A control method for variable load mode of fuel cell system

Abstract

The present invention proposes a control method for a fuel cell system load variation mode, which relates to the field of fuel cell technology. The present invention calculates the response time of the slowest responding functional component in the fuel cell system, and then calculates and fits the load variation rate of other functional components. Each functional component varies its load according to the fitted load variation rate, and the current is loaded at a rate slightly slower than the loading rate of the functional component. When the load is reduced, the current is reduced at a rate slightly faster than the load reduction rate of the functional component. Such a control method can improve the response rate of the fuel cell system and can effectively avoid the problem of low single-cell voltage in the stack during the load variation process, reduce the system error rate during the load variation process, and improve the service life of the fuel cell system.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and in particular to a method for controlling a variable load mode of a fuel cell system. Background Art

[0002] As the application scenarios of hydrogen fuel cell vehicles become more and more extensive, the power loading requirements for hydrogen fuel cell systems are also getting higher and higher. When the vehicle issues a power request, the fuel cell system must respond accurately and quickly.

[0003] Most of the existing control strategies are that the vehicle sends a power request, the fuel cell system determines the loading density point according to the required power, and determines the loading current corresponding to the density point and the loading operating point corresponding to the functional component based on the MAP (calibration curve) calibrated in the laboratory. The functional component is then directly loaded according to the MAP, and the current is loaded at the set operating loading rate.

[0004] The disadvantages of this method are: first, in order to avoid a single low in the stack during loading, the current loading rate will be relatively slow, affecting the power tracking performance of the entire vehicle; second, when the power demand changes greatly, due to the different response times of different functional parts, for example, the response speed of the air compressor in the air circuit is relatively slow, while the response speed of the throttle is relatively fast, the dynamic loading will affect the fluctuation of the air metering ratio, and there may be problems such as insufficient gas volume and large pressure difference, which will lead to low voltage of a single cell in the system, thus affecting the service life of the fuel cell system. Summary of the invention

[0005] In order to solve the problems of slow power following of existing vehicles and insufficient gas volume and large hydrogen-air pressure difference in the fuel cell system when the power demand of the vehicle changes greatly, the present invention proposes a control method for the variable load mode of the fuel cell system, by calculating the response time of the slowest responding functional component in the fuel cell system, and then calculating and fitting the variable load rate of other functional components, each functional component changes load according to the fitted variable load rate, and the current is loaded at a loading rate slightly slower than the functional component, and the current is reduced at a load reduction rate slightly faster than the functional component when reducing load. Such a control method can improve the response rate of the fuel cell system and can effectively avoid the problem of low single-cell voltage in the stack during the load change process, reduce the system error rate during the load change process, and improve the service life of the fuel cell system.

[0006] To this end, the present invention adopts the following technical solutions:

[0007] The present invention provides a method for controlling a variable load mode of a fuel cell system, the method comprising:

[0008] S1: Judge the system variable load state based on the original system power and the vehicle's transmitted demand power; the system variable load state includes: loading state, unloading state, and maintaining the original power state;

[0009] S2: If the system is in the loading state or the unloading state, calculate and output the variable load rate of the functional components in real time. The variable load rate includes the air compressor speed, throttle opening, and hydrogen pump speed until the loading or unloading reaches the target value; among them, calculating the variable load rate of the functional components includes: calculating the response time of the functional component with the slowest response in the fuel cell system; the functional component with the slowest response in the fuel cell system is the air compressor speed; based on the response time, fit and calculate the variable load rates of other functional components;

[0010] S3: If the system is in the loading state or the unloading state, calculate and output the variable load rate of the current in real time based on the variable load rate of the functional components. Calculate the corresponding air stoichiometry ratio at the end of each step, and judge whether the air stoichiometry ratio is reasonable. If there is an under-air situation, re-correct the variable load rate of the current in the next step. If there is no under-air situation, continue to vary the load at the original rate until the loading or unloading reaches the target value; when loading, the current decreases at a rate slightly slower than, and when unloading, the current decreases at a rate slightly faster than the unloading rate of the functional components;

[0011] S4: The hydrogen pressure set value changes equally with the air inlet pressure into the stack until the loading or unloading reaches the target value.

[0012] Further, judging the system variable load state based on the original system power and the vehicle's transmitted demand power includes:

[0013] The original system power is P0, and the vehicle's transmitted demand power is P1. Judge the relationship between the vehicle's transmitted demand power P1 and the original power P0. If P1 > P0, it is the system loading state. If P1 < P0, it is the system unloading state. If P1 = P0, it is the system maintaining the original power state.

[0014] Further, calculating the loading rate of the functional components includes:

[0015] Use linear slope loading to calculate the loading rate of the functional components.

[0016] Further, calculating and outputting the variable load rate of the functional components in real time includes:

[0017] When the fuel cell system is initially calibrated, calibrate the system power, air compressor speed, throttle opening, and hydrogen pump speed at each current density point;

[0018] According to the calibration curve graph, find out the variable load current density point Iden corresponding to the vehicle's transmitted demand power P1;

[0019] According to the calibration curve, the calibration values ​​of the air compressor speed AC_Set, the throttle opening TV_Set and the hydrogen pump speed CP_Set corresponding to the variable load electric density point Iden are found, which are the target values ​​of the variable load;

[0020] Get the minimum loading rate AC_Ramp1 of the air compressor speed;

[0021] According to the lowest load change rate AC_Ramp1 of the air compressor speed, the longest average total time T0 required for the air compressor to change from the original speed value AC_Original to the target value AC_Set is calculated: T0 = (AC_Set-AC_Original) / AC_Ramp1;

[0022] According to the time t0 of one step of program load change, calculate the total number of steps N of load change: N = T0 / t0;

[0023] According to time T0, the first throttle load rate TV_Ramp1 and the first hydrogen pump load rate CP_Ramp1 are fitted:

[0024] TV_Ramp1=(TV_Set-TV_Original) / T0;

[0025] CP_Ramp1=(CP_Set-CP_Original) / T0;

[0026] Among them, TV_Original is the original value of throttle speed; CP_Original is the original value of hydrogen pump speed;

[0027] Calculate the output value of the variable load rate of the air compressor, the variable load rate of the throttle and the hydrogen pump after each step; the output value of the variable load rate of the air compressor after each step is the output value of the previous step, which increases or decreases the minimum loading rate AC_Ramp1 of the air compressor speed, the output value of the variable load rate of the throttle after each step is the output value of the previous step, which increases or decreases the first throttle variable load rate TV_Ramp1, and the output value of the variable load rate of the hydrogen pump after each step is the output value of the previous step, which increases or decreases the first hydrogen pump variable load rate CP_Ramp1;

[0028] Output the air compressor speed, throttle opening and hydrogen pump speed after each step load change until the load reaches the target value, stop the calculation and maintain the set value output.

[0029] Furthermore, the current loading rate is calculated and output in real time, including:

[0030] When the fuel cell system is calibrated for the first time, the variable load current at each electrical density point is calibrated;

[0031] According to the calibration curve, find out the current target value Itarget corresponding to the vehicle's required power P1;

[0032] According to time T0, fit the first current loading rate I1_Ramp1:

[0033] I1_Ramp1=(I target-Ioriginal) / (T0+Tn);

[0034] Where Tn=(α1~αn)*T0, initial Tn1=α1*T0; Ioriginal is the original value of current;

[0035] Calculate and output the current value I and air stoichiometry AirStoich after each step is loaded; the current output value after each step is loaded is the current output value of the previous step plus the first current loading rate I1_Ramp1;

[0036] After each step, compare whether the current value reaches the target current I target. If it reaches the target current, stop loading. If it does not reach the target current value, determine whether the loading process is short of air based on the air metering ratio AirStoich. If it is short of air, refit the current loading rate I1_Ramp2. If it is not short of air, continue loading with the current loading rate I1_Ramp1. Repeat the cycle. If it is short of air, fit the current loading rate I1_Rampn again until the target current is loaded and stop loading.

[0037] The refitting current loading rate includes: taking Tn1=α1*T0 in the first current loading rate I1_Ramp1 formula, taking Tn2=α2*T0 in the second current loading rate I1_Ramp2 formula, and taking Tnn=αn*T0 in the nth current loading rate I1_Rampn formula; wherein α1<α2<…<αn.

[0038] Furthermore, the load reduction rate of the current is calculated and output in real time, including:

[0039] When the fuel cell system is calibrated for the first time, the variable load current at each electrical density point is calibrated;

[0040] According to the calibration curve, find out the current target value Itarget corresponding to the vehicle's required power P1;

[0041] According to the time T0, the first current load reduction rate I2_Ramp1 is fitted:

[0042] I2_Ramp1=(I target-Ioriginal) / (T0-Tn);

[0043] Among them, Tn=(α1~αn)*T0, initial Tn1=αn*T0;

[0044] Calculate and output the current value I and the air stoichiometric ratio AirStoich after each step of load reduction; the current output value after each step of load reduction is the current output value of the previous step minus the first current load reduction rate I2_Ramp1;

[0045] After each step, compare whether the current value reaches the target current I target. If it reaches the target current, stop reducing the load. If it does not reach the target current value, determine whether the load reducing process is insufficient in air based on the air metering ratio AirStoich. If it is insufficient in air, refit the current load reduction rate I2_Ramp2. If it is not insufficient in air, continue reducing the load with the current load reduction rate I2_Ramp1. Repeat the cycle. If it is insufficient in air, refit the current load reduction rate I2_Rampn until the load is reduced to the target current, and stop reducing the load.

[0046] Refitting the current load reduction rate includes: taking Tn1=αn*T0 in the first current load reduction rate I2_Ramp1 formula, taking Tn2=αn-1*T0 in the second current load reduction rate I2_Ramp2 formula, and taking Tnn=α1*T0 in the nth current load reduction rate I1_Rampn formula; wherein αn<αn-1<…<α1.

[0047] Furthermore, the hydrogen pressure setting value changes arithmetic following the air inlet pressure, including:

[0048] When the fuel cell system is calibrated for the first time, the hydrogen-air pressure difference ΔP corresponding to each electrical density point is calibrated;

[0049] After determining the system load change state, the air inlet pressure of the system will be collected at each step. During the load change process, the hydrogen pressure setting value changes with the air pressure at a difference of ΔP.

[0050] Furthermore, if the system maintains the original power state, there is no need to calculate the loading rate and the load reduction rate.

[0051] Advantages and positive effects of the present invention:

[0052] The control method of the variable load mode of the fuel cell system in the present invention can effectively avoid the low air metering ratio caused by the inconsistency between the air compressor speed and the throttle opening during dynamic load change, avoid the problem of low single-cell voltage of the system due to the low air metering ratio, and improve the service life of the hydrogen fuel cell system. In addition, by real-time monitoring of the air metering ratio, the current loading rate is appropriately adjusted, which can not only ensure the response speed of the hydrogen fuel cell system, but also ensure the air inlet flow and pressure, and avoid the problem of low single-cell voltage of the fuel cell system due to insufficient gas volume. In addition, the hydrogen side stack pressure changes in real time with the air side stack pressure, which can reasonably and accurately control the hydrogen-air pressure difference, avoid the decrease in the efficiency of the stack reaction due to excessive hydrogen-air pressure difference, or cause irreversible damage to the proton exchange membrane, and improve the service life of the fuel cell system. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0054] Figure 1 This is a flow chart of determining the variable load state of a hydrogen fuel cell system in an embodiment of the present invention;

[0055] Figure 2 This is a load change flow chart of functional components in the load change process of the hydrogen fuel cell system in an embodiment of the present invention;

[0056] Figure 3 This is a current loading flow chart of the variable load process of the hydrogen fuel cell system in an embodiment of the present invention;

[0057] Figure 4 This is a flow chart of current load reduction during the load change process of the hydrogen fuel cell system in an embodiment of the present invention. DETAILED DESCRIPTION

[0058] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0059] It should be noted that the terms "first", "second", etc. in the description, claims and above-mentioned drawings of the present invention are used to distinguish similar objects, and do not necessarily describe a specific order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or units does not necessarily limit to those clearly listed steps or units, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0060] A control method for a variable load mode of a fuel cell system in an embodiment of the present invention specifically includes the following steps:

[0061] S1: Determine the system variable load state:

[0062] As Figure 1 shown, in specific implementation, S1 can be executed according to the following steps: For the original power P0 of the system and the required power P1 sent by the vehicle, judge the relationship between the required power P1 sent by the vehicle and the original power P0. If P1 > P0, it is the system loading state; if P1 < P0, it is the system unloading state; if P1 = P0, it is the system maintaining the original power state.

[0063] S2: After determining the system variable load state, if the system is in the loading (unloading) state, calculate the loading (unloading) rate of the functional components, which mainly includes the air compressor speed, throttle opening and hydrogen pump speed, and output in real time until it is loaded (unloaded) to the target value; if the system maintains the original power state, there is no need to calculate the loading (unloading) rate.

[0064] As Figure 2 shown, in specific implementation, S2 specifically includes the following steps:

[0065] S21: When the fuel cell system is initially calibrated, calibrate the system power, variable load current, air compressor speed, throttle opening, hydrogen pump speed, etc. at each current density point;

[0066] S22: According to the calibration curve MAP, find the variable load current density point Iden and current target value I target corresponding to the required power P1 sent by the vehicle;

[0067] S23: According to the calibration MAP, find the calibration values of the air compressor speed AC_Set, throttle opening TV_Set and hydrogen pump speed CP_Set corresponding to the variable load current density point Iden, which are the target values of the variable load;

[0068] S24: Since the air compressor speed response speed is the slowest among the above-mentioned functional parts, the lowest loading rate AC_Ramp1 of the air compressor speed is considered when changing the load;

[0069] S25: Calculate the longest average total time T0 required for the air compressor to change load to the set value according to the lowest load change rate AC_Ramp1 of the air compressor speed, that is:

[0070] T0=(AC_Set-AC_Original) / AC_Ramp1;

[0071] S26: According to the time t0 of one step of program load change, the total number of steps N of load change is calculated:

[0072] N = T0 / t0;

[0073] S27: According to time T0, fitting the first throttle load rate TV_Ramp1:

[0074] TV_Ramp1=(TV_Set-TV_Original) / T0;

[0075] S28: According to time T0, fit the first section hydrogen pump load rate CP_Ramp1:

[0076] CP_Ramp1=(CP_Set-CP_Original) / T0;

[0077] S29: According to the loading rate of the air compressor, the variable load rate of the throttle and the hydrogen pump, the output value after each step is calculated, that is, AC_Speed ​​(output value of the next step) = AC_Speed ​​(output value of the previous step) ± AC_Ramp1, TV (output value of the next step) = TV (output value of the previous step) ± TV_Ramp1, CP_Speed ​​(output value of the next step) = CP_Speed ​​(output value of the previous step) ± CP_Ramp1;

[0078] S210: Output the air compressor speed AC_Speed, throttle opening TV and hydrogen pump speed CP_Speed ​​after each step load change until the load changes to the target value, stop calculation and maintain the set value output.

[0079] In this embodiment, the calculation method of the functional component loading rate is to load with a linear slope. In another embodiment, other function forms can also be fitted according to the characteristic curve of the functional component.

[0080] S3: After determining the system variable load state, if the system is in the loading state, calculate the loading rate of the current and output it in real time. At the end of each step, calculate the corresponding air-fuel ratio and determine whether the air-fuel ratio is reasonable. If there is an under-air situation, re-correct the current loading rate for the next step. If there is no under-air situation, continue loading at the original rate until the target value is reached.

[0081] As Figure 3 shown, in specific implementation, S3 specifically includes the following steps:

[0082] S31: According to the time T0, fit the first current loading rate I1_Ramp1:

[0083] I1_Ramp1 = (I target - Ioriginal) / (T0 + Tn);

[0084] where Tn = (α1~αn) * T0, and the initial Tn1 = α1 * T0;

[0085] S32: According to the first current loading rate I1_Ramp1, calculate the output value after each step, that is: I (output value of the next step) = I (output value of the previous step) + I1_Ramp1, and calculate the air-fuel ratio AirStoich at this time.

[0086] S33: Output the current value I and the air-fuel ratio AirStoich after each step of loading.

[0087] S34: After each step, compare whether the current value reaches the target current I target. If it reaches, stop loading. If the current value is not reached, determine whether there is under-air in the loading process (that is, the air-fuel ratio AirStoich < Aref). If there is under-air, re-fit the current loading rate I1_Ramp2. If there is no under-air, continue loading at the above current loading rate I1_Ramp1; loop. If there is under-air, fit the current loading rates I1_Ramp3, I1_Ramp4...I1_Rampn again until the target current is reached and stop loading.

[0088] The principle of the above re-fitting of the current loading rate is: in the initial first current loading rate formula, take Tn1 = α1 * T0, in the second current loading rate formula, take Tn2 = α2 * T0, in the third current loading rate formula, take Tn2 = α3 * T0, and so on...; where α1 < α2 < α3 <…αn.

[0089] S4: After determining the system's variable load state, if the system is in a load reduction state, calculate the load reduction rate of the current and output it in real time. At the end of each step, calculate the corresponding air-fuel ratio and determine whether the air-fuel ratio is reasonable. If there is an under-air situation, re-correct the current load reduction rate for the next step. If there is no under-air situation, continue to reduce the load at the original rate until the load is reduced to the target value.

[0090] As Figure 4 shown, in specific implementation, S4 specifically includes the following steps:

[0091] S41: According to time T0, fit the first current load reduction rate I2_Ramp1:

[0092] I2_Ramp1 = (I target - Ioriginal) / (T0 - Tn);

[0093] where Tn = (α1~αn)*T0, and the initial Tn1 = αn*T0;

[0094] S42: According to the first current load reduction rate, calculate the output value after each step, that is:

[0095] I = I - I2_Ramp1, and calculate the air-fuel ratio AirStoich at this time;

[0096] S43: Output the current value I and the air-fuel ratio AirStoich after load reduction for each step;

[0097] S44: After each step, compare whether the current value reaches the target current I target. If it reaches, stop reducing the load. If the current value is not reached, determine whether there is an under-air situation during the load reduction process (i.e., the air-fuel ratio AirStoich < Aref). If there is an under-air situation, re-fit the current load reduction rate I2_Ramp2. If there is no under-air situation, continue to reduce the load at the above current load reduction rate I2_Ramp1; loop. If there is an under-air situation, fit the current load reduction rate I2_Ramp3 again until the load is reduced to the target current and stop reducing the load.

[0098] The principle of re-fitting the current load reduction rate is as follows: In the initial first current load reduction rate formula, take Tn1 = αn*T0. In the second current load reduction rate formula, take Tn2 = αn - 1*T0. In the third current load reduction rate formula, take Tn2 = αn - 2*T0, and so on...; where αn < αn - 1 < αn - 2 < …α1.

[0099] S5: After determining the system's variable load state, the hydrogen pressure set value changes equally with the air inlet pressure into the reactor until it is loaded (unloaded) to the target value.

[0100] In specific implementation, S5 specifically includes the following steps:

[0101] S51: When the fuel cell system is calibrated for the first time, the hydrogen-air pressure difference ΔP corresponding to each electrical density point is calibrated.

[0102] S52: After the system state is determined, the air inlet pressure of the system is collected at each step. During the load change process, the hydrogen pressure setting value changes with the air pressure at a difference of ΔP.

[0103] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for controlling a variable load mode of a fuel cell system, characterized in that: The method comprises: S1: judging the system load change state based on the system original power and the vehicle transmission demand power; the system load change state includes: loading state, load reduction state and maintaining the original power state; S2: If the system is in a loading state or a deloading state, calculate and output in real time the load variation rate of the functional component, the load variation rate includes the air compressor speed, the throttle opening and the hydrogen pump speed, until the load is loaded or deloaded to the target value; wherein, calculating the load variation rate of the functional component includes: calculating the response time of the slowest responding functional component in the fuel cell system; the slowest responding functional component in the fuel cell system is the air compressor speed; and calculating and fitting the load variation rates of other functional components based on the response time; S3: If the system is in a loading state or a load reduction state, the load change rate of the current is calculated and output in real time based on the load change rate of the functional component. The corresponding air metering ratio is calculated at the end of each step to determine whether the air metering ratio is reasonable. If there is a lack of air, the current load change rate of the next step is revised. If there is no lack of air, the load continues to change at the original rate until the load is loaded or reduced to the target value; when loading, the current is slightly slower than, and when reducing the load, the current is slightly faster than the load reduction rate of the functional component; S4: The hydrogen pressure setting value changes arithmetic with the air inlet pressure until it is loaded or unloaded to the target value; Calculating the loading rate of the functional part, including: calculating the loading rate of the functional part by using linear slope loading; Calculate and output the load change rate of functional parts in real time, including: When the fuel cell system is calibrated for the first time, the system power, air compressor speed, throttle opening and hydrogen pump speed at each electrical density point are calibrated; According to the calibration curve, find out the variable load density point Iden corresponding to the vehicle's required power P1; According to the calibration curve, the calibration values ​​of the air compressor speed AC_Set, the throttle opening TV_Set and the hydrogen pump speed CP_Set corresponding to the variable load electric density point Iden are found, which are the target values ​​of the variable load; Get the minimum loading rate AC_Ramp1 of the air compressor speed; According to the lowest load change rate AC_Ramp1 of the air compressor speed, the longest average total time T0 required for the air compressor to change from the original speed value AC_Original to the target value AC_Set is calculated: T0 = (AC_Set-AC_Original) / AC_Ramp1; According to the time t0 of one step of program load change, calculate the total number of steps N of load change: N = T0 / t0; According to time T0, the first throttle load rate TV_Ramp1 and the first hydrogen pump load rate CP_Ramp1 are fitted: TV_Ramp1=(TV_Set-TV_Original) / T0; CP_Ramp1=(CP_Set-CP_Original) / T0; Among them, TV_Original is the original value of throttle speed; CP_Original is the original value of hydrogen pump speed; Calculate the output values after each step of the variable load rate of the air compressor, throttle valve, and hydrogen pump; the output value of the variable load rate of the air compressor after each step is the output value of the previous step increased or decreased by the minimum load rate AC_Ramp1 of the air compressor speed, the output value of the variable load rate of the throttle valve after each step is the output value of the previous step increased or decreased by the first throttle valve variable load rate TV_Ramp1, and the output value of the variable load rate of the hydrogen pump after each step is the output value of the previous step increased or decreased by the first hydrogen pump variable load rate CP_Ramp1; Output the air compressor speed, throttle valve opening, and hydrogen pump speed after variable load for each step until the variable load reaches the target value, stop the calculation, and maintain the set value output.

2. A method for controlling a variable load mode of a fuel cell system according to claim 1, characterized in that: Judge the system variable load state based on the original power of the system and the power demand sent by the whole vehicle, including: The original power of the system P0, the power demand sent by the whole vehicle P1, judge the relationship between the power demand P1 sent by the whole vehicle and the original power P0. If P1 > P0, it is the system loading state; if P1 < P0, it is the system unloading state; if P1 = P0, it is the system maintaining the original power state.

3. The control method of a fuel cell system variable load mode according to claim 1, characterized in that: Calculate and output the loading rate of the current in real time, including: When the fuel cell system is initially calibrated, calibrate the variable load current at each current density point; According to the calibration curve graph, find out the current target value Itarget corresponding to the power demand P1 sent by the whole vehicle; According to the time T0, fit the first current loading rate I1_Ramp1: I1_Ramp1 = (I target - Ioriginal) / (T0 + Tn); where Tn = (α1~αn)*T0, the initial Tn1 = α1*T0; Ioriginal is the original current value; Calculate and output the current value I and air stoichiometry AirStoich after loading for each step; the current output value after loading for each step is the current output value of the previous step increased by the first current loading rate I1_Ramp1; After each step, compare whether the current value reaches the target current I target. If it reaches, stop loading. If the current value is not reached, judge whether the loading process is under-air based on the air stoichiometry AirStoich. If it is under-air, re-fit the current loading rate I1_Ramp2. If it is not under-air, continue loading at the above current loading rate I1_Ramp1; loop. If it is under-air, re-fit the current loading rate I1_Rampn again until the target current is loaded and stop loading; Re-fitting the current loading rate includes: taking Tn1 = α1*T0 in the formula of the first current loading rate I1_Ramp1, taking Tn2 = α2*T0 in the formula of the second current loading rate I1_Ramp2, and taking Tnn = αn*T0 in the formula of the nth current loading rate I1_Rampn; where α1 < α2 <…< αn.

4. A method for controlling a variable load mode of a fuel cell system according to claim 1, characterized in that: Calculate and output the unloading rate of the current in real time, including: When the fuel cell system is initially calibrated, calibrate the variable load current at each current density point; According to the calibration curve graph, find out the current target value Itarget corresponding to the power demand P1 sent by the whole vehicle; According to the time T0, the first current load reduction rate I2_Ramp1 is fitted: I2_Ramp1=(I target-Ioriginal) / (T0-Tn); Among them, Tn=(α1~αn)*T0, initial Tn1=αn*T0; Calculate and output the current value I and the air stoichiometric ratio AirStoich after each step of load reduction; the current output value after each step of load reduction is the current output value of the previous step minus the first current load reduction rate I2_Ramp1; After each step, compare whether the current value reaches the target current I target. If it reaches the target current, stop reducing the load. If it does not reach the target current value, determine whether the load reducing process is insufficient in air based on the air metering ratio AirStoich. If it is insufficient in air, refit the current load reduction rate I2_Ramp2. If it is not insufficient in air, continue reducing the load with the current load reduction rate I2_Ramp1. Repeat the cycle. If it is insufficient in air, refit the current load reduction rate I2_Rampn until the load is reduced to the target current, and stop reducing the load. Refitting the current load reduction rate includes: taking Tn1=αn*T0 in the first current load reduction rate I2_Ramp1 formula, taking Tn2=αn-1*T0 in the second current load reduction rate I2_Ramp2 formula, and taking Tnn=α1*T0 in the nth current load reduction rate I1_Rampn formula; wherein αn<αn-1<…<α1.

5. The method for controlling a variable load mode of a fuel cell system according to claim 1, characterized in that: The hydrogen pressure setting value changes with the air inlet pressure, including: When the fuel cell system is calibrated for the first time, the hydrogen-air pressure difference ΔP corresponding to each electrical density point is calibrated; After determining the system load change state, the air inlet pressure of the system will be collected at each step. During the load change process, the hydrogen pressure setting value changes with the air pressure at a difference of ΔP.

6. A method for controlling a variable load mode of a fuel cell system according to claim 1, characterized in that: If the system maintains the original power state, there is no need to calculate the loading rate and load reduction rate.

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