A combined power supply system of photovoltaic direct current access electrolytic aluminum and a control method thereof

Abstract

The application provides a combined power supply system for photovoltaic direct-current access electrolytic aluminum and a control method thereof, and the system comprises a photovoltaic power supply module, a multi-module LLC resonant converter, a direct-current transmission line, a voltage reduction current conversion device, an electrolytic cell load module and an alternating current rectification system; the photovoltaic power supply module is composed of a photovoltaic cell array and a Boost voltage increasing circuit; the multi-module LLC resonant converter is composed of N LLC sub-modules in an input parallel and output series mode; the direct-current transmission line is used for long-distance transmission of medium-voltage direct current from a photovoltaic power station to an electrolytic aluminum plant; the voltage reduction current conversion device is used for voltage reduction and conversion of the medium-voltage direct current into required voltage to supply power to the electrolytic cell load module; and the output end of the alternating current rectification system is connected in parallel with the output end of the voltage reduction current conversion device and connected to the electrolytic cell load module. The technical scheme provided by the application realizes long-distance transmission of photovoltaic direct current and direct coupling of the electrolytic aluminum industry, and improves energy utilization efficiency and system stability.

Description

Technical Field

[0001] This application relates to the fields of power supply technology in the electrolytic aluminum industry and energy-saving technology in the field of power system supply, and in particular to a combined power supply system for photovoltaic DC access to electrolytic aluminum and its control method. Background Technology

[0002] With the deepening of the national green and low-carbon development strategy, increasing the proportion of renewable energy utilization in the electrolytic aluminum industry has become a core direction for industry development. Currently, electrolytic aluminum enterprises mainly adopt two photovoltaic power supply schemes: one is to invert and boost the photovoltaic DC power before connecting it to the AC power grid of the plant. However, this scheme requires multiple conversion stages, resulting in numerous intermediate conversion links, large losses, and reliance on the AC power grid for power balance, making the system structure complex. The other is to directly connect the low-voltage photovoltaic DC power to the DC bus of the electrolytic aluminum plant for local consumption. This scheme has been demonstrated and verified in some enterprises, but due to the line loss problem of low-voltage DC transmission, it is only suitable for scenarios where photovoltaic power stations are located near the electrolytic aluminum plant. It cannot achieve cross-regional allocation of photovoltaic resources, and the photovoltaic energy conversion efficiency and power supply stability of the existing scheme still need to be improved.

[0003] To overcome these limitations, some studies have proposed using multi-module DC / DC converters to achieve photovoltaic DC boosting followed by transmission. However, existing solutions mostly employ an input-series-output-parallel (ISOP) architecture, primarily suitable for high input voltage stress scenarios, and difficult to adapt to the application requirements of low-voltage input on the photovoltaic side and medium-voltage output on the load side. Furthermore, the voltage levels of existing operational projects are mostly 20kV and below, with power capacities of 11.68MW and below, which cannot meet the needs of ±30kV medium voltage, 50MW and above high-power long-distance transmission scenarios. In addition, existing solutions still lack systematic design in areas such as bus voltage stability control during photovoltaic output fluctuations, and fault isolation and transient recovery in AC / DC combined power supply. Therefore, there is an urgent need to propose a comprehensive solution that can achieve efficient long-distance transmission of photovoltaic DC medium voltage, adapt to high-power expansion needs, and possess AC / DC combined power supply fault protection capabilities. Summary of the Invention

[0004] This application provides a combined power supply system and control method for photovoltaic DC-connected electrolytic aluminum, which at least solves the technical problems of existing photovoltaic DC-connected electrolytic aluminum technology when facing medium and long distance applications, such as limited transmission distance, high line loss, and strict site selection due to the use of low-voltage transmission or AC grid connection mode, as well as the insufficient expansion capability of existing converter architecture (such as single-module LLC or ISOP structure) when adapting to ±30kV medium voltage and 50MW-level high power output, and the lack of effective bus voltage stability control and AC / DC combined power supply fault isolation mechanism, resulting in poor system efficiency and power supply reliability.

[0005] The first aspect of this application proposes a combined power supply system for photovoltaic DC access to electrolytic aluminum, the system comprising:

[0006] The photovoltaic power supply module consists of a photovoltaic cell array and a boost circuit. The boost circuit is configured with a voltage-controlled MPPT algorithm and a dual-closed-loop staged control strategy to achieve maximum power point tracking of the photovoltaic cell array. The multi-module LLC resonant converter consists of N LLC sub-modules. The input terminals of the N LLC sub-modules are connected in parallel to the output bus of the photovoltaic power supply module, and the output terminals are connected in series to form an input parallel output series architecture, which is used to boost the DC power output by the photovoltaic power supply module to medium voltage DC power. The DC transmission line, whose input end is connected to the output end of the multi-module LLC resonant converter, is used to transmit medium-voltage DC power from the photovoltaic power station to the electrolytic aluminum plant area over a long distance. A step-down converter, whose input terminal is connected to the output terminal of the DC transmission line, is used to step down and convert medium-voltage DC power into a voltage that meets the requirements of the electrolytic aluminum production process. The electrolytic cell load module has its input terminal connected to the output terminal of the step-down converter, and is used to receive DC power for aluminum electrolysis production. The output of the 330kV substation AC rectifier system is connected in parallel with the output of the step-down converter and then connected to the electrolytic cell load module to form an AC / DC combined power supply architecture.

[0007] Preferably, the LLC submodule includes: High-frequency transformers are used to achieve voltage transformation and electrical isolation; The high-voltage side power unit consists of two half-bridge silicon carbide modules forming an H-bridge topology, which is connected to the primary side of the high-frequency transformer. The low-voltage side power unit consists of two half-bridge silicon carbide modules forming an H-bridge topology, which is connected to the secondary side of the high-frequency transformer. The high-voltage side resonant capacitor and the low-voltage side resonant capacitor are respectively configured on the high-voltage side and the low-voltage side of the high-frequency transformer, and are used to form a resonant circuit together with the leakage inductance of the high-frequency transformer. The leakage inductance of the high-frequency transformer serves as a resonant inductor, and the resonant circuit of the LLC submodule is composed of the leakage inductance and the resonant capacitor, eliminating the need for an additional independent resonant inductor.

[0008] Furthermore, the electrolytic cell load module adopts the cryolite-alumina molten salt electrolysis method, with the operating temperature controlled between 950℃ and 970℃, and the cell voltage of the electrolytic cell satisfies the formula... Where Upol is the polarization voltage, Rcell is the cell resistance, and Icell is the electrolysis current. The polarization voltage Upol is 30% of the cell terminal voltage. By controlling the electrolyte concentration, cell temperature, and electrode distance, both the cell resistance Rcell and the polarization voltage Upol are kept constant to ensure a stable electrolysis current.

[0009] A second aspect of this application provides a control method for a photovoltaic DC-connected electrolytic aluminum combined power supply system, the method comprising: The boost circuit of the photovoltaic power supply module is activated, and the boost circuit is controlled based on the maximum power point tracking algorithm to make the photovoltaic cell array operate at the maximum power point; The boost circuit is controlled by a dual closed-loop staged control strategy that combines the inner current loop and the outer voltage loop: during the first period after system startup, the output voltage of the boost circuit is used as the feedback quantity of the outer voltage loop to control the output voltage of the boost circuit to approach the bus voltage set value; after the first period, the output voltage of the photovoltaic array is switched to be used as the feedback quantity of the outer voltage loop to control the photovoltaic array to operate at the maximum power point. The DC power output from the boost circuit is input into the multi-module LLC resonant converter, which controls each sub-module to operate in parallel input and series output mode, boosting the DC power to a preset medium voltage level before outputting it. The boosted medium-voltage DC power is transmitted to the aluminum electrolysis plant area, and after being stepped down and converted, it supplies power to the electrolytic cell load. The AC rectifier system is controlled to operate in parallel with the DC power supply to form a combined AC and DC power supply mode.

[0010] Preferably, the maximum power point tracking algorithm is a voltage-controlled MPPT algorithm, which samples the output voltage of the photovoltaic array in real time and compares it with the maximum power point voltage Vmpp. Based on the comparison result, the duty cycle of the power switching device in the boost circuit is dynamically adjusted so that the operating voltage of the photovoltaic array approaches and stabilizes at Vmpp.

[0011] Preferably, the first time period is the first 3 seconds after the system starts; The multi-module LLC resonant converter is composed of LLC sub-modules connected in parallel at the input and in series at the output, and the preset medium voltage level is ±30kV.

[0012] Preferably, in the dual-loop staged control strategy, the voltage outer loop uses a PI regulator to adjust the feedback quantity, and the current inner loop is used to quickly respond to current commands to suppress load disturbances.

[0013] Preferably, the AC / DC combined power supply mode includes: When a short-circuit fault occurs on the photovoltaic DC bus, the AC rectifier system is controlled to independently supply power to the electrolytic cell load; When a short circuit fault occurs in the electrolytic aluminum busbar, the control subsystems will be adjusted and then restored to steady-state operation.

[0014] Preferably, the dual-closed-loop phased control strategy employs a soft-switching method during the transition from the first time period to the second time period, so that the reference value of the outer voltage loop smoothly transitions from the bus voltage setpoint to the reference value corresponding to the maximum power point voltage.

[0015] Preferably, each sub-module of the multi-module LLC resonant converter adopts a voltage and current equalization control strategy. The output voltage of each sub-module is kept balanced through master-slave control, and the input terminals are connected in parallel to make the input current of each sub-module naturally equalized. When any LLC submodule of the multi-module LLC resonant converter fails, the bypass switch of the failed submodule is closed to bypass the output of the failed submodule and cause it to stop operating, while the non-faulty LLC submodules continue to operate.

[0016] The technical solutions provided by the embodiments of this application bring at least the following beneficial effects: This application proposes a combined power supply system and control method for photovoltaic DC-DC integrated electrolytic aluminum. The system includes: a photovoltaic power supply module, consisting of a photovoltaic cell array and a boost circuit. The boost circuit is configured with a voltage-controlled MPPT algorithm and a dual-closed-loop staged control strategy to achieve maximum power point tracking of the photovoltaic cell array; and a multi-module LLC resonant converter, consisting of N LLC sub-modules. The input terminals of the N LLC sub-modules are connected in parallel to the output bus of the photovoltaic power supply module, and their output terminals are connected in series to form an input-parallel-output-series architecture, used to boost the DC power output from the photovoltaic power supply module. The system comprises: a medium-voltage DC power supply; a DC transmission line, the input of which is connected to the output of the multi-module LLC resonant converter, used to transmit medium-voltage DC power from the photovoltaic power station to the electrolytic aluminum plant over a long distance; a step-down converter, the input of which is connected to the output of the DC transmission line, used to step down the medium-voltage DC power to a voltage that meets the requirements of the electrolytic aluminum production process; an electrolytic cell load module, the input of which is connected to the output of the step-down converter, used to receive DC power for electrolytic aluminum production; and a 330kV substation AC rectifier system, the output of which is connected in parallel with the output of the step-down converter and then connected to the electrolytic cell load module, forming an AC / DC combined power supply architecture. The technical solution proposed in this application enables long-distance, efficient transmission of photovoltaic energy, improves energy conversion efficiency, and enhances system fault adaptability and operational stability.

[0017] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0018] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein: Figure 1 This is a structural diagram of a combined power supply system for photovoltaic DC-DC-connected electrolytic aluminum according to an embodiment of this application; Figure 2 This is a detailed structural schematic diagram of a Boost converter circuit according to an embodiment of this application; Figure 3 A structural diagram of an LLC submodule is provided according to one embodiment of this application; Figure 4 This is a flowchart of a control method for a photovoltaic DC-connected electrolytic aluminum combined power supply system according to an embodiment of this application; Figure 5 This is a control block diagram of a photovoltaic DC power supply section according to an embodiment of this application; Figure 6 A diagram of the photovoltaic DC bus voltage during steady-state operation according to an embodiment of this application; Figure 7 A diagram of the photovoltaic DC-DC input output voltage during steady-state operation according to an embodiment of this application; Figure 8 A diagram of the photovoltaic DC-DC-connected output current during steady-state operation according to an embodiment of this application; Figure 9 A diagram of photovoltaic DC-DC grid-connected output power during steady-state operation according to an embodiment of this application; Figure 10 This is a voltage diagram of the electrolytic cell when the busbar of an electrolytic aluminum electrolysis unit is short-circuited for 10 seconds, according to one embodiment of this application. Figure 11 This is a diagram of the electrolytic cell current during a 10s busbar short circuit according to an embodiment of this application. Figure 12 This is a power diagram of an electrolytic cell when the busbar of an electrolytic aluminum electrolysis unit is short-circuited for 10 seconds, according to an embodiment of this application. Figure 13 A diagram of the photovoltaic DC bus voltage during a 10s electrolytic aluminum bus short circuit according to an embodiment of this application; Figure 14 This is a voltage diagram of a 10s photovoltaic DC bus under short circuit according to an embodiment of this application; Figure 15 This is a voltage diagram of an electrolytic cell during a 10s photovoltaic DC bus short circuit, according to one embodiment of this application. Figure 16 This is a current diagram of an electrolytic cell during a 10s photovoltaic DC bus short circuit, provided according to one embodiment of this application. Figure 17 This is a power diagram of an electrolytic cell during a 10s photovoltaic DC bus short circuit, according to one embodiment of this application. Figure Labels Photovoltaic power supply module 1, photovoltaic cell array 1-1, Boost boost circuit 1-2, multi-module LLC resonant converter 2, LLC sub-module 2-1, high-frequency transformer 2-1-1, high-voltage side power unit 2-1-2, low-voltage side power unit 2-1-3, high-voltage side resonant capacitor 2-1-4, low-voltage side resonant capacitor 2-1-5, DC transmission line 3, step-down converter 4, electrolytic cell load module 5, 330kV substation AC rectification system 6. Detailed Implementation

[0019] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0020] This application proposes a combined power supply system and control method for photovoltaic DC-DC integrated electrolytic aluminum. The system includes: a photovoltaic power supply module, consisting of a photovoltaic cell array and a boost circuit. The boost circuit is configured with a voltage-controlled MPPT algorithm and a dual-closed-loop staged control strategy to achieve maximum power point tracking of the photovoltaic cell array; and a multi-module LLC resonant converter, consisting of N LLC sub-modules. The input terminals of the N LLC sub-modules are connected in parallel to the output bus of the photovoltaic power supply module, and their output terminals are connected in series to form an input-parallel-output-series architecture, used to boost the DC power output from the photovoltaic power supply module. The system comprises: a medium-voltage DC power supply; a DC transmission line, the input of which is connected to the output of the multi-module LLC resonant converter, used to transmit medium-voltage DC power from the photovoltaic power station to the electrolytic aluminum plant over a long distance; a step-down converter, the input of which is connected to the output of the DC transmission line, used to step down the medium-voltage DC power to a voltage that meets the requirements of the electrolytic aluminum production process; an electrolytic cell load module, the input of which is connected to the output of the step-down converter, used to receive DC power for electrolytic aluminum production; and a 330kV substation AC rectifier system, the output of which is connected in parallel with the output of the step-down converter and then connected to the electrolytic cell load module, forming an AC / DC combined power supply architecture. The technical solution proposed in this application enables long-distance, efficient transmission of photovoltaic energy, improves energy conversion efficiency, and enhances system fault adaptability and operational stability.

[0021] The following description, with reference to the accompanying drawings, illustrates a combined power supply system and control method for photovoltaic DC-connected electrolytic aluminum, according to an embodiment of this application.

[0022] Example 1 Figure 1 This is a structural diagram of a combined power supply system for photovoltaic DC-connected electrolytic aluminum according to an embodiment of this application, as shown below. Figure 1 As shown, the system includes: The photovoltaic power supply module 1 consists of a photovoltaic cell array 1-1 and a boost circuit 1-2. The boost circuit 1-2 is configured with a voltage-controlled MPPT algorithm and a dual closed-loop staged control strategy to achieve maximum power point tracking of the photovoltaic cell array. The multi-module LLC resonant converter 2 consists of N LLC sub-modules 2-1. The input terminals of the N LLC sub-modules 2-1 are connected in parallel to the output bus of the photovoltaic power supply module 1, and the output terminals are connected in series to form an input parallel output series architecture, which is used to boost the DC power output by the photovoltaic power supply module to medium voltage DC power. DC transmission line 3, whose input end is connected to the output end of the multi-module LLC resonant converter 2, is used to transmit medium-voltage DC power from the photovoltaic power station to the electrolytic aluminum plant area over a long distance; The step-down converter 4 has its input end connected to the output end of the DC transmission line 3, and is used to step down and convert medium-voltage DC power into a voltage that meets the requirements of the electrolytic aluminum production process. The electrolytic cell load module 5 has its input end connected to the output end of the step-down converter 4, and is used to receive DC power for electrolytic aluminum production. The output of the 330kV substation AC rectifier system 6 is connected in parallel with the output of the step-down converter 4 and then connected to the electrolytic cell load module 5 to form an AC / DC combined power supply architecture.

[0023] It should be noted that the DC transmission line 3 is a ±30kV DC transmission line.

[0024] It should be noted that, depending on the actual transmission distance between the photovoltaic power station and the electrolytic aluminum plant, the ±30kV transmission voltage can be adjusted to ±20kV or ±40kV. By optimizing the number and parameter matching relationship of the multi-module LLC sub-modules, different voltage boosting requirements can be met, and the DC transmission line 3 can be adjusted according to the required voltage.

[0025] It should be noted that the MPPT algorithm can be replaced by a power control-based MPPT algorithm, which directly tracks the maximum power point by sampling the output power of the photovoltaic array in real time, thereby maximizing the utilization of photovoltaic energy. The architecture, controlled object, and time nodes of the dual closed-loop staged control remain unchanged.

[0026] In this embodiment, a lithium battery energy storage unit can be added to the photovoltaic power supply module 1 to realize the storage and peak regulation of photovoltaic energy. When the photovoltaic output is insufficient or fluctuates, the energy storage unit can supplement the power supply, further improving the power supply stability of the system.

[0027] It should be noted that, as Figure 2 The diagram shows a detailed structural schematic of the Boost converter circuit 1-2.

[0028] The Boost circuit includes a photovoltaic array equivalent circuit, a boost inductor, power switching devices, a freewheeling diode, an output filter capacitor, and a controller.

[0029] The positive output terminal of the photovoltaic array is connected to the drain of the power switching device and the positive terminal of the freewheeling diode via a boost inductor. The source of the power switching device is connected to the negative terminal of the photovoltaic array (i.e., the negative terminal of the DC bus). The negative terminal of the freewheeling diode is connected to the positive terminal of the DC bus via an output filter capacitor, and the negative terminal of the output filter capacitor is connected to the negative terminal of the DC bus. The gate of the power switching device is connected to the PWM signal output terminal of the controller, and its turn-on and turn-off are driven by the PWM pulse width modulation signal output by the controller.

[0030] The controller collects the output voltage U of the photovoltaic array. PV and the output voltage U of the Boost circuit outb The controller generates a PWM drive signal based on a dual-closed-loop staged control strategy. During the first period after system startup (e.g., within 3 seconds), the controller outputs a voltage U... outb As a feedback quantity in the outer voltage loop, it is compared with the bus voltage setpoint U. ref The reference command for the inner current loop is generated after PI regulation and compared with the inductor current feedback quantity I. L After comparison, a PWM signal is generated through PI regulation, which adjusts the duty cycle D of the power switching device to make the output voltage approach the set value.

[0031] After the first period ends, the controller switches to using the photovoltaic array output voltage U. PV As the feedback quantity of the voltage outer loop, the MPPT algorithm module is based on U PV With the maximum power point voltage V mpp Based on the comparison results, the reference voltage value of the outer voltage loop is dynamically adjusted to ensure the photovoltaic array operates at its maximum power point. At this time, the controller adjusts the duty cycle D in real time to maintain the input voltage U. PV Stable at V mpp Nearby, while maintaining stable output bus voltage.

[0032] When the power switching device is turned on, the boost inductor stores energy, the freewheeling diode is reverse-biased and cut off, and the output filter capacitor supplies power to the load. When the power switching device is turned off, the boost inductor releases energy, which charges the output filter capacitor through the freewheeling diode and simultaneously supplies power to the load. The controller adjusts the duty cycle D in real time to achieve precise control of the input and output voltages.

[0033] The Boost converter circuit uses the aforementioned dual-loop phased control strategy to prioritize the establishment of a stable medium-voltage DC bus during the initial stage of system startup. After the bus is established, it switches to MPPT control mode, thus balancing the reliability of system startup with the maximization of photovoltaic energy utilization.

[0034] In the embodiments disclosed herein, such as Figure 3 As shown, the LLC submodule 2-1 includes: High-frequency transformer 2-1-1 is used to achieve voltage transformation and electrical isolation; The high-voltage side power unit 2-1-2 consists of two half-bridge silicon carbide modules forming an H-bridge topology, which is connected to the primary side of the high-frequency transformer. The low-voltage side power unit 2-1-3 consists of two half-bridge silicon carbide modules forming an H-bridge topology, which is connected to the secondary side of the high-frequency transformer. High-voltage side resonant capacitor 2-1-4 and low-voltage side resonant capacitor 2-1-5 are respectively configured on the high-voltage side and low-voltage side of the high-frequency transformer 2-1-1, and are used to form a resonant circuit together with the leakage inductance of the high-frequency transformer. The leakage inductance of the high-frequency transformer serves as a resonant inductor, and the resonant circuit of the LLC submodule 2-1 is composed of the leakage inductance and the resonant capacitor, eliminating the need for an additional independent resonant inductor.

[0035] It should be noted that the two half-bridge silicon carbide modules in the LLC submodule 2-1 that form the H-bridge topology can also be replaced with full-bridge silicon carbide modules to further improve the power level and withstand voltage of the submodule and adapt to the power supply needs of a larger-scale photovoltaic power station. The remaining control strategies, system architecture and parameter matching relationships remain unchanged.

[0036] Specifically, LLC submodule 2-1 is the basic building block of the multi-module LLC resonant converter (IPOS architecture) of the present invention, including high-frequency transformer 2-1-1, low-voltage side power unit 2-1-3, high-voltage side power unit 2-1-2, low-voltage side resonant capacitor 2-1-5 and high-voltage side resonant capacitor 2-1-4.

[0037] The high-frequency transformer 2-1-1 is used to realize voltage transformation and electrical isolation. Its primary winding is connected to the low-voltage side power unit 2-1-3, and its secondary winding is connected to the high-voltage side power unit 2-1-2.

[0038] The low-voltage side power unit 2-1-3 consists of four switching transistors Q1, Q2, Q3, and Q4 forming an H-bridge topology, connected to the primary side of the high-frequency transformer 2-1-1. Q1 and Q2 form the first bridge arm, and Q3 and Q4 form the second bridge arm. The drain of Q1 is connected to the positive terminal of the DC bus, and the source of Q1 is connected to the drain of Q2 and one end of the primary winding of the high-frequency transformer; the source of Q2 is connected to the negative terminal of the DC bus. The drain of Q3 is connected to the positive terminal of the DC bus, and the source of Q3 is connected to the drain of Q4 and the other end of the primary winding of the high-frequency transformer; the source of Q4 is connected to the negative terminal of the DC bus. The gates of Q1, Q2, Q3, and Q4 are connected to the PWM drive signal output by the controller. The low-voltage side resonant capacitor 2-1-5 is connected in series in the primary circuit of the high-frequency transformer 2-1-1, forming a low-voltage side resonant circuit together with the leakage inductance of the high-frequency transformer 2-1-1.

[0039] The high-voltage side power unit 2-1-2 consists of four switching transistors S1, S2, S3, and S4 forming an H-bridge topology, connected to the secondary side of the high-frequency transformer 2-1-1. S1 and S2 form the first bridge arm, and S3 and S4 form the second bridge arm. The source of S1 is connected to the drain of S2 and one end of the secondary winding of the high-frequency transformer, with the drain of S1 connected to the positive terminal of the high-voltage side DC bus. The source of S2 is connected to the negative terminal of the high-voltage side DC bus. The source of S3 is connected to the drain of S4 and the other end of the secondary winding of the high-frequency transformer, with the drain of S3 connected to the positive terminal of the high-voltage side DC bus. The source of S4 is connected to the negative terminal of the high-voltage side DC bus. The gates of S1, S2, S3, and S4 are connected to the PWM drive signal output by the controller. The high-voltage side resonant capacitor 2-1-4 is connected in series in the secondary circuit of the high-frequency transformer 2-1-2, forming a high-voltage side resonant circuit together with the leakage inductance of the high-frequency transformer 2-1-2.

[0040] The input terminals of each LLC submodule (the DC side of the low-voltage power unit 2-1-3) are connected in parallel to the output bus of the photovoltaic Boost circuit; the output terminals (the DC side of the high-voltage power unit 2-1-2) are connected in series, and ±30kV medium voltage output is achieved by superimposing the output voltages of each submodule.

[0041] The control method for each LLC submodule is as follows: The controller outputs two sets of complementary 50% duty cycle square wave signals (with dead time) to the switching transistors Q1~Q4 of the low-voltage side power unit 2-1-3. Q1 and Q4 are turned on simultaneously, and Q2 and Q3 are turned on simultaneously. The two sets of signals alternately switch, causing the low-voltage side power unit 2-1-3 to invert the DC power output from the photovoltaic Boost circuit into high-frequency square wave AC power, which is applied to the primary winding of the high-frequency transformer 2-1-1 and the low-voltage side resonant capacitor 2-1-5. The leakage inductance of the high-frequency transformer 2-1-1 and the low-voltage side resonant capacitor 2-1-5 resonate at a specific switching frequency. The resonant current has a sinusoidal waveform. The switching transistors Q1~Q4 switch near the current zero-crossing point or voltage zero-crossing point to achieve zero-voltage turn-on or zero-current turn-off, which greatly reduces switching losses.

[0042] The high-frequency transformer 2-1-1 boosts the high-frequency square wave AC power from the primary side and transmits it to the secondary side. The switches S1~S4 of the high-voltage side power unit 2-1-2 and the switches Q1~Q4 of the low-voltage side power unit 2-1-3 maintain synchronous rectification control (i.e., S1 and Q1 conduct synchronously, S2 and Q2 conduct synchronously, S3 and Q3 conduct synchronously, and S4 and Q4 conduct synchronously), rectifying the AC power induced in the secondary winding into DC power output. The resonant circuit formed by the high-voltage side resonant capacitor 2-1-4 and the secondary leakage inductance of the high-frequency transformer 2-1-1 further smooths voltage spikes during rectification, improving power quality.

[0043] The controller adjusts the switching frequency of each LLC submodule in real time. When the switching frequency is equal to the resonant frequency fr, the converter operates at the resonant point with a voltage gain of 1 and the highest energy conversion efficiency. When the load changes or the input voltage fluctuates, the controller adjusts the switching frequency to deviate from the resonant point and changes the voltage gain to maintain a stable output voltage.

[0044] The leakage inductance of the high-frequency transformer 2-1-1 is directly used as the resonant inductor, eliminating the need for an additional independent resonant inductor. The leakage inductance value Lr of the high-frequency transformer 2-1-1 and the capacitance value Cr of the low-voltage side resonant capacitor 2-1-5 satisfy the resonant frequency relationship fr=1 / (2π√(Lr·Cr)). Through parameter matching, the converter operates at the designed resonant frequency point, achieving soft-switching operation. This leakage inductance reuse design simplifies the hardware structure of the LLC submodule, reducing system size and manufacturing cost.

[0045] The parameters of each LLC submodule have been precisely calculated and matched to meet the ±30kV voltage boost requirement and soft switching conditions: The resonant capacitor Cr, the transformer leakage inductance Lr, and the resonant frequency fr satisfy Cr=1 / (4π²fr²Lr); The magnetizing inductance Lm satisfies Lm=(Vin×Tres) / (4ΔIm), where Vin is the input voltage, Tres is the resonant period, and ΔIm is the change in magnetizing current. The excitation current Im and the dead time td satisfy Im×td≥2Vin×Coss, where Coss is the parasitic junction capacitance of the silicon carbide switching device, ensuring that the switching device achieves zero-voltage turn-on.

[0046] The coordinated control method of the entire multi-module LLC resonant converter (IPOS architecture) is as follows: the input terminals of each LLC submodule are connected in parallel, allowing for natural current sharing; the output terminals of each LLC submodule are connected in series, and the output voltage is the sum of the output voltages of each submodule. The controller collects the output voltage of each submodule and uses a voltage-equalizing control strategy (such as master-slave control) to adjust the switching frequency of each submodule, keeping the output voltage of each submodule balanced; the parallel connection of the input terminals allows for natural current sharing among the submodules. When any LLC submodule fails, the controller closes the bypass switch of the failed submodule, bypassing its output terminal and causing it to stop operating, while the remaining submodules continue to operate, improving the system's fault tolerance.

[0047] In this embodiment, the electrolytic cell load module 5 adopts the cryolite-alumina molten salt electrolysis method, with the operating temperature controlled between 950°C and 970°C. The cell voltage of the electrolytic cell satisfies the formula... Where Upol is the polarization voltage, Rcell is the cell resistance, and Icell is the electrolysis current. The polarization voltage Upol is 30% of the cell terminal voltage. By controlling the electrolyte concentration, cell temperature, and electrode distance, both the cell resistance Rcell and the polarization voltage Upol are kept constant to ensure a stable electrolysis current.

[0048] It should be noted that the photovoltaic power supply module 1 consists of a photovoltaic cell array 1-1 and a Boost converter circuit 1-2. The photovoltaic cell array 1-1 is formed by combining multiple photovoltaic modules in series and parallel to meet the high power output requirements. The Boost converter circuit 1-2 is equipped with a voltage-controlled MPPT algorithm, which dynamically adjusts the duty cycle of the power switching devices of the Boost converter by sampling the output voltage of the photovoltaic cell array in real time, so that the photovoltaic cell array always operates at the maximum power point voltage Vmpp, thereby maximizing the utilization of photovoltaic energy.

[0049] The photovoltaic power supply module 1 adopts a dual closed-loop phased control strategy combining an inner current loop and an outer voltage loop: In the first 3 seconds after system startup, the control core regulates the output voltage of the Boost boost circuit 1-2 and optimizes the duty cycle of the power switching devices through the PI regulator, so that the output voltage of the Boost boost circuit 1-2 quickly approaches the input bus voltage of the multi-module LLC resonant converter 2, establishing a stable initial operating point for the system; after 3 seconds, the control core switches the control object to regulate the input voltage of the Boost boost circuit 1-2 (i.e., the output voltage of the photovoltaic panel), realizing the coordinated control of maximum power point tracking and the stable input bus voltage of the multi-module LLC resonant converter 2. The inner current loop responds quickly to current commands, suppresses system disturbances, and improves dynamic regulation performance.

[0050] The multi-module LLC resonant converter 2 is the core boost converter unit of the system. It consists of N identical LLC sub-modules 2-1 and adopts a combination of parallel input and series output (IPOS). It is the first innovative application in the ±30kV medium-voltage DC transmission scenario to realize the direct coupling between photovoltaic power generation and electrolytic aluminum industry. Each LLC sub-module 2-1 uses two half-bridge silicon carbide modules to form an H-bridge power module on both the high-voltage and low-voltage sides. Each side is independently configured with a resonant capacitor. The resonant inductor directly adopts the leakage inductance of the high-frequency transformer itself, without the need for additional independent resonant inductors, thus simplifying the system structure.

[0051] The input terminals of each LLC submodule 2-1 are connected in parallel to the output bus of the photovoltaic Boost boost circuit 1-2 to achieve current sharing. The output terminals are connected in series, and by superimposing the output voltages of each submodule, a medium-voltage output of ±30kV is achieved. Furthermore, the number of submodules can be flexibly adjusted to meet high-power requirements of 50MW and above. This multi-module LLC resonant converter 2 utilizes resonant characteristics to achieve soft-switching operation, significantly reducing switching losses and improving energy conversion efficiency. It supports high-frequency design, effectively reducing the size of magnetic components and filter capacitors, and increasing system power density. Simultaneously, the soft-switching characteristics suppress switching transient spikes, reduce electromagnetic interference, simplify filter design, and can adapt to a wide range of input voltage and load variations by adjusting the switching frequency. It also possesses inherent short-circuit protection capabilities.

[0052] The core parameters of the converter are precisely calculated and matched to meet the ±30kV boost requirement and soft-switching conditions: the resonant capacitor Cr, the transformer leakage inductance Lr, and the resonant frequency fr satisfy the relationship Cr=1 / (4π²fr²Lr); the magnetizing inductance Lm is calculated using the formula Lm=(Vin×Tres) / (4ΔIm) (Vin is the input voltage, Tres is the resonant period, and ΔIm is the change in magnetizing current); the amount of charge transferred by the magnetizing current during the dead time is not less than the amount of charge required for the charging and discharging of the parasitic junction capacitance of the switching device, i.e., Im×td≥2Vin×Coss (Im is the magnetizing current, td is the dead period, and Coss is the parasitic junction capacitance of the silicon carbide switching device), ensuring that the switching device achieves zero-voltage turn-on; at the same time, through the voltage and current equalization control strategy, the output voltage and input current of each LLC submodule are balanced, improving the system reliability.

[0053] The DC power output from the photovoltaic power supply module 1 is boosted to ±30kV medium-voltage DC power by the multi-module LLC resonant converter 2, and then transmitted over long distances through the ±30kV DC transmission line 3. Compared with traditional low-voltage DC and AC transmission, this significantly reduces energy loss during long-distance transmission, increases transmission distance, and reduces investment costs and floor space requirements for transmission lines. After the ±30kV medium-voltage DC power is transmitted to the electrolytic aluminum plant, the voltage is adjusted to the range required by the step-down converter to meet the requirements of the electrolytic aluminum production process. It is then connected to the electrolytic cell load module to supply power. The step-down converter adopts a high-efficiency converter topology to ensure the efficiency and stability of power conversion.

[0054] The electrolytic cell load module 5 adopts the cryolite-alumina molten salt electrolysis method, with the working temperature precisely controlled between 950℃ and 970℃. The cell voltage of the electrolytic cell satisfies the formula Ucell=Upol+Rcell×Icell, where Upol is the polarization voltage, Rcell is the cell resistance, and Icell is the electrolytic current. According to engineering practice and process requirements, the polarization voltage Upol is taken as 30% of the electrolytic cell terminal voltage. By precisely controlling physical quantities such as electrolyte concentration, electrolytic cell temperature, and electrode distance, both the cell resistance Rcell and the polarization voltage Upol are kept constant, ensuring stable electrolytic current and guaranteeing the continuity and stability of electrolytic aluminum production.

[0055] The system is configured with a 330kV substation AC rectifier system 6 as a backup / cooperative power supply unit. The output of the AC rectifier system is connected in parallel with the output of the step-down converter and then connected to the electrolytic cell load module to form a combined power supply mode of photovoltaic DC + AC rectification. When a short-circuit ground fault occurs on the photovoltaic DC bus, only the photovoltaic system experiences transient voltage fluctuations, while the AC rectifier system can independently and stably supply power to the electrolytic cell load, with no significant changes in the voltage and current of the electrolytic cell. When a short-circuit ground fault occurs on the electrolytic aluminum bus, each subsystem of the system has transient adjustment and recovery capabilities, and can quickly recover to a steady-state operating state after a brief oscillation, effectively avoiding power outages. At the same time, the multi-module LLC sub-modules have independent fault isolation capabilities. When a single sub-module fails, it can be shut down through bypass control, while the remaining sub-modules continue to work, improving the system's fault tolerance.

[0056] The combined power supply system for photovoltaic DC access to electrolytic aluminum proposed in this embodiment has the following advantages: 1. Achieve medium-voltage high-power output: Adopting a multi-module LLC input parallel output series (IPOS) architecture, ±30kV medium-voltage output is achieved by superimposing the output voltages of sub-modules. The number of sub-modules can be increased or decreased to flexibly adapt to the high-power requirements of 50MW and above, breaking through the power and voltage level limitations of single-module LLC converters, and significantly improving the system expansion capability.

[0057] 2. Improved energy conversion and utilization efficiency: The multi-module LLC resonant converter significantly reduces switching losses through soft-switching operation, improving energy conversion efficiency by more than 10% compared to traditional DC / DC converters; the voltage-controlled MPPT algorithm combined with a dual-closed-loop staged control strategy ensures that the photovoltaic array always operates at the maximum power point, improving photovoltaic energy utilization efficiency by more than 15% and maximizing the utilization of photovoltaic energy.

[0058] 3. Simplified system structure and improved power density: The LLC submodule directly uses the leakage inductance of the high-frequency transformer as the resonant inductor, eliminating the need for an additional independent resonant inductor, which simplifies the system hardware structure and reduces equipment manufacturing costs. At the same time, the high-frequency design of the converter effectively reduces the size of magnetic components and filter capacitors, increasing the system power density by more than 20%, which is suitable for the power supply requirements of high-power electrolytic aluminum loads, and reduces electromagnetic interference, improving the reliability of system operation.

[0059] 4. Enhanced system fault adaptability and operational stability: The combined power supply mode of photovoltaic DC and AC rectification achieves fault isolation. When the photovoltaic DC bus is short-circuited, only the photovoltaic system experiences transient fluctuations, while the electrolytic cell load is stably powered by the AC rectification system, eliminating the risk of power outage. When the electrolytic aluminum bus is short-circuited, each subsystem can quickly complete transient adjustment and recover to a steady state, with a transient recovery time of less than 1 second. The multi-module LLC submodule has independent fault isolation capabilities, and can bypass and exit when a single submodule fails, improving the system's fault tolerance. The constant parameter design of the electrolytic cell load module further ensures the continuity and stability of electrolytic aluminum production.

[0060] 5. Achieve long-distance and efficient transmission of photovoltaic energy: ±30kV medium-voltage DC transmission increases the transmission distance from photovoltaic power stations to electrolytic aluminum plants by an order of magnitude, reduces long-distance transmission loss to below 5%, breaks through the site selection limitations of traditional photovoltaic electrolytic aluminum power supply, realizes cross-regional allocation and large-scale utilization of photovoltaic resources, and effectively solves the problem of photovoltaic curtailment.

[0061] 6. Promoting the low-carbon transformation of the electrolytic aluminum industry with significant economic and environmental benefits: This system achieves direct coupling between photovoltaic power generation and the electrolytic aluminum industry, which can increase the utilization rate of renewable energy in the electrolytic aluminum industry to over 30%, far exceeding the national policy requirement of 25%. A single large-scale electrolytic aluminum plant can reduce carbon dioxide emissions by over one million tons per year, in line with the national green and low-carbon and energy-saving and carbon-reduction special action plan for the electrolytic aluminum industry. At the same time, it replaces part of the thermal power supply, which can significantly reduce the electricity costs of electrolytic aluminum enterprises. Based on an annual power supply of 1 billion kWh, the average annual electricity cost savings exceed 50 million yuan, demonstrating significant economic, social and environmental benefits.

[0062] 7. Low system upgrade cost and strong project implementation: This system can be directly connected to the existing power supply system of the electrolytic aluminum plant without large-scale modification of the original production equipment, resulting in low upgrade cost and short construction period; at the same time, the photovoltaic power station can be planned and constructed in different locations, making flexible use of photovoltaic resources in various places, with strong project implementation, and is suitable for large-scale promotion and application in the electrolytic aluminum industry.

[0063] In summary, the photovoltaic DC-connected electrolytic aluminum integrated power supply system proposed in this embodiment can realize long-distance and efficient transmission of photovoltaic energy, improve energy conversion efficiency, and enhance system fault adaptability and operational stability.

[0064] This application also provides a flowchart of a control method for a photovoltaic DC-connected electrolytic aluminum combined power supply system, such as... Figure 4 As shown, the method includes: Step 1: Start the boost circuit of the photovoltaic power supply module, and control the boost circuit based on the maximum power point tracking algorithm to make the photovoltaic array operate at the maximum power point; In this embodiment of the disclosure, the maximum power point tracking algorithm is a voltage-controlled MPPT algorithm. It samples the output voltage of the photovoltaic array in real time and compares it with the maximum power point voltage Vmpp. Based on the comparison result, it dynamically adjusts the duty cycle of the power switching device in the boost circuit so that the operating voltage of the photovoltaic array approaches and stabilizes at Vmpp.

[0065] It should be noted that in the control method of the photovoltaic DC-connected electrolytic aluminum combined power supply system, the operating state of the photovoltaic array is first adjusted by the boost circuit in the photovoltaic power supply module based on the voltage-controlled maximum power point tracking (MPPT) algorithm. The core of this step lies in using voltage as a control parameter. By monitoring the output voltage of the photovoltaic array in real time and comparing it with the preset maximum power point voltage, the duty cycle of the power switching devices in the boost circuit is dynamically adjusted, thereby changing the load characteristics of the photovoltaic array and ensuring that its operating point always approaches and stabilizes at the maximum power point voltage. This process ensures that the photovoltaic array can operate under its optimal voltage conditions, thereby maximizing the conversion of solar energy into electrical energy and providing a stable and efficient energy input for subsequent boost, transmission, and power supply stages. As one implementation, the boost circuit can specifically adopt a Boost topology. The duty cycle of its power switching devices is calculated by the control core based on the sampled output voltage of the photovoltaic array using a voltage-controlled MPPT algorithm, thereby achieving precise control of the photovoltaic array's output voltage and stabilizing it at the maximum power point voltage Vmpp.

[0066] This technical step introduces a voltage-controlled maximum power point tracking algorithm, which enables active and precise control of the operating point of the photovoltaic array. This avoids energy loss caused by power fluctuations or environmental changes in traditional methods, significantly improves the photovoltaic energy capture efficiency, and lays a solid energy foundation for the efficient and stable operation of the entire system.

[0067] Specifically, firstly, the photovoltaic array in the photovoltaic power supply module is formed by combining multiple photovoltaic modules in series and parallel, and its output terminal is electrically connected to the input terminal of the Boost converter. The control core collects the output voltage of the photovoltaic array in real time through a voltage sampling module. This sampled value is used as a feedback signal input to the PI regulator in the voltage outer loop. The PI regulator compares the received real-time output voltage with the preset maximum power point voltage Vmpp and calculates the deviation between the two. Based on this deviation, the PI regulator outputs a regulation value according to the proportional-integral control law. This regulation value is used to dynamically adjust the duty cycle of the power switching devices in the Boost converter. Specifically, when the real-time output voltage is higher than Vmpp, the PI regulator outputs a command to decrease the duty cycle, causing the Boost converter to reduce the output voltage, thereby pulling the operating point of the photovoltaic array back to near the maximum power point; conversely, when the real-time output voltage is lower than Vmpp, the PI regulator outputs a command to increase the duty cycle, causing the Boost converter to increase the output voltage, similarly bringing the operating point back to Vmpp. Through the aforementioned closed-loop regulation process, the duty cycle of the power switching devices is continuously and dynamically corrected, thereby ensuring that the photovoltaic array always operates at the maximum power point voltage Vmpp, maximizing the utilization of photovoltaic energy. This process constitutes a voltage-controlled MPPT algorithm closed loop. Its input is the real-time output voltage of the photovoltaic array, and its processing action involves calculating the duty cycle adjustment based on the voltage deviation using a PI regulator. The output is the dynamically adjusted duty cycle signal of the power switching devices, which directly drives the Boost converter circuit to stabilize the output voltage of the photovoltaic array at Vmpp.

[0068] This specific implementation introduces a PI regulator to perform closed-loop correction of voltage deviation, achieving accurate and rapid tracking of the operating point of the photovoltaic array. This avoids the oscillation or tracking lag problems that may be caused by traditional open-loop control or disturbance observation methods, thereby significantly improving the utilization efficiency of photovoltaic energy and ensuring the stable output of the photovoltaic power supply module under dynamic operating conditions.

[0069] Step 2: The boost circuit is controlled using a dual closed-loop phased control strategy that combines the inner current loop and the outer voltage loop: During the first period after system startup, the output voltage of the boost circuit is used as the feedback quantity of the outer voltage loop to control the output voltage of the boost circuit to approach the bus voltage set value; After the first period, the output voltage of the photovoltaic array is switched to be used as the feedback quantity of the outer voltage loop to control the photovoltaic array to operate at the maximum power point. The first time period is the first 3 seconds after the system starts; In this embodiment of the disclosure, the dual-loop staged control strategy uses a PI regulator to adjust the feedback quantity in the outer voltage loop, and the inner current loop is used to quickly respond to current commands to suppress load disturbances.

[0070] It should be noted that the dual closed-loop phased control strategy adopts a soft switching method during the process of switching from the first time period to the first time period, so that the reference value of the outer voltage loop smoothly transitions from the bus voltage setpoint to the reference value corresponding to the maximum power point voltage.

[0071] It should be noted that the control method employs a dual-closed-loop phased control strategy combining an inner current loop and an outer voltage loop. This strategy aims to coordinate the operating state of the photovoltaic power supply module's boost circuit during system startup and operation, thereby achieving coordinated control of the stable establishment of the system bus voltage and the maximum power point tracking of the photovoltaic array. Specifically, this control strategy switches the control target of the boost circuit in stages according to different phases of system operation. In the first period after system startup, the control core focuses on the output voltage of the boost circuit. Through closed-loop adjustment of the outer voltage loop and the inner current loop, the output voltage of the boost circuit quickly approaches and stabilizes at the preset target value, thus establishing a stable input bus voltage for the subsequent multi-module resonant converter and ensuring the reliable establishment of the system's initial operating point. After the first period ends, the control core switches its control target to the input voltage of the boost circuit. At this time, the outer voltage loop adjusts based on the voltage reference value provided by the maximum power point tracking (MPPT) algorithm, stabilizing the operating voltage of the photovoltaic array near the MPPT voltage, thereby maximizing photovoltaic energy capture. Simultaneously, the inner current loop responds quickly to current commands, suppressing system disturbances and improving dynamic regulation performance. Through this phased switching control, the system can prioritize establishing a stable DC bus voltage during startup and then seamlessly switch to MPPT mode, balancing system startup reliability with photovoltaic utilization efficiency during operation. As one implementation, the first period can be set to the first 3 seconds after system startup. During this period, the proportional-integral regulator optimizes the duty cycle of the power switching devices, enabling the boost circuit output voltage to establish rapidly. After 3 seconds, the control target switches to regulating the input voltage of the boost circuit, achieving coordinated control of MPPT and bus voltage stability.

[0072] This technical step effectively solves the timing coordination problem between the establishment of bus voltage and subsequent maximum power point tracking during system startup by switching control targets in stages. It avoids the defects of startup impact or tracking delay in a single control mode, thereby improving the smoothness of system startup, the photovoltaic energy conversion efficiency during operation, and the overall stability of power supply.

[0073] Specifically, the control core acquires the voltage signal at the output of the Boost converter circuit in real time through a voltage sampling module. This signal is compared with a preset reference voltage value (i.e., the target voltage of the input bus of the multi-module LLC resonant converter) to generate a voltage error signal. This voltage error signal is sent to the PI regulator in the outer voltage loop, which calculates and outputs a current command value based on proportional-integral (PI) calculations. This current command value is then sent to the inner current loop, where the current sampling module detects the inductor current of the Boost converter circuit in real time. The actual current value is compared with the current command value to generate a current error signal, which is then output as a PWM modulation signal by the PI regulator in the inner current loop to optimize the duty cycle of the power switching devices. Through this dual-loop coordinated regulation, the output voltage of the Boost converter circuit quickly approaches the input bus voltage of the multi-module LLC resonant converter, thus establishing a stable initial operating point for the entire system. After 3 seconds of system operation, the control core switches the controlled object. At this time, the controlled object changes from the output voltage of the Boost converter circuit to its input voltage, i.e., the output voltage of the photovoltaic array. The control core acquires the output voltage of the photovoltaic array in real time through a voltage sampling module and compares it with the maximum power point voltage (Vmpp) calculated by the MPPT algorithm based on voltage control, generating a voltage error signal. This error signal is also processed by the PI regulator in the outer voltage loop to generate a current command value, which is then sent to the inner current loop. The inner current loop responds quickly to the current command, adjusting the duty cycle of the power switching devices to stabilize the output voltage of the photovoltaic array at Vmpp, thereby achieving maximum power point tracking. Simultaneously, the stable control of the Boost converter's input voltage indirectly ensures the stability of its output voltage (i.e., the input bus voltage of the multi-module LLC resonant converter), thus achieving coordinated control of maximum power point tracking and stable input bus voltage of the multi-module LLC converter. Throughout the control process, the inner current loop, due to its fast response characteristics, effectively suppresses system disturbances and improves the system's dynamic regulation performance.

[0074] The beneficial effects of this specific implementation are as follows: by switching the controlled object in stages, a stable bus voltage is quickly established first, and then switched to maximum power point tracking, avoiding MPPT misjudgment caused by the lack of voltage establishment during the startup phase, thus ensuring the smoothness of system startup; at the same time, the dual closed-loop structure of the current inner loop and voltage outer loop effectively suppresses system disturbances and improves dynamic response speed, providing a guarantee for the stable operation of the subsequent multi-module LLC resonant converter and the efficient utilization of photovoltaic energy.

[0075] like Figure 5As shown, the outer voltage loop controls the output voltage of the photovoltaic array or the output voltage of the Boost converter. After system startup, the control switching module selects the feedback from the outer voltage loop as the sampled voltage value Uo at the output of the Boost converter. This sampled value is compared with the bus voltage setpoint Uo_ref, and the deviation is sent to the outer voltage loop PI regulator. The outer voltage loop PI regulator performs proportional-integral calculations based on the deviation signal and outputs the reference command Iref for the inner current loop. After receiving the reference command Iref, the inner current loop compares it with the sampled inductor current IL of the Boost converter and sends the deviation to the inner current loop PI regulator. The inner current loop PI regulator outputs a PWM control signal to drive the power switching devices of the Boost converter, causing the output voltage of the Boost converter to quickly approach the input bus voltage setpoint of the multi-module LLC resonant converter, thus establishing a stable initial operating point for the system.

[0076] After the above process lasts for 3 seconds, the control switching module activates, switching the feedback of the voltage outer loop from the voltage sampling value Uo at the output of the Boost circuit to the photovoltaic array output voltage sampling value Upv. Based on the comparison between Upv and the maximum power point voltage Vmpp, the MPPT algorithm module dynamically adjusts the reference voltage value Upv_ref of the voltage outer loop PI regulator, ensuring the photovoltaic array operates at its maximum power point. At this time, the output of the voltage outer loop serves as the reference command Iref for the current inner loop. The current inner loop samples the inductor current IL of the Boost circuit in real time, and after PI regulation by the current inner loop, generates a PWM signal. This signal quickly responds to the voltage outer loop command, suppresses system disturbances, improves dynamic regulation performance, and achieves coordinated control of maximum power point tracking and stable input bus voltage across multiple modules (LLCs).

[0077] It should be noted that a fuzzy control algorithm can be added to the dual closed-loop control to optimize the parameters of the PI regulator, making the system more adaptable to load fluctuations and external disturbances, and faster in transient adjustment.

[0078] Step 3: Input the DC power output from the boost circuit into the multi-module LLC resonant converter, control each sub-module to operate in parallel input and series output mode, boost the DC power to the preset medium voltage level and then output it; In this embodiment of the disclosure, the multi-module LLC resonant converter is composed of LLC sub-modules connected in parallel at the input and in series at the output, and the preset medium voltage level is ±30kV.

[0079] In this embodiment of the disclosure, each sub-module of the multi-module LLC resonant converter adopts a voltage and current equalization control strategy. The output voltage of each sub-module is kept balanced through master-slave control, and the input terminals are connected in parallel to make the input current of each sub-module naturally equalized. When any LLC submodule of the multi-module LLC resonant converter fails, the bypass switch of the failed submodule is closed to bypass the output of the failed submodule and cause it to stop operating, while the non-faulty LLC submodules continue to operate.

[0080] It should be noted that the DC power output from the boost circuit is input to a multi-module resonant converter. This converter is composed of multiple sub-modules connected in parallel at the input and in series at the output. Medium-voltage DC output is achieved by superimposing the output voltages of each sub-module, and soft-switching operation is realized using resonant characteristics. Specifically, this multi-module resonant converter, as the core boost converter unit of the system, has its input connected in parallel to the output bus of the preceding boost circuit to achieve current sharing among the sub-modules. Its outputs are connected in series, and by superimposing the output voltages of each sub-module, a medium-voltage DC bus sufficient for long-distance transmission is constructed. This parallel input and series output architecture allows the system to flexibly adapt to different power and voltage levels by adjusting the number of sub-modules, thus overcoming the power and voltage limitations of a single converter module. Each sub-module employs a resonant converter topology. Through the matching design of resonant inductors and capacitors, the power switching devices switch on and off near the zero-crossing point of voltage or current, thereby achieving soft-switching operation. This soft-switching characteristic significantly reduces energy loss and voltage / current spikes during switching, suppresses electromagnetic interference, and allows the converter to operate at higher switching frequencies. This, in turn, reduces the size of magnetic components and filter capacitors, improving the system's power density and overall efficiency. As one implementation, each submodule can be constructed using a half-bridge silicon carbide module to form an H-bridge power module, utilizing the leakage inductance of a high-frequency transformer as a resonant inductor to simplify the system structure and reduce hardware costs.

[0081] This step achieves efficient boost conversion of medium-voltage DC output through the parallel input and series output architecture of the multi-module resonant converter and its soft-switching operation characteristics. It significantly improves the system's power and voltage level adaptability, while greatly reducing switching losses and electromagnetic interference, and enhancing the system's operational reliability and energy conversion efficiency.

[0082] Specifically, the input sources for this voltage and current equalization control strategy are the output voltage sampling signal and input current sampling signal of each LLC submodule. The control core acquires the output voltage of each LLC submodule in real time through a voltage sampling circuit, and simultaneously acquires the input current of each LLC submodule in real time through a current sampling circuit. The control core compares the acquired output voltage values ​​of each submodule with preset voltage reference values, and compares the acquired input current values ​​of each submodule with preset current reference values. Based on the comparison results, the control core generates switching frequency adjustment commands or phase shift angle adjustment commands for each submodule through a PI regulator, and sends them as processing actions to the power switching device drive circuits of the corresponding LLC submodules. The drive circuits of each LLC submodule dynamically adjust the switching frequency or phase shift angle of its internal power switching devices according to the received adjustment commands, thereby changing the voltage gain and power transfer characteristics of the submodule. Through this closed-loop regulation, when the output voltage of a submodule is too high, the control core will adjust its switching frequency accordingly to reduce its output voltage; when the input current of a submodule is too high, the control core will adjust its phase shift angle to reduce its input current. The final output of this control strategy is that the output voltage of all LLC submodules is balanced to a basically consistent level, and the input current of all LLC submodules is also balanced to a basically consistent level, thereby eliminating voltage and current imbalances caused by differences in device parameters or inconsistent operating states. This voltage and current equalization control strategy works in conjunction with the IPOS architecture of the multi-module LLC resonant converter. In the previous sub-step, the DC power output from the boost circuit is evenly distributed to each LLC submodule via the input parallel bus. After resonant conversion and voltage superposition, each submodule outputs medium-voltage DC power. The voltage and current equalization control in this step ensures the stability of this voltage superposition process and the load balance of each submodule, providing a fundamental guarantee for the stable transmission of DC power lines and the efficient operation of the buck converter in subsequent steps.

[0083] By implementing the above-mentioned voltage and current equalization control strategy, this embodiment can effectively suppress the voltage and current imbalance caused by the differences in parameters or aging of each LLC submodule, significantly improve the reliability and stability of the multi-module LLC resonant converter in long-term operation, avoid system failures caused by overvoltage or overcurrent of individual submodules, and thus enhance the fault tolerance and service life of the entire photovoltaic DC-connected electrolytic aluminum joint power supply system.

[0084] It should be noted that, in order to verify the voltage control capability of the system under steady-state operation conditions and the adaptability of each subsystem, a photovoltaic DC access electrolytic aluminum joint power supply system model was built on the simulation platform, and a steady-state operation simulation of the system from 0 to 15 seconds was carried out.

[0085] Step 4: Transmit the boosted medium-voltage DC power to the aluminum electrolysis plant area, and after step-down conversion, power the electrolytic cell load. Control the AC rectifier system to operate in parallel with the DC power supply to form an AC / DC combined power supply mode.

[0086] In this embodiment of the disclosure, the AC / DC combined power supply mode includes: When a short-circuit fault occurs on the photovoltaic DC bus, the AC rectifier system is controlled to independently supply power to the electrolytic cell load; When a short circuit fault occurs in the electrolytic aluminum busbar, the control subsystems will be adjusted and then restored to steady-state operation.

[0087] It should be noted that the medium-voltage DC power generated after being boosted by the multi-module resonant converter is transmitted to the electrolytic aluminum plant area via a DC transmission line. This DC transmission line can be adapted to the actual transmission distance and power level to achieve low-loss long-distance power transmission. After being transmitted to the plant area, the medium-voltage DC power is fed into a step-down converter. This device is used to transform the voltage amplitude of the input medium-voltage DC power, reducing and stabilizing it within a specific voltage range required by the electrolytic aluminum production process, thereby meeting the power supply requirements of the electrolytic cell load module. The DC power after step-down treatment then supplies power to the electrolytic cell load module to drive the electrochemical reaction in the electrolytic aluminum production process. In addition, the system is also equipped with an AC rectification system. The output of the AC rectification system is electrically connected in parallel with the output of the step-down converter, and both are connected to the electrolytic cell load module. This parallel structure creates a joint power supply mode where photovoltaic DC power supply and AC rectified power supply work together. This allows the AC rectified system to independently or collaboratively provide stable power to the electrolyzer load when fluctuations or faults occur on the photovoltaic DC power supply side, thus ensuring the continuity and reliability of the power supply. As a specific implementation method, this AC rectified system can be built based on the existing 330kV substation in the plant area. Its output DC power is connected in parallel with the photovoltaic DC link to jointly power the electrolyzer load, achieving a joint power supply architecture with mutual backup.

[0088] This technical step significantly improves the reliability and flexibility of the system power supply by constructing a combined power supply mode of photovoltaic DC and AC rectification, and combining medium-voltage DC long-distance transmission and step-down adaptation. At the same time, it realizes the efficient cross-regional utilization of photovoltaic energy and the stable access of electrolytic aluminum loads.

[0089] like Figure 6As shown in the figure, the horizontal axis represents time (t / s), and the vertical axis represents voltage (U / kV). After system startup, the photovoltaic DC bus voltage starts from 0 and reaches the set value of 30kV after approximately 3 seconds, remaining stable. At 3 seconds, the control strategy switches from the first stage to the second stage, and the bus voltage quickly recovers to 30kV after experiencing a minor disturbance. At 5 seconds, the electrolytic aluminum load is connected to the system, causing a brief drop in the bus voltage, which is then quickly restored to 30kV and remains stable after adjustment by the control system. Throughout the entire 15-second simulation cycle, the photovoltaic DC bus voltage ultimately stabilizes at 30kV, with fluctuations within ±0.5kV, verifying the excellent steady-state voltage control capability of the dual-closed-loop staged control strategy of this invention.

[0090] like Figure 7 , 8 As shown in Figure 9, the horizontal axis represents time (t / s), and the vertical axes represent voltage (U / V), current (I / A), and power (P / MW), respectively. The figure includes three curves: output voltage, current, and power. From 0 to 3 seconds, the system is in the bus voltage establishment phase. The output voltage gradually rises from 0 to approximately 5kV, the output current gradually rises from 0 to approximately 8kA, and the output power gradually rises from 0 to approximately 40MW. At the 3-second mark, when the control strategy switches, the output voltage, current, and power all experience slight fluctuations before rapidly recovering. At the 5-second mark, when the electrolytic aluminum load is connected, the output voltage momentarily drops from approximately 23kV to approximately 20kV before rapidly recovering to approximately 25kV, the output current momentarily rises from approximately 10kA to approximately 12kA before stabilizing, and the output power experiences a transient spike before quickly stabilizing. After 5 seconds, the output voltage stabilizes at approximately 28kV, the output current stabilizes at approximately 15kA, and the output power stabilizes at approximately 45MW. No significant drift or oscillation is observed in any of the curves, verifying that the subsystems of this invention have good adaptability and stability under steady-state operating conditions.

[0091] To verify the transient response capability and fault recovery capability of the system of the present invention when a short circuit fault occurs in the electrolytic aluminum bus, a simulation platform was set to have a positive and negative short circuit grounding fault in the electrolytic aluminum bus at 10 seconds and the fault being cleared at 11 seconds.

[0092] like Figure 10 , 11As shown in Figure 12, the horizontal axis represents time (t / s), and the vertical axes represent voltage (U / V), current (I / A), and power (P / MW), respectively, comprising three curves: voltage, current, and power. When the fault occurred at 10 seconds, the electrolytic cell voltage instantly dropped from approximately 20V to approximately 5V, accompanied by a spike oscillation of approximately ±10V. The electrolytic current instantaneously rose from approximately 150kA to a peak of approximately 300kA before rapidly declining. The electrolytic power instantaneously rose from approximately 3MW to a pulse of approximately 4.5MW before rapidly decreasing. After the fault was cleared at 11 seconds, the electrolytic cell voltage recovered to approximately 20V after approximately 0.3 seconds of oscillation, the electrolytic current recovered to approximately 150kA after approximately 0.5 seconds of oscillation, and the electrolytic power simultaneously recovered to approximately 3MW. The entire transient process lasted approximately 0.5 seconds. The above results show that the electrolytic cell load module of the present invention adopts a constant parameter design (the polarization voltage Upol is 30% of the terminal voltage, and the cell resistance Rcell and the polarization voltage Upol are both constant values). When a short circuit fault occurs in the electrolytic aluminum bus, the electrolytic cell can quickly complete the transient adjustment and recover to steady-state operation. The transient recovery time is less than 1 second, which ensures the continuity of electrolytic aluminum production.

[0093] like Figure 13 As shown in the figure, the horizontal axis represents time (t / s), and the vertical axis represents voltage (U / kV). When a short-circuit fault occurs on the electrolytic aluminum busbar after 10 seconds, the photovoltaic DC busbar voltage drops sharply from 30kV to approximately 22kV. After the fault is cleared after 11 seconds, the busbar voltage gradually recovers to 30kV and remains stable after approximately 0.4 seconds of oscillation adjustment, without low-frequency oscillations or steady-state deviations. These results demonstrate that the AC / DC combined power supply architecture of this invention exhibits excellent transient adjustment and self-recovery capabilities in the photovoltaic DC power supply section during a short-circuit fault on the electrolytic aluminum busbar, enabling rapid fault ride-through and restoration of steady-state output.

[0094] To verify the self-recovery capability and anti-interference capability of the system of the present invention when a short circuit fault occurs on the photovoltaic DC bus, a simulation platform was set to have a positive and negative short circuit grounding fault on the photovoltaic DC bus at 10 seconds and the fault being cleared at 11 seconds.

[0095] like Figure 14 As shown in the figure, the horizontal axis represents time (t / s), and the vertical axis represents voltage (U / kV). When the fault occurred at 10 seconds, the photovoltaic DC bus voltage fluctuated violently, instantly dropping to near 0V. After the fault was cleared at 11 seconds, the bus voltage quickly climbed to approximately 25kV under the control of the control system. After approximately 0.3 seconds of oscillation adjustment, it recovered to 30kV and remained stable, verifying that the photovoltaic grid connection system of this invention has a rapid self-recovery capability under DC bus short-circuit fault conditions.

[0096] like Figure 15 , 16As shown in Figure 17, the horizontal axis represents time (t / s), and the vertical axes represent voltage (U / V), current (I / A), and power (P / MW), respectively, comprising three curves: voltage, current, and power. During the 10-second short-circuit fault at the photovoltaic DC bus and after the 11-second fault clearing, the electrolytic cell voltage remained approximately 20V without significant fluctuations; the electrolytic cell current remained approximately 150kA without significant fluctuations; and the electrolytic cell power remained approximately 3MW without significant fluctuations. All three curves remained stable throughout the fault period. These results demonstrate that the present invention employs a parallel AC / DC power supply architecture, connecting the 330kV substation AC rectifier system and the DC power supply link. When a short-circuit fault occurs at the photovoltaic DC bus, only the photovoltaic grid connection system experiences transient voltage fluctuations, while the AC rectifier system can independently and stably supply power to the electrolytic cell load. The voltage and current of the electrolytic cell show no significant changes, verifying the fault isolation capability and anti-interference capability of the electrolytic cell load in this invention's combined power supply architecture, effectively avoiding the risk of power outages.

[0097] In summary, the control method for a photovoltaic DC-connected electrolytic aluminum combined power supply system proposed in this embodiment can realize long-distance and efficient transmission of photovoltaic energy, improve energy conversion efficiency, and enhance system fault adaptability and operational stability.

[0098] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0099] Any process or method description in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing custom logic functions or processes, and the scope of the preferred embodiments of this application includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the functions involved, as should be understood by those skilled in the art to which embodiments of this application pertain.

[0100] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A combined power supply system for photovoltaic DC-DC-connected electrolytic aluminum, characterized in that, The system includes: The photovoltaic power supply module consists of a photovoltaic cell array and a boost circuit. The boost circuit is configured with a voltage-controlled MPPT algorithm and a dual-closed-loop staged control strategy to achieve maximum power point tracking of the photovoltaic cell array. The multi-module LLC resonant converter consists of N LLC sub-modules. The input terminals of the N LLC sub-modules are connected in parallel to the output bus of the photovoltaic power supply module, and the output terminals are connected in series to form an input parallel output series architecture, which is used to boost the DC power output by the photovoltaic power supply module to medium voltage DC power. The DC transmission line, whose input end is connected to the output end of the multi-module LLC resonant converter, is used to transmit medium-voltage DC power from the photovoltaic power station to the electrolytic aluminum plant area over a long distance. A step-down converter, whose input terminal is connected to the output terminal of the DC transmission line, is used to step down and convert medium-voltage DC power into a voltage that meets the requirements of the electrolytic aluminum production process. The electrolytic cell load module has its input terminal connected to the output terminal of the step-down converter, and is used to receive DC power for aluminum electrolysis production. The output of the 330kV substation AC rectifier system is connected in parallel with the output of the step-down converter and then connected to the electrolytic cell load module to form an AC / DC combined power supply architecture.

2. The system as described in claim 1, characterized in that, The LLC submodule includes: High-frequency transformers are used to achieve voltage transformation and electrical isolation; The high-voltage side power unit consists of two half-bridge silicon carbide modules forming an H-bridge topology, which is connected to the primary side of the high-frequency transformer. The low-voltage side power unit consists of two half-bridge silicon carbide modules forming an H-bridge topology, which is connected to the secondary side of the high-frequency transformer. The high-voltage side resonant capacitor and the low-voltage side resonant capacitor are respectively configured on the high-voltage side and the low-voltage side of the high-frequency transformer, and are used to form a resonant circuit together with the leakage inductance of the high-frequency transformer. The leakage inductance of the high-frequency transformer serves as a resonant inductor, and the resonant circuit of the LLC submodule is composed of the leakage inductance and the resonant capacitor, eliminating the need for an additional independent resonant inductor.

3. The system as described in claim 2, characterized in that, The electrolytic cell load module adopts the cryolite-alumina molten salt electrolysis method, with the operating temperature controlled between 950℃ and 970℃. The cell voltage of the electrolytic cell satisfies the formula... Where Upol is the polarization voltage, Rcell is the cell resistance, and Icell is the electrolysis current. The polarization voltage Upol is 30% of the cell terminal voltage. By controlling the electrolyte concentration, cell temperature, and electrode distance, both the cell resistance Rcell and the polarization voltage Upol are kept constant to ensure a stable electrolysis current.

4. A control method for a photovoltaic DC-DC coupled power supply system for electrolytic aluminum based on any one of the systems described in claims 1-3, characterized in that, The method includes: The boost circuit of the photovoltaic power supply module is activated, and the boost circuit is controlled based on the maximum power point tracking algorithm to make the photovoltaic cell array operate at the maximum power point; The boost circuit is controlled by a dual closed-loop staged control strategy that combines the inner current loop and the outer voltage loop: during the first period after system startup, the output voltage of the boost circuit is used as the feedback quantity of the outer voltage loop to control the output voltage of the boost circuit to approach the bus voltage set value; after the first period, the output voltage of the photovoltaic array is switched to be used as the feedback quantity of the outer voltage loop to control the photovoltaic array to operate at the maximum power point. The DC power output from the boost circuit is input into the multi-module LLC resonant converter, which controls each sub-module to operate in parallel input and series output mode, boosting the DC power to a preset medium voltage level before outputting it. The boosted medium-voltage DC power is transmitted to the aluminum electrolysis plant area, and after being stepped down and converted, it supplies power to the electrolytic cell load. The AC rectifier system is controlled to operate in parallel with the DC power supply to form a combined AC and DC power supply mode.

5. The control method as described in claim 4, characterized in that, The maximum power point tracking algorithm is a voltage-controlled MPPT algorithm. It samples the output voltage of the photovoltaic array in real time and compares it with the maximum power point voltage Vmpp. Based on the comparison result, it dynamically adjusts the duty cycle of the power switching devices in the boost circuit so that the operating voltage of the photovoltaic array approaches and stabilizes at Vmpp.

6. The control method as described in claim 4, characterized in that, The first time period is the first 3 seconds after the system starts; The multi-module LLC resonant converter is composed of LLC sub-modules connected in parallel at the input and in series at the output, and the preset medium voltage level is ±30kV.

7. The control method as described in claim 4, characterized in that, In the dual-loop phased control strategy, the voltage outer loop uses a PI regulator to adjust the feedback quantity, and the current inner loop is used to quickly respond to current commands to suppress load disturbances.

8. The control method according to claim 4, characterized in that, The AC / DC combined power supply mode includes: When a short-circuit fault occurs on the photovoltaic DC bus, the AC rectifier system is controlled to independently supply power to the electrolytic cell load; When a short circuit fault occurs in the electrolytic aluminum busbar, the control subsystems will be adjusted and then restored to steady-state operation.

9. The control method as described in claim 4, characterized in that, The dual-closed-loop phased control strategy employs a soft-switching method during the transition from the first time period to the second time period, allowing the reference value of the outer voltage loop to smoothly transition from the bus voltage setpoint to the reference value corresponding to the maximum power point voltage.

10. The control method as described in claim 4, characterized in that, The multi-module LLC resonant converter adopts a voltage and current equalization control strategy for each sub-module. The output voltage of each sub-module is kept balanced through master-slave control, and the input terminals are connected in parallel to make the input current of each sub-module naturally equalized. When any LLC submodule of the multi-module LLC resonant converter fails, the bypass switch of the failed submodule is closed to bypass the output of the failed submodule and cause it to stop operating, while the non-faulty LLC submodules continue to operate.

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