Fuel cell power generation high-voltage direct-hanging grid-connected system and application method and system thereof

Abstract

The invention discloses a fuel cell power generation high-voltage direct-hanging grid-connected system and an application method and system thereof. The fuel cell power generation high-voltage direct-hanging grid-connected system comprises a hydrogen fuel cell power generation system, a Z-source network-based high-gain DC / DC converter and an MMC inverter system, the output end of the hydrogen fuel cell power generation system is connected with the input end of the high-gain DC / DC converter based on the Z-source network; the output end of the high-gain DC / DC converter based on the Z-source network is connected with the DC side of the MMC inverter system; the AC side of the MMC inverter system is connected with an AC power grid. By innovating the topological structure, the Z-source network is used for converting electric energy into stable voltage and current, a transformer is omitted, the high-voltage direct-hanging grid-connected method adapting to the thermoelectric rapid adjustment and wide fluctuation characteristics of the fuel cell is achieved, the method has high adaptability and flexibility, and the method has the advantages of improving the system operation efficiency, reducing the operation cost and improving the operation reliability. The obvious effect and application value are realized in the aspects of reducing the environmental influence and the like.

Description

Technical Field

[0001] The present invention relates to the field of grid connection of hydrogen fuel cells, and particularly relates to a high-voltage direct connection grid connection system for fuel cell power generation, and an application method and system thereof. Background Art

[0002] With the increasingly serious problems such as the global energy crisis and environmental pollution, renewable energy sources such as photovoltaic and wind energy are widely applied to microgrids. However, since the current wind and light energy cannot be fully utilized, the phenomenon of curtailment of wind and light has become a key issue in the current new energy research. Some domestic and foreign scholars have proposed to generate clean hydrogen by using surplus wind energy and solar energy, and integrate it into the microgrid through hydrogen fuel cells for power generation, and assist in power supply during peak hours. Therefore, it is necessary to conduct research on the integration of hydrogen fuel cells into the microgrid. Among them, as a power generation device that is both efficient, safe, clean and environmentally friendly, hydrogen fuel cells have the characteristics of high power generation efficiency, good stability and no pollution, and have received extensive attention in recent years. However, in the existing technology, since the IGBT is connected in parallel after the Z-source impedance network output by the fuel cell, and the output current of the fuel cell can reach up to 697A at most, the required economic cost of the IGBT increases, and the requirements for its current-carrying capacity and reliability are increased. Moreover, the output voltage of the fuel cell is relatively low and the output characteristics are also relatively soft, and the boost capacity of this method is limited, and the problem of high-voltage direct connection cannot be realized. Summary of the Invention

[0003] In order to solve the problems in the prior art that it is difficult to achieve high-voltage direct connection in the hydrogen fuel cell grid connection system, and the output voltage, current and power of the fuel cell fluctuate in a wide range, making it difficult to stabilize the DC bus voltage, the present invention provides a high-voltage direct connection grid connection system for fuel cell power generation, and an application method and system thereof.

[0004] The technical solution provided by the present invention is as follows:

[0005] The present invention provides a high-voltage direct connection grid connection system for fuel cell power generation, and the system includes: a hydrogen fuel cell power generation system, a high-gain DC / DC converter based on a Z-source network, and an MMC inverter system;

[0006] The output end of the hydrogen fuel cell power generation system is connected to the input end of the high-gain DC / DC converter based on the Z-source network;

[0007] The output end of the high-gain DC / DC converter based on the Z-source network is connected to the DC side of the MMC inverter system;

[0008] The AC side of the MMC inverter system is connected to the AC grid.

[0009] Preferably, the high-gain DC / DC converter based on the Z-source network internally includes a specific number of Z-source boost converters. The hydrogen fuel cell power generation system is connected to the input end of each Z-source boost converter. The input ends of each Z-source boost converter are connected in parallel, and the output ends are cascaded in series to the MMC inverter system.

[0010] Preferably, the sub-module of the MMC inverter system adopts a half-bridge structure.

[0011] Based on the same inventive concept, the present invention also provides an application method for a fuel cell power generation high-voltage direct connection and grid-connected system, and the method includes:

[0012] Adjust the DC-side voltage V dc of the MMC inverter system to obtain the DC-side modulation wave d1 of the MMC inverter system;

[0013] By comparing the DC-side modulation wave d1 of the MMC inverter system with the preset triangular carrier voltage, control the short circuit or connection of the high-gain DC / DC converter based on the Z-source network to the circuit, so as to control the DC-side voltage V dc of the MMC inverter system to be stable;

[0014] By controlling the opening and closing of the IGBT switches in the sub-modules of the MMC inverter system, invert the DC-side voltage V dc of the MMC inverter system to achieve inverter grid connection;

[0015] Preferably, the step of comparing the DC-side modulation wave d1 of the MMC inverter system with the preset triangular carrier voltage and controlling the short circuit or connection of the high-gain DC / DC converter based on the Z-source network to the circuit to control the DC-side voltage V dc of the MMC inverter system to be stable includes:

[0016] Compare the DC-side modulation wave d1 of the MMC inverter system with the preset triangular carrier voltage:

[0017] When the output value of the DC-side modulation wave d1 of the MMC inverter system is greater than the triangular carrier wave, short-circuit the high-gain DC / DC converter based on the Z-source network;

[0018] When the output value of the DC-side modulation wave d1 of the MMC inverter system is less than the triangular carrier wave, connect the high-gain DC / DC converter based on the Z-source network to the circuit to control the DC-side voltage V dc of the MMC inverter system to be stable.

[0019] Preferably, the adjustment of the DC-side voltage V dc of the MMC inverter system is proportional-integral controller adjustment.

[0020] Preferably, the modulation wave d1 on the DC side of the MMC inverter system is determined by the following formula:

[0021] d1 = (V dc * - V dc )(K p + K i / s);

[0022] In the formula, d1 is the modulation wave on the DC side of the MMC inverter system, V dc * is the reference voltage on the DC side of the MMC inverter system, V dc is the voltage on the DC side of the MMC inverter system, K p is the proportionality coefficient, K i is the integral coefficient, and s is the complex frequency variable in the Laplace transform.

[0023] Based on the same inventive concept, the present invention also provides an application system of a fuel cell power generation high-voltage direct connection and grid-connected system, and the system includes: a voltage regulation module, a comparison module, and an inverter grid-connection module;

[0024] Voltage regulation module: regulates the voltage V dc on the DC side of the MMC inverter system to obtain the modulation wave d1 on the DC side of the MMC inverter system;

[0025] Comparison module: controls the short circuit or connection of the high-gain DC / DC converter based on the Z-source network to the circuit by comparing the modulation wave d1 on the DC side of the MMC inverter system with a preset triangular carrier voltage, so as to control the stability of the voltage V dc on the DC side of the MMC inverter system;

[0026] Inverter grid-connection module: realizes inverter grid-connection by controlling the on and off of the IGBT switches in the sub-modules of the MMC inverter system for the voltage V dc on the DC side of the MMC inverter system;

[0027] Preferably, the comparison module is specifically used for:

[0028] Comparing the modulation wave d1 on the DC side of the MMC inverter system with a preset triangular carrier voltage:

[0029] When the output value of the modulation wave d1 on the DC side of the MMC inverter system is greater than the triangular carrier wave, short-circuit the high-gain DC / DC converter based on the Z-source network;

[0030] When the output value of the modulation wave d1 on the DC side of the MMC inverter system is less than the triangular carrier wave, connect the high-gain DC / DC converter based on the Z-source network to the circuit to control the stability of the voltage V dc on the DC side of the MMC inverter system.

[0031] Preferably, the regulation of the DC-side voltage V of the MMC inverter system in the voltage regulation module dc is carried out by a proportional-integral controller.

[0032] Preferably, the modulation wave d1 on the DC side of the MMC inverter system in the comparison module is determined by the following formula:

[0033] d1 = (V dc * - V dc )(K p + K i / s);

[0034] In the formula, d1 is the modulation wave on the DC side of the MMC inverter system, V dc * is the reference voltage on the DC side of the MMC inverter system, V dc is the DC-side voltage of the MMC inverter system, K p is the proportionality coefficient, K i is the integral coefficient, and s is the complex frequency variable in the Laplace transform.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] The present invention provides a high-voltage direct connection and grid-connected system for fuel cell power generation, its application method and system, including: a hydrogen fuel cell power generation system, a high-gain DC / DC converter based on a Z-source network, and an MMC inverter system; the output end of the hydrogen fuel cell power generation system is connected to the input end of the high-gain DC / DC converter based on the Z-source network; the output end of the high-gain DC / DC converter based on the Z-source network is connected to the DC side of the MMC inverter system; the AC side of the MMC inverter system is connected to the AC grid. Through an innovative topology structure, the present invention uses the Z-source network to convert electrical energy into stable voltage and current, eliminating the transformer, and realizing a high-voltage direct connection and grid-connected method that adapts to the characteristics of rapid regulation and wide-range fluctuations of fuel cell thermal power. It has strong adaptability and flexibility, and has remarkable effects and application values in improving system operation efficiency, reducing operation costs, improving operation reliability, and reducing environmental impacts. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a schematic diagram of the high-voltage direct connection and grid-connected system for fuel cell power generation of the present invention;

[0038] Figure 2 is a topological structure diagram of the high-voltage direct connection and grid-connected system for fuel cell power generation of the present invention;

[0039] Figure 3 is a topological control flow chart of the high-voltage direct connection and grid-connected system for fuel cell power generation of the present invention;

[0040] Figure 4Schematic diagram of the specific connection mode between the stack and the Z-source network in the topology of the fuel cell power generation high-voltage direct connection and grid-connected system of the present invention;

[0041] Figure 5 Flowchart of the application method of the fuel cell power generation high-voltage direct connection and grid-connected system of the present invention;

[0042] Figure 6 Schematic diagram of the application system of the fuel cell power generation high-voltage direct connection and grid-connected system of the present invention. Detailed implementation manners

[0043] The following further details the detailed implementation manners of the present invention with reference to the accompanying drawings.

[0044] Embodiment 1

[0045] The present invention provides a fuel cell power generation high-voltage direct connection and grid-connected system, as Figure 1 shown, including: a hydrogen fuel cell power generation system, a high-gain DC / DC converter based on a Z-source network, and an MMC inverter system;

[0046] The output end of the hydrogen fuel cell power generation system is connected to the input end of the high-gain DC / DC converter based on the Z-source network;

[0047] The output end of the high-gain DC / DC converter based on the Z-source network is connected to the DC side of the MMC inverter system;

[0048] The AC side of the MMC inverter system is connected to the AC grid.

[0049] Specifically, as Figure 2 shown, the grid-connected system is divided into the following three parts: one is the hydrogen fuel cell power generation system, the second is the high-gain DC / DC converter based on the Z-source network, and the third is the MMC inverter system. As Figure 3 shown is the grid-connected control method.

[0050] Figure 2 In:

[0051] SM: represents a sub-module, which is the basic component unit of the MMC inverter. Each sub-module consists of two switches (usually IGBTs) and a small capacitor.

[0052] SM1, SM2, SMN: represent sub-module numbers (e.g., SM1 represents the first sub-module, SM2 represents the second sub-module... SMN represents the Nth sub-module)

[0053] u pa and u na : respectively represent the a-phase voltage and the n-phase voltage (the n-phase usually refers to the neutral point or the reference phase) output by the MMC inverter.

[0054] i pa and i na : respectively represent the phase-a current and phase-n current output by the MMC inverter.

[0055] L: represents the filter inductor, which is used to filter out the high-frequency harmonic components in the inverter output current and improve the current waveform.

[0056] i a 、i b 、i c : represent the currents corresponding to the three-phase AC power grid of phase-a, phase-b, and phase-c respectively.

[0057] v a 、v b 、v c : represent the voltages corresponding to the three-phase AC power grid of phase-a, phase-b, and phase-c respectively.

[0058] Further, the hydrogen fuel cell power generation system is characterized in that: the hydrogen fuel cell power generation system can be equivalent to a DC power supply with a wide range of voltage, current, and power fluctuations in output.

[0059] Further, the Z-source network-based high-gain DC / DC converter is characterized in that: the input end of the DC / DC converter is connected to a DC power supply, the output end is connected to an MMC inverter, and the inside of the DC / DC converter is composed of eight Z-source boost converters. The input ends of each Z-source boost converter are connected in parallel, and the output ends are cascaded one by one. Each Z-source is composed of two inductors L1, L2, four capacitors C0, C1, C2, C3, two diodes D0, D1, and a switch S7. The switch S7 is located in each Z-source module and is connected between the traditional Z-source circuit and the series circuit formed by the diode D1 and the capacitor C3. As Figure 4 shown.

[0060] Further, the MMC inverter is characterized in that: the sub-module adopts a half-bridge structure.

[0061] Embodiment 2

[0062] Based on the same inventive concept, the present invention also provides an application method for a fuel cell power generation high-voltage direct connection and grid-connected system, as Figure 5 shown, including:

[0063] S1. Adjust the DC-side voltage V dc of the MMC inverter system to obtain the DC-side modulation wave d1 of the MMC inverter system;

[0064] S2. By comparing the modulation wave d1 on the DC side of the MMC inverter system with the preset triangular carrier voltage, control the short - circuit or connection of the high - gain DC / DC converter based on the Z - source network to the circuit, so as to control the stability of the DC - side voltage V of the MMC inverter system dc ;

[0065] S3. By controlling the on - off of the IGBT switches in the sub - modules of the MMC inverter system, invert the DC - side voltage V of the MMC inverter system dc to achieve inverter grid - connection.

[0066] Step S1 specifically includes:

[0067] Set the reference voltage V dc * of the inverter DC side, measure the voltage V dc of the inverter DC side in real - time, and adjust the capacitor voltage V dc through a PI controller to obtain the modulation wave d1.

[0068] Step S2 specifically includes:

[0069] Compare the modulation wave d1 with the triangular carrier. When the output value of the modulation wave d1 is greater than the value of the triangular carrier, the comparator outputs a high level at this time, and the switch S7 is turned on. The switch S7 is connected between the traditional Z - source circuit and the series circuit formed by the diode D1 and the capacitor C3. When the switch S7 is turned on, the traditional Z - source circuit is short - circuited, so that the voltage at the corresponding Z - source output end is 0 at this time; when the output value of the modulation wave d1 is less than the value of the triangular carrier, the comparator outputs a low level at this time, and the switch S7 is turned off. When the switch S7 is turned off, the voltage of the Z - source circuit can be normally output through the diode.

[0070] Apply the PWM technology described above to control the on - off of the S7 of the 8 Z - source boost networks, so as to control the stability of V dc .

[0071] Furthermore, the feature of the modulation wave d1 obtained by the PI controller adjustment is that:

[0072] d1=(V dc * - V dc )(K p + K i / s)

[0073] where K p is the proportionality coefficient, K i is the integral coefficient, and s is the complex - frequency variable in the Laplace transform.

[0074] Furthermore, the feature of the triangular carrier is that: the upper and lower limits of the triangular carrier are 1 and 0 respectively, and the period is Ts.

[0075] Step S3 specifically includes: realizing inverter grid connection by controlling the on and off of IGBT switches in the sub-modules of the MMC inverter, where the sub-modules of the MMC inverter adopt a half-bridge structure.

[0076] The present invention realizes a high-voltage direct connection grid connection method suitable for the characteristics of rapid thermal power regulation and wide-range fluctuation of fuel cells through an innovative topological structure. First, based on the scheduling requirements, with the comprehensive energy system evaluation system such as environmental protection, energy efficiency, and economy as the optimization goal, the function expressions and constraint equations of each optimization goal are determined as the multi-node input of the tensor flow. Secondly, the objective function and constraint equations are transmitted to the preprocessing node by using directed edges. The node converts the multi-objective optimization into a single-objective optimization based on the anti-entropy weight method. Finally, the preprocessed optimization problem is also transmitted to the calculation node by using directed edges. The node performs power flow calculation through the Newton-Raphson method and takes the result as the output of the tensor flow.

[0077] Embodiment 3

[0078] Based on the same inventive concept, the present invention also provides an application system of a fuel cell power generation high-voltage direct connection grid connection system, as Figure 6 shown, the system includes: a voltage regulation module, a comparison module, and an inverter grid connection module;

[0079] Voltage regulation module: adjusts the DC-side voltage V dc of the MMC inverter system to obtain the DC-side modulation wave d1 of the MMC inverter system;

[0080] Comparison module: by comparing the DC-side modulation wave d1 of the MMC inverter system with a preset triangular carrier voltage, controls the short-circuit or connection of the high-gain DC / DC converter based on the Z-source network to the circuit, so as to control the stability of the DC-side voltage V dc of the MMC inverter system;

[0081] Inverter grid connection module: realizes inverter grid connection by controlling the on and off of IGBT switches in the sub-modules of the MMC inverter system for the DC-side voltage V dc of the MMC inverter system;

[0082] Preferably, the comparison module is specifically used for:

[0083] comparing the DC-side modulation wave d1 of the MMC inverter system with a preset triangular carrier voltage:

[0084] when the output value of the DC-side modulation wave d1 of the MMC inverter system is greater than the triangular carrier wave, short-circuits the high-gain DC / DC converter based on the Z-source network;

[0085] When the output value of the modulation wave d1 on the DC side of the MMC inverter system is less than the triangular carrier wave, the high-gain DC / DC converter based on the Z-source network is connected to the circuit to control the stability of the DC side voltage V of the MMC inverter system. dc of the MMC inverter system.

[0086] Preferably, the regulation of the DC side voltage V of the MMC inverter system in the voltage regulation module is dc performed by a proportional-integral controller.

[0087] Preferably, the modulation wave d1 on the DC side of the MMC inverter system in the comparison module is determined by the following formula:

[0088] d1 = (V dc * - V dc )(K p + K i / s);

[0089] where d1 is the modulation wave on the DC side of the MMC inverter system, V dc * is the reference voltage on the DC side of the MMC inverter system, V dc is the voltage on the DC side of the MMC inverter system, K p is the proportional coefficient, K i is the integral coefficient, and s is the complex frequency variable in the Laplace transform.

[0090] Embodiment 4

[0091] The following further illustrates the present invention with the specific implementation of the hydrogen energy demonstration project in Yili, Xinjiang.

[0092] As Figure 2 shown, both fuel cells are DC power supplies with an output voltage ranging from 323 V to 213 V, a current ranging from 0 A to 697 A, and a power ranging from 0 kW to 148 kW. In the Z-source network, C0 = 1 mF, L1 = L2 = 0.2 mH, and C1 = C2 = C3 = 1.5 mF. In the inverter, the sub-module capacitor C = 4.7 mF, N = 30, and the AC grid side is an AC power supply with a line voltage of 10 kV and phases differing by 120° in sequence.

[0093] Set the reference voltage V dc * of the inverter DC side to 14.69 kV, and detect the actual magnitude V dc of the inverter DC side voltage in real time. Using a PI controller, d1 = (V dc * - V dc )(K p + K i / s) is obtained, where K p and K i can be obtained through simulation experiments.

[0094] Then compare d1 with a triangular carrier wave with a period of Ts = 1 / 20000 s, an upper limit of 1, and a lower limit of 0 in terms of magnitude, and then obtain the on-off signals of eight switches S7 through PWM technology. Here, the on and off times and moments of each S7 are kept consistent. After reaching a steady state, V dc stabilizes at around 14.69 kV.

[0095] Then, through the MMC inverter control strategy, control the MMC to achieve the purpose of directly connecting the provincial transformer to the high voltage and realizing inverter grid connection.

[0096] In summary, a fuel cell power generation high-voltage direct connection and grid-connected system and its application method and system provided by the present invention include: a hydrogen fuel cell power generation system, a high-gain DC / DC converter based on a Z-source network, and an MMC inverter system; the output end of the hydrogen fuel cell power generation system is connected to the input end of the high-gain DC / DC converter based on the Z-source network; the output end of the high-gain DC / DC converter based on the Z-source network is connected to the DC side of the MMC inverter system; the AC side of the MMC inverter system is connected to the AC grid. Through the innovative topology structure, the present invention uses the Z-source network to convert electrical energy into stable voltage and current, eliminating the transformer, and realizes a high-voltage direct connection and grid-connected method that adapts to the characteristics of rapid adjustment and wide-range fluctuation of fuel cell thermoelectricity. It has strong adaptability and flexibility, and has remarkable effects and application values in improving the system operation efficiency, reducing the operation cost, enhancing the operation reliability, and reducing the environmental impact.

[0097] Those skilled in the art should understand that the embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0098] The present invention is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram can be implemented by computer program instructions, and the combination of the processes and / or blocks in the flowchart and / or block diagram can also be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate a device for realizing the functions specified in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0099] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to operate in a particular manner, such that the instructions stored in the computer-readable memory produce a manufacture including an instruction means that implements the functions specified in one or more of the processes and / or blocks Figure 1 of one or more of the processes and / or blocks Figure 1 of the functions specified in one or more of the blocks.

[0100] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, whereby the instructions executed on the computer or other programmable apparatus provide steps for implementing the functions specified in one or more of the processes and / or blocks Figure 1 of one or more of the processes and / or blocks Figure 1 of the functions specified in one or more of the blocks.

[0101] The above are only embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention are included within the scope of the claims of the present invention pending approval.

Claims

1. A high-voltage direct connection and grid-connected system for fuel cell power generation, characterized in that, The system includes: a hydrogen fuel cell power generation system, a high-gain DC / DC converter based on a Z-source network, and an MMC inverter system; The output end of the hydrogen fuel cell power generation system is connected to the input end of the high-gain DC / DC converter based on the Z-source network; The output end of the high-gain DC / DC converter based on the Z-source network is connected to the DC side of the MMC inverter system; The AC side of the MMC inverter system is connected to the AC grid.

2. The system according to claim 1, characterized in that, The high-gain DC / DC converter based on the Z-source network internally includes a specific number of Z-source boost converters. The hydrogen fuel cell power generation system is connected to the input end of each Z-source boost converter. The input ends of each Z-source boost converter are connected in parallel, and the output ends are cascaded in series to the MMC inverter system.

3. The system according to claim 1, wherein The sub-modules of the MMC inverter system adopt a half-bridge structure.

4. Application method of a high-voltage direct connection and grid-connected system for fuel cell power generation, characterized in that, The method includes: Adjust the DC-side voltage V of the MMC inverter system dc to obtain the DC-side modulation wave d1 of the MMC inverter system; By comparing the modulation wave d1 on the DC side of the MMC inverter system with the preset triangular carrier voltage, the short-circuit or connection to the circuit of the high-gain DC / DC converter based on the Z-source network is controlled to control the stability of the DC-side voltage V of the MMC inverter system dc ; By controlling the opening and closing of the IGBT switches in the sub-modules of the MMC inverter system, the DC-side voltage V of the MMC inverter system dc is inverted to achieve grid connection by inversion; Among them, the fuel cell power generation high-voltage direct connection grid-connected system is the fuel cell power generation high-voltage direct connection grid-connected system according to any one of claims 1-4.

5. The method according to claim 4, characterized in that By comparing the modulation wave d1 on the DC side of the MMC inverter system with the preset triangular carrier voltage, controlling the short - circuit or connection of the high - gain DC / DC converter based on the Z - source network to the circuit, so as to control the stability of the DC - side voltage V of the MMC inverter system, including: dc The stability of: Compare the modulation wave d1 on the DC side of the MMC inverter system with a preset triangular carrier voltage: When the output value of the modulation wave d1 on the DC side of the MMC inverter system is greater than the triangular carrier, short-circuit the high-gain DC / DC converter based on the Z-source network; When the output value of the modulation wave d1 on the DC side of the MMC inverter system is less than the triangular carrier wave, a high-gain DC / DC converter based on the Z-source network is connected to the circuit to control the stability of the DC-side voltage V dc of the MMC inverter system.

6. The method according to claim 4, wherein The adjustment of the DC-side voltage V of the MMC inverter system dc is carried out by a proportional-integral controller.

7. The method according to claim 4, characterized in that The modulation wave d1 on the DC side of the MMC inverter system is determined by the following formula: d1=(V dc *-V dc )(K p +K i / s); Where, d1 is the modulation wave on the DC side of the MMC inverter system, V dc * is the reference voltage on the DC side of the MMC inverter system, V dc is the voltage on the DC side of the MMC inverter system, K p is the proportionality coefficient, K i is the integral coefficient, and s is the complex frequency variable in the Laplace transform.

8. An application system of a high-voltage direct connection and grid-connected system for fuel cell power generation, characterized in that, The system includes: a voltage regulation module, a comparison module, and an inverter grid-connection module; Voltage regulation module: regulates the DC-side voltage V of the MMC inverter system dc to obtain the modulation wave d1 on the DC side of the MMC inverter system; Comparison module: By comparing the modulation wave d1 on the DC side of the MMC inverter system with the preset triangular carrier voltage, it controls the short circuit or connection of the high-gain DC / DC converter based on the Z-source network to the circuit, so as to control the stability of the DC side voltage V of the MMC inverter system dc ; Inverter grid-connection module: By controlling the on and off of IGBT switches in the sub-modules of the MMC inverter system, the DC-side voltage V of the MMC inverter system is inverted to achieve inverter grid-connection; dc and perform inversion to achieve grid connection; Among them, the fuel cell power generation high-voltage direct connection grid-connected system is the fuel cell power generation high-voltage direct connection grid-connected system according to any one of claims 1-4.

9. The system according to claim 8, wherein The comparison module is specifically used for: Compare the modulation wave d1 on the DC side of the MMC inverter system with a preset triangular carrier voltage: When the output value of the modulation wave d1 on the DC side of the MMC inverter system is greater than the triangular carrier, short-circuit the high-gain DC / DC converter based on the Z-source network; When the output value of the modulation wave d1 on the DC side of the MMC inverter system is less than the triangular carrier wave, the high-gain DC / DC converter based on the Z-source network is connected to the circuit to control the stability of the DC side voltage V dc of the MMC inverter system.

10. The system according to claim 8, wherein The voltage regulation performed on the DC-side voltage V of the MMC inverter system in the voltage regulation module dc is proportional-integral controller regulation.

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