Patent Publication Number: US-2022239235-A1

Title: Adaptable DC-AC Inverter Drive System and Operation

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of commonly assigned, co-pending U.S. patent application Ser. No. 17/033,060, filed Sep. 25, 2020, entitled “DC-AC INVERTER DRIVE SYSTEM AND OPERATION,” which is incorporated by reference in its entirety herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to systems and methods of operation of direct current-to-alternating current (DC-AC) inverters operating as an interconnected array for applications such as solar energy harvesting in off-grid mode, and also to systems and methods of operation of an adaptable DC-AC inverter with varying number of DC source inputs. 
     BACKGROUND 
     Modular DC-AC inverters are becoming very popular for applications in solar energy harvesting due to several advantages such as modularity, safety, and lower sensitivity to shading and single module failure, as compared to traditional central inverters. The majority of the inverters are used in the grid connected mode where the reference voltage and frequency are provided by the grid. In addition, the grid acts as a sink for excess power generated by the inverters over and above the local load demand, and as a source for additional power when the power generated by the inverters is less than the local load demand. This enables a stable operation of the inverters in the grid connected mode of operation. However, there is a significant need for inverters operating reliably in the off-grid mode in several applications where access to grid power is not always available and even when available not very reliable. Conventional methods of paralleling output of multiple inverters to support a load do not provide a solution that is stable under varying load conditions. In most of these cases synchronization of output voltage, frequency and phase angle are very difficult, and the output voltage becomes unstable under varying load conditions. 
     The present disclosure is targeted to address these limitations of operating DC-AC inverters and to enable a well-synchronized, stable output voltage to support varying load conditions, both in grid-connected and off-grid modes of operation. 
     SUMMARY 
     Described herein is a drive system and its operating methods for an array of DC-AC inverters operating in off-grid mode of operation wherein the alternating current (AC) output of the inverters are connected in parallel. One of the DC-AC inverters in the array is configured as a control unit and the remaining DC-AC inverters in the array are configured as follower units. The control unit computes the duty cycle for the pulse-width modulation (PWM) drive signals at the switching frequency of the DC-AC converter based on the input direct current (DC) voltage and the required output AC characteristics such as, without limitation, voltage, wave form and frequency. The PWM drive signal duty cycles are adjusted (e.g., constantly) based on the voltage feedback signals from an AC bus. The PWM drive signals are used for generating the required AC output waveform in the control unit. In addition, a reference PWM signal along with a PWM synchronization signal and zero cross synchronization signal from the control unit are transmitted to the follower units via a communication port of the control unit. 
     The follower units receive the reference PWM drive signal along with the PWM synchronization signal and the zero crossing synchronization signal via respective communication ports of the follower units. Reference PWM signal duty cycles are determined by the follower units via respective electronic capture modules of the follower units. Respective PWM modules in the follower units replicate the PWM drive signals based on the determined duty cycle, and the PWM drive signals are used for generating respective AC output waveforms in the follower units. 
     The synchronization signals (e.g., the PWM synchronization signal and the zero crossing synchronization signal) received by the follower units from the control unit are used to ensure synchronization of the AC output waveforms of the follower units with the AC output waveform of the control unit. 
     The use of common PWM drive and synchronization signals across all the inverters ensures the stability of the AC output waveform for the array of inverters connected in parallel under varying load conditions. 
     This summary is provided to introduce a selection of concepts in a simplified form described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of claimed subject matter. 
     Also described herein is an adaptable DC-AC inverter system and its operating methods. The inverter system includes multiple DC input sources configured to operate in a grid-connected mode of operation or an off-grid mode of operation. Each DC input source is connected to a respective power conversion module, wherein the alternating current (AC) outputs of the respective power conversion modules are connected in parallel. The power conversion modules are configured to be driven by common pulse-width modulation (PWM) drive signals generated by a PWM module in a controller sub-system of the inverter system. The controller sub-system is configured to compute the duty cycle for the PWM drive signals at the switching frequency of the power conversion modules based on the input direct current (DC) voltage and the output AC characteristics such as, without limitation, voltage, wave form, and frequency. The duty cycles for the PWM drive signals may be adjusted (e.g., constantly) based on the voltage feedback signals from an AC bus. The PWM drive signals may be transmitted to the power conversion modules through a communication port of the controller sub-system to a communication sub-system of the inverter system. 
     The power conversion modules are configured to receive the PWM drive signals from the communication sub-system at the respective communication ports of the power conversion modules. The PWM drive signals may be used for generating respective AC output waveforms in the power conversion modules. 
     The AC outputs from the power conversion modules may be transmitted to the AC bus in the communication sub-system. The combination of the AC outputs from the power conversion modules form the AC output of the inverter system. 
     The use of common PWM drive signals across the power conversion modules ensures the stability of the combined AC output waveform of the inverter system under varying load conditions. The disclosed inverter system also offers flexibility to add DC input sources to or remove DC input sources from the system without impacting the functionality of the system. In addition, the system maintains the advantages of modularity at the same time reducing cost by eliminating duplication of components in the system. 
     This summary is provided to introduce a selection of concepts described below in the detailed description in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is set forth with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical components or features. 
         FIG. 1  is a schematic block diagram of an example DC-AC inverter system with multiple DC sources and associated DC-AC inverters working in off-grid mode of operation along with reference PWM and synchronization signals. 
         FIG. 2  is a schematic block diagram of an example DC-AC inverter configured as a control unit illustrating the power converter subsystem and controller subsystem. Also noted is a communication port(s) configured as the transmitter of the reference PWM and synchronization signals. 
         FIG. 3  is a schematic block diagram of an example DC-AC inverter configured as a follower unit illustrating the power converter subsystem and controller subsystem. Also noted is a communication port(s) configured as the receiver of the reference PWM and synchronization signals. 
         FIG. 4  is a flow diagram of an illustrative process for the operation of a DC-AC inverter system operating in the off-grid mode of operation. Also noted are the process for generating and communicating a reference PWM and synchronization signals from the control unit to the follower unit. 
         FIG. 5  is a schematic block diagram of an example Adaptable DC-AC inverter system with multiple DC sources. Controller sub-system, communication sub-system, power conversion sub-system along with the power conversion modules, system output port and the associated interconnections are illustrated. Also illustrated are the PWM drive signals generated by the controller sub-system feeding into the power conversion modules along the AC voltage output bus. 
         FIG. 6  is a schematic block diagram of an example controller sub-system generating the PWM drive signals and a power conversion module converting the DC input to AC output voltage employing the PWM drive signals. Also illustrated are the input signals coming into the controller sub-system for the purpose of computing the PWM duty cycles. 
         FIG. 7  is a flow diagram of an illustrative process for the operation of an adaptable DC-AC inverter system. Also illustrated is a process for generating and communicating the PWM drive signals from the controller sub-system to the power conversion modules. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of embodiments, specific detailed examples are given to provide an understanding of the embodiments. However, it is to be appreciated that the embodiments may be practiced without these specific details. Furthermore, the techniques and systems disclosed herein are limited to the described embodiments. Numerous modifications, changes, variation, substitutions, and equivalents will be apparent to those skilled in the art. 
       FIG. 1  is a schematic block diagram of an example DC-AC inverter system  100  with multiple DC input sources  101 - 1 ,  101 - 2 ,  101 - 3 , . . .  101 -N, and associated DC-AC inverters  102 - 1 ,  102 - 2 ,  102 - 3 , . . .  102 -N (sometimes referred to herein as “inverters  102 ”) working in off-grid mode of operation. In this system the inverter  102 - 1  is configured as the control unit and the remaining inverters  102 - 2 ,  102 - 3 , . . .  102 -N are configured as follower units. Also presented are the reference PWM signal  103 , PWM synchronization (sync.) signal  104  and V ac  zero crossing signal  105 . The V ac  output  106 - 1 ,  106 - 2 ,  106 - 3 , . . .  106 -N represent the output from each of the inverters  102 - 1 ,  102 - 2 ,  102 - 3 , . . .  102 -N respectively which are connected to off-grid AC bus  107 . The off-grid AC bus  107  is connected to the loads  108 . 
     As an illustrative example, the DC-AC inverter system  100  may be a 3000 Watt roof top solar energy system operating in off-grid mode with ten 300 Watt solar panels as DC input sources ( 101 - 1 ,  101 - 2 ,  101 - 3 , . . .  101 - 10 ) connected to ten inverters ( 102 - 1 ,  102 - 2 ,  102 - 3 , . . .  102 - 10 ), each inverter  102  of a 300 watt capacity. The inverter  102 - 1  is configured as the control unit and the inverters  102 - 2 ,  102 - 3 , . . .  102 - 10  are configured as follower units. The reference PWM signal  103  generated in the control unit  102 - 1  is shared with (e.g., sent to, transmitted to, communicated to, etc.) the follower units  102 - 2 ,  102 - 3 , . . .  102 - 10  for replication and for driving the DC-AC converters in these follower units. The AC output  106 - 1 ,  106 - 2 ,  106 - 3 , . . .  106 - 10  are connected to the off-grid AC bus  107  which supplies the energy to the load  108 . 
     While in the above illustrative example the DC input sources  101 - 1 ,  101 - 2 ,  101 - 3 , . . .  101 -N are identified as solar panels, the techniques and systems described herein are equally applicable to different types of DC input source connected to the inverters. Some specific examples of possible DC input sources are (i) a photovoltaic solar panel, (ii) a fuel cell, (iii) a battery, (iv) a wind energy generator, or (v) an ultracapacitor. 
     It is to be noted that while in the above example inverter  102 - 1  is configured as the control unit, in practice, any one of the inverters ( 102 - 1 ,  102 - 2 ,  102 - 3 , . . .  102 -N) can be configured as a control unit and the remaining inverters configured as follower units. Also, the reconfiguration of a control unit into a follower unit and a follower unit into a control unit is possible. 
     The inverters ( 102 - 1 ,  102 - 2 ,  102 - 3 , . . .  102 -N) employ at least one of (i) one or more high frequency transformers, or (ii) one or more high frequency inductors; and operate in at least one of (i) a single-stage DC-AC conversion mode, or (ii) a two-stage DC-DC-AC conversion mode. 
     Also to be noted is that the inverter(s) ( 102 - 1 ,  102 - 2 ,  102 - 3 , . . .  102 -N) is/are one of: (i) a microinverter(s), (ii) a modular inverter(s), or (iii) a string inverter(s) with specifications of an (i) off-grid inverter, (ii) hybrid inverter, or (iii) dual mode inverter operating in off-grid mode of operation 
     Also to be noted is that the inverters ( 102 - 1 ,  102 - 2 ,  102 - 3 , . . .  102 -N) can be of the same or different power ratings as long as they share a common switching frequency. 
     For the purpose of operating and controlling the inverters ( 102 - 1 ,  102 - 2 ,  102 - 3 , . . .  102 -N), the DC-AC inverter system  100  is equipped with one or more sensors and one or more fault protection features such as: a input DC voltage high and low limits sensor(s), an AC output voltage sensor(s), an output current limit sensor(s) and/or a temperature sensor(s). 
       FIG. 2  is a schematic block diagram of an example DC-AC inverter  102 - 1  configured as a control unit  200  illustrating the power converter subsystem  201  and controller subsystem  202  of the control unit  200 . Controller subsystem  202  has a PWM module  203  for generating the reference PWM signal  103  (sometimes referred to herein as a “reference PWM signal  103 ”) and additional PWM drive signals  206  for driving power switches  204  through gate drivers  207  in the DC-AC converter  210 . Also noted is a communication port(s)  205  configured as the transmitter of the reference PWM signal  103 , PWM sync. signal  104 , and V ac  zero crossing signal  105  (sometimes referred to herein as a “zero crossing signal  105 ” or “zero crossing synchronization signal  105 ”). 
     As an illustrative example, the PWM module  203  in the control unit  200  initially generates reference PWM signal  103  and additional PWM drive signals  206  at the switching frequency (for example 56 kilohertz (KHz)) of the power converter subsystem  201  whose duty cycles are computed based on the input DC voltage V dc    101 - 1  (for example 36 Volts (V)) and required output AC characteristics such as, without limitation, voltage (for example 220 V), wave form (for example sine wave) and frequency (for example 50 hertz (Hz)). Once initiated, the duty cycle of the reference PWM signal  203  and PWM drive signals  206  are dynamically controlled by the closed loop feedback of the AC bus  107  voltage to maintain the AC bus  107  voltage V ac  within specified limits (for example between 210 and 230 V). 
       FIG. 3  is a schematic block diagram of an example DC-AC inverter  102 - 2  configured as a follower unit  300  illustrating the power converter subsystem  201  and controller subsystem  302  of the follower unit. Also noted is a communication port(s)  305  configured as the receiver of the reference PWM signal  103 , PWM sync. signal  104 , and V ac  zero crossing signal  105  transmitted from the control unit  200 . 
     Controller subsystem  302  has an electronic capture module  308  for determining the duty cycle of the received reference PWM signal  103 , and a PWM module  303  for generating the PWM drive signals  206  by replicating the reference PWM signal  103  for driving the power switches  204  through gate drivers  207  in the DC-AC converter  210 . 
     As an illustrative example, the communication port  305  configured as the receiver receives the reference PWM signal  103  at the switching frequency (for example 56 KHz) from the control unit  200 . The capture module  308  determines the duty cycle of the reference PWM signals  103 . Controller subsystem  302  configures the PWM module  303  for generating the PWM drive signals  206  by replicating the frequency and duty cycle of the reference PWM signal  103  for driving the power switches  204  through gate drivers  207  in the DC-AC converter  210 . The power converter subsystem  201  in the follower unit  300  generates output AC voltage of required voltage, frequency, and waveform (for example 220 volts, 50 Hz, and sine wave). PWM sync. signal  104  and V ac  zero crossing signal  105  transmitted from the control unit  200  and received by the follower unit  300  are utilized for ensuring the AC output V ac    106 - 2  waveform generated by the follower unit  300  is in synchronization with that generated by the control unit  200 . 
     The communication port  205  for the control unit  200  and the communication port  305  for the follower unit  300  can be of wired or wireless type using industry standard or custom protocols. 
     The controller subsystem  202  for the control unit  200  and the controller subsystem  302  for the follower unit  300  can be of any one of a digital signal processor (DSP), a microcontroller, a Field Programmable Gate Array (FPGA), or an Application Specific Integrated Circuit (ASIC). 
       FIG. 4  is a flow diagram of an illustrative process for the operation of a DC-AC inverter system  100  operating in the off-grid mode of operation. The process flow  400  outlines the functions of the control unit  102 - 1  and the follower unit  102 - 2 . Also noted are the process for generating and communicating a reference PWM signal  103 , PWM synchronization signal  104 , and V ac  zero crossing signal  105  from the control unit  102 - 1  to the follower unit  102 - 2 . The processes are illustrated as a collection of blocks in a logical flow graph, which represent a sequence of operations that can be implemented in hardware, software, or a combination thereof. In the context of software, the blocks represent computer-executable instructions stored on one or more computer-readable storage media that, when executed by one or more processors, perform the recited operations. Generally, computer-executable instructions include routines, programs, objects, components, data structures, and the like that perform particular functions or implement particular abstract data types. The order in which the operations are described is not intended to be construed as a limitation, and any number of the described blocks can be combined in any order and/or in parallel to implement the processes. Moreover, in some embodiments, one or more blocks of the processes may be omitted entirely. 
     As an illustrative example, the initialization step  401  of the control unit  102 - 1  involves configuring the PWM module  203  for a switching frequency (for example 56 KHz) of the power converter subsystem  201  and configuring the communication port  205  as a transmitter. The initialization step  425  of the follower unit  102 - 2  involves configuring the PWM module  303  for a switching frequency (for example 56 KHz) of the power converter subsystem  201  and configuring the communication port  305  as a receiver. The initialization step  425  may further involve configuring the capture module  308  in the controller subsystem  302  of the follower unit  102 - 2 . 
     The initialization step  403  sets the V ac  minimum limit  405 , V ac  maximum limit  406  and the V ac  output frequency  407  for the output voltage V ac  (for example 210 V, 230 V and 50 Hz, respectively). 
     The initialization step  409  sets the start clock time, t=0 for the control unit  102 - 1  and the initialization steps  427  sets the start clock time, t=0 for the follower unit  102 - 2 . 
     The process steps  411 - 423  are associated with control unit  102 - 1 . In process step  411  PWM module  203  acquires the DC input voltage V dc    101 - 1  and AC output voltage V ac    106 - 1  from the respective sensors. Process step  413  compares the value of V ac    106 - 1  with the V ac  minimum limit  405  and V ac  maximum limit  406  and verifies if the logic V ac  minimum limit  405 &lt;V ac    106 - 1 &lt;V ac  maximum limit  406 . If this logic is satisfied, the process step  415  is skipped, and the process step  417  is performed. If the logic is not satisfied, process step  415  is performed where controller subsystem  202  computes, using V dc  and V ac  values, the duty cycle for the reference PWM signal  103 . In process step  417  the PWM module  203  in the controller subsystem  202  generates the reference PWM signal  103 , PWM drive signals  206 , PWM synchronization signal  104  and V ac  zero crossing signal  105 . In process step  419  the communication module  205  in the controller subsystem  202  set in the transmit mode transmits the reference PWM signal  103 , PWM synchronization signal  104  and V ac  zero crossing signal  105  to the follower unit  102 - 2  communication module  305  set in the receiver mode. In process step  421 , the PWM drive signals  206  are used to drive the power switches  204  in the DC-AC converter  210  of the control unit  102 - 1 . This process flow (steps  411 - 421 ) is repeated at high frequency (for example 56 KHz) with time interval Δt (for example 17.85 microseconds) as indicated in step  423 . 
     The process steps  429 - 435  are associated with follower unit  102 - 2 . In process step  429  the communication module  305  in the controller subsystem  302  set in the receiver mode receives the reference PWM signal  103 , PWM synchronization signal  104  and V ac  zero crossing signal transmitted from the control unit  102 - 1  communication module  205  set in the transmitter mode. In process step  431 , the capture module  308  determines the duty cycle of the reference PWM signal  103 . Controller subsystem  302  configures the PWM module  303  for generating the PWM drive signals  206  by replicating the frequency and duty cycle of the reference PWM signal  103 . PWM synchronization signal  104  is used to ensure the generated PWM drive signals are in synchronization with the reference PWM signal. In process step  433 , the PWM drive signals  206  are used to drive the power switches  204  in the DC-AC converter  210  of the follower unit  102 - 2 . V ac  zero crossing signal  105  is used to ensure the AC output from the follower unit  102 - 2  is in synchronization with the AC output from the control unit  102 - 1 , This process flow (steps  429 - 433 ) is repeated at high frequency (for example 56 KHz) with time interval Δt (for example 17.85 microseconds) as indicated in step  435 . 
     The techniques and systems disclosed herein can be implemented in different embodiments of DC-AC inverter systems operating in off-grid mode of operation. For example, the controller subsystem  202  of the control unit  200  can be configured to integrate the functions of PWM drive signal generation and transmission to the follower unit(s)  300  instead of generating and transmitting a reference PWM signal  103  to the follower unit(s)  300 . In this embodiment, the follower unit(s) may not include a controller subsystem  302 , but may include the power converter subsystem  201 . The controller subsystem  202  of the control unit  200  may comprise the PWM module  203  to generate PWM drive signals  206  at the switching frequency of the power converter subsystem  201  for driving the power switches  204  of both the control unit  200  and the follower unit(s)  300 . Accordingly, these PWM drive signals  206  may be transmitted via the communication port  205  of the control unit  200  for receipt by the follower unit(s)  300 , and the control unit  200  may refrain from transmitting a reference PWM drive signal  103  in this embodiment. The follower unit(s)  300  may receive, via their respective communication port(s)  305 , the PWM drive signals  206  from the control unit  200  and may use the PWM drive signals  206  to drive the power switches  204  of the power converter subsystem(s)  201  of the follower unit(s)  300 . 
       FIG. 5  is a schematic block diagram of an example adaptable DC-AC inverter system  500  including a controller sub-system  501 , a power conversion sub-system  503 , a communication sub-system  504 , multiple DC input source providing DC input voltages  502 - 1 ,  502 - 2 , . . .  502 -N to associated DC-AC power conversion modules  503 - 1 ,  503 - 2 , . . .  503 -N working in conjunction with the communication sub-system  504 . In some examples, the adaptable DC-AC inverter system  500  is auto-configurable to a number of the DC sources and associated power conversion modules  503 . This offers flexibility to add DC input sources and associated power conversion modules  503  to or remove DC input sources and associated power conversion modules  503  from the system  500  without impacting the functionality of the system  500 , as the system  500  is auto-configurable. Also illustrated are the PWM drive signals  505 , and the Vac output  506 - 1 ,  506 - 2 , . . .  506 -N (or “AC output voltages”  506 - 1 ,  506 - 2 , . . .  506 -N). The Vac output  506 - 1 ,  506 - 2 , . . .  506 -N represent the output from each of the power conversion modules  503 - 1 ,  503 - 2 , . . .  503 -N, respectively, which are connected to AC bus  507  (sometimes referred to herein as “AC voltage output bus”  507 ). The AC bus  507  passes through a current transformer (CT)  508  for sensing the output current Iac  509  before connecting to the system output port  510  which is connected to the loads  511 . In the case of grid-connected operation, the grid voltage (Vac Grid)  512  is connected to the controller sub-system  501  as well as the system output port  510 . In certain embodiments of the adaptable DC-AC inverter system  500 , the system output port  510  may incorporate an unfolding H-Bridge circuit. 
     As an illustrative example, the DC-AC inverter system  500  may be a 5000 Watt roof top solar energy system operating in off-grid mode with ten 500 Watt solar panels as DC input sources connected to ten power conversion modules ( 503 - 1 ,  503 - 2 , . . .  503 -N, where N=10), each of a 500 watt capacity. The PWM drive signals  505  generated in the controller sub-system  501  is shared with (e.g., sent to, transmitted to, communicated to, etc.) the power conversion modules  503 - 1 ,  503 - 2 , . . .  503 -N for driving the DC-AC converters in these power conversion modules  503 - 1 ,  503 - 2 , . . .  503 -N. The AC outputs  506 - 1 ,  506 - 2 , . . .  506 -N are connected to the AC bus  507  which supplies the energy to the load(s)  511  through the system output port  510 . 
     While in the above illustrative example the DC input sources providing the DC input voltages  502 - 1 ,  502 - 2 , . . .  502 -N are identified as solar panels, the techniques and systems described herein are equally applicable to different types of DC input sources connected to the inverters. Some specific examples of possible DC input sources are (i) a photovoltaic solar panel, (ii) a fuel cell, (iii) a battery, or (iv) an ultracapacitor. Furthermore, it is to be appreciated that the adaptable DC-AC inverter system  500  may represent (i) an off-grid inverter, (ii) a hybrid inverter, or (iii) a dual-mode inverter. 
     The power conversion modules ( 503 - 1 ,  503 - 2 , . . .  503 -N) of the power conversion subsystem  503  may employ at least one of (i) one or more transformers (e.g., high frequency transformers), or (ii) one or more inductors (e.g., high frequency inductors); and operate in at least one of (i) a single-stage DC-AC conversion mode, or (ii) a two-stage DC-DC-AC conversion mode. 
     Also to be noted is that the power conversion modules ( 503 - 1 ,  503 - 2 , . . .  503 -N) can be of the same or different power ratings as long as they share a common switching frequency. That is, the multiple power conversion modules  503 - 1 ,  503 - 2 , . . .  503 -N operate at the same switching frequency corresponding to the switching frequency set by the controller sub-system  501 . 
     For the purpose of operating and controlling the power conversion modules ( 503 - 1 ,  503 - 2 , . . .  503 -N), the adaptable DC-AC inverter system  500  is equipped with one or more sensors and one or more fault protection features such as: an input DC voltage high and low limits sensor(s) for input DC voltage high and low limits, an AC output voltage sensor(s) for voltage high and low limits for the AC output voltage, an output current limit sensor(s) for an output current limit, and/or a temperature sensor(s) for temperature. 
       FIG. 5  also illustrates that the communication sub-system  504  includes connectors  520 - 1 ,  520 - 2 ,  520 - 3 , . . .  520 -N. Transmission of the PWM drive signals  505  and the AC output voltages  506 - 1 ,  506 - 2 , . . .  506 -N by the communication sub-system  504  may be performed using one or more of the connectors  520 - 1 ,  520 - 2 ,  520 - 3 , . . .  520 -N. In some examples, the connectors  520 - 1 ,  520 - 2 ,  520 - 3 , . . .  520 -N represent one or more printed circuit boards with connector interfaces to the controller sub-system  501  and the multiple power conversion modules  503 - 1 ,  503 - 2 , . . .  503 -N. In some examples, the connectors  520 - 1 ,  520 - 2 ,  520 - 3 , . . .  520 -N represent one or more cables and a connector mechanism(s) connecting the controller sub-system  501  and the multiple power conversion modules  503 - 1 ,  503 - 2 , . . .  503 -N. 
       FIG. 6  is a schematic example of controller sub-system  501  generating the PWM drive signals  505  and a power conversion module  503 - 1  converting the DC input voltage  502 - 1  from the DC input source  602 - 1  (sometime referred to herein as “DC source”  602 - 1  or “DC voltage source”  602 - 1 ) to AC or rectified AC output voltage  506 - 1  employing the PWM drive signals  505 . Controller sub-system  501  has a PWM module  603  for generating the PWM drive signals  505 . Communication port  604  set in transmit mode transmits the PWM drive signals  505  to the power conversion module  503 - 1  via the communication sub-system  504 . The communication port  606  in the power conversion module  503 - 1  set in the receiver mode receives the PWM drive signals  505  which are used for driving power switches  607  through gate drivers  609  in the power conversion module  503 - 1 . The communication ports  604  and/or  606  may utilize a wired protocol, a wireless protocol, or combination of both the wired protocol and the wireless protocol. Also noted are the input signals from various sensors to the adaptable DC-AC inverter system  500 , the input signals including AC current (Iac)  509 , Vdc  502 - 1 , Vac  506 , Temperature  514  and Vac Grid  512 . These input analog signals are converted to digital input by the ADC module  611  and fed to the micro controller  605 . In some examples, the controller sub-system  501  incorporates at least one of a digital signals processor (DSP), a microcontroller, a Field Programmable Gate Array (FPGA), or an Application Specific Integrated Circuit (ASIC). 
     As an illustrative example, the PWM module  603  in the controller sub-system  501  initially generates PWM drive signals  505  at the switching frequency (for example 56 kilohertz (KHz)) of the power conversion sub-system  503  whose duty cycles are computed based on the input DC voltage Vdc  502 - 1  (for example 36 Volts (V)) and output AC characteristics such as, without limitation, voltage (for example 220 V), wave form (for example sine wave), and frequency (for example 50 hertz (Hz)). The PWM drive signals  505  are fed to the power conversion module  503 - 1  for driving the power switches  607  through the gate drives  609 . The power conversion module  503 - 1  generates output AC voltage  506 - 1  of voltage, frequency, and waveform (for example 220 volts, 50 Hz, and sine wave). Once initiated, the duty cycle of the PWM drive signals  505  is dynamically controlled by the closed-loop feedback of the AC bus voltage  506  to maintain the AC bus voltage Vac  506  within specified limits (for example between 210 and 230 V). Here, the AC bus voltage (See Vac  506  in  FIG. 5 ) may represent the system output voltage (e.g., system output AC voltage). The AC bus voltage can be the average of the output AC voltages  506 - 1  to  506 -N from the multiple power conversion modules  503 - 1  to  503 -N. 
       FIG. 7  is a flow diagram of an illustrative process for the operation of an adaptable DC-AC inverter system  500 . The process flow  700  outlines the functions of the controller sub-system  501  and a representative power conversion module  503 - 1 . Also illustrated as part of the process flow  700  is a process for generating and communicating a PWM drive signals  505  from the controller sub-system  501  to the representative power conversion module  503 - 1 . The processes are illustrated as a collection of blocks in a logical flow graph, which represent a sequence of operations that can be implemented in hardware, software, or a combination thereof. In the context of software, the blocks represent computer-executable instructions stored on one or more computer-readable storage media that, when executed by one or more processors, perform the recited operations. Generally, computer-executable instructions include routines, programs, objects, components, data structures, and the like that perform particular functions or implement particular abstract data types. The order in which the operations are described is not intended to be construed as a limitation, and any number of the described blocks can be combined in any order and/or in parallel to implement the processes. Moreover, in some embodiments, one or more blocks of the processes may be omitted entirely. 
     As an illustrative example, the initialization step  701  of the controller sub-system  501  involves configuring the controller sub-system  501  (or auto-configurating the adaptable DC-AC inverter system  500 ) for the number of DC sources  602  (and associated series of power conversion modules  503 ) connected to the adaptive DC-AC inverter system  500  (for example ten DC sources  602 ), selecting a mode of operation specified as on-grid or off-grid (for example off-grid) and the switching frequency of the power conversion module  503 - 1  (for example 56 KHz). The initialization step  723  of the representative power conversion module  503 - 1  involves configuring the communication port  606  as a receiver, such as to receive PWM drive signals  505 . 
     The initialization step  703  sets the Vac minimum limit  305 , Vac maximum limit  306 , and the Vac output frequency  307  for the system output voltage Vac  506  (for example 210 V, 230 V, and 50 Hz, respectively). 
     The initialization step  709  sets the start clock time, t=0 for the controller sub-system  501 , and the initialization step  725  sets the start clock time, t=0 for the power conversion module  503 - 1 . 
     The process steps  711 - 721  are associated with controller sub-system  501 . In process step  711 , PWM module  603  acquires from the ADC module  611 , the DC input voltage Vdc  502 - 1  and AC output voltage Vac  506  from the respective sensors. Process step  713  compares the value of Vac  506  with the Vac minimum limit  305  and Vac maximum limit  306  and verifies if the logic Vac minimum limit  305 &lt;Vac  506 &lt;Vac maximum limit  306 . If this logic is satisfied, the process step  715  is skipped, and the process step  717  is performed. If the logic is not satisfied, process step  715  is performed where controller sub-system  501  computes, using Vdc  502 - 1  and Vac  506  values, the duty cycle for the PWM drive signals  505 . In process step  717 , the PWM module  603  in the controller sub-system  501  generates the PWM drive signals  505 . In process step  719 , the communication port  604  in the controller sub-system  501  set in the transmit mode transmits the PWM drive signals  505  to the power conversion module  503 - 1  via the communication sub-system  504 . This process flow (steps  711 - 721 ) is repeated at high frequency (for example 56 KHz) with time interval Δt (for example 17.85 microseconds) as indicated in step  721 . 
     The process steps  727 - 733  are associated with power conversion module  503 - 1 . In process step  727 , the communication port  606  in the power conversion subsystem  503 - 1  set in the receiver mode receives the PWM drive signals  505 . In process step  729 , the PWM drive signals  505  are used to drive the power switches  607  through the gate driver  609  in the power conversion module  503 - 1 . In process step  731 , the AC output voltage  506  is transmitted to an AC bus  507  of the communication sub-system  504 . This process flow (steps  727 - 733 ) is repeated at high frequency (for example 56 KHz) with time interval Δt (for example 17.85 microseconds) as indicated in step  733 . 
     In some examples, the duty cycle of the PWM drive signals  505  is dynamically controlled by the closed-loop feedback of the AC bus voltage  506  to maintain the AC bus voltage Vac  506  within specified limits (for example between 210 and 230 V). That is, in some examples, the process flow  700  may further include using a closed-loop control system to maintain the AC output voltage  506  in a predefined voltage range, wherein the predefined voltage range is defined with preset limit values for an AC output voltage minimum and an AC output voltage maximum. Such a closed-loop control system may implement a closed-loop voltage control algorithm, a closed-loop power control algorithm, or a combination thereof to maintain the AC output voltage in the predefined voltage range. 
     The techniques and systems disclosed herein can be implemented in different embodiments of an adaptable DC-AC inverter systems operating. As an example, power conversion modules  503 - 1 ,  503 - 2 , . . . ,  503 -N in the power conversion sub-system  503  may be configured to generate a rectified AC output instead of full sine wave AC output. The rectified AC output from the power conversion modules can be transferred via the AC bus  507  of the communication sub-system  504  to a system output port  510  with a H-Bridge circuit for unfolding the rectified AC output to full sine wave AC output (e.g., to convert the rectified AC output into the full sinewave AC output). This embodiment of the adaptable DC-AC inverter eliminates the need for unfolding H-Bridge circuit in every power conversion module whose functions can be carried by single H-Bridge circuit in the system output port.