Abstract:
A system and process of its operation for monitoring and managing load circuits connected to a renewable energy generation system are disclosed. A programmable load manger circuit continuously monitors the available energy from the generation system and manages the load circuits connected to the system in a manner such that the energy demand from the active load circuits is below the level of available energy. The load circuits can be prioritized and programmed such that the lower priority loads are deactivated prior to the higher priority loads when the available energy from the generation system is not sufficient to satisfy demand from all the active load circuits. When the renewable energy generation system incorporates more than one generator, a load balancing control algorithm, continuously monitoring the load connected to the system and allocates the load in a balanced manner to each of the generators in the system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority to U.S. Provisional Application No. 62/325,888, filed on Apr. 21, 2016, entitled, “LOAD MANAGER FOR RENEWABLE ENERGY SYSTEMS,” the contents of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to load management and balancing systems operating in conjunction with a renewable energy source such as one or more photovoltaic solar energy panels and methods of their operation. 
       BACKGROUND 
       [0003]    Adoption of renewable energy is becoming very popular across the globe. Distributed energy generation employing photovoltaic solar energy systems is gaining popularity as the cost of these systems are coming down. However due to the varying nature of the solar radiation, even during the day time, a majority of the photovoltaic solar energy systems rely on an expensive battery storage subsystem and inefficiently manage the load connected to the system in order to store the direct current (DC) energy which is then converted to usable alternating current (AC) energy using a DC-AC inverter subsystem. 
         [0004]    Currently radiation meters located in the site near the solar panels provide an estimate of the available power in a location. However such estimates are not specific to the solar panels and does not take into account various factors that may affect the output of a solar panel. 
       SUMMARY 
       [0005]    Disclosed herein is a load management circuit that is configured to work in conjunction with a solar energy system with one or more photovoltaic generators and associated DC-AC inverters. In the off-grid mode of operation of the system, the load management circuit is configured to constantly monitor the power demand from the active load circuits and the available power from the photo voltaic generators and manage the active load circuits in a manner such that the power demand from the active loads does not exceed the available power. 
         [0006]    Also disclosed is a programmable load balancing algorithm residing in one of the DC-AC inverters designated as the master to enable sharing of the load by all the DC-AC inverters in a balanced manner through communication with the remaining DC-AC inverters in the array. During the off-grid mode of operation of the solar energy system, the algorithm computes the total power demand from the system and allocates a power limit for each one of the DC-AC inverters in the system and communicates the limit to each one of the DC-AC inverters through power-line communication. 
         [0007]    This summary is provided to introduce a selection of concepts in a simplified form described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The detailed description is set forth with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical components or features. 
           [0009]      FIG. 1  is a schematic block diagram of an example photovoltaic (PV) solar energy system with multiple PV sources and associated DC-AC inverters working in off-grid mode of operation in conjunction with a load manager. 
           [0010]      FIG. 2  is an example power Vs. voltage diagram for a solar photo voltaic panel at 25 Deg. C. at different radiation levels with details of an example method of computing available power based on power drawn and the panel voltage measurement at any given time. 
           [0011]      FIG. 3  is a flow diagram of an illustrative process for the off-grid operation of the load manager circuit in order to constantly monitor the power demand from the active load circuits and the available power from the photo voltaic generators and manage the active load circuits in a manner such that the power demand from the active loads does not exceed the available power. 
           [0012]      FIG. 4  is a flow diagram of an illustrative process for the off-grid operation of the photo voltaic solar energy system through which the system operates within a prescribed range(s) of output voltage and the inverters in the system share the load in a balanced manner. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    In the following detailed description of embodiments, specific detailed examples are given in order to provide an understanding of the embodiments. However, it is to be appreciated that the embodiments may be practiced without these specific details. Furthermore, the techniques and systems disclosed herein are limited to the described embodiments. Numerous modifications, changes, variation, substitutions and equivalents will be apparent to those skilled in the art. 
         [0014]      FIG. 1  is a schematic block diagram of an example of an photovoltaic (PV) solar energy system  100  with multiple PV sources  101 - 1 ,  101 - 2 ,  101 - 3 , . . .  101 -N and associated DC-AC inverters  105 - 1 ,  105 - 2 ,  105 - 3 , . . .  105 -N working in off-grid mode of operation, in conjunction with Load Manager circuit  109 . As an example the photovoltaic solar energy system  100  may be a 3000 Watt roof top solar energy system consisting of ten 300 watt solar panels each connected to a 300 watt DC-AC inverter. 
         [0015]    The DC voltage output from the PV sources V dc1,2,3 . . . N  are connected to the input  103 - 1 , 2 , 3  . . . N of the DC-AC inverters  105 - 1 , 2 , 3  . . . N. In the off-grid mode of operation of the solar energy system, the AC output  107 - 1 , 2 , 3  . . . N are connected to the off-grid voltage V ac  bus  108 . 
         [0016]    The communication modules  111 - 1 , 2 , 3  . . . N of the DC-AC inverters  105 - 1 , 2 , 3  . . . N are interconnected via data links  135 - 1 , 2 , 3  . . . N and are in turn connected to the communication module  126  of the of the controller subsystem  127  of the load manager circuit  109  via the data link  136 . 
         [0017]    The controller subsystem  127  in the load manager circuit  109  is configured to sense the presence or absence of grid voltage  123 , and/or sense off-grid voltage  129 , and to receive inputs from the AC load current sensors  119 - 1 , 2 , 3  . . . M, and input DC voltage values V dc1,2,3 . . . N  from the from the DC-AC inverters  105 - 1 , 2 , 3  . . . N. 
         [0018]    When the grid voltage V grid  is present, the load manger circuit is connected to the grid via link  115  and selector relay  117 . In this case the load manager simply acts like a pass-through and allows all the load segments  137  to be connected to the grid. 
         [0019]    When the grid voltage V grid  is absent, the load manger circuit operates in the off-grid mode. In the off-grid mode of operation of the solar energy system, the off-grid voltage V ac  bus  108  is connected to the selector relay  117  through connector  113 , and the selector relay  117  is connected to the load selector relays  118 - 1 , 2 , 3  . . . M of the load selector  118 . The load selector relays  118 - 1 , 2 , 3  . . . M are associated with the load segments  137 - 1 , 2 , 3  . . . M respectively. Each of the load segments  137 - 1 , 2 , 3  . . . M may be assigned a priority which may be stored in the controller subsystem  127  of the load management circuit  109 . The AC load current sensors  119 - 1 , 2 , 3  . . . M are configured to measure the AC current in the load segments  137 - 1 , 2 , 3  . . . M respectively and transmit the values of measured AC current to the controller subsystem  127 . 
         [0020]    One of the DC-AC inverters  105 - 1 , 2 , 3  . . . N may be designated as the master (for example  105 - 1  in this example). The designated master is configured to receive the AC current output from each of the other DC-AC inverters  105 - 2 , 3  . . . N, compute the total power demanded from the PV solar energy system  100  and allocate the power demand to each of the DC-AC inverters  105  in a balanced manner by setting a current limit for each one of the other DC-AC inverters  105 - 2 , 3  . . . N and communicating the current limit to the other DC-AC inverters  105 - 2 , 3  . . . N via the data links  135 - 1 , 2 , 3  . . . N. 
         [0021]      FIG. 2  is an example power Vs. voltage diagram for a solar photo voltaic panel at 25 Deg. C. at different radiation levels with details of an example process of computing available power based on power drawn and the panel voltage measurement at any given time. Characteristics of a panel with a peak power capacity of 250 watts is presented in this example. 
         [0022]    The horizontal axis represents the DC voltage output of the solar panel, for example the solar panel may be a 60 or 72 cell polycrystalline solar module representing any one of the PV sources  101 - 1 ,  101 - 2 ,  101 - 3 , . . .  101 -N in the example photovoltaic (PV) solar energy system  100 , which ranges from 0 to 40 Volts DC in this example. The vertical axis represents the corresponding solar power in watts that can be generated from the solar panel at different solar radiation levels ranging from 400 W/m 2  (watts/meter squared) to 1000 W/m 2 ·in this example. Photovoltaic solar energy systems are designed to operate in the voltage range to the right of the DC output voltage corresponding to the peak power points (P 1 , P 2 , P 3  and P 4  in this example). The DC output voltage range to the left of the peak power points (P 1 , P 2 , P 3  and P 4  in this example) is considered a non-operating region as shown in  FIG. 2 . 
         [0023]    Focusing on the operating range (30-40 Volt in this example), for a given radiation level, as the power drawn by the load connected to the solar panel increases, the DC output voltage decreases along the Power Vs. Voltage curve corresponding to the radiation level. In the case of a fully characterized solar panel (Power vs. Voltage curves at different radiation levels), a predictive algorithm may predict the total available power from the solar panel employing the measured DC voltage of the panel at a known level of power drawn from the panel by the connected load. 
         [0024]    The illustration of the “Zoomed view of voltage vs. Power curve at 50 W load” in  FIG. 2  is an example of the predictive algorithm for calculating the available power from the solar panel at any given time based on the Panel DC voltage and the power drawn by the load connected to the panel at that time, taking into account various factors affecting the panel performance. In this example where the power drawn by the load is 50 Watts, a panel voltage of 33.5 Volts establishes a radiation level of 400 Watts/meter squared and a total available power of P 1  equivalent to 90 Watts. Similarly, a panel voltage of 35 Volts will represent a total available power of 140 Watts (P 2  in  FIG. 2 ) a panel voltage of 36 Volts will represent a total available power of 180 Watts (P 3  in  FIG. 2 ), and a panel voltage of 37.5 Volts will represent a total available power of 220 Watts (P 4  in  FIG. 2 ). 
         [0025]    Table 1 is an example of Power available computed employing the predictive algorithm and real time data of Solar panel voltage and the load power drawn at four different power output levels drawn from the solar panel. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Available Power Computed using predictive Algorithm 
               
             
          
           
               
                 Solar panel 
                 Solar Panel output 
                 Solar Panel 
               
               
                 output power 
                 DC Voltage 
                 Available Power 
               
               
                 Watts 
                 Volts 
                 Watts 
               
               
                   
               
             
          
           
               
                 50 
                 33.5 
                 90 
               
               
                   
                 35 
                 140 
               
               
                   
                 36 
                 180 
               
               
                   
                 37.5 
                 225 
               
               
                 100 
                 33.5 
                 140 
               
               
                   
                 35 
                 180 
               
               
                   
                 36.5 
                 225 
               
               
                 150 
                 34.5 
                 180 
               
               
                   
                 36 
                 225 
               
               
                 200 
                 35 
                 225 
               
               
                   
               
             
          
         
       
     
         [0026]    From the data available in the look up table, it is possible to derive an empirical relationship between total power available and solar panel voltage and power drawn from the panel at any given time. In the case of look up tables, interpolation techniques can be used with the values from the look up tables to get more accurate values for the available power from the PV source. 
         [0027]    Such lookup tables or empirical formula representing the look up table for each of the solar PV sources  101 - 1 , 2 , 3  . . . N may be computed and stored in the DC-AC inverters  105 - 1 , 2 , 3  . . . N or the controller subsystem  127  of load manager circuit  109  associated with the solar energy system  100 , and these lookup tables or empirical formula can be readily used to determine the available power P AV-i  for each of the PV source “i”. The total available power for the PV solar energy system  100  P AV-Total  may be computed by summation of the available power from each PV source  101 - 1 , 2 , 3  . . . N (P AV-Total =P AV-1 +P AV-2 +P AV-3 + . . . P AV-N ). 
         [0028]      FIGS. 3 and 4  illustrate example processes that may be carried out to perform the techniques described herein. The processes are illustrated as a collection of blocks in a logical flow graph, which represent a sequence of operations that can be implemented in hardware, software, or a combination thereof. In the context of software, the blocks represent computer-executable instructions stored on one or more computer-readable storage media that, when executed by one or more processors, perform the recited operations. Generally, computer-executable instructions include routines, programs, objects, components, data structures, and the like that perform particular functions or implement particular abstract data types. The order in which the operations are described is not intended to be construed as a limitation, and any number of the described blocks can be combined in any order and/or in parallel to implement the processes. Moreover, in some embodiments, one or more blocks of the processes may be omitted entirely. 
         [0029]      FIG. 3  is a flow diagram of an illustrative process  300  for the off-grid operation of the load manager circuit  109  in order to continuously monitor the power available from solar energy sources  101 - 1 , 2 ,  3  . . . N connected to the DC-AC inverters  105 - 1 , 2 , 3  . . . N and power demand from the active loads  137 - 1 , 2 , 3  . . . M connected to the load manager circuit  109 , and actively managing the loads such that total demanded power P D-Total  is less than the total available power P AV-Total . 
         [0030]    In step  302  the system initialization parameters are set to appropriate values. The number of DC-AC inverters in the system is set to value N, the number of load segments is set to value M. An excess power threshold value P Ex  and a reserve power threshold value P Res  are set to appropriate values. As an example for a Photovoltaic solar energy system of 3000 Watts capacity, N may be set to 10 (each inverter with 300 W capacity, for example), M may be set to 4 (the load circuits segmented into 4 segments), the excess power threshold P Ex  may be set to 500 Watts, and the reserve power threshold value P Res  may be set to 100 Watts. At the system start-up, the highest priority load segment may be set to active status through selector switch  118 - 1  and all other segments may be set to inactive status though selector switches  118 - 2 , 3 , . . . M. Set the active segment value Q−1 and set timer value t=0. 
         [0031]    In step  304  the total power demand P D-Total  from the active load segments is computed. For this computation the off-grid bus voltage V ac , AC current sensor  119 - 1 , 2 , 3  . . . M output values I ac-1 , I ac-2 , I ac-3 , . . . I ac-M  are acquired and total power demand is computed as P D-Total =V ac  (I ac-1 +I ac-2 +I ac-3 + . . . +I ac-M ). For the example system, V ac  may be equal to 220 Volts, and (I ac-1 +I ac-2 +I ac-3 + . . . +I ac-M ) value may be equal to 4 amps, representing a total power demand P D-Total  equal to 880 Watts. 
         [0032]    In step  306  the total available power P AV-Total  for the system is computed. The DC voltage input for each inverter V dc-1 , V dc-2 , V dc-3  . . . V dc-N , is combined with the corresponding DC current values I dc-1 , I dc-2 , I dc-3 , . . . I ac-N  and used to compute the DC power drawn from each PV source P dc-1 , P dc-2 , P dc-3  . . . P dc-N . Using the power and the associated DC voltage values, the available power from each PV source P AV-1 , P AV-2 , P AV-3  . . . P AV-N  is computed from the look up table (e.g., Table 1), as discussed in the previous section. Total available power for the system is then computed as P AV-Total =P AV-1 +P AV-2 +P AV-3  . . . +P AV-N . For the example system, P AV-Total  may be equal to 1200 Watts. 
         [0033]    In step  308  systems ability to activate a load segment that is not currently active is determined based on a comparison of total available power P AV-Total  with the total power demand P D-Total +excess power threshold, P Ex . If it is determined, based on the comparison at step  308 , that P AV-Total  is greater than P D-Total +P Ex , then the process  300  follows the “yes” route from step  308  to step  309  where selector switch with the highest priority among the ones which are currently inactive will be activated and the corresponding load segment connected to the system output. 
         [0034]    If, at step  308 , P AV-Total  is not greater than P D-Total +P Ex , then the process  300  follows the “no” route from step  308  to step  310 , where the system ability to de-activate a load segment that is currently active is determined based on a comparison of the total available power P AV-Total  with the total power demand P D-Total +a reserve power threshold, P Res . If it is determined, based on the comparison at step  310 , that P AV-Total  is less than P D-Total +P Res , then process  300  follows the “yes” route from step  310  to step  311  where the selector switch with the lowest priority among the ones which are currently active will be deactivated and the corresponding load segment disconnected from the system output. 
         [0035]    Either following the “no” route from step  310 , after activating the highest priority selector switch at step  309 , or after deactivating the lowest priority selector switch at step  311 , the process  300  proceeds to step  312  to repeat the steps  304  to  310  in periodic intervals of time Δt in order to continuously monitor the power available from solar energy sources  101 - 1 , 2 ,  3  . . . N connected to the DC-AC inverters  105 - 1 , 2 , 3  . . . N and power demand from the active loads  137 - 1 , 2 , 3  . . . M connected to the load manager circuit  109 , and actively managing the loads such that total demanded power P D-Total  is less than the total available power P AV-Total . 
         [0036]      FIG. 4  is a flow diagram of an illustrative process  400  for the off-grid operation of the Photo voltaic solar energy system  100  through which the system operates within a prescribed range of output voltage V ac-L-T  and V ac-H-T  and the inverters  105  in the system share the load in a balanced manner. 
         [0037]    In step  402  the system initialization parameters are set to appropriate values. The number of DC-AC inverters  105  in the system is set to value N, and one of the DC-AC inverters  105 - 1  is designated as the master. A low voltage threshold V ac-L-T  value and a high voltage threshold value V ac-H-T  are set. As an example V ac-L-T  may be set to 205 volts and V ac-H-T  may be set to 235 volts. In this step, a current increment or decrement size ΔI is set. As an example ΔI may be set to 25 milli amps. Set inverter current buffer value I ac -Buffer. As an example the value of I ac-Buffer  may be set as 100 milli amps. A time interval Δt at which the balancing process is repeated is also set at this step. As an example the value of Δt may be set as 400 milli seconds. Set initial timer value t=0. 
         [0038]    In step  404  total AC current drawn I ac-Total  by the loads is computed as the sum of current values I ac-1 , I ac-2 , I ac-3 , . . . I ac-M  from the current sensors  119 - 1 ,  119 - 2 ,  119 - 3  . . .  119 -M respectively. This total AC current value I ac-Total  is allocated to each inverter as a current limit value I ac-limit . As an example the total AC current value I ac-Total  may be allocated substantially equally to each of the N inverters  105  in the system. 
         [0039]    In step  405 , the inverter  105 - 1  designated as the master is set to operate in the voltage control mode within the operating voltage range of V ac-L-T &lt;V ac &lt;V ac-H-T  and the current limit value I ac-limit set  to (I ac-Total ÷N.)+I ac-Buffer . Examples of operating in the voltage control mode is described in U.S. Pat. No. 9,444,366, entitled “DUAL MODE MICRO-INVERTER SYSTEM AND OPERATION,” and U.S. Pat. No. 9,590,528, entitled “DUAL MODE DC-AC INVERTER SYSTEM AND OPERATION,” the contents of which are herein incorporated by reference. 
         [0040]    In step  406  the AC bus voltage V ac  is compared with low voltage threshold V ac-L-T  value. If it is determined, based on the comparison at step  406  V ac  is less than V ac-L-T  then the process  400  follows the “yes” route from step  406  to  407 , where the current injected by inverter “x” is incremented by value ΔI. 
         [0041]    If it is determined, based on the comparison at step  406  V ac  is not less than V ac-L-T  then the process  400  follows the “no” route from step  406  to  408 . 
         [0042]    In step  408  the AC bus voltage V ac  is compared with high voltage threshold V ac-H-T  value. If it is determined, based on the comparison at step  408  V ac  is greater than V ac-H-T  then the process  400  follows the “yes” route from step  408  to  409  If where the current injected by inverter “x” is decreased by value ΔI. 
         [0043]    If it is determined, based on the comparison at step  408  V ac  is not greater than V ac-H-T  then the process  400  follows the “no” route from step  408  to  410 . 
         [0044]    In step  410  the output current of inverter “x” is compared with current limit I ac-limit . it is determined, based on the comparison at step  410  output current of inverter “x” is greater than I ac-limit  then the process  400  follows the “yes” route from step  410  to  411  where the current injected by inverter “x” is decreased by value ΔI. it is determined, based on the comparison at step  410  output current of inverter “x” is not greater than I ac-limit  then the process  400  follows the “no” route from step  410  to  412 . 
         [0045]    In step  412  the inverter ID value “x” is incremented by one. In step  414  boundaries for the value of “x” set as between 2 and N. 
         [0046]    Steps  406 - 414  are executed on a continuous basis in order for the system  100  to adequately support the active load segments and maintain the off-grid AC bus voltage between low voltage threshold V ac-L-T  and high voltage threshold V ac-H-T . 
         [0047]    Step  416  is an indication for repeating the steps  404  to  414  in periodic intervals of time Δt in order to periodically compute the total AC current I ac-Total  and set the appropriate current limits I ac-limit  for all the inverters in order for the system to function in a balanced manner. 
       CONCLUSION 
       [0048]    In closing, although the various embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended representations is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as example forms of implementing the claimed subject matter.