Abstract:
A circuit with series-connected solar modules separated into a first and second substring, wherein each substring includes a first and a second terminal, and a solar inverter configured to supply electrical energy from the solar modules to an AC power grid. The circuit includes a first switch coupled to the first terminal of the first substring to a first power cable of the inverter, and a second switch coupled to the second terminal of the first substring to a first terminal of the second substring at a center point, thereby coupling the first and second substrings to form at least one string. The circuit further includes a third switch couple to second terminal of the second substring, and a shared actuator to open the first switch, second switch, and third switch if a current between the center point and a circuit ground exceeds a threshold value.

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to European Patent Application No. 09167378, filed on Aug. 6, 2009, and also claims priority to U.S. Provisional Patent Application Ser. No. 61/237,411, filed on Aug. 27, 2009, the contents both of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a system and method for connecting at least one string of a photovoltaic system that contains multiple solar modules to an inverter that supplies electrical power from the photovoltaic system to an AC power grid. 
     BACKGROUND 
     A high-performance photovoltaic system usually comprises multiple strings that contain multiple solar modules each. In such a system, the solar modules of each string are connected to each other either in series or in parallel to ensure that the desired output voltage and the desired output current are present at each string. To connect multiple strings of this type of photovoltaic system to a central inverter that supplies electrical power from the photovoltaic system to an AC power grid, it is often the case that long power cables have to be installed whose length is determined by the surface area of each string as well as the spatial arrangement of all the strings. These power cables are a significant cost factor given their large cross-sections and moreover suffer from continuous power loss due to the electrical resistance they cause. In general, it would be possible to reduce the requisite cross-section of the power cables and the amount of power loss if the output voltage of each string is increased through the serial connection of more solar modules while the same number of solar modules with less parallel connections would reduce the power accordingly. However, this concept cannot be implemented in all cases as it would cause the output voltages of the strings to rise to dangerous levels. In the USA, for example, voltages exceeding 600 V to ground are generally prohibited in conventional electrical systems. In practice, this limits not only the output voltages of the strings in a photovoltaic system but also the possibility of conducting smaller currents at higher voltages so as to minimize the cross-section of the power cables. 
     Known system features implemented in the “Solaron Remote PV Tie (RPT)” by Advanced Energy Industries, Inc., Fort Collins, Colorado, USA (see www.advanced-energy.com). Based on these features, a string of a photovoltaic system with multiple solar modules connected in series and parallel is split into two serially connected substrings with a safety switch installed between them. On the output side, two power cables with a power switch installed in each are used to connect the string to an inverter that supplies electrical power from the photovoltaic system to a three-phase 480 V AC power grid. While the photovoltaic system is operating, the AC power grid also provides a ground reference point for the system. If the photovoltaic system is disconnected from the inverter and thus the AC power grid when the power switch in the power cables opens, the safety switch between the two substrings opens and the switches in the potential cables connecting the substrings to the inverter on both sides of the safety switch close. Disconnecting both substrings limits the maximum voltage generated at each point of the photovoltaic system and thus the maximum voltage to ground to 600 volts. Via the potential cable, the two substrings can also be checked for the presence of ground faults, provided, of course, the photovoltaic system is switched off. If the photovoltaic system is switched on again, the switches in the potential cables to the substrings reopen at the same time the safety switch between the substrings closes to interrupt the contact provided by the potential cables between the substrings and the ground due to the potential reference point of the photovoltaic system via the 480 V AC power grid. Based on this known circuit arrangement, any ground fault that occurs while the photovoltaic system is operating gives rise to major short-circuit currents that must be interrupted by triggering a common line circuit breaker or residual current breaker. Moreover, while operating the photovoltaic system with the known circuit arrangement, the accumulation of voltages greater than 600 V to ground normally cannot be prevented. One advantage, however, is that only two power cables are needed to connect the string that supplies an output voltage of 1200 V to the inverter. All other cables between the inverter and the strings are not used to conduct power and can therefore have a comparatively small cross-section. 
     SUMMARY 
     The invention includes a method and a circuit arrangement for connecting at least one string of a photovoltaic system that contains multiple solar modules to an inverter that supplies electrical power from the photovoltaic system to an AC power grid and which can furthermore be used to detect ground faults in the area of the photovoltaic system while it is operating (i.e., under load) and prevent the resulting voltage to ground condition of more than half the output voltage of the string. 
     According to one embodiment of the method in which at least one safety switch is connected between the two substrings, the string is connected to the inverter via two power cables and an additional safety switch is installed in each of the power cables. The string that is split into two serially connected substrings is electrically isolated from the AC power grid at first. This isolation defines a fixed ground reference point for the photovoltaic system while it is operating. More specifically, the string&#39;s center point between the two substrings is connected to a circuit ground while the photovoltaic system is operating. During normal operation of the photovoltaic system, the grounding ensures that no voltage greater than half of the output voltage of the strings is present at any point in the system to ground. If, given a ground fault, a current flows from the center point of the string to the circuit ground and exceeds a maximum threshold value, to the extent it is desirable that the grounding of the string at the center point between its substrings be terminated to interrupt this current, the safety switches between the substrings and in the power cables from the string to the inverter are opened. Opening these safety switches disconnects the substrings from each other and from the power cables so that, even without grounding, no voltage greater than half of the output voltage of the string is present at any point in the photovoltaic system to ground. 
     Usually, the string is electrically isolated from the AC power grid using a transformer through which power is supplied to the grid by the inverter and which has a floating contact on the inverter side. 
     In one embodiment of the present invention, a shared potential bus bar is used to connect the center points of multiple strings to the circuit ground. If a current flows to the circuit ground, then all the strings will initially be disconnected from the inverter via the safety switches and split into substrings. Normally, however, a ground fault only occurs in one of the strings, which means the inverter can continue to function using the other strings of the photovoltaic system. To determine the defective string, a switch that disconnects all the strings from the potential bus bar and then reconnects them one by one is installed, in one embodiment, for each connection between the strings and the shared potential bus bar. A significant amount of ground current will arise when connecting the string that has the ground fault and consequently is identified as a defective string. 
     To allow for the localization of the ground fault even if the safety switch between the substrings of each string is opened, it is desired in one embodiment to have each side of the safety switch between the two substrings connected with the circuit ground individually (i.e., the shared potential bus bar) and then wait and see if the ground current reappears. 
     According to one embodiment, once the ground fault is localized, only the strings that do not have a ground fault will be reconnected to the inverter. With these strings, the supply of electrical energy from the photovoltaic system to the AC power grid can continue without interruption. 
     The power cables of multiple strings are also combined to power bus bars in one embodiment. In one embodiment, a power switch is included in each of the two power bus bars that most of the strings are connected to. This power switch resides inside the inverter&#39;s enclosure while the safety switches in the individual power cables are installed in the same location as the strings in one embodiment. 
     In one embodiment, the new circuit arrangement, a GFDI circuit breaker is used to connect the center point between the two substrings to the circuit ground while the photovoltaic system is operating. The GFDI circuit breaker triggers or trips when a ground current exceeds its maximum permissible value as defined by the GFDI circuit breaker, which, in turn, indicates the presence of a ground fault. If the GFDI circuit breaker trips, then the safety switches are opened accordingly, in one embodiment. 
     Alternatively, in one embodiment the center point between the two substrings are grounded via a resistor to achieve a so-called soft grounding in which the ground fault protection devices of the circuit arrangement detect any voltage drop across the resistor. If this voltage rises to a specific level, then it serves as another indicator that a ground current limit has been exceeded. The safety switches are then opened accordingly. In one embodiment it is advantageous to have the resistor serially connected with an additional circuit breaker that opens to interrupt the excessively high ground current. 
     In one embodiment of the invention, the center point between the two substrings is connected to a circuit ground in the inverter&#39;s enclosure. The center point is thus connected to circuit ground in the central inverter, which means a GFDI circuit breaker or other circuit breaker for limiting the ground current can easily be reset to either localize the ground fault or continue operating the strings that are unaffected by any of the ground faults actually found and whose center points remain grounded. 
     To allow for the grounding of the center point in an inverter&#39;s enclosure, in one embodiment it is not necessary to route power cables from the center point to the enclosure. A basic potential cable with a small cross-section routed from the string to the enclosure is sufficient in this instance. In one embodiment, multiple basic cables with small cross-sections are routed from the string to the enclosure in addition to the two power cables. In this embodiment, at least one of the switches assigned to a substring is placed between the circuit ground and each substring and then closed individually by the ground fault protection devices in order to localize a ground fault. In one embodiment, these two switches for each string, which are used to localize ground faults, comprise two serially connected safety switches installed between the two substrings. Note, however, that these safety switches are placed in the same location as the string since they carry the power current between the substrings. They are also individually closable in order to localize ground faults in one embodiment. It is nonetheless possible and contemplated in one embodiment to arrange the switches assigned to the individual substrings in an inverter&#39;s enclosure. In such an embodiment one potential cable for each substring are routed to the enclosure where the switches are located. In one embodiment, the switches are connected between the relevant potential cable and a shared potential bus bar is used to ground multiple strings. 
     While the photovoltaic system is operating, the switches assigned to the individual substrings in the inverter&#39;s enclosure remain closed in one embodiment. Should a current arise from the shared potential bus bar to circuit ground, the switches will open so that any circuit breaker that the ground current causes to open can be reset without having it triggered directly. If the switches in the individual potential cables are closed successively, the ground current will reappear after the switch assigned to the substring with the ground fault closes. The string that contains this substring will not be used until the ground fault is eliminated, i.e., the safety switches assigned to the string and the switches in the potential cables remain open. 
     In one embodiment, the safety switches are used to actually connect or disconnect the entire photovoltaic system to or from the inverter. A central power switch in the power bus bars inside the inverter can also be used for this purpose. Note that the power cables from multiple strings are connected to these power bus bars. 
     In one embodiment the method and the circuit arrangement are configured to connect multiple strings to a remote inverter (i.e., 10 to 1000 meters away) using just two power cables, whereby all of the safety switches of each string are placed in the same location as this string. Only cables that do not carry a power current and therefore have a small cross-section are routed from the respective string to the inverter. 
     Further advantageous embodiments of the invention can be found in the patent claims, the description and drawings. The advantages of the features and combinations of features mentioned in the introduction of this description merely serve as examples and may be applied either alternatively or cumulatively without the advantages being necessarily achieved by embodiments according to the invention. Additional features can be found in the drawings—particularly in the depicted geometries and the relative measurements of components with respect to each other as well as their relative arrangement and functional interaction. The combination of features in different embodiments of the invention or the combination of features from different patent claims may also deviate from the selected retroactive applications of the patent claims and is hereby encouraged. This also applies to any features depicted in separate drawings or mentioned in their description. These features can also be combined with the features of different patent claims. The features defined in the patent claims may also be excluded in other embodiments of the invention. 
    
    
     
       DESCRIPTION OF THE FIGURES 
       The invention shall be further explained and described in the following with reference to the enclosed figures and based on examples of the various embodiments. 
         FIG. 1  depicts the layout of a string in a circuit arrangement specified by the invention. 
         FIG. 2  shows the connection of multiple strings (as per  FIG. 1 ) with an inverter in one of the first embodiments of the circuit arrangement specified by the invention. 
         FIG. 3  depicts a second circuit arrangement specified by the invention in which both the individual strings as well as the inverter vary in relation to  FIG. 2 . 
         FIG. 4  shows yet another embodiment of the circuit arrangement specified by the invention. 
     
    
    
     DESCRIPTION 
       FIG. 1  depicts a string  1  of a photovoltaic system which contains a multitude of solar modules  2  and which is split into two substrings  3  and  4  with the same number of solar modules  2  each, for example. In each string, the solar modules are arranged in multiple serial circuits that are individually protected by a fuse  5  and connected to each other in parallel. The substrings  3  and  4  are serially connected through two safety switches  6  and  7 . Additional safety switches  8  and  9  are installed in power cables  10  and  11 . These cables are connected to the free ends of substrings  3  and  4  and carry the electrical power generated by string  1 . A shared actuator  12  is provided for all safety switches  6  through  9 . In one embodiment the actuator  12  is configured so that the switches  6  through  9  only close when the control signal is present. At the safety switches  6  and  7 , the potential cables  13  and  14  lead away from the two substrings  3  and  4 . Fuses  15  are provided in the potential cables. The string  1  depicted in  FIG. 1  features a total of six terminals  16  and  17 , whereby only the two terminals  16  of the power cables  10  and  11  with large cross-sections have to be connected while the terminals  17  for the potential cables  13  and  14  and the actuator  12  can be connected with the wires of a basic multicore control cable with a comparatively small cross-section. To connect the string  1  shown in  FIG. 1  to an inverter at a greater distance, relatively little effort is needed compared to the output power of string  1  provided with the sum of output voltages of the substrings  3  and  4  amounting to, say, 600 V each (i.e., 1200 V) and thus by only half the current that would be supplied if substrings  3  and  4  were connected in parallel to limit the voltage. 
     In the given example, to avoid voltages to ground of more than 600 V even if the two substrings are serially connected when safety switches  6  and  7  close, a center point  18  of string  1  between the two substrings  3  and  4  is connected to the circuit ground  23  while the photovoltaic system  19  is operating with the circuit arrangement depicted in  FIG. 2 . The photovoltaic system  19  contains multiple strings  1  (three in this example). Note that  FIG. 2  does not show all the details of the string  1  as depicted in  FIG. 1 . The strings  1  are connected to the circuit ground  23  via the potential cables  13  and  14  in the enclosure  20  of an inverter  21  that is used to supply electrical energy from the photovoltaic system  19  to an AC power grid  22 . In order to ground the center points  18  of the strings  1  while the photovoltaic system  19  is operating, the photovoltaic system  19  is electrically isolated from the AC power grid  22  and hence from the system&#39;s ground, for example by a transformer  24  comprised in the inverter  21  or installed between the inverter  21  and the AC power grid  22 . The center points  18  of the individual strings  1  are grounded via a shared potential bus bar  25  to which the potential cables  13  and  14  are connected in the enclosure  20 . A switch  26  is provided in each potential cable  13  and  14  at the shared potential bus bar  25  in the enclosure  20 . The fuses  15  in the potential cables  13  and  14  prevent a short-circuit current from arising through the potential bus bar  25 . The potential bus bar  25  is grounded by means of a soft-grounding device  27  in which a circuit breaker  28  is serially connected to a resistor  29  between the potential bus bar  25  and the circuit ground  23 . While the photovoltaic system  1  is operating, both switch  26  and switch  28  are closed, and the voltage drop across the resistor is used to measure the current flowing from the center points  18  to the circuit ground  23  is monitored by a voltage measurement device  30 . When the voltage drop indicates that the ground current exceeds a maximum threshold value, the actuators  12  open the safety switches  6  through  9  after receiving a signal from a controller  31  via the activation lines  32 . At the same time, switch  28  and switch  26  are opened. By opening the safety switches  6  through  9 , the substrings  3  and  4  of all strings  1  are disconnected so that the maximum amount of output voltage of a substring  3  or  4  to ground is present in the area of the photovoltaic system  19 , even if a ground fault has occurred at the end of substring  3  or  4  that is facing away from the center point  18  and the ground connection of the center point  18  has been terminated when the switch  28  opens to interrupt the unwanted ground current. To localize this ground fault on the substring  3  or  4  in question without generating voltages greater than, for example, 600 V within the area of the photovoltaic system  19 , the safety switches  6  through  9  remain open. Switch  28  is closed and the switches  26  will be closed successively until the ground current caused by the voltage drop across the resistor  29  reappears. This is the case if the switch  26  for the substring  3  or  4  that is affected by the ground fault is closed. The string  1  with the substring  3  or  4  that is identified in this manner will stop working until the ground fault is eliminated. This means that the safety switches  6  through  9  of this string remain open. The associated switch  26  also remains open. The other strings  1 , however, can continue to be operated safely by closing their safety switches  6  through  9  and the associated switches  26  together with switch  28  to ground their center points. 
     In the embodiment of the circuit arrangement depicted in  FIG. 3 , the center points  18  are grounded over the shared potential bus bar  25  using a GFDI circuit breaker  33 , which trips (i.e., opens) when a ground current that exceeds a maximum threshold value is detected. In this respect, the GFDI circuit breaker  33  offers an alternative to the soft grounding device  27  shown in  FIG. 2 . Only one safety switch  6  is provided between the substrings  3  and  4  for each string shown in  FIG. 3  and the fuses  15  in the potential cables  13  and  14  may be excluded. As per  FIGS. 1 and 2 , these fuses  15  typically have a lower trigger value than the soft grounding device  27  or the GFDI circuit breaker  33 . Finally,  FIG. 3  also indicates that power switches  36  for connecting the photovoltaic system  19  to the inverter  21  are provided in the power bus bars  34  and  35 . The power cables  10  and  11  from the individual strings  1  are connected to these bus bars. In addition, a buffer capacitor  37  between power bus bars  34  and  35  is connected to the input of the inverter  21 . However, the basic function of the circuit arrangement shown in  FIG. 3  matches the function of the arrangement shown in  FIG. 2 . 
     The circuit arrangement in  FIG. 4  is a modification of the circuit arrangement in  FIG. 3  insofar as each string  1  has its own GFDI circuit breaker  33  located in the same location as the string  1  and a signal line  38  is routed from this circuit breaker to the central controller  31  in the enclosure  20  of the inverter  21 . In this arrangement, the actuators  12  that open the safety switches  6  through  9  are not activated by the controller  31  via the activation lines  32 . Instead, each of the actuators  12  are connected to the GFDI circuit breaker via a coupling device  39  in such a way that the safety switches  6  through  9  are opened directly by the circuit breaker whenever it trips. The controller  31  can then be used to selectively close the two safety switches  6  and  7  that are also included in this arrangement in order to localize the substring  3  or  4  in which the ground fault that triggered the GFDI circuit breaker has occurred. As a matter of course, the GFDI circuit breaker must be reset (i.e., closed again) before the ground fault can be localized. This selective checking of the substrings  3  and  4  is particularly advantageous given that multiple GFDI circuit breakers  33  can be tripped when a ground fault occurs in just one substring  3  or  4  of the photovoltaic system  19 . Before the substrings  3  and  4  of any string  1  can be reconnected to each other without risk, it must be assured that the ground fault is not present in any of these substrings  3  and  4  in one embodiment. If the controller  31  is unable to activate the safety switches  6  and  7  separately from each other and separately from the other safety switches  8  and  9 , each GFDI circuit breaker  33  can also be connected to the center point  18  via the two potential cables  13  and  14  that contain switch  26  as per  FIG. 3 . This would also make it possible to exclude one of the safety switches  6  and  7  in each string  1 . Similar to the activation lines  32 , the signal lines  38  can have the same small cross-section as the potential cables  13  and  14  as depicted in  FIGS. 1 ,  2 , and  3 . 
     While the invention has been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular regard to the various functions performed by the above described components or structures (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.