Abstract:
A photovoltaic power generation system, having a photovoltaic panel, which has a direct current (DC) output and a micro-inverter with input terminals and output terminals. The input terminals are adapted for connection to the DC output. The micro-inverter is configured for converting an input DC power received at the input terminals to an output alternating current (AC) power at the output terminals. A bypass current path between the output terminals may be adapted for passing current produced externally to the micro-inverter. The micro-inverter is configured to output an alternating current voltage significantly less than a grid voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a contiuation of U.S. patent application Ser. No. 13/348,214 filed Jan. 11, 2012, which claims priority to patent application GB1100450.4, filed Jan. 12, 2011, in the United Kingdom Intellectual Property Office. U.S. patent application Ser. No. 13/348,214 and GB Application No. 1100450.4 are herein incorporated by reference 
    
    
     FIELD OF THE INVENTION 
     Aspects generally relate to distributed power system and more particularly to the use of multiple micro-inverters. 
     BACKGROUND 
     Recent increased interest in renewable energy has led to research and development of distributed power generation systems including photovoltaic cells and fuel cells. Various topologies have been proposed for connecting these power sources to the load, taking into consideration various parameters, such as voltage/current requirements, operating conditions, reliability, safety, costs. These sources provide low voltage direct current output (normally below 3 Volts), so they are connected serially to achieve the required voltage. Conversely, a serial connection may fail to provide the required current, so that several strings of serial connections may be connected in parallel to provide the required current. 
     Power generation from each of these sources typically depends on manufacturing, operating, and environmental conditions of the power sources, e.g. photovoltaic panels. For example, various inconsistencies in manufacturing may cause two identical sources to provide different output characteristics. Similarly, two identical sources may react differently to operating and/or environmental conditions, such as load, temperature, etc. In practical installations, different source may also experience different environmental conditions, e.g. in solar power installations some panels may be exposed to full sun, while others be shaded, thereby delivering different power output. 
     Islanding is a condition where a power generation system is severed from the utility network, but continues to supply power to portions of the utility network after the utility power supply is disconnected from those portions of the network. Photovoltaic systems must have anti-islanding detection in order to comply with safety regulations. Otherwise, the photovoltaic installation may electrically shock or electrocute repairpersons after the grid is shut down from the photovoltaic installation generating power as an island downstream. The island condition poses a hazard also to equipment. Thus, it is important for an island condition to be detected and eliminated. 
     The process of connecting an alternating current (AC) generator or power source (e.g. alternator, inverter) to other AC power sources or the power grid is known as synchronization and is crucial for the generation of AC electrical power. There are five conditions that are met for the synchronization process. The power source must have equal line voltage, frequency, phase sequence, phase angle, and waveform to that of the power grid. Typically, synchronization is performed and controlled with the aid of synch relays and micro-electronic systems. 
     The term “grid voltage” as used herein is the voltage of the electrical power grid usually 110V or 220V at 60 Hz or 220V at 50 Hz. 
     BRIEF SUMMARY 
     According to various aspects there is provided a micro-inverter having input terminals and output terminals. The micro-inverter may be adapted for inverting an input DC power received at the input terminals to an output alternating current (AC) power at the output terminals, which have a voltage significantly less than a grid voltage. A bypass current path between the output terminals may be adapted for passing current produced externally to the micro-inverter. An optional synchronization module may be adapted for synchronizing the output AC power to the grid voltage. A control loop may be configured to set the input DC power received at the input terminals according to a previously determined criterion. The previously determined criterion typically sets a maximum input power. 
     According to various aspects there is provided a photovoltaic power generation system having multiple photovoltaic panels with direct current (DC) outputs connectible to multiple micro-inverters. Each micro-inverter has input terminals connectible to the DC outputs and output terminals. The micro-inverters are configured for inverting input DC power received at the input terminals to an output alternating current (AC) at the output terminals with an output voltage substantially less than a grid voltage. The output terminals are connectible in series into a serial string and an output voltage of the serial string may be substantially equal to the grid voltage. Each micro-inverter includes a bypass current path between the output terminals for passing current produced externally in the serial string. The alternating current (AC) micro-inverter may have a control loop configured to set the input DC power received at the input terminals according to a previously determined criterion. An optional central control unit may be operatively attached to the serial string and the grid voltage. The central control unit may be adapted for disconnecting the system from the grid upon detecting a less than minimal grid voltage. The central control unit optionally monitors the synchronization of the voltage of the serial string to the grid voltage and disconnects the serially connected micro-inverters from the grid or disables the micro-inverters upon a lack of synchronization between the grid voltage and the output voltage of the serially connected micro-inverters. 
     According to various aspects there is provided a method for photovoltaic power generation in a system having multiple of photovoltaic panels with direct current (DC) outputs and multiple micro-inverters each including input terminals and output terminals. The input terminals of the micro-inverters are connectible to respective DC outputs of the photovoltaic panels. The output terminals are connected serially to a serial voltage output. The DC power received at the input terminals may be inverted to an output alternating current (AC) power at the output terminals while maintaining the serial voltage output substantially equal to a grid voltage. The output terminals preferably have a current bypass in the event of failure of inverting the DC power received at the input terminals to the output alternating current (AC) power at the output terminals or upon the micro-inverter being shut down in the event of a failure to maintain the serial voltage output at the level of the grid voltage. 
     Upon connecting the input terminals and the output terminals, inversion of input DC power to output power may be enabled after a previously determined time delay. The serial voltage output may be synchronized to the grid voltage. The output terminals preferably have a current bypass in the event of failure of inverting the DC power received at the input terminals to the output alternating current (AC) power at the output terminals or upon the micro-inverter being shut down in the event of a failure to maintain the serial voltage output at the level of the grid voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments are described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a conventional installation of a solar power system. 
         FIG. 2  illustrates one serial string of DC sources. 
         FIG. 3  illustrates a power harvesting system. 
         FIG. 4 a    illustrates a power harvesting system in accordance with one or more embodiments of the disclosure. 
         FIG. 4 b    illustrates a power harvesting system in accordance with one or more embodiments of the disclosure. 
         FIG. 4 c    illustrates further details of a bypass in accordance with one or more embodiments of the disclosure. 
         FIG. 5 a    illustrates a method of operation of a power harvesting system in accordance with one or more embodiments of the disclosure. 
         FIG. 5 b    shows further details of connection and wake-up of a power harvesting system in accordance with one or more embodiments of the disclosure. 
         FIG. 5 c    shows further details of operation in accordance with one or more embodiments of the disclosure. 
     
    
    
     The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Various aspects are described below with reference to the figures. 
     A conventional installation of a solar power system  10  is illustrated in  FIG. 1 . Since the voltage provided by each individual photovoltaic panel  100  is low, several panels  100  are connected in series to form a string  102  of panels  100 . For a large installation, in order to achieve higher current, several strings  102  may be connected in parallel. Photovoltaic panels  100  are mounted outdoors, and are connected to a maximum power point tracking (MPPT) module  106  and to an inverter  104 . MPPT  106  is typically implemented in the same housing as inverter  104 . 
     Harvested power from the DC sources is delivered to inverter  104 , which converts the fluctuating direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which, for residential application, is usually 110V or 220V at 60 Hz or 220V at 50 Hz. AC current from inverter  104  may then be used for operating electric appliances or fed to the power grid. Alternatively, if the installation is not tied to the grid, the power extracted from inverter  104  may be directed to store the excess power in batteries. 
       FIG. 2  illustrates one serial string of DC sources according to conventional art, photovoltaic panels  100 , connected to MPPT circuit  106  and inverter  104  to form a power harvesting system  20  connected to load  108 . The current versus voltage (IV) characteristics are plotted to the left of each photovoltaic panel  100 . For each photovoltaic panel  100 , the current decreases as the output voltage increases. At some voltage value the current goes to zero, and in some applications may assume a negative value, meaning that some photovoltaic panels  100  instead of being sources of power become sinks of power. Bypass diodes (not shown) connected in parallel across each photovoltaic panel  100  output are used to prevent any photovoltaic panel  100  from becoming a sink of power. The power output of each photovoltaic panel  100  is equal to the product of current and voltage (P=I*V) and varies depending on the voltage drawn from the panel  100 . At a certain current and voltage, the power reaches its maximum (represented by the dot on the IV curve for each graph). It is desirable to operate a panel  100  at this maximum power point (MPP). The purpose of the maximum power point tracking (MPPT) module  106  is to find a suitable “average” maximum power point (MPP) for all panels  100 . The maximum power point of the string selected by MPPT module  106  is shown using a dotted line with label MPP. The maximum power point of the string of panels  100  is generally not the maximum power of all panels  100 . The dots indicating maximum power point of the individual panels  100  do not fall on the dotted line marked MPP. 
       FIG. 3  illustrates another power harvesting system  30  according to conventional art, which combines power of multiple photovoltaic panels  100 . Each photovoltaic panel  100  has a direct current (DC) output connected to the input of an inverter  104 . A bypass diode  310  is connected in parallel across the direct current (DC) output panel  100  for safety requirements. Inverter  104  receives the direct current (DC) output of photovoltaic panel  100  and converts the direct current (DC) to give an alternating current (AC) at the output of inverter  104 . Maximum power point tracking (MPPT) module  106  is typically implemented as part of the inverter  104 . The outputs of multiple inverters  104  (with inputs attached to multiple photovoltaic panels  100 ) are connected in parallel to produce an alternating current (AC) output  304 . Alternating current (AC) output  304  supplies load  108 . Load  108  typically is an alternating current (AC) power grid, alternating current (AC) motor or a battery charging circuit. 
     Before explaining various aspects in detail, it is to be understood that embodiments are not limited to the details of design and the arrangement of the components set forth in the following description and illustrated in the drawings. Other embodiments are capable of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     By way of introduction, aspects are directed to serially connected inverters in a grid connected photovoltaic system. In a system with serially connected inverters, as opposed to conventional system  30  which illustrates parallel connected inverters, each inverter is required to output a low voltage, for instance 24 volts AC root mean square (RMS) for ten serially connected inverters. Low output voltage of the micro-inverter is suitable for efficient and low cost micro-inverter topologies. One such topology is discussed in IEEE Transactions on Power Electronics, Vol. 22, No. 5, September 2007, entitled “A Single-Stage Grid Connected Inverter Topology for Solar PV Systems With Maximum Power Point Tracking, this paper proposes a high performance, single-stage inverter topology for grid connected PV systems. 
     The term “bypass” as used herein refers to an alternate low impedance current path around or through a circuit, equipment or a system component. The bypass is used to continue operation when the bypassed circuit is inoperable or unavailable. 
     The terms “wake-up” and “shut-down” as used herein refer to processes during, which a photovoltaic system is activated or de-activated respectively. A criterion for “wake-up”, i.e. activation of a photovoltaic panel, for instance, is that a photovoltaic panel is exposed to sufficient light such as at dawn A criterion for “shut-down”, i.e. de-activation of a photovoltaic panel, is that a photovoltaic panel is not exposed to sufficient light, for example at dusk. 
     Reference is now made to  FIG. 4 a   , which illustrates a power harvesting system  41  according to some embodiments. Photovoltaic inverting modules  410  each have panel  100 , bypass diode  310 , a control loop  404  and micro-inverter  402 . Micro-inverters  402  may have optional synchronization units  408  and current bypass paths  422 . Photovoltaic panels  100  have direct current (DC) outputs, which are connected respectively to the input of inverters  402 . Bypass diodes  310  may connected in parallel across the direct current (DC) outputs of each panel  100  for safety requirements (e.g. IEC61730-2 solar safety standards). Control loops  404  are configured according to a predetermined criterion, typically to maintain maximum power at the inputs of micro-inverters  402 , i.e. from the direct current (DC) outputs of photovoltaic panels  100 . Bypass paths  422  are optionally normally-closed relays, which open during operation, and which are connected respectively to the outputs of photovoltaic inverting modules  410 . Photovoltaic inverting modules  410  have alternating current (AC) outputs with voltage V a  and current I a  from module  410   a ; voltage V b  and current I b  from module  410   b ; voltage V n  and current In from module  410   n . Outputs of modules  410  are connected in series to give a voltage output V out , which is applied to a load  406  via switch  414 . Switch  414  is preferably controlled by control unit  418 . Load  406  typically is an alternating current (AC) power grid, alternating current (AC) motor or a battery charging circuit. Control units  418  typically provide control signals to synchronization units  408  in order to achieve synchronization with load or grid  406 . Synchronization units  408  or control unit  418  provide anti-islanding functionality for power harvesting system  41 . 
     Additionally, the outputs of photovoltaic inverting modules  410   a - 410   n  are bypassed (i.e. the output of modules  410   a - 410   n  are short circuited) by bypass  422  in the event of under voltage production by micro inverter modules  402  or the bypass is opened (i.e. modules  410   a - 410   n  are open circuit) in the event of over voltage by micro inverter modules  402  or during a situation of anti-islanding. 
     Reference is now made to  FIG. 4 c   , which illustrates further details of bypass  422  according to various embodiments. Bypass  422  is controlled by control logic module  460 , e.g. a microprocessor  460  controlling micro-inverter  402 . Microprocessor  460  has a sensing input connected to the output voltage (V microinverter ) of micro inverter  402 . Control logic module  460  has other inputs connected across the bypass path at nodes A and B. Control logic module  460  has two outputs; one output connects to the gate of a metal oxide semi-conductor field effect transistor (MOSFET) Q 1 , the other output connects to the gate of MOSFET Q 2 . The drain of MOSFET Q 1  is connected to node A and the source of MOSFET Q 1  is connected to the source of MOSFET Q 2 , the drain of MOSFET Q 2  is connected to node B. MOSFET Q 1  has a diode with an anode connected to the drain and a cathode connected to the source. MOSFET Q 2  has a diode with an anode connected to the drain and a cathode connected to the source. The bypass current (I bypass ) path is identified between nodes A and B. 
     A high impedance path is provided between nodes A and B when micro inverter  402  is producing an alternating current (AC) voltage synchronized to grid voltage  406 . The high impedance path is provided between nodes A and B when MOSFETs Q 1  and Q 2  are turned off by control logic unit  460 . When the high impedance path is provided between nodes A and B currents I b , I X , I in , I a , I Y  and I out  are equal according to Kirchhoff&#39;s current law. A low impedance path is provided between nodes A and B when micro inverter  402  is not producing an AC voltage and another serially-connected micro inverter  402  is producing an AC voltage. A low impedance path is provided between nodes A and B by alternately switching MOSFETs Q 1  and Q 2  on and off alternately via control logic unit  460 . When the load  406  is a grid voltage Q 1  and Q 2  are turned alternately on and off according to the frequency of the grid voltage. When the load  406  is a load, Q 1  and Q 2  are turned alternately on and off according to the frequency of synchronized inverters  402   a -  402   n . In the case of low impedance path being provided between nodes A and B in the embodiment according to  FIG. 4 a   ; switching MOSFETs Q 1  and Q 2  on and off by control logic unit  460  is achieved via communication signals between central control unit  408  and control units  408   a - 408   n . In the case of low impedance path being provided between nodes A and B in the embodiment according to  FIG. 4 b   ; switching MOSFETs Q 1  and Q 2  on and off alternately by control logic unit  460  is achieved via communication signals between control units  408   a - 408   n  and information of grid voltage  406  via sensor  416 . A low impedance path provided between nodes A and B means that currents I b , I bypass  and I out  are substantially equal according to Kirchhoff&#39;s current law. A low impedance path provided between nodes A and B means that current I bypass  flows alternately from drain to source of Q 2  and the diode of Q 1  for one half cycle and for the other half cycle I bypass  flows alternately through from drain to source of Q 1  and the diode of Q 2 . 
     Reference is now made to  FIG. 4 b   , which illustrates a power harvesting system  42  according to further embodiments. As in power harvesting system  41  photovoltaic inverting modules  410   a - 410   n  each has a photovoltaic panel  100 , bypass diode  310 , control loops  404  and inverters  402  having synchronization units  408  and current bypasses  422 . Modules  410   a - 410   n  have outputs connected in series to give a voltage output V out , which is applied to load  406 . Sensor  416  preferably senses the live voltage applied to load  406  optionally via electromagnetic pickup on the power line connected to load  406  or directly by having visibility of the grid by virtue of bypasses  422 . Sensor unit  412  transfers details of the load voltage (e.g. amplitude, phase, and frequency) to synchronization unit  408   a  via control line  420 . Control signals are optionally sent over power line communications, wireless or over a separate interface. 
     Although only one control line  420  is shown, optionally multiple or all synchronization units  408  receive synchronization signals from sensor  412 . 
     Reference is now made to  FIG. 5 a   , which shows a flow chart of a method  50  illustrating operation of power harvesting systems  41  and  42  according to various aspects. Method steps include installation (step  500 ) wake-up (step  501 ), normal operation (step  503 ), and shut down (step  505 ). 
       500  Installation and  501  Wake-Up 
     During installation (step  500 ), photovoltaic modules  410  are preferably not producing power so as not to be a safety hazard to the installers. Optionally, a “keep-alive” signal is transmitted for instance by control unit  418  over the AC power lines. When the “keep-alive” signal is not received by micro-inverters  402 , AC output power is disabled or not produced. Alternatively, if the grid is “visible” to micro-inverters  402 , then in the absence of grid voltage, (e.g. switch  414  in  FIG. 4 a    is open) micro-inverters  402  do not produce AC power. Reference is now made to  FIG. 5 b   , which illustrates an installation method  500  according to certain aspects. In step  500   a , input terminals of micro-inverters  402  are connected to the output of photovoltaic panels  100 . In step  500   b , the output terminals of photovoltaic panels  100  are connected serially to give a serial voltage output. After an optional predetermined time delay (step  501   a ), power inversion is enabled (step  501   b ).The enabling (step  501   b ) of power inversion may be performed by synchronization modules  408  when grid voltage is sensed or by control unit  418  when switch  414  is closed. 
       503  Operation and  505  Shutdown 
     Reference is now made again to  FIG. 5 c   , which shows a flow chart of a method  503  for operating serially connected micro-inverter module according to various embodiments. Micro-inverters  402  invert (step  503   b ) the direct current (DC) power output of photovoltaic panels  100  to alternating current (AC) power at the outputs of micro-inverters  402  while maintaining output voltage equal to the grid voltage. Synchronization (step  503   a ) between the voltage outputs of micro-inverters  402   a - 402   n  and the grid voltage is maintained. Control unit  418  optionally monitors AC synchronization between output voltage Vout and load  406 , e.g. grid. Control unit  418  also may provide anti-islanding functionality for power harvesting system  41 . If either synchronization and/or voltage of power harvesting system  41  is incompatible with the grid, control unit  418  disconnects power harvesting system from the grid by signaling switch  414 . Alternatively, synchronization (step  503   a ) including maintenance of grid voltage is achieved using synchronization units  408  which can sense the grid by virtue of bypass paths  422 . Upon failure of either synchronization (step  503   a ) or inverting power at grid voltage (step  503   b ) by any of the serially connected micro-inverter modules  402 , then current bypass occurs (step  503   d ). Current bypass is optionally an active current bypass using active switches as shown in  FIG. 4 c    or preferably a passive current bypass. Shutdown (step  505 ) occurs for instance at dusk when light levels are two low to maintain the grid voltage at any current level. During shutdown, the photovoltaic system is optionally disconnected from the grid using switch  414  in system  41  or in system  42  each of micro-inverter modules  402  stop and present high impedance to the grid. 
     According to yet further embodiments, the regulation of output voltage of photovoltaic inverting modules  410   a - 410   n  is achieved directly by the grid  406 . The regulation does not require control unit  418  and switch  414  as shown in  FIG. 4 a    and relies on the fact that grid  406  is almost infinitely greater in terms of potential supply of power by comparison to the AC power produced by photovoltaic inverting modules  410   a - 410   n . The greater power of grid  406  forces photovoltaic inverting modules  410   a - 410   n  to adjust to the grid voltage and as such, photovoltaic inverting modules  410   a - 410   n  are preferably operated to give as much voltage as possible at their outputs. Typically, photovoltaic inverting modules  410   a - 410  are capable of sensing grid voltage  406  so as to provide anti-islanding. 
     The definite articles “a”, “an” is used herein, such as “a photovoltaic panel”, have the meaning of “one or more” that is “one or more photovoltaic panels”. 
     Although selected embodiments have been shown and described, it is to be appreciated that changes may be made to these embodiments without departing from the principles and spirit of the invention.