Converter for achieving optimal efficiency of hydrogen production system and its control method

Abstract

The present invention relates to the field of power electronic converter control, aiming to provide a converter and its control method for achieving the optimal efficiency of a hydrogen production system. The converter is connected to a three-phase AC power supply and an electrolytic cell, and is an isolated AC / DC converter composed of two-stage converters, including a three-phase power correction rectifier module and a phase-shifted full-bridge module that are electrically connected in sequence, and an electrolytic capacitor is provided between the two modules; wherein, the three-phase power correction rectifier module is composed of six switching tubes, and the switching tubes are interconnected to form three H-bridge structures; the phase-shifted full-bridge module includes a DC / AC sub-module and an AC / DC sub-module; wherein, the DC / AC sub-module includes a primary H-bridge composed of four switching tubes; the AC / DC sub-module is located on the secondary side of the transformer and includes a synchronous rectifier switching tube and an inductor-capacitor filter circuit, and its output terminal is connected to the electrolytic cell. The present invention has the advantages of low switching loss and wide load voltage regulation range, and can meet the requirements of the hydrogen production system.

Description

Technical Field

[0001] The present invention relates to the field of power electronic converter control, and particularly to a converter based on the optimal efficiency of a hydrogen production system and its control method. Background Art

[0002] In the context of the global advocacy of "carbon neutrality and carbon peak", the development of high-efficiency and zero-emission renewable energy electrolytic hydrogen technology has become the key to "carbon neutrality and carbon peak". As the core device for electrolytic water hydrogen production, the electrolyzer requires DC power supply. Thus, AC-DC power electronic converters have become the focus of research and development. In this process, it is also very important to improve the efficiency of the hydrogen production system. To increase the hydrogen production rate of the electrolyzer, the converter needs to have the characteristics of as small output current ripple as possible and output low voltage and large current.

[0003] Although the Buck converter of the traditional hydrogen production power supply can achieve voltage reduction, its voltage reduction ability is limited, and the voltage and current stresses of the switching tubes are large, resulting in an increase in switching losses and affecting the overall efficiency of the system. In addition, the hydrogen production system in the traditional technology uses a method of fixing the DC link voltage, and adjusts the electrolyzer terminal voltage and current to improve the energy conversion efficiency from the input electrical energy of the electrolyzer to chemical energy, but does not consider the power loss of the converter, which often results in a low energy utilization efficiency of the hydrogen production system. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a converter based on the optimal efficiency of a hydrogen production system and its control method.

[0005] To solve the technical problem, the solution of the present invention is:

[0006] Provide a converter for realizing the optimal efficiency of a hydrogen production system. The converter is connected to a three-phase AC power supply and an electrolyzer, and is an isolated AC / DC converter composed of two-stage converters, including a three-phase power correction rectifier module (AC / DC module) and a phase-shifted full-bridge module (DC / DC module) that are electrically connected in sequence. An electrolytic capacitor is provided between the two modules; wherein,

[0007] The three-phase power correction rectifier module is composed of six switching tubes, and the switching tubes are interconnected to form three H-bridge structures;

[0008] The phase-shifted full-bridge module includes a DC / AC sub-module and an AC / DC sub-module; wherein, the DC / AC sub-module includes a primary H-bridge composed of four switching tubes; the AC / DC sub-module is located on the secondary side of the transformer and includes synchronous rectifier switching tubes and an inductor-capacitor filter circuit, and its output terminal is connected to the electrolyzer.

[0009] The present invention further provides a control method for achieving the optimal efficiency of a hydrogen production system based on the aforementioned converter, specifically including the following steps:

[0010] (1) Collect the voltage and current magnitudes at both ends of the electrolyzer, and calculate the energy conversion efficiency from electrical energy to chemical energy during the electrolysis process;

[0011] (2) Conduct gradient iteration of the optimal efficiency of the electrolyzer to obtain the voltage loop reference value V 0_ref ;

[0012] (3) Voltage loop control of the phase-shifted full-bridge module: Subtract the voltage sampling value V 0_ref from the voltage loop reference value V 0 to obtain the voltage error. Use the voltage error as the input of the PI controller. After the operation of the PI controller, obtain the effective duty cycle of the phase-shifted full-bridge module and the current compensation term I comp of the three-phase power correction rectifier module:

[0013] (4) Use the effective duty cycle obtained in step (3) to generate the PWM wave of the H-bridge of the phase-shifted full-bridge module, which is used to control the time length of the simultaneous conduction of the two pairs of switching tubes in the H-bridge respectively;

[0014] (5) Establish a loss model of the cascaded AC-DC converter and linearize the fractional-order terms in the loss polynomial;

[0015] (6) Based on the loss polynomial of the cascaded AC-DC converter, solve the number of roots of the polynomial in V DC ∈ [V min , V max ;

[0016] (7) Use the gradient method to solve the DC link voltage V DC under the optimal efficiency of the converter;

[0017] (8) Voltage outer loop control of the three-phase power correction rectifier module: Subtract the DC link voltage sampling value V DC_ref from the voltage loop reference value V DC to obtain the voltage error. Use the voltage error as the input of the PI controller. After the operation of the PI controller, add the current compensation term I comp in step (3) to obtain the current amplitude adjustment coefficient;

[0018] (9) Multiply the current amplitude adjustment coefficient by the phase voltage to obtain the three-phase phase current command values for the three-phase power correction rectifier module;

[0019] (10) Current inner loop control of the three-phase power correction rectifier module: Generate three-phase modulation waves, compare them with the carrier wave to obtain six-way SPWM signals for controlling six switching tubes respectively.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1. The present invention adopts a cascaded AC-DC converter. The front-stage three-phase boost power factor correction rectifier inputs three-phase alternating current, which can reduce the harmonic rate of electric energy at the input end of the conversion and provide a wide-range adjustable high DC link voltage for the rear-stage converter. The rear-stage phase-shifted full-bridge DC converter inputs high voltage and uses the step-down ability of the transformer to provide low voltage and large current for the electrolyzer, with the advantages of low switching loss and wide load voltage adjustment range, and can meet the requirements of the hydrogen production system.

[0022] 2. The present invention is based on the optimal gradient iteration method, which can improve the energy conversion efficiency from the electric energy input to the electrolyzer to chemical energy during the electrolysis process;

[0023] 3. The present invention adopts the method of adjustable DC link voltage, which can reduce the power loss of the cascaded AC-DC converter. Compared with the traditional Buck converter, the present invention can significantly improve the overall efficiency of the hydrogen production system. Description of the Drawings

[0024] Figure 1 It is the topology and control process of the cascaded AC-DC converter with the electrolyzer as the load.

[0025] Figure 2 It is the optimal gradient iteration flow chart for determining the optimal energy conversion efficiency of the electrolyzer and the optimal electrolyzer terminal voltage.

[0026] Figure 3 It is the phase-shifted full-bridge PWM control mode.

[0027] Figure 4 It is the flow chart for determining the DC link voltage V DC under the optimal efficiency of the AC-DC converter.

[0028] Figure 1 In the drawings, reference numerals: 1 external heat source; 2 electrolyzer. Detailed Embodiment

[0029] The following details the present invention according to the drawings.

[0030] The converter used to achieve the optimal efficiency of the hydrogen production system in the present invention is an isolated AC / DC converter composed of two-stage converters, connecting a three-phase AC power supply and an electrolyzer; the converter includes a three-phase power correction rectifier module and a phase-shifted full-bridge module connected in sequence, and an electrolytic capacitor is arranged between the two modules; wherein,

[0031] The three-phase power correction rectifier module consists of six switching tubes, and the switching tubes are interconnected to form three H-bridge structures; three-phase symmetrical sine modulation wave signals with a phase difference of 120 degrees are applied to different bridge arms; among them, the phase references of the three-phase modulation signals are respectively determined by the phase of the phase voltage of that phase, and the amplitudes of the three-phase modulation signals are determined by the output signal of the voltage outer loop.

[0032] The phase-shifted full-bridge module includes a DC / AC sub-module and an AC / DC sub-module; among them, the DC / AC sub-module includes a primary H-bridge composed of four switching tubes; the AC / DC sub-module is located on the secondary side of the transformer, including synchronous rectifier switching tubes and an inductor-capacitor filter circuit, and its output terminal is connected to the electrolytic cell. For the DC / AC sub-module, the duty cycle of the gate drive signals of the switching tubes in the same bridge arm of its primary H-bridge is adjusted to 50% and the phase-shift angle is 180 degrees. By controlling the phase-shift angles of the upper and lower tubes on different bridge arms, square waves with different duty cycles are obtained at the primary end of the transformer; then, through the isolation transformation of the transformer with a primary-secondary turns ratio of n, the secondary side voltage becomes a square wave attenuated by n times relative to the primary side voltage; finally, the AC / DC sub-module rectifies the secondary side voltage to obtain a DC voltage and current with smaller ripple to meet the requirements of the electrolytic cell.

[0033] Based on this converter, the present invention provides a control method for realizing the optimal efficiency of the hydrogen production system, which specifically includes the following steps:

[0034] 1. Collect the voltage V 0 across the electrolytic cell and the current magnitude I 0 , and calculate the energy conversion efficiency η from electrical energy to chemical energy in the electrolytic hydrogen:

[0035]

[0036] In the above formula, Q H is the chemical energy of hydrogen, Q heat is the energy supplied by the external heat source, Q power is the input electrical energy, K is the electrochemical coefficient, R h is the calorific value of the chemical energy of hydrogen, β is the temperature adjustment coefficient, F is the Faraday constant, V rev is the reversible voltage of the electrolysis unit, λ is the heat dissipation coefficient, r 1 , r 2 , s 1 , s 2 , s 3 , t 1 , t 2 , t 3 are empirical constants, T 1 is the temperature of the electrolytic cell, S is the entropy value of hydrogen at this temperature, A is the electrode area, and N cell is the number of electrolysis units.

[0037] 2. Perform gradient iteration of the optimal efficiency of the electrolytic cell to obtain the voltage loop reference value V of the phase-shifted full-bridge module 0_ref .

[0038] The optimal gradient iteration process is as follows Figure 2 shown:

[0039] Subtract the electrolytic cell terminal voltage V 0 (n) collected at the nth moment from the electrolytic cell terminal voltage V 0 (n - 1) collected at the (n - 1)th moment to obtain the voltage difference △V 0 (n) of the electrolytic cell at adjacent times. Subtract the energy efficiency η(n) calculated at the nth moment from the energy efficiency η(n - 1) calculated at the (n - 1)th moment to obtain the energy efficiency difference △η(n) of the electrolytic cell at adjacent times.

[0040] Divide the energy efficiency difference △η(n) at adjacent times by the voltage difference △V 0 (n) of the electrolytic cell at adjacent times. If △η(n) / △V 0 (n)>0, then the electrolytic cell terminal voltage V 0 (n + 1) = V 0 (n)+ε. If △η(n) / △V 0 (n)<0, then the electrolytic cell terminal voltage V 0 (n + 1) = V 0 (n)-ε, and take V 0 (n + 1) as the phase-shifted full-bridge voltage outer loop reference value V 0_ref = V 0 (n + 1).

[0041] 3. Voltage loop control of the phase-shifted full-bridge module:

[0042] Subtract the electrolytic cell terminal voltage sampling value V 0_ref from the voltage loop reference value V 0 to obtain the voltage error V 0_ref -V 0 . Take the voltage error as the input of the PI controller. After the operation of the PI controller, the effective duty cycle of the phase-shifted full-bridge converter is obtained where n is the turns ratio of the primary and secondary windings of the transformer, and V DC is the DC link voltage; at the same time, the current compensation term I comp of the power factor correction rectifier can also be obtained.

[0043] 4. Use the effective duty cycle obtained in step 3 to generate the PWM wave of the H-bridge of the phase-shifted full-bridge module to control the time length of the simultaneous conduction of the two pairs of switching tubes in the H-bridge.

[0044] As shown Figure 1 in the figure, the drive signals G 7 ~G 10 correspond to the switching transistors S 7 ~S 10 . S 7 and S 9 are the upper and lower switching transistors of the same bridge arm, and their drive waveforms are complementary. A dead time t dead is set. Within the switching period T of one switching transistor, the conduction time of S 7 and S 9 is Similarly, S 8 and S 10 are the upper and lower switching transistors of the same bridge arm, and their drive waveforms are complementary. A dead time t dead is set. Within one PWM period, the conduction time of S 8 and S 10 is Then, the effective duty cycle in the third step is used to control the length of the time when the switching transistors S 7 , S 10 conduct simultaneously and the length of the time when the switching transistors S 8 , S 9 conduct simultaneously.

[0045] 5. Establish a loss model for the cascaded AC-DC converter and linearize the fractional-order terms in the loss polynomial;

[0046] This step is one of the cores of the present invention and specifically includes the following sub-steps.

[0047] (1) Calculate the loss of the power factor correction rectifier.

[0048] The conduction loss P Cond,PFC of the power factor correction rectifier is respectively composed of the switching conduction loss P switcn , the filter inductor winding resistance loss P ind , and the output filter capacitor equivalent series resistance loss P ESR,PFC :

[0049]

[0050] Among them, I rms is the effective value of the current passing through the switching transistors S 1 ~S 6 , I in,RMS is the effective value of the current passing through the three-phase filter inductor, I DC,rms is the effective value of the current passing through the output filter capacitor, P is the three-phase AC input power, and R DS,PFC is the resistance of the switching transistors S 1 ~S 6On-state resistance, r L is the resistance of the three-phase AC input filter inductor winding, r C is the equivalent series resistance of the DC-link filter capacitor, V pk is the peak value of the single-phase voltage.

[0051] The switching loss P of the power factor correction rectifier tube switching tube S 1 ~S 6 is: sw,PFC :

[0052]

[0053] In the above formula, C oss is the parasitic output resistance of the MOSFET, t tr is the switching tube S 1 ~S 6 The sum of the turn-on and turn-off times, f is the switching frequency of the switching tube S 1 ~S 6 (Switching tube S 7 ~S 10 )

[0054] (2) Calculate the losses of the phase-shifted full-bridge converter.

[0055] Calculate the on-state losses of the phase-shifted full-bridge transformer, including the conduction loss P of the synchronous rectifier MOSFET SR , the secondary winding loss P of the transformer trans,sec , the choke coil loss P cond,Ls , the equivalent series resistance loss P of the filter capacitor ESR,PSFB :

[0056]

[0057] In the above formula, i s,RMS is the effective value of the choke coil current, i sec,RMS is the effective value of the secondary side winding current of the transformer, r L ′ is the resistance of the choke coil, r T ′ is the resistance of the secondary side winding of the transformer, r c ′ represents the equivalent series resistance of the phase-shifted full-bridge output filter capacitor, R′ DS represents the on-resistance of the synchronous rectifier MOSFET, x = ψT / 2π, L s is the inductance of the choke coil.

[0058] The turn-off loss P of the synchronous rectifier MOSFET sw,sec and the buffer absorption power P of the synchronous rectifier MOSFET snubber can be expressed as:

[0059]

[0060] In the above formula, t s,off is the turn-off time of the rectifier diode, L lks is the leakage inductance of the secondary side of the transformer, and β is the peak value of the choke coil current.

[0061] The on-state loss P cond,pri of the primary side of the phase-shifted full-bridge includes the on-state losses of the switching transistors S 7 ~S 10 and the loss of the primary side winding of the transformer:

[0062]

[0063] R DS is the on-state resistance of the switching transistors S 7 ~S 10 r T is the resistance of the primary winding, and a and b are the valley value and peak value of the primary current of the transformer respectively.

[0064] The turn-off loss P 7 ~S 10 of the switching transistors S sw,pri is:

[0065]

[0066] In the above formula, t off is the turn-off time of the switching transistors S 7 ~S 10 .

[0067] The loss P core of the transformer is:

[0068]

[0069] K, α, and β are Steinmetz parameters, L e is the magnetic path length of the transformer, L m is the magnetizing inductance of the transformer, and N is the number of turns of the primary side of the transformer.

[0070] (3) Based on sub-steps (1) and (2), substituting the magnetic material characteristics of the transformer α = 1 and β = 2, a polynomial of the loss P DS of the cascaded AC-DC converter with the DC voltage V tot as the independent variable is established:

[0071]

[0072] Among them,

[0073]

[0074]

[0075] Linearize the fractional-order terms in the loss model polynomial around the steady-state operating point of the DC-link voltage as follows:

[0076]

[0077] where

[0078]

[0079] In the above equations, P PFC is the loss of the power factor correction rectifier, P PSFB is the loss of the phase-shifted full-bridge converter; n is the turns ratio of the primary and secondary windings of the transformer, N is the number of turns of the primary winding of the transformer, T is the switching period of the switching device, f is the switching frequency of the switching device, L s is the inductance of the choke coil, P is the input power of the three-phase alternating current, R′ DS is the on-resistance of the switching devices Q 1 , Q 2 , R DS is the on-resistance of the switching devices S 7 ~S 10 , R DS,PFC is the on-resistance of the switching devices S 1 ~S 6 , r T is the primary resistance of the transformer, r T ′ is the secondary resistance of the transformer, r L is the resistance of the winding of the input filter inductor of the three-phase alternating current, r L ′ is the resistance of the winding of the choke coil, r c ′ is the equivalent series resistance of the output filter capacitor of the phase-shifted full-bridge, r c is the equivalent series resistance of the DC-link filter capacitor, L p is the primary resonant inductance of the transformer, L lks is the leakage inductance of the secondary winding of the transformer, t tr is the sum of the turn-on and turn-off times of the switching devices S 1 ~S 6 , V DC is the DC-link voltage, L e is the magnetic path length, L m is the magnetizing inductance of the transformer, V pk is the peak voltage of the single-phase alternating current, C oss is the parasitic output resistance of the MOSFET, and K is the Steinmetz parameter.

[0080] 6. Based on the loss polynomial of the cascaded AC-DC converter, solve the number of roots of the polynomial in V DC ∈[V min , V max .

[0081] This step is one of the cores of the present invention and is divided into the following sub-steps.

[0082] (1) Solve the polynomial according to the coefficients of the first column of the Routh-Hurwitz matrix The number of roots in V DC ∈ [V min , V max . V min , V max respectively refer to the minimum and maximum values in the adjustable range of the bus voltage.

[0083] Establish the equation to be solved:

[0084]

[0085] Let V DC = q + V min , and substituting it into the above polynomial gives:

[0086] α 5 q 5 + α 4 q 4 + α 2 q 2 + α 1 q + a 0 = 0

[0087] In the formula, α 0 ~α 5 are expressions about a, b, c′, e′, F′, V min . The coefficients of the first column of the above Routh-Hurwitz matrix are:

[0088]

[0089] If the number of sign changes of the above coeff 1 , coeff 2 , coeff 3 , coeff 4 values is n, that is, the number of roots in [V min , +∞] is n.

[0090] Let V DC = r + V max and substituting it into the above polynomial gives:

[0091] β 5 r 5 + β 4 r 4 + β 2 r 2 + β 1 r + β0 = 0

[0092] wherein, β 0 ~β 5 are expressions regarding a, b, c′, e′, F′, V max The coefficients of the first column of the Routh-Hurwitz matrix of the above formula are:

[0093]

[0094] If the number of sign changes of the above coeff 5 , coeff 6 , coeff 7 , coeff 8 is m, that is, the number of roots in [V max , +∞] is m.

[0095] In summary, the number of roots of the polynomial in V DC ∈ [V min , V max is n - m.

[0096] (2) If the polynomial has no solution in V DC ∈ [V min , V max , that is, n - m = 0.

[0097] Then in V DC ∈ [V min , V max , if V max is used as the reference value of the voltage outer loop; if then V max is used as the reference value of the voltage outer loop, and then directly go to step 8.

[0098] (3) If the polynomial has a solution in V DC ∈ [V min , V max , that is, n - m > 0, then go to step 7.

[0099] 7. Use the gradient method to solve the DC link voltage V DC .

[0100] This step is one of the cores of the present invention, and the process is as shown in Figure 4 and specifically includes the following sub-steps:

[0101] (1) First, initialize the starting point at V i-1 = V min, and define the number of iteration steps as A; iterate δ with a smaller step size to get V i =V min +Aδ, if Then proceed to the next step, otherwise continue iterating in step (1).

[0102] (2) Use the binary method to select V i+1 =(V i +V i-1 ) / 2; if Then V i+2 =(V i-1 +V i+1 ) / 2; if satisfied Then V i+2 =(V i +V i+1 ) / 2.

[0103] (3) Determine the product Positive and negative The degree of convergence is, if both and Then use array opm to save V i+2 , otherwise return to sub-step (2).

[0104] (4) If the number of poles found is less than nm, return to sub-step (1). If the number of poles found is equal to nm, bring the voltage values ​​in the array opm into the loss model and select the voltage value with the minimum loss. Serves as the reference value for the outer loop voltage of the power correction rectifier.

[0105] 8. Voltage outer loop control of three-phase power correction rectifier module: Based on the voltage loop reference value V DC_ref Subtract the DC link voltage sampling value V DC The voltage error is obtained and used as the input of the PI controller. After the operation of the PI controller, the current compensation term I in step 3 is added. comp , and obtain the current amplitude adjustment coefficient.

[0106] 9. Collect V in the three-phase line voltage AB 、V AC , the three-phase voltage can be expressed as:

[0107]

[0108] The three-phase voltage provides the phase for the current command value. The phase current command value can be obtained by multiplying the three-phase voltage with the current amplitude adjustment coefficient.

[0109] 10. Current inner-loop control of three-phase power correction rectifier module: The phase current command value minus the phase current sampling value gives the current error. Taking the current error as the input of the PI controller, after the operation of the PI controller, three-phase modulation waves are obtained. Then, by comparing with the carrier wave, six SPWM signals can be output to drive the switching transistors S 1 ~S 6 .

Claims

1. A control method for achieving the optimal efficiency of a hydrogen production system, characterized in that, an isolated AC / DC converter composed of two - stage converters is set between a three - phase AC power supply and an electrolyzer, including a three - phase power correction rectifier module and a phase - shifted full - bridge module connected in sequence electrically, and an electrolytic capacitor is set between the two modules; wherein, the three - phase power correction rectifier module is composed of six switching tubes, and the switching tubes are interconnected to form three H - bridge structures; the phase - shifted full - bridge module includes a DC / AC sub - module and an AC / DC sub - module; wherein, the DC / AC sub - module includes a primary - side H - bridge composed of four switching tubes; the AC / DC sub - module is located on the secondary - side of the transformer, including synchronous rectifier switching tubes and an inductor - capacitor filter circuit, and its output terminal is connected to the electrolyzer; For the three - phase power correction rectifier module, three - phase symmetric sine modulation wave signals with a phase difference of 120 degrees are applied to different bridge arms; wherein the phase references of the three - phase modulation signals are respectively determined by the phase voltages of the corresponding phases, and the amplitudes of the three - phase modulation signals are determined by the output signal of the voltage outer loop; For the DC / AC sub - module of the phase - shifted full - bridge module, the duty ratios of the gate drive signals of the switching tubes in the same bridge arm of its primary - side H - bridge are all adjusted to 50%, and the phase - shift angle is 180 degrees. By controlling the phase - shift angles of the upper and lower tubes on different bridge arms, square waves with different duty ratios are obtained at the primary end of the transformer; then through the isolation transformation of a transformer with a primary - to - secondary turns ratio of n, the secondary - side voltage becomes a square wave attenuated by n times relative to the primary - side voltage; finally, the AC / DC sub - module rectifies the secondary - side voltage to obtain a DC voltage and current with smaller ripple to meet the requirements of the electrolyzer; The control method specifically includes the following steps: (1) Collect the voltage and current magnitudes at both ends of the electrolyzer, and calculate the energy conversion efficiency from electrical energy to chemical energy during the electrolysis process; (2) Perform gradient iteration of the optimal efficiency of the electrolytic cell to obtain the voltage loop reference value V of the phase-shifted full-bridge module 0_ref ; (3) Voltage loop control of the phase-shifted full-bridge module: Using the voltage loop reference value V 0_ref subtract the voltage sampling value V 0 to obtain the voltage error. Take the voltage error as the input of the PI controller. After the operation of the PI controller, the effective duty cycle of the phase-shifted full-bridge module and the current compensation term I of the three-phase power correction rectifier module are obtained comp : (4) Use the effective duty ratio obtained in step (3) to generate the PWM waves of the H - bridge of the phase - shifted full - bridge module, which are used to control the time lengths of the simultaneous conduction of the two pairs of switching tubes in the H - bridge respectively; (5) Establish a loss model of the cascaded AC - DC converter and linearize the fractional - order terms in the loss polynomial; (6) Solve the polynomial based on the loss polynomial of the cascaded AC-DC converter at V DC ∈ [V min , V max ; the number of roots of P tot is the loss of the cascaded AC-DC converter with the DC voltage V DS as the independent variable, and V min , V max respectively refer to the minimum and maximum values in the adjustable range of the bus voltage; (7) Solve for the DC link voltage V at the optimal efficiency of the converter using the gradient method DC ; (8) Voltage outer loop control of three-phase power correction rectifier module: Using the voltage loop reference value V DC_ref minus the DC link voltage sampling value V DC to obtain the voltage error. Taking this voltage error as the input of the PI controller, after the operation of the PI controller, adding the current compensation term I in step (3) comp , to obtain the current amplitude adjustment coefficient; (9) Multiply the current amplitude adjustment coefficient by the phase voltage to obtain the three - phase phase - current command values for the three - phase power correction rectifier module; (10) Current inner - loop control of the three - phase power correction rectifier module: Generate three - phase modulation waves, compare them with the carrier wave to obtain six - channel SPWM signals for controlling the six switching tubes respectively.

2. The method according to claim 1, characterized in that, the specific content of step (1) includes: Collect the voltages V at both ends of the electrolytic cell 0 and the magnitudes I of the currents 0 , and calculate the energy conversion efficiency η from electrical energy to chemical energy during the electrolysis process: In the above formula, Q H is the chemical energy of hydrogen, Q heat is the energy supplied by the external heat source, Q power is the input electrical energy, K is the electrochemistry coefficient, R h is the calorific value of the chemical energy of hydrogen, β is the temperature adjustment coefficient, F is the Faraday constant, V rev is the reversible voltage of the electrolysis unit, λ is the heat dissipation coefficient, r 1 、r 2 、s 1 、s 2 、s 3 、t 1 、t 2 、t 3 are empirical constants, T 1 is the temperature of the electrolyzer, S is the entropy value of hydrogen at this temperature, A is the electrode area, N cell is the number of electrolysis units.

3. The method according to claim 1, characterized in that, the specific content of step (2) includes: The electrolytic cell terminal voltage V 0 (n) collected at the nth moment is subtracted from the electrolytic cell terminal voltage V 0 (n - 1) collected at the (n - 1)th moment to obtain the voltage difference △V 0 (n) of the electrolytic cell at adjacent times; the energy efficiency η(n) calculated at the nth moment is subtracted from the energy efficiency η(n - 1) calculated at the (n - 1)th moment to obtain the energy efficiency difference △η(n) of the electrolytic cell at adjacent times; the energy efficiency difference △η(n) at adjacent times is divided by the voltage difference △V 0 (n) of the electrolytic cell at adjacent times; if △η(n) / △V 0 (n)>0, then the electrolytic cell terminal voltage V 0 (n + 1) = V 0 (n)+ε; if △η(n) / △V 0 (n)<0, then the electrolytic cell terminal voltage V 0 (n + 1) = V 0 (n)-ε; and V 0 (n + 1) is used as the phase - shifted full - bridge voltage outer - loop reference value V 0_ref = V 0 (n + 1).

4. The method according to claim 1, characterized in that, the specific content of step (5) includes: (5.1) Calculate the loss of the power factor correction rectifier; (5.2) Calculate the loss of the phase - shifted full - bridge converter; (5.3) On the basis of the above two steps, bring in the material properties to establish the loss P of the cascaded AC-DC converter with the DC voltage as the independent variable tot Polynomial: wherein, Linearize the fractional-order terms in the loss model polynomial around the steady-state operating point of the DC-link voltage as follows: wherein, In the above equations, P PFC is the loss of the power factor correction rectifier, and P PSFB is the loss of the phase-shifted full-bridge converter; n is the turns ratio of the primary and secondary sides of the transformer, N is the number of turns of the primary side of the transformer, T is the switching period of the switching device, f is the switching frequency of the switching device, L s is the inductance of the choke coil, P is the input power of the three-phase alternating current, R′ DS is the on-resistance of the switching devices Q 1 and Q 2 ; R DS is the on-resistance of the switching devices S 7 to S 10 ; R DS,PFC is the on-resistance of the switching devices S 1 to S 6 ; r T is the primary-side resistance of the transformer, r T ′ is the secondary-side resistance of the transformer, r L is the resistance of the winding of the input filter inductor of the three-phase alternating current, r L ′ is the resistance of the winding of the choke coil, r c ′ is the equivalent series resistance of the output filter capacitor of the phase-shifted full-bridge, r c is the equivalent series resistance of the DC-link filter capacitor, L p is the primary-side resonant inductance of the transformer, L lks is the leakage inductance of the secondary side of the transformer, t tr is the sum of the turn-on and turn-off times of the switching devices S 1 to S 6 , V DC is the DC-link voltage, L e is the magnetic path length, L m is the magnetizing inductance of the transformer, V pk is the peak voltage of the single-phase alternating current, C oss is the parasitic output resistance of the MOSFET, and K is the Steinmetz parameter.

5. The method according to claim 1, characterized in that, the specific content of step (6) includes: (6.1) Solve the polynomial according to the coefficients in the first column of the Routh-Hurwitz matrix in V DC ∈ [V min , V max for the number of roots; In the above formula, α 0 ~α 5 is an expression about a, b, c′, e′, F′, V min ; if the number of sign changes of the values of coeff 1 , coeff 2 , coeff 3 , coeff 4 is n, that is, the number of roots in [V min , +∞] is n; In the above formula, β 0 ~β 5 is an expression about a, b, c′, e′, F′, V max ; if the number of sign changes of the above coeff 5 , coeff 6 , coeff 7 , coeff 8 is m, that is, the number of roots in [V max , +∞] is m; In summary, the polynomial in V DC ∈ [V min , V max has n - m roots; (6.2) If the polynomial has no solution in V DC ∈ [V min , V max , that is, n - m = 0; At V DC ∈ [V min , V max , if V max serves as the reference value of the voltage outer loop; if then V max serves as the reference value of the voltage outer loop and then directly proceeds to step (8); (6.3) If the polynomial has a solution in V DC ∈ [V min , V max , that is, n - m > 0, then go to step (7).

6. The method according to claim 1, characterized in that, the specific content of step (7) includes: (7.1) Initialize by setting the starting point at V i-1 = V min , and define the number of iteration steps A; Iterate δ with a smaller step size to obtain V i = V min + Aδ, if then proceed to the next step, otherwise keep iterating at (7.1); (7.2) Use the bisection method to take V i+1 =(V i +V i-1 ) / 2. If then V i+2 =(V i-1 +V i+1 ) / 2; if is satisfied, then V i+2 =(V i +V i+1 ) / 2; (7.3) Determine the product sign and convergence. If both and |(V i+2 ) - (V i+1 )| < △ are satisfied, then use the array opm to save V i+2 , otherwise return to (7.2); (7.4) If the number of poles is less than n - m, return to (7.1); if the number of poles is equal to n - m, substitute the voltage values in the array opm into the loss model and select the voltage value V with the minimum loss. (Ptot)min As the reference value of the voltage outer loop of the power correction rectifier.

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