Abstract:
The invention relates to a module for locally controlling a photovoltaic panel that includes: first and second terminals (B 1 , B 2 ) for connecting in series by a single conductor ( 13 ) having homologous modules; a first terminal (A 1 ) for connecting the photovoltaic panel, said first terminal being connected to the first terminal (B 1 ) for connecting in series; a switcher (S) that is connected between the second terminal (B 2 ) for connecting in series and a second terminal (A 2 ) connecting the panel; a diode (D 0 ) that is connected between the first and second terminals (B 1 , B 2 ) for connecting in series; a converter ( 70 ) that is provided so as to supply power to the module on the basis of the voltage that is developed by the panel between the first and second terminals (A 1 , A 2 ) connecting the panel; a sensor (R 3 ) for measuring the current flowing within the single conductor ( 13 ); and a means ( 60, 62 ) for closing the switcher when the current flowing within the single conductor exceeds a threshold.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to the management of a fleet of photovoltaic panels. 
       BACKGROUND OF THE INVENTION 
       [0002]    A traditional photovoltaic panel comprises several parallel/serial associations of photovoltaic cells and develops a direct voltage of approximately forty volts at its terminals under nominal light conditions. In a minimal facility, approximately ten panels are serially connected to produce a direct voltage, in the vicinity of 400 V, that can be exploited with a good output by an inverter to transfer the energy produced onto the network. 
         [0003]    One advantage of the serial connection of the panels is that the connector technology is reduced to two connection terminals per panel, plus one ground terminal, which facilitates installation. The panels are thus equipped with standardized junction boxes comprising the necessary terminals. 
         [0004]    Nevertheless, the serial connection may have a number of problems. 
         [0005]    The current produced by a serial string of panels is determined by the weakest link, i.e. the panel generating the weakest current. That panel may simply be a panel located in the shade. In such a situation, it is necessary to establish a path short-circuiting the panel, such that the panels operating under normal conditions can throw their nominal current. To that end, the panels are equipped with so-called “bypass” diodes, connected between the terminals of the panel, in the direction of the current, which is generally the blocked direction of the diodes relative to the voltage generated by the panel. When a panel no longer generates any voltage, the current of the string passes through its bypass diodes. 
         [0006]    However, when a panel is partially in shade, it will produce a voltage below its nominal voltage, but sufficient to avoid activating the bypass diodes. 
         [0007]    To manage such a situation more smartly, it has been provided to equip each photovoltaic panel with a control module electrically powered by the panel, as described in U.S. Pat. No. 7,602,080. 
         [0008]      FIG. 1  diagrammatically illustrates a local control module  10  (LCU) associated with a panel  12 , as described in the aforementioned patent. The LCU control module is connected to the panel  12  by two connection terminals A 1  and A 2 , terminal A 1  being connected to the “+” of the panel, and terminal A 2  to the “−”. The module includes two terminals B 1  and B 2  for connecting it in series by a single conductor  13  to homologous modules. The cathode of a bypass diode D 1  is connected to the terminal B 1  and the anode of said bypass diode is connected to the terminal B 2 . The direction of the serial current in the conductor  13  is thus from the terminal B 2  toward the terminal B 1 . A switch S, controlled by a circuit  14 , is connected between the terminals A 1  and B 1 . A capacitor C 1  is connected between the terminals A 1  and A 2 . 
         [0009]    The control circuit  14  is powered by the panel  12 , between the terminals A 1  and A 2 . It communicates with a shared central control unit located at the inverter through a COM link. To avoid multiplying the number of connections between panels, this link may be done by carrier current on the serial link conductor or by wireless communication. 
         [0010]    The purpose of this management system is to control, in switching mode, the switch S of a module associated with a lowly-lit panel to optimize the energy transfer. 
         [0011]    As indicated, the LCU control modules are powered by the associated panel  12 . If the electricity production of the panel is insufficient, the module no longer works. In that case, the module is unable to communicate with the central control unit, in particular to indicate the permanent or temporary out-of-service status of the panel. 
         [0012]    The system described in the aforementioned patent uses complex communication means between the modules and the central control unit. Each module must incorporate a microcontroller and a modem by carrier current or by wireless communication. These means are too costly for bottom-of-the-line facilities into which one nevertheless wishes to integrate certain basic functions. 
         [0013]    A fleet of photovoltaic panels has a risk of electrocution during assembly. In fact, a lit panel, even a disconnected one, begins to produce electricity. As the panels are connected in series, the difference in potential between the end terminals of the mounted panels increases, that potential difference reaching the vicinity of 400 V when it is time to connect the last panel. 
         [0014]    In current fleets, it is difficult to locate the site of an accidental cut in the serial link conductor. In fact, the cut of the serial conductor cancels the current therein. All the units of the panels see cancellation of the current at the same time, such that a module, even a smart module, cannot determine that the cut has occurred at its level to indicate that fact. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0015]    Thus, it is desirable for a local control module of a photovoltaic panel to be able to be electrically powered even if the panel is not producing electricity, without using links other than the serial link conductor of the panels. 
         [0016]    To meet that need, a module is provided for locally controlling a photovoltaic panel that includes first and second terminals for connecting it in series with homologous modules by a single conductor, and means for supplying the module with electricity from the current flowing within the single conductor. 
         [0017]    One embodiment of a central control unit for a set of modules of this type includes a sensor for measuring the current flowing within the single conductor and means for injecting a current into the single conductor sufficient to power the modules when the measured current is below a threshold. 
         [0018]    It may also be desirable for the module to have a minimum level of intelligence, in particular to control a safety device limiting the risk of electrocution, without providing complex communication means. 
         [0019]    To meet this need, a module is provided for locally controlling a photovoltaic panel that includes first and second terminals for connecting it in series by a single conductor with homologous modules; a first terminal for connecting the photovoltaic panel, said first terminal being connected to the first terminal for connecting in series; a switch that is connected between the second terminal for connecting in series and a second terminal connecting the panel; a diode that is connected between the first and second terminals for connecting in series; a converter that is provided so as to supply power to the module on the basis of the voltage that is developed by the panel between the first and second terminals connecting the panel; a sensor for measuring the current flowing within the single conductor; and a means for closing the switch when the current flowing within the single conductor exceeds a threshold. 
         [0020]    One embodiment of a central control unit for a set of modules of this type includes a means for determining a power-on of the set of modules; and a means for injecting a current into the single conductor that is above the threshold when the power-on is determined, resulting in closing the switches of the modules associated with panels supplying electricity. 
         [0021]    Lastly, it is desirable to be able to locate the position of a cut of the serial link conductor of the panels in a simple manner. 
         [0022]    To meet this need, a module is provided for locally controlling a photovoltaic panel including first and second terminals for connecting it in series with homologous modules by a single conductor; a diode element allowing current to flow between the first and second terminals for connecting in series when the photovoltaic panel does not produce electricity; a ground terminal; and a steady current source connected between the ground terminal and the single conductor. 
         [0023]    One embodiment of a central control unit for a set of modules of this type includes first and second input terminals, for connecting to the ends of the single conductor, one of the input terminals being grounded; a sensor for measuring the current flowing in the single conductor; and means for locating the module at which the cut is located from the measured current. 
         [0024]    The central control unit may use a method including the following steps: detecting the cut by the fact that the current within the single conductor drops to a residual value below or equal to the sum of the currents of the steady current sources of the modules; and determining the rank of the module at which the cut is located by dividing the residual value of the current by the value of the steady current sources. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Other advantages and features will appear more clearly from the following description of specific embodiments provided as non-limiting examples and illustrated using the appended drawings, in which: 
           [0026]      FIG. 1 , previously described, shows a local control module for a panel of a traditional system for managing a fleet of photovoltaic panels; 
           [0027]      FIG. 2  shows one embodiment of a local control module for a panel that can be supplied with electricity independently of the electricity production of the panel; 
           [0028]      FIGS. 3   a  and  3   b  show two operating modes of the module of  FIG. 2  when the panel is producing electricity; 
           [0029]      FIGS. 4   a  and  4   b  show two operating modes of the module of  FIG. 2  when the panel is not producing electricity; 
           [0030]      FIG. 5  shows one embodiment of a system for locating an outage of the conductor for connecting the panels in series and an adapted central control unit; 
           [0031]      FIG. 6  shows one embodiment of a local control module of a panel incorporating a simple communication means, in particular to control a safety device limiting the risks of electrocution; 
           [0032]      FIG. 7  shows an alternative of the module of  FIG. 2 ; 
           [0033]      FIG. 8  shows another alternative of the module of  FIG. 7 ; and 
           [0034]      FIGS. 9   a  and  9   b  show two operating modes of the alternative of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]      FIG. 2  diagrammatically shows one embodiment of a local control module LCU of a photovoltaic panel  12 , which does not depend on the production of electricity by the panel to be powered. The module is powered from the serial current flowing in the serial link conductor  13  of the panels. 
         [0036]    The module, which is intended to be incorporated into a standardized junction box, comprises the same terminals A 1 , A 2 , B 1  and B 2  as the module of  FIG. 1 . The connection terminal A 1  of the panel is connected to the terminal B 1  for connecting in series. The switch S, formed with an N-channel MOS transistor, for example, is connected between the connection terminal A 2  of the panel and the terminal B 2  for connecting in series. Instead of finding a single diode between the terminals B 1  and B 2 , there is a stack of diodes Dn forming a diode element whereof the conduction threshold is higher than that of a diode. The cathodes of the stack of diodes Dn are on the side of the terminal B 1 . A diode D 0  is connected by its cathode to the terminal A 1 , and by its anode to the terminal A 2 . This diode D 0  preferably has a low conduction threshold, and to that end may be a Schottky diode. 
         [0037]    The gate of the transistor S is controlled by a circuit  14  that is powered between the potential supplied by a converter  16  and the terminal A 2 . The supply voltage Vin of the converter  16  is taken at the terminals of the transistor S. In this way, the voltage Vin at the terminals of the transistor S is particularly low when the transistor S is closed. The transistor S is chosen with a resistance in the on state (Rdson) that is high enough so that the voltage at its terminals, produced by the current passing through the resistance, is able to power the converter  16 . It will be shown below that the converter  16  can be powered in all operating modes of the module. 
         [0038]    The converter  16  is preferably a switching step-up converter. Step-up converters exist on the market that can produce, in a state of equilibrium, a sufficient supply voltage from less than 100 mV, such as the L6920 circuit marketed by STMicroelectronics. This circuit nevertheless requires a higher voltage to start, which will be provided to it as seen below. 
         [0039]    Such a converter  16  is generally provided to work with a maximum input voltage of several volts, whereas the voltage Vin can reach the voltage Vp of the panel. Preferably, at the input of the converter  16 , a voltage limiting circuit  17  is provided based on a transistor and a Zener diode to keep the input voltage of the converter within the required boundaries. 
         [0040]    The control circuit  14  here integrates, as communication means with a central control unit, a carrier current modem connected to a current transformer  18  inserted within the serial link conductor at the terminal B 2 . The module can thus for example transmit, to the central control unit, the value of the voltage provided by the panel  12 , measured by a resistance bridge  20 , and receive switching commands from the transistor S. 
         [0041]    In order to improve the transmission by carrier current, a capacitor C 2  is provided connected to the terminals of the diode element Dn. This capacitor offers a low impedance at the modulation frequency of the carrier current, and therefore makes it possible to short-circuit the complex impedances introduced by the various elements connected between the terminals B 1  and B 2 . 
         [0042]      FIGS. 3   a  and  3   b  show two operating modes of the module of  FIG. 2  when the panel  12  produces electricity. The transistor S is symbolized by a switch, and it is assumed that the circuit  14  (not shown here) controls permanently the closing of the transistor S, which is equivalent, as shown, to control of the transistor S by the output of the converter  16 . It is also assumed that the ends of the string of panels are connected to the inverter, which closes the current circuit. 
         [0043]      FIG. 3   a  shows a case wherein the fleet is started up in full daylight. The module was not power supplied, since the serial current was equal to zero. The transistor S is therefore open. The voltage Vp at the terminals of the panel  12  establishes a current that can flow through the diode elements Dn of the other modules (not shown), the inverter (not shown), and the converter  16 . This current is equal to: 
         [0000]        I=[Vp −( n− 1) Vn ]/( Zinv+Zsmps ),
 
         [0000]      and  Vin=Zsmps·I,    
         [0000]    where Vn is the conduction threshold of a diode element Dn, n is the number of panels, Zinv is the impedance of the inverter, and Zsmps is the impedance of the converter  16 . The inverter being configured to handle an important power, its impedance is low, while the impedance of the converter  16  is relatively high. Thus, the voltage Vin at the input of the converter is established practically at Vp−(n−1)Vn, which is greatly sufficient to start up the converter. The transistor S is closed right away, resulting in the mode of  FIG. 3   b.    
         [0044]    On the other hand, if the inverter is not connected to the string, for example if the panels are under installation, no current can be established and the transistor S remains open. This eliminates the risk of electrocution. 
         [0045]    In  FIG. 3   b , the transistor S has been therefore closed. This is the normal operating mode of the fleet. The current arrives from the previous module, flows through the transistor S and the panel  12 , and reaches the following module. In this way, the input voltage Vin of the converter  16  is taken at the terminals of a closed transistor S. 
         [0046]    As previously indicated, the resistance in the on state of the transistor S is chosen so that the voltage drop at its terminals is sufficient to power the converter  16 , once the latter has started. It is desirable that the voltage drop at the terminals of the transistor is sufficient to power the converter, but that the voltage drop does not to significantly affect the power production efficiency. A voltage drop in the vicinity of 100 mV would be a good trade off. 
         [0047]    In fact, in the mode of  FIG. 3   b , the voltage Vin is regulated at a sufficient value. In fact, if this voltage Vin becomes insufficient, the converter  16  can no longer control the transistor S, and the latter conducts less. Thus, the voltage Vin at its terminals increases until the voltage is again sufficient to power the converter  16 . 
         [0048]    From this operating mode, one may wish to control the opening of the transistor S, for example to disconnect the panel after having detected an anomaly. Upon opening the transistor S, the serial current begins to flow essentially through the diode element Dn, which undergoes a reverse voltage drop Vn equal to the conduction threshold of the element Dn. The input voltage of the converter  16  is then equal to Vin=Vp+Vn, which is the highest value among the possible operating modes. 
         [0049]      FIGS. 4   a  and  4   b  show two operating modes of the module of  FIG. 2  when the panel  12  does not produce electricity. 
         [0050]      FIG. 4   a  illustrates an operating mode reached after the one of  FIG. 3   b . The panel  12  stops producing electricity, for example because it is in the shade. The panel operates from an operating mode as a generator to an operating mode as a load through which the serial current flows. The voltage at its terminals reverses up to the conduction threshold V 0  of the diode D 0 , which then takes over to let circulate the serial current. 
         [0051]    It will be understood here that the conduction threshold Vn of the element Dn is preferably higher than V 0 , such that the serial current preferably flows through the diode D 0 , and therefore through the transistor S to power the converter  16 , instead of flowing through the element Dn without flowing through the transistor S. 
         [0052]    The string producing less power owing to the failure of a panel, the serial current also decreases. As a result, the voltage Vin at the terminals of the transistor S decreases. The converter  16  again reacts by decreasing the conductance of the transistor S until the voltage at its terminals sufficiently powers the converter. 
         [0053]    If all the panels stop producing electricity, for example upon night fall, the serial current becomes insufficient to power the converter  16 . The impedance thereof becomes lower than that of the transistor S, and the voltage Vin decreases below the operating threshold of the converter. The transistor S opens, and the serial current continues to flow through the converter  16  and the diode D 0 . 
         [0054]    If one wishes to continue powering the modules from that moment, it suffices for the central control unit to inject a sufficient serial current, as will be described below. 
         [0055]    In  FIG. 4   b , the transistor S is open in a case where the serial current is sufficient to power the modules, but where the panel does not produce electricity. This case occurs upon start-up of the fleet when day breaks and the panel is in the shade or is defective. The module can also have received a command to open the transistor S. 
         [0056]    The serial current is distributed between the diode D 0 , passing through the converter  16 , and the diode element Dn. The voltage Vin is then equal to Vn−V 0 . Thus, the threshold Vn of the element Dn is preferably chosen so that the voltage Vn−V 0  is higher than a value allowing the converter  16  to start up. 
         [0057]    In a start-up situation, the module has not received a command to open the transistor S. The converter closes the transistor S once its input voltage Vin reaches a sufficient value upon start-up. Then, the operating mode of  FIG. 4   a  has been reached. 
         [0058]      FIG. 5  diagrammatically illustrates a string of solar panels connected to an inverter  22  (INV) by both ends of the serial conductor  13 . The inverter is preceded by a central control unit  24  (CCU), carrying out the functions previously mentioned. In this embodiment, the central control unit includes, in series on the conductor  13 , a general switch Sg for turning off the fleet, and a resistance Rs for measuring the serial current. An auxiliary current source  26  is connected to inject a serial current Ia into the conductor  13 , in the same direction as the nominal current. 
         [0059]    A control circuit  28  manages the functions of the CCU. In particular it controls the switch Sg and the current source  26 , and determines the serial current by measuring the voltage at the terminals of the resistance Rs. It also includes a carrier current modem making it possible to communicate with homologous modems of the LCU modules through a current transformer  30  inserted into the line  13 . 
         [0060]    The general switch Sg is open to perform maintenance operations. Its opening cancels out the serial current, and therefore the power supply of the modules, the transistors S thereof open, eliminating any risk of electrocution. 
         [0061]    The general switch Sg is closed under normal operation. When the lighting of the panels decreases, the serial current decreases. The control circuit  28  turns on the auxiliary current source  26  to continue powering the LCU modules when the serial current reaches a minimal value. The CCU will draw its power from a battery recharged during the day, or from the electrical network. 
         [0062]    The LCU modules are thus powered day and night, and can communicate with the CCU at any time by carrier current. 
         [0063]    If the single conductor  13  is cut, as illustrated between the second and third modules starting from the bottom, the serial current is cancelled out and the modules are no longer powered. It would nevertheless be desirable to know where the cut of the conductor is located. Since the serial current cancels at the same time for all of the modules, the onboard intelligence in one module cannot be used to locate the cut. 
         [0064]      FIG. 5  also illustrates an embodiment of a system for locating a cut of the serial conductor. Each module  10  includes a steady current source  32  connected between the serial conductor  13 , for example at the terminal B 1  of the module, and a ground terminal E of the module. The function of the ground terminal E is standardized. It serves to ground the metal parts of the panel by a conductor  34  shared by all of the panels. This conductor is also connected to the negative input of the central control unit CCU and the negative input of the inverter, if the manufacturer has provided for grounding the inverter in that way. Certain inverters are grounded by their positive input, in which case the direction of the current of the sources  32  is reversed. 
         [0065]    Each source  32  is provided to circulate a steady watch current Iw from the ground terminal E toward the terminal B 1  of the module. In this way, a current Iw starting from each source  32  circulates, as shown in broken lines, in the clockwise direction following the serial current in the conductor  13 , passing through the central control unit CCU, up to the grounded connection of the conductor  13 . There, the currents Iw return toward the respective sources  32  through the ground conductor  34 . 
         [0066]    When the conductor  13  is sectioned, for example between the second and third modules starting from the bottom, the current sources  32  of the modules located under the cut can no longer circulate their current Iw. Conversely, the sources  32  of the modules located above the cut, as shown, still let circulate their current. The sum of the watch currents Iw arriving to the CCU is therefore representative of the rank of the module at which the cut is located. 
         [0067]    More specifically, during a cutting of the serial conductor  13 , the serial current is cancelled out. The control CCU detects it and turns on the auxiliary current source  26 . The auxiliary current Ia, intended to power the modules, has a nominal value higher than the sum of the watch currents. Since in this situation only the watch currents can circulate through the auxiliary source  26 , they impose their value, which is 3·Iw in this example. The control circuit  28  divides that residual current by the value Iw of a watch current, and thus finds the rank, 3 from the top, of the module where the cut is located. The residual current is at most equal to n·Iw (where n is the number of modules), which corresponds to the case where the cut occurs between the first module and the inverter. If the cut takes place between the last module and the inverter, the residual current is zero. 
         [0068]    This cut location system is independent from the type of module used. It may involve a module with no smart features. The current sources  32  will preferably be bipolar, so that they do not need power supply at the modules. A bipolar current source draws its power supply from the voltage existing between its two terminals, provided that voltage is sufficient. 
         [0069]    When all the panels are powered and there is no cut, the voltages at the terminals of the sources  32  are close to the input voltage of the inverter, to within several Vn thresholds. However, the sources  32  are reverse-polarized and are therefore inactive. 
         [0070]    When there is a cut, the auxiliary current source  26  reverses the input voltage on the inverter, the sources  32  then being polarized and becoming active. The source  32  having the lowest voltage at its terminals is that of the first module, which has a voltage Va−(n−1)Vn, where Va is the voltage at the terminals of the auxiliary current source  26 . The supply voltage of the auxiliary source  26  is preferably chosen so that the source  32  of the first module has a sufficient voltage at the terminals thereof. 
         [0071]    In basic facilities, certain functions may be skipped to reduce costs, in particular communication functions by carrier current. It is, however, desirable to keep safety functions, in particular those eliminating the risk of electrocution. It has been shown that a module of the type of  FIG. 2  ensures, without any communication with the control unit, automatic opening of the transistor S once it is no longer passed through by a current. In other words, once the serial current is cut, either by a general switch located at the inverter, or by the removal of a panel, the transistors S of all of the modules disconnect the panels from the serial conductor, thereby eliminating the risk of electrocution. 
         [0072]    However, a more in-depth safety function may be desired, i.e. a reconnection of the panels only upon explicit command. By using modules of the type of  FIG. 2 , such a function can be obtained by using a local control circuit  14  that waits for a specific order to close the transistor S. This order would reach the circuit from the control unit by carrier current. 
         [0073]      FIG. 6  shows an embodiment of a local control module making it possible to implement this function without complex communication means. Relative to  FIG. 2 , the diode element Dn has been replaced by a single diode D 1 , preferably of the Schottky type. The diode D 0  has been removed. The converter used to power the circuits of the module, here designated by  16 ′, takes its input voltage from the terminals of the panel  12 , i.e. on the terminals A 1  and A 2 . In other words, the module is only powered here if the panel  12  is producing electricity. Since it is desirable to power the module even if the panel is weakly lit, and is producing a low voltage, the converter  16  is preferably of the step-up type. Thus, it is preferable to provide a voltage limiter  17  at the input of the converter to adapt its input voltage when the panel is producing its nominal voltage. 
         [0074]    The transistor S is controlled by a comparator  60  that compares the voltage at the terminals of a resistance R 2  to a reference voltage Vref and closes the transistor S when the voltage at the terminals of the resistance R 2  exceeds the reference voltage Vref. A transconductance amplifier  62  injects a current indicative of the serial current in the conductor  13  into the resistance R 2 . The amplifier  62  measures a voltage representative of the serial current at the terminals of a resistance R 3  placed in the conductor  13  between the terminals A 1  and B 1 . 
         [0075]    The amplifier  62  is powered at the terminals of a Zener diode Dz whereof the cathode is connected to the terminal B 1  and the anode is connected to the terminal A 2  by a resistance R 4 . 
         [0076]    With this configuration, once the panel  12  produces electricity, the converter  16 ′ powers the circuits of the module. However, the transistor S remains open. The same is true for all of the modules of the chain. The panels therefore remain disconnected from the serial conductor  13 , even if the entire facility is powered on. 
         [0077]    In order to start the facility, the central control unit CCU ( FIG. 5 ) injects an auxiliary current into the serial conductor  13 . This current passes through the diodes D 1  and the resistances R 3  of the modules. This current is chosen to be sufficient to switch the comparators  60 . The transistors S are closed by connecting the panels to the serial conductor. In each module, the current flows through the transistor S, the panel  12 , and the resistance R 3 . The current flowing through the resistance R 3  being even higher than the auxiliary current, the transistor S is kept closed. 
         [0078]    Once a panel  12  no longer produces electricity, the corresponding module is no longer powered, and its transistor S opens. The serial current then flows through the diode D 1 . Once the panel again starts to produce electricity, the module is powered. The current in the resistance R 3  being sufficient, the comparator  60  immediately closes the transistor R 3 . 
         [0079]    In order to cause a new safety disconnection of the panels, the general switch Sg is opened ( FIG. 5 ). A subsequent closure of that switch powers the facility on, but does not cause the closure of transistors S—to that end, it is necessary to again inject a current into the serial conductor. 
         [0080]    One advantage of this embodiment, relative to that of  FIG. 2 , is that the parasitic voltage drops introduced by the module can be minimized. In fact, the transistor S can be chosen with a resistance in the on state as low as desired. The diode D 1 , of the Schottky type, has a very low conduction threshold. 
         [0081]      FIG. 7  shows an alternative of the module of  FIG. 2 . Relative to  FIG. 2 , the module includes a second converter  70  powered between the terminals A 1  and A 2 , supplying the converter  16 , in particular in the operating mode of  FIG. 3   b.    
         [0082]    The mode of  FIG. 3   b  corresponds to a normal mode of a panel producing electricity. This is the mode one seeks to have for as long as possible. This is also a mode where it will be desirable to use the most functionalities of the module. However, it is also a mode where the module of  FIG. 2  is powered the least (from the voltage drop at the terminals of the closed transistor S). The desired functionalities, carried out by the microcontroller, may require more current than can be supplied by the converter  16  from a voltage in the vicinity of 100 mV. 
         [0083]    The supplemental converter  70  makes it possible to power the module from the panel, and therefore to replace the converter  16  in the modes where the panel produces electricity. In the modes where the panel does not produce electricity, it is the converter  16  that powers the module in the manner previously described. The converter  70  is preferably a step-down voltage converter. 
         [0084]      FIG. 8  shows an alternative of the module of  FIG. 7 . Relative to  FIG. 7 , the two converters  16  and  70  are replaced by a single converter  80 , of the step-up type. The positive input of the converter  80  is connected to the terminal A 1  by a diode D 2  and to the terminal B 2  by a diode D 3 . These diodes are connected to provide the converter  80  with the highest of the voltages exhibited on the terminals A 1  and B 2 . 
         [0085]    The diode D 0  is replaced by a diode D 0 ′ whereof the cathode remains connected to the terminal A 1 , but whereof the anode is no longer connected to the terminal A 2 . The anode of the diode D 0 ′ is connected to the negative terminal of the converter  80 . A diode D 4  is connected by its cathode to the terminal A 2  and by its anode to the negative terminal of the converter  80 . 
         [0086]    The converter  80  is of the step-up type to work with a maximum input voltage of several volts. However, this input voltage can yield the voltage of the panel, through the diodes D 2  or D 3 , depending on the operating mode, which may be incompatible with the operating range of the converter. Preferably, as for  FIG. 2 , a voltage limiting circuit  17 , which brings the input voltage back to an acceptable value when the converter is powered by the panel, is provided at the input of the converter. 
         [0087]    The diodes D 0 ′ and D 2  to D 4  are preferably diodes with a low conduction threshold, for example of the Schottky type. 
         [0088]    This alternative, as will be seen later, enables the converter  80  to work under better conditions than in  FIGS. 2 and 7 , in the modes where the panel does not produce electricity. 
         [0089]      FIG. 9   a  illustrates the module of  FIG. 8  in an operating mode where the panel  12  produces electricity. Part of the current of the panel passes through the diode D 2 , the converter  80 , the diode D 4 , and returns into the panel. Regardless of the state of the transistor S, the latter is not passed through by the current that powers the converter. If the transistor is open, the serial current passes through the diode element Dn. If it is closed, the serial current passes through the transistor and through the panel. 
         [0090]    The converter is thus powered by a voltage Vin=Vp−2V 0 . 
         [0091]      FIG. 9   b  illustrates the module of  FIG. 8  in an operating mode where the panel  12  does not produce electricity. Part of the serial current flows through the diode element Dn, which has a reverse threshold voltage Vn at its terminals. This voltage Vn powers the converter  80 : a second portion of the serial current passes through the diode D 3 , the converter  80 , and the diode Do′. The transistor S, whether closed or open, does not affect the current that powers the converter  80 . 
         [0092]    The converter is thus powered by a voltage Vin=Vn−2V 0 . As a function of the current consumed by the circuits of the module, the threshold voltage Vn of the diode element Dn will be chosen so that the converter can supply the required power. 
         [0093]    A module of the type of  FIG. 7  or  8  remains powered even if it is disconnected from the serial conductor  13 , as long as its panel is lit. As a result, the opening of the transistor S is not automatic upon disconnection of the module. If one wishes to limit the risk of electrocution in the modules of  FIGS. 7 and 8 , it is possible to provide the same mechanism as in  FIG. 6 . 
         [0094]    According to one alternative using communication by carrier current, the module and the control unit  24  are configured to implement a “watch-dog” procedure. The control unit periodically emits a verification signal. Each time the verification signal is received, the module restarts a timer. If the verification is no longer received, because the module has been disconnected from the serial conductor  13 , the timer expires and the module orders the transistor S to be opened. Once the module receives the verification signal again, it controls the closure of the transistor S.