Abstract:
A maximum power point tracking method and system for use with a power generator comprises sampling instantaneous output voltage and current of the power generator at a first instant in time and at a second instant in time to obtain first and second power samples, generating a reference voltage or current signal from a difference of the first and second power samples; comparing the reference voltage or current to the instantaneous power generator voltage or current and generating at least one gating signal; and repeating so as to minimize the difference of the first and second power samples; wherein the gating signal affects magnitude of the output voltage and current of the power generator; wherein the maximum power point is tracked when the difference signal is minimized. The power generator may be at least one photovoltaic cell, wind turbine, or fuel cell.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/327,782, filed on 26 Apr. 2010, the contents of which are incorporated herein by reference in their entirety. 
     
    
     FIELD 
       [0002]    This invention relates to circuits, systems, and methods for obtaining power from a power generator. In particular, the circuits, systems, and methods described herein may be used to maximize the power obtained from the power generator. 
       BACKGROUND 
       [0003]    Grid-connected power generation systems typically include two major parts: power generators that produce the power and inverters that receive, condition, and inject the power into the power distribution grid. Power generators include, for example, photovoltaic (PV) cells and wind turbines. 
         [0004]    Power generators may be configured as: centralized, string, multi-string, and AC-module/cell technologies. To increase the overall efficiency of power generators under different conditions, such as varying wind conditions on wind turbines, partial shadowing of PV cells, or mismatches between PV cells, independent control and power extraction is required for each power generator. This requires using a separate inverter, i.e., a micro-inverter”, for each power generator. Power extraction from each power generator may enhanced if maximum power point tracking (MPPT) is be performed on each power generator independently. 
         [0005]    Maximum power point tracking of a PV cell in particular is challenging due to the nonlinear current-voltage characteristic and ever-changing nature of the irradiation source. Conventional MPPT systems use an algorithm that finds the best operating point and creates a reference signal, or logical and relational operators, and are based on trial and error or seek and find to find the best operating point. These may be implemented in software running on microprocessors. Such approaches may lead to oscillation around the optimum point, which adversely impacts overall efficiency of the system. Moreover, trial and error approaches degrades efficiency for fast changing conditions. This drawback and the low speed characteristic of such approaches may be problematic in conditions such as monotonic and fast increase of the irradiation level. 
       SUMMARY 
       [0006]    Described herein is a maximum power point tracking method, comprising: (i) sampling instantaneous output voltage and current of a power generator at a first instant in time and at a second instant in time to obtain first and second power samples, wherein the instantaneous voltage and current at the first instant in time are always greater than the instantaneous voltage and current at the second instant in time, or vice versa; (ii) generating a reference voltage or current signal from a difference of the first and second power samples; (iii) comparing the reference voltage or current to the instantaneous power generator voltage or current and generating at least one gating signal; and (iv) repeating (i) to (iii) so as to minimize the difference of the first and second power samples; wherein the gating signal affects magnitude of the output voltage and current of the power generator; wherein the maximum power point is tracked when the difference signal is minimized. In one embodiment, generating a reference voltage or current signal from a difference of the first and second power samples may include using a proportional-integral (PI) controller. 
         [0007]    The method may include sweeping the output voltage of the power generator for a range of maximum power points, determining a global maximum power point, and setting a starting point for maximum power point tracking as close as possible to the global maximum power point. 
         [0008]    Also described herein is a maximum power point tracker, comprising: (i) a sampling means that samples instantaneous output voltage and current of a power generator at a first instant in time and at a second instant in time to obtain first and second power samples, wherein the instantaneous voltage and current at the first instant in time are always greater than the instantaneous voltage and current at the second instant in time, or vice versa; (ii) a subtractor that subtracts the first and second power samples to produce a difference signal; (iii) a means that generates a reference voltage or current signal from the difference signal; and (iv) a means that generates at least one gating signal by comparing the reference voltage or current signal to the instantaneous power generator voltage or current; wherein the gating signal minimizes the difference of the first and second power samples; wherein the maximum power point is tracked when the difference signal is minimized. In one embodiment, the means that generates the reference voltage or current signal may comprise a proportional-integral (PI) controller. 
         [0009]    In one embodiment a maximum power point tracker comprises means that sweeps the output voltage of the power generator for a range of maximum power points, determines a global maximum power point, and sets a starting point for maximum power point tracking as close as possible to the global maximum power point. 
         [0010]    Also described herein is a micro-inverter for a power generator, comprising a maximum power point tracker as described above and a power converter. The power converter may comprise a DC-DC converter and a DC-AC inverter. The power converter may generate a sinusoidal output current from the power generator output. 
         [0011]    Also described herein is a power generation system, comprising a micro-inverter as described above and at least one power generator. 
         [0012]    In the methods, circuits, and systems described herein, the power generator may be a photovoltaic cell, a wind turbine, or a fuel cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a better understanding of the invention, and to show more clearly how it may be carried into effect, embodiments will now be described, by way of example, with reference to the accompanying drawings, wherein: 
           [0014]      FIGS. 1(   a ) and ( b ) are generalized block diagrams of micro-inverter systems. 
           [0015]      FIG. 2(   a ) is a plot showing typical current-voltage and power-voltage characteristics and the maximum power point of a PV cell, and  FIG. 2(   b ) is a plot showing how the characteristics change with amount of irradiation. 
           [0016]      FIGS. 3(   a ) and ( b ) are diagrams of maximum power point tracking schemes according to two embodiments. 
           [0017]      FIGS. 4(   a )-( d ) are plots showing PV cell waveforms for different operating points relative to the maximum power point. 
           [0018]      FIGS. 5(   a )-( c ) are plots showing results of a simulation of the embodiment of  FIG. 3(   a ), for a step change and a sinusoidal change in the irradiation level at (a) 20 ms and (b) 40 ms, respectively, with the error signal shown in (c). 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0019]    Described herein are systems, circuits, and methods for obtaining power from a power generator. A power generator may be, for example, a wind turbine, a fuel cell, or a photovoltaic cell. The power generator may be a distributed power generator. Whereas embodiments of the systems, circuits, and methods are described herein primarily with respect to photovoltaic cells, it will be appreciated that the systems, circuits, and methods are not limited thereto. 
         [0020]    The systems, circuits, and methods described herein may be used in a micro-inverter for a power generator. As used herein, the term “micro-inverter” refers to a device that interfaces a power generator with a load, such as a power distribution grid. A system including a micro-inverter is shown in the generalized block diagram of  FIG. 1(   a ). The micro-inverter  20  receives power from a power generator  10 , and outputs power to a load  30 . The micro-inverter  20  may include a power section  200  that may perform one or more functions, such as, for example, DC-DC conversion, DC to AC conversion, or a combination thereof. The micro-inverter may include a control section  300  that may perform one or more functions such as, for example, maximum power point tracking of the power generator, and/or providing gating signals to the power section  200 . The gating signals may be determined by sensing the power generator voltage and/or current, and/or the load voltage and/or current. 
         [0021]    The generalized block diagram of  FIG. 1(   b ) shows an embodiment of a micro-inverter that includes a power converter section  210  (e.g., DC-DC converter) that receives power from the power generator  10 , and an inverter section  220  that receives power from the converter section  210  and produces output power. The output power may optionally be conditioned (e.g. filtered)  250  to be compatible with the load  30  (e.g., a power distribution grid). One or more passive component (e.g., a capacitor and/or an inductor) may optionally be employed at the power converter section  210  input and/or between the power converter section  210  and the inverter section  220  (i.e., at  230  and  240 , respectively). A filter  250  may optionally be employed at the inverter section  220  output. A first controller  310  may sense the power generator voltage and/or current and produce gating signals for switches of the power converter section  210 . The first controller may perform maximum power point tracking. A second controller  320  may sense the voltage and/or current delivered to the load and produce gating signals for switches of the inverter section  220 . 
         [0022]    A micro-inverter as described herein increases the overall efficiency of the power generator under different circumstances and conditions. For example, in the case of a PV cell or a PV cell string that forms a PV module, partial shadowing of the PV cell or mismatches between PV cells can degrade the overall efficiency of the system. However, use of a micro-inverter for each PV cell, or for each PV cell string or module, permits independent control and power extraction from each PV cell or PV cell string or module, maximizing efficiency of the system despite varying conditions of individual PV cells, strings, or modules. 
         [0023]    A micro-inverter as described herein is compact, so as to be attached to a power generator (e.g., to the back of a PV cell). Since micro-inverters are exposed to a wide range of environmental conditions, such as extremes of temperature and humidity, reliability and maintenance are major issues. This exposure also adversely affects the life expectancy and performance of the inverter. These factors demand robust design and construction, and may require more expensive components that lead to a higher manufacturing cost. Consequently, challenges in the design of a micro-inverter are achieving compactness and low cost, e.g., by reducing the number and size of circuit components. Advantageously, a micro-inverter as described herein does not require costly high voltage components and wiring. The controller system may be implemented in whole or in part using discrete components, using digital technology (e.g., in a digital signal processor (DSP), field programmable gate array (FPGA), or application specific integrated circuit (ASIC) device), or using a combination thereof. For example, one or more components of the controller may be implemented in an algorithm using a suitable hardware language such as, for example, very high speed integrated circuit (VHSIC) hardware descriptive language (VHDL), register transfer language (RTL), or Verilog. Such an algorithm may be implemented in, for example, a FPGA or ASIC device, or other suitable logic device. Use of digital technology provides a controller that is compact and robust. 
         [0024]    A micro-inverter as described herein may include maximum power point tracking (MPPT). As a result, MPPT may be performed on each power generator independently. A challenge in using PV cells is presented by their nonlinear current-voltage (I-V) characteristics, which result in a unique maximum power point (MPP) on the power-voltage (P-V) curve, as shown in  FIGS. 2(   a ) and ( b ). Thus, in the case of PV cells, where partial shading cannot be avoided, MPPT allows the maximum power to be extracted from each PV cell for any instantaneous condition. MPPT removes any mismatch losses between PV cells in the system. Further, micro-inverters as described herein provide modularity to distributed power generators, allowing a “plug and play” approach to their use in a distributed power generation system. 
         [0025]    As used herein, the terms “maximum power point tracking (MPPT)” and “maximum power point tracker (MPP tracker)” are distinct. “MPPT” refers to an algorithm and “MPP tracker” refers to hardware (i.e., a circuit). The MPPT calculates the optimum operating point for a power generator, and provides a reference point for MPP tracker to steer the system toward the optimum operating point. 
         [0026]    As used herein, the term “photovoltaic cell” refers to any cell having a light absorbing material to absorb photons and generate electrons via a photoelectric effect. A non-limiting example of a photovoltaic cell is a solar cell. The light absorbing material may absorb light in any wavelength or combination of wavelengths, including, for example, wavelengths of solar light that reach the earth&#39;s surface, and/or wavelengths of solar light beyond the earth&#39;s atmosphere. Two or more light absorbing materials having specific wavelengths of light absorption may be used in combination to take advantage of different light absorption and charge separation mechanisms. The light absorbing material may be configured as, for example, bulk material, thin-film (e.g., inorganic layers, organic dyes, and organic polymers), and/or nanocrystals. The photovoltaic cells may be combined into arrays, strings, modules, or panels. 
         [0027]    As used herein, the term “photovoltaic cell string” refers to a plurality of photovoltaic cells connected together in a series, parallel, series-parallel, or other configuration. A PV cell string may form a PV cell module. 
         [0028]    Maximum power point tracking as described here may be used with any converter, such as, for example, a resonant-mode converter, a voltage source converter, a current source converter, etc. 
         [0029]    Exemplary embodiments of a MPP tracker and a MPPT scheme are shown in  FIGS. 3(   a ) and  3 ( b ). In the following description, the embodiment shown in  FIG. 3(   a ) is described with reference to the exemplary waveforms at different operating points shown in  FIGS. 4(   a ) to ( d ). Operation of the embodiment shown in  FIG. 3(   b ) will be readily apparent to one or ordinary skill in the art based on the principle of operation described below. 
         [0030]    Referring to  FIGS. 3(   a ) and  4 ( a ) to ( d ), the principle of operation may be explained as follows, using a PV cell as an example of a power generator  10 . In this embodiment an input capacitor C 1  is provided at the input of the DC-DC converter section  210 . When the DC-DC converter  210  switches are in positions such that the converter does not draw power from the PV cell, the PV cell will charge the input capacitor C 1 . As shown in  FIG. 4(   c ), if the average PV cell voltage at the operating point is much lower than the maximum power point (MPP), the power curve has the same slope as the voltage (the opposite condition is shown in  FIG. 4(   a )). However, as shown in  FIGS. 4(   b ) and ( d ), for the case where the PV cell voltage is increasing, the PV cell power increases and then decreases, and passes through the MPP. Therefore, as shown in  FIG. 4(   d ), if the control strategy forces the PV cell to satisfy P pv (t 1 )=P pv (t 2 ), the MPP will always be tracked. Sampling of the PV cell voltage and current may be performed such that from t 1  to t 2  the voltage of the PV cell increases. Similarly for  FIG. 3(   b ), sampling of the PV cell voltage and current may be performed such that the current of the PV cell increases from t 1  to t 2 . Sampling is controlled by a timing circuit as described below. 
         [0031]    The embodiment of the control section  310  shown in  FIG. 3(   a ) or ( b ) includes closed-loop feedback control. For example, control may be implemented by a proportional-integral (PI) controller  371  as shown in the figures. As noted above, all or part of the control section may be implemented in analog and/or digital (hardware/software) platforms. For example, an algorithm in a digital implementation may include the PI controller. In the embodiment shown in  FIG. 3(   a ) or ( b ) the PV cell instantaneous voltage and current are sampled, and the instantaneous power from the PV cell P in  is determined at  321 . Using two delay and sample blocks  341 ,  361 , the instantaneous power P in  from the PV cell is sampled at two switching instances (t 1  and t 2 ) where the voltage increases monotonically. The sampling times are controlled by a timing circuit  351 . The difference of these two power values, (P in (t 1 ) and P in (t 2 )) is determined at  381 , and gives a correct direction towards the MPP. That is, if the difference is negative, the PI controller  371  increases the set point V pv   ref  to a point where the two values (P pv (t 1 ) and P pv (t 2 )) are equal, and vice versa (i.e., if the difference is positive, the PI controller  371  decreases the set point V pv   ref  to a point where the two values (P pv (t 1 ) and P pv (t 2 )) are equal). The objective is to make this difference zero. A control and timing block  391  produces gating signals for the switches of the converter  210 . The control and timing block  391  may also provide power decoupling. Increasing or decreasing the set point V pv   ref  is performed while minimizing or avoiding any possible oscillations and within a short time interval. It will be appreciated that the method provides a very fast response time and does not use trial and error or any logical/relational operations, and thereby avoids misleading results typical of prior methods. 
         [0032]    It is noted that the embodiments described herein permit the use of a small value of C 1 . The resulting short charge/discharge times of the capacitor facilitate very fast maximum power point tracking. 
         [0033]    The embodiments of  FIGS. 3(   a ) and ( b ) may be used substantially as shown or with other circuitry to produce a DC output power for use with a DC load or a DC power distribution system. The embodiments of  FIGS. 3(   a ) and ( b ) may also be used with further circuitry such as an inverter circuit and suitable rectifier/filter circuits (i.e., one or more of sections  220 ,  240 ,  250 , and  320  of  FIG. 1(   b )) to produce AC output power (e.g., 50 or 60 Hz) for use with an AC load or injected into a power distribution grid. 
         [0034]    In some cases a PV cell characteristic may have more than one maximum power point. Depending on the starting point of the algorithm, this may lead to a situation in which a power point tracking method, such as described herein, gets trapped in local maximum power points. To avoid such a situation, the method may include sweeping the voltage of the PV cell for the range of MPPs, determining a global maximum power point, and setting the starting point as close as possible to the global maximum power point. This ensures that the MPPT algorithm will always track the global maximum power point. The interval at which the voltage range of the PV cell is swept may be programmable and may depend on factors such as the PV cell/module configuration. Since the sweep can be performed very fast, and this situation normally does not happen often, the search for a global maximum power point does not affect the overall efficiency of the system. It is worth mentioning that such a condition is not specific to the method described herein; rather, any MPPT algorithm may be similarly affected. 
         [0035]    The following non-limiting example is provided to further illustrate the invention. 
       Working Example 
       [0036]    An analogue simulation of the MPP tracker and MPPT control scheme shown in  FIG. 3(   a ) was carried out for a PV cell, using the PV cell model in PSIM™ version 9 (Powersim Inc., Woburn, Mass.). Irradiation was varied using a step change and a sinusoidal change at 20 Hz.  FIG. 5(   a )-( c ) shows the simulation results at the different irradiation levels. From  FIG. 5(   c ) it can be seen that for very fast changing irradiation (the step change) the error was maintained at a very low level. This improves the overall efficiency of the system. 
         [0037]    The contents of all references cited herein are hereby expressly incorporated by reference. 
       Equivalents 
       [0038]    Those skilled in the art will recognize or be able to ascertain variants of the embodiments described herein. Such variants are within the scope of the invention and are covered by the appended claims.