Dc to ac conversion apparatus and method based on battery dynamic access

Abstract

This invention discloses a DC-AC conversion device and method based on dynamic battery access, belonging to the field of power electronics technology. The device includes: a battery module (10) containing multiple independently connectable battery sub-modules; a switch array module (20) for selectively accessing battery modules, changing output polarity, and controlling capacity balance; and a control module (30) that dynamically controls the switch array module (20) to access different combinations of battery sub-modules according to the target AC waveform and battery state (e.g., SOC), and controls the switch array module (20) to switch polarity to directly synthesize a stepped or multi-level AC voltage waveform. Optionally, the device also includes a filter module (40) and a BMS module (50). This invention avoids high-frequency switching losses and dependence on transformers through low-frequency dynamic access, polarity switching, and capacity balance, achieving high-efficiency, low-cost, simple-to-control, high- and low-voltage adaptability, and low electromagnetic interference DC-AC conversion. It is applicable to fields such as solar power generation, energy storage systems, electric vehicles, and microgrids.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a device and method for efficiently converting direct current (DC) to alternating current (AC). More specifically, this invention relates to DC-AC conversion using dynamic connection and polarity switching technology of battery modules, applicable to applications requiring the conversion of low-voltage or high-voltage DC to AC, such as solar power generation, wind power generation, electric vehicles, energy storage systems, uninterruptible power supplies (UPS), and microgrids. Background Technology

[0002] Existing DC-AC conversion technologies mainly include:

[0003] (1) Scheme based on high-frequency switching modulation (such as pulse width modulation PWM) and transformer boosting: This scheme uses high-frequency switches (such as MOSFETs or IGBTs) to chop the DC voltage and uses filters to generate AC waveforms. If boosting is required, an additional transformer is usually needed. The main disadvantages of this scheme are: high-frequency switching operation leads to significant switching losses, reducing conversion efficiency; the high-frequency switching devices themselves and their drive circuits increase cost and complexity; the use of transformers increases the size, weight and cost of the device and may introduce additional losses; high-frequency switching is prone to electromagnetic interference (EMI) problems.

[0004] (2) Modular Multilevel Inverter (MMC): MMC synthesizes multilevel AC voltage waveforms by cascading multiple sub-modules (usually including capacitors and switches), which has advantages such as good output waveform quality, low harmonic content, and high efficiency. However, MMC technology also has inherent challenges: the control is extremely complex, especially requiring precise sub-module capacitor voltage equalization control strategies to ensure stable system operation and voltage equalization of each module; the system cost is high, with a large number of sub-modules and their control and drive circuits leading to a significant increase in hardware costs; for scenarios requiring a wide range of voltage input (especially low voltage input), its efficiency and cost advantages may not be obvious, and its adaptability is relatively poor.

[0005] Therefore, existing technologies still have shortcomings in achieving efficient, low-cost, simple-to-control DC-AC conversion that can adapt to a wide voltage range input. Especially in applications that directly utilize battery energy storage for DC-AC conversion, how to avoid high-frequency switching losses and transformer dependence, while simplifying control and reducing costs, is a pressing technical problem that needs to be solved. Summary of the Invention

[0006] (I) Technical Issues

[0007] This invention aims to solve the problems of high switching losses, low efficiency (especially in high-frequency switching schemes), complex control (especially in MMC schemes), high cost (due to high-frequency devices, transformers or a large number of sub-modules), and limited adaptability to high and low voltage inputs in existing DC-AC conversion technologies. It provides a high-efficiency, reliable, low-cost DC-AC conversion device and method that is suitable for a wide voltage range input.

[0008] (II) Technical Solution

[0009] To address the aforementioned technical problems, this invention provides a DC-to-AC conversion device based on dynamic battery access, comprising:

[0010] (1) Battery module: It has a specific voltage output capability and contains at least two battery sub-modules that can be independently connected or disconnected. Each battery sub-module consists of one or more individual battery cells connected in series (at least one).

[0011] (2) Switching array module: Connected to each battery sub-module and load of the battery pack module, it is used to selectively connect or disconnect one or more battery sub-modules to the output circuit according to the control signal. Simultaneously, it changes the polarity of the output voltage according to the control signal, and its switching frequency is the same as or a low multiple of the fundamental frequency of the target AC waveform (e.g., switching polarity only near the voltage zero-crossing point). Furthermore, it adjusts the control strategy according to the state of charge (SOC) or other capacity-related parameters of each battery sub-module (e.g., when selecting battery sub-modules to connect, prioritizing the discharge of battery sub-modules with higher SOCs, or rotating the use of each sub-module) to ensure balanced output or consistent SOC among the battery sub-modules, thereby extending battery life. The switching array module may employ low-frequency switching devices, such as MOSFETs, IGBTs, or relays.

[0012] (3) Control Module: Electrically connected to and controls the switch array module, BMS module, and filtering and compensation module. The control module is configured to dynamically determine the combination of battery sub-modules to be connected at different times and the polarity of the output voltage based on the target AC voltage waveform (e.g., sine wave), polarity, and the state of each battery sub-module (e.g., state of charge, SOC), and generate corresponding control signals to drive the switch array module and the switch array module to synthesize a stepped or multi-level AC voltage waveform, while ensuring the capacity balance of the battery sub-modules, i.e., balanced output or consistent SOC, to extend battery life. This control module preferably adopts a low-frequency control strategy.

[0013] (4) Filtering and compensation module (optional): Connected to the output of the switch array module, it is used to filter out high-order harmonics in the output voltage waveform, smooth the waveform, and may perform reactive power compensation to obtain a high-quality AC output that is close to the target waveform. For example, an LC low-pass filter can be used.

[0014] (5) BMS Module: This module performs the functions of a battery management system (BMS) to monitor the voltage, current, temperature, and other statuses of individual battery cells, battery sub-modules, and battery pack modules, and provides overvoltage, undervoltage, overcurrent, and overtemperature protection functions. It may also include overload and short-circuit protection circuits for the control module, switch array module, and load to ensure safe and reliable system operation.

[0015] Accordingly, the present invention also provides a DC-to-AC conversion method based on dynamic battery access, comprising the following steps:

[0016] (1) Battery grouping and configuration: The DC power supply (battery pack) is divided into multiple battery sub-modules that can be independently controlled to be connected or disconnected.

[0017] (2) Target waveform synthesis control:

[0018] a. Determine the access sequence: Based on the preset target AC voltage waveform (amplitude, frequency, phase), the control module calculates which battery sub-modules need to be connected in series in each time period of the AC waveform to approximate the target voltage value at that moment.

[0019] b. Control module access: According to the determined access sequence, the control module sends a control signal to the switch array module at the corresponding time point to turn on the switch corresponding to the required battery sub-module and connect it in series to the output circuit.

[0020] c. Balanced output control: The control module controls which battery submodules need to be connected in series and the connection order based on the real-time calculated current capacity (SOC) of each battery submodule, so that the capacity of each battery submodule is balanced.

[0021] d. Polarity Switching Control: The control module sends a control signal to the switch array module based on whether the target AC waveform is currently in the positive or negative half-cycle, to output a positive or negative voltage. Polarity switching is preferably performed at the voltage zero-crossing point or in the low-voltage region to reduce switching stress.

[0022] (3) Waveform filtering and compensation (optional): The synthesized stepped or multi-level voltage waveform is filtered and compensated to remove harmonics and obtain a smooth target AC voltage waveform.

[0023] (4) Real-time monitoring and protection: Continuously monitor the status (voltage, current, temperature, SOC) of individual battery cells, battery sub-modules and battery pack modules, execute capacity balancing and control adjustment access strategies, and trigger protection mechanisms to disconnect output or take other protection measures when abnormal conditions are detected.

[0024] (III) Beneficial effects.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) High efficiency: By directly synthesizing voltage through dynamic access to the battery sub-module, the high-frequency switching losses in the traditional PWM scheme are avoided. The switch array and switch array module can operate at a lower frequency (even the base frequency), which significantly reduces switching losses and improves the overall conversion efficiency of the system, making it especially suitable for applications with high efficiency requirements.

[0027] (2) Low cost: No expensive high-frequency switching devices and drive circuits are required; low-cost low-frequency switches can be used instead. More importantly, this solution can directly output the required voltage level (adjusted by the number of series battery modules), eliminating the need for step-up / step-down transformers in many applications, thus significantly reducing system cost, size, and weight.

[0028] (3) Simple control: Compared with the complex sub-module voltage equalization control of MMC, the equalization control based on the battery sub-module SOC of this invention is relatively simple. The control module mainly executes low-frequency switching logic, which reduces the complexity of the control algorithm and the requirements for processor performance.

[0029] (4) Good adaptability to high and low voltage: By adjusting the voltage specifications of a single battery sub-module and the number of series sub-modules, the present invention can be flexibly configured to adapt to different DC input voltage levels (low voltage or high voltage) and AC output voltage requirements, and can maintain good efficiency under different voltage levels.

[0030] (5) High reliability: Modular design facilitates redundancy. A robust SOC-priority discharge strategy or a rotating control strategy for each battery submodule, along with a BMS protection mechanism, ensures balanced and safe operation of the battery and system. Low-frequency operation reduces device stress, contributing to improved long-term system reliability.

[0031] (6) Low electromagnetic interference (EMI): The electromagnetic interference generated by low-frequency switching operation is much less than that of high-frequency PWM scheme, which simplifies the EMI filtering design requirements.

[0032] (7) Directly utilize battery characteristics: The output is constructed by directly using the battery module as the voltage source, eliminating the need for DC link capacitors and complex voltage balancing issues in the MMC, resulting in a simpler structure. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 : A block diagram of a DC-to-AC conversion device according to one embodiment of the present invention.

[0035] Figure 2 This is a schematic diagram of a battery pack module in this invention, showing the composition of the battery sub-module and the battery module.

[0036] Figure 3 This invention provides a timing diagram of dynamic access control, illustrating the logic of access and polarity switching for different sub-modules.

[0037] Figure 4 The schematic diagram of the output voltage waveform synthesis of this invention shows the stepped waveform and the filtered sine waveform.

[0038] Figure 5 This diagram illustrates a specific implementation of the switch array module of the present invention, demonstrating the implementation methods for connecting different battery sub-modules and switching polarity. Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] Implementation Case 1.

[0041] This embodiment provides a device for converting approximately 14.8V DC power into 220V / 50Hz AC power.

[0042] Device configuration:

[0043] Reference Figure 1 The device includes a battery pack module 10, a switch array module 20, a control module 30, a filter supplement module 40, a BMS module 50, and a load 60.

[0044] (1) Battery pack module 10: such as Figure 2As shown, it consists of multiple lithium-ion battery cells (nominal voltage 3.7V) connected in series. Assuming a peak voltage of approximately 311V is required, if four battery sub-modules are used, each sub-module would need approximately 22 3.7V battery cells connected in series (considering voltage fluctuations and margin), resulting in an output voltage of approximately 81V for each sub-module. In this example, four battery sub-modules (B1, B2, B3, B4) are configured, with each sub-module having a voltage of approximately 81V.

[0045] (2) Switch array module 20: such as Figure 5 As shown, the circuit consists of multiple bidirectional switches, which can control the timing, quantity, and polarity of each battery submodule's connection to the main circuit. Polarity control can also employ an H-bridge or similar circuit to switch voltage polarity.

[0046] (3) Control module 30: Utilizes MCU, FPGA, or DSP, with internal logic implementation as follows:

[0047] Store sampling point data or generation algorithm for the target 50Hz sine wave.

[0048] Real-time monitoring of SOC information from each module of the BMS.

[0049] Based on the instantaneous value of the target sine wave, determine the number N of modules that need to be connected (e.g., instantaneous voltage V ≈ N * V). i V i (Submodule voltage).

[0050] Select N specific modules based on their SOC (e.g., prioritize modules with high SOC).

[0051] A control signal is generated to drive the corresponding switch of the switch array module 20 to conduct, and the selected N modules are connected in series.

[0052] Based on whether the sine wave is in the positive or negative half-cycle, the corresponding switch (or H-bridge) is driven to achieve polarity output. The control frequency can be low, for example, synchronized with the step changes of the stepped waveform, or switching only at zero crossings, with a frequency much lower than the PWM frequency. In this example, if the number of steps is large, the frequency of the control switch array may be from several hundred hertz to several thousand hertz.

[0053] Execute the capacity balancing algorithm and adjust the priority of module selection.

[0054] (4) Filtering module 40: An LC low-pass filter is used to filter out the high-order harmonics in the stepped wave synthesized by the switch array and output a smooth 50Hz sine wave. For example, select appropriate L and C values ​​so that the cutoff frequency is slightly higher than 50Hz, such as 150Hz-300Hz.

[0055] (5) BMS module 50: Monitors the voltage and temperature of each battery, calculates the SOC, and provides overcharge, over-discharge, and over-temperature protection signals to the control module 30. At the same time, it monitors the output current to realize overcurrent and short-circuit protection.

[0056] Workflow:

[0057] (1) Positive half-cycle: The control module 30 determines the target sine wave instantaneous voltage value V_target(t)>0 based on the following:

[0058] The required number of modules N is approximately equal to V_target(t) / Vdc_module.

[0059] like Figure 3 According to the SOC balancing strategy, N sub-modules are selected from the four modules for access. For example, if one sub-module is needed at time t1, B1 with the highest SOC is selected for access; if two sub-modules are needed at time t2, B1 and B2 with the highest SOC are selected and connected in series; if three modules are needed at time t3, and so on.

[0060] The control module 30 drives the switch array 20 to turn on the corresponding switches.

[0061] The control module 30 drives the switch to output positive polarity.

[0062] like Figure 3 and Figure 4 As shown, the switch array outputs a positive stepped wave.

[0063] (2) Negative half-cycle: The control module 30 determines the target sine wave instantaneous voltage value V_target(t) < 0:

[0064] The required number of modules N ≈ |V_target(t)| / Vdc_module.

[0065] Based on the SOC load balancing strategy, select N modules for access.

[0066] The control module 30 drives the switch array 20 to turn on the corresponding switches.

[0067] The control module 30 drives the switch array module to output a negative polarity, and the polarity switching occurs when the voltage is close to zero.

[0068] The switch array outputs a negative stepped wave.

[0069] Filtering: The LC filter 40 filters the output stepped wave to obtain an approximate sine wave output of 220V / 50Hz.

[0070] Balancing and Protection: The BMS module 50 continuously monitors the system, and the control module 30 dynamically adjusts the module selection strategy based on SOC feedback to achieve balancing. In the event of a fault, the protection module is triggered, and the control module executes shutdown or other protective actions.

[0071] Implementation Case 2

[0072] This embodiment describes a method for converting a higher DC voltage (such as the battery cluster voltage in an energy storage system) into three-phase AC power.

[0073] Method and steps:

[0074] Battery grouping: The high-voltage battery cluster is divided into M battery modules with basically the same voltage level.

[0075] Three-phase independent control: Configure a separate set of switch arrays and switch array modules for each of the three phases (A, B, C). The control module needs to be able to independently control the output voltage synthesis of each phase.

[0076] Dynamic access and polarity switching (taking phase A as an example):

[0077] The control module calculates the combination and polarity of the sub-modules that need to be connected at time t based on the A-phase target sine wave V_A_target(t) = Vm * sin(ωt) and the SOC of each sub-module.

[0078] If V_A_target(t)>0, select N_A sub-modules to connect, and output positive voltage through the A-phase switch array module.

[0079] If V_A_target(t)<0, select N_A sub-modules to connect, and output negative voltage through the A-phase switch array module.

[0080] Phase B and Phase C perform similar operations, but their target waveforms are V_B_target(t) = Vm * sin(ωt- 120°) and V_C_target(t) = Vm * sin(ωt + 120°), respectively.

[0081] Capacity balancing: When selecting modules for three phases, the control module comprehensively considers the SOC status of all available sub-modules and implements a global or grouped capacity balancing strategy to ensure balanced discharge of the entire battery cluster.

[0082] Filtering: The stepped wave output of each phase passes through its own filter to obtain a three-phase balanced sinusoidal alternating current.

[0083] Digital compensation (optional): The control module can perform closed-loop control or digital compensation based on feedback from the output voltage and current to further correct waveform distortion and improve the quality of output power.

[0084] Monitoring and Protection: Continuously monitor the BMS, balance the SOC, and protect the system.

[0085] The above method can efficiently and flexibly convert high-voltage DC energy storage into three-phase AC power, which is suitable for grid-connected inverters or large UPS systems.

[0086] The core of this invention lies in utilizing the battery module itself as a switchable DC voltage source. Through low-frequency dynamic access and polarity switching, multi-level AC waveforms are directly synthesized, thereby avoiding the high-frequency switching losses and transformer dependence of traditional solutions. Simultaneously, it simplifies control, reduces costs, and exhibits good voltage adaptability and reliability. The above embodiments are merely specific application examples of this invention. Any modifications or improvements made by those skilled in the art based on the core ideas of this invention should fall within the protection scope of this invention.

Claims

1. A DC-to-AC conversion device based on dynamic battery access, characterized in that, include: (1) Battery module (10), comprising at least two battery sub-modules that can be independently controlled to be connected or disconnected, each battery sub-module providing DC voltage; (2) Switch array module (20), connected to each battery sub-module and load of the battery pack module, for selectively connecting one or more battery modules in series to the output circuit or disconnecting them according to the first control signal; and for changing the polarity of the output voltage of the output circuit according to the second control signal; (3) Control module (30), electrically connected to and generating the first control signal and the second control signal to control the switch array module (20) respectively; the control module (30) is configured to: dynamically determine the combination of battery modules to be connected at different times and the polarity of the output voltage according to the target AC voltage waveform and the state of the at least two battery modules, so as to synthesize a stepped or multi-level AC voltage waveform. It is also configured to perform capacity balancing control, for monitoring the state of charge (SOC) of the at least two battery modules, and adjusting the first control signal according to the state of charge, and adopting a closed-loop control algorithm based on SOC sorting or SOC prediction to balance the output of each battery module. (4) Control capacity balance: When the control module (30) adjusts the selected battery sub-module combination according to the SOC of each battery sub-module, the determined battery sub-module combination is connected to the output circuit to achieve output balance.

2. The apparatus according to claim 1, characterized in that, The switch array module (20) contains multiple low-frequency bidirectional switching devices or H-bridges.

3. The apparatus according to claim 1, characterized in that, The control module (30) is implemented using a microprocessor (MCU), digital signal processor (DSP) or field programmable gate array (FPGA) and executes a low-frequency control strategy. The switching frequency of the switching array module (20) is basically the same as or a low multiple of the fundamental frequency of the target AC voltage waveform.

4. A DC-to-AC conversion method based on dynamic battery access, characterized in that, Includes the following steps: (1) Module division: Divide the DC battery pack module into multiple (at least two) battery sub-modules that can be independently controlled to be connected or disconnected, so as to achieve a balance between the target AC voltage waveform requirements and control complexity; (2) Determine the access sequence: The control module (30) dynamically determines the combination of battery modules that need to be accessed at different times based on the target AC voltage waveform and the status of the at least two battery modules; (3) Control capacity balance: When the control module (30) adjusts the selected battery sub-module combination according to the SOC of each battery sub-module, the determined battery sub-module combination is connected to the output circuit to achieve output balance; (4) Control polarity: The control module (30) generates a second control signal to drive the switch array module (20) to adjust the polarity of the output voltage of the output circuit according to the determined polarity requirements; (5) By repeatedly performing the control access, control capacity balancing and control polarity steps, a stepped or multi-level AC voltage waveform is synthesized.

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