Hybrid power flow controller system with off-grid starting function and control method

Abstract

The invention provides a hybrid power flow controller system with an off-grid starting function and a control method, and belongs to the field of power grid regulation and control. The system comprises a series transformer, a state change-over switch group, an exciting transformer and a cascaded H-bridge converter, the two ends of the primary side of the series transformer are connected with a power grid, the movable end is connected with the primary side of the exciting transformer, the secondary side is connected with the movable end of the secondary side of the exciting transformer through the state switching switch group, the fixed end of the secondary side of the exciting transformer is connected with the cascaded H-bridge converter, and the cascaded H-bridge converter is provided with an energy storage unit; the state switching switch group is used for switching to an off-grid starting state when the power grid is powered off, so that the series transformer and the exciting transformer are isolated, the cascaded H-bridge converter supplies power to the exciting transformer through the energy storage unit, and the state switching switch group is switched to a grid-connected state when the power grid operates normally, so that the series transformer and the exciting transformer are connected in series. The method can help the power grid to realize black start and other functions in an off-grid state.

Description

Technical Field

[0001] The present invention relates to the technical field of power grid control, and in particular to a hybrid power flow controller system with an off-grid starting function and a control method. Background Art

[0002] With the access of a large number of renewable energy sources to the power grid, the power system faces problems such as volatility and intermittency, which leads to prominent problems in grid stability and power quality. At present, the mainstream power flow regulation devices in the power grid include two categories: mechanical electromagnetic devices and flexible alternating current transmission systems (FACTS). The typical representative of the former is the phase-shifting transformer (PST), which has a large regulation capacity but low accuracy and cannot be adjusted continuously; the typical representative of the latter is the unified power flow controller (UPFC). FACTS has high regulation accuracy, but the structure is complex and the cost is high, which is not conducive to the promotion of engineering applications. PST and UPFC can form a hybrid unified power flow controller (HUPFC), which can achieve both high-power regulation and high-precision power regulation.

[0003] However, existing HUPFCs are all operated in a grid-connected state. They absorb part of the energy through the excitation transformer, then use the cascaded H Bridge (CHB) to generate an adjustable voltage, and then couple with the main grid through a series transformer. When the grid is out of power, the existing HUPFC cannot be started and can only operate normally after the grid is restored, which does not give full play to the flexible regulation function of HUPFC. Summary of the invention

[0004] The embodiment of the present invention provides a hybrid power flow controller system and a control method with an off-grid startup function to solve the problem that the HUPFC cannot be started when the power grid is out of power.

[0005] In a first aspect, an embodiment of the present invention provides a hybrid power flow controller system with an off-grid startup function, including a series transformer, a state switching switch group, an excitation transformer, and a cascaded H-bridge converter;

[0006] The two ends of the primary side of the series transformer are connected to the power grid, the active end is connected to the primary side of the excitation transformer, the secondary side is connected to the active end of the secondary side of the excitation transformer through a state switching switch group, the fixed end of the secondary side of the excitation transformer is connected to the cascade H-bridge converter, and the cascade H-bridge converter has an energy storage unit;

[0007] The state switching switch group is used to switch to the off-grid starting state when the power grid is out of power, so as to isolate the series transformer from the excitation transformer. The cascaded H-bridge converter supplies power to the excitation transformer through the energy storage unit, and switches to the grid-connected state when the power grid is operating normally, so as to connect the series transformer in series with the excitation transformer.

[0008] In a possible implementation, the state switching switch group includes a first switch, a second switch, and a third switch;

[0009] The first end of the A phase of the secondary side of the series transformer is connected to the active end of the A phase of the secondary side of the excitation transformer through the first switch, the first end of the B phase of the secondary side of the series transformer is connected to the active end of the B phase of the secondary side of the excitation transformer through the second switch, and the first end of the C phase of the secondary side of the series transformer is connected to the active end of the C phase of the secondary side of the excitation transformer through the third switch;

[0010] When the state switching switch group is in the off-grid startup state, the first switch, the second switch and the third switch are disconnected;

[0011] When the state switching switch group is in the grid-connected state, the first switch, the second switch and the third switch are closed.

[0012] In a possible implementation, the state switching switch group further includes a fourth switch and a fifth switch;

[0013] The active end of the A phase of the secondary side of the excitation transformer is connected to the active end of the B phase through the fourth switch, and the active end of the B phase is connected to the active end of the C phase through the fifth switch;

[0014] When the state switching switch group is in the off-grid startup state, the fourth switch and the fifth switch are closed;

[0015] When the state switching switch group is in the grid-connected state, the fourth switch and the fifth switch are disconnected.

[0016] In a possible implementation, it also includes a battery charging and discharging control module and a DC voltage average value control module;

[0017] When the state switching switch group is in the off-grid starting state, the output voltage of the battery charge and discharge control module is used as the reference value of the DC voltage average value control module, and the output of the DC voltage average value control module is used as the output voltage d-axis reference value of the cascaded H-bridge converter.

[0018] In a possible implementation, the output voltage q-axis reference value of the cascaded H-bridge converter is 0, and the output phase angle reference value is determined based on the frequency of the synchronous motor.

[0019] In a possible implementation, it also includes a PST gear and CHB coordinated control module, a battery charge and discharge control module, and a DC voltage average value control module;

[0020] When the state switching switch group is in the grid-connected state, the output of the PST gear and CHB collaborative control module is used as the output voltage d-axis reference value of the cascaded H-bridge converter, the output voltage of the battery charge and discharge control module is used as the reference value of the DC voltage average value control module, and the output of the DC voltage average value control module is used as the output voltage q-axis reference value of the cascaded H-bridge converter. The output phase angle reference value of the cascaded H-bridge converter is determined based on the PLL phase-locked module.

[0021] In a possible implementation, the secondary side of the series transformer adopts a triangle connection mode, and the vertex outgoing line of the triangle is connected to the state switching switch group.

[0022] In a possible implementation, the cascaded H-bridge converter includes a plurality of single-phase full-bridge converters, each of which includes four fully-controlled switching devices, one capacitor, one inductor, and one battery pack.

[0023] In a second aspect, an embodiment of the present invention provides a control method for a hybrid power flow controller system with an off-grid startup function, the system comprising a series transformer, a state switching switch group, an excitation transformer and a cascaded H-bridge converter;

[0024] The two ends of the primary side of the series transformer are connected to the power grid, the active end is connected to the primary side of the excitation transformer, the secondary side is connected to the active end of the secondary side of the excitation transformer through a state switching switch group, the fixed end of the secondary side of the excitation transformer is connected to the cascade H-bridge converter, and the cascade H-bridge converter has an energy storage unit;

[0025] The method includes:

[0026] The state switching switch group switches to the off-grid starting state when the power grid fails to power on, so as to isolate the series transformer from the excitation transformer. The cascaded H-bridge converter supplies power to the excitation transformer through the energy storage unit, and switches to the grid-connected state when the power grid operates normally, so as to connect the series transformer in series with the excitation transformer.

[0027] The embodiment of the present invention provides a hybrid power flow controller system and control method with off-grid starting function. When the power grid is out of power, the state switching switch group is switched to the off-grid starting state, the connection relationship between the series transformer and the excitation transformer is changed, and then a partially adjustable voltage is generated by the energy storage unit of the cascaded H-bridge converter. The excitation transformer is used to reversely convert the low voltage into high voltage, which can help the power grid to achieve black start and other functions in the off-grid state. In the grid-connected state, the series transformer is connected in series with the excitation transformer, which can continue to assume the phase adjustment function and has flexible power grid regulation function. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, 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 drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 is a structural schematic diagram of a hybrid power flow controller system with an off-grid startup function provided by an embodiment of the present invention;

[0030] Figure 2 is a structural schematic diagram of a hybrid power flow controller system with an off-grid startup function provided by an embodiment of the present invention;

[0031] Figure 3 is a control block diagram of a hybrid power flow controller system with an off-grid startup function provided by an embodiment of the present invention;

[0032] Figure 4 It is a flow chart of the implementation of the control method of the hybrid power flow controller system with off-grid startup function provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0033] In the following description, specific details such as specific system structures, technologies, etc. are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments of the present invention. However, it should be clear to those skilled in the art that the present invention may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present invention.

[0034] In order to make the purpose, technical solutions and advantages of the present invention more clear, specific embodiments will be described below in conjunction with the accompanying drawings.

[0035] The existing HUPFCs are all operated in a grid-connected state. They absorb part of the energy through the excitation transformer, and then use the cascaded H Bridge (CHB) to generate an adjustable voltage, and then couple with the main grid through the series transformer. When the power grid is out of power, the existing HUPFC cannot be started, and cannot help the power grid to achieve off-grid startup. It can only wait for the power grid to recover before it can operate normally, and does not give full play to the flexible regulation function of HUPFC. The applicant found that the main reason is that the cascaded H bridge converter in the existing HUPFC can only cooperate with the excitation transformer to transmit energy to the series transformer. When the power grid is out of power, the excitation transformer cannot provide energy, and the DC side of the cascaded H bridge converter lacks voltage support and cannot operate. Therefore, this patent proposes a hybrid power flow controller system and control method with off-grid startup function. The system can not only complete the basic functions of the traditional HUPFC and realize the flexible regulation of the power grid flow, but also can operate in an off-grid state to help the power grid resume operation. The present invention can be applied to lines with flexible power flow regulation and power systems with black start requirements and medium and low voltage distribution circuits, and can help the distribution system to operate in an off-grid operation mode.

[0036] Figure 1 A schematic diagram of the structure of a hybrid power flow controller system with an off-grid startup function provided by an embodiment of the present invention, the system comprising a series transformer 11, a state switching switch group 12, an excitation transformer 13 and a cascaded H-bridge converter 14;

[0037] Both ends of the primary side of the series transformer 11 are connected to the power grid, the active end is connected to the primary side of the excitation transformer 13, the secondary side is connected to the active end of the secondary side of the excitation transformer 13 through the state switching switch group 12, and the fixed end of the secondary side of the excitation transformer 13 is connected to the cascade H-bridge converter 14, and the cascade H-bridge converter 14 has an energy storage unit;

[0038] The state switching switch group 12 is used to switch to the off-grid starting state when the power grid is out of power, so as to isolate the series transformer 11 from the excitation transformer 13. The cascaded H-bridge converter 14 supplies power to the excitation transformer 13 through the energy storage unit, and switches to the grid-connected state when the power grid is operating normally, so that the series transformer 11 is connected in series with the excitation transformer 13.

[0039] In this embodiment, the existing HUPFCs are all operated in a grid-connected state, which absorbs part of the energy through the excitation transformer, then uses the cascaded H-bridge converter to generate an adjustable voltage, and then couples with the main grid through the series transformer. When the grid is out of power, since the cascaded H-bridge converter in the existing HUPFC can only cooperate with the excitation transformer to transmit energy to the series transformer, when the grid is out of power, the excitation transformer cannot provide energy, and the DC side of the cascaded H-bridge converter lacks voltage support, resulting in the HUPFC being unable to start and unable to help the grid achieve off-grid startup, and can only wait for the grid to recover before it can operate normally.

[0040] In this regard, in this embodiment, a cascaded H-bridge converter with an energy storage unit is selected, and a state switching switch group is added to adjust the connection relationship between the excitation transformer and the series transformer. When the power grid is out of power, it is switched to the off-grid start state, and the energy storage unit is used to generate a partially adjustable voltage, and then the excitation transformer is used to reversely convert the low voltage into high voltage, which can help the power grid to achieve black start and other functions in the off-grid state. In the grid-connected state, the state switching switch group connects the series transformer and the excitation transformer in series, and the hybrid power flow controller system can continue to assume the function of phase adjustment.

[0041] In the embodiment of the present invention, when the power grid is out of power, the state switching switch group is switched to the off-grid starting state, the connection relationship between the series transformer and the excitation transformer is changed, and then a partially adjustable voltage is generated through the energy storage unit of the cascaded H-bridge converter, and the low voltage is reversely converted into high voltage using the excitation transformer, which can help the power grid to achieve black start and other functions in the off-grid state. In the grid-connected state, the series transformer is connected in series with the excitation transformer, and can continue to assume the phase adjustment function, and has a flexible power grid regulation function.

[0042] In a possible implementation, the state switching switch group includes a first switch, a second switch, and a third switch;

[0043] The first end of the A phase of the secondary side of the series transformer is connected to the active end of the A phase of the secondary side of the excitation transformer through the first switch, the first end of the B phase of the secondary side of the series transformer is connected to the active end of the B phase of the secondary side of the excitation transformer through the second switch, and the first end of the C phase of the secondary side of the series transformer is connected to the active end of the C phase of the secondary side of the excitation transformer through the third switch;

[0044] When the state switching switch group is in the off-grid startup state, the first switch, the second switch and the third switch are disconnected;

[0045] When the state switching switch group is in the grid-connected state, the first switch, the second switch and the third switch are closed.

[0046] In this embodiment, the internal structure of the state switching switch group and the connection relationship with the series transformer and the excitation transformer are as follows: Figure 2As shown. The control system of the system mainly includes a state switching switch group control module, a PST gear and CHB coordinated control module, a battery charging and discharging control module, a DC voltage average value control module, a PLL phase-locked module, an ABC phase DC voltage average value control module, and an ABC phase bridge arm SOC balancing control module. The state switching switch group control module controls the switch state of each switch, and the first switch K1, the second switch K2, and the third switch K3 are series switches.

[0047] One end of K1, K2, and K3 is connected to the secondary side of the series transformer, and the other end is connected to the secondary side of the excitation transformer. When in the off-grid starting state, the K1, K2, and K3 switches are disconnected, and the secondary side of the series transformer does not participate in the operation;

[0048] When in the grid-connected state, the K1, K2, and K3 switches are closed. At this time, the cascaded H-bridge converter and the secondary side of the excitation transformer are connected in series and connected to the secondary side of the series transformer. The HUPFC is in the grid-connected operation state.

[0049] In a possible implementation, the state switching switch group further includes a fourth switch and a fifth switch;

[0050] The active end of the A phase of the secondary side of the excitation transformer is connected to the active end of the B phase through the fourth switch, and the active end of the B phase is connected to the active end of the C phase through the fifth switch;

[0051] When the state switching switch group is in the off-grid startup state, the fourth switch and the fifth switch are closed;

[0052] When the state switching switch group is in the grid-connected state, the fourth switch and the fifth switch are disconnected.

[0053] In this embodiment, if Figure 2 As shown, K4 and K5 are parallel switches, one end of K4 is connected to the secondary side of the excitation transformer phase A, and the other end is connected to the secondary side of the excitation transformer phase B; one end of K5 is connected to the secondary side of the excitation transformer phase A, and the other end is connected to the secondary side of the excitation transformer phase B. When the K4 and K5 switches are closed, the secondary side of the excitation transformer forms a Y-type connection, and the secondary side of the excitation transformer is connected to the cascaded H-bridge converter, acting as the load of the cascaded H-bridge converter. When the K4 and K5 switches are open, the excitation transformer is directly connected in series with the series transformer.

[0054] In a possible implementation, it also includes a battery charging and discharging control module and a DC voltage average value control module;

[0055] When the state switching switch group is in the off-grid starting state, the output voltage of the battery charge and discharge control module is used as the reference value of the DC voltage average value control module, and the output of the DC voltage average value control module is used as the output voltage d-axis reference value of the cascaded H-bridge converter.

[0056] In this embodiment, Figure 3 The output voltage reference value dq coordinate system reference value V d 、V q and angle θ determine the output of the cascaded H-bridge converter.

[0057] When in off-grid startup state, V d Connected to signal C3, C3 comes from the DC voltage average value control module, and its specific control method is: according to the output V of the battery charging and discharging module B_ref Determine the reference value of the DC voltage, and lower V when the output power needs to be increased. B_ref The capacitor voltage decreases, and the energy injected into the capacitor from the battery increases. In order to maintain the constant capacitor voltage, V needs to be increased. d On the contrary, when the output power needs to be reduced, increase V B_ref The energy injected into the capacitor from the battery decreases. In order to maintain the constant voltage of the capacitor, it is necessary to reduce V d During the off-grid startup process, V B_ref value, thereby continuously increasing the output power until V d The value reaches the rated value.

[0058] In a possible implementation, the output voltage q-axis reference value of the cascaded H-bridge converter is 0, and the output phase angle reference value is determined based on the frequency of the synchronous motor.

[0059] In this embodiment, if Figure 3 As shown, when in off-grid startup state, V q Connected to signal C2, V q is controlled to 0, θ is connected to signal C5, using an internally generated phase signal.

[0060] In addition, Va_bal is a parameter used to control the balance of the DC bus between bridge arms, and Va_1_SOC is a parameter used to control the balance of the SOC state of the battery inside each module.

[0061] In a possible implementation, it also includes a PST gear and CHB coordinated control module, a battery charge and discharge control module, and a DC voltage average value control module;

[0062] When the state switching switch group is in the grid-connected state, the output of the PST gear and CHB collaborative control module is used as the output voltage d-axis reference value of the cascaded H-bridge converter, the output voltage of the battery charge and discharge control module is used as the reference value of the DC voltage average value control module, and the output of the DC voltage average value control module is used as the output voltage q-axis reference value of the cascaded H-bridge converter. The output phase angle reference value of the cascaded H-bridge converter is determined based on the PLL phase-locked module.

[0063] In this embodiment, if Figure 3 As shown, when in grid-connected operation, V d Connected to signal C1, C1 comes from the PST gear and CHB collaborative control module, which adjusts V according to the grid phase modulation requirements during operation. d Value. q Connected to signal C3, the battery is in standby mode and does not output power to the grid. V q The size of ensures that the input power of the cascaded H-bridge converter is equal to its power loss, thereby ensuring the constant DC voltage.

[0064] In a possible implementation, the secondary side of the series transformer adopts a triangle connection mode, and the vertex outgoing line of the triangle is connected to the state switching switch group.

[0065] In this embodiment, in the delta connection mode, one end of each winding of the transformer is connected to the other end of the adjacent winding to form a closed triangle shape, and the line voltage is equal to the phase voltage.

[0066] In a possible implementation, the cascaded H-bridge converter includes a plurality of single-phase full-bridge converters, each of which includes four fully-controlled switching devices, one capacitor, one inductor, and one battery pack.

[0067] In this embodiment, the cascaded H-bridge converter is a multi-level converter, which is formed by cascading multiple H-bridge converter units to achieve higher voltage levels and better output voltage waveforms. The battery pack, as the energy storage unit of the cascaded H-bridge converter, can store the electric energy provided by the power grid when the power grid is operating normally, and supply power to the cascaded H-bridge converter when a power outage occurs in the power grid, thereby providing electric energy to the excitation transformer and the series transformer, helping the power grid to complete operations such as black start.

[0068] As can be seen from the above, the hybrid power flow controller system and control method with off-grid starting function proposed in the present invention adopts a cascaded H-bridge structure as the converter of the HUPFC. The DC side of the H-bridge is connected to the power grid, and a state switching switch group is added. It is isolated from the series transformer during off-grid operation and connected in series with the series transformer during grid-connected operation. The cascaded H-bridge converter can generate an adjustable voltage function through the energy storage unit part, and the excitation transformer is used to reversely convert the low voltage into high voltage, which can help the power grid achieve black start and other functions in the off-grid state. At the control level, a variety of state switching control strategies are constructed. When in the off-grid startup state, K1, K2, and K3 in the state switching switch group are disconnected, and K4 and K5 are closed. The Vd in the control loop comes from the DC voltage average value control module to ensure the input and output energy balance. The angle uses the internally generated angle, and Vq is set to 0; when in the grid-connected state, Vd comes from the PST gear and CHB collaborative control module, Vq comes from the DC voltage average value control module, and the angle uses the phase-locked loop output result, which can allow HUPFC to assist in black start in the off-grid state and continue to assume the phase adjustment function in the grid-connected state.

[0069] It should be understood that the order of execution of the steps in the above embodiment does not necessarily mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiment of the present invention.

[0070] The following is an embodiment of the device of the present invention. For details not described in detail therein, reference may be made to the corresponding method embodiment described above.

[0071] Figure 4 The following is a flowchart showing a control method for a hybrid power flow controller system with an off-grid startup function provided by an embodiment of the present invention. For ease of explanation, only the part related to the embodiment of the present invention is shown, which is described in detail as follows:

[0072] like Figure 4 As shown, the system includes a series transformer, a state switching switch group, an excitation transformer and a cascade H-bridge converter;

[0073] The two ends of the primary side of the series transformer are connected to the power grid, the active end is connected to the primary side of the excitation transformer, the secondary side is connected to the active end of the secondary side of the excitation transformer through a state switching switch group, the fixed end of the secondary side of the excitation transformer is connected to the cascade H-bridge converter, and the cascade H-bridge converter has an energy storage unit;

[0074] The control method of the hybrid power flow controller system with off-grid starting function includes:

[0075] Step 401, the state switching switch group switches to the off-grid starting state when the power grid is out of power, so that the series transformer is isolated from the excitation transformer, and the cascaded H-bridge converter supplies power to the excitation transformer through the energy storage unit, and switches to the grid-connected state when the power grid is operating normally, so that the series transformer is connected in series with the excitation transformer.

[0076] In a possible implementation, the state switching switch group includes a first switch, a second switch, and a third switch;

[0077] The first end of the A phase of the secondary side of the series transformer is connected to the active end of the A phase of the secondary side of the excitation transformer through the first switch, the first end of the B phase of the secondary side of the series transformer is connected to the active end of the B phase of the secondary side of the excitation transformer through the second switch, and the first end of the C phase of the secondary side of the series transformer is connected to the active end of the C phase of the secondary side of the excitation transformer through the third switch;

[0078] When the state switching switch group is in the off-grid startup state, the first switch, the second switch and the third switch are disconnected;

[0079] When the state switching switch group is in the grid-connected state, the first switch, the second switch and the third switch are closed.

[0080] In a possible implementation, the state switching switch group further includes a fourth switch and a fifth switch;

[0081] The active end of the A phase of the secondary side of the excitation transformer is connected to the active end of the B phase through the fourth switch, and the active end of the B phase is connected to the active end of the C phase through the fifth switch;

[0082] When the state switching switch group is in the off-grid startup state, the fourth switch and the fifth switch are closed;

[0083] When the state switching switch group is in the grid-connected state, the fourth switch and the fifth switch are disconnected.

[0084] In a possible implementation, the system further includes a battery charging and discharging control module and a DC voltage average value control module;

[0085] The method further includes:

[0086] When the state switching switch group is in the off-grid starting state, the output voltage of the battery charge and discharge control module is used as the reference value of the DC voltage average value control module, and the output of the DC voltage average value control module is used as the output voltage d-axis reference value of the cascaded H-bridge converter.

[0087] In a possible implementation, the method further includes:

[0088] The output voltage q-axis reference value of the cascaded H-bridge converter is 0, and the output phase angle reference value is determined based on the frequency of the synchronous motor.

[0089] In a possible implementation, the system further includes a PST gear and CHB coordinated control module, a battery charge and discharge control module, and a DC voltage average value control module;

[0090] The method further includes:

[0091] When the state switching switch group is in the grid-connected state, the output of the PST gear and CHB collaborative control module is used as the output voltage d-axis reference value of the cascaded H-bridge converter, the output voltage of the battery charge and discharge control module is used as the reference value of the DC voltage average value control module, and the output of the DC voltage average value control module is used as the output voltage q-axis reference value of the cascaded H-bridge converter. The output phase angle reference value of the cascaded H-bridge converter is determined based on the PLL phase-locked module.

[0092] In a possible implementation, the secondary side of the series transformer adopts a triangle connection mode, and the vertex outgoing line of the triangle is connected to the state switching switch group.

[0093] In a possible implementation, the cascaded H-bridge converter includes a plurality of single-phase full-bridge converters, each of which includes four fully-controlled switching devices, one capacitor, one inductor, and one battery pack.

[0094] In the embodiment of the present invention, when the power grid is out of power, the state switching switch group is switched to the off-grid starting state, the connection relationship between the series transformer and the excitation transformer is changed, and then a partially adjustable voltage is generated through the energy storage unit of the cascaded H-bridge converter, and the low voltage is reversely converted into high voltage using the excitation transformer, which can help the power grid to achieve black start and other functions in the off-grid state. In the grid-connected state, the series transformer is connected in series with the excitation transformer, and can continue to assume the phase adjustment function, and has a flexible power grid regulation function.

[0095] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0096] Those of ordinary skill in the art will appreciate that the templates, units, and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0097] If the module / unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the present invention implements all or part of the processes in the above-mentioned embodiment method, and can also be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium. When the computer program is executed by the processor, it can implement the steps of the above-mentioned control method embodiments of the hybrid power flow controller system with off-grid startup function. Among them, the computer program includes computer program code, and the computer program code can be in source code form, object code form, executable file or some intermediate form. The computer-readable medium may include: any entity or device that can carry the computer program code, recording medium, U disk, mobile hard disk, disk, optical disk, computer memory, read-only memory, random access memory, electrical carrier signal, telecommunication signal and software distribution medium.

[0098] The embodiments described above 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 may be replaced by equivalents. Such 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, and should all be included in the protection scope of the present invention.

Claims

1. A hybrid power flow controller system with off-grid startup function, characterized in that: It includes a series transformer, a state switching switch group, an excitation transformer and a cascade H-bridge converter; The two ends of the primary side of the series transformer are connected to the power grid, the active end is connected to the primary side of the excitation transformer, the secondary side is connected to the active end of the secondary side of the excitation transformer through the state switching switch group, the fixed end of the secondary side of the excitation transformer is connected to the cascaded H-bridge converter, and the cascaded H-bridge converter has an energy storage unit; The state switching switch group is used to switch to an off-grid starting state when the power grid is out of power, so as to isolate the series transformer from the excitation transformer. The cascaded H-bridge converter supplies power to the excitation transformer through the energy storage unit, and switches to a grid-connected state when the power grid is operating normally, so as to connect the series transformer to the excitation transformer in series.

2. The hybrid power flow controller system with off-grid startup function according to claim 1, characterized in that: The state switching switch group includes a first switch, a second switch and a third switch; The first end of the A phase of the secondary side of the series transformer is connected to the active end of the A phase of the secondary side of the excitation transformer through the first switch, the first end of the B phase of the secondary side of the series transformer is connected to the active end of the B phase of the secondary side of the excitation transformer through the second switch, and the first end of the C phase of the secondary side of the series transformer is connected to the active end of the C phase of the secondary side of the excitation transformer through the third switch; When the state switching switch group is in an off-grid startup state, the first switch, the second switch and the third switch are disconnected; When the state switching switch group is in a grid-connected state, the first switch, the second switch and the third switch are closed.

3. The hybrid power flow controller system with off-grid startup function according to claim 2, characterized in that: The state switching switch group also includes a fourth switch and a fifth switch; The active end of the A phase of the secondary side of the excitation transformer is connected to the active end of the B phase through the fourth switch, and the active end of the B phase is connected to the active end of the C phase through the fifth switch; When the state switching switch group is in the off-grid startup state, the fourth switch and the fifth switch are closed; When the state switching switch group is in the grid-connected state, the fourth switch and the fifth switch are disconnected.

4. The hybrid power flow controller system with off-grid startup function according to claim 1, characterized in that: It also includes the battery charge and discharge control module and the DC voltage average value control module; When the state switching switch group is in the off-grid startup state, the output voltage of the battery charge and discharge control module is used as the reference value of the DC voltage average value control module, and the output of the DC voltage average value control module is used as the output voltage d-axis reference value of the cascaded H-bridge converter.

5. The hybrid power flow controller system with off-grid startup function according to claim 4, characterized in that: The output voltage q-axis reference value of the cascaded H-bridge converter is 0, and the output phase angle reference value is determined based on the frequency of the synchronous motor.

6. The hybrid power flow controller system with off-grid startup function according to claim 1, characterized in that: It also includes the PST gear and CHB collaborative control module, the battery charge and discharge control module and the DC voltage average value control module; When the state switching switch group is in the grid-connected state, the output of the PST gear and CHB collaborative control module is used as the output voltage d-axis reference value of the cascaded H-bridge converter, the output voltage of the battery charge and discharge control module is used as the reference value of the DC voltage average value control module, and the output of the DC voltage average value control module is used as the output voltage q-axis reference value of the cascaded H-bridge converter. The output phase angle reference value of the cascaded H-bridge converter is determined based on the PLL phase-locked module.

7. The hybrid power flow controller system with off-grid startup function according to claim 1, characterized in that: The secondary side of the series transformer adopts a triangle connection mode, and the vertex outgoing line of the triangle is connected to the state switching switch group.

8. The hybrid power flow controller system with off-grid startup function according to claim 1, characterized in that: The cascaded H-bridge converter includes a plurality of single-phase full-bridge converters, each of which includes four fully-controlled switch devices, one capacitor, one inductor and one battery pack.

9. A control method for a hybrid power flow controller system with off-grid startup function, characterized in that: The system comprises a series transformer, a state switching switch group, an excitation transformer and a cascaded H-bridge converter; The two ends of the primary side of the series transformer are connected to the power grid, the active end is connected to the primary side of the excitation transformer, the secondary side is connected to the active end of the secondary side of the excitation transformer through the state switching switch group, the fixed end of the secondary side of the excitation transformer is connected to the cascaded H-bridge converter, and the cascaded H-bridge converter has an energy storage unit; The method comprises: The state switching switch group switches to an off-grid starting state when the power grid fails, so as to isolate the series transformer from the excitation transformer. The cascaded H-bridge converter supplies power to the excitation transformer through the energy storage unit, and switches to a grid-connected state when the power grid operates normally, so as to connect the series transformer to the excitation transformer in series.

10. The control method of the hybrid power flow controller system with off-grid startup function according to claim 9, characterized in that: The state switching switch group includes a first switch, a second switch and a third switch; The first end of the A phase of the secondary side of the series transformer is connected to the active end of the A phase of the secondary side of the excitation transformer through the first switch, the first end of the B phase of the secondary side of the series transformer is connected to the active end of the B phase of the secondary side of the excitation transformer through the second switch, and the first end of the C phase of the secondary side of the series transformer is connected to the active end of the C phase of the secondary side of the excitation transformer through the third switch; When the state switching switch group is in an off-grid startup state, the first switch, the second switch and the third switch are disconnected; When the state switching switch group is in a grid-connected state, the first switch, the second switch and the third switch are closed.

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