Composite control device for stable production of green hydrogen by photovoltaic and energy storage with three-stage smooth switching

Abstract

The present invention provides a composite control device for stable production of green hydrogen by photovoltaic and energy storage systems considering three-stage smooth switching, which includes a photovoltaic and energy storage device, a power converter, a control module, a DC bus, and an electrolytic hydrogen production module; the photovoltaic and energy storage device is controlled by the power converter to transmit electric energy between the DC bus; the DC bus transmits electric energy to the electrolytic hydrogen production module; the control module is used to collect electrical quantity data, obtain the operating state of the electrolytic hydrogen production module according to the electrolytic voltage of the electrolyzer, and transmit the operating state and electrical quantity data to the power converter; the power converter smoothly switches the control state according to the operating state, the electrical quantities of the photovoltaic and energy storage device, the DC bus voltage, and the load current, and the control state includes grid-following control and grid-forming control. The composite control device provided by the present invention can achieve safe, stable, and efficient operation of electrolytic hydrogen production, ensure the continuity and stability of the hydrogen production process, and improve the efficiency and safety of electrolytic green hydrogen production.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic energy storage and hydrogen production, and particularly to a composite control device for stable production of green hydrogen by photovoltaic energy storage considering three-stage smooth switching. Background Art

[0002] Using new energy such as photovoltaic power to directly produce hydrogen generates green hydrogen with zero carbon emissions; in addition, hydrogen production is also an effective technical means to improve the consumption capacity of new energy. However, the volatility and intermittency of photovoltaic power generation pose challenges to the stable power supply required by hydrogen production equipment. Photovoltaic power generation systems rely on sunlight irradiation, so their power generation is significantly affected by weather conditions and day-night changes. This inherent volatility and intermittency cause photovoltaic power generation to be unable to continuously and stably supply power, especially at night or when sunlight is insufficient. Hydrogen production equipment, especially electrolyzers, requires stable current and voltage to operate efficiently. The volatility of photovoltaic power generation will cause unstable input to the electrolyzer, and the sharp fluctuation of current may cause the electrolyzer to deviate from the stable operating condition, affecting the electrode reaction rate, and further affecting the hydrogen production efficiency and safety.

[0003] To solve this problem, researchers have proposed various technical solutions. For example, using a DC / DC converter to track the change of photovoltaic power in real time can effectively suppress the adverse effect of photovoltaic energy fluctuation on the energy utilization rate. In addition, by designing an intelligent control algorithm, the DC / AC inverter can be controlled to optimize the performance when the photovoltaic direct-coupled hydrogen production system is connected to the grid. Adding an energy storage system, such as a storage battery, can be used as a "regulation stabilizer" for the hydrogen production system, store the peak power of photovoltaic power generation, and release it when sunlight is insufficient to provide a continuous and stable power supply. The above-mentioned "photovoltaic energy storage" grid-connected hydrogen production can use intelligent energy scheduling, grid optimization and operation strategies to synchronously adjust the operating power of the electrolyzer to fully ensure the stable operation of the hydrogen production system. However, hydrogen production with grid power is not green hydrogen production, and carbon emissions are generated during the hydrogen production process. In addition, although the above methods focus on studying how to achieve a stable state of hydrogen production and try to improve the stability of the photovoltaic hydrogen production system through an energy storage system and grid inertia control, they do not fully consider the changing load characteristics during the start-stop of hydrogen production.

[0004] For a photovoltaic hydrogen production system, a solution of directly coupling a photovoltaic power generation system to electrolyze water to produce hydrogen is adopted. These solutions usually rely on the electricity generated by a photovoltaic array to directly power an electrolyzer for electrolyzing water to produce hydrogen. However, these solutions may not fully consider the impact of the volatility and intermittency of photovoltaic power generation on the stability of the electrolyzer. An energy storage system such as a battery is combined with the photovoltaic power generation system to provide a more stable power supply to the electrolyzer. This "photovoltaic + energy storage" integrated solution can release the stored electricity when the light is insufficient to maintain the stable operation of the electrolyzer. But these solutions may not be specifically optimized for the start-stop characteristics of hydrogen production equipment. In addition, a DC / DC converter is used to track the change of photovoltaic power in real time to effectively suppress the adverse impact of photovoltaic energy fluctuations on the energy utilization rate. At the same time, the DC / AC inverter is controlled by an intelligent control algorithm to optimize the performance of the photovoltaic direct coupling hydrogen production system when it is connected to the grid. These technologies mainly focus on improving the stability of the photovoltaic hydrogen production system, but may not cover the load characteristics in the start-stop stage. For the control of the photovoltaic array, a maximum power point tracking (MPPT) control strategy is adopted to optimize the working state of the photovoltaic array and ensure a constant current input to the electrolyzer. This control strategy helps to maintain the stable operation of the electrolyzer under changing light conditions or partial shading of the photovoltaic array. Existing technologies may adopt a single control strategy, such as only using MPPT control or only using grid-forming control, without implementing a composite control strategy that automatically switches between grid-following and grid-forming control strategies according to the bus voltage level of the hydrogen production system. Summary of the Invention

[0005] Aiming at the problems existing in the prior art and to adapt to the volatility of photovoltaic power generation and the hydrogen production load in the three stages of steady state, stop, and start, and to ensure the safe and efficient operation of the electrolyzer, the present invention provides a composite control device for stable photovoltaic + energy storage green hydrogen production considering smooth switching in three stages.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows: A composite control device for stable photovoltaic + energy storage green hydrogen production considering smooth switching in three stages, comprising a photovoltaic + energy storage device, a power converter, a control module, a DC bus, and an electrolytic hydrogen production module;

[0007] The photovoltaic + energy storage device is controlled by the power converter to transmit electric energy to the DC bus, or the photovoltaic + energy storage device is controlled by the power converter to mutually transmit electric energy with the DC bus;

[0008] The DC bus transmits electric energy to the electrolytic hydrogen production module;

[0009] The control module is used to collect electrical quantity data, obtain the operating state of the electrolytic hydrogen production module according to the electrical quantity data, and transmit the operating state and the electrical quantity data to the power converter; the electrical quantity data includes the electrical quantity of the photovoltaic and energy storage device, the DC bus voltage, the electrolytic voltage of the electrolyzer of the electrolytic hydrogen production module, and the load current of the electrolytic hydrogen production module; the control module obtains the operating state of the electrolytic hydrogen production module according to the electrolytic voltage of the electrolyzer; the operating state includes a stable operation stage, a stop process, and a start process;

[0010] The power converter smoothly switches the control state according to the operating state, the electrical quantity of the photovoltaic and energy storage device, the DC bus voltage, and the load current, and the control state includes grid-following control and grid-forming control.

[0011] In some embodiments, when the operating state is the stable operation stage, the control state of the power converter is grid-following control;

[0012] When the operating state is the stop process: when the DC bus voltage is lower than the preset voltage or the load current is higher than the preset current, the control state of the power converter is grid-following control; when the DC bus voltage is not lower than the preset voltage and the load current is not higher than the preset current, the control state of the power converter smoothly switches to grid-forming control;

[0013] When the operating state is the start process: when the DC bus voltage is not lower than the preset voltage or the load current is not higher than the preset current, the control state of the power converter is grid-forming control; when the DC bus voltage is lower than the preset voltage and the load current is higher than the preset current, the control state of the power converter smoothly switches to grid-following control.

[0014] In some embodiments, the photovoltaic and energy storage device includes a new energy power generation device and an energy storage device, and the power converter includes a first power converter and a second power converter;

[0015] The new energy power generation device is controlled by the first power converter to transmit electric energy to the DC bus;

[0016] The energy storage device is controlled by the second power converter to mutually transmit electric energy with the DC bus.

[0017] In some embodiments, the grid-following control includes MPPT control and constant current discharge control, and the grid-forming control includes droop control.

[0018] In some of these embodiments, when the operating state is the stable operation stage: the control state of the first power converter is MPPT control, and the control state of the second power converter is constant current discharge control;

[0019] When the operating state is the stop process: when the DC bus voltage is lower than the preset voltage or the load current is higher than the preset current, the control state of the first power converter is MPPT control, and the control state of the second power converter is constant current discharge control; when the DC bus voltage is not lower than the preset voltage and the load current is not higher than the preset current, the first power converter and the second power converter smoothly switch to droop control;

[0020] When the operating state is the start process: when the DC bus voltage is not lower than the preset voltage or the load current is not higher than the preset current, the control states of the first power converter and the second power converter are droop control respectively; when the DC bus voltage is lower than the preset voltage and the load current is higher than the preset current, the control state of the first power converter smoothly switches to MPPT control, and the control state of the second power converter smoothly switches to constant current discharge control.

[0021] In some of these embodiments, the first power converter includes a boost conversion control circuit and a buck conversion control circuit;

[0022] The output end of the new energy power generation device is connected to the input end of the boost conversion control circuit, the output end of the boost conversion control circuit is connected to the input end of the buck conversion control circuit, and the output end of the buck conversion control circuit is connected to the DC bus.

[0023] In some of these embodiments, the boost conversion control circuit includes a new energy side circuit, a boost converter, a transmission line circuit, and a first control circuit;

[0024] The input voltage of the new energy side circuit is the output voltage of the new energy power generation device;

[0025] The first control circuit includes a first outer voltage loop, an intermediate voltage loop, and a first inner current loop;

[0026] The input values of the first outer voltage loop include the DC bus reference voltage and the input voltage of the new energy side circuit, and the output value of the first outer voltage loop is the reference voltage of the intermediate voltage loop;

[0027] The input values of the intermediate voltage loop include the reference voltage of the intermediate voltage loop and the output voltage of the transmission line circuit, and the output value of the intermediate voltage loop is the reference current of the first inner current loop;

[0028] The input value of the first inner current loop includes the reference current of the first inner current loop and the input current of the new energy side circuit, and the output value of the first inner current loop is the duty cycle of the boost converter.

[0029] In some embodiments, the buck conversion control circuit includes a buck converter, an output circuit, and a second control circuit;

[0030] The second control circuit includes a second outer voltage loop and a second inner current loop;

[0031] The input value of the second outer voltage loop includes the DC bus reference voltage and the DC bus voltage, and the output value of the second outer voltage loop is the reference current of the second inner current loop;

[0032] The input value of the second inner current loop includes the reference current of the second inner current loop and the output current of the output circuit, and the output value of the second inner current loop is the duty cycle of the buck converter.

[0033] In some embodiments, the second power converter is a bidirectional DC-DC power converter.

[0034] In some embodiments, when the electrolytic voltage of the electrolytic cell is the first load voltage, the operating state is the stable operation stage;

[0035] The stopping process includes a stopping start moment, a stopping duration stage, and a stopping end moment;

[0036] During the stopping process, the electrolytic voltage of the electrolytic cell first drops from the first load voltage to the second load voltage and then rises to the third load voltage; the moment when the electrolytic voltage of the electrolytic cell starts to drop from the first load voltage is the stopping start moment; the moment when the electrolytic voltage of the electrolytic cell rises to the third load voltage is the stopping end moment;

[0037] The starting process includes a starting start moment, a starting duration stage, and a starting end moment;

[0038] During the starting process, the electrolytic voltage of the electrolytic cell first rises from the third load voltage to the fourth load voltage and then drops to the first load voltage; the moment when the electrolytic voltage of the electrolytic cell starts to rise from the third load voltage is the starting start moment; the moment when the electrolytic voltage of the electrolytic cell drops to the first load voltage is the starting end moment.

[0039] In some embodiments, the preset current is 450A and the preset voltage is 41.4V.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] Based on the dynamic load characteristics of hydrogen production, the present invention combines the control of the power converter for the photovoltaic and energy storage devices with the electrolytic hydrogen production module through the control module, avoiding the mutation of the DC bus and the DC bus during the three stages of steady state, stop, and start, realizing the safe, stable, and efficient operation of the electrolytic hydrogen production module, ensuring the continuity and stability of the hydrogen production process, and improving the efficiency and safety of electrolytic green hydrogen production.

[0042] The traditional photovoltaic power generation control strategy is MPPT grid-following control, and energy storage is mostly constant current charge and discharge control. These grid-following controls do not have the ability to regulate the bus voltage level. The present invention realizes the full stability of the DC bus voltage and DC bus current at different operating stages during the entire operation period through the automatic smooth switching mode between grid-following control and grid-forming control, so that no sudden changes in voltage, current, and temperature are introduced during the hydrogen production process, and the start, stop, and stable operation conditions of hydrogen production can be matched.

[0043] Grid-forming control and grid-following control belong to different types of control. In the prior art, switches are often used to complete the switching between the two control loops, but there will be large transient fluctuations during the switching process. The present invention innovatively designs a control circuit that combines a boost conversion control circuit and a buck conversion control circuit, which can integrate the two controls of grid-following control and grid-forming control in a complete set of controls in stages, and realizes the automatic smooth switching between the two control methods by setting voltage thresholds. This control method can not only reduce the loss during the power transmission process of photovoltaic power generation, but also realize the automatic smooth switching between the two control modes through a control that only contains proportional and limiting links. The control strategy does not add a differential link, avoiding system oscillation or instability caused by unreasonable parameter settings, and enhancing the robustness of the control system. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a schematic structural diagram of a composite control device for stable green hydrogen production by photovoltaic and energy storage considering three-stage smooth switching according to the present invention;

[0045] Figure 2 It is a schematic structural diagram of the first power converter according to the present invention;

[0046] Figure 3 It is a control block diagram of the first control circuit according to the present invention;

[0047] Figure 4 It is a control block diagram of the second control circuit according to the present invention;

[0048] Figure 5 It is a change diagram of the electrolytic voltage of the electrolyzer at different operating stages in the calculation example according to the present invention;

[0049] Figure 6It is a schematic structural diagram of the electrolytic hydrogen production model in the calculation example of the present invention;

[0050] Figure 7 It is a comparison diagram of the simulation results of the actual water tank temperature and the water tank temperature data of the simulation model;

[0051] Figure 8 It is a control block diagram of the feedforward PID controller with an adaptive delay time in the calculation example of the present invention;

[0052] Figure 9 It is a change diagram of the electrolysis current of the electrolyzer in different operation stages in the calculation example of the present invention;

[0053] Figure 10 It is a change diagram of the temperature of the electrolyzer in different operation stages in the calculation example of the present invention;

[0054] Figure 11 It is a schematic diagram of the composite control method for stable production of green hydrogen by photovoltaic energy storage considering three-stage smooth switching in the calculation example of the present invention;

[0055] Figure 12 It is the volt-ampere characteristic curve of the photovoltaic power generation system and the energy storage device in the calculation example of the present invention;

[0056] Figure 13 It is the control output characteristic curve of the 1.8MW photovoltaic power generation system in the calculation example of the present invention;

[0057] Figure 14 It is the output voltage characteristic curve of the DC bus in the simulation experiment of the calculation example of the present invention;

[0058] Figure 15 It is the output current characteristic curve of the DC bus in the simulation experiment of the calculation example of the present invention. Specific embodiments

[0059] To clearly illustrate the technical features of the present solution, the following will combine the accompanying drawings and embodiments to detail the implementation manner of the present application, so as to fully understand how the present application uses technical means to solve technical problems and the implementation process of achieving corresponding technical effects and implement accordingly. Each feature in the embodiments of the present application and the embodiments can be combined with each other on the premise of not conflicting, and the formed technical solutions are all within the protection scope of the present application.

[0060] Fully considering the varying load characteristics during the start-up and shut-down of hydrogen production, first of all, the load characteristics of hydrogen production equipment during start-up and shut-down are very different from those during stable operation: during start-up, the electrolyzer needs to set an appropriate current density to achieve the cold start of the hydrogen production equipment; while during shut-down, the current cannot be suddenly interrupted to avoid rapid changes in the internal pressure of the electrolyzer, which may affect the safety and lifespan of the equipment. Secondly, photovoltaic power generation has changing light intensity due to day-night alternation, thus resulting in start-stop processes. Therefore, the start-stop conditions must be considered during the hydrogen production process. Ignoring the varying load characteristics during the start-up and shut-down of hydrogen production is not conducive to the safety and stability of hydrogen production.

[0061] Currently, there is little research on the control strategy for "photovoltaic-storage" green hydrogen production involving the three stages of "start-up, shut-down, and stability", and there is a lack of an integrated "photovoltaic-storage" composite control solution. In order to achieve the stable operation of the photovoltaic-storage hydrogen production system and ensure the hydrogen production efficiency, the present invention fully considers the load characteristics of different stages of "start-up, shut-down, and stability" during the green hydrogen production process, ensuring efficient and stable control at each stage; further combining various working conditions, a three-stage smoothly switched "photovoltaic-storage" integrated composite control device is proposed.

[0062] See Figure 1 , an embodiment of the present invention provides a composite control device for stable photovoltaic-storage green hydrogen production considering three-stage smooth switching, including a photovoltaic-storage device, a power converter, a control module, a DC bus, and an electrolytic hydrogen production module;

[0063] The photovoltaic-storage device is controlled by the power converter to transmit electric energy to the DC bus, or the photovoltaic-storage device is controlled by the power converter to mutually transmit electric energy with the DC bus;

[0064] The DC bus transmits electric energy to the electrolytic hydrogen production module;

[0065] The control module is used to collect electrical quantity data, to obtain the operating state of the electrolytic hydrogen production module based on the electrical quantity data, and to transmit the operating state and electrical quantity data to the power converter; See Figure 1 , the control module includes an electrical quantity acquisition unit, the electrical quantity acquisition unit is used to collect electrical quantity data, and the electrical quantity data includes the electrical quantity of the photovoltaic-storage device, the DC bus voltage, the electrolytic voltage of the electrolyzer in the electrolytic hydrogen production module, and the load current of the electrolytic hydrogen production module; the control module obtains the operating state of the electrolytic hydrogen production module based on the electrolytic voltage of the electrolyzer;

[0066] The power converter smoothly switches the control state according to the operating state, the electrical quantity of the photovoltaic-storage device, the DC bus voltage, and the load current, and the control state includes grid-following control and grid-forming control.

[0067] By combining the control of the power converter over the photovoltaic and energy storage device with the electrolytic hydrogen production module through the control module, stable control of voltage, current, and temperature is achieved. The start-up process and the stop process are inverse processes of each other. The control states of the power converter over the photovoltaic and energy storage device all involve smooth switching between grid-following control and grid-forming control. Under the composite control of smooth switching between grid-following control and grid-forming control, smooth conversion between different operating stages, i.e., different operating state conditions, during the entire operation period is realized, enabling the bus voltage and bus current to be fully stable during different operating stages of the entire operation period without sudden changes in voltage and current. It can match different operating conditions of the electrolytic hydrogen production module operation state, ensuring the safety and stability during the hydrogen production process and ensuring the maximum hydrogen production efficiency at the same time.

[0068] In some of these embodiments, the operating state includes a stable operation stage, a stop process, and a start-up process, that is, the device includes three different operating stages, namely, a stable operation stage, a stop process, and a start-up process, during the entire operation period;

[0069] When the operating state is the stable operation stage, the control state of the power converter is grid-following control; that is, when the operating state is the stable operation stage, the power converter adopts grid-following control. The temperature is stable during the stable operation stage, with constant current and constant voltage, that is, constant power operation;

[0070] When the operating state is the stop process: when the DC bus voltage is lower than the preset voltage or the load current is higher than the preset current, the control state of the power converter is grid-following control; when the DC bus voltage is not lower than the preset voltage and the load current is not higher than the preset current, the control state of the power converter smoothly switches to grid-forming control; that is, when the operating state is the stop process, the power converter first adopts grid-following control, and when the DC bus voltage is not lower than the preset voltage and the load current is not higher than the preset current, the power converter smoothly switches to grid-forming control. The stop process first has constant current boost and then constant current current reduction to achieve temperature reduction control; in some embodiments, the preset current is 450A and the preset voltage is 41.4V;

[0071] When the operating state is the start-up process: when the DC bus voltage is not lower than the preset voltage or the load current is not higher than the preset current, the control state of the power converter is grid-forming control; when the DC bus voltage is lower than the preset voltage and the load current is higher than the preset current, the control state of the power converter smoothly switches to grid-following control, that is, when the operating state is the start-up process, the power converter first adopts grid-forming control, and when the DC bus voltage is higher than the preset voltage and the load current is lower than the preset current, the power converter smoothly switches to grid-following control. The start-up process first has constant voltage current increase and then constant current voltage reduction to achieve temperature increase control.

[0072] By combining the control of the power converter over the photovoltaic and energy storage device with the electrolytic hydrogen production module through the control module, stable control of voltage, current, and temperature is achieved. The startup process and the stop process are inverse processes of each other. The control of the power converter over the photovoltaic and energy storage device involves smooth switching between grid-following control and grid-forming control. During the stable operation stage, the electrolytic hydrogen production module operates stably, and the control of the power converter over the photovoltaic and energy storage device is grid-following control, providing the maximum power for the system. Under the composite control of smooth switching between grid-following control and grid-forming control, smooth conversion between different operating conditions in three stages can be achieved throughout the operation period, enabling the bus voltage and bus current to be fully stable during the entire three stages without sudden changes in voltage and current, matching the start-stop and stable operation conditions of the electrolytic hydrogen production module, ensuring the safety and stability during the hydrogen production process, and at the same time ensuring the maximum hydrogen production efficiency. Meanwhile, the hydrogen production efficiency during the stable operation stage reaches 86%, meeting the standard requirements.

[0073] See Figure 1 , in some of these embodiments, the photovoltaic and energy storage device includes a new energy power generation device and an energy storage device, and the power converter includes a first power converter and a second power converter; modularizing the photovoltaic and energy storage device into a new energy power generation device and an energy storage device enables precise control for different modules, simplifies the control while ensuring control accuracy; preferably, the new energy power generation device is a photovoltaic power generation system, and the energy storage device is a storage battery;

[0074] The new energy power generation device is controlled by the first power converter to transmit electric energy to the DC bus.

[0075] The energy storage device is controlled by the second power converter to mutually transmit electric energy with the DC bus. Preferably, the second power converter is a bidirectional DC-DC power converter, which can achieve bidirectional energy flow. It can not only output the electric energy in the energy storage device to the DC bus to supply power to the electrolytic hydrogen production module, but also recover the excess electric energy into the energy storage device when there is surplus electric energy and has a voltage regulation function, that is, it can automatically adjust the output voltage of the energy storage device according to the change of the DC bus voltage and the load demand, ensuring a stable power supply for the electrolytic hydrogen production module and further ensuring the stability of the electrolytic hydrogen production process.

[0076] The photovoltaic power generation system generates electric energy, is controlled by the first power converter, and is transmitted to the DC bus. The storage battery is controlled by the bidirectional DC-DC power converter to provide stable DC bus voltage support. Preferably, the control module is connected to the DC bus, obtains electric energy from the DC bus, and controls the bidirectional DC-DC power converter and the first power converter according to the load voltage and load current. Electric quantity acquisition is used to monitor the operation status of the entire device to ensure stable power conversion.

[0077] In some of these embodiments, the grid-following control includes MPPT control and constant-current discharge control, and the grid-forming control includes droop control.

[0078] In some of these embodiments, when the operating state is the stable operating stage: the control state of the first power converter is MPPT control, and the control state of the second power converter is constant-current discharge control;

[0079] When the operating state is the stopping process: when the DC bus voltage is lower than the preset voltage or the load current is higher than the preset current, the control state of the first power converter is MPPT control, and the control state of the second power converter is constant-current discharge control; when the DC bus voltage is not lower than the preset voltage and the load current is not higher than the preset current, the first power converter and the second power converter smoothly switch to droop control;

[0080] When the operating state is the starting process: when the DC bus voltage is not lower than the preset voltage or the load current is not higher than the preset current, the control states of the first power converter and the second power converter are droop control respectively; when the DC bus voltage is lower than the preset voltage and the load current is higher than the preset current, the control state of the first power converter smoothly switches to MPPT control, and the control state of the second power converter smoothly switches to constant-current discharge control.

[0081] See Figure 2 , in some of these embodiments, the first power converter includes a boost conversion control circuit and a buck conversion control circuit; Figure 2 , the photovoltaic panel represents a new energy power generation device, the Boost converter represents a boost converter, the Buck converter represents a buck converter, the circuit between the photovoltaic panel and the Buck converter is a boost conversion control circuit, and the Buck converter and the circuit between the Buck converter and the DC bus are a buck conversion control circuit;

[0082] The output end of the new energy power generation device is connected to the input end of the boost conversion control circuit, the output end of the boost conversion control circuit is connected to the input end of the buck conversion control circuit, and the output end of the buck conversion control circuit is connected to the DC bus.

[0083] See Figure 2 , in some of these embodiments, the boost conversion control circuit includes a new energy side circuit, a boost converter, a transmission line circuit, and a first control circuit; Figure 2 , is the new energy side capacitor, is the new energy side inductor, is the transmission line capacitor; the input voltage of the new energy side circuit is the output voltage of the new energy power generation device ;

[0084] See Figure 3, the first control circuit includes a first outer - loop voltage loop, an intermediate voltage loop, and a first inner - loop current loop;

[0085] The input value of the first outer - loop voltage loop includes the DC - bus reference voltage and the input voltage of the new - energy - side circuit, which is the output voltage of the new - energy power generation equipment , and the output value of the first outer - loop voltage loop is the reference voltage of the intermediate voltage loop , the deviation between the input voltage of the new - energy - side circuit and the DC - bus reference voltage is amplified by the new - energy - side voltage proportionality coefficient and the reference voltage of the intermediate voltage loop is obtained after passing through a limiting link ;

[0086] The input value of the intermediate voltage loop includes the reference voltage of the intermediate voltage loop and the output voltage of the transmission - line circuit , and the output value of the intermediate voltage loop is the reference current of the first inner - loop current loop , the deviation between the output voltage of the transmission - line circuit and the reference voltage of the intermediate voltage loop is amplified by the transmission - line voltage proportionality coefficient and the reference current of the first inner - loop current loop is obtained after passing through a limiting link ;

[0087] The input value of the first inner - loop current loop includes the reference current of the first inner - loop current loop and the input current of the new - energy - side circuit , and the output value of the first inner - loop current loop is the duty ratio of the boost converter , the deviation between the input current of the new - energy - side circuit and the reference current of the first inner - loop current loop is amplified by the new - energy - side current proportionality coefficient and the duty ratio of the boost converter is obtained after passing through a limiting link , and the boost converter is controlled according to the duty ratio of the boost converter .

[0088] Similarly, referring to Figure 2 , in some embodiments, the buck - conversion control circuit includes a buck converter, an output circuit, and a second control circuit; Figure 3 In , is the output - side capacitor,

[0089] Referring to Figure 4 , the second control circuit includes a second outer - loop voltage loop and a second inner - loop current loop;

[0090] The input value of the second outer - loop voltage loop includes the DC - bus reference voltage and the DC - bus voltage The output value of the second outer - loop voltage loop is the reference current of the second inner - loop current loop The deviation between the DC - bus reference voltage and the DC - bus voltage is amplified by the output - side voltage proportionality coefficient and the reference current of the second inner - loop current loop is obtained after passing through a limiting link;

[0091] The input value of the second inner - loop current loop includes the reference current of the second inner - loop current loop and the output current of the output circuit The output value of the second inner - loop current loop is the duty cycle of the buck converter The deviation between the reference current of the second inner - loop current loop and the output current of the output circuit is amplified by the output - side current proportionality coefficient and the duty cycle of the buck converter is obtained after passing through a limiting link. The buck converter is controlled according to the duty cycle of the buck converter ;

[0092] By combining the boost - conversion control circuit and the buck - conversion control circuit to implement the control strategy of the first power converter, the grid - following control and the grid - forming control can be fused in stages in a complete set of control. By setting the voltage threshold, i.e., the preset voltage, the automatic and smooth switching between the two control modes is realized. This control method combining the boost - conversion control circuit and the buck - conversion control circuit can not only reduce the loss in the process of photovoltaic power transmission, but also realize the automatic and smooth switching between the grid - following control and the grid - forming control through the control with only proportional and limiting links.

[0093] The control strategy does not add a differential link, avoiding system oscillation or instability caused by unreasonable parameter settings and enhancing the robustness of the control system.

[0094] In some of the embodiments, when the electrolytic voltage of the electrolytic cell is the first load voltage, the operating state is the stable - operation stage;

[0095] The stopping process includes a stopping start time, a stopping duration stage, and a stopping end time;

[0096] During the stopping process, the electrolytic voltage of the electrolytic cell first drops from the first load voltage to the second load voltage and then rises to the third load voltage; the moment when the electrolytic voltage of the electrolytic cell starts to drop from the first load voltage is the stopping start time; the moment when the electrolytic voltage of the electrolytic cell rises to the third load voltage is the stopping end time;

[0097] The startup process includes a startup start time, a startup duration phase, and a startup end time;

[0098] During the startup process, the electrolytic voltage of the electrolyzer first rises from the third load voltage to the fourth load voltage and then drops to the first load voltage; the moment when the electrolytic voltage of the electrolyzer starts to rise from the third load voltage is the startup start time; the moment when the electrolytic voltage of the electrolyzer drops to the first load voltage is the startup end time.

[0099] Example

[0100] Considering the dynamic load characteristics of the hydrogen production module of the electrolyzer, mainly its start-stop characteristics, on the premise of considering the influence of gas flow and temperature change on hydrogen production efficiency, the present invention provides an electrolytic hydrogen production model for electrolytic hydrogen production simulation experiments, which is used to replace the electrolytic hydrogen production module in the composite control device for stable production of green hydrogen by integrating photovoltaics and energy storage with three-stage smooth switching in this example. The model includes a hydrogen production module, a temperature dynamic module, and an electrolysis control model. The hydrogen production module includes an electrolyzer voltage model and an electrolytic heat generation model. The temperature dynamic module includes an electrolyzer temperature model, a water tank temperature model, a heat exchanger temperature model, and a chiller; the electrolysis control model is used to control the hydrogen production module and the temperature dynamic module; considering that most startup methods of the electrolyzer hydrogen production module are cold startups, the operating temperature after startup can only reach the expected optimal operating state by generating heat through the electrolytic heat generation model. Therefore, the electrolytic hydrogen production model adopts the interaction between the hydrogen production module containing electricity, heat, and gas and the temperature dynamic model to fit the stable, stop, and start three operating stages of the actual electrolyzer hydrogen production module.

[0101] Preferably, the electrolytic hydrogen production model is built using Simulink; see Figure 6 , is the electrolytic voltage of the electrolyzer, is the water tank temperature, is the opening degree of the cold water outlet valve, is the electrolytic heat generation of the electrolyzer reaction, is the electrolyzer temperature. The connecting lines in the figure respectively represent the transfer of relevant quantities between models; the chiller reduces the temperature of the cooling water to a preset temperature, usually the preset temperature is 20 °C, and then the chiller transports the cooling water at the preset temperature to the heat exchanger temperature model. The heat generated by the electrolytic heat generation model, that is, the electrolytic heat generation of the electrolyzer reaction is transferred to the electrolyzer temperature model to increase the temperature of the electrolyzed water in the electrolyzer temperature model to form the electrolyzer temperature , the electrolyzer temperature is transferred to the water tank temperature model along with the electrolyzed water to form the water tank temperature , the water tank temperature The electrolyzed water exchanges heat with the cooling water in the heat exchanger temperature model, that is, the cooling water absorbs the heat released by the electrolyzed water, thereby reducing the temperature of the electrolyzed water. The electrolyzed water transfers heat to the cooling water, and its own temperature decreases and then flows into the electrolyzer temperature model. To verify Figure 6 the effectiveness of the temperature dynamic simulation model of the electrolyzer in Figure 7 this example, the temperature data of the water tank (actual water tank) in the real electrolytic hydrogen production module collected by the sensor (the actual temperature curve in Figure 7 ) is compared with the simulation result of the temperature data in the water tank temperature model of the electrolytic hydrogen production model (simulation model) (the simulation temperature curve in

[0102] ). It can be seen that the cold start temperature change curve of the simulation model is basically consistent with the temperature change of the actual water tank, and the maximum temperature deviation is 1.6 °C. Based on the hydrogen production module and the temperature dynamic module, considering the stability and high efficiency of electrolytic hydrogen production at the same time, this example proposes a feedforward PID controller based on adaptive delay time as the electrolysis control model to control the temperature of the temperature dynamic model, ensuring that the temperature of the electrolyzer is about 65 °C under the operating conditions of the stable operation stage. Figure 8 In is the temperature reference value, is the feedforward control quantity, is the feedback control quantity of the PID controller. By comparing with the PID control effect, it is found that the feedforward PID controller with adaptive delay time proposed by the present invention can better fit the actual temperature control requirements.

[0103] Three interpolation points , and are selected. The feedforward control quantity in the feedforward PID controller with adaptive delay time is shown in formula (1):

[0104] (1)

[0105] The reference quantity transmitted to the proportional integral controller after adaptive start adjustment is shown in formula (2):

[0106] (2)

[0107] In the formula, is the step function, is the parameter switching temperature point, is the difference between the electrolyzer temperature and the temperature reference value;

[0108] The feedback control quantity of the PID controller is shown in formula (3):

[0109] (3)

[0110] Wherein, is the proportional coefficient of the PID controller, is the integral coefficient of the PID controller, is the derivative coefficient of the PID controller.

[0111] Based on fully considering the gas flow and temperature dynamics, the electrolytic hydrogen production model of the device can better fit the actual working state. At the same time, through the feedforward PID controller with an adaptive delay time of variable starting points, the stability of the electrolytic cell temperature in the stable operation stage of the electrolytic hydrogen production model can be ensured.

[0112] Replace the electrolytic hydrogen production module in the hybrid control device for stable production of green hydrogen from photovoltaic and energy storage considering three-stage smooth switching with the electrolytic hydrogen production model. The cross-sectional area of the connecting wire is 250 cm -2 , and the electrolytic cell current of the electrolytic hydrogen production model at different operation stages during the entire operation period is shown in Figure 9 , the electrolytic voltage of the electrolytic cell is shown in Figure 5 , and the temperature of the electrolytic cell is shown in Figure 10 :

[0113] (1) In the stable operation stage, the current density is 1.8 A·cm -2 , at this time, the temperature of the electrolytic cell tends to be constant, and the load voltage, that is, the electrolytic voltage of the electrolytic cell, is constant, realizing constant power hydrogen production.

[0114] (2) During the stopping process, first ensure that the current density decreases step by step ((1.8 → 1 → 0.6 → 0.28) A·cm -2 ), set the external cold water switch to the maximum, and start to lower the temperature. Gradually increase the cooling rate while avoiding a large range drop in the load voltage.

[0115] (3) During the starting process, the current density is maintained at 1.8 A·cm -2 , and the maximum power is used to increase the temperature, simulating the working condition of the electrolytic hydrogen production model starting with a constant current density. The temperature of hydrogen production rises to 65°C and operates stably. The current density is still 1.8 A·cm -2 , at this time, the temperature is constant, the voltage is constant, and constant power hydrogen production is achieved.

[0116] The above three-stage control can ensure that the current density during the electrolytic hydrogen production process is between 0.2 and 1.8·cm -2 , ensuring the safety and stability of hydrogen production.

[0117] Considering the coupling relationship among the three variables of electrolyzer temperature, load voltage, and load current (i.e., electrolysis current of the electrolyzer), the temperature is regarded as the key intermediate variable, and the load characteristic changes of the electrolyzer in the three stages of steady state, shutdown, and startup are discussed. The change curve of the electrolyzer temperature is verified through simulation, as Figure 10 . The change of the load voltage during the electrolytic hydrogen production in the three stages of simulation steady state, shutdown, and startup is shown in Appendix Figure 5 . Thus far, based on the electrolytic hydrogen production model, by simulating the electrical characteristics of the electrolyzer and using an equivalent resistance to fit the working characteristic curve of the electrolyzer, the relationship between the load current and the load voltage during the operation of the electrolyzer is obtained.

[0118] The electrolytic hydrogen production model controls and fits the change of the hydrogen production load characteristics in the three stages, as Figure 11 shown.

[0119] During the startup process:

[0120] Firstly, the voltage is kept constant and the current is increased to complete the pre-startup. At this time, the electrolyzer operates within the set voltage range (between 41V - 43V), and the power converter adopts droop control with the voltage remaining basically unchanged. A small power is used for preheating to raise the current density to 1.8 A·cm -2 , that is, the preset current of 450 A is reached. At this time, the electrolyzer utilizes the maximum heat production power, which is the heating stage.

[0121] Secondly, the current is kept constant and the voltage is decreased for heating. At this time, the current density remains at 1.8 A·cm -2 , achieving the maximum power heating, simulating the operating condition of the electrolytic hydrogen production module starting at a constant current density; the electrolyzer is cold-started to raise its operating temperature to the expected optimal operating state through electrolytic heat production. During the startup process of this stage, the input current of the electrolyzer needs to be kept stable to ensure the continuity of the electrolysis reaction development. The first power converter executes MPPT control. By optimizing the operating state of the photovoltaic array, it ensures that the electrolyzer obtains a constant current input, and can remain stable even under changing light conditions or partial shading of the photovoltaic array. However, as the temperature of the electrolyzer increases, the resistivity of the electrolyte decreases, which causes the operating voltage of the electrolyzer to drop. Therefore, adopting MPPT control in this stage can adjust the output voltage of the photovoltaic power generation system according to the real-time monitored voltage change to meet the operating requirements of the electrolyzer.

[0122] During the stable operation stage: The voltage and current are kept constant for stable operation. Under this operating condition, the current density is still 1.8 A·cm -2 , at this time the temperature is constant, the voltage is constant, and hydrogen is produced at a constant power. When the electrolyzer operates stably, the photovoltaic panel produces hydrogen at a constant power through the maximum power point tracking (MPPT) technology, while the energy storage system provides the necessary energy buffering and regulation to cope with the volatility of photovoltaic power generation.

[0123] During the stop process:

[0124] First, boost the voltage at a constant current to cool down. The first power converter performs MPPT control to keep the current still stable at 1.8 A·cm -2 , but by opening the external total cold water switch, the temperature control loop loses its effectiveness, and the temperature drops first. Considering the relationship between the volt-ampere characteristics of the electrolyzer and the temperature, the voltage exceeds the limit (41V - 43V) as the temperature drops.

[0125] Secondly, boost the current at a constant voltage. After the pre-cooling process, the electrolysis voltage of the electrolyzer exceeds the limit (41V - 43V) as the temperature drops. At this time, droop control is adopted during the electrolysis process, and the electrolysis voltage of the electrolyzer is stabilized between 41V and 43V. At the same time, the cooling system is still working continuously, and the temperature drops. Considering the influence of the temperature on the volt-ampere characteristics of the electrolyzer, the current drops, and the load reduction stop is achieved.

[0126] In this disclosure, the preset current is 450A. According to the volt-ampere characteristics of the photovoltaic power generation system and the energy storage device, see Figure 12 , the voltage range of the droop control is 41V - 43V.

[0127] Table 1 Design parameters of the photovoltaic panel

[0128]

[0129] It can be concluded that when the DC bus voltage drops to 41V, the Figure 2 output current of the photovoltaic and energy storage device in it needs to be 450A, which is the output current when smoothly switching from the droop control mode to the MPPT control mode. It can be calculated that the output power at the turning point, that is, the switching point from the droop control mode to the MPPT control mode, is 18450W. According to the detailed values of the design parameters of the photovoltaic panel in Table 1, if photovoltaic panels with a maximum output power of 200W per panel are used, 90 photovoltaic panels are required in this example, and the calculated maximum output current is 450A.

[0130] From the P-V curve of the photovoltaic panel, it can be seen that when the operating voltage at the maximum power point is 64V, when the operating voltage is between 64V and the turning point voltage, the P-V curve is a non-linear curve, that is, the maximum power operating area. At this time, the photovoltaic power generation system operates in the MPPT control mode to supply power to the electrolytic hydrogen system; when the voltage is lower than the turning point voltage, the P-V curve is an approximately linear curve, and at this time, the photovoltaic power generation system operates in the droop control mode to provide voltage support for the DC system bus. From the Figure 12 it can be seen that when the photovoltaic power generation system switches from the maximum power point area to the low power area on the left, the current remains basically constant. In order to achieve the goal of "outputting low voltage in the photovoltaic maximum power area and high voltage in the low power area", it is necessary to design a power converter and its control strategy.

[0131] From the appendixFigure 3 It can be seen that the first control circuit corresponding to the Boost converter includes a first outer-loop voltage loop, an intermediate voltage loop, and a first inner-loop current loop; the DC bus reference voltage is set to 64V, and the output voltage of the new energy power generation device fed back and the DC bus reference voltage The difference gradually increases after passing through the proportional and limiting links, so the output voltage of the new energy power generation device gradually decreases, and at the same time, the reference voltage of the intermediate voltage loop is output . Through the first outer-loop voltage loop, the reference voltage of the intermediate voltage loop output by the Boost gradually increases. Therefore, the voltage control of the first outer-loop voltage loop realizes the transformation of the input link with gradually decreasing voltage into the output link with gradually increasing voltage. The droop interval operating point of the photovoltaic is set on the left side of the maximum power point because the current and power change greatly on the right side of the maximum power point, and it is not easy to achieve stable control. The intermediate voltage loop also outputs the reference current of the first current inner loop through the proportional and limiting links, approaching the photovoltaic constant current, that is, the preset current of 450A. Since the outer-loop proportional coefficient is large and the voltage increases rapidly, although the proportional coefficient of the intermediate link is limited, the Boost output is likely to exceed the DC bus reference voltage, so the voltage over-limit is restricted by limiting. The current inner loop adjusts the duty cycle according to the difference between the reference current of the first inner-loop current loop and the input current of the new energy side circuit .

[0132] The second control circuit corresponding to the Buck converter includes a second outer-loop voltage loop and a second inner-loop current loop, that is, double-loop control, to reduce the voltage to 41V; in addition, adding a buck conversion control circuit after the boost conversion control circuit can, to a certain extent, prevent the problem of open circuit of the boost conversion control circuit and infinite rise of the output voltage, and the buck conversion control circuit can limit the output voltage below the input voltage, thus avoiding this problem.

[0133] Taking the photovoltaic power generation system as an example, a simulation experiment is carried out to verify this example. The simulation experiment uses a device with a power rating of 1.8MW, the upper voltage limit of the DC bus is 43V, and the lower voltage limit is 41V, as shown in the appendix Figure 13 . The photovoltaic control block diagram and simulation results are as shown in Figure 2-4 , and the MPPT control and droop control are unified into the same control loop. The simulation output results are as shown in Figure 14 , 15 . It can be seen that according to the bus voltage level, this example can achieve smooth switching in three stages during operation.

[0134] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than limiting the protection scope of the present invention. Any simple modification or equivalent replacement made by those of ordinary skill in the art to the technical solution of the present invention shall not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A composite control device for stable production of green hydrogen by photovoltaic storage considering three-stage smooth switching, characterized by: It includes photovoltaic storage equipment, power converter, control module, DC bus and electrolysis hydrogen production module; The photovoltaic storage device is controlled by the power converter to transmit electric energy to the DC bus, or the photovoltaic storage device is controlled by the power converter to transmit electric energy to and from the DC bus; The DC bus transmits electric energy to the electrolysis hydrogen production module; The control module is used to collect electrical quantity data, to obtain the operating state of the electrolytic hydrogen production module according to the electrical quantity data, and to transmit the operating state and the electrical quantity data to the power converter; the electrical quantity data includes the electrical quantity of the photovoltaic storage device, the DC bus voltage, the electrolysis voltage of the electrolytic cell of the electrolytic hydrogen production module and the load current of the electrolytic hydrogen production module; the control module obtains the operating state of the electrolytic hydrogen production module according to the electrolysis voltage of the electrolytic cell; the operating state includes a stable operation stage, a stop process and a start process; The power converter smoothly switches the control state according to the operating state, the electrical quantity of the photovoltaic storage device, the DC bus voltage and the load current, and the control state includes grid-following control and grid-forming control; The photovoltaic storage device includes a new energy power generation device and an energy storage device, and the power converter includes a first power converter and a second power converter; The new energy power generation equipment is controlled by the first power converter to transmit electric energy to the DC bus; The energy storage device is controlled by the second power converter to transmit electric energy to and from the DC bus; When the operating state is the stable operating stage: the control state of the first power converter is MPPT control, and the control state of the second power converter is constant current discharge control; When the operating state is the stop process: when the DC bus voltage is lower than the preset voltage or the load current is higher than the preset current, the control state of the first power converter is MPPT control, and the control state of the second power converter is constant current discharge control; when the DC bus voltage is not lower than the preset voltage and the load current is not higher than the preset current, the first power converter and the second power converter are smoothly switched to droop control; When the operating state is the startup process: when the DC bus voltage is not lower than the preset voltage or the load current is not higher than the preset current, the control states of the first power converter and the second power converter are respectively droop control; when the DC bus voltage is lower than the preset voltage and the load current is higher than the preset current, the control state of the first power converter is smoothly switched to MPPT control, and the control state of the second power converter is smoothly switched to constant current discharge control.

2. According to claim 1, the composite control device for stable green hydrogen production by photovoltaic storage taking into account three-stage smooth switching is characterized by: The first power converter includes a boost conversion control circuit and a buck conversion control circuit; The output end of the new energy power generation equipment is connected to the input end of the boost conversion control circuit, the output end of the boost conversion control circuit is connected to the input end of the buck conversion control circuit, and the output end of the buck conversion control circuit is connected to the DC bus.

3. The composite control device for stable green hydrogen production by photovoltaic storage considering three-stage smooth switching according to claim 2 is characterized in that: The boost conversion control circuit includes a new energy side circuit, a boost converter, a transmission line circuit and a first control circuit; The input voltage of the new energy side circuit is the output voltage of the new energy power generation equipment; The first control circuit includes a first outer voltage loop, an intermediate voltage loop and a first inner current loop; The input value of the first outer voltage loop includes the DC bus reference voltage and the input voltage of the new energy side circuit, and the output value of the first outer voltage loop is the reference voltage of the intermediate voltage loop; The input value of the intermediate voltage loop includes the reference voltage of the intermediate voltage loop and the output voltage of the transmission line circuit, and the output value of the intermediate voltage loop is the reference current of the first inner current loop; The input value of the first inner current loop includes a reference current of the first inner current loop and an input current of the new energy side circuit, and the output value of the first inner current loop is a duty cycle of the boost converter.

4. The composite control device for stable green hydrogen production by photovoltaic storage considering three-stage smooth switching according to claim 2 is characterized in that: The step-down conversion control circuit includes a step-down converter, an output circuit and a second control circuit; The second control circuit includes a second outer voltage loop and a second inner current loop; The input value of the second outer voltage loop includes a DC bus reference voltage and a DC bus voltage, and the output value of the second outer voltage loop is a reference current of the second inner current loop; The input value of the second inner current loop includes a reference current of the second inner current loop and an output current of the output circuit, and the output value of the second inner current loop is a duty cycle of the buck converter.

5. According to claim 1, the composite control device for stable green hydrogen production by photovoltaic storage taking into account three-stage smooth switching is characterized by: When the electrolysis voltage of the electrolytic cell is the first load voltage, the operation state is the stable operation stage; The stopping process includes a stopping start time, a stopping duration phase and a stopping end time; During the stopping process, the electrolysis voltage of the electrolytic cell is first stepped down from the first load voltage to the second load voltage and then stepped up to the third load voltage; the moment when the electrolysis voltage of the electrolytic cell starts to step down from the first load voltage is the stopping start moment; the moment when the electrolysis voltage of the electrolytic cell is stepped up to the third load voltage is the stopping end moment; The startup process includes a startup start time, a startup duration phase, and a startup end time; During the startup process, the electrolysis voltage of the electrolytic cell is first boosted from the third load voltage to the fourth load voltage and then reduced to the first load voltage; the moment when the electrolysis voltage of the electrolytic cell starts to be boosted from the third load voltage is the startup start time; the moment when the electrolysis voltage of the electrolytic cell is reduced to the first load voltage is the startup end time.

6. The composite control device for stable green hydrogen production by photovoltaic storage considering three-stage smooth switching according to any one of claims 1 to 5, characterized in that: The preset current is 450A, and the preset voltage is 41.4V.

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