A power control method for a wind-solar coupled off-grid electrolysis hydrogen production system

Abstract

This invention discloses a power control method for a wind-solar coupled off-grid electrolysis hydrogen production system, comprising the following steps: Step 1: Designing output control equations for the bottom-level controller of the wind-solar coupled off-grid electrolysis hydrogen production system, and controlling the DC / DC converter in the system according to the designed control equations; Step 2: Controlling the operation of the wind-solar coupled off-grid electrolysis hydrogen production system according to a set strategy. This invention not only achieves bus voltage control of the microgrid electrolysis hydrogen production system using energy storage units, water electrolysis hydrogen production units, and unloading units—with the energy storage unit establishing bus voltage support, the water electrolysis hydrogen production unit implementing auxiliary control, and the unloading unit providing over-limit protection—but also enables the protection and recovery of the lithium battery SoC, thereby improving the service life of the energy storage unit. Simultaneously, this control method fully considers the system's transient response, utilizing the lithium battery and unloading unit to collaboratively absorb unbalanced power and maintain stable bus voltage, thus ensuring stable system operation.

Description

Technical Field

[0001] This invention relates to the fields of microgrid technology, distributed generation technology, and hydrogen production technology, and particularly to a power control method for a wind-solar coupled off-grid electrolysis hydrogen production system. Background Technology

[0002] In recent years, with the increasing prevalence of environmental degradation such as global warming, countries have imposed increasingly stringent carbon emission targets. To achieve sustainable development, distributed energy technologies have been developed, and the use of clean energy sources such as wind and solar power has significantly reduced the proportion of energy derived from traditional oil and natural gas, alleviating the energy crisis to some extent. Producing hydrogen using clean energy can achieve a closed-loop, pollution-free process from preparation to use. This energy conversion method can reduce harmful gas emissions, improve air quality, and align with sustainable development and emission reduction goals. However, due to the intermittent and random nature of renewable energy sources such as wind and solar power, large-scale grid connection can cause voltage fluctuations and deteriorate power quality. To address the grid connection issues of renewable energy power, microgrid technology has emerged.

[0003] A microgrid is a miniature power supply system that integrates distributed power sources, energy storage systems, and loads through an interface converter. Microgrids can operate in grid-connected or islanded DC mode, depending on actual needs. Based on bus type, microgrids can be divided into AC microgrids and DC microgrids. Compared to AC microgrids, DC microgrids do not require consideration of reactive power and frequency, making voltage tracking control easier to implement and ensuring system stability. Furthermore, photovoltaic and energy storage units in microgrids are DC micro-sources, and DC loads such as alkaline electrolyzers and electric vehicles are becoming increasingly common. Connecting these micro-sources and loads to an AC microgrid system requires cascading DC / DC and DC / AC converters, which inevitably reduces energy conversion efficiency and increases system control complexity. Conversely, connecting them to a DC microgrid system requires only a single DC / DC converter, significantly improving energy conversion efficiency while simplifying system structure and enhancing system stability.

[0004] Against this backdrop, DC microgrids are increasingly attracting attention from academia and industry due to their high efficiency and reliability. In DC microgrid systems, bus voltage is a key indicator of system stability. Power imbalances between sources and loads can cause voltage rises or falls, while the intermittency of distributed energy sources can also cause bus voltage fluctuations. In microgrid systems, incorporating energy storage devices and employing appropriate control strategies can mitigate bus power fluctuations and improve system reliability and power quality. Currently, to address voltage fluctuation issues, scholars both domestically and internationally have conducted extensive research, and relevant literature has been published on DC bus voltage control methods, such as:

[0005] 1. In their Chinese patent application No. 202010878014.9, entitled "Adaptive Method for Isolated Photovoltaic-Storage DC Microgrid Mode," Zhang Kaitao et al. implemented system control at different stages of bus voltage by real-time monitoring. This included controlling the voltage using a photovoltaic unit bus voltage control loop when the bus voltage exceeded a first preset voltage; controlling the bus voltage using the energy storage unit when the bus voltage reached a second preset voltage; and controlling the bus voltage continuously when the bus voltage fell below a third preset voltage, indicating an overload zone. While this method considered the charging and discharging conditions of the energy storage system and the segmented control of the bus voltage, it failed to take measures to control the bus voltage when it fell below the third preset voltage. This resulted in large fluctuations in the bus voltage during sudden load changes, failing to guarantee stable system operation. Furthermore, due to the intermittent nature of the photovoltaic units, changes in light intensity inevitably caused bus voltage variations, making bus voltage control unsuitable.

[0006] 2. In the article titled "Coordinated Control Strategy for Energy Management of Photovoltaic-Storage AC Microgrids under Islanded DC Mode," author Wen Sufang utilizes lithium batteries as the main power source to establish bus voltage support. The photovoltaic units operate in maximum power point tracking (MPPT) or power-limited load matching mode based on the internal energy relationships of the microgrid. Simultaneously, load switching control is employed to prevent battery over-discharge. However, this control strategy has the following problems: when the batteries cannot absorb excess power, excess photovoltaic power is discarded, resulting in light curtailment; the system uses an AC bus, leading to low energy conversion efficiency; furthermore, frequent switching is not possible for alkaline electrolyzer-type loads. Therefore, this control strategy is not applicable to alkaline electrolyzer hydrogen production systems.

[0007] 3. In the paper "Modeling and Coordinated Control of Island DCHybrid System of Wind / PV / Electrolyzer and BESS" published by X. Xiaowen et al., 2021 International Conference on Power System Technology (POWERCON), 2021, pp. 1117-1122, a novel hierarchical control strategy was proposed. Distributed control was used at the bottom layer to achieve autonomous control of the underlying equipment; power monitoring control was adopted at the top layer to ensure power balance between the source and load. However, this control strategy limits the source-side input power when the source-side power exceeds the sum of the alkaline electrolyzer and battery power, resulting in wind and solar power curtailment. Furthermore, the system established in this paper does not consider the intermittency of wind and solar power, nor the dynamic response time of the alkaline electrolyzer, relying too heavily on the regulation capability of the alkaline electrolyzer. When the power supply and load power are momentarily unbalanced, the system cannot operate according to the original strategy. At the same time, the battery charging and discharging power of this system fluctuates significantly, which has a certain impact on battery life. Summary of the Invention

[0008] In view of this, the purpose of this invention is to provide a power control method for a wind-solar coupled off-grid electrolysis hydrogen production system, so as to solve technical problems such as voltage fluctuation of the off-grid bus in wind-solar coupled systems, control of alkaline electrolyzer units, and protection and recovery of the SOC of the energy storage system.

[0009] The power control method for the wind-solar coupled off-grid electrolysis hydrogen production system of the present invention includes the following steps:

[0010] Step 1: Design the output control equation for the bottom-level controller of the wind-solar coupled off-grid electrolysis hydrogen production system. The system includes a DC bus, distributed generation units, an energy storage unit, a water electrolysis hydrogen production unit, and an unloading unit. The energy storage unit includes a lithium battery and a bidirectional DC / DC converter connecting the lithium battery to the DC bus. The water electrolysis hydrogen production unit includes an alkaline electrolyzer and a unidirectional DC / DC converter connecting the alkaline electrolyzer to the DC bus. The unloading unit includes an unloading resistor and a chopper connecting the unloading resistor to the DC bus. The chopper includes a switching transistor and a diode. Control the DC / DC converter in the wind-solar coupled off-grid electrolysis hydrogen production system according to the designed control equation:

[0011] a. The bidirectional DC / DC converter of the energy storage unit is controlled using a dual closed-loop PI control, with an outer loop being the voltage loop and an inner loop being the current loop. The outer loop PI controller takes the nominal value and the measured value of the DC bus voltage as inputs, and the output of the outer loop PI controller is connected to a selector D. bat Selector Dbat It is connected to the input terminal of the inner loop PI controller, which outputs drive signals to the switching transistors of the bidirectional DC / DC converter of the energy storage unit.

[0012] When the lithium battery meets the bus voltage support condition, the inner loop input reference is generated by the outer loop, i.e., D. bat =1; When the lithium battery does not meet the bus voltage support condition, the inner loop input reference is selected as 0, i.e., D bat =0, equivalent to disconnecting the lithium battery; the control equation for the energy storage unit is:

[0013]

[0014] In the formula, V ref This is the nominal value of the DC bus voltage; V bus This is the measured value of the DC bus voltage; i ref1 and i ch These are the inner loop current reference and the battery current measurement, respectively; d1 is the duty cycle of the bidirectional DC / DC converter switch of the energy storage unit; k ip1 and k ii1 These are the inner loop PI parameters, k vp1 and k vi1 These are the outer ring PI parameters;

[0015] b. The control of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit also adopts dual closed-loop PI control, with an outer loop being a voltage loop and an inner loop being a current loop. The outer loop PI controller uses the nominal value and measured value of the DC bus voltage as inputs, and a two-stage switch selector is used to select the inner loop current reference. The output of the outer loop PI controller is connected to the first-stage selector, and the second-stage selector is connected to the input of the inner loop PI controller. The inner loop PI controller outputs drive signals to the switching transistors of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit. The first-stage selector is S... ael The selection reference is the outer loop output or the lowest hydrogen production current, and the second-stage selector D... ael Select the connection or disconnection of the first-stage selector; in addition, limit the output at the outer loop to ensure that the current input reference of the alkaline electrolyzer is at most the rated current value; the control equation of the water electrolysis hydrogen production unit is:

[0016]

[0017] In the formula, V ref This is the nominal value of the DC bus voltage; V bus This refers to the measured value of the bus voltage; i ref2 and i ael These are the inner loop current reference and the battery current measurement, respectively, where i ref2 The selection depends on the control strategy, Iaelmin For the minimum hydrogen production current, when i ref2 Selecting this value indicates that the alkaline electrolyzer is operating at its lowest hydrogen production power; d2 is the duty cycle of the DC / DC converter switch S3; k ip2 and k ii2 These are the inner loop PI parameters, k vp2 and k vi2 These are the outer ring PI parameters;

[0018] c. The unloading unit is controlled by a single-loop PI controller. The PI controller takes the nominal value and the measured value of the DC bus voltage as inputs and outputs drive signals to the switching transistors of the unloading unit. The control equation of the unloading unit is as follows:

[0019]

[0020] In the formula, d3 is the duty cycle of the unloading unit switch transistor; V refmax The maximum value of the DC bus voltage, V bus This is the measured value of the DC bus voltage; k p and k i For PI parameters; additionally, the unloading unit operates when the bus voltage exceeds the set value V. refmax Run at time;

[0021] Step 2: Control the operation of the wind-solar coupled off-grid electrolysis hydrogen production system according to the following strategy:

[0022] When the power generated by the distributed generation unit is connected to the DC bus, the upper-level controller provides the lower-level controller with the switching state based on the source-side power and the SOC state of the lithium battery, which is the selector D of the bidirectional DC / DC converter of the energy storage unit. bat The secondary selector S of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit ael and D ael Provide a switch signal, in three cases:

[0023] Scenario 1: When the input power is less than 30% of the rated power of the alkaline electrolyzer, and the SOC of the lithium battery is higher than the lower limit, the lithium battery meets the bus voltage support condition, the alkaline electrolyzer operates at 30% of its rated power, and the energy storage unit provides the shortfall power to the alkaline electrolyzer. At this time, selector D... bat =1, Second-order selector S ael D ael Selecting 0 and 1 respectively indicates that the lithium battery connection and the alkaline electrolyzer are operating at 30% of their rated power;

[0024] When the input power is less than 30% of the rated power of the alkaline electrolyzer and the SOC of the lithium battery is below the lower limit, the lithium battery does not meet the bus voltage support condition and cannot provide bus voltage support. In this case, the second-stage selector D... ael Select 0, which means the alkaline electrolyzer is disconnected and all input power is used to charge the lithium battery until the SOC limit of the lithium battery is reached.

[0025] Scenario 2: When the input power is greater than 30% of the rated power of the alkaline electrolyzer but less than or equal to the rated power, the alkaline electrolyzer operates at the input power, and the energy storage unit does not work; in this case, selector D... bat =0, Second-order selector S ael D ael Selecting 1 for all options indicates that the lithium battery is disconnected. At this time, the unidirectional DC / DC converter of the water electrolysis hydrogen production unit controls the bus voltage, and the alkaline electrolyzer operates at the input power.

[0026] Scenario 3: When the input power exceeds the rated power of the alkaline electrolyzer and the battery's SOC is below the upper limit, the alkaline electrolyzer operates at its rated power, and the energy storage unit absorbs excess power from the source side beyond the alkaline electrolyzer's rated power. In this case, selector D... bat =1, Second-order selector S ael D ael Selecting 1 for all options indicates that the lithium battery is connected and the bidirectional DC / DC converter of the energy storage unit controls the bus voltage; at this time, the outer loop output of the unidirectional DC / DC converter of the water electrolysis hydrogen production unit reaches the limiting value, which plays an auxiliary control role.

[0027] When the input power exceeds the rated power of the alkaline electrolyzer, and the battery's SOC is higher than or equal to the upper limit, the alkaline electrolyzer operates at its rated power. The energy storage unit cannot absorb excess power from the source side beyond the alkaline electrolyzer's rated power. Selector D... bat When option 0 is selected, the lithium battery is disconnected. At this time, the unidirectional DC / DC converter of the water electrolysis hydrogen production unit cannot control the bus voltage, and the bus voltage rises. When the bus voltage rises to the set maximum value, the unloading unit works to control the bus voltage at the set maximum value and waits for the input power to change so as to enter the next state.

[0028] Furthermore, the upper limit of the SOC of the lithium battery is 80%, and the lower limit of the SOC is 20%.

[0029] Furthermore, the distributed power generation unit includes a wind power generation unit and a photovoltaic power generation unit.

[0030] The beneficial effects of this invention are:

[0031] This invention discloses a power control method for a wind-solar coupled off-grid electrolysis hydrogen production system. This method not only enables control of the bus voltage of the microgrid electrolysis hydrogen production system using energy storage units, water electrolysis hydrogen production units, and unloading units—with the energy storage unit providing bus voltage support, the water electrolysis hydrogen production unit implementing auxiliary control, and the unloading unit providing over-limit protection—but also protects and restores the lithium battery SoC, thereby improving the lifespan of the energy storage unit. Furthermore, this control method applies control only to the load side, reducing wind and solar power curtailment; and the energy consumed by the unloading unit can be used for practical applications. Simultaneously, this control method fully considers the system's transient response, utilizing the collaborative efforts of the lithium battery and the unloading unit to absorb unbalanced power and maintain stable bus voltage, thus ensuring stable system operation. Attached Figure Description

[0032] Figure 1 This is a block diagram illustrating the power control principle of a wind-solar coupled off-grid hydrogen production system. The energy storage unit uses a lithium battery pack, and the distributed power sources, namely the wind turbine and photovoltaic units, convert wind and solar energy into electrical energy and inject it into the system's source side, providing input power for the entire system. Both are equivalent to current sources, i.e., I0... wind I pv Input current is injected into the system; a bidirectional DC / DC converter and its controller are established on the energy storage unit side to support the bus voltage and smooth power fluctuations; a unidirectional DC / DC converter and its controller are established on the alkaline electrolyzer side to assist the bidirectional DC / DC converter of the energy storage unit in controlling the bus voltage; when the input power of the distributed power source is greater than the sum of the power of the electrolyzer and the lithium battery, or when there is a sudden change in wind speed (the bus voltage support established by the lithium battery loses control), the unloading unit absorbs the overshoot power from the grid to absorb the excess power input on the system source side that exceeds the load and energy storage power, maintaining the safe and stable operation of the system and avoiding grid voltage runaway. An upper-level coordination controller is established in the system to collect battery status and input power levels through communication equipment, accurately allocate system power, and ensure stable system operation. In the figure, V bus The nominal value of the bus voltage is taken as 1050V in the following embodiments; I ch I ael The currents are the battery current and the electrolytic cell current, respectively. After sampling, they are input into the controller to control the output current.

[0033] Figure 2 This is a simplified circuit diagram of the wind-solar coupled off-grid hydrogen production system described in the embodiment. The distributed generation units are DG1 and DG2, representing the wind turbine and photovoltaic power respectively, both of which are equivalent to current sources; the bidirectional DC / DC converter of the energy storage unit adopts a Buck / Boost structure, R... outL1 is the line resistance, L2 is the filter inductor, C1 is the filter capacitor, and S1 and S2 are the drive signals for the MOSFETs, generated by the controller. The DC / DC converter of the alkaline electrolytic cell adopts a Buck circuit structure (the bus voltage is higher than the electrolytic cell terminal voltage). out L1 is the line resistance, L2 is the filter inductor, C2 is the filter capacitor, and S3 is the drive signal for the MOSFET. The unloading unit uses a chopper circuit. When the bus voltage reaches the upper limit, the controller starts to control the bus voltage at the upper limit, and the power is absorbed by the resistor. This resistor can be used for heating water, etc., to convert electrical energy into other forms of energy for storage, avoiding energy waste. In the following embodiment, R out Take 0.001Ω, L1 and L2 take 4mH, C1 and C2 take 0.3mF. In other examples, the values ​​can be set according to the power level and performance indicators.

[0034] Figure 3 This is a control principle diagram of the bidirectional DC / DC converter controller for the energy storage unit.

[0035] Figure 4 The control principle diagram is shown for the controller of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit.

[0036] Figure 5 This is for the upper-level controller to execute the control strategy. Where P... AEL P is the rated power of the electrolytic cell. source For input power, P bat The rated power of the battery is specified. Power is allocated based on the input power level and the lithium battery's state of charge (SOC). In the following embodiments, the lower limit of the lithium battery's SOC is 20%, and the upper limit is 80%.

[0037] Figure 6 The figure shows the system simulation results when the input power is less than 30% of the rated power of the electrolytic cell.

[0038] Figure 7 The figure shows the system simulation results when the input power is greater than 30% of the rated power of the electrolytic cell but less than the rated power of the electrolytic cell.

[0039] Figure 8 The figure shows the system simulation results when the input power is greater than the rated power of the electrolytic cell. Detailed Implementation

[0040] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0041] The power control method for the wind-solar coupled off-grid electrolysis hydrogen production system in this embodiment is characterized by the following steps:

[0042] Step 1: Design the output control equations for the bottom-level controller of the wind-solar coupled off-grid electrolysis hydrogen production system. For example... Figure 1 As shown, the wind-solar coupled off-grid electrolysis hydrogen production system includes a DC bus, a distributed generation unit, an energy storage unit, a water electrolysis hydrogen production unit, and an unloading unit. In this embodiment, the distributed generation unit includes a wind power generation unit and a photovoltaic power generation unit; however, in different embodiments, the distributed generation unit may also be either a wind power generation unit or a photovoltaic power generation unit. The energy storage unit includes a lithium battery and a bidirectional DC / DC converter connecting the lithium battery and the DC bus; the water electrolysis hydrogen production unit includes an alkaline electrolyzer and a unidirectional DC / DC converter connecting the alkaline electrolyzer and the DC bus; the unloading unit includes an unloading resistor and a chopper connecting the unloading resistor and the DC bus, the chopper including a switching transistor and a diode. The DC / DC converter in the wind-solar coupled off-grid electrolysis hydrogen production system is controlled according to the designed control equations:

[0043] a. The bidirectional DC / DC converter of the energy storage unit is controlled using a dual closed-loop PI control, with an outer loop being the voltage loop and an inner loop being the current loop. The outer loop PI controller takes the nominal value and the measured value of the DC bus voltage as inputs, and the output of the outer loop PI controller is connected to a selector D. bat Selector D bat It is connected to the input terminal of the inner loop PI controller, which outputs drive signals to the switching transistors of the bidirectional DC / DC converter of the energy storage unit.

[0044] When the lithium battery meets the bus voltage support condition, the inner loop input reference is generated by the outer loop, i.e., D. bat =1; When the lithium battery does not meet the bus voltage support condition, the inner loop input reference is selected as 0, i.e., D bat =0, equivalent to disconnecting the lithium battery; the control equation for the energy storage unit is:

[0045]

[0046] In the formula, V ref This is the nominal value of the DC bus voltage; V bus This is the measured value of the DC bus voltage; i ref1 and i ch These are the inner loop current reference and the battery current measurement values, respectively; d1 is the duty cycle of DC / DC converter switch S1, and 1-d1 is the duty cycle of switch S2; k ip1 and k ii1 These are the inner loop PI parameters, k vp1 and k vi1 These are the outer loop PI parameters, which were obtained by trial and error in this implementation.

[0047] b. The control of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit also adopts dual closed-loop PI control, with an outer loop being a voltage loop and an inner loop being a current loop. The outer loop PI controller uses the nominal value and measured value of the DC bus voltage as inputs, and a two-stage switch selector is used to select the inner loop current reference. The output of the outer loop PI controller is connected to the first-stage selector, and the second-stage selector is connected to the input of the inner loop PI controller. The inner loop PI controller outputs drive signals to the switching transistors of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit. The first-stage selector is S... ael The selection reference is the outer loop output or the lowest hydrogen production current, and the second-stage selector D... ael Select the connection or disconnection of the first-stage selector; in addition, limit the output at the outer loop to ensure that the current input reference of the alkaline electrolyzer is at most the rated current value; the control equation of the water electrolysis hydrogen production unit is:

[0048]

[0049] In the formula, V ref This is the nominal value of the DC bus voltage; V bus This refers to the measured value of the bus voltage; i ref2 and i ael These are the inner loop current reference and the battery current measurement, respectively, where i ref2 The selection depends on the control strategy, I aelmin To achieve the lowest possible hydrogen production current, this example uses 700A; when i ref2 Selecting this value indicates that the alkaline electrolyzer is operating at its lowest hydrogen production power; d2 is the duty cycle of the DC / DC converter switch S3; k ip2 and k ii2 These are the inner loop PI parameters, k vp2 and k vi2 These are the outer loop PI parameters, which were obtained by trial and error in this implementation.

[0050] In this embodiment, the alkaline electrolyzer is the main load of the system, with a rated power of 816 kW. Hydrogen production in the off-grid system is achieved by controlling the electrolyzer. The electrochemical equation for the alkaline electrolyzer is:

[0051]

[0052] In the formula, U rev The reversible voltage is constant, taken as 1.229V, r1 and r2 are the ohmic resistance parameters of the electrolyte, and T... el Where is the temperature of the electrolytic cell, I is the DC current, A is the effective surface area of ​​the electrode, s1, s2, s3 are the electrode overvoltage coefficients, and t1, t2, t3 are the electrode overvoltage coefficients.

[0053] As can be seen from the electrochemical equations of an electrolyzer, its voltage level is related to the current; therefore, the operation of the electrolyzer can be controlled by controlling the current. However, due to the inherent characteristics of electrolyzers—namely, the long start-up and shutdown times of alkaline electrolyzers, and the fact that frequent start-ups and shutdowns affect their lifespan—and the fact that the purity of hydrogen produced by an electrolyzer is related to its power, if the input power is below 30% of the rated power, the purity of the produced hydrogen will be substandard, potentially leading to safety accidents.

[0054] c. The unloading unit is controlled by a single-loop PI controller. The PI controller takes the nominal value and the measured value of the DC bus voltage as inputs and outputs drive signals to the switching transistors of the unloading unit. The control equation of the unloading unit is as follows:

[0055]

[0056] In the formula, d3 is the duty cycle of the unloading unit switch transistor; V refmax This represents the maximum value of the DC bus voltage; in this embodiment, it is specifically set to 1070V. bus This is the measured value of the DC bus voltage; k p and k i For PI parameters; additionally, the unloading unit operates when the bus voltage exceeds the set value V. refmax Runs at any time.

[0057] Step 2: Control the operation of the wind-solar coupled off-grid electrolysis hydrogen production system according to the following strategy:

[0058] When the power generated by the distributed generation unit is connected to the DC bus, the upper-level controller provides the lower-level controller with the switching state based on the source-side power and the SOC state of the lithium battery, which is the selector D of the bidirectional DC / DC converter of the energy storage unit. bat The secondary selector S of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit ael and D ael Provide a switch signal, in three cases:

[0059] Scenario 1: When the input power is less than 30% of the rated power of the alkaline electrolyzer, and the SOC of the lithium battery is higher than the lower limit (in this embodiment, the lower limit of the lithium battery's SOC is 20%), the lithium battery meets the bus voltage support condition, the alkaline electrolyzer operates at 30% of its rated power, and the energy storage unit provides the deficit power to the alkaline electrolyzer. At this time, selector D... bat =1, Second-order selector S ael D ael Selecting 0 and 1 respectively indicates that the lithium battery connection and the alkaline electrolyzer are operating at 30% of their rated power;

[0060] When the input power is less than 30% of the rated power of the alkaline electrolyzer and the SOC of the lithium battery is below the lower limit, the lithium battery does not meet the bus voltage support condition and cannot provide bus voltage support. In this case, the second-stage selector D... ael Selecting option 0 means the alkaline electrolyzer is disconnected, and all input power is used to charge the lithium battery until the upper limit of the lithium battery's SOC is reached. In this embodiment, the upper limit of the lithium battery's SOC is 80%.

[0061] Scenario 2: When the input power is greater than 30% of the rated power of the alkaline electrolyzer but less than or equal to the rated power, the alkaline electrolyzer operates at the input power, and the energy storage unit does not work; in this case, selector D... bat =0, Second-order selector S ael D ael Selecting 1 for all options indicates that the lithium battery is disconnected. At this time, the unidirectional DC / DC converter of the water electrolysis hydrogen production unit controls the bus voltage, and the alkaline electrolyzer operates at the input power.

[0062] Scenario 3: When the input power exceeds the rated power of the alkaline electrolyzer and the battery's SOC is below the upper limit, the alkaline electrolyzer operates at its rated power, and the energy storage unit absorbs excess power from the source side beyond the alkaline electrolyzer's rated power. In this case, selector D... bat =1, Second-order selector S ael D ael Selecting 1 for all options indicates that the lithium battery is connected and the bidirectional DC / DC converter of the energy storage unit controls the bus voltage; at this time, the outer loop output of the unidirectional DC / DC converter of the water electrolysis hydrogen production unit reaches the limiting value, which plays an auxiliary control role.

[0063] When the input power exceeds the rated power of the alkaline electrolyzer, and the battery's SOC is higher than or equal to the upper limit, the alkaline electrolyzer operates at its rated power. The energy storage unit cannot absorb excess power from the source side beyond the alkaline electrolyzer's rated power. Selector D... bat When option 0 is selected, the lithium battery is disconnected. At this time, the unidirectional DC / DC converter of the water electrolysis hydrogen production unit cannot control the bus voltage, and the bus voltage rises. When the bus voltage rises to the set maximum value, the unloading unit works to control the bus voltage at the set maximum value and waits for the input power to change so as to enter the next state.

[0064] The effectiveness of the power control method for the wind-solar coupled off-grid electrolysis hydrogen production system in this embodiment is verified by simulation.

[0065] Figure 6The simulation results are for a system where the input power is less than 30% of the electrolyzer's rated power. As mentioned above, the electrolyzer's rated power in this example is 816kW, and its lowest hydrogen production power is selected as 243kW, which is 30% of the electrolyzer's rated power. In the simulation system, this example uses state-average equation modeling. Unlike ordinary modeling methods, this method has a fast simulation speed, and the battery's SOC can change significantly during the simulation. In this example, the simulation time is selected as 9000s, with power jumps every 3000s. When the input power is less than 243kW, according to the control strategy, the battery's SOC state is collected, and it is determined whether the SOC is less than 20%. If it is less, the input power is used for battery charging, and the charging power becomes the input power; the electrolyzer does not operate. If the State of Charge (SOC) is greater than 20%, the battery status in the previous state is assessed. If the battery is discharging, the electrolyzer operates at 30% of its rated power, and the battery replenishes the missing power until the SOC discharges to 20%. If, after reaching 20%, the input power is still less than the electrolyzer's minimum hydrogen production power, all input power is used for battery charging. If the battery is charging, the input power is used for battery charging, and the electrolyzer is in standby mode. Figure 6 In Phase 1, the input power is 100kW, which is less than the minimum hydrogen production power. The initial SOC of the battery is 40% (battery discharge at the start). Therefore, the electrolyzer operates at 30% of its rated power, and the battery discharges. When the battery discharges to the lower SOC limit of 20%, it can no longer discharge to support the electrolyzer's operation. Therefore, the electrolyzer goes into standby mode, and all the input power is used to charge the battery at a charging power of 100kW. At 3000s of simulation time, Phase 2 begins, and the input power jumps to 200kW. According to the control strategy, the SOC is now greater than 20%, but the battery was charging in the previous state, so it continues to charge at a charging power of 200kW. The charging speed increases, which is reflected in an increased increase in the battery SOC. When the battery is charged to the upper limit of 80%, the electrolyzer operates at the minimum hydrogen production power, and the battery replenishes the missing power by 43kW. At a simulation time of 6000s, the system entered stage 3, with the input power increasing to 150kW. At this point, the battery's SOC had not reached the lower limit of 20%, and the battery was in a discharging state in the previous stage. In this stage, the electrolyzer operated at its minimum hydrogen production power, and the battery discharged at a power of 93kW. Compared to the previous stage, the battery provided 50kW more of the deficit power, which is represented by an increased SOC decline rate in the figure. Furthermore, throughout the simulation, the bus voltage of the wind-solar-storage-hydrogen production system remained stable at the set value of 1050V, with only occasional fluctuations during power switching or when reaching the upper or lower SOC limits. However, these fluctuations remained within the performance requirements, achieving stable system operation and verifying the effectiveness of the wind-solar-storage-hydrogen production system control method of this invention.

[0066] Figure 7The simulation results are for a system where the input power is 30% greater than but less than the electrolyzer's rated power. According to the operating control strategy, the battery does not operate within this power range, the battery SOC remains unchanged, and the electrolyzer's operating power is equal to the input power. Analysis of the graphs shows that at 3000 s, the system input power is 600 kW, the battery SOC remains at its initial value of 40%, and the electrolyzer power is 600 kW. At 3000 s, the system input power abruptly increases to 400 kW, and the electrolyzer power decreases accordingly to 400 kW, while the battery SOC remains unchanged. During the simulation period of 6000-9000 s, the system enters stage 3, with the input power increasing to 700 kW, and the electrolyzer power increasing accordingly, while the battery SOC remains at 40%. Furthermore, throughout all three simulation stages, the system's DC bus voltage remains stable at 1050 V, and the lithium battery power is 0, meaning the battery neither charges nor discharges, indicating stable system operation and verifying the effectiveness of the system control method.

[0067] Figure 8 The simulation results show the system when the input power exceeds the electrolyzer's rated power. According to the operation control strategy, when the source-side input power exceeds the electrolyzer's rated power, the electrolyzer operates at its rated power of 816kW. If the battery's SOC is less than 80%, the battery absorbs the excess power from the source-side input power beyond the electrolyzer's rated power. If the excess power exceeds the battery's maximum absorbable power, the unloading unit operates. When the battery's SOC is greater than 80%, the battery power is 0, and the unloading unit absorbs the excess power. Analysis of the graphs shows that during the simulation period of 0-3000s, the system input power is 1000kW, the electrolyzer power is 816kW, and the initial battery SOC is 40%. Since the SOC is less than the upper limit of 80%, the system's surplus power is used for battery charging, with a charging power of 184kW. At approximately 1800 seconds, the battery charged to 80% of its State of Charge (SOC). At this point, the battery could no longer absorb excess power, and the bus voltage rose above 1070V. The unloading unit activated, controlling the voltage at 1070V, and the excess power was absorbed by the unloading resistor, converting electrical energy into heat. At 3000 seconds, the system input power abruptly increased to 2000kW, the electrolytic cell power stabilized at its rated power of 816kW, the unloading unit power became 1184W, and the battery SOC remained constant at 80%. During the simulation period of 6000-9000 seconds, the system entered stage 3, the input power decreased to 1000kW, the electrolytic cell power stabilized at its rated power of 816kW, the unloading unit power became 184kW, and the battery SOC remained unchanged. Furthermore, in all three simulation stages, the system DC bus voltage rose from 1050V to 1070V after the SOC reached 80%, remaining constant in the remaining states, and the power distribution met expectations, indicating stable system operation and verifying the effectiveness of the system control method.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A power control method for a wind-solar coupled off-grid electrolysis hydrogen production system, characterized in that: Includes the following steps: Step 1: Design the output control equation for the bottom-level controller of the wind-solar coupled off-grid electrolysis hydrogen production system. The system includes a DC bus, distributed generation units, an energy storage unit, a water electrolysis hydrogen production unit, and an unloading unit. The energy storage unit includes a lithium battery and a bidirectional DC / DC converter connecting the lithium battery to the DC bus. The water electrolysis hydrogen production unit includes an alkaline electrolyzer and a unidirectional DC / DC converter connecting the alkaline electrolyzer to the DC bus. The unloading unit includes an unloading resistor and a chopper connecting the unloading resistor to the DC bus. The chopper includes a switching transistor and a diode. Control the DC / DC converter in the wind-solar coupled off-grid electrolysis hydrogen production system according to the designed control equation: a. The bidirectional DC / DC converter of the energy storage unit is controlled using a dual closed-loop PI control, with an outer loop being the voltage loop and an inner loop being the current loop. The outer loop PI controller takes the nominal value and the measured value of the DC bus voltage as inputs, and the output of the outer loop PI controller is connected to a selector D. bat Selector D bat It is connected to the input terminal of the inner loop PI controller, which outputs drive signals to the switching transistors of the bidirectional DC / DC converter of the energy storage unit. When the lithium battery meets the bus voltage support condition, the inner loop input reference is generated by the outer loop, i.e., D. bat =1; When the lithium battery does not meet the bus voltage support condition, the inner loop input reference is selected as 0, i.e., D bat =0, equivalent to disconnecting the lithium battery; the control equation for the energy storage unit is: In the formula, V ref This is the nominal value of the DC bus voltage; V bus This is the measured value of the DC bus voltage; i ref1 and i ch These are the inner loop current reference and the battery current measurement, respectively; d1 is the duty cycle of the bidirectional DC / DC converter switch of the energy storage unit; k ip1 and k ii1 These are the inner loop PI parameters, k vp1 and k vi1 These are the outer ring PI parameters; b. The control of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit also adopts dual closed-loop PI control, with an outer loop being a voltage loop and an inner loop being a current loop. The outer loop PI controller uses the nominal value and measured value of the DC bus voltage as inputs, and a two-stage switch selector is used to select the inner loop current reference. The output of the outer loop PI controller is connected to the first-stage selector, and the second-stage selector is connected to the input of the inner loop PI controller. The inner loop PI controller outputs drive signals to the switching transistors of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit. The first-stage selector is S... ael The selection reference is the outer loop output or the lowest hydrogen production current, and the second-stage selector D... ael Select the connection or disconnection of the first-stage selector; in addition, limit the output at the outer loop to ensure that the current input reference of the alkaline electrolyzer is at most the rated current value; the control equation of the water electrolysis hydrogen production unit is: In the formula, V ref This is the nominal value of the DC bus voltage; V bus This refers to the measured value of the bus voltage; i ref2 and i ael These are the inner loop current reference and the battery current measurement, respectively, where i ref2 The selection depends on the control strategy, I aelmin For the minimum hydrogen production current, when i ref2 Selecting this value indicates that the alkaline electrolyzer is operating at its lowest hydrogen production power; d2 is the duty cycle of the DC / DC converter switch S3; k ip2 and k ii2 These are the inner loop PI parameters, k vp2 and k vi2 These are the outer ring PI parameters; c. The unloading unit is controlled by a single-loop PI controller. The PI controller takes the nominal value and the measured value of the DC bus voltage as inputs and outputs drive signals to the switching transistors of the unloading unit. The control equation of the unloading unit is as follows: In the formula, d3 is the duty cycle of the unloading unit switch transistor; V refmax The maximum value of the DC bus voltage, V bus This is the measured value of the DC bus voltage; k p and k i For PI parameters; additionally, the unloading unit operates when the bus voltage exceeds the set value V. refmax Run at time; Step 2: Control the operation of the wind-solar coupled off-grid electrolysis hydrogen production system according to the following strategy: When the power generated by the distributed generation unit is connected to the DC bus, the upper-level controller provides the lower-level controller with the switching state based on the source-side power and the SOC state of the lithium battery, which is the selector D of the bidirectional DC / DC converter of the energy storage unit. bat The secondary selector S of the unidirectional DC / DC converter in the water electrolysis hydrogen production unit ael and D ael Provide a switch signal, in three cases: Scenario 1: When the input power is less than 30% of the rated power of the alkaline electrolyzer, and the SOC of the lithium battery is higher than the lower limit, the lithium battery meets the bus voltage support condition, the alkaline electrolyzer operates at 30% of its rated power, and the energy storage unit provides the shortfall power to the alkaline electrolyzer. At this time, selector D... bat =1, Second-order selector S ael D ael Selecting 0 and 1 respectively indicates that the lithium battery connection and the alkaline electrolyzer are operating at 30% of their rated power; When the input power is less than 30% of the rated power of the alkaline electrolyzer and the SOC of the lithium battery is below the lower limit, the lithium battery does not meet the bus voltage support condition and cannot provide bus voltage support. In this case, the second-stage selector D... ael Select 0, which means the alkaline electrolyzer is disconnected and all input power is used to charge the lithium battery until the SOC limit of the lithium battery is reached. Scenario 2: When the input power is greater than 30% of the rated power of the alkaline electrolyzer but less than or equal to the rated power, the alkaline electrolyzer operates at the input power, and the energy storage unit does not work; in this case, selector D... bat =0, Second-order selector S ael D ael Selecting 1 for all options indicates that the lithium battery is disconnected. At this time, the unidirectional DC / DC converter of the water electrolysis hydrogen production unit controls the bus voltage, and the alkaline electrolyzer operates at the input power. Scenario 3: When the input power exceeds the rated power of the alkaline electrolyzer and the battery's SOC is below the upper limit, the alkaline electrolyzer operates at its rated power, and the energy storage unit absorbs excess power from the source side beyond the alkaline electrolyzer's rated power. In this case, selector D... bat =1, Second-order selector S ael D ael Selecting 1 for all options indicates that the lithium battery is connected and the bidirectional DC / DC converter of the energy storage unit controls the bus voltage; at this time, the outer loop output of the unidirectional DC / DC converter of the water electrolysis hydrogen production unit reaches the limiting value, which plays an auxiliary control role. When the input power exceeds the rated power of the alkaline electrolyzer, and the battery's SOC is higher than or equal to the upper limit, the alkaline electrolyzer operates at its rated power. The energy storage unit cannot absorb excess power from the source side beyond the alkaline electrolyzer's rated power. Selector D... bat When option 0 is selected, the lithium battery is disconnected. At this time, the unidirectional DC / DC converter of the water electrolysis hydrogen production unit cannot control the bus voltage, and the bus voltage rises. When the bus voltage rises to the set maximum value, the unloading unit works to control the bus voltage at the set maximum value and waits for the input power to change so as to enter the next state.

2. The power control method for a wind-solar coupled off-grid electrolysis hydrogen production system according to claim 1, characterized in that: The lithium battery has an upper limit of 80% and a lower limit of 20% in terms of SOC.

3. The power control method for a wind-solar coupled off-grid electrolysis hydrogen production system according to claim 1, characterized in that: The distributed generation unit includes a wind power generation unit and a photovoltaic power generation unit.

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