Patent Publication Number: US-2005139258-A1

Title: Solar cell array control device

Description:
FIELD OF THE INVENTION  
      The present invention relates to a solar cell array control device and, more particularly, to a device capable of detecting whether a plurality of solar cell modules is abnormal and properly controlling each solar cell module to operate at the maximum power.  
     BACKGROUND OF THE INVENTION  
      A solar cell is an energy storage device for converting solar energy into electric energy. In practical applications, a plurality of solar cells is usually assembled into a plurality of solar cell modules, which are connected together in series to form a solar cell array for enhancing the output power. Finally, a solar cell converter is used for the maximum power output conversion of the whole solar cell array.  
      When the solar cell array is used, however, the whole output power may be affected due to different environments. For instance, part of the solar cell modules in the solar cell array may be shielded to lower their output power or even have no output power to form a load mode, hence affecting the output power of the whole solar cell array.  
      In the prior art, a KW or above solar cell converter integrates the powers of all solar cell modules to provide the output power of the whole solar cell array. This kind of solar cell converter does not allow each solar cell module to operate at its maximum output power. Therefore, the output power provided by this kind of solar cell converter is in fact not the maximum.  
      Conventionally, the problem of series connection between solar cell modules is not taken into consideration in the design of a solar cell converter. In a solar cell array formed by connecting several solar cell modules in series, the solar cell modules may differ due to different environmental factors. Operation of that each solar cell module at the maximum output power cannot be ensured. If part of the solar cell modules operate in a load mode, the power of other solar cell modules will be dissipated, hence not accomplishing the expected performance when the solar cell modules are connected together in series to form the solar cell array.  
      Accordingly, the present invention proposes a solar cell array control device, which makes use of a detection and compensation mechanism to detect whether one of the series-connected solar cell modules is abnormal. Through properly controlling the electric power conversion of a bidirectional DC converter, each solar cell module can operate at the maximum power point.  
     SUMMARY OF THE INVENTION  
      The primary object of the present invention is to provide a solar cell array control device, which makes use of a bidirectional DC converter to connect several solar cell modules in series and a detection and compensation mechanism to detect whether each solar cell module is abnormal. Through properly controlling the power conversion of the bidirectional DC converter, each solar cell module can operate at the maximum power.  
      To achieve the above object, the present invention provides a solar cell array control device, which comprises a plurality of solar cell modules, a bidirectional DC converter, at least a voltage sensor and a control unit. The bidirectional DC converter corresponds to and is electrically connected to the solar cell modules to series connect these solar cell modules in series to form a solar cell array. The voltage sensor is electrically connected to the solar cell modules, and can generate an abnormal voltage when detecting that one of the solar cell modules is abnormal. The control unit is electrically connected to the voltage sensor and the bidirectional DC converter, and outputs a pulse width modulation (PWM) signal by detecting the abnormal voltage to control the bidirectional DC converter for compensating the conversion current and thus enhancing the output power of this solar cell module.  
      It is preferred that the solar cell array control device of the present invention further comprise an AC/DC converter capable of tracking the maximum power. The AC/DC converter is electrically connected to the output terminals of the bidirectional DC converter to convert the output power of the solar cell array into an AC power connected in shunt with the public electric power grid or used as an independent power source.  
      It is preferred that the bidirectional DC converter in the solar cell array control device of the present invention further comprise a flyback transformer and a plurality of electronic switches. The flyback transformer corresponds to and is electrically connected to the output terminals of the solar cell modules to connect all the solar cell modules in series. Each electronic switch corresponds to and is connected in series to an input terminal of the flyback transformer, and has a pulse width modulation input terminal through which the pulse width modulation signal can be input to compensate the conversion current of the solar cell module for enhancing the output power.  
      It is also preferred that the solar cell control device of the present invention further comprise a drive circuit electrically connected between the bidirectional DC transformer and the control unit to amplify the pulse width modulation signal and drive the bidirectional DC converter to switch the pulse width for compensating the conversion current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawings, in which:  
       FIG. 1  is an architecture diagram according to an embodiment of the present invention;  
       FIG. 2  is a control flowchart of the present invention;  
       FIGS. 3A and 3B  are equivalent circuit diagrams of the present invention;  
       FIGS. 4A and 4B  are waveform diagrams showing the pulse width modulation signal and switching of the electronic switch of the present invention; and  
       FIG. 5  shows the voltage-current characteristic curve of a solar cell array according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      As shown in  FIG. 1 , the present invention applies to a solar cell array  1  formed by series connecting a plurality of solar cell modules  10  to control all the solar cell modules  11  to operate at the maximum power. Each solar cell module  11  is composed of at least a solar cell panel and at least a storage battery. The solar panel can convert solar energy into electric energy stored into the storage battery.  
      A bidirectional DC converter  11  is used to series connect all the solar cell modules  10  in the present invention. The bidirectional DC converter  11  is composed of a flyback transformer  111  and a plurality of electronic switches  112 . The turn ratio of the flyback transformer  111  is unified. The flyback transformer  111  has a plurality of input terminals corresponding to and electrically connected to output terminals of the solar cell modules  10  for series connecting all the solar cell modules  10 . The electronic switch  112  can be a high power MOSFET, and corresponds to and is connected in series to one of the input terminals of the flyback transformer  111 . The electronic switch  112  has a pulse width modulation input terminal (the gate of the MOSFET) through which a pulse width modulation signal can be input to control the compensation current to enhance the output power when one of the solar cell modules is abnormal.  
      An AC/DC converter  12  capable of tracking the maximum power can be electrically connected to an output terminal of the bidirectional DC converter  11  to convert the output power of the solar cell array  1  into an AC power connected in shunt with the public electric power grid or used as an independent power source.  
      In the present invention, in order to let all the solar cell modules  11  operate at the maximum power, a detection and compensation mechanism is designed to detect the voltage value of each solar cell module and to determine whether the solar cell module is abnormal (e.g., operating in the load mode). When there is an abnormal situation, the power factor enhancement technique, i.e., the pulse width modulation technique for switching power conversion, is used to compensate the conversion current of the solar cell module  10  for accomplishing the object of enhancing the whole power factor.  
      In the present invention, in addition to the bidirectional DC converter  11 , the detection and compensation mechanism also comprises at least a voltage sensor  13 , a control unit  14  and a drive circuit  15 . The voltage sensor  13  can be a comparator having a plurality of input terminals electrically connected to the solar cell modules  11 . The voltage sensor  13  is used to compare the voltage values (Vpvm 1 , Vpvm 2 ) of at least an arbitrary two of the solar cell modules  10  to see whether the absolute value of the voltage difference of these two solar cell modules  10  is larger than a set value (Vset). If one of the solar cell modules  10  is abnormal, an abnormal voltage (Verror) will be generated and output to the control unit  14 .  
      The control unit  14  is electrically connected to the voltage sensor  13  and the drive circuit  15 . The drive circuit  15  is electrically connected to the bidirectional DC converter  11 . The control unit  14  can be a microprocessor for executing a pulse width modulation procedure. The control unit  14  can output the pulse width modulation signal for controlling the bidirectional DC converter  11  to the drive circuit  15  by determining the abnormal voltage (Verror). The drive circuit  15  will amplify the pulse width modulation signal and then inputs to the pulse width modulation input terminal (the gate of the MOSFET) of the electronic switch  112  in the bidirectional DC converter  11  to switch the power conversion pulse width for compensating the conversion current and thus enhancing the output power of this solar cell module  10 .  
       FIG. 2  is a control flowchart of the present invention. First, the bidirectional DC converter  11  is built to connect all the solar cell modules  10  in series (Step  200 ). The voltage values V 1  and V 2  of a first solar cell module  101  and a second solar cell module  102  are then read (Step  201 ). Next, the absolute value of the difference of the two voltages V 1  and V 2  (i.e., |V 1 -V 2 |) is calculated to see whether it is larger than the set value Vset (Step  202 ). If the answer is yes, the abnormal voltage Verror&gt;0 is output; otherwise the abnormal voltage Verror is set to zero, meaning that the first and second solar cell modules  101  and  102  are operating at the maximum power without compensating the conversion current.  
      If the second solar cell module  102  is abnormal, V 1  will be larger than V 2  (e.g., point A shown in  FIG. 5 ), and the abnormal voltage Verror&gt;0 will be output to the control unit  14  (Step  203 ). The control unit  14  will then generate a pulse width modulation signal (shown in  FIG. 4 ) sent to the pulse width modulation input terminals (MOSFET gates Sg 1 , Sg 2 ) of electronic switches S 1  and S 2  (Step  204 ). The pulse width modulation signal is used to control the electronic switches S 1  and S 2  to let the current on the second solar cell module  102  (I 2 ) be equal to the current on the first solar cell module  101  (I 1 ) minus the total current (Ic) for compensating the conversion current of the second solar cell module  102  (e.g., the point B shown in  FIG. 5 ) (Step  205 ). It is evident that the output power of the second solar cell module  102  has been enhanced.  
      To sum up, the present invention provides a solar cell array control device, which makes use of a bidirectional DC converter to series connect all solar cell modules and a detection and compensation mechanism to detect whether each solar cell modules is abnormal. Through properly controlling the electric power conversion of the bidirectional DC converter, each solar cell module can operate at the maximum power.  
      Although the present invention has been described with reference to the preferred embodiments thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.