Abstract:
Systems, methods, and devices for use with photovoltaic micro-inverters. A control system for controlling power switches in a full bridge inverter in a DC/DC converter uses an estimate of the output current of a photovoltaic (PV) panel. The estimate is provided by a current estimator that takes as input the output voltage of the PV panel as well as the bus voltage of the DC/DC converter. Also used as input to the current estimator are two of the control signals for specific power switches in the full bridge inverter in the DC/DC converter.

Description:
RELATED APPLICATIONS 
     This application is a non-provisional of U.S. Provisional Patent Application No. 62/073,431 filed Oct. 31, 2014. 
     TECHNICAL FIELD 
     The present invention relates to circuit and control systems for photovoltaic (PV) micro-inverters. In particular, the present invention relates to methods, circuits, and devices for use with PV micro-inverters and for maximum power point tracking (MPPT) of PV panels. 
     BACKGROUND 
     Photovoltaic (PV) power conditioning systems usually include a maximum power point tracker (MPPT) in order to ensure that the maximum available power is extracted from the PV panels. The PV power conditioning system usually consists of two stages. The first stage is a DC/DC converter. The second stage is usually a DC/AC inverter. The DC/DC converter is responsible for providing galvanic isolation as required by grid interconnection regulatory standards. The DC/DC converter also boosts the voltage at the output of the PV panel. As well, the DC/DC converter control system is responsible for extracting the maximum power from the PV panel. The DC/AC converter converts the DC power to AC power which can be delivered to the utility grid. 
       FIG. 1  shows an exemplary arrangement of a two-stage 
     PV power conditioning system. In  FIG. 1 , illustrated are: a PV power conditioning system  1 , a PV-panel  2 , a DC/DC converter  3 , a DC-link  4 , a DC/AC converter  5 , and a utility grid  6 . The DC/DC converter  3  performs the maximum power point tracking of the PV-panel  2 . Also, present is an intermediate DC-link  4  between the two stages of the power conditioning system. This component is used so that energy can be stored and to provide decoupling between the first-stage and the second-stage of the system. As well, the DC-link  4  attenuates the double frequency ripple caused by the power ripple at the output of second-stage DC/AC converter  5 . The DC/AC converter  5  produces power compatible to the utility grid  6 . 
     The arrangement in  FIG. 2  shows an exemplary arrangement of the DC/DC converter control system. This figure shows that the control DC/DC converter control system  30 , includes: maximum power point tracker (MPPT)  32 , and DC/DC converter controller  34 . The MPPT  32  produces the reference value for the PV voltage/current according to the feedback signals. These feedback signals are the PV output voltage v PV  and the PV output current i PV . The MPPT  32  requires both the PV output voltage and current in order to find the “maximum power point”. With the PV output voltage v PV , and the PV output current i PV , the MPPT  32  produces the reference value for either the PV output voltage, V* PV , or the PV output current, i* PV . This reference is the input to the DC/DC converter controller  34 . The DC/DC converter controller  34  produces appropriate gate pulses for the DC/DC converter in order to regulate the converter input voltage or the converter input current. 
     In order to measure the output current of the PV panel, a current sensor able to measure direct current (DC) is required. Two types of current sensors are commonly used to measure this DC current: Hall-effect current sensors, and resistive current sensors. 
     Hall effect sensors suffer from several practical difficulties. Due to the remnant flux, such Hall effect sensors introduce a time varying DC-bias into the control system. A correction algorithm therefore has to be added to compensate for this time varying DC bias. This correction algorithm increases the complexity of the implementation of the control system and reduces the reliability of the converter. As well, the bandwidth of the Hall effect sensors are limited and they introduce delay into the closed-loop control system. Such delay may jeopardise the stability of the control system. Finally, Hall effect sensors are very costly and can significantly contribute to the overall cost of the converter. 
     The second technique to measure the input current is through the use of resistive current sensors. Resistive current sensors require a very precise and noise-free differential amplifier. These types of sensors also increase the power losses of the converter. These power losses are not preferable due to the efficiency deterioration and thermal management. The arrangement in  FIG. 3  shows an exemplary arrangement of the resistive current sensor used to measure the output current of a PV panel. Referring to  FIG. 3 , illustrated are: a resistive current sense  36 , a precise difference amplifier  38 , and an analogue-to-digital converter (ADC)  40 . This circuitry in  FIG. 3  produces the digital value of the PV panel output current for the MPPT  32 . According to  FIG. 3 , the resistive current sensor requires a precise difference amplifier in order to accurately measure the current. Also, at light loads the accuracy of the resistive current sensor is highly compromised due to the small value of the current sense resistor R sense . Therefore, at light loads the performance of MPPT is significantly deteriorated. 
     For multi-input PV power conditioning systems, in particular, the input current sensors add a lot of complexity and cost to the power conditioning system. The arrangement in  FIG. 4  shows an exemplary arrangement of a four-input PV power conditioning system. This power conditioning system is able to perform maximum power point tracking on each individual panels. However, the power conditioning system is required to measure each PV panel&#39;s output current in order to perform maximum power point tracking. The power conditioning system therefore requires current sense resistors, difference amplifiers and ADCs for each individual panel. Because of this, current sensors significantly contribute to the overall cost of the power conditioning system. 
     In addition to the above, there is another difficulty related to using resistive current sensors for multi-input power conditioning systems. The arrangement in  FIG. 5  shows the multi-input PV power conditioning system with resistive current sensors. In order to minimize the required isolation circuitry for the DC/DC converter, the control system should have the same ground as the control system. This is because gate pulses produced by the control system can be directly applied to the DC/DC converters power switches without isolation. However, in this arrangement, the multiple inputs of the power conditioning system cannot operate in parallel, since the current sense resistors will be in parallel and cannot indicate the precise value of the current for each panel. This difficulty restricts the application of the multi-input PV power conditioning systems for different applications. In particular, this arrangement cannot be applied to applications where the PV panels are arranged in parallel. 
     There is therefore a need for a simple and practical solution which can provide the output current information for the PV panel. Preferably, such a solution should not add extra circuitry to the power conditioning system. It would also be preferable if such a solution can provide the current information very precisely and reliably under different conditions. 
     SUMMARY 
     The present invention provides systems, methods, and devices for use with photovoltaic micro-inverters. A control system for controlling power switches in a full bridge inverter in a DC/DC converter uses an estimate of the output current of a photovoltaic (PV) panel. The estimate is provided by a current estimator that takes as input the output voltage of the PV panel as well as the bus voltage of the DC/DC converter. Also used as input to the current estimator are two of the control signals for specific power switches in the full bridge inverter in the DC/DC converter. 
     In one aspect, the present invention provides a control system for controlling a DC/DC converter for a photovoltaic panel, the control system comprising:
         a current estimator for providing an estimated output current of said photovoltaic panel, said current estimator basing said output current on an output voltage of said photovoltaic panel and on a bus voltage of said DC/DC converter;   a maximum power point tracker (MPPT) for producing a reference value for an output voltage for said photovoltaic panel, said MPPT receiving said estimated output current from said current estimator;   an input voltage controller for producing a desired phase shift for regulating an input voltage of said DC/DC converter, said input voltage controller receiving said reference value from said MPPT;   a phase-shift modulator for producing control signals for power switches in said DC/DC converter based on said desired phase shift;       

     wherein said current estimator is based on nonlinear dynamics of said DC/DC converter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which: 
         FIG. 1  shows an exemplary arrangement of a two-stage PV power conditioning system according to the prior art; 
         FIG. 2  shows an exemplary arrangement of the DC/DC converter control system according to the prior art; 
         FIG. 3  shows an exemplary arrangement of the resistive current sensor used to measure the output current of a PV panel according to the prior art; 
         FIG. 4  shows an exemplary arrangement of a four-input PV power conditioning system according to the prior art; 
         FIG. 5  shows the multi-input PV power conditioning system with resistive current sensors according to the prior art; 
         FIG. 6  illustrates a PV power conditioning system with a current sensorless MPPT according to one aspect of the invention; 
         FIG. 7  illustrates the circuit implementing the current observer for the output current of PV panel; 
         FIG. 8  shows the switching pulses of the power switches in the DC/DC converter when the invention is implemented; 
         FIG. 9  shows the step size for the discrete calculation of the digital observer in each switching cycle; 
         FIG. 10  shows the block diagram of the current observer for the output current of the PV panel; 
         FIG. 11  show various simulation waveforms for the DC/DC converter using one aspect of the invention; 
         FIG. 12  show further waveforms illustrating the accuracy of the observer according to one aspect of the invention; 
         FIG. 13  illustrate various waveforms which show that the observer of the invention is able to accurately estimate the output current of the PV panel; 
         FIG. 14  illustrates the transient performance of the closed loop control system; 
         FIG. 15  shows the steady-state performance of the prototype converter using the invention; and 
         FIG. 16  illustrates the transient performance of the prototype converter using the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In one aspect, the present invention relates to a current sensorless MPPT for PV power conditioning systems. The technique of the invention eliminates the need for current sense circuitry for measuring the output current of PV panels. As well, the technique of the invention can precisely estimate the output current of the PV panel for maximum power point tracking. The technique can greatly reduce the power conditioning system circuitry and, in turn, can reduce the overall cost of the power conditioning system. Additionally, the technique of the invention is not sensitive to the operating condition of the power conditioning system. Thus, this aspect of the invention can precisely estimate the current for different load conditions. 
     The arrangement in  FIG. 6  shows a system according to one aspect of the invention. In  FIG. 6 , a PV power conditioning system  100  with the current sensorless MPPT is illustrated. Referring to  FIG. 6 , illustrated are: a DC/DC converter  110 , and a DC/DC converter control system  120 . The DC/DC converter  110  is a series resonant converter. The DC/DC converter control system  120  includes a current estimator  130 , an MPPT  32 , an input voltage controller  150 , and a phase-shift modulator  160 . The current estimator  130  estimates the output current of the PV panel using the output voltage of the PV panel and the DC-bus voltage. The estimated current and the PV output voltage are applied to the MPPT  32 . The MPPT  32  produces the reference value for the PV panel output voltage, v* PV . This reference value is sent to the input voltage controller  150 . The input voltage controller  150  produces the appropriate phase-shift, ψ in order to regulate the input voltage of the DC/DC converter. The phase-shift ψ is sent to the phase-shift modulator  160  and the phase-shift modulator  160  generates the gate pulses for the power switches S 1 -S 4  according to the inserted phase-shift ψ. 
     The current estimator is designed based on the DC/DC converter mathematical dynamics. According to  FIG. 6 , the dynamics of the DC/DC converter  110  are given by: 
     
       
         
           
             
               
                 
                   
                     
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     The objective is to design an observer which is able to estimate the output current of the PV panel or, more precisely, the output current of the PV panel in the DC component of the DC/DC converter input current, i in . 
     Since the DC/DC converter input current, i in , can be derived from the series resonant current, i p , the objective becomes one of estimating the series resonant current i p  using the dynamics given by Eqn. (1), Eqn. (2), and Eqn. (3). The only measurable state variable in these dynamics is the DC-bus voltage, v BUS . Therefore, the system dynamics can be rewritten as: 
     
       
         
           
             
               
                 
                   
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     Because the system is observable, the adaptive observer introduced in the present invention is given by: 
                         x   ^     .     1     =         1     L   s       ⁢     v   AB       -       1     L   s       ⁢       x   ^     2       -       1     L   s       ⁢     sign   ⁡     (       x   ^     1     )       ⁢     x   3       +       μ   1     ⁢       x   ~     3                 (   6   )                     x   ^     .     2     =         1     C   s       ⁢       x   ^     1       +       μ   2     ⁢       x   ~     3                 (   7   )                     x   ^     .     3     =         1     C   BUS       ⁢            x   ^     1            -       β   ^     ⁢     x   3       +       μ   3     ⁢       x   ~     3                 (   8   )                   β   ^     .     =     γ   ⁢           ⁢     x   3     ⁢       x   ~     3               (   9   )                   i   ^     in     =         x   ^     1     ⁢     sign   ⁡     (     v   AB     )                 (   10   )                   i   ^     PV     =     LPF   ⁡     (     i   in     )               (   11   )               
where the coefficients μ 1 , μ 2 , μ 3 , γ are the observer gains.  FIG. 7  shows the current observer for the output current of PV panel. In  FIG. 7 , S 1 (t) and S 3 (t) are the switching pulses for power switch S 1  and S 3  respectively. It should be noted that the circuit in  FIG. 7  merely implements Eqn. (6)-Eqn.(11) above. The switching pulses of the power switches in the DC/DC converter are illustrated in  FIG. 8  for clarity. Also, in  FIG. 7 , sgn represents the sign function (i.e. for the sgn function, if the input is positive the output of the sgn function is +1, if the input is negative the output of the sgn function is −1, and if the input is zero the output of the sgn function is 0), LPF represents a generic low pass filter.
 
     It should be noted that there are a few subtleties in the digital implementation of the observer of the present invention. One of the main subtleties stems from the fact that the observer subsystem is nonlinear. Because of this, special attention should be paid to discretizing the observer in order to accurately estimate the output current of the PV panel. The digital observer introduced in the present invention can be represented by the following difference equations: 
     
       
         
           
             
               
                 
                   
                     
                       
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     Another subtlety relates to the sampling frequency for the DC-bus voltage, v BUS , the PV panel output voltage, v PV , and the step size for the digital observer calculations. It should be noted that the series resonant current, x 1  is a nearly sinusoidal current with the same frequency as the inverter output voltage, v AB . Because of this, the observer difference equations must be calculated in a much finer step size than the switching frequency so that one may precisely estimate the series inductor current.  FIG. 9  shows the step size for the discrete calculation of the digital observer in each switching cycle.  FIG. 9  shows that the step size should be much finer than the switching period in order to achieve precise calculation of the series resonant current. Also, the sampling frequency of the DC-bus voltage and the output voltage of the PV panel is much coarser than the step size of the discrete observer calculation (usually once or a few times for each switching cycle). This is the reason that the discrete value of the DC-bus voltage v BUS (n) and the PV output voltage v PV (n) has an argument different from the step size k. 
     The estimated series inductor current {circumflex over (x)} 1 (k) and the switching pattern of the full-bridge inverter, in particular S 1 (t)-S 3 (t), determines the input current of the series resonant converter, î in (k). Finally, the estimated PV panel output current, î PV  is the DC-value of the estimated input current of the converter.  FIG. 10  shows the block diagram of the current observer for the output current of the PV panel.  FIG. 10  shows the digital current observer  130 , which includes: series resonant current observer  170 , adaptive law  180 , input current estimator  190 , and finite impulse response (FIR) filter  200 . The series resonant current observer  170  estimates the series resonant current î p (k) as the main output. The other outputs are the estimated series capacitor voltage, {circumflex over (x)} 2 (k), and the estimated DC-bus voltage, {circumflex over (x)} 3 (k). The inputs to the series resonant current observer  170  are the switching signals S 1 (k) and S 3 (k), the sampled PV panel output voltage, v PV (n), the sampled DC-bus voltage v BUS  (n), and the parameter {circumflex over (β)}(k). The series resonant current observer  170  implements Eqn. (12), Eqn. (13), and Eqn. (14). The adaptive law  180  implements Eqn. (15). The adaptive law  180  estimates the parameter {circumflex over (β)}(k) as the output using the sampled DC-bus voltage v BUS (n) and the estimated DC-bus voltage {circumflex over (x)} 3 (k) as the inputs. The input current estimator  190  implements Eqn. (16). The input current estimator  190  receives the estimated series resonant current, î p (k), and the switching signals, S 1 (k)-S 3 (k), as the inputs and produces the estimated input current î in (k) as the output. The FIR  200  implements Eqn. (17). Eqn. (17) represents the simplest form of an FIR filter. However, any appropriately designed FIR can be used to extract the DC-value of the estimated input current î in (k). The FIR  200  receives the estimated input current î in (k) as the input and produces the estimated PV panel output current î PV (k) as the output. 
     Simulation waveforms of the DC/DC converter with the current sensorless MPPT are shown in  FIG. 11 ,  FIG. 12 ,  FIG. 13 , and  FIG. 14 . The waveforms in  FIG. 11  include: the inverter output voltage, v AB , the estimated series resonant current, î p , the estimated series capacitor voltage, {circumflex over (v)} Cs , and the transformer primary voltage, v pri . Referring to  FIG. 12 , illustrated are: the actual series resonant current, i p , the estimated series resonant current, î p , the actual series capacitor voltage, v Cs , and the estimated series capacitor voltage {circumflex over (v)} Cs .  FIG. 12  shows the accuracy of the observer according to one aspect of the invention.  FIG. 13  shows the estimated series resonant current, î p , the estimated converter input current, î in  and the estimated PV output current, î PV .  FIG. 13  shows that the observer according to one aspect of the invention is able to accurately estimate the output current of the PV panel. The transient performance of the closed loop control system is illustrated in  FIG. 14 . Referring to  FIG. 14  illustrated are: the transient performance of the estimated series resonant current, î p , the estimated PV output current, î PV , and the estimated converter input current, î in . 
     Experimental waveforms of the DC/DC converter prototype using the current sensorless MPPT according to one aspect of the present invention are shown in  FIG. 15  and  FIG. 16 .  FIG. 15  shows the steady-state performance of the converter and  FIG. 16  illustrates the transient performance of the converter. 
     It should be noted that the observer according to one aspect of the invention may be implemented as an ASIC (application specific integrated circuit). Similarly, the observer may be implemented using a general purpose computer and suitable programming and hardware. 
     The embodiments of the invention may be executed by a computer processor or similar device programmed in the manner of method steps, or may be executed by an electronic system which is provided with means for executing these steps. Similarly, an electronic memory means such as computer diskettes, CD-ROMs, Random Access Memory (RAM), Read Only Memory (ROM) or similar computer software storage media known in the art, may be programmed to execute such method steps. As well, electronic signals representing these method steps may also be transmitted via a communication network. 
     Embodiments of the invention may be implemented in any conventional computer programming language. For example, preferred embodiments may be implemented in a procedural programming language (e.g.“C”) or an object-oriented language (e.g.“C++”, “java”, “PHP”, “PYTHON” or “C#”). Alternative embodiments of the invention may be implemented as pre-programmed hardware elements, other related components, or as a combination of hardware and software components. 
     Embodiments can be implemented as a computer program product for use with a computer system. Such implementations may include a series of computer instructions fixed either on a tangible medium, such as a computer readable medium (e.g., a diskette, CD-ROM, ROM, or fixed disk) or transmittable to a computer system, via a modem or other interface device, such as a communications adapter connected to a network over a medium. The medium may be either a tangible medium (e.g., optical or electrical communications lines) or a medium implemented with wireless techniques (e.g., microwave, infrared or other transmission techniques). The series of computer instructions embodies all or part of the functionality previously described herein. Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, optical or other memory devices, and may be transmitted using any communications technology, such as optical, infrared, microwave, or other transmission technologies. It is expected that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation (e.g., shrink-wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server over a network (e.g., the Internet or World Wide Web). Of course, some embodiments of the invention may be implemented as a combination of both software (e.g., a computer program product) and hardware. Still other embodiments of the invention may be implemented as entirely hardware, or entirely software (e.g., a computer program product). 
     A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow.