Abstract:
An apparatus for power conversion includes an inverter; a converter configurable to function as a DC voltage booster; and a controller for selectively causing the converter to provide a boosted DC voltage to the inverter.

Description:
FIELD OF DISCLOSURE 
       [0001]    This disclosure relates to electric power conditioning, and in particular, to converting DC into AC. 
       BACKGROUND 
       [0002]    Many modes of generating or storing electricity involve generation and storage of a DC voltage. For example, voltages maintained across an energy storage element, such as a battery or capacitor, and voltage developed across a fuel cell of solar cell, are all typically DC voltages. 
         [0003]    Electric power utilities typically require AC voltages, not DC voltages. Accordingly, it is common to provide a source of DC voltage with a device, such as an inverter, for converting DC to AC. 
         [0004]    A typical inverter uses an input DC voltage level to generate AC having a specified amplitude with a peak not exceeding the input DC voltage level. Thus, an inverter provided with a high DC voltage can generate an AC output waveform having a high amplitude. Conversely, an inverter provided with only a low DC voltage level will only be able to generate a low AC voltage output. Such an inverter would no longer be able to generate the high AC voltage output that it could when it was receiving a higher DC voltage as an input. Instead, it would output a “clipped” AC waveform. 
         [0005]    Most electric power utilities require, from a power generating source, an AC voltage having a particular amplitude. In some cases, a DC voltage source cannot develop a DC voltage sufficient to provide an AC voltage having the requisite amplitude. For example, in the case of a solar cell, this may occur at dusk or dawn, or when passing clouds obscure the sun. In the case of an energy storage device, this might occur when the stored charge is close to exhausted. 
         [0006]    A DC source that fails to develop sufficient voltage to satisfy the requirements of a utility grid is nevertheless still generating power. However, this power is essentially wasted. 
       SUMMARY 
       [0007]    In one aspect, the invention features an apparatus for power conversion. Such an apparatus includes an inverter; a converter configurable to function as a DC voltage booster; and a controller for selectively causing the converter to provide a boosted DC voltage to the inverter. 
         [0008]    In some embodiments, the controller is configured to cause the converter to transition from a first state, in which the converter converts DC into AC, to a second state, in which the converter converts DC into boosted DC. Among these embodiments are those that also include a set of contactors, the set having a first subset of contactors and a second subset of contactors, the second subset of contactors being the complementary subset of the first subset of contactors, wherein the controller is configured to transition between the first state and the second state by causing a change in state of all contactors in the first subset and causing a change in state in all contactors of the second subset. Also among these embodiments are those in which the controller is configured to transition between the first state and the second state by opening all contactors in the first subset and closing all contactors in the second subset. 
         [0009]    In other embodiments, the apparatus also includes a common bus connecting a DC terminal of the inverter to a DC terminal of the converter. Among these embodiments are those that further include a first contactor for selectively connecting the common bus to a DC source, and those in which the controller is configured to close the first contactor, thereby enabling DC voltage to be provided as DC inputs to the inverter and the converter, and to open the first contactor, thereby disconnecting the inverter from the DC source. 
         [0010]    In yet other embodiments, the apparatus further includes a voltage sensor in communication with the controller for determining a DC voltage, with the controller being configured to selectively cause the converter to provided the boosted DC voltage to the inverter upon determining that a DC voltage has crossed a threshold. 
         [0011]    In another aspect, the invention features an apparatus for causing AC having a specified amplitude to be generated from DC having a variable voltage level. Such an apparatus includes means for determining whether a first DC voltage level is sufficient to generate the AC having the specified amplitude; and means for selectively boosting the first voltage level to a second DC voltage level in response to a determination, from the means for determining, that the first voltage level is inadequate for generating the AC having the specified amplitude. 
         [0012]    In some embodiments, the means for determining includes a controller in communication with a sensor for measuring, or determining, a voltage level. 
         [0013]    Other embodiments include those in which the means for selectively boosting includes a controller configured to control an inverter, and those in which the means for selectively boosting includes a plurality of contactors, with the plurality having first and second configurations, in which the second configuration causes a DC voltage to be boosted. 
         [0014]    In another aspect, the invention features a method for generating AC having a specified amplitude from DC having a variable voltage level. Such a method includes determining that a DC voltage provided by a DC source has a DC voltage level that is inadequate to generate the AC; boosting the DC voltage; providing the boosted DC voltage to an inverter for conversion into the AC; determining that the DC voltage level provided by the DC source has become adequate to generate the AC; and providing the DC voltage from the DC source to the inverter for conversion into AC. 
         [0015]    Among the practices of the foregoing method are those in which providing the boosted DC voltage to an inverter includes disconnecting the inverter from the DC source, and those in which providing the boosted DC voltage level to an inverter includes disconnecting an inverter from an AC output, 
         [0016]    Also among the practices of the foregoing method are those in which boosting the DC voltage level includes causing a converter to switch from generating an AC voltage from a DC voltage to generating a first DC voltage from a second DC voltage, those in which boosting the DC voltage level includes dynamically reconfiguring a connection between the inverter and the DC voltage, and those in which boosting the DC voltage level includes carrying out double-conversion of the DC voltage, and wherein providing the DC voltage from the DC source to the inverter includes carrying out single-conversion of the DC voltage. 
         [0017]    These and other features of the invention will be apparent from the following detailed description and the accompanying figures, in which: 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0018]      FIG. 1  shows a DC-AC inverter system; 
           [0019]      FIG. 2  shows the DC-AC inverter system of  FIG. 1  configured for operation when insufficient voltage has been developed by the DC power source; and 
           [0020]      FIG. 3  shows the DC-AC inverter system of  FIG. 1  configured for operation when sufficient voltage is developed by the DC power source. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring to  FIG. 1 , a DC-AC conversion system  10  converts a DC voltage from a DC source  28  into an AC voltage for an AC load  38 . Examples of a DC source  28  include a solar array, a fuel cell, a battery, and a capacitor. An example of an AC load  38  is an electric power grid. 
         [0022]    One embodiment of a DC-AC conversion system  10  includes an inverter  12  and a converter  14 , both of which can convert DC into AC. As is well known, an inverter is a species of power converter. Many power converters can be configured to convert DC into AC, as well as many other power conversion functions. Thus, the inverter  12  can, in some embodiments, be implemented by a multi-function power converter that is configured to operate as an inverter, i.e. to convert DC into AC. However, in some embodiments, the inverter  12  is implemented by a device that can only convert DC into AC. 
         [0023]    The converter  14  is implemented by a multifunctional unit that can also convert DC at a first voltage into DC at a second voltage, with the second voltage being greater than the first voltage. Thus, the inverter  12  has a DC terminal  16  and at least an AC terminal  18 , whereas the converter  14  has a DC terminal  20  and an AC/DC terminal  22 . A common bus  24  connects the DC terminal  16  of the inverter  12  and the DC terminal  20  of the converter  14 . 
         [0024]    An input terminal  26  of the DC-AC conversion system  10  provides a connection between the common bus  24  and a DC voltage source  28  by way of a first contactor  30  that can be selectively opened and closed by a controller  32 . 
         [0025]    An output terminal  30  of the conversion system  10  directly connects to the AC terminal  18  of the inverter  12 . The output terminal  30  of the conversion system  10  also connects to the AC/DC terminal  22  of the converter  14 , but via a second contactor  34  that is selectively opened and closed by the controller  32 . Finally, the AC/DC terminal  22  of the converter  14  also connects to the input terminal  26  of the DC-AC conversion system  10  by way of a third contactor  36 . Like the first and second contactors  30 ,  34 , the third contactor  36  can also be selectively opened and closed by the controller  32 . 
         [0026]    When the DC voltage is inadequate to support an AC waveform having the required amplitude, the controller  32  causes the DC-AC conversion system  10  to operate in “double-conversion mode,” as shown in  FIG. 2 . To do so, the controller  32  closes the third contactor  36  but leaves the first and second contactors  30 ,  34  open. In addition, the controller  32  configures the converter  14  to boost an input DC voltage. 
         [0027]    In double-conversion mode, the DC source  28  provides a DC voltage to the AC/DC terminal  22  of the converter  14 . The converter  14 , having been programmed to do so by the controller  32 , boosts this DC voltage and provides it to the DC terminal  16  of the inverter  12 . The inverter  12  then uses this boosted DC voltage to generate an output AC voltage having the required amplitude. 
         [0028]    When the DC voltage is adequate to support an AC waveform having the required amplitude, the controller  32  causes the DC/AC conversion system  10  to operate in “single-conversion mode,” as shown in  FIG. 3 . In single-conversion mode, the controller  32  closes the first and second contactors  30 ,  34  but leaves the third contactor  36  open. In addition, the controller  32  configures the converter  14  to generate AC from DC. 
         [0029]    The DC boosting process carried out by the converter  14  in double-conversion mode is inherently an inefficient one. By adaptively switching between the two conversion modes, the system  10  avoids having to carry out the inefficient DC boosting process except when rendered necessary by the unavailability of adequate DC voltage for generating an AC voltage waveform having the required amplitude. 
         [0030]    The inverter  12  and the converter  14  are rated to have a particular power-handling capacity. Typically, the rating of a single inverter is inadequate to handle the power generated by the DC source when operating at or near full capacity. For this reason, a DC-AC conversion system  10  would ordinarily have two or more inverters that cooperate to generate the required AC voltage. Accordingly, the DC-AC converter system  10  would require an inverter  12  and a converter  14  anyway just to handle the power generated by the DC source, as well as a controller  32  to control the inverter  12  and the converter  14 . Thus, other than three extra contactors, no additional hardware is required to implement the double-conversion mode. Instead, the second converter is simply used for a different function. 
         [0031]    The particular topology of the embodiment described herein offers particular ease of implementation because switching from one mode to another amounts to switching the state of each contactor  30 ,  34 ,  36 . The set of contactors  30 ,  34 ,  36  defines two subsets: a first subset containing only the third contactor  36  and a second subset containing only the first and second contactors  30 ,  34 . As such, the second subset is a complementary subset of the first subset since the union of the first and second subsets defines the original set. Each contactor  30 ,  34 ,  36  is in one of two states: open or closed. Transition between states, at least in the illustrated embodiment, thus amounts to changing the state of each contactor from its current state to the opposite of its current state. This is particularly easy to implement on a controller  32  since it amounts to implementing a logical “NOT” operator on a register containing one bit for each contactor, with the state of the bit corresponding to the state of the contactor. 
         [0032]    In some embodiments, a voltage sensor  40  in communication with the controller  32  determines whether the voltage provided by the DC source has fallen below a critical value. Based on a measurement provided by this sensor  40 , the controller  32  automatically reconfigures the contactors  30 ,  34 ,  36  and inverters  12 ,  14  to operate in either single-conversion mode or double-conversion mode. 
         [0033]    For example, in one embodiment, upon detecting that the voltage provided by the DC source has risen past the critical value, the controller  32  automatically reconfigures the contactors  30 ,  34 ,  36  and inverters  12 ,  14  to operate in single-conversion mode. Conversely, upon detecting that the voltage provided by the DC source has fallen below the critical value, the controller  32  automatically reconfigures the contactors  30 ,  34 ,  36  and inverters  12 ,  14  to operate in double-conversion mode. In either case, the transition occurs seamlessly and without human intervention. 
         [0034]    The DC-AC conversion system  10  as described herein greatly extends the range over which a DC source can operate. For example, using the DC-AC conversion system  10  enables a solar array to continue providing power to a utility grid closer to dawn or dusk, during when it would normally no longer be providing such power.