Abstract:
An electrical power converter system adjusts a wakeup voltage periodically, to permit earlier connection and/or operation, to increase performance. The electrical power converter system selects between a mathematically adjusted wakeup voltage based on at least one previous period, and a table derived wakeup voltage that takes into account historical information. The electrical power converter system is particularly suited to applications with periodicity such as solar based photovoltaic power generation.

Description:
REFERENCE TO RELATED APPLICATION 
   This application is a Divisional of U.S. application Ser. No. 10/844,060, which was filed on May 12, 2004 which claims priority to U.S. Provisional Application No. 60/470,323, which was filed on May 12, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present systems and methods relate to electrical power converter systems, and particularly to electrical power converter systems suitable for photovoltaic and other periodic applications. 
   2. Description of the Related Art 
   Electrical power converter systems are used to transform and/or condition electrical power in a variety of applications. For example, electrical power converter systems may transform AC power from a power grid to a form suitable for a standalone application (e.g., powering an electric motor, lights, electric heater, household or commercial equipment, telecommunications equipment, computing equipment, uninterruptible power supply (UPS)). Also for example, electrical power converter systems may transform power from a standalone power source such as an array of photovoltaic cells, fuel cell system, micro-turbine, or flywheel, for use in a standalone application and/or for export to, or by, a power grid. 
   The electrical power converter system may comprise one or more subsystems such as a DC/AC inverter, DC/DC converter, and/or AC/DC rectifier. Typically, electrical power converter systems will include additional circuitry and/or programs for controlling the various subsystems, and for performing switching, filtering, noise and transient suppression, and device protection. 
   In many power conversion applications, it is highly desirable to realize the maximum efficiency possible. For example, in photovoltaic applications the cost of photovoltaic arrays is still relatively high, and the physical area occupied by photovoltaic arrays may be undesirably large, particularly where real estate is at a premium. Thus it is desirable to use the least number of photovoltaic cells as possible to achieve the desired power output. 
   Many power conversion applications are periodic in nature. For example, typical photovoltaic applications are strongly influenced by the relative movement of the sun. The time that the sun rises and/or sets, and the relative position of the sun in the sky throughout the day, determines the amount of power that the photovoltaic cells may generate. Further, the relative position of the sun throughout the year determines the amount of power the photovoltaic cells may generate, and determines the time of sunrise and sunset. Similarly, micro-turbine applications such as hydroelectric power generation may experience a similar periodicity, for example, as water flows increase due to rain or melting snow, on a daily and/or seasonal basis. 
   It is desirable to have a method and apparatus for converting power that makes the maximum use of the power generated by a power source, such as a photovoltaic array or micro-turbine. However, coupling the electrical power converter system to the power grid before sufficient power is being generated by the power source will result in the electrical power converter system disadvantageously dissipating some power from the power grid, for example, via a transformer in the electrical power converter system. Thus, it is desirable to have a relatively inexpensive, reliable and electrically efficient method and apparatus for electrical power conversion that switches ON or “wakes” the electrical power converter system as early as possible in the power generation cycle, but not before sufficient power is being generated. The present systems and methods are directed at these issues and provide further related advantages. 
   BRIEF SUMMARY OF THE INVENTION 
   An electrical power converter system adjusts a wakeup voltage periodically, to permit earlier connection and/or operation, to increase performance. The electrical power converter system selects between a mathematically adjusted wakeup voltage based on at least one previous period, and a table derived wakeup voltage that takes into account historical information. The electrical power converter system is particularly suited to applications with periodicity such as solar based photovoltaic power generation. 
   In one aspect, a method of operating an electrical power converter system to transform power for a power source comprises determining an amount of power available from a power source at approximately a startup time; and adjusting a value of a wakeup voltage parameter of the electrical power converter system from a value of the wakeup voltage parameter of the electrical power converter system for a previous period based at least in part on the determined amount of power available from the power source at approximately the startup time. Adjusting a value of a wakeup voltage parameter from that of a previous period may comprise increasing the value of the wakeup voltage parameter if the amount of power at approximately the startup time is less than a startup power threshold. Adjusting a value of a wakeup voltage parameter from that of a previous period may comprise decreasing the value of the wakeup voltage parameter if the amount of power at approximately the startup time is greater than a startup power threshold. The method may further comprise selecting between the value of the wakeup voltage parameter calculated in real time and a value of the wakeup voltage parameter stored in a historically based lookup table, where the value of the wakeup voltage parameter from the historically based lookup table is based at least in part on a real time clock. 
   In another aspect, a method of operating an electrical power converter system to transform power from a power source comprises selecting between a value of a wakeup voltage parameter calculated in real time and a value of the wakeup voltage parameter stored in a historically based lookup table, where the value of the wakeup voltage parameter from the historically based lookup table is based at least in part on a real world time; determining a voltage available from the power source; determining if the determined voltage available from the power source exceeds the selected value of the wakeup voltage parameter; if the determined voltage available from the power source exceeds the selected value of the wakeup voltage parameter, determining a power available from the power source; determining if the power available from the power source is less than a startup power threshold; providing power to a load if the power available from the power source is not less than the startup power threshold; and calculating a value of the wakeup voltage parameter based on whether the power available from the power source is less than or greater than the startup power threshold. 
   In yet another aspect, an electrical power converter system comprises at least a first switch operable between an ON state and an OFF state; and a controller coupled to control the at least first switch, and configured to determine an amount of power available from a power source at approximately a startup time and to adjust a value of a wakeup voltage parameter of the electrical power converter system from a value of the wakeup voltage parameter of the electrical power converter system for a previous period based at least in part on the determined amount of power available from the power source at approximately the startup time. 
   In a further aspect, an electrical power converter system comprises an input bus; an output bus; at least a first switch electrically coupled between the input and the output buses; and a controller coupled to control the at least one switch, the controller configured to select between a value of a wakeup voltage parameter calculated in real time and a value of the wakeup voltage parameter stored in a historically based lookup table, where the value of the wakeup voltage parameter from the historically based lookup table is based at least in part on a real world time. The controller may be configured to select the value of the wakeup voltage parameter stored in the historically based lookup table if both an operating state is a sleep state and a real time is at least equal to a wakeup time. 

   
     BRIEF DESCRIPTION OF THE DRAWING(S) 
     In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. 
       FIG. 1  is a schematic diagram of an environment suitable for practicing the present systems and methods, comprising an electrical power converter system coupling a periodic source such as a PV array to a power grid. 
       FIG. 2  is a block diagram of a controller suitable for controlling the electrical power converter system of  FIG. 1 . 
       FIG. 3  is a flow diagram of a method of operating the electrical power converter system according to one embodiment of the present systems and methods. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following description, certain specific details are set forth in order to provide a thorough understanding of the various embodiments of the present systems and methods. However, one skilled in the art will understand that the present systems and methods may be practiced without these details. In other instances, well-known structures associated with photovoltaic cells, power converters such as AC→DC rectifiers, DC→AC inverters, and/or DC/DC converters, have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the present systems and methods. 
   Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
   The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention. 
     FIG. 1  shows an electrical power converter system  10  coupling a power source, such as a PV array  12 , to a load, such as a power grid  14 . The PV array  12  typically produces DC power in response to light. The amount of power produced by the PV array  12  is a function of the intensity of light received by the PV array  12 . Thus, where the source of light is the sun, the amount of power produced will be periodic, changing throughout the day, and/or throughout the year. 
   The power grid  14  typically carries 3-phase AC power, and is capable of supplying and receiving power. The electrical power converter system  10  inverts the DC power from the PV array  12  into AC power for supply to the power grid  14   
   The electrical power converter system  10  comprises a variety of subsystems and components. For example, a main inverter  16  separates a DC bus  18  from an AC bus  20 , and inverts the DC power into AC power. A bus capacitor C BUS  is electrically coupled across the DC bus  18  in parallel with the PV array  12 . The main inverter  16  may be of a variety of types, for example, one or more bridges configured from a number of transistors such as three half bridges formed from six insulated gate bipolar transistors (IGBT) and associated diodes. As will be apparent to those of skill in the art, the same DC to AC conversion may be accomplished using other numbers of half bridges, which correspond to a phase, and each switching pair may contain any number of switching devices. For simplicity and clarity, the discussion herein assumes a common three phase/three switching pair configuration, although other configurations are of course possible. 
   A grid interface transformer  22  inductively couples the output of the main inverter  16  to the power grid  14 , providing electrical isolation therebetween. Inductors L are electrically coupled on the AC bus  20  between the grid interface transformer  22  and the main inverter  16 . Capacitors C are electrically coupled across the phases of the AC bus  20 . 
   A number of contactors K_ 1  are operable for connecting and disconnecting the electrical power converter system  10  with the power grid  14 . 
   A controller  24  is coupled to receive various signals indicative of system operation and to provide signals to control various elements of the electrical power conversion system  10 . The controller  24  may take the form of a microprocessor, micro-controller or application specific integrated circuit (ASIC), and may be implemented in software, hardware and/or firmware. 
   The controller  24  receives signals indicative of phase currents from a set of phase current sensors  26 . The controller  24  also receives signals indicative of phase voltages V S1  (e.g., voltages for each of the three phases) from a first set of phase voltage sensors  28  on the power grid  14  side of the contactors K_ 1 , and receives signals indicative of phase voltages V S2  (e.g., voltages for each of the three phases) from a second set of phase voltage sensors  30  on the main inverter  16  side of the contactors K_ 1 . The controller  24  also receives signals indicative the current being produced by the photovoltaic array  12 . 
   The controller  24  may further receive signals via a customer interface unit  32 , for example from another electrical power conversion system  34 , customer  36  and/or communications I/O  38 . 
   The controller  24  provides signals to operate the switches (e.g., IGBTs) of the main inverter  16 , for example, via a gate drive controller. The controller  24  also provides signals to operate the contactors K_ 1  for connecting and disconnecting the electrical power converter system  10  with the power grid  14 . 
     FIG. 2  illustrates certain aspects of the controller  24 . An adaptive DC voltage reference generator  51  produces a DC voltage source reference V DC     —     SOURCE1     —     REF . The adaptive DC voltage reference generator  51  computationally determines the DC voltage source reference V DC     —     SOURCE1     —     REF , adjusting the wakeup voltage WK_UP_V from the previous period based on the power produced at the startup of the main inverter  16 . For example, if the most recent wakeup voltage WK_UP_V results in sufficiently more power at the start up, the adaptive DC voltage reference generator  51  will lower the wake up voltage WK_UP_V for the next cycle. This ensures that the main inverter  16  will connect to the power grid  14  earlier in the next period (e.g., the next day) to export more power to the power grid  14 . If, however, the most recent wake up voltage WK_UP_V does not provide sufficient power (e.g., sufficient to maintain the power delivery state or some other threshold), the adaptive DC voltage reference generator  51  increases the wake up voltage WK_UP_V for the next cycle. This approach prevents the main inverter  16  from repeatedly attempting to connect to the power grid  14  when the PV array  12  is not producing sufficient power. This approach also adjusts the wake up voltage WK_UP_V to extend the time of power delivery so that maximum energy is extracted from the PV array  12  and delivered to the power grid  14 . 
   A voltage reference source selector  55  receives clock and/or date information and power to grid status information, and in response produces a clock and/or date signal. 
   A DC voltage reference generator  52  receives clock and/or date information from the voltage reference source selector  55 , and produces a DC voltage source reference V DC     —     SOURCE2     —     REF . The DC voltage reference generator  52  employs a lookup table based on history data, for example, daily times of sun-rise at the particular location of installation of the electrical power converter system  10 . While the DC voltage reference generator  52  may rely on the data from the lookup table for performing one or more calculations, in many cases it will be more efficient to store the end result in the lookup table, for example, the table based wake up voltage values. The lookup table may be based on additional or different historical information. This approach ensures that electrical power converter system  10  will not be unduly hindered by aberrations, such as where the previous day was cloudy, and also avoids unnecessary oscillation in the starting point of the electrical power converter system  10 . 
   In addition to producing the clock/date signal, the voltage reference source selector  55  also operates a switch or switching logic  53  for selectively switching between the DC voltage source references V DC     —     SOURCE1     —     REF , V DC     —     SOURCE2     —     REF  produced by the adaptive DC voltage reference generator  51  and the DC voltage reference generator  52 , respectively, and placed on nodes  57 ,  58 , respectively. The output V DC     —     REF  of the switch or switching logic  53  is summed with V DC  by an adder or summer  56 . The sum is provided to an inverter current reference generator  54 , which produces a current reference I REFERENCE     —     FOR     —     INVERTER  for operating the inverter. 
     FIG. 3  shows a method  60  of operating the electrical power converter system  10 , starting in step  62 . In step  64 , the controller  24  selects a wakeup voltage WK_UP_V employing the switch  53 . In step  66 , the controller  24  determines if a voltage V DC  across the DC bus  18  is greater than the wakeup voltage WK_UP_V, and determines whether the characteristics of the power grid  14  are suitable for exporting power to the power grid  14 . If not, the controller  24  places the main inverter  16  into a sleep mode in step  68 . If the voltage V DC  across the DC bus  18  is greater than the wakeup voltage WK_UP_V, the controller  24  stores the wakeup voltage WK_UP_V in a register WK_UP_V_OLD in step  70 . 
   In step  72 , the controller  24  enables a maximum power point tracking algorithm. 
   Various alternatives for maximum power point tracking are discussed in detail in commonly assigned U.S. provisional patent application Ser. No. 60/467,332, filed May 2, 2003, entitled “NEW MAXIMUM POWER POINT TRACKING TECHNIQUE FOR A GRID-LINK PHOTOVOLTAIC INVERTER”; U.S. provisional patent application Ser. No. 60/470,321, filed May 12, 2003, entitled “METHOD AND APPARATUS FOR TRACKING MAXIMUM POWER POINT FOR A GRID-LINKED PHOTOVOLTAIC INVERTER” U.S. Provisional Patent Application Ser. No. 60/470,323, filed May 12, 2003, entitled “METHOD AND APPARATUS FOR ADJUSTING WAKEUP TIME IN ELECTRICAL POWER CONVERTER SYSTEMS AND TRANSFORMER ISOLATION”; and U.S. patent application Ser. No. 10/836,121, entitled “METHOD AND APPARATUS FOR DETERMINING A MAXIMUM POWER POINT OF PHOTOVOLTAIC CELLS,” and filed Apr. 30, 2004. 
   In step  74 , the controller  24  starts a first timer T 1 . In step  76 , the controller determines whether the first timer T 1  has exceeded a first timer threshold T 1THRESHOLD . If the first timer T 1  has not exceeded the first timer threshold T 1THRESHOLD , the controller  24  executes a wait loop, returning to step  76 . If the first timer T 1  has exceeded the first timer threshold T 1THRESHOLD , the controller  24  passes control to step  78 . 
   In step  78 , the controller  24  sets a variable startup power STARTUP_PWR equal to the power PV_PWR from the photovoltaic array  12 . In step  80 , the controller  24  determines whether the power PV_PWR from the photovoltaic array  12  is less than a defined power threshold P THRESHOLD . If the power PV_PWR from the photovoltaic array  12  is not less than the defined power threshold P THRESHOLD , the controller  24  resets a second timer T.sub. 2  in step  81  and provides signals that causes the inverter to deliver power in step  83 . 
   If the power PV_PWR from the photovoltaic array  14  is less than the defined power threshold P THRESHOLD , the controller  24  starts the second timer T 2  in step  82 . In step  84 , the controller  24  determines whether the second timer T 2  has exceeded a second timer threshold T 2THRESHOLD . If the second timer T 2  has not exceeded a second timer threshold T 2THRESHOLD , the controller  24  provides signals that cause the inverter to deliver power in step  83 . If the second timer T 2  has exceeded a second timer threshold T 2THRESHOLD , the controller  24  calls or executes a function to produce a new wakeup voltage WK_UP_V_NEW in step  86 . In step  88 , the new wakeup voltage WK_UP_V_NEW is supplied to the one node  57  of the switch  53 , as indicated by the broken line. The controller  24  then causes the main inverter  16  to enter the sleep mode in step  68 . 
   In step  90 , a real time clock produces a signal indicative of the real time (i.e., in contrast to a system clock or bus clock). In step  92 , the controller  24  employs a lookup table to provide a table based wakeup voltage WK_UP_VOLT based on the real time. The table based wakeup voltage value WK_UP_VOLT is supplied to other node  58  of the switch  53 . The step of providing the real time may be provided in parallel with one or more of the other steps. 
   In step  96 , the controller  24  determines 1) whether the main inverter  16  is in a sleep state (i.e., logical EQUAL); and (i.e., logical AND) 2) whether the time is equal to the wakeup time (i.e., logical EQUAL). If the answer to both questions is TRUE, the converter  24  sets the switch  53  to node  58  to select the table derived wakeup voltage WK_UP_VOLT. If the answer to either or both questions is FALSE, the controller  24  sets the switch  53  to node  57 , to select the mathematically calculated wakeup voltage WAKE_UP_V_NEW. 
   Although specific embodiments of, and examples for, the power system and associated methods are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the invention, as will be recognized by those skilled in the relevant art. The teachings provided herein of the present systems and methods can be applied to other power systems, not necessarily the exemplary power systems and methods generally described above. For example, the present method and apparatus may be employed in electrical power converter systems  10  having power sources other than PV arrays  12 . For example, the method and apparatus may be employed in micro-turbine applications where appropriate (e.g., hydro-electrical applications), with or without modifications. For example, in micro-turbine applications, the electrical power converter system  10  may employ a rectifier to transform an AC output from the micro-turbine to DC power, a DC/DC converter to condition and adjust the voltage of the DC power, and an inverter to transform the DC power to AC for export to the power grid  14 . The electrical power converter system  10  may include additional elements such as a DC/DC converter. Also for example, the methods discussed herein can include additional steps, eliminate some steps, and/or perform steps in a different order. 
   The various embodiments described above can be combined to provide further embodiments. All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in the this specification and/or listed in the Application Data Sheet, including but not limited to U.S. provisional patent application Ser. No. 60/467,332, filed May 2, 2003, entitled “NEW MAXIMUM POWER POINT TRACKING TECHNIQUE FOR A GRID-LINK PHOTOVOLTAIC INVERTER”; U.S. provisional patent application Ser. No. 60/470,321, filed May 12, 2003, entitled “METHOD AND APPARATUS FOR TRACKING MAXIMUM POWER POINT FOR A GRID-LINKED PHOTOVOLTAIC INVERTER”; U.S. Provisional Patent Application Ser. No. 60/470,323, filed May 12, 2003, entitled “METHOD AND APPARATUS FOR ADJUSTING WAKEUP TIME IN ELECTRICAL POWER CONVERTER SYSTEMS AND TRANSFORMER ISOLATION”; and U.S. patent application Ser. No. 10/836,121, entitled “METHOD AND APPARATUS FOR DETERMINING A MAXIMUM POWER POINT OF PHOTOVOLTAIC CELLS,” and filed Apr. 30, 2004, are incorporated herein by reference in their entirety. Aspects of the present systems and methods can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments of the present systems and methods. 
   These and other changes can be made to the present systems and methods in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all power systems and methods that operate in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.