Abstract:
A power generation system including a photovoltaic (PV) module to generate direct current (DC) power is provided. The system includes a controller to determine a maximum power point for the power generation system and a boost converter for receiving control signals from the controller to boost the power from the PV module to a threshold voltage required to inject sinusoidal currents into the grid. A DC to alternating current (AC) multilevel inverter is provided in the system to supply the power from the PV module to a power grid. The system also includes a bypass circuit to bypass the boost converter when an input voltage of the DC to AC multilevel inverter is higher than or equal to the threshold voltage.

Description:
BACKGROUND 
       [0001]    This invention relates generally to electric energy conversion, and, more specifically, to efficient connection of a photovoltaic module to a power grid or a load. 
         [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. The electricity generated by PV modules is transmitted to the power grid through one or multiple power electronic converters. 
         [0003]    Power losses of the power electronic converters are an important issue in unit sizing of photovoltaic systems because of the influence of such losses on the total energy delivered to the load. The power losses occur in the converters mainly because of losses in switching devices such as Insulated Gate Bipolar Transistors (IGBTs), Metal Oxide Field Effect Transistors (MOSFETs) and diodes, which are generally used in the converters or passive components such as filter inductors. The switching devices generally have three major types of losses: conduction losses, switching losses and gate drive losses. Gate drive losses are not present in diodes, however, conduction losses and switching losses are very much part of the diode losses. Losses in fast switching diodes are typically higher than slow switching diodes. The switching losses correspond to the losses that occur during state changes of the switching device (during turn on and turn off). The conduction losses correspond to losses that occur in the switching device during its conduction (when the device is carrying a current). Gate drive losses refer to the energy required to charge and discharge gate-source and gate-drain capacitances of the switching devices and are affected by switching frequency, drain-source capacitance, and the voltage traversed. Filter inductor losses consist typically of copper and iron losses. Copper losses in power converters are typically increased by skin and proximity effects. Therefore, it is desirable to determine a method and a system that will address the foregoing issues. 
       BRIEF DESCRIPTION 
       [0004]    In accordance with an exemplary embodiment of the present invention, a power generation system is provided. The system comprises a photovoltaic module for generating direct current (DC) power and a controller to determine a maximum power point for the power generation system. The system further includes a boost converter for receiving control signals from the controller to boost the power from the PV module to a threshold voltage required to inject sinusoidal currents into the grid and a DC to alternating current (AC) multilevel inverter to supply the power from the PV module to a power grid. A bypass circuit is also provided in the system to bypass the boost converter when an input voltage of the DC to AC multilevel inverter is higher than or equal to the threshold voltage. 
         [0005]    In accordance with another exemplary embodiment of the present invention, a method for delivering solar power from a power generation system comprising a PV module is provided. The method includes determining a threshold voltage for generating sinusoidal currents for the power generation system and determining whether an output voltage of the PV module at maximum power production is higher than the threshold voltage. The method further includes boosting the power from the PV module to achieve the threshold voltage level when the output voltage of the PV module is less than the threshold voltage and converting the boosted power to AC power. The method also includes bypassing the boosting and directly converting the power from the PV module to AC power when the output voltage of the PV module is higher than the threshold voltage. 
         [0006]    In accordance with yet another exemplary embodiment of the present invention, a method for controlling a power generation system is provided. The method includes controlling a stability of a center point of a direct current (DC) link using a boost converter when an output voltage of a PV module is less than a threshold voltage for generating sinusoidal currents for the power generation system. The method further includes controlling the stability of the center point using a DC to AC multi-level converter when the output voltage of the PV module is higher than the threshold voltage,. 
     
    
     
       DRAWINGS 
         [0007]    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: 
           [0008]      FIG. 1  is a diagrammatical representation of a conventional solar power generation system; 
           [0009]      FIG. 2  is a diagrammatical representation of a solar power generation system in accordance with an embodiment of the present invention; 
           [0010]      FIG. 3  is a schematic representation of a two inductor boost converter along with a split DC link in accordance with an embodiment of the present invention; 
           [0011]      FIG. 4  is a schematic representation of another two inductor boost converter and a split DC link in accordance with an embodiment of the present invention; and 
           [0012]      FIG. 5  is a diagrammatical representation of one leg of a diode clamped multilevel inverter and its output waveform in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    As discussed in detail below, embodiments of the present invention function to provide a system and a method for efficient power transfer from a solar power generation system to a load or a power grid. 
         [0014]      FIG. 1  illustrates a conventional solar power generation system  10 . The power generation system includes a PV module  12 . The PV module 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 . The DC/AC converter  20  maintains a constant DC voltage at the DC link  18 , and thus the energy flow from the DC-LINK  18  to the power grid  14  is managed. The DC/DC converter  16  is controlled by a controller  22 , 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 grid side converter  20 . In other systems, the grid side three-phase converter may be replaced by multiple single-phase converters and/or a single controller may be used for the multiple control functions shown in  FIG. 1 . 
         [0015]      FIG. 2  illustrates a solar power generation system  40  in accordance with an embodiment of the present invention. In one embodiment, the system includes a PV module  12 , a boost converter  42 , a bypass circuit  44 , a split DC link  46 , a multilevel three phase inverter  48 , a boost side maximum power point controller (MPPT) controller  50  and an inverter side MPPT controller  52 . A grid voltage Vac generally determines the minimum DC link voltage. As long as the DC link voltage is higher than a threshold value or a threshold voltage, typically set by the maximum peak line-to-line voltage, the current injected into the grid by the multilevel inverter  48  will have a sinusoidal waveform. If the maximum power point operation of the PV module is associated with a voltage below the threshold voltage the boost side MPPT controller  50  is used to determine a maximum power point for the current-voltage (I-V) characteristics of the PV module and to provide a switching signal to the boost converter to operate the module close to that point at all times. Thus, the boost converter is used to raise the DC link voltage to make it at least equal to the threshold voltage so as to extract maximum power from the PV module. 
         [0016]    In one embodiment, the MPPT controller  50  utilizes a perturbation and observation method. In the perturbation and observation method the current drawn from the solar array is perturbed and the power change is observed. If the perturbation results in an increase in power, the subsequent perturbation is made in the same direction and vice versa. 
         [0017]    When the PV module voltage at maximum power output is higher than the threshold voltage needed for injecting a sinusoidal current into the grid, the bypass circuit  44  bypasses the boost converter. In other words, the boost converter is bypassed when the DC voltage at the photovoltaic (PV) module is higher than the minimum voltage at the split DC link. As the boost converter in the solar power generation system gets bypassed for certain duration of time, it improves the efficiency of the system by eliminating the losses in the boost converter and the boost inductor for that duration. In one embodiment, the bypass circuit comprises a power diode or a voltage controlled switch. In another embodiment, the power diode comprises a silicon carbide (SiC) diode to further improve efficiency of the solar power generation system. In one embodiment of the present solar power generation system, when the bypass circuit  44  bypasses the boost converter  42 , the MPPT is operated on the multilevel inverter  48 . The inverter side MPPT controller  52  senses the voltages and currents from the PV module and it also receives a signal from the bypass circuit  44  to determine when the circuit  44  is working. If the boost converter is bypassed the controller  52  provides switching command signals to the multilevel inverter  48  such that the multilevel inverter  48  will extract maximum power from the PV module. In one embodiment, the controllers  50  and  52  may be combined into a single controller. 
         [0018]      FIG. 3  illustrates a schematic  60  of a two inductor boost converter along with a split DC link in accordance with an embodiment of the present invention. The boost converter  42  includes switching devices  62 ,  64 , the boost inductors  66 ,  68  and the diodes  70 ,  72  and  74 . In one embodiment, the switching devices comprise insulated gate bipolar transistors (IGBTs) or power metal oxide semiconductor field effect transistors (MOSFETs). The switching devices are typically turned on and turned off by a gate drive circuit and in one embodiment comprise silicon carbide devices to improve the efficiency of the circuit. The split DC link  46  includes two capacitors  76  and  78 . In operation, in one step, the switching devices  62  and  64  are turned on, which causes energy to be stored in the boost inductors  66  and  68 . In another step, the switching device  62  is turned off while the switching device  64  is still on. This step results in charging of the capacitor  76  through the charging path formed by the boost inductor  66 , the diode  70 , the capacitor  76 , the device  64  and the boost inductor  68 . In yet another step, the switching device  64  is turned off while the device  62  is turned on. This results in charging of the capacitor  78  through the charging path formed by the boost inductor  66 , the device  62 , the capacitor  78 , diode  72  and the boost inductor  68 . When both the devices  62  and  64  are turned off, the capacitors  76  and  78  may get charged through the charging path formed by the boost inductor  66 , the diode  70 , capacitors  76 ,  78 , the diode  72  and the boost inductor  68 . 
         [0019]      FIG. 4  illustrates a schematic  90  of another two inductor boost converter and a split DC link in accordance with an embodiment of the present invention. In the embodiment of  FIG. 4 , a single switching device  92  is used along with two diodes  94  and  96  instead of two switching devices  62  and  64  as in the embodiment of  FIG. 3 . In operation, when the device  92  is turned on, the current flows from the boost inductor  66 , the device  92  and the boost inductor  68 . Thus, storing energy in the two inductors. During this time, the diodes  70  and  72  block the voltages V 1  and V 2  respectively. When the device  92  is turned off, the current flows from the boost inductor  66 , the diode  70 , capacitors  76  and  78 , the diode  72  and the boost inductor  68 . Thus, charging the capacitors  76  and  78 . 
         [0020]      FIG. 5  illustrates a schematic  110  of one leg or one phase of a diode clamped multilevel inverter and its output waveform in accordance with an embodiment of the present invention. In one embodiment, the one leg  112  of the multilevel inverter includes four switching devices  114 ,  116 ,  118  and  120  and two diodes  122  and  124 . The voltages V 1  and V 2  are controlled to maintain at Vdc/2 by the boost converter  42  of  FIG. 3  or  FIG. 4  and voltage V 3  is the phase A voltage. Further the device  114  is complementary of device  118 , i.e., when the device  114  is conducting, device  118  is not conducting and vice versa. Similarly, devices  116  and  120  are complementary of each other. 
         [0021]    In operation, the one leg of the diode clamped multilevel inverter has three switching stages. In the first switching stage, the devices  114  and  116  are turned on and the devices  118  and  120  are turned off. Assuming a stable operation, V 1 =V 2 =Vdc/2, V 3  becomes Vdc. In the second switching stage, devices  116  and  118  are turned on while devices  112  and  120  are turned off. In this stage, V 3  is equal to Vdc/2. In this stage the output voltage is equal to a voltage at a center tap or a center point  126  of the multilevel converter. The center point  126  refers to a connection point between two DC link capacitors. In one embodiment, where there are more than two DC link capacitors, there may be more than one center point depending on number of DC link capacitors utilized. In operation depending on a load current supplied by the multilevel converter to the power grid, the center point voltage may not remain stable and hence voltage V 1  and V 2  may fluctuate from the value Vdc/2. In one embodiment, when an output voltage of the PV module is less than a threshold voltage, the stability of the center point is controlled by the boost converter; and when the output voltage of the PV module is higher than the threshold voltage, the stability of the center point is controlled by the multi-level converter. 
         [0022]    In the third switching stage, devices  114  and  116  are turned on whereas devices  118  and  120  are turned off. This results in V 3  becoming zero as shown in a waveform  130 . Thus, it can be seen that the phase voltage V 3  has three levels Vdc, Vdc/2 and 0. All three legs of the diode clamped three phase inverter are then combined and the resulting line voltages having five levels namely Vdc, Vdc/2, 0, −Vdc/2 and −Vdc are fed to the power grid as shown in  FIG. 2 . In another embodiment (not shown), the multilevel inverter may comprise a flying capacitor inverter comprising a ladder structure of DC capacitors or a cascaded H-bridge inverter, wherein various single phase inverters are connected in series. It will be appreciated by those skilled in the art that the three-level inverter  112  of  FIG. 5  can be increased to any level depending on the circuit topology and thus depending on number of devices and diodes in the circuit. As the number of levels in the inverter increase, the output waveform of the inverter approaches a pure sine wave, resulting in lower harmonics in the output voltage. 
         [0023]    The advantages of multilevel inverter are the dv/dt stresses on the passive devices, such as inductors, are reduced due to the small increment in voltage steps, reduced electromagnetic compatibility, smaller rating of switching devices and filter components and better feature of output voltage in term of less distortion, lower harmonics contents and lower switching losses. 
         [0024]    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.