A three-phase fuel cell grid-connected system

Abstract

The present invention discloses a three-phase fuel cell grid-connected system, which adopts a reversible solid oxide fuel cell coordinated grid-connected microgrid structure. The entire microgrid system is based on a two-stage control structure of a DC bus and an AC bus. The DC / DC converter boosts and stabilizes the output voltage of the fuel cell to meet the grid-connected operation index. At the same time, the stable voltage output can suppress the current ripple and improve the output efficiency and service life of the fuel cell. The DC / AC inverter matches the grid-connected current and the grid voltage as well as the phase sequence and frequency consistency, thereby solving the fuel cell grid-connected problem. The invention solves the technical problem that when the fuel cell is connected to an unbalanced three-phase grid, the output power of the DC / AC inverter generates a pulsating component of the double frequency grid frequency, and under power coupling, the output current of the fuel cell has a low-frequency ripple component, which affects the output and service life of the fuel cell.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected systems, and in particular to a three-phase fuel cell grid-connected system. Background Art

[0002] Fuel cells have become the preferred choice for distributed power generation due to their high power generation efficiency, no pollution to the environment and few installation restrictions, and are widely used in grid-connected power generation systems. When fuel cells are connected to a three-phase power grid as a power generation source, their research is similar to that of a three-phase photovoltaic power generation grid-connected power generation system. The control objectives are mainly focused on improving the power quality of the three-phase grid-connected power and controlling the DC side voltage, because there is no power coupling in the case of an ideal balanced grid. However, the actual grid is susceptible to factors such as non-ideal loads, faults, and resonance, and often exhibits unbalanced non-ideal characteristics. When a fuel cell is connected to an unbalanced three-phase grid, the output power of the DC / AC inverter will generate a pulsating component of twice the grid frequency. Due to the existence of power coupling, the output current will have a low-frequency ripple component, which will affect the output and service life of the fuel cell (for example: accelerating electrode melting, causing the catalyst layer cathode electrode to fail, slowing down the cathode surface reaction, resulting in reduced output capacity, and overload-induced interference tripping). Summary of the invention

[0003] The present invention provides a three-phase fuel cell grid-connected system, which is used to solve the technical problem that when the fuel cell is connected to an unbalanced three-phase power grid, the output power of the DC / AC inverter generates a pulsating component of twice the grid frequency, and under power coupling, the output current of the fuel cell has a low-frequency ripple component, which affects the output and service life of the fuel cell.

[0004] In view of this, the present invention provides a three-phase fuel cell grid-connected system, including a fuel cell system, a new energy generator set, a first DC / DC converter, a second DC / DC converter, a DC bus, an AC bus, an AC / DC rectifier, a DC / AC inverter, an external power grid and a power load;

[0005] The new energy generator set is connected to the DC bus through the first DC / DC converter or AC / DC rectifier, and the fuel cell system is connected to the DC bus through the second DC / DC converter, and the second DC / DC converter is a bidirectional DC / DC converter;

[0006] The input end of the DC / AC inverter is connected to the DC bus, and the output end of the DC / AC inverter is connected to the AC bus;

[0007] The power load is connected to the AC bus;

[0008] The external power grid is connected to the AC busbar via a common connection point;

[0009] The fuel cell system includes a reversible solid oxide fuel cell and a hydrogen storage system, and the reversible solid oxide fuel cell is connected to the second DC / DC converter and the hydrogen storage system respectively;

[0010] The fuel cell system is configured with a high-temperature solid oxide water electrolysis hydrogen production mode and a solid oxide fuel cell mode. When the output power fluctuation of the new energy generator set is greater than the preset value or the output power of the new energy generator set is lower than the preset proportion of the rated power, the fuel cell system operates in the high-temperature solid oxide water electrolysis hydrogen production mode. When the power grid is at peak power consumption, the fuel cell system operates in the solid oxide fuel cell mode.

[0011] Optionally, it further includes a first capacitor, a second capacitor, an LC series circuit and an LC filter circuit;

[0012] One end of the first capacitor is connected to the positive electrode of the reversible solid oxide fuel cell, the other end of the first capacitor is connected to the negative electrode of the reversible solid oxide fuel cell, the second DC / DC converter is connected in parallel to the rear stage of the first capacitor, the second capacitor is connected in parallel to the rear stage of the second DC / DC converter, the LC series circuit is connected in parallel to the rear stage of the second capacitor, the DC / AC inverter is connected in parallel to the rear stage of the LC series circuit, the LC filter circuit is connected in parallel to the rear stage of the DC / AC inverter, and the LC filter circuit is connected to the AC bus.

[0013] Optionally, the new energy generator set includes a photovoltaic generator set and a wind generator set;

[0014] The photovoltaic generator set is connected to the DC bus through the first DC / DC converter;

[0015] The wind turbine generator set is connected to the DC bus through an AC / DC rectifier.

[0016] Optionally, a power harvester is also included;

[0017] The power collector is connected to the output ends of the first DC / DC converter and the AC / DC rectifier respectively.

[0018] Optionally, the preset value is 10%.

[0019] Optionally, the preset ratio is 30%.

[0020] Optionally, a power sensor is further included, and the power sensor is connected to the output end of the DC / AC inverter.

[0021] Optionally, an AC grid-connected switch is also included;

[0022] Two ends of the AC grid-connected switch are connected to the AC bus and the DC / AC inverter respectively.

[0023] Optionally, a DC grid-connected switch is further included, the first DC / DC converter is connected to the DC bus through the DC grid-connected switch, and the AC / DC rectifier is connected to the DC bus through the DC grid-connected switch.

[0024] Optionally, the grid-connected inverter is composed of a three-phase full-bridge DC / AC circuit.

[0025] It can be seen from the above technical solutions that the three-phase fuel cell grid-connected system provided by the present invention has the following advantages:

[0026] The three-phase fuel cell grid-connected system provided by the present invention adopts a reversible solid oxide fuel cell coordinated grid-connected microgrid structure in order to suppress the output power fluctuation of renewable energy, combined with the advantages of the electrolyzer with fast and flexible power adjustment. The entire microgrid system is based on a two-stage control structure of a DC bus and an AC bus. The DC / DC converter boosts and stabilizes the output voltage of the fuel cell to meet the grid-connected operation indicators. At the same time, the stable voltage output can suppress current ripple and improve the output efficiency and service life of the fuel cell; the DC / AC inverter matches the grid-connected current and the grid voltage as well as the phase sequence and frequency consistency, thereby solving the fuel cell grid-connected problem and solving the problem that when the fuel cell is connected to an unbalanced three-phase grid, the output power of the DC / AC inverter generates a pulsating component of twice the grid frequency, and under power coupling, the output current of the fuel cell has a low-frequency ripple component, which affects the output and service life of the fuel cell.

[0027] At the same time, considering that when the three-phase grid voltage is unbalanced, the output power of the three-phase grid-connected inverter will have double frequency pulsation, resulting in the output current of the fuel cell containing a large double frequency current ripple. In order to reduce the output current ripple of the fuel cell without using a large DC side capacitor, the three-phase fuel cell grid-connected system provided by the present invention proposes a power decoupling circuit structure for a three-phase SOFC grid-connected system, including a reversible solid oxide fuel cell, a second DC / DC converter, a DC / AC inverter, a first capacitor, a second capacitor, an LC series circuit and an LC filter circuit. The reversible solid oxide fuel cell works in a solid oxide fuel cell mode, one end of the first capacitor is connected to the positive electrode of the reversible solid oxide fuel cell, and the other end of the first capacitor is connected to the negative electrode of the reversible solid oxide fuel cell. The second DC / DC converter is connected in parallel to the rear stage of the first capacitor, the second capacitor is connected in parallel to the rear stage of the second DC / DC converter, the LC series circuit is connected in parallel to the rear stage of the second capacitor, the DC / AC inverter is connected in parallel to the rear stage of the LC series circuit, the LC filter circuit is connected in parallel to the rear stage of the DC / AC inverter, and the LC filter circuit is connected to the AC bus. Therefore, through the power decoupling circuit structure, the pulsating power is provided by the decoupling circuit, so that the bus voltage will not pulsate, and the fuel cell output current will remain constant, suppressing the double frequency ripple of the fuel cell output current, significantly reducing the harmonic distortion rate of the AC side grid-connected current, and improving the dynamic response characteristics of the fuel cell grid-connected. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 A schematic structural diagram of a three-phase fuel cell grid-connected system provided in the present invention;

[0030] Figure 2 A topological structure diagram of a fuel cell DC / DC boost voltage stabilization circuit of a three-phase fuel cell grid-connected system provided in the present invention;

[0031] Figure 3 A schematic diagram of a DC / AC inverter circuit topology of a three-phase fuel cell grid-connected system provided in the present invention;

[0032] Figure 4 It is a schematic diagram of the power decoupling circuit structure of the three-phase SOFC grid-connected system provided in the present invention. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] For easier understanding, see Figures 1 to 3 , the present invention provides an embodiment of a three-phase fuel cell grid-connected system, comprising a fuel cell system, a new energy generator set, a first DC / DC converter, a second DC / DC converter, a DC bus, an AC bus, an AC / DC rectifier, a DC / AC inverter, an external power grid and a power load;

[0035] The new energy generator set is connected to the DC bus through the first DC / DC converter or AC / DC rectifier, and the fuel cell system is connected to the DC bus through the second DC / DC converter, and the second DC / DC converter is a bidirectional DC / DC converter;

[0036] The input end of the DC / AC inverter is connected to the DC bus, and the output end of the DC / AC inverter is connected to the AC bus;

[0037] The power load is connected to the AC bus;

[0038] The external power grid is connected to the AC busbar via a common connection point;

[0039] The fuel cell system includes a reversible solid oxide fuel cell and a hydrogen storage system, and the reversible solid oxide fuel cell is connected to the second DC / DC converter and the hydrogen storage system respectively;

[0040] The fuel cell system is configured with a high-temperature solid oxide water electrolysis hydrogen production mode and a solid oxide fuel cell mode. When the output power fluctuation of the new energy generator set is greater than the preset value or the output power of the new energy generator set is lower than the preset proportion of the rated power, the fuel cell system operates in the high-temperature solid oxide water electrolysis hydrogen production mode. When the power grid is at peak power consumption, the fuel cell system operates in the solid oxide fuel cell mode.

[0041] It should be noted that, different from the traditional microgrid system, the grid-connected microgrid structure based on reversible solid oxide fuel cell (RSOC) in the present invention is as follows Figure 1As shown, it includes a fuel cell system, a new energy generator set, a first DC / DC converter, a second DC / DC converter, a DC bus, an AC bus, an AC / DC rectifier, a DC / AC inverter, an external power grid and power loads. The entire microgrid system is based on a two-stage control structure of a DC bus and an AC bus. Through multiple conversion links such as inverters, converters and rectifiers, the real-time supply and demand balance of the operating power of the microgrid system is coordinated. The AC bus is connected to the external power grid through a common connection point PCC (Point of Common Coupling).

[0042] In order to suppress the output power fluctuation of renewable energy, combined with the advantages of fast and flexible power adjustment of electrolyzer, a reversible solid oxide fuel cell (RSOC) coordinated grid-connected microgrid structure is adopted. The fuel cell system is configured with a high-temperature solid oxide water electrolysis hydrogen production (SOEC) mode and a solid oxide fuel cell (SOFC) mode. When the output power fluctuation of the new energy generator set is greater than the preset value or the output power of the new energy generator set is lower than the preset proportion of the rated power, the fuel cell system operates in the high-temperature solid oxide water electrolysis hydrogen production (SOEC) mode to achieve efficient electrolysis hydrogen storage; when the power grid is at a peak power consumption, the fuel cell system operates in the solid oxide fuel cell (SOFC) mode to achieve stable energy storage and grid connection. In the present invention, when the output power fluctuation of the new energy generator set is greater than 10% or the output power of the new energy generator set is lower than 30% of the rated power, the fuel cell system operates in the high-temperature solid oxide water electrolysis hydrogen production (SOEC) mode. The output power of the new energy generator set is collected by a power collector, and the power collector is respectively connected to the output ends of the first DC / DC converter and the AC / DC rectifier. The new energy generator set includes a photovoltaic generator set and a wind generator set. The photovoltaic generator set is connected to the DC bus through a first DC / DC converter, and the wind generator set is connected to the DC bus through an AC / DC rectifier.

[0043] An AC grid-connected switch can also be configured, and the two ends of the AC grid-connected switch are connected to the AC bus and the DC / AC inverter respectively. The opening and closing of the AC grid-connected switch is used to control whether the microgrid is connected to the external power grid. A DC grid-connected switch can also be configured, and the first DC / DC converter is connected to the DC bus through the DC grid-connected switch, and the AC / DC rectifier is connected to the DC bus through the DC grid-connected switch. Whether the new energy unit is connected to the DC bus is controlled by the opening and closing of the DC grid-connected switch.

[0044] The DC power output by the three-phase fuel cell is boosted and stabilized by a DC / DC converter. The circuit principle is as follows: Figure 2 As shown, Figure 2In the figure, Udc is the DC bus voltage, and D is the rectifier diode. When the switch tube S0 is turned on, the output voltage Us of the SOFC charges the input inductor L, and its stored magnetic field energy increases. At this time, the capacitor C1 and the DC bus form a loop and provide power to the load, and the voltage on the capacitor C1 decreases. When the switch tube S0 is turned off, the power supply, the input inductor L, and the DC side capacitor C1 form a loop. At this time, the energy stored in the input inductor L is released, and the power supply and the input inductor L jointly charge the capacitor C1. The DC power after the DC / DC converter steps up and stabilizes the voltage is converted into AC power by the DC / AC inverter and connected to the power grid. The grid-connected inverter is composed of a three-phase full-bridge DC / AC circuit, such as Figure 3 shown. Figure 3 In the figure, ia, ib, ic are the grid currents; ua, ub, uc are the grid voltages; Lf and Cf are filter inductors and capacitors, which are used to suppress current ripples. When the circuit is working normally, the two switch tubes on the same bridge arm are turned on complementary to each other. The two switch power tubes on the same bridge arm cannot be turned on at the same time, otherwise it will cause a power short circuit.

[0045] The three-phase fuel cell grid-connected system provided in the present invention has a two-stage control structure based on a DC bus and an AC bus. The DC / DC converter boosts and stabilizes the output voltage of the fuel cell to meet the grid-connected operation index. At the same time, the stable voltage output can suppress the current ripple and improve the output efficiency and service life of the fuel cell. The DC / AC inverter matches the grid-connected current and the grid voltage as well as the phase sequence and frequency consistency, thereby solving the fuel cell grid-connected problem and solving the problem that when the fuel cell is connected to an unbalanced three-phase grid, the output power of the DC / AC inverter generates a pulsating component of twice the grid frequency. Under power coupling, the output current of the fuel cell has a low-frequency ripple component, which affects the output and service life of the fuel cell.

[0046] In one embodiment, the three-phase fuel cell grid-connected system may also be configured with a power sensor, which is connected to the output end of the DC / AC inverter. The power sensor is used to detect the output power of the microgrid system.

[0047] When the three-phase grid voltage is unbalanced, the output power of the three-phase grid-connected inverter will pulsate at a double frequency, causing the output current of the fuel cell to contain a large double frequency current ripple. The existing method uses a large capacitor on the DC side to absorb the generated ripple. This method can reduce the ripple to a certain extent, but it will lead to a decrease in system power density and an increase in cost and volume. Therefore, in order to reduce the output current ripple of the fuel cell without using a large DC side capacitor, the present invention proposes a power decoupling circuit structure for a three-phase SOFC grid-connected system, such as Figure 4As shown, it includes a reversible solid oxide fuel cell, a second DC / DC converter, a DC / AC inverter, a first capacitor C1, a second capacitor C2, an LC series circuit and an LC filter circuit. The reversible solid oxide fuel cell works in a solid oxide fuel cell mode. One end of the first capacitor C1 is connected to the positive electrode of the reversible solid oxide fuel cell, and the other end of the first capacitor C1 is connected to the negative electrode of the reversible solid oxide fuel cell. The second DC / DC converter is connected in parallel to the rear stage of the first capacitor, the second capacitor C2 is connected in parallel to the rear stage of the second DC / DC converter, the LC series circuit is connected in parallel to the rear stage of the second capacitor C2, the LC series circuit includes a series inductor L and a third capacitor C3, the DC / AC inverter is connected in parallel to the rear stage of the LC series circuit, the LC filter circuit is connected in parallel to the rear stage of the DC / AC inverter, and the LC filter circuit is connected to the AC bus. Through the power decoupling circuit structure, the pulsating power is provided by the decoupling circuit, so that the bus voltage will not pulsate, the fuel cell output current will remain constant, the double frequency ripple of the fuel cell output current will be suppressed, the harmonic distortion rate of the AC side grid-connected current will be significantly reduced, and the dynamic response characteristics of the fuel cell grid-connected will be improved.

[0048] The terms "first", "second", "third", "fourth", etc. in the specification of the present invention are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein, for example. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0049] As described above, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A three-phase fuel cell grid-connected system, characterized in that: Including fuel cell system, new energy generator set, first DC / DC converter, second DC / DC converter, DC bus, AC bus, AC / DC rectifier, DC / AC inverter, external power grid and power load; The new energy generator set is connected to the DC bus through the first DC / DC converter or AC / DC rectifier, and the fuel cell system is connected to the DC bus through the second DC / DC converter, and the second DC / DC converter is a bidirectional DC / DC converter; The input end of the DC / AC inverter is connected to the DC bus, and the output end of the DC / AC inverter is connected to the AC bus; The power load is connected to the AC bus; The external power grid is connected to the AC busbar via a common connection point; The fuel cell system includes a reversible solid oxide fuel cell and a hydrogen storage system, and the reversible solid oxide fuel cell is connected to the second DC / DC converter and the hydrogen storage system respectively; The fuel cell system is configured with a high-temperature solid oxide water electrolysis hydrogen production mode and a solid oxide fuel cell mode. When the output power fluctuation of the new energy generator set is greater than the preset value or the output power of the new energy generator set is lower than the preset proportion of the rated power, the fuel cell system works in the high-temperature solid oxide water electrolysis hydrogen production mode. When the power grid is at a peak power consumption, the fuel cell system works in the solid oxide fuel cell mode. Also includes a first capacitor, a second capacitor, an LC series circuit and an LC filter circuit; One end of the first capacitor is connected to the positive electrode of the reversible solid oxide fuel cell, the other end of the first capacitor is connected to the negative electrode of the reversible solid oxide fuel cell, the second DC / DC converter is connected in parallel to the rear stage of the first capacitor, the second capacitor is connected in parallel to the rear stage of the second DC / DC converter, the LC series circuit is connected in parallel to the rear stage of the second capacitor, the DC / AC inverter is connected in parallel to the rear stage of the LC series circuit, the LC filter circuit is connected in parallel to the rear stage of the DC / AC inverter, and the LC filter circuit is connected to the AC bus.

2. The three-phase fuel cell grid-connected system according to claim 1, characterized in that: New energy generators include photovoltaic generators and wind generators; The photovoltaic generator set is connected to the DC bus through the first DC / DC converter; The wind turbine generator set is connected to the DC bus through an AC / DC rectifier.

3. The three-phase fuel cell grid-connected system according to claim 1, characterized in that: Also included is a power harvester; The power collector is connected to the output ends of the first DC / DC converter and the AC / DC rectifier respectively.

4. The three-phase fuel cell grid-connected system according to claim 3, characterized in that: The default value is 10%.

5. The three-phase fuel cell grid-connected system according to claim 4, characterized in that: The default ratio is 30%.

6. The three-phase fuel cell grid-connected system according to claim 1, characterized in that: It also includes a power sensor, which is connected to the output end of the DC / AC inverter.

7. The three-phase fuel cell grid-connected system according to claim 1, characterized in that: It also includes an AC grid-connected switch; Two ends of the AC grid-connected switch are connected to the AC bus and the DC / AC inverter respectively.

8. The three-phase fuel cell grid-connected system according to claim 1, characterized in that: It also includes a DC grid-connected switch, through which the first DC / DC converter is connected to the DC bus, and the AC / DC rectifier is connected to the DC bus through the DC grid-connected switch.

9. The three-phase fuel cell grid-connected system according to claim 1, characterized in that: The grid-connected inverter consists of a three-phase full-bridge DC / AC circuit.

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