Patent Publication Number: US-9407161-B2

Title: Parallel connected inverters

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 13/596,308, filed Aug. 28, 2012, which is a continuation application of U.S. application Ser. No. 12/329,520, filed Dec. 5, 2008, which claims priority benefit from U.S. Application Ser. No. 60/992,589, filed Dec. 5, 2007. Each of these prior applications is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to distributed power systems and, more particularly, a system and method for sharing power inversion/conversion between parallel connected power inverters/converters connected to the distributed power system. 
     DESCRIPTION OF RELATED ART 
     A conventional installation of a solar distributed power system  10 , including multiple solar panels  101 , is illustrated in  FIG. 1 . Since the voltage provided by each individual solar panel  101  is low, several panels  101  are connected in series to form a string  103  of panels  101 . For a large installation, when higher current is required, several strings  103  may be connected in parallel to form overall system  10 . The interconnected solar panels  101  are mounted outdoors, and connected to a maximum power point tracking (MPPT) module  107  and then to an inverter  104 . MPPT  107  is typically implemented as part of inverter  104  as shown in  FIG. 1 . The harvested power from DC sources  101  is delivered to inverter  104 , which converts the direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which is usually 110V or 220V at 60 Hz, or 220V at 50 Hz. The AC current from inverter  104  may then be used for operating electric appliances or fed to the power grid. 
     As noted above, each solar panel  101  supplies relatively very low voltage and current. A problem facing the solar array designer is to produce a standard AC current at 120V or 220V root-mean-square (RMS) from a combination of the low voltages of the solar panels. The delivery of high power from a low voltage requires very high currents, which cause large conduction losses on the order of the second power of the current i 2 . Furthermore, a power inverter, such as inverter  104 , which is used to convert DC current to AC current, is most efficient when its input voltage is slightly higher than its output RMS voltage multiplied by the square root of 2. Hence, in many applications, the power sources, such as solar panels  101 , are combined in order to reach the correct voltage or current. A large number of panels  101  are connected into a string  103  and strings  103  are connected in parallel to power inverter  104 . Panels  101  are connected in series in order to reach the minimal voltage required for inverter  104 . Multiple strings  103  are connected in parallel into an array to supply higher current, so as to enable higher power output. 
       FIG. 1B  illustrates one serial string  103  of DC sources, e.g., solar panels  101   a - 101   d , connected to MPPT circuit  107  and inverter  104 . The current (ordinate) versus voltage (abscissa) or IV characteristics are plotted ( 110   a - 110   d ) to the left of each DC source  101 . For each DC power source  101 , the current decreases as the output voltage increases. At some voltage value, the current goes to zero, and in some applications the voltage value may assume a negative value, meaning that the source becomes a sink. Bypass diodes (not shown) are used to prevent the source from becoming a sink. The power output of each source  101 , which is equal to the product of current and voltage (P=i*V), varies depending on the voltage drawn from the source. At a certain current and voltage, close to the falling off point of the current, the power reaches its maximum. It is desirable to operate a power generating cell at this maximum power point (MPP). The purpose of the MPPT is to find this point and operate the system at this point so as to draw the maximum power from the sources. 
     In a typical, conventional solar panel array, different algorithms and techniques are used to optimize the integrated power output of system  10  using MPPT module  107 . MPPT module  107  receives the current extracted from all of solar panels  101  together and tracks the maximum power point for this current to provide the maximum average power such that if more current is extracted, the average voltage from the panels starts to drop, thus lowering the harvested power. MPPT module  107  maintains a current that yields the maximum average power from system  10 . 
     However, since power sources  101   a - 101   d  are connected in series to single MPPT  107 , MPPT  107  selects a maximum power point which is some average of the maximum power points of the individual serially connected sources  101 . In practice, it is very likely that MPPT  107  would operate at an I-V point that is optimum for only a few or none of sources  101 . In the example of  FIG. 1B , the selected point is the maximum power point for source  101   b , but is off the maximum power point for sources  101   a ,  101   c  and  101   d . Consequently, the arrangement is not operated at best achievable efficiency. 
     The present applicant has disclosed in co-pending U.S. application Ser. No. 11/950,271 entitled “Distributed Power Harvesting Systems Using DC Power Sources”, the use of an electrical power converter, e.g. DC-to-DC converter, attached to the output of each power source, e.g. photovoltaic panel. The electrical power converter converts input power to output power by monitoring and controlling the input power at a maximum power level. 
     SUMMARY 
     The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
     Aspects of the invention provide load balancing of a parallel connected power converter, wherein each converter autonomously determine its own power conversion load. 
     According to an embodiment of the present invention there is provided a distributed power system including a direct current (DC) power source and multiple inverters. The inverter inputs are adapted for connection in parallel to the DC power source. The inverter outputs adapted for connection in parallel. Multiple control modules connect respectively to the inverters&#39; inputs. The control modules respectively control current drawn by the inverters from the DC input responsive to either the voltage or power of the DC input so that a voltage or power equilibrium, i.e., specified draw, is reached in the DC input. That is, the control module continuously monitors the power provided by the DC power source and adjust the current or power conversion of the power converter according to a specified function. Consequently, the inverters share the load of inverting power from the DC power source to output power. A power module may be attached between the DC power source and the inverters and include an input coupled to said DC power source and an output to the inverter inputs. The power module may be configured to maintain maximum peak power at the input coupled to the DC power source or the power module may be configured to control at maximum peak power at its output. Alternatively, a single maximum peak power tracking module connects the DC power source to the control modules. The control modules include a voltage loop block which upon comparing the voltage of the serial string to a previously specified reference voltage, outputs a current reference signal based on the comparison. A current loop block compares the current reference signal with a current signal proportional to the current in the DC power source. 
     According to embodiments of the present invention there is provided a method for sharing load in a distributed power system. Multiple inverters are coupled in parallel to the DC power source. The inverters invert power from the DC power source to an output power. 
     Current drawn by the inverters from the DC power source is autonomously controlled by each inverter responsive to selectably either the voltage or power of the DC input. In this manner, the inverters share the load of the inverting power from the DC power source to the output power according to a prescribed power conversion sharing function. A power module disposed between the DC power source and the inverters includes an input coupled to the DC power source and an output to inputs of the inverters. The power module optionally maintains maximum peak power at the input coupled to the DC power source. 
     According to another embodiment of the present invention there is provided a distributed power system including a direct current (DC) power source and multiple power converters. The power converter inputs are adapted for connection in parallel to the DC power source. The power converter outputs are adapted for connection in parallel. Multiple control modules connect respectively to the power converter&#39;s inputs. The control modules respectively control current drawn by the power converters from the DC input responsive to either the voltage or power of the DC input until either a voltage or power equilibrium is reached in the DC input. The power converters share the load of inverting power from the DC power source to output power. 
     According to embodiments of the present invention there is provided a method for sharing load in a distributed power system. Current drawn from a DC input by the inverters is individually controlled by each inverter responsive to the DC input. An equilibrium is reached in the DC input for each given DC power input, such that DC power conversion is shared among the inverters according to a prescribed formula. The inverter autonomously draws a portion of the load of inverting power from the DC input to output power. 
     The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate various features of the illustrated embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not necessarily drawn to scale. 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIGS. 1 and 1B  are block diagram of conventional power harvesting systems using photovoltaic panels as DC power sources; 
         FIG. 2  illustrates a distributed power harvesting circuit, based on the disclosure of U.S. application Ser. No. 11/950,271; 
         FIG. 3  illustrates a simplified system, according to an embodiment of the present invention; 
         FIG. 4 , is a simplified flow diagram of a method, illustrating a feature of the present invention; 
         FIG. 5  illustrates a simplified system, according to another embodiment of the present invention; 
         FIG. 6  which illustrates details of a control module integrated inside an inverter, in accordance with different embodiments of the present invention; 
         FIG. 7  is a graph showing a typical control current-voltage characteristic for controlling current response to input voltage, according to a feature of the present invention; and 
         FIGS. 8A and 8B  which illustrate racks and connections to the racks with parallel connected inverters, according to a feature of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
     It should be noted, that although the discussion herein relates primarily to photovoltaic systems and more particularly to those systems previously disclosed in U.S. application Ser. No. 11/950,271, the present invention may, by non-limiting example, alternatively be configured as well using conventional photovoltaic distributed power systems and other distributed power systems including (but not limited to) wind turbines, hydroturbines, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitors, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. 
     By way of introduction, distributed power installations have inverters which invert DC power to AC power. In large scale installations, a large inverter may be used, but a large inverter is more difficult to maintain and repair, leading to long downtime. The use of a number of small inverters has a benefit of modularity. If one inverter constantly is operating and a second inverter begins to operate when there is a larger load to handle, there is more wear on the working inverter. Hence load balancing between the inverters is desired. If the control of the two inverters is through a master/slave technique there is an issue of a single point of failure. The single master may break down and take the rest of the system out of whack. A good solution would be a load-balancing, not master-slave driver modular inverter. This disclosure shows a system and method for doing so. To be sure, in the context of this disclosure, load balancing does not necessarily mean that the load is spread among the converters in equal amounts, but rather that the load is distributed among the converters such that each converter assumes a certain part of the load, which may be predetermined or determined during run time. 
     It should be noted, that although the discussion herein relates primarily to grid tied power distribution systems and consequent application to inversion (i.e. power conversion from direct current (DC) to alternating current (AC), the teachings of the present invention are equally applicable to DC-DC power conversion systems such as are applicable in battery storage/fuel cell systems. Hence the terms “inverter” and “converter” in the present context represent different equivalent embodiments of the present invention. 
     Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or 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. 
     Reference is now made to  FIG. 2  which illustrates a distributed power harvesting circuit  20 , based on the disclosure in U.S. application Ser. No. 11/950,271. Circuit  20  enables connection of multiple distributed power sources, for example solar panels  101   a - 101   d , to a single power supply. Series string  203  of solar panels  101  may be coupled to an inverter  204  or multiple connected strings  203  of solar panels  101  may be connected to a single inverter  204 . In configuration  20 , each solar panel  101   a - 101   d  is connected individually to a separate power converter circuit or a module  205   a - 205   d . Each solar panel  101  together with its associated power converter circuit  205  forms a power source or power generating element  222 . (Only one such power generating element  222  is marked in  FIG. 2 .) Each converter  205   a - 205   d  adapts optimally to the power characteristics of the connected solar panel  101   a - 101   d  and transfers the power efficiently from input to output of converter  205 . Converters  205   a - 205   d  are typically microprocessor controlled switching converters, e.g. buck converters, boost converters, buck/boost converters, flyback or forward converters, etc. The converters  205   a - 205   d  may also contain a number of component converters, for example a serial connection of a buck and a boost converter. Each converter  205   a - 205   d  includes a control loop  221 , e.g. MPPT loop that receives a feedback signal, not from the converter&#39;s output current or voltage, but rather from the converter&#39;s input coming from solar panel  101 . The MPPT loop of converter  205  locks the input voltage and current from each solar panel  101   a - 101   d  at its optimal power point, by varying one or more duty cycles of the switching conversion typically by pulse width modulation (PWM) in such a way that maximum power is extracted from each attached panel  101   a - 101   d . The controller of converter  205  dynamically tracks the maximum power point at the converter input. Feedback loop  221  is closed on the input power in order to track maximum input power rather than closing a feedback loop on the output voltage as performed by conventional DC-to-DC voltage converters. 
     As a result of having a separate MPPT circuit in each converter  205   a - 205   d , and consequently for each solar panel  101   a - 101   d , each string  203  may have a different number or different specification, size and/or model of panels  101   a - 101   d  connected in series. System  20  of  FIG. 2  continuously performs MPPT on the output of each solar panel  101   a - 101   d  to react to changes in temperature, solar radiance, shading or other performance factors that effect one or more of solar panels  101   a - 101   d . As a result, the MPPT circuit within the converters  205   a - 205   d  harvests the maximum possible power from each panel  101   a - 101   d  and transfers this power as output regardless of the parameters effecting other solar panels  101   a - 101   d.    
     The outputs of converters  205   a - 205   d  are series connected into a single DC output that forms the input to inverter  204 . Inverter  204  converts the series connected DC output of converters  205   a - 205   d  into an AC power supply. 
     Reference is now made to  FIG. 3  which illustrates a simplified system  30 , according to an embodiment of the present invention. A solar panel array  20  in different embodiments may have serial and/or parallel power generating modules  222 , each of which includes solar panel  101  and MPPT power converter  205 . In system  30 , five strings  203  are connected in parallel. Connected to solar panel array  20  are multiple, e.g. two inverters  304  which are parallel connected both at their inputs and their outputs. 
     Reference is now also made to  FIG. 4 , a simplified flow diagram illustrating a method  40 , according to an embodiment of the present invention. Operation of system  30  is characterized by inverters  304  controlling their input currents based on the voltage input to inverters  304 . Under these circumstances, a drop in power (step  401 ), for instance caused by a cloud moving in front of the sun causes a drop (step  403 ) in voltage input to inverter  304 . The drop (step  403 ) in voltage input to inverters  304  causes inverters  304  to reduce (step  405 ) respective input currents which in turn tends to raise the input voltage respectively to inverters  304 . An equilibrium is reached (decision box  407 ) as both inverters  304  handle reduced power (step  409 ) from solar panel array  20 . This process is repeated continuously or intermittently to respond to changes in the operational characteristics of the DC power source. 
     Referring back to  FIG. 3 , in an example of an embodiment of the present invention using solar panel array  20  includes five parallel connected strings  203 , each string of ten power generating modules  222  each connected in series to parallel-connected inverters  304  which output a grid voltage of 220V RMS. Nominal input voltage to parallel-connected inverters  304  at maximum power conversion, e.g. 10 kiloWatts, is 400 Volts with 5 kiloWatts through each of two inverters  304 . Hence, ignoring power conversion/inversion efficiency losses, each of fifty solar panels  101  output 200 Watt of electrical power at 40 Volts. Current through each string is 2000 W/400V=5 amperes. Power generating modules  222  are configured to maximize their power input (or power output from solar panels  101 ). Voltage output from power generating modules  222  is typically floating. If the power output from power generating modules  222  decreases (for instance as a result of solar shading, e.g., cloud) input power to inverters  304  drops (step  401 ). Inverters  304  are configured to adjust their current draw (step  405 ) based on input voltage. Reference is now made to  FIG. 7  a graph showing a typical control current-voltage characteristic for controlling current response to input voltage, according to a feature of the present invention. In the example, the horizontal axis is Voltage in volts and the vertical axes indicate respectively and Power in Watts and Current in amperes. Of course, while in this example a linear function is shown for use by all inverters, other functions may be used and/or each individual inverter may have a different function. According to the graph, 5 kW inverters  304  are configured to draw close to zero Watts at 350 V DC  input, 2.5 kiloWatt at 375 V DC  input, and the full 5 kiloWatt at 400 V DC  input. In this case, if the direct current power is 10 kiloWatt, each inverter  304  operates at full peak load with an input voltage of 400 V DC  (each inverter  304  drawing each 12.5 ampere, so that total current draft is 25 ampere=10 kiloWatt/400 Volt). If the power input to inverters  304  drops to, e.g., 5 kW total power, both inverters  304  experience a drop in the input voltage (since the DC input is now 5 kW, if inverters  304  keep on drawing 12.5 A each, then the voltage would be 200V). However, each inverter  304  starts reducing its input current until an equilibrium is reached (decision box  407 ), which in this case is with each inverter  304  drawing 6.25 ampere at 375 VDC input to a total of 2.5 kW power inverted by each inverter  304  and 5 kW for the total both inverters  304 . 
     Reference is now made to  FIG. 5  which illustrates a simplified system  50 , according to an embodiment of the present invention. A solar panel array  10  in different embodiments may have serial and/or parallel connected solar cells/panels  101 . An MPPT power circuit  107  maintains a maximum power output of solar panel array  10  typically by drawing current at the peak power output level of solar panel array  10 . The output voltage of MPPT circuit  107  is preferably floating. Connected to MPPT  107  are multiple inverters, e.g. two inverters,  304  which are parallel connected both at their inputs and their outputs. 
     The operation of system  50  is illustrated by referring back to  FIG. 4 . If the power output from solar panel array  10  decreases (for instance as a result of solar shading, e.g., cloud) input power to inverters  304  drops (step  401 ). Inverters  304  are configured to adjust their current draw (step  405 ) based on input voltage. Each inverter  304  starts reducing (step  405 ) its input current until an equilibrium is reached (decision box  407 ) and each inverter  304  handles (step  409 ) a reduced power load. 
     Reference is now made to  FIG. 6  which illustrates a simplified system diagram of inverter  304  with an integrated control module  60  according to an embodiment of the present invention. Control module  60  includes two control loops a voltage control loop  601  and a current control loop block  605 . A previously specified voltage reference block  603  specifies two voltage references, a lower voltage reference and an upper voltage reference. As previously stated, in this example inverter  304  operates with a DC input voltage of 400V in order to invert to 220V RMS. Hence, in this specific example both the lower and upper voltage references are in the vicinity of 400 V DC. In the previous example used in reference to  FIG. 3  the lower reference voltage is 350 VDC and the upper reference voltage is 400 VDC. Voltage control loop block  601  compares the actual input DC voltage to the voltage references and outputs a current reference I ref  signal. The current reference signal I ref  is used as an input to current control loop block  605 . Current control loop block  605  receives also a signal  609  proportional to its output current. Typically, a current sensor provides signal  609  from within a pulse width modulation (PWM) block  607  of inverter  304 , which performs the power inversion. Current control loop block  605  compares output current signal  609  with the current reference signal I ref  and adjusts the output current accordingly until the current (and output power) equilibrate. Thus each inverter  304  typically handles an equal load of power from solar panel array  10  or  20 . 
     As can be understood, in general, embodiments of the invention provide a system whereby a plurality of power converters, e.g., inverters, are connected in parallel and share the power conversion load according to a prescribed function, but each power converter autonomously determines its share of power conversion. That is, each power converter operates according to its own power conversion formula/function, such that overall the parallel-connected converters share the power conversion load in a predetermined manner. That is, while the power conversion sharing scheme is designed according to the system as a whole, i.e., division of duty to all of the converters, each individual inverter operates individually to draw power according to its own formula. In one specific case, e.g., where all of the converters are of the same model and same rating, the formula is the same for all of the converters. On the other hand, in other implementations the formula can be individually tailored to each converter. For example, in installation where one converter has double the conversion capacity as all the other converters in the system, its formula may dictate its power conversion share to be double as the other converters. Also, while the formula exemplified in  FIG. 7  is linear, other functions or formulas may be used, as this is given as one particular example. 
     Reference is now made to  FIGS. 8A and 8B  which illustrate racks with parallel connected inverters, according to a feature of the present invention. In this embodiment some or all of inverters  304  may be configured for operating in a load-balancing mode, according to an embodiment of the present invention, but inverters  304  may actually share some components. One such embodiment might be parallel inverters  304  with a shared enclosure for the electrically separate inverters, as depicted in  FIG. 8A . Other embodiments might also include shared electrical elements of the inverters, and example of which as depicted in  FIG. 8B  which shows parallel connected inverters with a shared EMI/RFI filter bank (these filters might be at the DC input, AC input, or both). Joint connections are shown in the racks, shared by inverters  304 , a joint AC connection  81  to the grid and a joint DC connection  83  to DC power source  20 . According to a further feature of the present invention, a joint electromagnetic interference filter is used to filter all the outputs of inverters  304  and electromagnetic radiation thereform, whether they are actually load balancing or not, according to the present invention. 
     The articles “a”, “an”, as used hereinafter are intended to mean and be equivalent to “one or more” or “at least one”. For instance, “a direct current (DC) power source” means “one or more direct current (DC) power sources”. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. 
     The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and/or components of the described embodiments may be used singly or in any combination in the server arts. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.