Patent Publication Number: US-2019190272-A1

Title: Recovery of degraded photovoltaic panel

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of photovoltaic panels. In particular, the invention relates to an inverter system, a photovoltaic panel system and a method for operating the photovoltaic panel system. 
     BACKGROUND OF THE INVENTION 
     Potential induced degradation (PID) is a phenomenon which causes significant reduction in power generation capability of photovoltaic panels. Exposure of a photovoltaic panel to a high negative potential is the major cause of PID, which may gradually reduce the rated output power of a photovoltaic panel after a few years of service. 
     In the case of a galvanically isolated inverter system, which is used for transforming the DC voltage from the photovoltaic panel into an AC voltage to be supplied to a grid, the problem of PID can be easily solved by grounding the negative pole of the photovoltaic panel. On the other hand, typical transformerless inverters, which normally apply a negative potential to the negative pole of the photovoltaic panel, do not allow connecting the negative pole directly to the earth ground since this would cause a ground short circuit. 
     One type of PID solution for a photovoltaic panel interconnected with a transformerless inverter is based on a special topology which enables direct connection of the negative pole of the photovoltaic panel to the ground. For example, US 2011 235 384 A1 shows a photovoltaic panel system with an inverter that allows to connect the negative pole of the photovoltaic panel to ground. A drawback of such a solution may be that the performance of the special topology is not optimal in terms of efficiency, cost, and complexity. 
     Further solutions exploit that PID may be corrected by applying a positive potential to the photovoltaic panel. For example, a PID corrector, which applies a positive voltage to the photovoltaic panel during the night, may be connected to the photovoltaic panel. Such a corrector only may be used when the transformerless inverter is disconnected from the grid, when the corrector is operating. That is, ancillary services may not be available during operation of the corrector. 
     EP 2 773 036 A1 relates to a method for DC-AC-conversion for photovoltaic panels, wherein each photovoltaic panel is connected with its outputs with a converter. The converter has a DC link and a switch for disconnecting a negative output from the DC link. 
     EP 2 234 264 A1 and US 2014/0139031 A1 show photovoltaic panels interconnected via a converter with an electrical grid. 
     DESCRIPTION OF THE INVENTION 
     It is an objective of the invention to provide a simple PID recovery solution without the need for restricting to specific topologies for the inverter system. 
     This objective is achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description. 
     An aspect of the invention relates to an inverter system for a photovoltaic panel. The inverter system is used for converting the electrical DC voltage produced by the photovoltaic panel into an AC voltage to be supplied to an electrical grid. In general, the inverter system may comprise an inverter with semiconductor switches that are controlled by a controller to generate the AC voltage. 
     According to an embodiment of the invention, the inverter system comprises an inverter adapted for transforming a DC voltage from the photovoltaic panel into an AC voltage to be supplied to an electrical grid; a DC link interconnected with the inverter and providing a positive DC link output connectable to a positive pole of the photovoltaic panel and a negative DC link output connectable to a negative pole of the photovoltaic panel; a switch, which may be positioned between the negative pole and the negative DC link output, for disconnecting the negative pole of the photovoltaic panel from the DC link; and a controller for controlling the inverter and the switch. The controller is adapted for opening the switch, such that the photovoltaic panel is solely supplied by the positive DC link output. 
     For example, the inverter, which may be seen as the main inverter of the inverter system, may comprise a three-phase or single-phase input at the AC side, which may be interconnected with the electrical grid. On the DC side, the inverter may be interconnected with a DC link, which may comprise a capacitor. To this DC link, which provides the positive and the negative DC link output, the photovoltaic panel may be connected. 
     During normal operation of the inverter and the photovoltaic panel, when sun is shining on the photovoltaic panel, the positive pole of the photovoltaic panel is on positive potential (such as +V DC /2, where V DC  is the DC link voltage) and the negative pole is on negative potential (such as −V DC /2). It has to be noted that all voltages mentioned herein may be determined from ground, i.e. the ground potential may have a voltage of 0 V. Thus, during normal operation, PID may take place. 
     For recovery operation of the photovoltaic panel, for example during night-time, the inverter system comprises a switch that is positioned in the negative output and/or that may be opened by the controller. For example, the switch may be a mechanical relay or a semiconductor switch. When the switch is opened, the negative pole of the photovoltaic panel is disconnected from the negative DC link output. The positive pole of the photovoltaic pole remains on the potential of the positive DC link output. Since during the night, when the recovery operation may take place, the photovoltaic panel does not generate a voltage or nearly no voltage between its positive and negative pole, also the negative pole is shifted (or nearly shifted) to the potential of the positive DC link output. Thus, the effect of PID may be reversed during recovery operation. 
     No special topology for the inverter is necessary. The inverter may stay connected with the electrical grid and/or also an auxiliary power supply of the inverter system supplied by the DC link may stay in operation. Thus, the performance of the inverter is not affected, while a connection of the inverters to the electrical grid is still allowed during recovery operation, for example during night-time for performing ancillary services. Furthermore, the solution can be implemented and integrated into an inverter, without the need for a complicated additional system. 
     According to an embodiment of the invention, the controller is adapted for determining, when a voltage produced by the photovoltaic panel falls below a threshold voltage and for then opening the switch. The switch may be opened and closed automatically by the controller. The determination, whether the photovoltaic panel may be recovered, i.e. produces no or nearly no voltage, may be performed directly or indirectly. 
     For example indirectly, the determination may be based on a clock of the controller, i.e. it may be assumed that during a specific time interval (that may be called night), the sun is down and therefore no light is falling onto the photovoltaic panel. For example directly, that the controller receives measurement values from the inverter system, upon which the actual voltage of the photovoltaic panel may be determined. 
     According to an embodiment of the invention, the controller is adapted for maintain a DC link voltage, when the switch is opened. During recovery operation, the inverter may supply the DC link with voltage from the electrical grid to maintain the voltage at the positive DC link output. 
     According to an embodiment of the invention, the inverter system further comprises an additional power supply for generating an additional DC voltage; and a second switch, which may be interconnected between the positive pole and the positive DC link output, for interconnecting the photovoltaic panel with the additional power supply, such that the additional DC voltage from the additional power supply is added to a positive DC link voltage. For example, the second switch may be connected in parallel to the additional power supply and during recovery, the controller may open the switch, such that the additional power supply is connected in series with the DC link. The additional power supply may be supplied with electrical power from the electrical grid. 
     Since day-time is usually longer than night-time, the exposure time of the photovoltaic panel to the negative potential from the negative DC link output may be longer than that to the positive potential of the positive DC link output during the PID recovery operation. The higher voltage produced by the additional power supply may be used to fully recover from PID during a short time. 
     Alternatively or additionally, a DC link voltage may be increased during the recovery operation by corresponding control of the controller. 
     According to an embodiment of the invention, the additional DC voltage is higher than 100 V, for example more than 1,000 V. It has to be noted that for effective recovery from PID, higher voltages as provided by the DC link may be needed. With the additional power supply, which may have lesser power than the main inverter, these high voltages may be reached. 
     According to an embodiment of the invention, the inverter system further comprises an auxiliary power supply for supplying the controller with power. Usually, every inverter system has an auxiliary power supply, which is used for supplying electronic components of the system with power and/or which produces a rather low voltage (for example below 20 V). Such auxiliary power supply may be supplied by the DC link of the inverter or directly by the electrical grid. 
     According to an embodiment of the invention, the additional power supply has a common transformer with the auxiliary power supply. The additional power supply and the auxiliary power supply may be integrated into each other. Both may be flyback converters with windings on a common core. 
     According to an embodiment of the invention, the additional power supply is supplied by the auxiliary power supply. On the other hand, the additional power supply may be connected to an output of the auxiliary power supply and may transform the voltage from the auxiliary power supply into a higher voltage. 
     According to an embodiment of the invention, the inverter is a transformerless inverter, such that the photovoltaic panel is galvanically connected with the electrical grid during normal operation of the inverter system. Since the negative pole of the photovoltaic panel is galvanically disconnected from the inverter system during recovery operation, there is no need for a galvanically separating inverter system, which is usually done with a transformer. 
     A further aspect of the invention relates to a photovoltaic panel system, which comprises an inverter system as described in the above and in the following and a photovoltaic panel connected to the converter system. 
     According to an embodiment of the invention, the negative pole of the photovoltaic panel is floating, when the switch in the negative DC link output is opened. In other words, the negative pole of the photovoltaic panel may be completely disconnected from a potential and/or may only be connected via the photovoltaic panel with the positive DC link output. 
     A further aspect of the invention relates to a method for recovering a degraded photovoltaic panel. For example, the method may be performed by the controller of the photovoltaic panel system. 
     According to an embodiment of the invention, the method comprises: during normal operation, converting a DC voltage from the photovoltaic panel with an inverter into an AC voltage supplied to an electrical grid, wherein the photovoltaic panel is connected with a positive pole and a negative pole to a DC link of the inverter; and during recovery operation, disconnecting the negative pole of the photovoltaic panel from the DC link of the inverter and supplying the positive pole of the photovoltaic panel with a positive voltage from the DC link. When recovery operation is started, the controller may simply open the switch. 
     According to an embodiment of the invention, the method further comprises: closing a switch for connecting the negative pole of the photovoltaic panel with a negative output of the DC link during normal operation; opening the switch for disconnecting the negative pole of the photovoltaic panel from the negative output of the DC link during recovery operation. 
     According to an embodiment of the invention, the method further comprises: maintaining a DC link voltage in the DC link during recovery operation. 
     According to an embodiment of the invention, the method further comprises: 
     interconnecting an additional power supply between the positive pole of the photovoltaic panel and a positive output of the DC link, for raising a voltage supplied to the positive pole of the photovoltaic panel. This may be done with a further, second switch in the positive DC link output, which is controlled by the controller. 
     According to an embodiment of the invention, the method further comprises: determining whether the photovoltaic panel is producing a voltage (for example higher than a threshold) between its positive and negative pole. As mentioned above, this determination may be performed directly, i.e. via measurement in the photovoltaic panel system or indirectly, with a clock. In the latter case, it may be assumed that during night-time, which may be defined as a time interval in the controller, no sun is shining and the photovoltaic panel does not produce a voltage between its positive and negative pole. 
     When night-time is determined, the controller may switch to recovery operation. When day-time is determined, i.e. the opposite of night-time, the controller may switch to normal operation. 
     According to an embodiment of the invention, the determination, whether the photovoltaic panel is producing a voltage, is based on an actual time, which may be provided by a clock of the controller. It also may be possible that the determination, whether the photovoltaic panel is producing a voltage, is based on a voltage measurement. 
     It has to be understood that features of the method as described in the above and in the following may be features of the inverter system and/or the photovoltaic panel system as described in the above and in the following, and vice versa. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings. 
         FIG. 1  schematically shows a photovoltaic panel system according to an embodiment of the invention. 
         FIG. 2  schematically shows a photovoltaic panel system according to a further embodiment of the invention. 
         FIG. 3  schematically shows an embodiment of an additional power supply for the photovoltaic panel system of  FIG. 2 . 
         FIG. 4  schematically shows a further embodiment of an additional power supply for the photovoltaic panel system of  FIG. 2 . 
         FIG. 5  shows a flow diagram for a method for recovering a photovoltaic panel according to an embodiment of the invention. 
         FIG. 6  shows a diagram with a voltage at the negative pole of a photovoltaic panel produced with the method of  FIG. 5 . 
         FIG. 7  shows a diagram with a voltage at the negative pole of a photovoltaic panel produced with the method of  FIG. 5 . 
     
    
    
     The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows a photovoltaic panel system  10  comprising a photovoltaic panel  12 , which is interconnected via an inverter system  14  with an electrical grid  16 . 
     The photovoltaic panel  12  has a positive pole  18   a  and a negative pole  18   b.  When light (usually sunlight) is shining on the photovoltaic panel  12 , a positive voltage V PV  is generated by the photovoltaic panel  12  between the positive pole  18   a  and the negative pole  18   b.  When no or nearly no light is shining on the photovoltaic panel, the voltage V PV  between the poles  18   a,    18   b  is substantially zero. 
     The photovoltaic panel is connected with its positive pole  18   a  with a positive DC link output  20   a  of the inverter system  14  and with its negative pole  18   b  with a negative DC link output  20   b  of the inverter system  14 . The outputs  20   a,    20   b  are provided by a DC link  22 , which are interconnected with a main inverter  24  of the inverter system. The inverter  24  may be a voltage source inverter, for example, a two or more level converter based on half-bridges. The DC link may comprise a capacitor interconnected between the outputs  20   a,    20   b.    
     On an AC side, the inverter  24  may be connected via a filter  26  with the electrical grid  16 . The grid  16  may be a three-phase or single-phase grid and/or the inverter  24  may be a single-phase or three-phase inverter  24 . It may be possible that a relay is situated between the filter  26  and the grid  16 , which may be used for disconnecting the inverter system  14  from the grid  16 . It has to be noted that the inverter system  14  may be transformerless, i.e. may contain no transformer that galvanically decouples the photovoltaic panel  12  from the electrical grid  16 . 
     Furthermore, the inverter system  14  comprises a controller  28 , which is adapted for controlling the inverter  24 . During a normal operation of the system  10 , the photovoltaic panel produces a voltage V PV  which is supplied to the DC link, which then has the equal voltage V DC . The positive voltage V PV+  at the positive pole  18   a  is equal to the positive DC link voltage V DC+ , which has a value of +V DC /2 with respect to ground. The negative voltage V PV− , at the negative pole  18   b  is equal to the negative DC link voltage V DC , which has a value of −V DC /2 with respect to ground. The inverter  24 , under the control of the controller  28 , converts the DC link voltage V DC  into the single-phase or three-phase voltage V g  supplied to the electrical grid  16 . 
     Since the negative voltage V PV−  is negative with respect to ground, potential induced degradation (PID) takes place during normal operation. 
     For mitigating or counterbalancing this PID, the inverter system  14  comprises a switch  30  in the negative DC link output  20   b,  which is adapted for disconnecting the negative pole  18   b  of the photovoltaic panel  12  from the negative DC link  20   b.  The switch  30  may be opened and closed by the controller  28 . 
     When the switch  30  is opened, the negative pole  18   b  of the photovoltaic panel  12  is disconnected from the potential provided by the negative output  20   b  of the DC link  22  and therefore may be free floating, when this is the only connection point of the negative pole  18   b  with the system  10 . Thus, the voltage at the negative pole  18   b  is the voltage of the positive pole  18   a,  which, in  FIG. 1 , is the potential of the positive DC link output  20   a,  i.e. +V DC /2, minus the voltage V PV  generated by the photovoltaic panel  12 . In the night-time, the voltage V PV  is substantially zero and the negative pole  18   b  is set to about the voltage provided by the positive output  20   a  of the DC link  22 . 
     Thus, when opening the switch  30  during night-time in a recovery operation mode, the PID of the photovoltaic panel  30  may be reversed. 
     The effect of the recovery is dependent on the height of the positive voltage applied to the photovoltaic panel  12 . For example, the voltage at the positive output  20   a  of the DC link may be raised by operating the inverter  24  in a corresponding way. 
     As shown in  FIG. 2 , an alternative (or addition) to this is an additional power supply  32 , which provides a voltage V aux  that is added to the voltage provided by the positive output  20   a.  For example, the additional power supply  32  may be connected between the positive pole  18   a  and the positive output  20   a  of the DC link  22 . A further switch  34  connected in parallel to the additional power supply  32  may be opened to connect the additional power supply  32  to the positive pole  18   a  and may be closed to disconnect it. Also, the further switch  34  may be controlled by the controller  28 . 
     During normal day-time operation, both of the switches  30 ,  34  are closed. During night-time, for recovery operation, both switches may be opened and the additional voltage V aux  is applied between the photovoltaic panel  12  and the DC link  22 . In this case, the potential of the negative pole  18   b  of the photovoltaic panel  12  may reach the sum of V DC /2 and V aux , i.e., the potential is increased by V aux , which accelerates PID recovery. 
       FIG. 3  shows an embodiment of an additional power supply  32 , which is integrated into an auxiliary power supply  36  of the inverter system  14 . The auxiliary power supply  36 , which usually is used to feed the power for electronics, gate drivers, fans inside the inverter  24 , and in particular the controller  28 , may be supplied by the DC link  22  and/or may be a flyback converter as shown in  FIG. 3 . 
     The auxiliary power supply  36  and the additional power supply  32  share a transformer  38  with a primary winding. A first secondary winding of the transformer  38 , which is part of a first flyback converter, may generate the auxiliary supply voltage V 0 . A second secondary winding of the transformer  38 , which is part of a second flyback converter for the additional power supply  32 , generates the additional voltage V aux . 
     With a switch  40 , the additional power supply  32  may be disconnected from the negative pole  18   a  and the negative output  20  (see  FIG. 2 ). This switch  40 , under the control of the controller  28 , may be closed during recovery operation and opened during normal operation. Thus, V aux  may be generated by extending the secondary winding of the transformer  38  of the auxiliary power supply  36 . 
       FIG. 4  shows an alternative embodiment for an additional power supply  32 , which may comprise a separate converter, which is supplied by the auxiliary power supply  36 . The auxiliary power supply  36  may be designed like in  FIG. 3 . 
     In  FIG. 4 , the additional power supply  32  is connected to the output of the auxiliary power supply  36 . In this case, no modification of an existing auxiliary power supply  36  is required and the additional power supply may only need a small isolation. Since the additional power supply  32  only need to sustain the specific voltage V aux  without feeding power, a very low-power and/or low-cost implementation may be feasible. 
       FIG. 5  illustrates a method for recovering a degraded photovoltaic panel  12 , which may be performed by the system  10  under the control of the controller  28 . 
     In step S 10 , the system  10  is in normal operation  42  (see  FIG. 6  and  FIG. 7 ). During normal operation  42 , the system  10  converts the DC voltage V PV  from the photovoltaic panel  12  with the inverter  24  into the AC voltage V g , which is supplied to the electrical grid  16 . 
     Normal operation  42  is usually performed during daytime, when the voltage V PC  produced by the photovoltaic panel  12  is substantially different from 0. 
     In step S 12 , the system  10  is in recovery operation  44 . During recovery operation  44 , the negative pole  18   b  of the photovoltaic panel  12  is disconnected from the negative DC link output  20   b  of the inverter system  14 . Only a positive voltage is supplied from the inverter system  10  to the positive pole  18   a  of the photovoltaic panel  12 . It may be that the DC link voltage V DC  in the DC link  22  is maintained during recovery operation  44  by switching the inverter  24  correspondingly. The power for maintaining the DC link voltage V DC  may be supplied by the grid  16 . 
     Recovery operation  42  is usually performed during night-time, when the voltage V PC  produced by the photovoltaic panel  12  is substantially 0. 
       FIGS. 6 and 7  show diagrams with the voltage V PV  of the negative pole  18   b  of the photovoltaic panel  12 . During normal operation  42 , the voltage V PV+  is equal to the negative DC link voltage V DC− , since the negative pole  18   b  is connected via the switch  30  with the negative DC link output  20   b.    
     When the system  10  is in normal operation  42  (and also in recovery operation), the controller  10  determines, for example continuously and/or repeatedly, whether the photovoltaic panel  12  is producing a voltage V PV  between its positive pole  18   a  and negative pole  18   b.  The determination, whether the photovoltaic panel  12  is producing a voltage, may be based on an actual time and/or a voltage measurement. 
     For example, a timer in the controller  28  may determine that recovery operation  44  should be started, when a specific daytime has been reached, i.e. the night-time has begun. It also may be possible that the controller receives measurement values, from which the voltage V PC  may be determined and switches to recovery operation  44 , when the voltage V PC  falls below a threshold value. 
     For starting recovery operation  44 , the controller  28  disconnects the negative pole  18   b  of the photovoltaic panel  12  from the DC link  22  of the inverter  24  and supplies only the positive pole  18   a  of the photovoltaic panel  12  with at least the positive voltage V DC+  (i.e. V DC /2) from the DC link  22 . The switch  30  maintains open during recovery operation  44 . 
     For the system  10  from  FIG. 1 , during recovery operation  44 , the voltage V PV+  may be equal to the positive DC link voltage V DC+  as shown in  FIG. 6 . 
     With the system  10  of  FIG. 2  it also may be possible that the controller  28  interconnects the additional power supply  32  between the positive pole  18   a  of the photovoltaic panel  12  and the positive output  20   a  of the DC link  22 . This may be done by opening the switch  34  and closing the switch  40 . Both switches may stay in this position during the whole recovery operation  44 . 
     Thus, the voltage V PV+  at the positive pole  18   a  and, as shown in  FIG. 7 , the voltage V PV−  at the negative pole  18   b  may be raised up to V DC+ +V aux  (i.e. V DC /2+V aux ), the sum of the positive DC link voltage V DC+  and the additional voltage V aux . In general, the voltage V PV−  is V PV+  minus V PC , which however, may be assumed to be 0 during night-time. 
     In both cases ( FIG. 6  and  FIG. 7 ), a recovery of the photovoltaic panel  12  takes place during recovery operation  44 , which is completely on positive potential. 
     When the controller  28  determines that recovery operation should be ended and/or normal operation  42  should be started, which again may be based on an actual time and/or measurements, the switch  30  is closed for connecting the negative pole  18   b  of the photovoltaic panel  12  with the negative output  20   b  of the DC link  22 . During normal operation, the switch  30  stays closed. Additionally, for the embodiment of  FIG. 2  switch  34  may be closed and switch  40  may be opened. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. 
     LIST OF REFERENCE SYMBOLS 
     
         
           10  photovoltaic panel system 
           12  photovoltaic panel 
           14  inverter system 
           16  electrical grid 
           18   a  positive pole 
           18   b  negative pole 
           20   a  positive output 
           20   b  negative output 
           22  DC link 
           24  inverter 
           26  filter 
           28  controller 
           30  switch 
           32  additional power supply 
           34  switch 
           36  auxiliary power supply 
           38  transformer 
           40  switch 
           42  normal operation 
           44  recovery operation