Patent Publication Number: US-11641110-B2

Title: Apparatus and method for reactive power control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/001,534, filed Jan. 20, 2016, which claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/105,886, entitled “Power Converter Having Reactive Power Control” and filed Jan. 21, 2015, the entire contents of each of these applications is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Embodiments of the present invention generally relate to power conversion and, more popularly, to a power converter having reactive power control. 
     Description of the Related Art 
     Alternative power systems such as solar, wind, and the like generally produce DC power that is converted to AC power for injection onto the AC power grid. Conversion from DC power to AC power must be performed very efficiently to enable these alternative power systems to be commercially viable. One form of highly efficient power converter uses a cycloconverter. A cycloconverter converts a constant voltage, constant frequency AC waveform to another AC waveform of a lower frequency by synthesizing the output waveform from segments of the AC supply without an intermediate DC link. To facilitate DC to AC conversion, a DC full or half bridge circuit is coupled between a DC power source and the cycloconverter. The combination of the DC bridge and the cycloconverter provides a highly efficient DC to AC power converter (also referred to as an inverter). Cycloconverters are available in single phase and three-phase configurations. For purposes of this description, a switched mode cycloconverter switches a cycloconverter at a frequency that is higher than the frequency of the AC grid. 
     Switched mode three-phase cycloconverters were described in literature as early as 1985. Improvements that increased efficiency include enhanced control requirements to achieve zero volt switching (ZVS) operation and a simplification that reduced the number of switches used in the cycloconverter by adopting a half-bridge configuration. A further advance used a half-bridge cycloconverter that included a series-resonant circuit employing a variable frequency control, where a transformer center tap was used in conjunction with an LLC series-resonant circuit relying upon a gapped transformer core to facilitate efficient cycloconversion. 
     These advances in switched mode cycloconverter circuitry made available highly efficient power converters for use with alternative power systems. The widespread use of alternative power systems has raised concern with traditional power generation companies regarding reactive power control for the AC power grid. 
     Regulations and standards (e.g., IEC 1000-3-2) have been adopted to ensure that circuitry coupled to the power grid utilizes power factor correction techniques to ensure that the power factor at the connection to the power grid is unity. This regulation applies to power converters as well as power loads. For purposes of this description, power factor correction (PFC) is a technique used to provide harmonic correction of nonlinear loads that ensures that the power converter couples energy to the grid having the sinusoidal current in phase with the sinusoidal voltage of the AC grid. A power factor of unity is used even if the power factor of the power grid is not unity. 
     Power consumers coupled to the power grid can cause reactive power to be present on the power grid. As such, the grid power factor is no longer unity. In some instances the power generation companies require power consumers (e.g., large industrial power consumers) to perform reactive power control to reduce the amount of reactive power on the power grid, i.e., the large consumers are asked to absorb the reactive power. The power generation utility also compensates for reactive power on the grid in an attempt to maintain a power factor of unity. 
     Power converters used in alternative energy systems have not been designed to facilitate reactive power control; these power converters are designed for power factor control to ensure the energy that they are producing has a power factor of unity. As alternative energy systems become larger and larger, in some areas, the power they generate by alternative generator sources may dominate the power on the power grid without any reactive power control. 
     Therefore, there is a need in the art for power converters used in alternative energy systems to facilitate reactive power control. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention generally relate to a method and apparatus for controlling reactive power substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims. 
     Various advantages, aspects and novel features of the present disclosure, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which embodiments of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG.  1    is a block diagram of a power generation system in accordance with embodiments of the present invention; 
         FIG.  2    is a set of graphs representing magnitude versus time of: voltage/current of a single phase of the AC power being created by a power converter without any reactive power control (i.e., power factor of unity), a power representation of the voltage/current, and energy flow within a power converter having no reactive power control; 
         FIG.  3    is a set of graphs representing magnitude versus time of: voltage/current of a single phase of the AC power being created by the power converter having reactive power control (i.e., power factor that is not unity); and energy flow within a power converter being used to create reactive power; 
         FIG.  4    is a phasor representation of AC power; 
         FIG.  5    is a block diagram of a bidirectional power converter in accordance with an embodiment of the present invention; 
         FIG.  6    is block diagram of a controller for the bidirectional power converter of  FIG.  5   ; 
         FIG.  7    is a schematic diagram of one embodiment of a single phase, inverter that can be used within the bidirectional power converter of  FIG.  5   ; 
         FIG.  8    is a schematic diagram of one embodiment of a three-phase inverter that can be used within the bidirectional power converter of  FIG.  5   ; 
         FIG.  9    is a block diagram of an alternative power system utilizing the bidirectional power converter of the present invention; 
         FIG.  10    is a block diagram of a second type of alternative power system utilizing the bidirectional power converter of the present invention; 
         FIG.  11    is a block diagram of a third type of alternative power system utilizing the bidirectional power converter of the present invention; 
         FIG.  12    is a schematic diagram of one embodiment of a three-phase VAr compensator that uses a switched mode cycloconverter; and 
         FIG.  13    is a flow diagram of a method for controlling reactive power in accordance with one or more embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention include a power converter having reactive power control. More specifically, embodiments of the invention include a bidirectional power converter having the capability of flowing power into and out of a storage element within the power converter. The power converter comprises a DC-side bridge coupled via a resonant tank and transformer to a switch mode cycloconverter coupled to a controller. The controller is adapted to receive a power utility defined reactive power control schedule that is implemented by the power converter. As such, the bidirectional power converter may create reactive power at times and with a magnitude defined in this schedule. 
       FIG.  1    is a block diagram of a power generation system  100  in accordance with embodiments of the present invention. The power generation system  100  comprises a power generator  102  coupled to a bidirectional power converter  104 . The power generator  102  (or power source) may be any form of DC power generator including, but not limited to, a wind turbine, solar panel or panels, a battery or batteries, and the like. Power generator  102  provides DC power to the bidirectional power converter  104 . The bidirectional power converter  104  produces AC power that is coupled to an AC grid  106 . To facilitate reactive power control (RPC), a power utility RPC schedule is provided to the bidirectional power converter  104 . The utility RPC schedule, in one simple form, may comprise a list of reactive power amounts and the time of day at which the reactive power is to be supplied to the AC grid  106 . Typically, the local power generation company or utility that manages the AC grid  106  provides the RPC schedule. However, in other embodiments, an RPC schedule may contain a schedule of reactive power as a function of: AC grid voltage (Mains Voltage), inverter output power, change in AC grid voltage, fixed value, and the like. See  Common Functions for Smart Inverters , Version 3, EPRI, Palo Alto, Calif.: 2013. 3002002233. 
       FIG.  2    is a set of graphs  200 ,  202 ,  204  respectively representing magnitude versus time of: voltage/current of a single phase of AC power being created by a power converter without any reactive power (i.e., power factor of unity), a power representation of the voltage/current, and energy flow within a power converter having no reactive power control. More specifically, graph  200  depicts the magnitude of both voltage  206  and current  208  created by a power converter having no reactive power control, where the AC power has a power factor of unity (no reactive power). Note that the voltage  206  and the current  208  are phase synchronized. Typically, output of the power converter is synchronized to the voltage of the AC grid to which the power converter is supplying the power. With power factor of unity, the power graph  202  depicts the power fluctuating between 0 and a positive magnitude at a frequency that is twice the frequency of the AC grid voltage. 
     Because the input from the power generator  102  is a constant DC power and the output power from the bidirectional power converter  104  is a pulsatile AC power during DC to AC conversion, the bidirectional power converter  104  must buffer the input power to create the oscillating AC output power. Typically, this energy buffering is accomplished using a storage device such as a capacitor within the bidirectional power converter. Graph  204  depicts the energy flow into and out of the power converter&#39;s storage device (line  212 ) to provide the necessary energy buffering. The average power delivered by the DC source  102  is represented by dashed line  218 . The energy  214  above the average power line  218  represents energy being released from the storage device and the energy  216  below the average power line  218  represents energy being stored in the storage device. When no reactive power is needed, the energy storage and release is synchronized at twice the frequency of the voltage of the AC grid, i.e., synchronous with the power of the AC grid. 
       FIG.  3    is a set of graphs  300 ,  302  respectively representing magnitude versus time of: voltage/current of a single phase of the AC power being created by the power converter  104  when employing reactive power control (i.e., power factor that is not unity); and energy flow within the power converter  104  being used to create reactive power. More specifically, the graph  300  depicts the magnitude of both voltage  304  and current  306  created by the power converter  104  when employing reactive power control. Note that the voltage  304  and the current  306  are not phase synchronized. The phase offset represents the amount of reactive power being created by the power converter  104 . Reactive power is 90° out of phase with the AC grid voltage and the reactance may be lagging or leading. The ability to create reactive power requires a bidirectional power converter because power must be able to flow to the AC grid  106  as well as from the AC grid  106 . 
     The graph  302  depicts the energy flow (i.e., line  308 ) into and out of the storage device within the bidirectional power converter  104 . Note that a portion of the curve at  314  is below zero magnitude level indicating that energy must flow from the grid  106  during this period. For magnitudes above zero, energy is flowing into the grid  106 . As such, the production of reactive power is only possible with a bidirectional power converter. 
       FIG.  4    is a phasor representation  400  of AC power. Phasor  402  represents real power of which the current is in phase with the AC grid voltage. Phasor  404  represents imaginary power (units of VAr—Volt-Amps reactive) of which the current is quadrature to the AC grid voltage. Phasor  406  represents the vector addition that results if the real and imaginary power vectors  402  and  404  are combined to obtain a reactive resultant phasor  406 . This reactive phasor  406  represents the reactive power being generated and this will be specified in units of VA (Volt-Amps). The units VA are used for reactive loads and the way of calculating VA is to simply multiply the magnitude of the voltage by the magnitude of the current in amps—ignoring the fact that voltage and current are not in phase. The reactive load (in units of VA) has two components—a real power component (in units of VV) and an imaginary power component (in units of VAr). This relationship can be expressed as a vector identity: VA=W+VAr (where all three entities represent vector components). The imaginary power component is also referred to herein as a reactive power component. 
       FIG.  5    is a block diagram of a bidirectional power converter  104  in accordance with an embodiment of the present invention that is capable of creating reactive power. The bidirectional power converter  104  comprises a controller  500  and an inverter  502 . Embodiments of the inverter  502  are described in detail with respect to  FIGS.  7  and  8    below. The controller  500  comprises a first sampler  504 , a second sampler  508 , a current sampler  526 , a maximum power point tracking (MPPT) controller  506 , a phase lock loop (PLL)  512 , a cosine table  516 , a sine table  518 , a first multiplier  520 , a second multiplier  522 , a reactive power controller (RPC)  514  and a summer  524 . In some other embodiments, one or more of the first sampler  504 , the second sampler  508 , and the current sampler  526  may be components external to the controller  500 . 
     The power controller  500  receives as input: DC power of the DC source (e.g., the first sampler  504  samples values of DC voltage and DC current), AC voltage sample of the AC grid voltage (e.g., the second sampler  508  samples values of the AC voltage), a utility RPC schedule  510 , and the time of day (e.g., from a “real time clock” function residing within the controller  500 ). In one embodiment, the time of day is used in conjunction with the utility RPC schedule  510  to define the time during the day when particular values of reactive power need to be created and applied to the AC grid. In other embodiments, the RPC schedule  510  may contain a schedule of reactive power as a function of: AC grid voltage (mains voltage), inverter output power, change in AC grid voltage, fixed value, and the like. 
     In operation, the second sampler  508  creates a digital signal representative of an instantaneous voltage of the AC grid voltage. The digital representation is coupled to the PLL  512  as well as the RPC  514 . The PLL  512  generates a phase counter signal that indexes two look up tables  516  and  518 —the table that is in phase with the AC grid voltage is referred to as the sine table  518  and the table that is quadrature to the AC grid voltage is referred to as the cosine table  516  (other conventions could be used). The outputs from the sine and cosine tables  518  and  516  represent normalized (by definition to unity—i.e., max value for sine=1) representations of the AC grid voltage (for sine) and the quadrature of the AC grid voltage (for cosine). 
     The MPPT controller  506  generates a signal Dreq and couples the generated Dreq signal to the first multiplier  520 , while the RPC  514  generates a signal Qreq and couples the generated Qreq signal to the second multiplier  522 . The two signals Dreq and Qreq represent the requested real and imaginary currents, respectively, that are to be delivered to the AC grid. Dreq is the direct current request—i.e., the real current component and is supplied from the MPPT controller  506 . In order to determine the signal Dreq, the MPPT controller  506  receives a representation of the DC source voltage from the sampler  504  and receives a representation of the current from the DC source from a current sampler  526  coupled between the DC source  102  and the MPPT controller  506 . The MPPT controller  506  operates in a well-known manner known to those skilled in the art to derive a value Dreq for the desired real output current, while maintaining the DC source  102  operating at a maximum power point. The signal Qreq is the quadrature current request—i.e., the imaginary current request and is supplied from an algorithm that is, for example, ultimately specified by the power utility company. Typically, this algorithm, performed by the reactive power controller  514 , would adjust the requested imaginary current as a function of the AC grid voltage as this will help regulate the voltage on the AC grid. 
     The signals Dreq and Qreq scale the outputs from the sine and cosine tables  518  and  516  respectively using a multiplication operation (x)—i.e., multipliers  520  and  522  respectively. The results of these two multiplications are summed together with an addition operation (+) (summer  524 ) and the output becomes the Ireq signal, where Ireq is a vector representation of the desired current to be supplied to the AC grid. Ireq has a polarity that mirrors the polarity of the AC current flowing out of the power converter  104 . When the instantaneous AC voltage and current are of the same polarity, the condition is referred to as forward power flow and, when the instantaneous AC voltage and current are of opposite polarity, the condition is referred to as reverse power flow. There are different conventions for assigning polarity to the AC voltage, AC current, and power flow direction and defining polarity differently will result in a different but equally valid alternate convention. 
       FIG.  6    is a block diagram of one embodiment of a controller  600  for the embodiment of a bidirectional power converter of  FIG.  5    (the controller  600  is one embodiment of the controller  500 ). The control functions defined above with respect  FIG.  5    can be implemented in hardware, software, or a combination of hardware and software. Inputs to the controller  600  include a digital representation of the DC source voltage, a digital representation of the DC source current and a digital representation of the AC grid voltage. These signals are created using hardware to sample and digitize the voltages and current, e.g., an analog to digital (A/D) converter (not shown), (although in other embodiments the controller  600  may comprise one or more modules or components for sampling DC voltage, DC current, and/or AC voltage and generating digital representations thereof). 
     The controller  600  comprises a central processing unit (CPU)  602  coupled to each of support circuits  604  and memory  606 ; in some embodiments, the CPU  602  may further be coupled to a transceiver for communication to and from the power converter  102  (e.g., using power line communications). The CPU  602  may be any commercially available processor, microprocessor or microcontroller, or combinations thereof, configured to execute non-transient software instructions to perform various tasks such as those described herein. In some embodiments, the CPU  602  may be a microcontroller comprising internal memory for storing controller firmware that, when executed, provides the controller functionality. The controller  600  may be implemented using a general purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present invention. 
     The support circuit  604  may include, but are not limited to, such circuits as power supplies, cache memory, clock circuits, and the like. The memory  606  may include read-only memory and/or random access memory that stores data and software instructions to be utilized by the CPU  602 . 
     The memory  606  stores an operating system (OS)  620  (when needed) of the controller  600 , where the OS  620  may be one of a number of commercially available operating systems such as, but not limited to, Linux, Real-Time Operating System (RTOS), and the like. The memory  606  stores non-transient processor-executable instructions and/or data that may be executed by and/or used by the CPU  602 . These processor-executable instructions may comprise firmware, software, and the like, or some combination thereof. In the embodiment described with respect to  FIG.  6   , the memory  606  comprises a sine table  608 , a cosine table  610 , an RPC schedule  612 , and inverter control software  614 . The inverter control software includes MPPT control software  616  and RPC control software  618 . When executed by the CPU  602 , the MPPT control software  616  functions as the MPPT controller  506  and the RPC control software  618  functions as the RPC controller  514 . The controller  600  may also be implemented as an application specific integrated circuit (ASIC) that is specifically programmed to perform the operations described herein (e.g., with respect to  FIG.  5   ). 
       FIG.  7    is a schematic diagram of one embodiment of a single phase inverter  700  that can be used within the bidirectional power converter  104  of  FIG.  5    (i.e., the inverter  700  is one embodiment of the inverter  502 ). The inverter  700  comprises a storage device  702  (e.g., a capacitor), a DC full bridge  704 , a resonant circuit  706 , and isolation transformer  708 , and a switched mode cycloconverter  712 . As described above, the storage device  702 , coupled across the input to the inverter  700 , stores energy to facilitate both creation of AC power from DC power as well as facilitating creating reactive power. The bridge  704  converts DC power into a high-frequency AC signal. The DC bridge  704  is coupled to the resonant circuit  706  that has a resonance that is commensurate with the high-frequency AC signal. 
     The AC signal is coupled to the cycloconverter  712  via the isolation transformer  708 . The cycloconverter  712  converts the high-frequency AC signal into a signal having a power profile commensurate with the AC power on the AC grid. The controller  500  of  FIG.  5    or the controller  600  of  FIG.  6    is used to control the timing of the switches within the bridge  704  and the cycloconverter  712  to achieve single-phase AC power containing reactive power. 
     To reiterate, the storage device  702  operates to buffer energy during the power conversion process. In addition, when the bidirectional power converter  104  must flow power from the grid  106  to facilitate reactive power generation, the storage device  702  stores the necessary energy. 
     Generally, a cycloconverter converts an AC signal of a particular voltage/current, frequency, and phase order directly to a different voltage/current and/or frequency and/or phase order without the use of an intermediate DC bus or DC energy storage. Although a single-phase cycloconverter is depicted in  FIG.  7   , a cycloconverter can have a single-phase, three-phase, or any general polyphase input. Likewise, a cycloconverter can have a single-phase, three-phase or any general polyphase output. Accordingly, cycloconverters can be used to convert from a polyphase system of any order (n=1,2,3,4 . . . ) to any other polyphase system order (n=1,2,3,4 . . . ). Cycloconverters rely on bidirectional switches. These switches are sometimes referred to as four-quadrant switches as they can handle voltage and current of any polarity (i.e., the four quadrants (++, +−, −+, −−). Four-quadrant or bidirectional switches can be made by connecting two unidirectional switches in series such that the two switches are orientated such that they conduct the same current in opposite directions. Alternatively, bidirectional switches can be facilitated using a bridge rectifier and a single unidirectional switch. 
       FIG.  8    is a schematic diagram of an embodiment of a three-phase inverter  800  that can be used within the bidirectional power converter  104  of  FIG.  5    (i.e., the inverter  800  is one embodiment of the inverter  502 ). The inverter  800  comprises a storage device  802  (e.g., a capacitor), a DC full bridge  804 , a resonant circuit  808 , an isolation transformer  806 , and a switched mode cycloconverter  810 . As described above, the storage device  802  stores energy to facilitate both creation of AC power from DC power as well as facilitating creating reactive power. The DC bridge  804  converts DC power into a high-frequency AC signal. The DC bridge  804  is coupled to the isolation transformer  806  via a resonant circuit  808  that has a resonance that is commensurate with the high-frequency AC signal. The AC signal is coupled to the cycloconverter  810 . The cycloconverter  810  converts the high-frequency AC signal into a signal having a power profile commensurate with the three-phase AC power on the AC grid. The controller  500  of  FIG.  5    or the controller  600  of  FIG.  6    is used to control the timing of the switches within the bridge  804  and the cycloconverter  810  to achieve three-phase AC power containing reactive power. 
     Further information can be found on cycloconverter operation in commonly assigned U.S. application publication number 2012/0170341, published on Jul. 5, 2012 having a title of “Method and Apparatus for Resonant Power Conversion” and herein incorporated by reference in its entirety. 
       FIG.  9    is a block diagram of one embodiment of an alternative power system  900  utilizing the bidirectional power converter  104  of the present invention to produce AC power from DC power, where the AC power includes reactive power. The system  900  comprises a plurality of power sources (PS)  902   1 ,  902   2  . . .  902   n , collectively referred to as power sources  902 , coupled to the bidirectional power converter  104 . The string of power sources  902  combines to provide DC power to the bidirectional power converter  104  for conversion to AC power having reactive power as described herein. The power sources  902  may be arranged in a large array coupled to a single bidirectional power converter  104  (as depicted in  FIG.  9   ) or alternatively to a small number of bidirectional power converters  104 . 
       FIG.  10    is a block diagram of one embodiment of a second type of alternative power system  1000  utilizing the bidirectional power converter  104  of the present invention to create AC power with reactive power. The system  1000  comprises a plurality of power sources (PS)  1002   1 ,  1002   2  . . .  1002   n , collectively referred to as power sources  1002 , each coupled to an associated bidirectional power converter  104   1 ,  104   2  . . .  104   n , collectively referred to as power converters  104 . Each power source  1002  provides its DC power output to a corresponding bidirectional power converter  104  for conversion to AC power having reactive power as described herein. The output AC power from the power converters  104  is coupled to a bus  1006 . The power sources  1002  and their associated bidirectional power converters  104  may be arranged in a large array. The AC output power is coupled to the AC power grid  106 . 
       FIG.  11    is a block diagram of one embodiment of a third type of alternative power system  1100  utilizing the bidirectional power converter  104  of the present invention to produce AC power with reactive power. The system  1100  comprises a plurality of power sources (PS)  1102   1 ,  1102   2  . . .  1102   n , collectively referred to as power sources  1102 , coupled to a DC power aggregator  1104  such that the DC power from the power sources  1102  is combined in the aggregator  1104  to a high-voltage DC power. A multitude of DC power sources (such as power sources  1106   1 ,  1106   2  . . .  1106   n , collectively referred to as power sources  1106 ) and additional aggregators (such as the aggregator  1108  coupled to the power sources  1106 ) can be used to form a large power array. The outputs of the aggregators  1104  and  1108  are coupled to a high-voltage DC bus  1112  that is coupled to a bidirectional power converter  104 . The strings of power sources  1102  and  1106  and the aggregators  1104  and  1108  combine to provide DC power to the bidirectional power converter  104  for conversion to AC power having reactive power as described herein. 
       FIGS.  9 ,  10  and  11    are intended to show a sample of the types of alternative power system arrangements in which the bidirectional power converter  104  may find use. The three systems  900 ,  1000 , and  1100  are not meant to be exhaustive. The bidirectional power converter  104  having reactive power control may find use in any power system where, for example, a utility desires to control the reactive power on the AC grid  106 . The utility RPC schedule  108  may be coupled to the bidirectional power converter  104  via wired and/or wireless communication techniques, such as Ethernet, wireless techniques based on standards such as IEEE 802.11, Zigbee, Z-wave, or the like, power line communications, and the like. In some embodiments, the utility RPC schedule  108  may be manually entered or pre-programmed into the bidirectional power converter  104 . 
       FIG.  12    is a schematic diagram of an embodiment of a three-phase static VAr compensator  1200  that uses a switched mode cycloconverter  1200 . The static VAr compensator  1200  comprises a storage device  1204  (e.g., a resonant tank comprising a capacitor  1206  and an inductor  1208 ) and the switched mode cycloconverter  1202 . A static VAr compensator is one type of bidirectional power converter, and the static VAr compensator  1200  may be used as part of the bidirectional power converter  104  in one or more embodiments. 
     The storage device  1204  stores and releases energy to facilitate creating reactive power. The cycloconverter  1202  couples energy to and from the AC grid  106  such that the AC current is quadrature to the AC mains voltage, with a magnitude that is commensurate with the desired amount of reactive power. The controller  500  of  FIG.  5    or the controller  600  of  FIG.  6    may be used as the VAr compensator controller to control the timing of the switches within the cycloconverter  1202  to achieve three-phase AC power containing reactive power. Since the VAr compensator  1200  is not coupled to a DC source, there is no need for the controller to create the variable Dreq that is generated by the MPPT controller  506 . As such, the VAr compensator controller does not include or has deactivated the MPPT controller  516 /MPPT control software  616  and the associated sine table  518 /sine table  608 . In the VAr compensator  1200 , the signal Dreq is unnecessary because the static VAr compensator  1200  is only capable of providing or consuming VAr, i.e., no real power is generated or consumed and there is only a reactive power component to the AC power. 
       FIG.  13    is a flow diagram of a method  1300  for controlling reactive power in accordance with one or more embodiments of the present invention. The method  1300  is implemented using a bidirectional power converter having a switched mode cycloconverter (e.g., the bidirectional power converter  104 ). The bidirectional power converter is coupled to an AC line or grid, such as a commercial AC grid. In some embodiments of the method  1300 , the bidirectional power converter is coupled to a renewable energy source (such as one or more photovoltaic (PV) modules) for receiving DC power that is converted to AC power. In one or more alternative embodiments, the bidirectional power converter is a static VAr compensator. 
     The method  1300  starts at step  1302  and proceeds to step  1304 . At step  1304 , a desired amount of a reactive power component to be generated by the bidirectional power converter is determined. In some embodiments, the desired amount of the reactive power component may be determined based on a reactive power control (RPC) schedule. The RPC schedule may be communicated to the bidirectional power converter using wired (e.g., power line communications) and/or wireless communication techniques. In certain alternative embodiments, the RPC schedule may be manually entered into the bidirectional power converter (e.g., through a web browser interface); in other alternative embodiments, the RPC schedule may be preprogrammed into the bidirectional power converter. 
     In order to facilitate reactive power control, the RPC schedule may comprise a list of reactive power amounts and the time of day at which the listed reactive power amounts are to be supplied to the AC grid. Additionally or alternatively, the RPC schedule may contain a schedule of reactive power to be generated as a function of one or more of the AC grid voltage, the power converter output power, a change in AC grid voltage, a fixed value, and the like. 
     Typically, the RPC schedule is provided by the local power generation company or utility that manages the AC grid to which the bidirectional power converter is coupled, although in some alternative embodiments the RPC schedule may be obtained from a different source. 
     The method  1300  proceeds from step  1304  to step  1306 . At step  1306  the bidirectional power converter generates AC power having the desired amount of the reactive power component determined in step  1304 . The bidirectional power converter generates the AC power using its switched mode cycloconverter as described above (e.g., with respect to  FIG.  5   ). In some embodiments, the bidirectional power converter generates single-phase AC power; in other embodiments, the bidirectional power converter generates three-phase AC power. 
     The method  1300  proceeds to step  1308 , where a determination is made whether to continue. If the result of the determination is yes, the method  1300  returns to step  1304 ; if the result of the determination is no, the method  1300  proceeds to step  1310  where it ends. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.