Patent Publication Number: US-2021184468-A1

Title: Bypass module for enhanced pv array dc-ac ratio capability

Description:
I. TECHNICAL FIELD 
     The present invention relates generally to photovoltaic (PV) array systems. In particular, the present invention relates to a bypass module for enhancing the PV array DC-AC ratio capability within a PV array system. 
     II. BACKGROUND 
     A PV array system is typically connected to an input of an electric power system to convert and transmit power to the electric power system. It includes PV arrays, a combiner box connected thereto and a PV inverter to convert the power from DC to AC power for the electric power system (e.g., a utility grid). In order to maximize power of the PV array system, it is common for the system to be designed with higher power-rated PV arrays than the power rating of the PV inverter. During operation, the system covers different environmental conditions, some resulting in a higher output power capability of the PV arrays which includes a higher voltage and current capability than the PV inverter is able to operate. Cold weather causes an increase of the PV open-circuit voltage, and high irradiance (e.g., &gt;full sun) causes an increase of the PV short-circuit current. These factors combined can result in a PV array with a much higher power capability than that of the PV inverter. 
     During operation it is desirable to operate the PV inverter at a power level that maximizes the AC power supplied to utility grid. Therefore, it is desirable to have the highest possible DC-to-AC ratio which increases the chance of a PV inverter tripping or being damaged. Thus, enhancement of the PV array DC-AC ratio in a PV array system without causing problems within the PV inverter is desired. 
     III. SUMMARY OF THE EMBODIMENTS 
     According to one embodiment, a bypass module is employed in a string at one or more of the PV panels to bypass the respective PV panel when the PV voltage is above an acceptable voltage range to avoid tripping or damaging a PV inverter of a PV array system. 
     Embodiments of the present invention provides a control system for a PV array system including a plurality of PV panels. The control system includes a bypass module having a first switch device and a second switch device disposed at at least one PV panel connected with others of the plurality of PV panels along a string, and configured to perform a switching operation when PV voltage of at least one PV panel is outside of an acceptable voltage range of the PV array system, and e bypass module short-circuits the PV panel when excess voltage at the PV panel is detected. The control system also including a control module configured to monitor and control operation of the bypass module. 
     Other embodiments of the present invention include a bypass method for performing a bypass operation at at least one of the PV panels of the PV array system. 
     The foregoing has broadly outlined some of the aspects and features of various embodiments, which should be construed to be merely illustrative of various potential applications of the disclosure. Other beneficial results can be obtained by applying the disclosed information in a different manner or by combining various aspects of the disclosed embodiments. Accordingly, other aspects and a more comprehensive understanding may be obtained by referring to the detailed description of the exemplary embodiments taken in conjunction with the accompanying drawings, in addition to the scope defined by the claims. 
    
    
     
       IV. DESCRIPTION OF THE DRAWINGS 
       The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the disclosure. Given the following enabling description of the drawings, the novel aspects of the present disclosure should become evident to a person of ordinary skill in the art. This detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of embodiments of the invention. 
         FIG. 1  is a schematic illustrating a PV array system that can be implemented within embodiments of the present invention. 
         FIG. 2  is a circuit schematic of PV panel string that includes a bypass module at one PV panel of a plurality of PV panels of the PV array system of  FIG. 1 , to bypass the PV panel during high voltage conditions, that can be implemented within the embodiments of the present invention. 
         FIG. 3  is a circuit schematic of a control module of the bypass module of  FIG. 2 , that can be implemented within embodiments of the present invention. 
         FIG. 4  is a flow chart illustrating a bypass process of the bypass module of  FIGS. 2 and 3 , that can be implemented within the embodiments. 
         FIG. 5  is a graph of a PV bypass waveforms formed upon implementation of the bypass module of  FIGS. 2 and 3 , that can be implemented within embodiments of the present invention. 
     
    
    
     V. DETAILED DESCRIPTION OF THE EMBODIMENTS 
     As required, detailed embodiments are disclosed herein. It must be understood that the disclosed embodiments are merely exemplary of various and alternative forms. As used herein, the word “exemplary” is used expansively to refer to embodiments that serve as illustrations, specimens, models, or patterns. The Figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. 
     In other instances, well-known components, apparatuses, materials, or methods that are known to those having ordinary skill in the art have not been described in detail in order to avoid obscuring the present disclosure. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art. 
     As noted above, the embodiments provide a bypass module that can be implemented in connection with one or more PV panels within a string to be connected with a combiner box (as depicted in  FIG. 1 ). As shown in  FIG. 1 , a PV array system  100  includes a plurality of PV panels  110  each formed of multiple cells connected into a combiner box  120  which is connected to a DC/AC PV inverter  130  and converts DC power to AC power to be supplied to a utility grid  150 . The PV panels  110  are arranged in series to stack up to a desired total voltage appropriate for the PV inverter  130 . The combiner box  120  brings together the multiple strings  200  from the PV cells, each string  200  at a low current and combines them together into a high current output for the PV inverter  130 . 
     Details regarding the bypass module  250  will be discussed below with reference to  FIG. 2 . As shown in  FIG. 2 , a string  200  having a negative terminal  200   a  and a positive terminal  200   b , connects multiple PV panels  110  (e.g., PV panel  1 , PV panel  2  . . . PV panel  19  and PV panel  20 ). Each PV panel  110  includes positive and negative terminals to be connected within the string  200 . The negative and positive terminals  200   a  and  200   b  of the string  200  are connected with a combiner box  120  (as depicted in  FIG. 1 ), to supply power to the PV inverter  130  (also depicted in  FIG. 1 ) The string  200  is not limited to connecting a particular number of PV panels  110 , and may vary as desired. The bypass module  250  is between the outputs of the PV panels  110 , for example, as shown between PV panel  19  and the PV panel  20 , i.e., at the input and output of PV panel  20 . The bypass module  250  includes a pair of solid state switches  260  and  262  (e.g., SS 1  and SS 2 ). According to some embodiments, the switches  260  and  262  are metal-oxide-semiconductor field-effect transistors (MOSFETs). However, the present invention is not limited hereto and any suitable switches may be used. 
     As shown in  FIG. 2 , in normal operation, if PV voltage is under the threshold for operation of the bypass module  250  then switch  260  is closed and conducting current to PV panel  20 . When excess voltage is detected, then switch  262  is closed to short circuit PV panel  20 . The current of the string  200  of approximately 8.66 amps (A) now flows into switch  262  and PV panel  20  short circuit current of approximately 9.15 amps (A) starts flowing in the reverse direction. Thus, the net current in switch  262  is approximately 0.49 amps (A). Optionally, according to an embodiment of the present invention, the switch  260  can then be open to reduce any increase in temperature of PV panel  20 . If the switch  260  is open, then the open circuit (OC) voltage at PV panel  20  can be measured to determine the temperature of PV panel  20  and to provide power supply to bypass module  250 . 
     When the total PV voltage across the string  200 , or the PV panel  20  OC voltage drops to a sufficient level, then switch  260  can be re-closed and switch  262  can be opened. 
     A control module  300  as shown in  FIG. 3  is also provided. The control module  300  is configured to monitor and control operation of all of the components shown in  FIG. 3  except PV panel  210  which corresponds to PV Panel  20  shown in  FIG. 2 , for example. As shown in  FIG. 3 , the PV panel  210  includes positive and negative terminals thereof connected within the string  200  which includes the negative and positive terminals  200   a  and  200   b . The PV panel  210  includes a photocell  160  comprising a diode  162  (e.g. a body diode) and a plurality of resistors  164  (e.g., Rs and Rsh). The control module  300  is connected to the PV panel  210  and includes a bypass module including a plurality of switches  310  and  312  (MOS 1  and MOS 2 ) which correspond to the bypass module  250  including switches  260  and  262 , respectively as shown in  FIG. 2 ), timer devices  314  and  316 , a voltage sensor  318 , filter device  320  and a comparator circuit  325  that includes a plurality of comparators  326  and  328  connected to a logic device  330  (e.g. an AND gate). 
     Operation of the bypass module as controlled by the control module  300  will be described below with reference to  FIGS. 3 and 4 . In  FIG. 3 , solar energy creates current (Iphoto) which flows in a direction towards the positive terminal  200   b  of the string  200  (as depicted in  FIG. 3 ); and the voltage of the PV panel  210  is dominated by the diode  162  which re-absorbs some of PV energy. The PV voltage can change due to changes in the temperature of the PV panel  210 . For example, heat (e.g., in a full sun environment) causes the PV voltage to decrease and lower temperatures, cool or cold (e.g., cloudy or cold environment) causes the voltage to be increased beyond normal operation. In  FIG. 4 , the process begins at operation  410  where during early hours of the day, when some voltage is created, it is used to power up the control module  300 . According to an embodiment, power supply of the control module  300  for the bypass module  250  can be derived from the PV panel  210  and it only operates with PV energy. 
     At operation  415 , the switch  310  is switched on and the string current travels through the PV panel  210  and out of the positive terminal  200   b  of the string  200 . At operation  420 , the voltage sensor  318  measures voltage directly across the PV panel  210  which ranges from approximately 30-40 volts (V). The filter device  320  removes any transient signals. Then at operation  425 , the comparator  326  detects that the PV voltage is high (Vhigh) and switch  312  is immediately switched on and switch  310  is delayed by timer device  314  for approximately a few seconds. As a result, at operation  430 , the switches  310  and  312  together short-circuit PV panel  210 . The voltage at the positive terminal  200   b  is immediately reduced by one PV panel (e.g., in this case by the short-circuit of PV panel  210 ). The current of string  200  is reduced since the voltage is lower with the excess short-circuit current from the PV panel  210  flowing down in switch  312 . At operation  435 , after the short delay switch  310  opens and the current (Iphoto) of the PV panel  210  has no external path so voltage increases and flows in diode  162 . The PV panel  210  now is an open circuit with a higher voltage and the current of the string  200  now flows up switch  312 . At operation  440 , once sufficient sun and time (several minutes) occurs, the PV panels  210  warm up the comparator  328  detects an acceptable voltage (V_OK) and latch is set at the logic device  330 . At operation  445 , the switch  310  is immediately switched on and switch  312  is delayed by timer device  316  by a few seconds. The PV panel  210  is again short-circuited, and at operation  450 , after the short delay the switch  312  is switched off and the PV voltage of the string  200  returns to its normal conditions. 
       FIG. 5  is graph  500  illustrating an example of PV bypass waveforms occurring under certain environmental conditions, when implementing the bypass module  250  as depicted in  FIGS. 2 and 3 . In the example, there are one string of 20 PV panels and another string of 20 PV Panels with one bypass module employed (i.e., a total of 40 PV Panels). In the example, the temperature is assumed to be constant at 25° C. The sun irradiance is assumed to be 40% at approximately four (4) seconds, increasing to 110% by approximately five (5) seconds and staying at 110%. Power is kW into a converter nominal rated 8 kilowatts (kW). PV voltage is DC voltage input into a converter with a maximum power rating of approximately 850 volts (V). As shown in  FIG. 4 , from zero (0) to four (4) seconds, the environmental conditions is cloudy, and PV array system is operating at a maximum power point tracking (MPPT) power to a maximum power of approximately 5 kilowatts (kW) with the DC voltage of approximately 750 volts (V) which falls within an acceptable voltage range. 
     From four (4) to five (5) seconds, the sun comes out, thereby causing the MPPT power to increase to approximately 14 kilowatts (kW) which is considered excess power. From five (5) to six (6) seconds, the converter reacts to curtail the power to near 8 kilowatts (kW) increasing the voltage to approximately 850 volts (V) but decreasing the current. From six (6) to seven (7) seconds, the bypass module operates and the switch  312  shown in  FIG. 3 , shorts the PV panel and the voltage falls to zero thereby reducing the overall DC voltage to approximately 840 volts (V). From seven (7) to eight (8) seconds the switch  310  switches to PV panel to an open circuit having a OC voltage of approximately 928/20 volts (V). Then, from eight (8) to ten (10) seconds the bypass module continues to operate for a few minutes until the heat from the sun warms the PV panels. With higher temperatures the OC voltage is reduced to a safer, acceptable voltage range and the bypass module is able to be switched off. 
     The bypass module of the embodiments of the present invention provides several advantages. Some of the advantages include enhancement of the PV array system DC to AC ratio capacity, and lowering of arc fault energy and manufacturing costs. 
     This written description uses examples to disclose the invention including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or apparatuses and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.