BACKGROUND 
       [0001]    This invention relates generally to electrical energy conversion and, more specifically, to connection of photovoltaic modules to a power grid. 
         [0002]    With the rising cost and scarcity of conventional energy sources and concerns about the environment, there is a significant interest in alternative energy sources such as solar power and wind power. Solar power generation uses photovoltaic (PV) modules to generate electricity from the sun. Multiple PV cells are connected electrically to one another in such systems. 
         [0003]    In solar power generation systems, in order to feed the electrical power to the power grid, power electronic components are used for power conditioning. In one example, the power electronic components include a direct current (DC) to DC converter and a DC to alternating current (AC) inverter connected back to back through a DC link. 
         [0004]    As new grid codes are developed for solar power generation systems, and as market penetration levels of solar power increase, variability of solar power is becoming less acceptable to utility companies. For example, utility companies are imposing or expected to impose requirements related to power ramp rate limits, power curtailment, and frequency stabilization. 
         [0005]    Power curtailment refers to the decrease in the output power of a power generation system and typically occurs when there is excess electric power production in an area and insufficient transmission capacity to move that electric power to demand centers. Power curtailment may also be demanded by a utility during certain types of grid disturbances. Frequency stabilization requirements of utilities are achievable when a solar power generation system is able to increase or decrease its output power in response to changes in grid conditions. Storing excess energy in energy storage devices is an option to achieve power curtailment and address frequency stabilization requirements but system costs increase with the use of and size of the storage devices. 
         [0006]    Therefore, there is a need for an improved, solar power transmission system to address one or more aforementioned issues. 
       BRIEF DESCRIPTION 
       [0007]    In accordance with an embodiment of the present invention, a power generation system including photovoltaic (PV) modules for generating power is provided. The system also includes power converters coupled to receive power from the PV modules and converter controllers to control the power converters. Each converter controller includes a maximum power point tracking control element. A master controller is provided in the system to coordinate power outputs of the power converters by controlling the timing and numbers of power converters having power voltage (P-V) curves sweeping between maximum power points and power points off the maximum power points. 
         [0008]    In accordance with another embodiment of the present invention, a method of controlling a solar power generation system is provided. The solar power generation system includes PV modules and power converters coupled to the PV modules. The method includes using at least one processor for coordinating power outputs of the power converters by controlling the timing and numbers of power converters having power-voltage (P-V) curves sweeping between maximum power points and power points off the maximum power points. 
     
    
     
       DRAWINGS 
         [0009]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0010]      FIG. 1  is a schematic illustration of a conventional solar power generation system; 
           [0011]      FIG. 2  is a graphical representation of a V-I characteristics and a P-V characteristic of a PV module; 
           [0012]      FIG. 3  is a diagrammatical representation a solar power generation system in accordance with an embodiment of the present invention; 
           [0013]      FIG. 4  is a graphical representation of a V-I curve, a P-V curve and a power waveform of a PV module of the embodiment of  FIG. 3 ; 
           [0014]      FIG. 5  is a graphical representation of output power waveforms of DC to DC converters and total output power waveform in accordance with embodiments of the present invention; and 
           [0015]      FIG. 6  is a graphical representation of another V-I curve, a P-V curve and a power waveform of a PV module in accordance with an embodiment of the present invention. 
           [0016]      FIG. 7  is a diagrammatical representation a solar power generation system in accordance with another embodiment of the present invention; 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  illustrates a conventional solar power generation system  10 . The power generation system includes a PV array  12  including a plurality of connected PV modules. The PV array is connected to a power grid  14  through a DC/DC converter  16 , a DC link  18 , and a grid side three-phase DC/AC converter  20 . In other systems, the grid side three-phase converter may be replaced by multiple single-phase converters. The DC/DC converter  16  is controlled by a DC/DC controller  22  which may comprise a maximum power point tracking (MPPT) controller, and the grid side converter  20  is controlled by a grid side controller  24 . A system controller  26  generates a reference DC voltage command, a reference output voltage magnitude command, and a reference frequency command for the DC/DC converter  22  and the grid side converter  20  respectively. In other systems, a single controller may be used for the multiple control functions shown in  FIG. 1 . Filters  28  are used in the system for removing harmonics from the system output power, and transformer  30  is used for matching grid side converter output voltage to the grid voltage. Power grid  14  may comprise a utility grid or any other system of connecting power generation systems and loads. 
         [0018]      FIG. 2  illustrates a graph  40  of a V-I characteristic  42  of a PV module and a graph  50  of a P-V characteristic  52  of the PV module. Horizontal axis  44  in graph  40  represents a voltage scale in volts, and vertical axis  46  represents a current scale in amperes. PV modules have a single operating point or a maximum power point (MPP)  48  where the values of the current (I) and voltage (V) of the PV module result in a maximum output power P MAX . These values correspond to a particular load resistance, which is equal to V/I as specified by Ohm&#39;s Law. As can be seen from graph  40 , the PV module has an exponential relationship between current and voltage, and the maximum power point occurs at the knee of the curve, where the resistance is equal to the negative of the differential resistance (V/I=−dV/dI). The voltage at maximum power point  48  is represented as V MP  and the current is represented as I MP . Maximum power point tracking controllers utilize a control circuit or a logic to search for this maximum power point and thus to allow the converter circuit to extract the maximum power available from the PV module. It should be noted that, the V-I characteristics of the PV module varies with the temperature and the insulation. Thus, at a different temperature, maximum power point  48  may be at a different voltage and a different current depending on the V-I characteristic at that temperature. Graph  50  shows corresponding power vs. voltage (P-V) curve  52  for V-I characteristic  42 . It can be seen from graph  50  that maximum power P MAX  occurs at voltage V MP . 
         [0019]      FIG. 3  shows a solar power generation system  210  in accordance with an embodiment of the present invention. Power generation system  210  includes a plurality of PV modules  212  to generate DC power, a plurality of DC to DC converters  214  which provide a controlled DC power to a feeder  216 , a plurality of converter controllers  222 , and a DC to AC inverter  218  which converts DC power from feeder  216  to AC power and then transmits the AC power to a power grid  220 . Each DC to DC converter  214  is connected to at least one respective PV module  212 . Although one PV module per DC to DC converter is included in  FIG. 3  for purposes of illustration, in some embodiments multiple PV modules may be included in an array to be coupled to a respective DC to DC converter, and the present invention is intended to encompass both embodiments. In one embodiment, there may be a plurality of DC to AC inverters connected to respective ones of the DC to DC converters  214 . In yet another embodiment, a plurality of DC to AC inverters may be directly coupled to the plurality of PV modules without any DC to DC converters in between. The control of DC to DC converters described with respect to  FIG. 3  is additionally applicable to the direct coupling embodiment  710  shown in  FIG. 7  wherein directly coupled DC to AC inverters  714  are used. Power generation system  210  further includes a master controller  224  to provide control signals to converter controllers  222  to operate the DC to DC converters  214  in an interleaved manner. In many embodiments converter controllers  222  and master controller  224  are housed in separate control units, however, if desired, these controllers may alternatively be integrated into a common control unit. 
         [0020]    Converter controller  222  includes a maximum power point tracking (MPPT) controller  221  for PV module  212 , a Pulse width modulation (PWM) generator  223  and a delay block  225 . As V-I characteristics of PV module  212  varies according to factors such as temperature and cloud cover, MPPT controller  221  continuously tracks a new modified maximum power point on the V-I curve. MPPT controller  221  receives Vdc and Idc as two inputs from PV module  212 , wherein Vdc and Idc are the DC voltage and DC current of the PV module. MPPT controller  221  may include any suitable MPPT algorithms with several examples including a perturbation and observation algorithm and an incremental conduction algorithm. 
         [0021]      FIG. 4  shows an enlarged view near the maximum power point of a V-I curve  230 , a P-V curve  240 , and a power waveform  250  of PV module  212  ( FIG. 3 ). As can be seen from the figure, while determining a maximum power (MP) point  242  on P-V curve  240 , MPPT controller  221  ( FIG. 3 ) sweeps P-V curve  240  from an off MP point  244  on the left side of MP point  242  to another off MP point  246  on the right side of MP point  242 . The corresponding voltage and current drawn from PV module are also varied as shown in V-I curve  230 . It should be noted that even though the power waveform  250  appears to be a smooth ripple waveform due to scaling, the ripple waveform will look like a stair case waveform if enlarged. As can be seen from power waveform  250 , it has an average value  254  and a MP value  252 . In one embodiment, once the maximum power point is determined by MPPT controller  221 , the voltage and current are kept constant at that particular value for a particular duration (i.e., a maintenance duration) of time which will depend on the application. Typically the maintenance duration is in the range of several seconds to several minutes. After the maintenance duration, the MPPT algorithm is then again utilized to see if the P-V curve has changed or the maximum power point has shifted to a new location on the curve. It should be noted that, since the MPPT algorithm is continuously (with continuously as used herein meaning either with or without maintenance durations separating sweeps) being run, it is easier to quickly switch to the relevant maximum power point when needed. 
         [0022]    The total output power is the addition of all the output powers from the DC to DC converters. Each of the DC to DC converters may operate in a curtailed mode or a normal mode. When operating in a normal mode, a DC to DC converter is either in a maintenance duration and operating at its maximum power point determined by the MPPT algorithm or is in an MPP sweep to redetermine the MPP. When operating in a curtailed mode, the DC to DC converter is operating with a sweep that is typically of a different nature than an MPP sweep. The difference in the nature of the sweep will depend on the type and characteristics of the MPP sweep. In some embodiments, the sweep will be slower in the curtailed mode (almost in a stepping rather than sweeping motion) in order to lower the power output. In some embodiments, the range of the sweep over the curve may be longer in the curtailed mode. The number of DC to DC converters that operate in the curtailed mode as well as the nature of the curtailed sweep may be determined based on the power curtailment requirement from the grid. 
         [0023]    To minimize the total overall power ripple, power outputs of curtailed DC to DC converters may be phase shifted. The larger the number of DC to DC converters that are controlled in the curtailed mode, the greater the flexibility for smoothly varying the total output power. To provide for consistent utilization of system components when less than all of the DC to DC converters are curtailed, a supervisory controller may command after a certain period of time that a different set of DC to DC converters to be operated in the curtailed mode and the original set of DC to DC converters return to the normal mode. The transition from the curtailed mode to the normal mode or vice versa may happen within few milliseconds to few seconds, for example, as for a transition from one mode to another mode only the nature or timing of sweeping of the converter needs to be altered. 
         [0024]    In another embodiment of the present invention, even in the absence of a curtailment requirement from the grid, the converter sweeps are controlled in a scheduled manner. Even in an embodiment wherein no curtailment is planned or required, there are benefits to staggering the MPP sweeps so as to minimize ripple in the output power. For example, if there are 60 converters each on a schedule for a 1 second MPPT sweep and a one minute MPP maintenance duration between MPP sweeps, in one embodiment, the converters may be controlled so that the 1st converter sweeps during the 1st second of each minute, and the 2nd converter sweeps during the 2nd second of each minute, and so forth. 
         [0025]    In another related embodiment wherein self curtailment is used to provide flexibility in the event that the grid requires extra power, scheduling of this self curtailment may be provided. For example, if there are total 1000 converters, then a schedule is determined well before hand as to which converters should operate in the curtailed mode and during what time. The first 200 converters may operate in the curtailed mode from 9 am to 11 am of the day. Then the next 200 converters may operate in the curtailed mode for the next 2 hours, and the first 200 converters will move back in the normal mode. If during a given period of time, the grid requires more power, some or all of the converters operating in a curtailment mode may be switched to a normal operating mode or have the degree of their curtailment reduced. If during a given period of time, curtailment requirements are demanded from the grid, additional converters may be switched from their normal operating mode to a curtailed operating mode. 
         [0026]      FIG. 5  shows output power waveforms  270 ,  280  of individual DC to DC converters and a total output power waveform  290  with phase shifted (interleaved) operation of DC to DC converters in accordance with an embodiment of the present invention. As can be seen from  FIG. 5 , power waveform  280  is phase shifted from power waveform  270  by a delay angle of approximately 180 degrees i.e., PWM waveforms for the second DC to DC converter are phase shifted by 180 degrees compared to the PWM waveforms of the first DC to DC converter. The total output power is the addition of output powers of the two DC to DC converters as shown in waveform  290  and it has an average value shown as  292 . 
         [0027]    With reference back to  FIG. 3 , in one embodiment, master controller  224  receives a total output power command Pdc and actual power output Pact as input signals from a supervisory controller (not shown) and a DC to DC converter  214  respectively and generates phase shift angles (or “delay” angles) by which the switching pulses for plurality of DC to DC converters  214  are shifted. Supervisory controller generates the total output power command Pdc based on a grid disturbance or upon excess power being produced by the solar power generation system. In one embodiment, based on the total output power command Pdc, master controller  224  also identifies which of the converters should operate in the curtailed mode and which of the converters should operate in normal mode. Further, master controller  224  may also determine the sweep duration for each of the converters. 
         [0028]    During the curtailed mode, the delay angle command provided by master controller  224  will be given by 
         [0000]    
       
         
           
             
               
                 
                   
                     φ 
                     n 
                   
                   = 
                   
                     
                       360 
                       * 
                       
                         ( 
                         
                           n 
                           - 
                           1 
                         
                         ) 
                       
                     
                     N 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where φ n  is the delay angle command for the n th  DC to DC converter, N is the total number of DC to DC converters operating the curtailed mode, and n refers to n th  number of the DC to DC converter operating in the curtailed mode. For example, when there are eight DC to DC converters operating in a curtailed mode, PWM pulses for the first DC to DC converter will be shifted by zero degrees, PWM pulses for the second DC to DC converter will be shifted by 45 degrees, PWM pulses for the third DC to DC converter will be shifted by 90 degrees, and so forth. Thus the total output power supplied by the PV modules and respective DC to DC converters will have a reduced effect from the ripple (as illustrated by  FIG. 5 ). 
         [0029]    Upon the occurrence of a curtailment request or a grid disturbance, the delay controller will change the delay angle command based on the number of converters to be operated in the curtailed mode. Delay blocks  225  shifts the PWM waveforms of DC to DC converters  214  by the delay angle provided by master controller  224 . 
         [0030]      FIG. 6  shows another enlarged view near the maximum power point of a V-I curve  300 , a P-V curve  310 , and an output power waveform  320  of PV module  192  ( FIG. 3 ). As can be seen from the figure, in one embodiment, MPPT controller  204  ( FIG. 3 ) is configured to sweep on only one side of a MP point  312 . Sweeping on only one side of MPP  312  results in a lower power ripple in the output power waveform  320 . In one embodiment, sweeping on only one side of MPP  312  is achieved by identifying the maximum power point on the P-V curve and once the maximum power point is identified sweeping the P-V curve only on one side of the maximum power point curve. 
         [0031]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
A power generation system includes photovoltaic (PV) modules for generating power and power converters coupled to receive power from the PV modules. The power generation system further includes converter controllers to control the power converters, each converter controller includes a maximum power point tracking control element. A master controller in the power generation system is configured to coordinate power outputs of the power converters by controlling the timing and numbers of power converters having power-voltage (P-V) curves sweeping between maximum power points and power points off the maximum power points.