-headed arrow  19  and endeavors to move the photovoltaic system to the MPP. This point changes continuously as a function of sun position, cloud cover, air pollution, and the like. 
     Because of the high number of installed modules M in a string S, the MPP regulator could exceed the maximum permissible value of the operating voltage. This is prevented by expanding the control algorithm by the condition that the predefined voltage value (1000 volts in the example here) must not be exceeded. This condition has priority over achieving an optimum power point MPP. In advantageous manner, an output is provided on the MPP regulator that causes the switch  15  to close if this condition is violated for any reason. 
       FIG. 4  shows a typical curve of the voltage present at the string S during morning startup of the system  1 . The illustrated course of the voltage as a function of the time of day is shown here by the curve  21  for the open-circuit case, and by the curve  21   a  with the inverter  11  connected in the operating case after it has been connected upon reaching the maximal permissible string voltage of 1000 volts in order to feed the energy that is generated into the grid through the output terminals  13 . In this context, the curve  21   a  follows the open-circuit curve  21  until connection of the inverter. When 16 modules M per string S are used by way of example, the curve  21  trends toward the open-circuit voltage of approximately 1440 volts. Accordingly, the curve  21   a  under load approaches the operating voltage of 960 volts (corresponding to 16 times 60 volts). Also shown in the diagram in  FIG. 4  is an additional curve  23 , drawn with a dotted-and-dashed line, which shows the behavior of the open circuit voltage with the switch  15  closed, which is to say with the five modules M bridged in the example. Up to the point in time when the inverter  11  is connected, this curve  23  approaches the open-circuit voltage of the remaining eleven series-connected, active modules M, thus approximately 990 volts (corresponding to 11 times 90 volts). 
     At morning startup without short-circuited modules M, the result would be the behavior shown in curve  21 , and the maximum permissible voltage level of 1000 volts would be reached at approximately 8:15 AM. Since the minimum power of 1 KW required for connection of the inverter  11  has not yet been reached, the voltage collapses and must be reestablished starting from zero as is shown by the behavior of the curve  21   a  from 8:15 AM onward. The minimum required power depends on the inverter  11  employed and can be approximately 15 KW for a 2.5 megawatt large-scale system. In like manner, a certain open-circuit voltage is necessary so that stable coupling of the inverter  11  to the grid can take place. In the exemplary embodiment shown in  FIG. 4 , it is assumed that the connection criteria are reached at a string voltage Ust of 700 volts when 11 modules M are present. 
     This is where the shorting switch  15  comes into action, the switch being switched on, which is to say closed, when the startup of the system begins. The photovoltaic system operates with eleven modules M per string S on the dotted-and-dashed line  23 , and at approximately 9:00 AM reaches a power point  25  at which the minimum power of 1 KW required for stable connection to the grid has been reached. At this point in time, the string voltage Ust is 700 volts. Starting at this point in time, the switch  15  is opened, which results in a brief drop in the voltage; this is represented by the circled zigzag in the enlarged detail, since the MPP regulator cannot immediately compensate for this situation. In reality, the zigzag is only a few seconds in duration. The MPP regulator applies its regulating behavior and, in an extremely short time, brings the voltage Ust on the curve  23   a  to a point  27  on an operating voltage curve  21   a ′ for the complete string S with all sixteen modules M. The curve  21   a ′ is drawn with short dashes and runs parallel to and offset to the left of the curve  21   a  starting at the point  27 . From that point, the curve  21   a ′ approaches the maximum of 960 volts of the operating voltage Ust in the further course of the day as the sun stands higher. As a result, power is fed into the grid earlier than would have been possible without the switch  15 . In the example shown, power feed to the grid begins at 8:45 AM, in contrast to which it would not have started until 9:20 AM with the photovoltaic system being operated along the curve. 
     The process is repeated in reverse order in the evening when the system  1  shuts down. Thus, in addition to the more favorable cable cross-sections, the invention also offers the further advantage of a more effective startup behavior in comparison to systems without shorting switches  15 . 
       FIG. 5  shows how six of the 16 modules M can be short-circuited in common for all strings S of the photovoltaic system. To this end, each of the line locations  29  located after the tenth module M are connected to one another and are then routed to the shorting switch  15 . 
       FIG. 6  shows the structure of a photovoltaic generator according to the invention which is provided with p=8 arrays F 1  to F 8 , each with n=10 strings S 1  to S 10 . The first array F 5 , preferably the one that is closest to the inverter  11 , is equipped with three strings S in accordance with  FIG. 5 , in which five modules M can be short-circuited by means of the shorting switch  15 . In the adjacent, second array F 6 , all strings S are equipped with the shorting switch  15 . In the event of a short circuit on the AC side  13 , the shorting switch or switches  15  is/are closed, and if adequate voltage limiting at the input side of the inverter  11  is not observed, disconnect switches  33 , which connect the arrays F to a bus bar  35  leading to the input  9  of the inverter  11 , are additionally opened. As many arrays F are provided with the shorting switch  15  as possible compensating currents between the arrays can be tolerated by the lines and the bus bar  35 . If a further reduction in the voltage at the inverter input  9  should be necessary because the aforesaid drawing-down of the voltage is not sufficient to reach a safe input voltage at the inverter  11 , the number of modules M per string S would have to be reduced, e.g. from 16 to 12. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.

Technology Category: 5