Two-stage parallel high-power hydrogen production power supply system and non-interconnected line current sharing control method thereof

Abstract

The application discloses a two-stage parallel high-power hydrogen production power supply system and a non-contact line current sharing control method thereof in the technical field of power supply or distribution circuit, which comprises a three-phase power supply, the three-phase power supply is connected with input ends of two PWM rectifiers respectively, output ends of each PWM rectifier are connected with input ends of two DC / DC converters, and output ends of the two DC / DC converters are connected with a hydrogen production PEM device, wherein the three-phase power supply, the PWM rectifier DC output end and the DC / DC output end are parallel operation; the conventional topology is improved, high reliability and low cost are considered, the application proposes a non-isolated three-phase power supply, PWM rectifier output end and DC / DC converter output end "three-end direct parallel" system architecture. The architecture effectively breaks the bottleneck that single-stage failure leads to the failure of the whole module, higher system-level reliability can be provided through fewer redundant devices; reasonable distribution and loop current suppression of the parallel system power are realized.

Description

Technical Field

[0001] This invention relates to a two-stage parallel high-power hydrogen production power supply system and its current sharing control method without tie lines in the field of power supply or distribution circuit technology. Background Technology

[0002] As the core power supply equipment for hydrogen production systems using renewable energy electrolysis, hydrogen production power supplies need to meet requirements such as low voltage and high current, low input harmonic content, high efficiency, and high power factor. With the development of larger electrolyzers, the power requirements for hydrogen production power supplies are becoming increasingly stringent, and a single hydrogen production power supply module cannot meet the power demands of the hydrogen production system. Currently, the rectifier equipment widely used in the field of hydrogen electrolysis mainly includes two types: thyristor phase-controlled rectifiers and IGBT-based fully controlled rectifiers, each with its own applicable scenarios and technical characteristics. Phase-controlled rectifiers are simple in structure, have large capacity, low cost, and mature technology, but they suffer from low power factor, slow response speed, high harmonics in the three-phase power grid, and large DC-side ripple current, reducing the efficiency of hydrogen electrolysis. With the continuous improvement of power supply requirements, hydrogen production power supplies are gradually being replaced by fully controlled rectifiers, whose main circuits generally adopt a front-stage PWM rectifier + rear-stage DC / DC structure. Due to the power limitations of IGBT devices, high-power hydrogen production power supplies usually need to connect multiple power supplies in parallel to increase power. A hydrogen production power supply system with multiple modules connected in parallel and scalable power levels can meet the demand for high-power water electrolysis hydrogen production, while improving the system's economic efficiency. When the input and output of the hydrogen production power supply are directly connected in parallel, there are issues with circulating current at the switching frequency level and low-frequency circulating current. Circulating current causes distortion of the system input current, excessive common-mode current, and in severe cases, machine failure and shutdown, affecting the safe and reliable operation of the system. Therefore, parallel power supply connections present numerous challenges, including current sharing control, circulating current control, and stability issues, all of which pose challenges to high-power hydrogen production power supplies.

[0003] To suppress circulating current in parallel inverter systems, the first approach involves using an isolated power supply on the DC side and adding a power frequency transformer or reactor to the three-phase power supply. This cuts off the circulating current path or increases the circulating current loop impedance, thus suppressing parallel circulating current. However, this approach increases equipment size, weight, and cost, and reduces system efficiency. The second approach optimizes and adjusts the waveform strategy to control parallel circulating current. For example, using multi-carrier methods to reduce common-mode voltage and suppress system circulating current, but this method has limited ability to suppress circulating current. Alternatively, PWM waveform parameters and carrier synchronization signals can be encoded and sent to the slave controller, which reconstructs a synchronized PWM signal to suppress parallel circulating current. Another approach is to use an improved SVPWM scheme to ensure zero common-mode voltage difference among multiple grid-connected inverters, thus solving the zero-sequence circulating current problem. These methods are open-loop circulating current suppression techniques, which are affected by the hardware parameters of the parallel devices, thus limiting their effectiveness. The third approach uses a zero-sequence current control strategy, a closed-loop control strategy, which is currently widely used.

[0004] When a hydrogen production power supply with a front-end PWM rectifier and a rear-end DC / DC structure is connected in parallel, it is usually connected in parallel on the three-phase power supply and the output side, but not on the middle DC side of the PWM rectifier output. This means that if either the front-end or rear-end fails, the unaffected stage will become unusable, reducing reliability and increasing operating costs. Current sharing control in parallel devices typically employs a current sharing line or a master-slave current sharing method. However, the master-slave method reduces system reliability, and delays in transmitting current sharing information can easily cause system oscillations. Another current sharing control method uses a masterless, tie-line-less current sharing method, which generally uses droop control. However, stable droop control is technically challenging and requires high sampling accuracy. Summary of the Invention

[0005] The purpose of this invention is to provide a two-stage parallel high-power hydrogen production power supply system and its tie-line-free current sharing control method, overcoming the defects of existing hydrogen production power supplies such as difficulty in current sharing control, serious circulating current problems, and insufficient system stability during multi-module parallel operation.

[0006] To achieve the above objectives, the present invention provides a two-stage parallel high-power hydrogen production power supply system, including a three-phase power supply, which is connected to the input terminals of two PWM rectifiers respectively. The output terminal of each PWM rectifier is connected to the input terminals of two DC / DC converters, and the output terminals of the two DC / DC converters are connected to the hydrogen production PEM device. The three-phase power supply, the DC output terminals of the PWM rectifiers, and the DC / DC output terminals all operate in parallel.

[0007] Compared with existing technologies, the advantages of this invention lie in its improvement of conventional topologies, balancing high reliability and low cost. This invention proposes a system architecture where the three-phase power supply, PWM rectifier output, and DC / DC converter output are directly connected in parallel ("three terminals directly in parallel"). Compared to the limitation of the non-parallel DC side in traditional two-stage architectures, this architecture effectively overcomes the bottleneck of a single-stage failure causing the entire module to fail, providing higher system-level reliability with fewer redundant devices; it also achieves reasonable power distribution and circulating current suppression in parallel systems, improving the stability and reliability of hydrogen production power systems operating in parallel.

[0008] To achieve the above objectives, the present invention also provides a tie-line-free current sharing control method for a two-stage parallel high-power hydrogen production power supply system, comprising the following steps:

[0009] S1. For the front-end PWM rectifier, an improved d-axis droop control strategy is constructed. The droop adjustment of the DC output voltage of the PWM rectifier is realized through single-loop control, so that the parallel PWM rectifier can automatically distribute power according to the load change and realize the current sharing control of the front-end PWM rectifier.

[0010] S2. To address the zero-sequence circulating current problem generated when PWM rectifiers are directly connected in parallel, proportional control is adopted to suppress the zero-sequence circulating current in the parallel system.

[0011] S3. To address the need for current sharing control during the startup phase of parallel operation of downstream DC / DC converters, which is voltage-controlled, a dual-closed-loop current sharing control method based on a droop strategy is proposed.

[0012] As a further improvement to the present invention, S1 is specifically described below.

[0013] S1.1, Establish the DC voltage droop target on the output side of the PWM rectifier, as shown in Equation (1).

[0014] (1);

[0015] In equation (1), This is the no-load voltage on the DC side of the PWM rectifier. The output value is the DC voltage of the PWM rectifier after being filtered by an inertial circuit. A virtual resistor, i.e., the droop slope, is inserted in series on the DC side. For the current filtering inductor of the PWM rectifier in the rotating coordinate system The d-axis current is obtained by rotating the coordinate system to form the dq coordinate system.

[0016] S1.2, for Differentiate and define the Lyapunov function. ,accomplish The goal;

[0017] S1.3, Establish the mathematical model of the PWM rectifier in the dq coordinate system, and obtain the values ​​of the three-phase modulation voltage in the dq coordinate system. .

[0018] As a further improvement to the present invention, the specific content of S1.2 is as follows:

[0019] right Differentiation yields the following equation (2):

[0020] (2);

[0021] In equation (2), This refers to the DC-side voltage output by the PWM rectifier. The droop control is achieved by using the inertial element's filtering time constant. The goal is to define the following Lyapunov function. As in equation (3):

[0022] (3);

[0023] in for The integral coefficient, where t is the time variable and du is the differential variable, for Differentiating gives , It can be represented as:

[0024] (4);

[0025] To achieve , can be made:

[0026] (5);

[0027] In formula (5) This is the droop control ratio coefficient.

[0028] As a further improvement to the present invention, the specific content of S1.3 is as follows:

[0029] The mathematical model of a PWM rectifier in the dq coordinate system can be expressed as:

[0030] (6);

[0031] In formula (6) This is the filter inductor for the three-phase power supply of the PWM rectifier. To convert the current of the three-phase power supply filter inductor to its value in the dq coordinate system. This is the value of the three-phase modulated voltage converted to the dq coordinate system. This represents the three-phase grid voltage converted to its dq coordinate system. Let be the angular frequency of the grid voltage; substituting equation (5) into the first equation of equation (6) yields:

[0032] (7);

[0033] For q-axis current control, a conventional feedforward decoupling strategy is adopted, assuming... Setting value Current error ,but The feedforward control strategy can be expressed as:

[0034] (8);

[0035] In formula (8) This is the proportional control coefficient for the current loop. is the integral control coefficient of the current loop.

[0036] As a further improvement to the present invention, the specific content of S2 is as follows.

[0037] Since the zero-sequence circulating current value only exists between directly parallel systems, the zero-sequence circulating current can be expressed as the sum of the three-phase currents, i.e. Furthermore, the zero-sequence circulating currents between the two hydrogen production power sources are equal in magnitude but opposite in direction; let the zero-sequence voltage at which the PWM rectifier suppresses the circulating current be... The integral I element of the PI controller exhibits steady-state error when tracking sinusoidal signals. Proportional control is used to suppress zero-sequence circulating current.

[0038] (9);

[0039] in This is the zero-sequence voltage proportional gain coefficient.

[0040] As a further improvement to the present invention, the specific content of S3 is as follows.

[0041] S3.1, the subsequent DC / DC converter consists of four interleaved parallel BUCK circuits, with the output currents of the four BUCK circuits being respectively... The total current of the 4 current channels is The hydrogen production PEM unit is a capacitive load, and the hydrogen production process is current-controlled. Therefore, current sharing control is not required for the parallel DC / DC units at this time. However, during the start-up phase, the process is voltage-controlled, and current sharing control is required.

[0042] The drooping strategy of the subsequent DC / DC converter is shown in equation (10):

[0043] (10);

[0044] In formula (10) The no-load voltage of the DC / DC output. The output voltage of the DC / DC converter. This refers to the virtual resistance inserted in series with the DC / DC converter, i.e., the droop slope. for The output value after filtering by the inertial element satisfies the equation (11):

[0045] (11);

[0046] The time constant for inertial element filtering;

[0047] S3.2, to achieve droop control, i.e. The goal is to define the Lyapunov function. And differentiate it;

[0048] S3.3, To achieve current sharing among the four BUCK circuits, the current setting values ​​for the four BUCK circuits should be equal. Here, the setting values ​​for the four BUCK circuits are not differentiated and are uniformly defined as follows: Its satisfaction .

[0049] As a further improvement to the present invention, S3.2 is specifically described below.

[0050] As in equation (12):

[0051] (12);

[0052] in for The integral coefficient, For the filter capacitor on the DC / DC output side; Differentiating gives , It can be represented as:

[0053] (13);

[0054] In formula (13) For DC / DC load current, therefore the load current change is relative to Slow, can be approximated as Thus, equation (13) can be transformed into:

[0055] (14);

[0056] If we let the intermediate variable for:

[0057] (15);

[0058] Equation (14) can be simplified to:

[0059] (16);

[0060] To achieve ,set up for The set value makes satisfy ,Right now

[0061] (17);

[0062] In equation (17) Let the droop control proportional coefficient of the DC / DC converter be... and The difference is ,so It can be transformed into:

[0063] (18).

[0064] As a further improvement to the present invention, S3.3 is specifically described below.

[0065] Assume the current control error of this BUCK circuit is Define the Lyapunov function V as:

[0066] (19);

[0067] Let V be the weighting coefficients of the Lyapunov function, then the derivative of V... for:

[0068] (20);

[0069] accomplish ,make ,in The proportional coefficient for the current control of the BUCK circuit is obtained by transformation:

[0070] ;

[0071] This allows us to obtain the BUCK control voltage. See equation (21):

[0072] (twenty one).

[0073] Compared with existing technologies, the advantages of this invention lie in its innovative proposal of parallel current sharing control and zero-sequence circulating current control strategies for PWM rectifiers in the front-end stage. This closed-loop control method achieves power sharing among multiple modules and effectively suppresses circulating current problems in parallel systems. For the voltage-controlled startup phase of the subsequent DC / DC converter (such as an interleaved parallel BUCK circuit), which requires current sharing control, this invention proposes a dual closed-loop control method based on a droop strategy, achieving high current sharing accuracy and stability. Attached Figure Description

[0074] Figure 1 This is a power topology diagram of a two-stage parallel system for hydrogen production power supply according to the present invention.

[0075] Figure 2 The waveform diagram shows the input current sharing of the parallel PWM rectifier system of the present invention.

[0076] Figure 3 This is a waveform diagram of the DC side voltage of the parallel PWM rectifier of the present invention.

[0077] Figure 4 The output current waveform of the DC / DC parallel module group of the present invention is shown.

[0078] Figure 5 This is a waveform diagram of the parallel branch current of the DC / DC converter of the present invention. Detailed Implementation

[0079] The present invention will be further described below with reference to the accompanying drawings:

[0080] like Figure 1 The two-stage parallel high-power hydrogen production power supply system shown includes a three-phase power supply, which is connected to the input terminals of two PWM rectifiers respectively. The output terminal of each PWM rectifier is connected to the input terminals of two DC / DC converters. The output terminals of the two DC / DC converters are connected to the hydrogen production PEM device. The three-phase power supply, the DC output terminals of the PWM rectifiers, and the DC / DC output terminals all operate in parallel.

[0081] like Figure 2-4 The tie-line-free current sharing control method for the two-stage parallel high-power hydrogen production power system shown includes the following steps:

[0082] S1. For the front-end PWM rectifier, an improved d-axis droop control strategy is constructed. The droop adjustment of the DC output voltage of the PWM rectifier is realized through single-loop control, so that the parallel PWM rectifier can automatically distribute power according to the load change and realize the current sharing control of the front-end PWM rectifier.

[0083] S1.1, Establish the DC voltage droop target on the output side of the PWM rectifier, as shown in Equation (1).

[0084] (1);

[0085] In equation (1), This is the no-load voltage on the DC side of the PWM rectifier. The output value is the DC voltage of the PWM rectifier after being filtered by an inertial circuit. A virtual resistor, i.e., the droop slope, is inserted in series on the DC side. For the current filtering inductor of the PWM rectifier in the rotating coordinate system The d-axis current is obtained by rotating the coordinate system to form the dq coordinate system.

[0086] S1.2, for Differentiate and define the Lyapunov function. ,accomplish The goal;

[0087] right Differentiation yields the following equation (2):

[0088] (2);

[0089] In equation (2), This refers to the DC-side voltage output by the PWM rectifier. The droop control is achieved by using the inertial element's filtering time constant. The goal is to define the following Lyapunov function. As in equation (3):

[0090] (3);

[0091] in for The integral coefficient, where t is the time variable and du is the differential variable, for Differentiating gives , It can be represented as:

[0092] (4);

[0093] To achieve , can be made:

[0094] (5);

[0095] In formula (5) This is the droop control ratio coefficient.

[0096] S1.3, Establish the mathematical model of the PWM rectifier in the dq coordinate system, and obtain the values ​​of the three-phase modulation voltage in the dq coordinate system. .

[0097] The mathematical model of a PWM rectifier in the dq coordinate system can be expressed as:

[0098] (6);

[0099] In formula (6) This is the filter inductor for the three-phase power supply of the PWM rectifier. To convert the current of the three-phase power supply filter inductor to its value in the dq coordinate system. This is the value of the three-phase modulated voltage converted to the dq coordinate system. This represents the three-phase grid voltage converted to its dq coordinate system. Let be the angular frequency of the grid voltage; substituting equation (5) into the first equation of equation (6) yields:

[0100] (7);

[0101] For q-axis current control, a conventional feedforward decoupling strategy is adopted, assuming... Setting value Current error ,but The feedforward control strategy can be expressed as:

[0102] (8);

[0103] In formula (8) This is the proportional control coefficient for the current loop. is the integral control coefficient of the current loop.

[0104] S2. To address the zero-sequence circulating current problem generated when PWM rectifiers are directly connected in parallel, proportional control is adopted to suppress the zero-sequence circulating current in the parallel system.

[0105] Since the zero-sequence circulating current value only exists between directly parallel systems, the zero-sequence circulating current can be expressed as the sum of the three-phase currents, i.e. Furthermore, the zero-sequence circulating currents between the two hydrogen production power sources are equal in magnitude but opposite in direction; let the zero-sequence voltage at which the PWM rectifier suppresses the circulating current be... The integral I element of the PI controller exhibits steady-state error when tracking sinusoidal signals. Proportional control is used to suppress zero-sequence circulating current.

[0106] (9);

[0107] in This is the zero-sequence voltage proportional gain coefficient.

[0108] S3. To address the need for current sharing control during the startup phase of parallel operation of downstream DC / DC converters, which is voltage-controlled, a dual-closed-loop current sharing control method based on a droop strategy is proposed.

[0109] S3.1, the subsequent DC / DC converter consists of four interleaved parallel BUCK circuits, with the output currents of the four BUCK circuits being respectively... The total current of the 4 current channels is The hydrogen production PEM unit is a capacitive load, and the hydrogen production process is current-controlled. Therefore, current sharing control is not required for the parallel DC / DC units at this time. However, during the start-up phase, the process is voltage-controlled, and current sharing control is required.

[0110] The drooping strategy of the subsequent DC / DC converter is shown in equation (10):

[0111] (10);

[0112] In formula (10) The no-load voltage of the DC / DC output. The output voltage of the DC / DC converter. This refers to the virtual resistance inserted in series with the DC / DC converter, i.e., the droop slope. for The output value after filtering by the inertial element satisfies the equation (11):

[0113] (11);

[0114] The time constant for inertial element filtering;

[0115] S3.2, to achieve droop control, i.e. The goal is to define the Lyapunov function. And differentiate it;

[0116] As in equation (12):

[0117] (12);

[0118] in for The integral coefficient, For the filter capacitor on the DC / DC output side; Differentiating gives , It can be represented as:

[0119] (13);

[0120] In formula (13) For DC / DC load current, therefore the load current change is relative to Slow, can be approximated as Thus, equation (13) can be transformed into:

[0121] (14);

[0122] If we let the intermediate variable for:

[0123] (15);

[0124] Equation (14) can be simplified to:

[0125] (16);

[0126] To achieve ,set up for The set value makes satisfy ,Right now

[0127] (17);

[0128] In equation (17) Let the droop control proportional coefficient of the DC / DC converter be... and The difference is ,so It can be transformed into:

[0129] (18).

[0130] S3.3, To achieve current sharing among the four BUCK circuits, the current setting values ​​for the four BUCK circuits should be equal. Here, the setting values ​​for the four BUCK circuits are not differentiated and are uniformly defined as follows: Its satisfaction .

[0131] Assume the current control error of this BUCK circuit is Define the Lyapunov function V as:

[0132] (19);

[0133] Let V be the weighting coefficients of the Lyapunov function, then the derivative of V... for:

[0134] (20);

[0135] accomplish ,make ,in The proportional coefficient for the current control of the BUCK circuit is obtained by transformation:

[0136] ;

[0137] This allows us to obtain the BUCK control voltage. See equation (21):

[0138] (twenty one).

[0139] In this embodiment of the invention, a non-isolated parallel connection method is constructed that can be used at the three-phase power supply, the output of the PWM rectifier, and the output of the DC / DC converter.

[0140] For the front-end PWM rectifier, an improved d-axis droop control strategy is constructed. The droop adjustment of the DC output voltage of the PWM rectifier is realized through single-loop control, so that the parallel PWM rectifier can automatically distribute power according to the load change and realize the current sharing control of the front-end PWM rectifier.

[0141] To address the zero-sequence circulating current problem generated when PWM rectifiers are directly connected in parallel, proportional control is adopted to suppress the zero-sequence circulating current in the parallel system.

[0142] To address the need for current sharing control during the startup phase of parallel operation of downstream DC / DC converters, which is voltage-controlled, a current sharing control method based on a droop strategy is proposed.

[0143] For the front-end PWM rectifier, an improved d-axis droop strategy was constructed, employing single-loop control to achieve DC voltage droop on the PWM rectifier output side, while the q-axis uses a conventional current control strategy. (Reference) Figure 1 ,by Figure 1 Taking the #1 PWM rectifier as an example (the parameters in the formula will have subscripts added), the droop target for DC-side improvement is shown in the following formula:

[0144] (twenty two);

[0145] In equation (22), This is the no-load voltage on the DC side of PWM rectifier #1. The output value is the DC voltage of PWM rectifier #1 after being filtered by an inertial circuit. A virtual resistor, i.e., the droop slope, is inserted in series on the DC side. The current filtering inductor of PWM rectifier #1 in the rotating coordinate system (dq coordinate system) d-axis current, The equation that satisfies this is equation (23):

[0146] (twenty three);

[0147] In equation (23), This is the DC-side voltage output by PWM rectifier #1. This is the filtering time constant for the inertial element. Droop control is then achieved. The goal is to define the following Lyapunov function. As in equation (24):

[0148] (twenty four);

[0149] in for The integral coefficient. For Differentiating gives , It can be represented as:

[0150] (25);

[0151] To achieve , can be made:

[0152] (26);

[0153] In equation (26) The droop control proportional coefficient, based on the mathematical model of the PWM rectifier in the dq coordinate system, taking PWM rectifier #1 as an example, can be expressed as:

[0154] (27);

[0155] In equation (27) This is the filter inductor for the three-phase power supply of the PWM rectifier. To convert the current of the three-phase power supply filter inductor to its value in the dq coordinate system. This is the value of the three-phase modulated voltage converted to the dq coordinate system. This represents the three-phase grid voltage converted to its dq coordinate system. Let be the angular frequency of the grid voltage. Substituting equation (26) into the first equation of equation (27), we get:

[0156] (28);

[0157] For q-axis current control, a conventional feedforward decoupling strategy is adopted, assuming... Setting value Current error ,but The feedforward control strategy can be expressed as:

[0158] (29);

[0159] In equation (29) This is the proportional control coefficient for the current loop. is the integral control coefficient of the current loop.

[0160] 2) Zero-sequence circulating current control

[0161] Since the zero-sequence circulating current value only exists between directly parallel systems, the zero-sequence circulating current can be expressed as the sum of the three-phase currents, i.e. Furthermore, the zero-sequence circulating currents between the two hydrogen production power sources are equal in magnitude but opposite in direction. Let the zero-sequence voltage that suppresses the circulating current in PWM rectifier #1 be... The integral I element of the PI controller exhibits steady-state error when tracking a sinusoidal signal. This invention employs proportional control to suppress zero-sequence circulating current, i.e.

[0162] (30);

[0163] Similarly, for PWM rectifier #2, because Therefore, the zero-sequence voltage of the #2 PWM rectifier suppresses the circulating current. ,therefore , (PI) has no effect on the potential of the neutral point N of the three-phase power supply, so that the suppression of zero-sequence circulating current will not interfere with the droop control and q-axis current control of PWM, thus improving stability.

[0164] Under the above control strategy, the parallel PWM rectifier can achieve uniform distribution of the input current and stable control of the DC-side voltage. Simulation results are as follows: Figure 2 As shown, Ia and Ia2 are the A-phase input currents of two parallel PWM rectifier modules, respectively. It can be seen that the two currents have the same amplitude, the same phase, and are basically coincident, indicating that the proposed droop control strategy can achieve the equal distribution of input current between rectifier modules, thereby achieving uniform power distribution.

[0165] At the same time, such as Figure 3 As shown, Udc and Udc1 are the DC-side output voltages of two parallel PWM rectifier modules, respectively. It can be seen that the two voltages are basically the same and the waveforms are stable with small ripples, indicating that the proposed control method can maintain the stable operation of the DC-side voltage and provide a stable input power supply for the subsequent DC / DC converter.

[0166] 3) Post-stage DC / DC current sharing control

[0167] The power-up DC / DC converter consists of four interleaved BUCK circuits connected in parallel. Taking DC / DC 1 as an example (the parameters in the formula will have subscripts added), the output currents of the four BUCK circuits are respectively... The total current of the 4 current channels is The hydrogen production PEM unit is a capacitive load, and the hydrogen production process is current-controlled. Therefore, current sharing control is not required for the parallel DC / DC units at this stage. However, during the startup phase, the process is voltage-controlled, and current sharing control is necessary.

[0168] The drooping strategy of the subsequent DC / DC converter is shown in equation (31):

[0169] (31);

[0170] In formula (31) This is the no-load voltage of the output of DC / DC converter #1. This is the output voltage of DC / DC converter #1. This refers to the virtual resistance, i.e., the droop slope, connected in series with DC / DC converter #1. for The output value after filtering by the inertial element satisfies the equation (32):

[0171] (32);

[0172] The droop control is achieved by using the inertial element's filtering time constant. The goal is to define the following Lyapunov function. As in equation (33):

[0173] (33)

[0174] in for The integral coefficient, This is the filter capacitor on the output side of DC / DC converter #1. Differentiating gives , It can be represented as:

[0175] (34);

[0176] In formula (13) The load current is for DC / DC converter #1, therefore the load current change is relative to... Slow, can be approximated as Thus, equation (34) can be transformed into:

[0177] (35);

[0178] If we let the intermediate variable for:

[0179] (36);

[0180] Equation (14) can be simplified to:

[0181] (37);

[0182] To achieve ,set up for The set value makes satisfy ,Right now

[0183] (38);

[0184] In equation (17) Let the droop control proportional coefficient of the DC / DC converter be... and The difference is ,so It can be transformed into:

[0185] (39)

[0186] Under the aforementioned control strategy, a uniform distribution of output current can be achieved among different DC / DC module groups. Simulation results are as follows: Figure 4 As shown, IoF and IoF1 represent the total output current of the two DC / DC parallel modules, respectively. It can be seen that their amplitudes are basically the same and their changing trends are consistent. After connecting the equivalent electrolytic cell load at 2 s, the output currents of the two groups remain consistent, indicating that the proposed droop control strategy can achieve current sharing control between module groups under dynamic operating conditions, thereby achieving system-level power balance distribution.

[0187] To achieve current sharing among the four BUCK circuits, the current settings for all four BUCK circuits should be equal. Here, we do not differentiate between the settings for the four BUCK circuits and define them uniformly as follows: Its satisfaction Taking the first BUCK circuit in DC / DC converter #1 as an example, let the current control error of this BUCK circuit be... Define the Lyapunov function V as:

[0188] (40);

[0189] Let V be the weighting coefficients of the Lyapunov function, then the derivative of V... for:

[0190] (41);

[0191] accomplish ,make ,in The proportional coefficient for the current control of the BUCK circuit is obtained by transformation: This will give us the control voltage for BUCK #1. See equation (42):

[0192] (42).

[0193] Under the aforementioned current control strategy, the parallel BUCK converter can achieve uniform current distribution across its branches. Simulation results are as follows: Figure 5 As shown, I1, I2, I3, and I4 represent the inductor currents of each branch in the parallel BUCK circuit, respectively. It can be seen that the current amplitudes of each branch are basically the same. After connecting the equivalent electrolytic cell load for 2 seconds, the currents of each branch can still remain consistent, indicating that the proposed control method can achieve current sharing control within the DC / DC module under load changes.

[0194] In summary, through the synergistic effect of droop control and zero-sequence circulating current control of the front-end PWM rectifier, and droop control and current inner loop control of the rear-end DC / DC converter, multi-level current sharing control from the rectifier module to the DC / DC module group and then to each branch is achieved. Simulation results verify the effectiveness of the proposed method and improve the stability and reliability of the system operation.

[0195] This invention addresses the power limitation of individual power modules in high-power water electrolysis hydrogen production systems by proposing a hydrogen production power supply system based on a two-stage structure of a PWM rectifier and a DC / DC converter, and establishing its parallel operation structure model. This system achieves parallel operation at the three-phase power supply, the DC output side of the PWM rectifier, and the output side of the DC / DC converter, thereby enabling power expansion and redundant operation of the power modules and improving the reliability of system operation.

[0196] To address the circulating current and current sharing issues in parallel operation of PWM rectifiers, a parallel current sharing control method for PWM rectifiers is proposed. By controlling the input current and DC-side voltage of each rectifier module, power sharing among multiple modules is achieved, and the circulating current problem in the parallel system is suppressed.

[0197] Furthermore, to address the current sharing problem during parallel operation of downstream DC / DC converters, a dual closed-loop control method based on a droop strategy is proposed on the basis of the staggered parallel structure of the BUCK converter. Through the coordinated control of the voltage outer loop and the current inner loop, the output current of the parallel modules is evenly distributed.

[0198] Based on the above two-stage parallel control strategy, two-stage current sharing control of the PWM rectifier stage and the DC / DC stage is realized, which effectively suppresses the circulating current in the parallel system, improves the stability and reliability of the system, and enables the efficient operation of the high-power hydrogen production power supply system without the need to add isolation devices.

[0199] This invention is not limited to the above embodiments. Based on the technical solutions disclosed herein, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A tie-line-free current sharing control method for a two-stage parallel high-power hydrogen production power supply system, characterized in that: Includes the following steps, S1. For the front-end PWM rectifier, an improved d-axis droop control strategy is constructed. The droop adjustment of the DC output voltage of the PWM rectifier is realized through single-loop control, so that the parallel PWM rectifier can automatically distribute power according to the load change and realize the current sharing control of the front-end PWM rectifier. S2. To address the zero-sequence circulating current problem generated when PWM rectifiers are directly connected in parallel, proportional control is adopted to suppress the zero-sequence circulating current in the parallel system. S3. To address the need for current sharing control during the startup phase of parallel operation of downstream DC / DC converters, which is voltage-controlled, a dual-closed-loop current sharing control method based on a droop strategy is proposed.

2. The tie-line-free current sharing control method for a two-stage parallel high-power hydrogen production power supply system according to claim 1, characterized in that: The specific content of S1 is as follows: S1.1, Establish the DC voltage droop target on the output side of the PWM rectifier, as shown in Equation (1). (1)； In equation (1), This is the no-load voltage on the DC side of the PWM rectifier. The output value is the DC voltage of the PWM rectifier after being filtered by an inertial circuit. A virtual resistor, i.e., the droop slope, is inserted in series on the DC side. For the current filtering inductor of the PWM rectifier in the rotating coordinate system The d-axis current is obtained by rotating the coordinate system to form the dq coordinate system. S1.2, for Differentiate and define the Lyapunov function. ,accomplish The goal; S1.3, Establish the mathematical model of the PWM rectifier in the dq coordinate system, and obtain the values ​​of the three-phase modulation voltage in the dq coordinate system. .

3. The tie-line-free current sharing control method for a two-stage parallel high-power hydrogen production power supply system according to claim 2, characterized in that: The specific content of S1.2 is as follows: right Differentiation yields the following equation (2): (2)； In equation (2), This refers to the DC-side voltage output by the PWM rectifier. The droop control is achieved by using the inertial element's filtering time constant. The goal is to define the following Lyapunov function. As in equation (3): (3)； in for The integral coefficient, where t is the time variable and du is the differential variable, for Differentiating gives , It can be represented as: (4)； To achieve , can be made: (5)； In formula (5) This is the droop control ratio coefficient.

4. The tie-line-free current sharing control method for a two-stage parallel high-power hydrogen production power supply system according to claim 3, characterized in that: The specific content of S1.3 is as follows: The mathematical model of a PWM rectifier in the dq coordinate system can be expressed as: (6)； In formula (6) This is the filter inductor for the three-phase power supply of the PWM rectifier. To convert the current of the three-phase power supply filter inductor to its value in the dq coordinate system. This is the value of the three-phase modulated voltage converted to the dq coordinate system. This is the value of the three-phase grid voltage converted to the dq coordinate system. Let be the angular frequency of the grid voltage; substituting equation (5) into the first equation of equation (6) yields: (7)； For q-axis current control, a conventional feedforward decoupling strategy is adopted, assuming... Setting value Current error ,but The feedforward control strategy can be expressed as: (8)； In formula (8) This is the proportional control coefficient for the current loop. is the integral control coefficient of the current loop.

5. The tie-line-free current sharing control method for a two-stage parallel high-power hydrogen production power supply system according to claim 4, characterized in that: The specific content of S2 is as follows. Since the zero-sequence circulating current value only exists between directly parallel systems, the zero-sequence circulating current can be expressed as the sum of the three-phase currents, i.e. Furthermore, the zero-sequence circulating currents between the two hydrogen production power sources are equal in magnitude but opposite in direction; let the zero-sequence voltage at which the PWM rectifier suppresses the circulating current be... The integral I element of the PI controller exhibits steady-state error when tracking sinusoidal signals. Proportional control is used to suppress zero-sequence circulating current. (9)； in This is the zero-sequence voltage proportional gain coefficient.

6. The tie-line-free current sharing control method for a two-stage parallel high-power hydrogen production power supply system according to claim 5, characterized in that: The specific details of S3 are as follows. S3.1, the subsequent DC / DC converter consists of four interleaved parallel BUCK circuits, with the output currents of the four BUCK circuits being respectively... The total current of the 4 current channels is The hydrogen production PEM unit is a capacitive load, and the hydrogen production process is current-controlled. Therefore, current sharing control is not required for the parallel DC / DC units at this time. However, during the start-up phase, the process is voltage-controlled, and current sharing control is required. The drooping strategy of the subsequent DC / DC converter is shown in equation (10): (10)； In formula (10) The no-load voltage of the DC / DC output. The output voltage of the DC / DC converter. This refers to the virtual resistance inserted in series with the DC / DC converter, i.e., the droop slope. for The output value after filtering by the inertial element satisfies the equation (11): (11) ； The time constant for inertial element filtering; S3.2, to achieve droop control, i.e. The goal is to define the Lyapunov function. And differentiate it; S3.3, To achieve current sharing among the four BUCK circuits, the current setting values ​​for the four BUCK circuits should be equal. Here, the setting values ​​for the four BUCK circuits are not differentiated and are uniformly defined as follows: Its satisfaction .

7. The method for current sharing control without tie lines in a two-stage parallel high-power hydrogen production power system according to claim 6, characterized in that: The specific content of S3.2 is as follows: As in equation (12): (12)； in for The integral coefficient, For the filter capacitor on the DC / DC output side; Differentiating gives , It can be represented as: (13)； In formula (13) For DC / DC load current, therefore the load current change is relative to Slow, can be approximated as Thus, equation (13) can be transformed into: (14)； If we let the intermediate variable for: (15)； Equation (14) can be simplified to: (16)； To achieve ,set up for The set value makes satisfy ,Right now (17)； In equation (17) Let the droop control proportional coefficient of the DC / DC converter be... and The difference is ,so It can be transformed into: (18)。 8. The tie-line-free current sharing control method for a two-stage parallel high-power hydrogen production power supply system according to claim 7, characterized in that: The specific content of S3.3 is as follows: Assume the current control error of this BUCK circuit is Define the Lyapunov function V as: (19)； Let V be the weighting coefficients of the Lyapunov function, then the derivative of V... for: (20)； accomplish ,make ,in The proportional coefficient for the current control of the BUCK circuit is obtained by transformation: ； This allows us to obtain the BUCK control voltage. See equation (21): (21)。 9. A two-stage parallel high-power hydrogen production power supply system, characterized in that: A tie-line-free current sharing control method for a two-stage parallel high-power hydrogen production power supply system as described in any one of claims 1-8 includes a three-phase power supply, wherein the three-phase power supply is connected to the input terminals of two PWM rectifiers respectively, the output terminal of each PWM rectifier is connected to the input terminals of two DC / DC converters, and the output terminals of the two DC / DC converters are connected to a hydrogen production PEM device, wherein the three-phase power supply, the DC output terminals of the PWM rectifiers, and the DC / DC output terminals all operate in parallel.

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