A quasi-single-stage Buck-LLC DCX converter and its control method

Abstract

The present invention relates to the technical field of power system control, and discloses a quasi-single-stage Buck-LLC DCX converter and a control method thereof. The converter includes a Buck circuit and an LLC DCX circuit; four switch tubes of the LLC DCX circuit constitute a full-bridge inverter circuit and are connected to the Buck circuit; the same-name end of each transformer primary side is connected to one output end of the full-bridge inverter circuit via a resonant inductor, and the opposite-name end of the primary side is connected to the other output end of the full-bridge inverter circuit via a resonant capacitor, and an excitation inductor is connected between the same-name and opposite-name ends, and the secondary side is a double-coil series structure. The control method includes collecting current and voltage to calculate power and comparing it with a preset power, and adjusting the Buck circuit duty cycle based on the comparison result. Among them, the quasi-single-stage Buck-LLC DCX converter has the technical advantages of strong voltage reduction capability, high efficiency, safety and reliability; when the radiation intensity is high, the control method can realize the power reduction operation of the load and the controllable output side.

Description

Technical Field

[0001] The present invention relates to the technical field of power system control, and in particular to a quasi-single-stage Buck-LLC DCX converter and a control method thereof. Background Art

[0002] As the most important link for indirect coupling between photovoltaic modules and loads, the design requirements of DC / DC converters should meet the requirements of high step-down conversion ratio, low output current ripple, large current carrying capacity and low cost. Buck circuit is one of the most common DC / DC converters. It is widely used in load power supply due to its simple circuit, low cost and certain step-down capability. However, the step-down capability of Buck circuit is limited, and when the input and output voltage gap is too large, the extremely low duty cycle will lead to low overall efficiency of DC off-grid system. At the same time, the control method in the existing DC off-grid system is generally photovoltaic maximum power tracking control, that is, the photovoltaic module is controlled to be in maximum power point tracking mode through DC / DC converter, and the entire system is controlled to operate at the maximum output power of the photovoltaic module. It does not take into account that under the condition of high irradiation intensity, the maximum power output of the photovoltaic module may exceed the rated power of the load, the load operation state limit or the load demand limit, and the controllability of the output side is not achieved.

[0003] In order to solve the problem of high voltage stress of power switch tubes, some researchers proposed a three-level Buck converter with flying capacitors, which can effectively reduce the voltage stress of power devices and the ripple of output voltage and current.

[0004] Internationally, some people have proposed an interleaved Buck converter with an extended duty cycle, adding a series active switch tube and a coupling capacitor for the power path. The advantage of this circuit structure is that when the duty cycle is less than 0.5, the voltage stress of the power switch tube is half of the input voltage, thereby reducing switching losses.

[0005] In order to further improve the step-down ratio of the converter and further reduce the voltage stress of the power switch tube, a research team proposed a cross-wound coupled inductor plus passive clamping circuit interleaved Buck converter. This circuit achieves a high step-down ratio by using coupled inductors. The peak voltage of the power tube is absorbed by the passive clamping circuit, and the reverse recovery problem of the diode is solved by using leakage inductance, thereby improving the efficiency of the converter.

[0006] Compared with non-isolated DC / DC converters, isolated DC / DC converters can not only reduce the secondary voltage but also play the role of electrical isolation due to the introduction of high-frequency transformers. A team proposed a half-bridge DC / DC converter topology suitable for interconnection with alkaline electrolyzers in off-grid photovoltaic hydrogen production systems. The converter has the advantages of low switching loss and wide voltage regulation range. Under the wide input voltage range of off-grid photovoltaic hydrogen production systems, it will further improve the hydrogen production efficiency of alkaline electrolyzers. However, due to the existence of energy circulation, the power factor and efficiency of the converter are reduced, and its fault tolerance is poor. When one of the switch tubes is damaged, the entire converter cannot work.

[0007] A team has proposed a half-bridge LLC resonant converter topology suitable for alkaline electrolyzers. This converter has the advantages of soft switching and high step-down ratio. However, when the LLC resonant converter operates over a wide range, the switching frequency varies greatly. When the switching frequency is far away from the series resonant frequency point, the converter reactive circulating current increases, resulting in a decrease in the transmission efficiency of the off-grid photovoltaic hydrogen production system. In addition, due to the limited fault tolerance of the half-bridge topology structure, the converter reliability is low and the isolation transformer current stress is large, so it is only suitable for low-power hydrogen production occasions.

[0008] In addition, the full-bridge resonant DC / DC converter has no duty cycle loss phenomenon, can achieve zero voltage turn-on in the full load range, has a wide voltage output range, can meet the output voltage range requirements of the alkaline electrolyzer, and improve the hydrogen production efficiency of the off-grid photovoltaic hydrogen production system. Compared with the half-bridge topology, the full-bridge topology has more advantages in high-power applications, but it has more switching devices and more complex control, which will further increase the cost of the hydrogen production power supply.

[0009] Another push-pull DC / DC converter suitable for off-grid photovoltaic hydrogen production system has a simple topology and control method, and has the advantages of low current ripple and high step-down ratio. It can meet the low voltage requirements of alkaline electrolyzers and can be better compatible with photovoltaic power sources. However, due to the use of hard switching, the converter efficiency is reduced and the voltage stress of the switching device is increased. In addition, the center-tapped transformer structure is too complicated, so the converter is limited in high-power hydrogen production occasions.

[0010] In off-grid photovoltaic hydrogen production systems, the control mode of photovoltaic modules is generally MPPT control. In most cases, photovoltaic modules are in the maximum power point tracking working state. When the photovoltaic output power is greater than the load power and the energy storage device is charged at a high state, the microgrid is in a power surplus state, and the photovoltaic modules will change to output constant voltage control. The MPPT control of photovoltaic modules is essentially to give a reference value of the input voltage of the photovoltaic DC / DC converter through various MPPT algorithms, and adjust the converter duty cycle through the input voltage controller to change the operating point; the constant voltage droop control is to give a reference value of the output voltage of the photovoltaic DC / DC converter through droop control, and adjust the converter duty cycle through the output voltage controller to change the operating point. There are significant differences in the control loops and transfer functions of the two. The transient process caused by mode switching will affect the operating stability of the off-grid photovoltaic hydrogen production system.

[0011] One team achieved smooth switching by comparing the size of the control signal given by different modes. When the DC bus voltage is low, the microgrid is in a power deficiency state, the constant voltage control output signal is forward saturated, and the photovoltaic module is in the MPPT working state; when the DC bus voltage gradually increases, the converter with a higher voltage feedback coefficient will exit the forward saturation first, and the photovoltaic module will be in the constant voltage control mode to control the DC bus voltage.

[0012] The foreign team used a dual voltage loop to establish a unified form of output voltage control and input voltage control. In MPPT mode, the MPPT algorithm gives a reference value for the input voltage, and the input voltage controller responds to the control signal, so that the photovoltaic module works in the maximum power point tracking state; in constant voltage control mode, the output voltage is mapped to the input voltage side through the proportional-integral (PI) link, and the input voltage control signal is given. The input voltage controller responds to the control signal to achieve the purpose of controlling the output voltage.

[0013] In summary, in off-grid photovoltaic hydrogen production systems, Buck converters are widely used in existing DC / DC converters used to power alkaline electrolyzers. However, the buck converter has limited step-down capability, and under high input voltage and low output voltage conditions, the duty cycle of the buck converter becomes very low, resulting in low transmission efficiency. At the same time, the off-grid photovoltaic hydrogen production system is a typical low-voltage and high-current application scenario, and the buck converter is a non-isolated DC / DC converter, and the input and output ends are not electrically isolated, which poses certain safety risks. The control strategy of existing off-grid photovoltaic hydrogen production systems is generally photovoltaic MPPT control, that is, the photovoltaic components are controlled to be in maximum power point tracking mode through DC / DC converters, so that the alkaline electrolyzer also operates at the photovoltaic output power. Obviously, the above control method does not take into account that under high irradiation intensity, the maximum power output of the photovoltaic components may exceed the rated power of the alkaline electrolyzer, the operating state limit of the alkaline electrolyzer or the hydrogen production demand limit. At this time, the working current of the alkaline electrolyzer is determined by the maximum photovoltaic output power, and the output side is uncontrollable. In actual production, it is usually hoped that the output side is controllable, that is, the alkaline electrolyzer should operate at an artificially set electrolysis current value.

[0014] In view of the above technical problems and production requirements, this embodiment provides a quasi-single-stage Buck-LLC DCX converter for an off-grid photovoltaic hydrogen production system and a control method thereof.

[0015] Specifically, the above technical problems are particularly prominent in off-grid photovoltaic hydrogen production systems. Summary of the invention

[0016] In order to solve the above technical problems existing in the prior art, the present invention aims to provide a converter with controllable output power for a photovoltaic power source, and in particular, proposes a quasi-single-stage Buck-LLC DCX converter and a control method thereof.

[0017] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0018] A quasi-single-stage Buck-LLC DCX converter comprises a Buck circuit and an LLC DCX circuit;

[0019] LLC DCX circuit includes switch tube , switch tube , switch tube , switch tube , resonant inductor , resonant inductor , Excitation inductance , Excitation inductance , resonant capacitor , resonant capacitor ,transformer ,transformer , Rectifier diode , Rectifier diode , Rectifier diode , Rectifier diode and output filter capacitor

[0020] Switching tube , switch tube , switch tube And switch tube A full-bridge inverter circuit is formed;

[0021] The two input terminals of the full-bridge inverter circuit are connected to the positive and negative terminals of the Buck circuit output respectively;

[0022] The first output of the full-bridge inverter circuit is connected to the resonant inductor Connect to transformer The same-name end of the primary side is connected through the resonant inductor Connect to transformer The same-name end of the original edge;

[0023] The second output of the full-bridge inverter circuit is converted through the resonant capacitor Connect to transformer The opposite end of the primary side is connected through the resonant capacitor Connect to transformer The antonym end of the original side;

[0024] Excitation inductance The two ends of the transformer are connected to The same-name and opposite-name terminals of the primary side, the excitation inductance The two ends of the transformer are connected to The homonymous and heteronymous ends of the original edge;

[0025] transformer The secondary side includes the first coil and the second coil, and the transformer The secondary side includes a third coil and a fourth coil;

[0026] The same end of the first coil is connected to the rectifier diode The positive pole of the rectifier diode is connected to the same-name end of the second coil. , Rectifier diode , Rectifier diode The negative pole of the load and the positive pole of the load are connected to the rectifier diode The negative electrode;

[0027] The same-name end of the second coil is connected to the same-name end of the fourth coil, and the opposite-name end is connected to the rectifier diode The positive electrode;

[0028] The same end of the third coil is connected to the rectifier diode The positive pole of the coil with the opposite pole is connected to the same pole of the fourth coil;

[0029] The same-name end of the fourth coil is connected to the negative pole of the load, and the opposite-name end is connected to the rectifier diode The positive electrode;

[0030] Output filter capacitor Connect between the positive and negative terminals of the output.

[0031] The present invention also provides a control method for the above-mentioned quasi-single-stage Buck-LLC DCX converter, wherein the input end of the Buck circuit is connected to a photovoltaic power source, comprising:

[0032] Sample the output voltage of the photovoltaic power source according to the preset period and output current , calculate the photovoltaic output power ;in, Indicates the periodic ordinal number;

[0033] If the photovoltaic output power Greater than target power , then increase the duty cycle of the Buck circuit;

[0034] If the photovoltaic output power Less than target power , and the output voltage Greater than the voltage at the maximum power output point of the photovoltaic power source , then increase the duty cycle of the Buck circuit;

[0035] If the photovoltaic output power Less than target power , and the output voltage Less than the maximum power point voltage output by the photovoltaic power source , then reduce the duty cycle of the Buck circuit;

[0036] Otherwise, the duty cycle of the Buck circuit is maintained.

[0037] Furthermore, the target power Determine by following these steps:

[0038] Step S1: Collect the load current and calculate the difference between the preset current and the load current ;

[0039] Step S2: The difference Input PI controller to get the corrected power , the expression is:

[0040] ;

[0041] In the formula, is the proportionality coefficient, is the integration coefficient, is the Laplace operator;

[0042] Step S3: Correction power The target power of the previous cycle Add and update the target power of the current cycle .

[0043] Preferably, the load is an alkaline electrolysis cell used for off-grid photovoltaic hydrogen production.

[0044] In the technical solution provided by the present invention, the quasi-single-stage Buck-LLC DCX converter has the technical advantages of strong voltage reduction capability, high efficiency, safety and reliability; the control method of the quasi-single-stage Buck-LLC DCX converter can realize the power reduction operation of the load under the condition of high radiation intensity, and at the same time realize the controllable output side. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 1 is a topological diagram of a quasi-single-stage Buck-LLC DCX converter in one embodiment of the present invention.

[0046] Figure 2 1 is a photovoltaic output voltage-power characteristic diagram at different temperatures in an embodiment of the present invention.

[0047] Figure 3 1 is a control flow chart of a quasi-single-stage Buck-LLC DCX converter in one embodiment of the present invention.

[0048] Figure 4 Schematic diagram of a constant current limiting power control strategy for an off-grid photovoltaic hydrogen production system in one embodiment of the present invention.

[0049] Figure 5 The structure diagram of the simulation model of a quasi-single-stage Buck-LLC DCX converter for an off-grid photovoltaic hydrogen production system in one embodiment of the present invention.

[0050] FIG6( a ) is a simulation waveform diagram of the load current when the working current of the simulation model is set to 160A in one embodiment of the present invention.

[0051] FIG6( b ) is a simulation waveform diagram of the load voltage when the working current of the simulation model is set to 160A in one embodiment of the present invention.

[0052] FIG6( c ) is a simulation waveform diagram of load power when the working current of the simulation model is set to 160A in one embodiment of the present invention.

[0053] FIG6( d ) is a simulation waveform diagram of photovoltaic output power when the working current of the simulation model is set to 160A in one embodiment of the present invention.

[0054] Figure 7 This is an error current diagram when the working current of the simulation model in one embodiment of the present invention is set to 160A.

[0055] Figure 8 1 is a diagram of the inductor current of the front-stage Buck converter of the simulation model in one embodiment of the present invention.

[0056] Fig. 9 This is a current diagram of the rear-stage LLC DCX converter of a simulation model in one embodiment of the present invention. DETAILED DESCRIPTION

[0057] The technical solution provided by the present invention will be further elaborated in detail in combination with the embodiments and drawings.

[0058] Example 1

[0059] Looking at the existing hydrogen production methods, water electrolysis has the advantages of simple process, low cost and no pollution compared with other hydrogen production methods. In addition, water electrolysis can help improve the absorption capacity of photovoltaic power generation, which can greatly reduce the occurrence of light abandonment. At present, photovoltaic hydrogen production systems can be roughly divided into grid-connected structure and off-grid structure according to whether they are connected to the power grid. The grid-connected structure can realize long-term uninterrupted operation of alkaline electrolyzers, but this structure needs to rely on the power grid. The application cost is relatively high for some remote areas with rich light resources. In addition to the DC / DC converter, the grid-connected structure also needs to include a rectifier or inverter, which is easy to generate harmonics and pollute the power grid. At the same time, the control of multi-stage converters is more complicated, and the overall efficiency of the system is lower. The system is not restricted by the power grid and can be used in remote areas without power grid conditions. In addition, the system has the advantages of low investment cost, short construction period, and more flexible capacity and scale. Therefore, with the continuous improvement of new energy photovoltaic installed capacity and hydrogen production scale, photovoltaic off-grid hydrogen production technology is gradually becoming a more suitable choice. Photovoltaic off-grid hydrogen production technology can be divided into two types according to the coupling method between the photovoltaic array and the alkaline electrolyzer. One is the direct coupling method, that is, the photovoltaic array is directly connected to the alkaline electrolyzer; the other is the indirect coupling method, that is, the photovoltaic array is first indirectly connected to the alkaline electrolyzer through a DC / DC converter. For the direct coupling method, the number of alkaline electrolyzers in series determines the voltage operating point of the photovoltaic array, so the realization of the MPPT of the photovoltaic array becomes difficult; while the indirect coupling method can flexibly match the IV curve of the photovoltaic array and the alkaline electrolyzer through the converter to easily realize MPPT (Maximum Power Point Tracking), and can also adjust the photovoltaic output power according to production needs. Therefore, it is of great significance to study the system DC / DC converter topology and its control strategy.

[0060] In the off-grid photovoltaic hydrogen production system of the existing technical solution, the photovoltaic is connected to the alkaline electrolyzer through a DC / DC converter. In order to meet the requirements of the alkaline electrolyzer, the DC / DC converter should have the characteristics of low output current ripple, large current carrying capacity, high step-down ratio and wide output voltage range. At the same time, the cost of the converter should be reduced as much as possible while ensuring the reliability and high efficiency of the DC / DC converter. The DC / DC converter connected to the alkaline electrolyzer can be divided into two types: isolated and non-isolated.

[0061] Among non-isolated DC / DC converters, Buck converters are widely used due to their simple structure and low cost. However, in order to reduce current ripple, the inductance needs to be increased or the switching frequency needs to be increased, which will increase the size of the Buck converter or increase the switching loss, thereby reducing its efficiency. In addition, in order to cope with the volatility of photovoltaics and the demand for hydrogen production in alkaline electrolyzers, Buck converters need to operate in a wide voltage range. However, the buck converter has limited step-down capability and has a low transmission efficiency under the conditions of high input voltage and low output voltage.

[0062] In order to be applicable to large-scale centralized off-grid photovoltaic hydrogen production systems, researchers proposed a converter input and output parallel connection technology, integrating multiple three-level Buck converters, and using staggered parallel control to further increase the power level, reduce output voltage and current ripple, and improve the quality of power conversion. However, its disadvantage is that the switch tube voltage stress and voltage reduction capability have not been improved compared with traditional Buck converters.

[0063] like Figure 1 As shown, the quasi-single-stage Buck-LLC DCX converter for off-grid photovoltaic hydrogen production system includes a photovoltaic module (photovoltaic power source), a front-stage Buck circuit, a rear-stage LLC DCX circuit and an alkaline electrolyzer; wherein the front-stage Buck circuit is connected to the positive and negative electrodes of the photovoltaic module, and the positive and negative electrodes of its output end are connected to the rear-stage LLC DCX circuit.

[0064] Specifically, the LLC DCX circuit includes a switch tube , switch tube , switch tube , switch tube , resonant inductor , resonant inductor , Excitation inductance , Excitation inductance , resonant capacitor , resonant capacitor ,transformer ,transformer , Rectifier diode , Rectifier diode , Rectifier diode , Rectifier diode and output filter capacitor .

[0065] Figure 1 middle, is the input filter capacitor, , and They are the main switch tube, freewheeling diode and inductor of the front-stage Buck circuit. It is the output filter capacitor of the previous Buck circuit.

[0066] Switching tube , switch tube , switch tube And switch tube A full-bridge inverter circuit is formed; the two input ends of the full-bridge inverter circuit are respectively connected to the positive and negative electrodes of the Buck circuit output end.

[0067] The first output of the full-bridge inverter circuit is connected to the resonant inductor Connect to transformer The same-name end of the primary side is connected through the resonant inductor Connect to transformer The same-name end of the original edge;

[0068] The second output of the full-bridge inverter circuit is converted through the resonant capacitor Connect to transformer The opposite end of the primary side is connected through the resonant capacitor Connect to transformer The synonym end of the original side.

[0069] Excitation inductance The two ends of the transformer are connected to The same-name and opposite-name terminals of the primary side, the excitation inductance The two ends of the transformer are connected to The homonymous and heteronymous ends of the original edge.

[0070] transformer The secondary side includes the first coil and the second coil, and the transformer The secondary side includes a third coil and a fourth coil;

[0071] The same end of the first coil is connected to the rectifier diode The positive pole of the rectifier diode is connected to the same-name end of the second coil. , Rectifier diode , Rectifier diode The negative electrode and the positive electrode of the output terminal are connected to the rectifier diode The negative electrode;

[0072] The same-name end of the second coil is connected to the same-name end of the fourth coil, and the opposite-name end is connected to the rectifier diode The positive electrode;

[0073] The same end of the third coil is connected to the rectifier diode The positive pole of the coil with the opposite pole is connected to the same pole of the fourth coil;

[0074] The same-name end of the fourth coil is connected to the negative pole of the output terminal, and the opposite-name end is connected to the rectifier diode The positive electrode;

[0075] Output filter capacitor Connect between the positive and negative terminals of the output.

[0076] Among them, the resonant inductor , resonant inductor , Excitation inductance , Excitation inductance , resonant capacitor and resonant capacitor As the resonant element, the resonant inductor and resonant inductor Including the primary leakage inductance of the corresponding transformer. and magnetizing inductance Connected in parallel with the corresponding transformer, it can be realized by the transformer's excitation inductance, so it is called excitation inductance. and resonant capacitor It is connected in series in the primary circuit and also acts as a DC isolation.

[0077] For the existing post-stage LLC DCX circuit, in order to further reduce the current stress on the secondary side of the transformer and improve the overall efficiency of the converter, a parallel structure of the primary and secondary sides of the transformer is adopted. , switch tube , switch tube , switch tube and resonant inductor , resonant inductor , Excitation inductance , Excitation inductance , resonant capacitor , resonant capacitor and rectifier diodes , Rectifier diode Based on the full-bridge LLC DCX converter, a resonant inductor is added , Excitation inductance , resonant capacitor and rectifier diodes , Rectifier diode , forming a transformer primary and secondary parallel structure.

[0078] At this time, the input voltage of the quasi-single-stage LLC DCX converter is , output voltage The relationship is as follows:

[0079] ;

[0080] In the formula, is the duty cycle of the Buck circuit, is the resonant cavity gain, is the transformer ratio.

[0081] Calculating the cavity gain The expression is as follows:

[0082] ;

[0083] In the formula, is the per-unit frequency, is the quality factor of the resonant circuit, It is the ratio of the magnetizing inductance to the resonant inductance.

[0084] The introduced post-stage LLC DCX circuit works in open-loop mode and at the resonant frequency, i.e. the switching frequency Equal to the resonant frequency .when = When the per unit frequency ,

[0085] ;

[0086] Then the resonant cavity gain is =1, the subsequent LLC DCX circuit only realizes the function of DC converter. At this time, the input and output voltage relationship of the converter can be rewritten as the following formula:

[0087] ;

[0088] In this way, the overall step-down capability of the quasi-single-stage Buck-LLC DCX converter can be greatly improved while ensuring the overall efficiency of the converter. Due to the introduction of the transformer, input and output isolation is achieved, and the safety and reliability of the off-grid photovoltaic hydrogen production system are improved.

[0089] Aiming at the low-voltage and high-current application scenarios of off-grid photovoltaic hydrogen production systems, in order to further improve the efficiency of the off-grid photovoltaic hydrogen production system and reduce the impact of the two-stage structure on the efficiency of the off-grid photovoltaic hydrogen production system, this embodiment also adopts a parallel structure of the primary and secondary sides of the transformer, which can effectively reduce the current on the secondary side through a single rectifier diode on the secondary side, thereby reducing diode losses and improving the overall efficiency of the off-grid photovoltaic hydrogen production system.

[0090] The control strategy of existing off-grid photovoltaic hydrogen production systems is generally photovoltaic MPPT control. The maximum power tracking control mode can improve the economic benefits of the operation of the off-grid photovoltaic hydrogen production system, but it does not take into account factors such as the maximum power output of the photovoltaic module may exceed the rated power of the alkaline electrolyzer, the operating state limit of the alkaline electrolyzer, or the hydrogen production demand limit under high irradiation intensity, and does not achieve output side control, which is not in line with actual production conditions. In order to solve the above problems, this embodiment proposes a constant current limited power control strategy for off-grid photovoltaic hydrogen production systems.

[0091] Photovoltaic output UP characteristics such as Figure 2 As shown. It can be observed that when the photovoltaic output power is constant, there are two possibilities for the output voltage, one is to work on the left side of the maximum power point, and the other is to work on the right side of the maximum power point. Obviously, the curve on the left is flatter, and the power output fluctuation caused by changing the photovoltaic output voltage is smaller, which is more conducive to the stability of the off-grid photovoltaic hydrogen production system, so the off-grid photovoltaic hydrogen production system should be operated in the area to the left of the maximum power point. In the off-grid photovoltaic hydrogen production system, the photovoltaic module is coupled with the alkaline electrolyzer through a DC / DC converter. At this time, adjusting the duty cycle of the DC / DC converter can change the photovoltaic output voltage, thereby changing the photovoltaic output power. For the quasi-single-stage Buck-LLC DCX converter proposed in this embodiment, since the rear-stage LLC DCX circuit only realizes the function of a DC transformer, the photovoltaic output power can be adjusted by modifying the duty cycle of the front-stage Buck circuit, such as increasing the duty cycle to reduce the photovoltaic output voltage, and reducing the duty cycle to increase the photovoltaic output voltage.

[0092] like Figure 3 As shown in the figure, the constant current limiting power control strategy mainly consists of constant power control and load current feedback control. is the change in duty cycle of the previous Buck circuit.

[0093] In the constant power control part, it is assumed that the target power of the off-grid photovoltaic hydrogen production system is , sample the current photovoltaic output voltage and output current , calculate the output power at this time If the photovoltaic output power is Greater than the set power Regardless of whether the off-grid photovoltaic hydrogen production system operates on the left or right side of the maximum power point, the duty cycle of the front-stage Buck circuit is increased to reduce the photovoltaic output voltage. Less than the set power ,and Greater than the maximum power point voltage At this time, the off-grid photovoltaic hydrogen production system operates on the right side of the maximum power point. The duty cycle of the front-stage Buck circuit should be increased to reduce the photovoltaic output voltage. If the photovoltaic output power Less than the set power ,and Less than the maximum power point voltage At this time, the off-grid photovoltaic hydrogen production system operates on the left side of the maximum power point. At this time, the duty cycle of the front-stage Buck circuit should be reduced to increase the photovoltaic output voltage. The duty cycle modulated output signal is converted into a PWM (Pulse Width Modulation) wave as a control signal Output.

[0094] Based on the above constant power control method, the load current is introduced The feedback loop can be used to obtain the constant current and power limit control strategy. Specifically, the electrolysis current is collected and compared with the artificially set value (reference current ) and input the error between them into the PI controller, which dynamically corrects the target power If the electrolysis current is less than the set value, increase the target power ; When the electrolysis current is greater than the set value, reduce the target power In other words, the electrolysis current and photovoltaic output voltage are collected at the same cycle. and output current . Then in the cycle , the target power of the off-grid photovoltaic hydrogen production system is .

[0095] Collect the load current and calculate the difference between the preset current and the load current ;

[0096] The difference Input PI controller to get the corrected power , the formula is as follows:

[0097] ;

[0098] In the formula, is the proportionality coefficient, is the integration coefficient, is the Laplace operator;

[0099] The corrected power The target power of the previous cycle Add and update to get the current cycle .

[0100] The overall control strategy proposed in this embodiment is as follows Figure 4As shown. The constant current limiting power control strategy consists of constant power control and load current loop. The load current feedback loop adjusts the target power in real time according to the error between the load current and the preset current during operation. If the load current is less than the set current, the target power is increased. If the load current is greater than the set current, the target power is reduced. The constant power control adjusts the duty cycle of the front-stage Buck circuit according to the target power adjusted by the load current feedback loop to achieve the output power regulation of the photovoltaic module.

[0101] like Figure 5 As shown, in this embodiment, a quasi-single-stage Buck-LLC DCX converter simulation model for an off-grid photovoltaic hydrogen production system is built in Matlab software.

[0102] When the load current is set to 160A, the simulation results of the system's load current, load voltage, load power, and photovoltaic output power are shown in Figures 6(a), 6(b), 6(c), and 6(d), respectively. As shown in Figures 6(c) and 6(d), the input and output of the entire off-grid photovoltaic hydrogen production system have reached a power balance state. At t=0s, the off-grid photovoltaic hydrogen production system is powered on, and the load current increases rapidly. Between t=0 and 1.5s, under the control of the constant current limiting power control strategy of the quasi-single-stage Buck-LLC DCX converter, the load current of the off-grid photovoltaic hydrogen production system begins to approach the load current setting value of 160A and fluctuates around it. In order to further investigate the reliability of the off-grid photovoltaic hydrogen production system, disturbances were set in the simulation. At t=1.5s, the irradiation intensity of the photovoltaic module was modified from 950 to 1050. It can be observed that the current, voltage and power of the off-grid photovoltaic hydrogen production system also changed accordingly due to the sudden change in irradiation intensity. At this time, the electrolysis current soared to 180A, but under the control of the constant current limiting power control strategy, the off-grid photovoltaic hydrogen production system soon stabilized at 160A.

[0103] At this time, the error between the load current and the set current changes as follows: Figure 7 It should be noted that, considering that the off-grid photovoltaic hydrogen production system is not in a steady state when it is powered on, it is meaningless to count the error current at this time, so the current error change is counted starting from t=0.2s. Figure 7 It can be observed more intuitively that the load current of the off-grid photovoltaic hydrogen production system can be controlled within the set value.

[0104] The inductor current waveform of the front-stage Buck circuit is as follows: Figure 8As shown. At this time, the inductor current exists in the entire switching cycle, that is, the inductor current is never zero, indicating that the Buck circuit is in normal CCM working mode (Continuous Conduction Mode). The resonant cavity current (including the excitation inductor current ILm, the resonant inductor current ILr and the resonant current ILp), the secondary diode current (including the first secondary current Idr1 and the second secondary current Idr2) and the secondary rectifier current Idr of the subsequent LLC converter are shown as follows: Fig. 9 As shown, it is obvious that the current waveform of the resonant cavity at this time conforms to the waveform when operating at the resonant frequency point.

[0105] In summary, it can be seen that in the technical solution provided by the present invention, the quasi-single-stage Buck-LLC DCX converter has the technical advantages of strong voltage reduction capability, high efficiency, safety and reliability; the control method of the quasi-single-stage Buck-LLC DCX converter can realize the power reduction operation of the load under the condition of high radiation intensity, and at the same time realize the controllable output side.

[0106] Furthermore, introducing load current to adjust the target power of the off-grid photovoltaic hydrogen production system can further improve the operating stability and safety of the quasi-single-stage Buck-LLC DCX converter; specifically, when the control method of the quasi-single-stage Buck-LLC DCX converter is used for DC off-grid hydrogen production, the alkaline electrolytic cell can be controlled to operate stably with a suitable reference current within the rated power range, which helps to improve the operating stability and safety of the off-grid photovoltaic hydrogen production system.

Claims

1. A quasi-single-stage Buck-LLC DCX converter, comprising a Buck circuit and an LLC DCX circuit, characterized in that: LLC DCX circuit includes switch tube Q 1. Switching tube Q 2. Switching tube Q 3. Switching tube Q 4. Resonant inductor Lr 1. Resonant inductor Lr 2. Excitation inductance Lm 1. Excitation inductance Lm 2. Resonant capacitor Cr 1. Resonant capacitor Cr 2. Transformer T 1. Transformer T 2. Rectifier diode D 2. Rectifier diode D 3. Rectifier diode D 4. Rectifier diode D 5 and output filter capacitor C 0; Switching tube Q 1. Switching tube Q 2. Switching tube Q 3 and switch tube Q 4 constitutes a full-bridge inverter circuit; The two input terminals of the full-bridge inverter circuit are connected to the positive and negative terminals of the Buck circuit output respectively; The first output of the full-bridge inverter circuit is connected to the resonant inductor Lr 1Connect to transformer T 1 The same-name end of the primary side, through the resonant inductor Lr 2Connect to transformer T 2 The same-name end of the original edge; The second output of the full-bridge inverter circuit is converted through the resonant capacitor Cr 1Connect to transformer T 1 The opposite end of the primary side, through the resonant capacitor Cr 2Connect to transformer T 2 The antonym end of the original side; Excitation inductance Lm 1 are connected to the transformer T 1. The same-name terminal and opposite-name terminal of the primary side, the excitation inductance Lm 2 are connected to the transformer T 2 The homonymous and heteronymous ends of the original edge; transformer T 1 The secondary side includes the first coil and the second coil, the transformer T 2 The secondary side includes a third coil and a fourth coil; The same end of the first coil is connected to the rectifier diode D The positive pole of 2, the opposite end is connected to the same end of the second coil; the rectifier diode D 3. Rectifier diode D 4. Rectifier diode D The negative pole of 5 and the positive pole of the load are connected to the rectifier diode D 2, the negative electrode; The same-name end of the second coil is connected to the same-name end of the fourth coil, and the opposite-name end is connected to the rectifier diode D 3's positive electrode; The same end of the third coil is connected to the rectifier diode D The positive pole of 4, the opposite end is connected to the same end of the fourth coil; The same-name end of the fourth coil is connected to the negative pole of the load, and the opposite-name end is connected to the rectifier diode D 5 is the positive electrode; Output filter capacitor C 0 is connected between the positive and negative poles of the output terminal; The control method of the quasi-single-stage Buck-LLC DCX converter includes connecting the input end of the Buck circuit to a photovoltaic power source: Sample the output voltage of the photovoltaic power source according to the preset period and output current , calculate the photovoltaic output power ;in, Indicates the periodic ordinal number; If the photovoltaic output power Greater than target power , then increase the duty cycle of the Buck circuit; If the photovoltaic output power Less than target power , and the output voltage Greater than the voltage at the maximum power output point of the photovoltaic power source , then increase the duty cycle of the Buck circuit; If the photovoltaic output power Less than target power , and the output voltage Less than the maximum power point voltage output by the photovoltaic power source , then reduce the duty cycle of the Buck circuit; Otherwise, keep the duty cycle of the Buck circuit; The target power Determine by following these steps: Step S1: Collect the load current and calculate the difference between the preset current and the load current ; Step S2: The difference Input PI controller to get the corrected power , the formula is as follows: ； In the formula, is the proportionality coefficient, is the integration coefficient, is the Laplace operator; Step S3: Correction power The target power of the previous cycle Add and update the target power of the current cycle .

2. The quasi-single-stage Buck-LLC DCX converter according to claim 1, characterized in that: The load is an alkaline electrolysis cell used for off-grid photovoltaic hydrogen production.

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