Common DC bus and common AC bus power electronic system and method

Abstract

The present invention provides an electrical system comprising inverters coupled together in parallel and coupled to a common DC bus. The electrical system also includes a common AC bus coupled between the outputs of the inverters and a power grid. The electrical system also includes a local controller coupled to the inverter and a decentralized controller coupled to the local controller. The decentralized controller measures voltage and current of the power grid and the inverter and generates decentralized control signals for the local controller based on the measured voltage and current. The electrical system also includes a centralized controller in communication with the local controller. The centralized controller predicts DC load and generates centralized control signals for the local controller to maintain a constant voltage on the common DC bus based on the predicted DC load.

Description

Technical Field

[0001] The technology of the present disclosure generally relates to a power electronics system and method for operating a local controller of an inverter through a decentralized controller and maintaining a constant DC voltage at the input of the inverter based on a DC load predicted by a centralized controller. Background Art

[0002] In power systems used for some solar applications, multiple parallel bidirectional inverters are coupled to multiple arrays of photovoltaic devices, and the bidirectional inverters are coupled to a DC bus. However, when a large number of bidirectional inverters (e.g., greater than 40 bidirectional inverters) are coupled together in this configuration, the power system may not function properly. In addition, the power system may not function properly when the common DC load alternates between positive and negative potentials. Summary of the Invention

[0003] The disclosed technology generally relates to power electronics systems and methods for operating local controllers of an inverter through decentralized controllers and maintaining a constant direct current (DC) voltage at the input of the inverter based on a DC load predicted by a centralized controller.

[0004] In one aspect, the present disclosure provides a system comprising an inverter, a common DC bus, a common alternating current (AC) bus, a local controller, a decentralized controller, and a centralized controller. The inverters are coupled in parallel. The common DC bus is coupled to the input of the inverter. The common AC bus is coupled between the output of the inverter and the power grid. The local controllers are respectively coupled to the inverters. The decentralized controllers are respectively coupled to the local controllers. The decentralized controllers measure the voltage and current of the power grid and the inverter and generate decentralized control signals for the local controllers based on the measured voltage and current of the power grid and the inverter. The centralized controller communicates with the local controllers. The centralized controller predicts a DC load and transmits a centralized control signal to the local controllers to maintain a constant voltage on the common DC bus based on the predicted DC load.

[0005] Embodiments of this aspect may include one or more of the following features. The inverter may be a three-phase inverter. The DC load may include a photovoltaic (PV) device, an energy storage device, or both. The centralized controller may operate at a slower speed than the decentralized controller. The number of inverters may be greater than 20, or greater than 40.

[0006] The centralized control signal may be a voltage control signal that results in a uniform distribution of the current at the output of the inverter.The centralized control signal may be a voltage control signal that causes the current at the output of the inverter to track a reference current.

[0007] The centralized controller may execute a polynomial droop control algorithm. Each decentralized controller may execute a droop control algorithm, such as a polynomial droop control algorithm.

[0008] The centralized controller can be incorporated into one of the local controllers and can be used as a master controller, and the remaining local controllers in the local controller can be used as slave controllers. The local controllers operating as the master controller and the slave controller can communicate with each other via the EtherCAT protocol.

[0009] In another aspect, the present disclosure provides a control method. The control method includes locally controlling, by a local controller, an inverter coupled in parallel between a common DC bus and a common AC bus. The control method also includes measuring, by a decentralized controller coupled to the local controller, a voltage and current of a power grid coupled to the common AC bus and a voltage and current of the inverter; and generating, by the decentralized controller, a decentralized control signal for the local controller based on the measured voltage and current of the power grid and the inverter. The control method also includes predicting a DC load by a centralized controller in communication with the local controller; and sending, by the centralized controller, a centralized control signal to the local controller to maintain a constant voltage on the common DC bus based on the predicted DC load.

[0010] Embodiments of this aspect may include one or more of the following features: The inverter may be a three-phase inverter. The DC load may include a photovoltaic (PV) device, an energy storage device, or both. The centralized controller may operate at a slower speed than the decentralized controllers.

[0011] The centralized control signal may be a voltage control signal that results in a uniform distribution of the current at the output of the inverter.The centralized control signal may be a voltage control signal that causes the current at the output of the inverter to track a reference current.

[0012] The centralized controller may execute a polynomial droop control algorithm. Each decentralized controller may execute a droop control algorithm, such as a polynomial droop control algorithm.

[0013] On the other hand, the present disclosure provides another control method. Another control method includes locally controlling the inverters coupled in parallel between a common DC bus and a common AC bus by a local controller. The other control method also includes measuring the voltage and current of the power grid coupled to the common AC bus and the voltage and current of the inverter by a decentralized controller coupled to the local controller. The other control method also includes generating a decentralized control signal for the local controller based on the measured voltage and current of the power grid and the inverter by the decentralized controller. The other control method also includes synchronously driving the inverters coupled in parallel by the decentralized controller using the decentralized control signal according to the EtherCAT protocol, whereby one of the decentralized controllers serves as a master controller and the remaining decentralized controllers in the decentralized controller serve as slave controllers.

[0014] The details of one or more aspects of the present disclosure are set forth in the accompanying drawings and the following description. Other features, objects, and advantages of the technology described in this disclosure will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a block diagram of the common DC bus power electronics system of the present disclosure.

[0016] Figure 2 is a block diagram of a coordinated centralized control architecture with parallel inverters.

[0017] Figure 3 is a block diagram of a coordinated decentralized control architecture with parallel inverters.

[0018] Figure 4A and Figure 4B is a graph of the sag curve.

[0019] Figures 5A-7 It is a circuit block diagram of a control system that combines centralized control features and decentralized control features.

[0020] Figure 8 is a circuit diagram of a parallel inverter system according to aspects of the present disclosure.

[0021] Figure 9 is a flow chart of a control method according to aspects of the present disclosure. DETAILED DESCRIPTION

[0022] Figure 1A power electronics system 10 is depicted that is in a common DC bus configuration and can be deployed as part of a larger array. It is expected that the present application can be used for solar tracker systems, fixed tilt solar systems, rooftop solar, and any type of solar array. The power electronics system 10 includes an inverter 40 that feeds an output voltage from a first power source (such as a solar power generation device 20) and a second power source (such as a DC storage device 30) via a common DC bus 50. Other types of power sources are contemplated, such as steam, nuclear, geothermal, hydro, wind, etc. It is contemplated that more than two power sources may be utilized. The inverter 40 can be sized to the AC output requirements of the grid to which it is connected. The common DC bus 50 allows for near-instantaneous response to changes in system power demand.

[0023] The DC storage device 30 typically includes a plurality of battery packs 31, a bidirectional DC / DC converter 32, and a centralized controller 60. The centralized controller 60 can control the charging and discharging rates. The bidirectional DC / DC converter 32 can be configured to charge the battery pack 31. The bidirectional DC / DC converter 32 can be sized for battery output or input. The bidirectional DC / DC converter 32 can employ a power droop algorithm that maintains constant power output in the normal MPPT region. When the voltage is above or below the MPPT region, the bidirectional DC / DC converter 32 can gradually increase the power output to the common DC bus 50 or gradually reduce the power output to the common DC bus. The power droop algorithm enables local control of the power output from the DC power generation device 30 based on the external load. In addition, the internal resistance of the bidirectional DC / DC converters 32 connected in parallel maintains relatively equal current sharing between the battery packs 31. The power droop algorithm combined with the battery bank 31 and photovoltaic panel array 21 sharing a common DC bus 50 eliminates the need for additional communication and allows for rapid response to microgrid applications (e.g., changes in the load on the inverter 40). Optionally, a DC / DC converter 22 may be electrically coupled between the common DC bus 50 and the respective photovoltaic panel array 21.

[0024] The inverter 40 receives power from the common DC bus 50 and converts it to an AC voltage. The inverter 40 can be sized to meet the AC output requirements of the power electronics system 10. The inverter 40 maintains output power at the maximum power point (MPP) using, for example, voltage tracking methods. Other methods known in the art are contemplated. Maximum power point tracking (MPPT) operates at the input port of the inverter 40. MPPT is a process that finds a load characteristic that maintains the system at the point where the system is optimized to deliver the highest power. The output power from the solar power plant is sampled, and an appropriate load characteristic (resistance) is applied to achieve maximum power. When a grid reduction command is received at the solar power plant 20 and the DC storage device 30, the inverter 40 experiences an increase in the system voltage at the common DC bus 50, and both the solar power plant 20 and the DC storage device 30 reduce power output without active control. Similarly, when the load increases, the inverter 40 experiences a decrease in the system voltage at the common DC bus 50, and both the solar power plant 20 and the DC storage device 30 increase power output without active control. These voltage changes are almost instantaneous at the inverter 40 .

[0025] Figure 2 is a block diagram of a coordinated centralized control architecture 200. In the coordinated centralized control architecture 200, local controllers 220 are respectively coupled to bidirectional inverters 230, and a centralized coordinated controller 210 is coupled to the local controllers 220. The local controllers 200 are responsible for high-frequency switching of the power electronic devices of the bidirectional inverters 230. The local controllers 220 also handle zero sorting. The centralized coordinated controller 210 can execute optimization-based and data-driven control algorithms. The centralized coordinated controller 210 also executes prediction-based and model-based control algorithms. The control algorithm is configured to maintain a constant DC voltage despite varying and uncertain loads. The control algorithm is also configured to adjust the current distribution so that the current distribution is evenly distributed and tracks a desired reference current distribution.

[0026] The coordinated centralized control feature of the present disclosure enables a large number of inverters to operate in parallel using a hierarchical scheme that operates at different times with limited communication and predicting DC loads (e.g., solar and battery loads). Measurement data is used to learn a model for predicting DC loads.

[0027] The coordinated centralized controller 210 utilizes an input / output architecture. The input / output architecture incorporates model-based optimization, whose objective can be expressed as minimizing a cost function. The input / output architecture also measures the voltage and current of the grid and bidirectional inverter 230. The output of the input / output architecture can be based on a reference voltage or P / Q control for each inverter 230. The output can be based on a polynomial droop curve for each inverter 230. The output is adjusted in real time based on predicted DC loads (e.g., solar and battery loads).

[0028] In various aspects, the coordinated centralized control architecture 200 can be configured to implement various aspects of EtherCAT technology. For example, one local controller 220 can function as an EtherCAT master, and the remaining local controllers 220 can function as EtherCAT slaves. In one embodiment, the EtherCAT master can perform all or part of the functions performed by the centralized coordinated controller 210.

[0029] The operation or control of the bidirectional inverter 230 can be synchronized based on the EtherCAT protocol, a deterministic Ethernet-based fieldbus protocol. The EtherCAT master sends a telegram that passes through each node. Each EtherCAT slave reads the data addressed to it (e.g., inverter synchronization data) "on the fly" and inserts its data into the frame as it moves downstream. The frame is delayed only by the hardware propagation delay time. The last node in the segment (or tap line) detects an open port and sends a message back to the EtherCAT master using the full-duplex nature of Ethernet technology.

[0030] The EtherCAT protocol specifies an update time, which may also be referred to as a cycle time. In one embodiment, the update time may be less than or equal to 100 ps. The EtherCAT protocol also specifies communication jitter for precise synchronization purposes. In one embodiment, the communication jitter may be specified to be less than or equal to 1 μs. The EtherCAT protocol also specifies a data rate greater than 100 Mbit / s, which is greater than 90% of the user data rate of 2×100 Mbit / s. For the EtherCAT protocol, a typical network update rate may be 1 kHz-30 kHz. The EtherCAT protocol is standardized under IEC 61158.

[0031] Figure 3A decentralized coordinated control architecture 300 is shown for use in the control system of the present disclosure. The decentralized coordinated control architecture 300 avoids communication and increases the modularity of the control system. The decentralized coordinated control architecture 300 includes a decentralized coordinated controller 310 that measures the voltage and current of the grid and the local inverter 230. The decentralized coordinated controller 310 can execute a control algorithm including a robust tube prediction algorithm to account for the uncertainty of other local inverters that are not connected to a given local inverter 230. In addition, the control algorithm of the decentralized coordinated controller 310 can use a polynomial droop curve for each inverter and adjust the output of the decentralized coordinated controller 310 according to the predicted DC load (e.g., solar load and battery load).

[0032] In one aspect, the decentralized coordination control architecture 300 can be configured to implement aspects of EtherCAT technology. For example, one of the decentralized coordination controllers 310 can function as an EtherCAT master, and the remaining decentralized coordination controllers 310 can function as EtherCAT slaves. In this configuration, the decentralized coordination controllers 310 can coordinate control of the inverters 230 via the local controllers 220.

[0033] In another aspect, the decentralized coordination control architecture 300 can be controlled according to the following control method. The control method includes locally controlling the inverters 230 coupled in parallel between a common DC bus and a common AC bus by the local controller 220. The control method also includes measuring the voltage and current of the power grid coupled to the common AC bus and the voltage and current of the inverter 230 by the decentralized coordination controller 310 coupled to the local controller 220. The control method also includes generating, by the decentralized coordination controller 310, a decentralized coordination control signal for the local controller 220 based on the measured voltage and current of the power grid and the inverter 230. The control method also includes synchronously driving the inverters 230 coupled in parallel by the decentralized coordination controller 310 using the decentralized coordination control signal according to the EtherCAT protocol, whereby one of the decentralized coordination controllers 310 acts as an EtherCAT master controller and the remaining decentralized coordination controllers 310 act as EtherCAT slave controllers.

[0034] Figure 4A and Figure 4B is an example of a graph of a droop curve used by the control system of the present disclosure. The droop curve can be represented as ω m –ω 基准 =-g(P m –P 基准 ) and V m –V 基准 =-h(Qm –Q 基准 ), where P 基准 and Q 基准 are the set points for active and reactive power, ω 基准 and V 基准 are the set points of frequency and inverter voltage amplitude, and g and h are the droop curve slopes. Figure 4A The graph shows the P-ω droop characteristic, and Figure 4B The QV droop characteristic is shown. The droop curve can be used to generate a droop control algorithm or a polynomial droop control algorithm, which can be incorporated into or used with the centralized coordinated control method or the decentralized coordinated control method of the present disclosure.

[0035] Figures 5A-7 Circuit diagram of a system that combines centralized control features with decentralized control features. Figure 5A As shown in the parallel inverter system 510, a plurality of PV panels (e.g., Figure 1 The power outputs 511 of the photovoltaic panel array 21 (e.g., a photovoltaic panel array 21) are electrically coupled to a common DC bus 513. A load 512 (e.g., a motor of a solar tracker) can be electrically coupled in parallel with each of the power outputs. The common DC bus 513 is in turn electrically coupled to a plurality of inverters 514, each of which includes a power switching device (e.g., a power transistor). The inverters 514 are in turn electrically coupled to LCL filters 516, which filter the current output from the inverters 514 to improve the quality of the current output from the inverters 514. The inverters 514 step up the voltage and supply AC power (e.g., three-phase AC power) to the grid 518.

[0036] Figure 5B Shown for operation Figure 5A The power switching devices of the inverter 514 (eg, Figure 8 814, 816 pairs of power switching devices 811, 812, 813) of the feedback control system 520. The current 528 through the inductor of the LCL filter 516 (or through Figure 8 The current i of the inductors L1 and L2 of the series RL circuit 820a and 820b is L1 、i L2 ) and the voltage 528 across the capacitor (e.g., vc) of the LCL filter 516 are measured at the output of the inverter 527 and fed to a centralized controller 521 including a microcontroller 522 via a feedback line 529.

[0037] The microcontroller 522 can execute Figure 25. The microcontroller 522 then outputs a control command signal to the decentralized coordination controller 310. The control command signal may include a desired voltage value 524. The control command signal may also include a desired voltage value 523, which is applied to other decentralized controllers 525, which control other corresponding inverters 527 arranged in parallel according to the present disclosure.

[0038] The decentralized coordination controller 310 generates switching signals to drive the operation of the power switching devices. The decentralized coordination control method executed by the decentralized controller 525 can be executed at a faster speed than the centralized coordination control method executed by the centralized controller 521. For example, the decentralized coordination control method can be executed by the decentralized controller 525 in the kHz range, while the centralized control method can be executed by the centralized controller 521 in the Hz range or more (e.g., every 10 seconds).

[0039] Figure 5B The microcontroller 522 executes various control methods or algorithms. Figure 6 As shown in FIG, the microcontroller 522 may execute a space vector modulation (SVM) algorithm that generates an odd-order 602 harmonic voltage control signal 524 or an even-order 602 harmonic voltage control signal 604. Alternatively, the microcontroller 522 may execute a sinusoidal pulse width modulation (SPWM) algorithm, a space vector PWM (SVPWM) algorithm, or a discontinuous PWM (DPWM) algorithm.

[0040] like Figure 7 As shown, the microcontroller 522 generates a voltage control signal or waveform 702 that maintains a fixed DC voltage V on the common DC bus 513. DC 704. The voltage control signal or waveform 702 may also cause a zero sequence current. The voltage control signal or waveform 702 may also evenly distribute the current (i L1 =i L2 =i Ln In some aspects, the microcontroller 522 can be programmed to generate a voltage control signal or waveform 702 that distributes current as needed. The microcontroller 522 also controls a converter (not shown) to provide and feed a DC voltage, which can be a fixed DC voltage, to the input of the inverter 514.

[0041] Figure 8 A parallel inverter system 810 is shown in accordance with aspects of the present disclosure. Figure 8Although a dual-inverter configuration is shown, the parallel inverter system 810 may include more than two inverters placed in parallel. Each inverter 810a, 810b includes three pairs 811, 812, 813 of series-connected power switches 814, 816, which may be implemented using suitable semiconductor switching devices. The three pairs 811, 812, 813 of series-connected power switches 814, 816 are connected in parallel with each other.

[0042] A series RL circuit 820a including an inductor 822 and a resistor 824 is coupled to each connection point between each pair of power switches 811, 812, 813. The inverter 810b also includes a series RL circuit 820b.

[0043] Each inverter 810a, 810b includes three outputs (a, b, c) coupled to each of the series RL circuits. Each of the three outputs is coupled to the grid and provides a voltage V 电网 The grid 809 provides one phase of a three-phase AC output signal. A load 802 is placed in parallel with the input of each inverter 810a, 810b via a common DC bus 806, 808. The load 802 can be positive or negative. In addition, the parallel inverter system 800 is controlled so that a constant DC voltage is applied to the inverter input.

[0044] Figure 9 9 is a flow chart of a control method 900 according to aspects of the present disclosure. At block 902, local controllers locally control inverters coupled in parallel between a common DC bus and a common AC bus. At block 904, decentralized controllers, each coupled to the local controller, measure a voltage and current of a power grid coupled to the common AC bus and a voltage and current of an inverter. At block 906, the decentralized controllers generate decentralized control signals for the local controllers based on the measured voltages and currents of the power grid and the inverter. At block 908, a centralized controller in communication with the local controllers predicts a DC load. Then, at block 910, the centralized controller sends centralized control signals to the local controllers to maintain a constant voltage on the common DC bus based on the predicted DC load.

[0045] It should be understood that the various aspects disclosed herein can be combined in combinations different from those specifically presented in the specification and drawings. It should also be understood that, depending on the example, certain actions or events of any of the processes and methods described herein may be performed in a different order, may be added, merged, or omitted altogether (e.g., all described actions or events may not be necessary for the practice of the technology). In addition, although certain aspects of the present disclosure are described as being performed by a single module or unit for purposes of clarity, it should be understood that the technology of the present disclosure may be performed by a combination of units or modules associated with, for example, a medical device.

[0046] In one or more examples, the techniques can be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions can be stored as one or more instructions or codes on a computer-readable medium and executed by a hardware-based processing unit. A computer-readable medium may include a non-transitory computer-readable medium, which corresponds to a tangible medium such as a data storage medium (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer).

[0047] Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general-purpose microprocessors, application-specific integrated circuits (ASICs), field-programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuits. Thus, as used herein, the term "processor" may refer to any of the foregoing structures or any other physical structure suitable for implementing the described techniques. Additionally, the techniques may be fully implemented in one or more circuits or logic elements.

Claims

1. An electronic system, comprising: Inverters, the inverters are coupled together in parallel; a common DC bus coupled to an input of the inverter; a common AC bus coupled between the output of the inverter and a power grid; local controllers, the local controllers being coupled to the inverters respectively; decentralized controllers, each coupled to the local controllers, configured to measure voltages and currents of the power grid and the inverter, and to generate decentralized control signals for the local controllers based on the measured voltages and currents of the power grid and the inverter; and A centralized controller is in communication with the local controllers and is configured to predict a DC load and send a centralized control signal to the local controllers to maintain a constant voltage on the common DC bus based on the predicted DC load. 2 . The electronic system according to claim 1 , wherein the inverter is a three-phase inverter. 3 . The electronic system according to claim 1 , wherein the DC load comprises a photovoltaic device, an energy storage device, or the photovoltaic device and the energy storage device.

4. The electronic system of claim 1, wherein the centralized controller operates at a slower speed than the decentralized controllers. The electronic system according to claim 1 , wherein the number of the inverters is greater than 20. The electronic system according to claim 1 , wherein the number of the inverters is greater than 40. 7 . The electronic system of claim 1 , wherein the centralized control signal is a voltage control signal that causes current to be evenly distributed at the output of the inverter. 8 . The electronic system of claim 1 , wherein the centralized control signal is a voltage control signal that causes the current at the output of the inverter to track a reference current.

9. The electronic system of claim 1, wherein the centralized controller executes a polynomial droop control algorithm. 10 . The electronic system of claim 1 , wherein each decentralized controller implements a droop control algorithm or a polynomial droop control algorithm.

11. The electronic system according to claim 1 , wherein the centralized controller is incorporated into one of the local controllers and serves as a master controller, wherein the remaining local controllers among the local controllers serve as slave controllers, and The local controllers communicate with each other via the EtherCAT protocol.

12. A method for controlling an electronic system, the method comprising: locally controlling, by a local controller, inverters coupled in parallel between a common DC bus and a common AC bus; measuring, by a decentralized controller coupled to the local controller, a voltage and a current of a power grid coupled to the common AC bus and a voltage and a current of the inverter, respectively; generating, by the decentralized controller, decentralized control signals for the local controller based on the measured voltages and currents of the power grid and the inverter; forecasting DC loads by a centralized controller in communication with the local controllers; and A centralized control signal is sent by the centralized controller to the local controller to maintain a constant voltage on the common DC bus based on the predicted DC load.

13. The method of claim 12, wherein the inverter is a three-phase inverter. 14 . The method according to claim 12 , wherein the DC load comprises a photovoltaic device, an energy storage device, or the photovoltaic device and the energy storage device.

15. The method of claim 12, further comprising operating the centralized controller at a slower speed than the decentralized controllers. 16 . The method of claim 12 , wherein the centralized control signal is a voltage control signal that causes current to be evenly distributed at the output of the inverter. 17 . The method of claim 12 , wherein the centralized control signal is a voltage control signal that causes the current at the output of the inverter to track a reference current.

18. The method of claim 12, further comprising executing a polynomial droop control algorithm by the centralized controller.

19. The method of claim 12, further comprising executing a droop control algorithm or a polynomial droop control algorithm by each decentralized controller.

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