Parallel connected inverters

TECHNICAL FIELD

0001The 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

0002A conventional installation of a solar distributed power system <b>10,</b> including multiple solar panels <b>101,</b> is illustrated in <figref idref="f0001">Figure 1</figref>. Since the voltage provided by each individual solar panel <b>101</b> is low, several panels <b>101</b> are connected in series to form a string <b>103</b> of panels <b>101.</b> For a large installation, when higher current is required, several strings <b>103</b> may be connected in parallel to form overall system <b>10.</b> The interconnected solar panels <b>101</b> are mounted outdoors, and connected to a maximum power point tracking (MPPT) module <b>107</b> and then to an inverter <b>104.</b> MPPT <b>107</b> is typically implemented as part of inverter <b>104</b> as shown in <figref idref="f0001">Figure 1</figref>. The harvested power from DC sources <b>101</b> is delivered to inverter <b>104,</b> which converts the direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which is usually 110V or 220V at 60Hz, or 220V at 50Hz. The AC current from inverter <b>104</b> may then be used for operating electric appliances or fed to the power grid.

0003As noted above, each solar panel <b>101</b> 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>i²</i>. Furthermore, a power inverter, such as inverter <b>104,</b> 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 <b>101</b>, are combined in order to reach the correct voltage or current. A large number of panels <b>101</b> are connected into a string <b>103</b> and strings <b>103</b> are connected in parallel to power inverter <b>104.</b> Panels <b>101</b> are connected in series in order to reach the minimal voltage required for inverter <b>104.</b> Multiple strings <b>103</b> are connected in parallel into an array to supply higher current, so as to enable higher power output.

0004<figref idref="f0002">Figure 1B</figref> illustrates one serial string <b>103</b> of DC sources, <i>e.g</i>., solar panels <b>101a -</b> 101 d, connected to MPPT circuit <b>107</b> and inverter <b>104.</b> The current (ordinate) versus voltage (abscissa) or IV characteristics are plotted (<b>110a - 110d</b>) to the left of each DC source <b>101.</b> For each DC power source <b>101,</b> 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 <b>101,</b> which is equal to the product of current and voltage (<i>P=i</i>*<i>V</i>), 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.

0005In a typical, conventional solar panel array, different algorithms and techniques are used to optimize the integrated power output of system <b>10</b> using MPPT module <b>107.</b> MPPT module <b>107</b> receives the current extracted from all of solar panels <b>101</b> 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 <b>107</b> maintains a current that yields the maximum average power from system <b>10.</b>

0006However, since power sources <b>101a - 101d</b> are connected in series to single MPPT <b>107,</b> MPPT <b>107</b> selects a maximum power point which is some average of the maximum power points of the individual serially connected sources <b>101.</b> In practice, it is very likely that MPPT <b>107</b> would operate at an I-V point that is optimum for only a few or none of sources <b>101.</b> In the example of <figref idref="f0002">Figure 1B</figref>, the selected point is the maximum power point for source <b>101b,</b> but is off the maximum power point for sources <b>101a, 101c</b> and <b>101d.</b> Consequently, the arrangement is not operated at best achievable efficiency.

0007The present applicant has disclosed in co-pending U.S. Application Serial No. <patcit id="pcit0001" dnum="US11950271B"><text>11/950,271</text></patcit> entitled "Distributed Power Harvesting Systems Using DC Power Sources", the use of an electrical power converter, <i>e.g.</i> DC-to-DC converter, attached to the output of <i>each</i> power source, <i>e.g</i>. photovoltaic panel. The electrical power converter converts input power to output power by monitoring and controlling the <i>input</i> power at a maximum power level.

0008Document <patcit id="pcit0002" dnum="EP1047179A1"><text>EP 1047179 A1</text></patcit> discloses a method of operating a power supply system consisting of a plurality of inverters with parallel inputs and outputs.

SUMMARY

0009The 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.

0010Aspects of the invention provide load balancing of a parallel connected power converter, wherein each converter autonomously determine its own power conversion load.

0011According 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' 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.

0012According 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.

0013Current 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.

0014According 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'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.

0015According 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.

0016The 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

0017The 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.

0018The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: <ul id="ul0001" list-style="none"><li><figref idref="f0001">Figures 1</figref> and <figref idref="f0002">1B</figref> are block diagram of conventional power harvesting systems using photovoltaic panels as DC power sources;</li><li><figref idref="f0003">Figure 2</figref> illustrates a distributed power harvesting circuit, based on the disclosure of <patcit id="pcit0003" dnum="US11950271B"><text>US application 11/950271</text></patcit>;</li><li><figref idref="f0004">Figure 3</figref> illustrates a simplified system, according to an embodiment of the present invention;</li><li><figref idref="f0004">Figure 4</figref>, is a simplified flow diagram of a method, illustrating a feature of the present invention;</li><li><figref idref="f0005">Figure 5</figref> illustrates a simplified system, according to another embodiment of the present invention;</li><li><figref idref="f0005">Figure 6</figref> which illustrates details of a control module integrated inside an inverter, in accordance with different embodiments of the present invention,</li><li><figref idref="f0006">FIG. 7</figref> 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 <figref idref="f0007">Figures 8A and 8B</figref> which illustrate racks and connections to the racks with parallel connected inverters, according to a feature of the present invention.</li></ul>

DETAILED DESCRIPTION

0019Reference 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.

0020It should be noted, that although the discussion herein relates primarily to photovoltaic systems and more particularly to those systems previously disclosed in <patcit id="pcit0004" dnum="US11950271B"><text>US application 11/950271</text></patcit>, 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.

0021By 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.

0022It 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.

0023Before 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.

0024Reference is now made to <figref idref="f0003">Figure 2</figref> which illustrates a distributed power harvesting circuit <b>20,</b> based on the disclosure in <patcit id="pcit0005" dnum="US11950271B"><text>US application 11/950271</text></patcit>. Circuit <b>20</b> enables connection of multiple distributed power sources, for example solar panels <b>101a - 101d,</b> to a single power supply. Series string <b>203</b> of solar panels <b>101</b> may be coupled to an inverter <b>204</b> or multiple connected strings <b>203</b> of solar panels <b>101</b> may be connected to a single inverter <b>204.</b> In configuration <b>20,</b> each solar panel <b>101a - 101d</b> is connected individually to a separate power converter circuit or a module <b>205a - 205d.</b> Each solar panel <b>101</b> together with its associated power converter circuit <b>205</b> forms a power source or power generating element <b>222.</b> (Only one such power generating element <b>222</b> is marked in <figref idref="f0003">Figure 2</figref>.) Each converter <b>205a - 205d</b> adapts optimally to the power characteristics of the connected solar panel <b>101a - 101d</b> and transfers the power efficiently from input to output of converter <b>205.</b> Converters <b>205a - 205d</b> are typically microprocessor controlled switching converters, <i>e.g</i>. buck converters, boost converters, buck/boost converters, flyback or forward converters, etc. The converters <b>205a - 205d</b> may also contain a number of component converters, for example a serial connection of a buck and a boost converter. Each converter <b>205a - 205d</b> includes a control loop <b>221,</b><i>e.g.</i> MPPT loop that receives a feedback signal, not from the converter's output current or voltage, but rather from the converter's input coming from solar panel <b>101.</b> The MPPT loop of converter <b>205</b> locks the input voltage and current from each solar panel <b>101a - 101d</b> 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 <b>101a - 101d.</b> The controller of converter <b>245</b> dynamically tracks the maximum power point at the converter input. Feedback loop <b>221</b> 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.

0025As a result of having a separate MPPT circuit in each converter <b>205a - 205d,</b> and consequently for each solar panel <b>101a - 101d,</b> each string <b>203</b> may have a different number or different specification, size and/or model of panels <b>101a - 101d</b> connected in series. System <b>20</b> of <figref idref="f0003">Figure 2</figref> continuously performs MPPT on the output of each solar panel <b>101a - 101d</b> to react to changes in temperature, solar radiance, shading or other performance factors that effect one or more of solar panels <b>101a - 101d.</b> As a result, the MPPT circuit within the converters <b>205a - 205d</b> harvests the maximum possible power from each panel <b>101a - 101d and</b> transfers this power as output regardless of the parameters effecting other solar panels <b>101a - 101d.</b> The outputs of converters <b>205a - 205d</b> are series connected into a single DC output that forms the input to inverter <b>204.</b> Inverter <b>204</b> converts the series connected DC output of converters <b>205a - 205d</b> into an AC power supply.

0026Reference is now made to <figref idref="f0004">Figure 3</figref> which illustrates a simplified system <b>30,</b> according to an embodiment of the present invention. A solar panel array <b>20</b> in different embodiments may have serial and/or parallel power generating modules <b>222,</b> each of which includes solar panel <b>101</b> and MPPT power converter <b>205.</b> In system <b>30,</b> five strings <b>203</b> are connected in parallel. Connected to solar panel array <b>20</b> are multiple, <i>e.g</i>. two inverters <b>304</b> which are parallel connected both at their inputs and their outputs.

0027Reference is now also made to <figref idref="f0004">Figure 4</figref>, a simplified flow diagram illustrating a method <b>40,</b> according to an embodiment of the present invention. Operation of system <b>30</b> is characterized by inverters <b>304</b> controlling their input currents based on the voltage input to inverters <b>304.</b> Under these circumstances, a drop in power (step <b>401</b>), for instance caused by a cloud moving in front of the sun causes a drop (step <b>403</b>) in voltage input to inverter <b>304.</b> The drop (step <b>403</b>) in voltage input to inverters <b>304</b> causes inverters <b>304</b> to reduce (step <b>405</b>) respective input currents which in turn tends to raise the input voltage respectively to inverters <b>304.</b> An equilibrium is reached (decision box <b>407</b>) as both inverters <b>304</b> handle reduced power (step <b>409</b>) from solar panel array <b>20.</b> This process is repeated continuously or intermittently to respond to changes in the operational characteristics of the DC power source.

0028Referring back to <figref idref="f0004">Figure 3</figref>, in an example of an embodiment of the present invention using solar panel array <b>20</b> includes five parallel connected strings <b>203</b>, each string of ten power generating modules <b>222</b> each connected in series to parallel-connected inverters <b>304</b> which output a grid voltage of 220V RMS. Nominal input voltage to parallel-connected inverters <b>304</b> at maximum power conversion, e.g. 10 kiloWatts, is 400 Volts with 5 kiloWatts through each of two inverters <b>304.</b> Hence, ignoring power conversion/inversion efficiency losses, each of fifty solar panels <b>101</b> output 200 Watt of electrical power at 40 Volts. Current through each string is 2000W/400V= 5 amperes. Power generating modules <b>222</b> are configured to maximize their power input (or power output from solar panels <b>101</b>). Voltage output from power generating modules <b>222</b> is typically floating. If the power output from power generating modules <b>222</b> decreases (for instance as a result of solar shading, e.g., cloud) input power to inverters <b>304</b> drops (step <b>401</b>). Inverters <b>304</b> are configured to adjust their current draw (step <b>405</b>) based on input voltage. Reference is now made to <figref idref="f0006">Figure 7</figref> 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, 5kW inverters <b>304</b> are configured to draw close to zero Watts at 350V_DC input, 2.5 kiloWatt at 375 V_DC input, and the full 5 kiloWatt at 400V_DC input. In this case, if the direct current power is 10 kiloWatt, each inverter <b>304</b> operates at full peak load with an input voltage of 400V_DC (each inverter <b>304</b> drawing each 12.5 ampere, so that total current draft is 25 ampere =10 kiloWatt/400Volt). If the power input to inverters <b>304</b> drops to, e.g., 5kW total power, both inverters <b>304</b> experience a drop in the input voltage (since the DC input is now 5kW, if inverters <b>304</b> keep on drawing 12.5A each, then the voltage would be 200V). However, each inverter <b>304</b> starts reducing its input current until an equilibrium is reached (decision box <b>407</b>), which in this case is with each inverter <b>304</b> drawing 6.25 ampere at 375VDC input to a total of 2.5kW power inverted by each inverter <b>304</b> and 5kW for the total both inverters <b>304.</b>

0029Reference is now made to <figref idref="f0005">Figure 5</figref> which illustrates a simplified system <b>50,</b> according to an embodiment of the present invention. A solar panel array <b>10</b> in different embodiments may have serial and/or parallel connected solar cells/panels <b>101.</b> An MPPT powder circuit <b>107</b> maintains a maximum power output of solar panel array <b>10</b> typically by drawing current at the peak power output level of solar panel array <b>10.</b> The output voltage of MPPT circuit <b>107</b> is preferably floating. Connected to MPPT <b>107</b> are multiple inverters, <i>e.g</i>. two inverters, <b>304</b> which are parallel connected both at their inputs and their outputs.

0030The operation of system <b>50</b> is illustrated by referring back to <figref idref="f0004">Figure 4</figref>. If the power output from solar panel array <b>10</b> decreases (for instance as a result of solar shading, <i>e.g</i>., cloud) input power to inverters <b>304</b> drops (step <b>401</b>). Inverters <b>304</b> are configured to adjust their current draw (step <b>405</b>) based on input voltage. Each inverter <b>304</b> starts reducing (step <b>405</b>) its input current until an equilibrium is reached (decision box <b>407</b>) and each inverter <b>304</b> handles (step <b>409</b>) a reduced power load.

0031Reference is now made to <figref idref="f0005">Figure 6</figref> which illustrates a simplified system diagram of inverter <b>304</b> with an integrated control module <b>60</b> according to an embodiment of the present invention. Control module <b>60</b> includes two control loops a voltage control loop <b>601</b> and a current control loop block <b>605.</b> A previously specified voltage reference block <b>603</b> specifies two voltage references, a lower voltage reference and an upper voltage reference. As previously stated, in this example inverter <b>304</b> 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 <figref idref="f0004">Figure 3</figref> the lower reference voltage is 350 VDC and the upper reference voltage is 400 VDC. Voltage control loop block <b>601</b> compares the actual input DC voltage to the voltage references and outputs a current reference <i>I_ref</i> signal. The current reference signal <i>I_ref</i> is used as an input to current control loop block <b>605.</b> Current control loop block <b>605</b> receives also a signal <b>609</b> proportional to its output current. Typically, a current sensor provides signal <b>609</b> from within a pulse width modulation (PWM) block <b>607</b> of inverter <b>304,</b> which performs the power inversion. Current control loop block <b>605</b> compares output current signal <b>609</b> with the current reference signal <i>I_ref</i> and adjusts the output current accordingly until the current (and output power) equilibrate. Thus each inverter <b>304</b> typically handles an equal load of power from solar panel array <b>10</b> or <b>20.</b>

0032As 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 <figref idref="f0006">Figure 7</figref> is linear, other functions or formulas may be used, as this is given as one particular example.

0033Reference is now made to <figref idref="f0007">Figures 8A and 8B</figref> which illustrate racks with parallel connected inverters, according to a feature of the present invention. In this embodiment some or all of inverters <b>304</b> may be configured for operating in a load-balancing mode, according to an embodiment of the present invention, but inverters <b>304</b> 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 <figref idref="f0007">figure 8A</figref>. Other embodiments might also include shared electrical elements of the inverters, and example of which as depicted in <figref idref="f0007">figure 8B</figref> 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 <b>304,</b> a joint AC connection <b>81</b> to the grid and a joint DC connection <b>83</b> to DC power source <b>20.</b> According to a further feature of the present invention, a joint electromagnetic interference filter is used to filter all the outputs of inverters <b>304</b> and electromagnetic radiation thereform, whether they are actually load balancing or not, according to the present invention.

0034The 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.

0035The 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 of the invention being indicated by the following claims.