Doubly-fed converter with slow shutdown function, wind power generation system and shutdown method

Abstract

The invention belongs to the technical field of power electronics, and provides a doubly-fed converter with a slow shutdown function, a wind power generation system and a shutdown method. A direct-current side energy storage circuit is arranged between a grid-side alternating-current circuit and a machine-side alternating-current circuit; when the power grid load is lost, controlling the grid side alternating current circuit to enter a grid construction voltage source mode, and controlling the machine side alternating current circuit to enter a grid following current source mode; the inertia and the maximum safety speed of the rotor are used as boundaries, the maximum energy capable of being absorbed by the rotor is given, and while the slip power of the rotor is increased, the whole machine power is distributed to a machine side converter; and the direct current side energy storage circuit is controlled to dynamically distribute the residual capacity and store or consume the energy, so that slow shutdown can be realized after the fan system loses the load, the torque and power impact caused by sudden load loss due to a fault and sudden off-grid of the fan system is reduced, a transmission chain system is protected, and the fan load is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a double-fed converter with a slow shutdown function, a wind power generation system and a shutdown method. Background Art

[0002] With the development of wind power conversion systems, doubly-fed converter systems have become the first choice for the wind power industry due to their small capacity, low cost and good economy. Currently, traditional wind power conversion systems do not have good countermeasures when facing a single unit disconnection from the grid.

[0003] When a wind turbine is suddenly disconnected from the grid due to an external fault, the loss of load will cause a huge torque shock to the wind turbine, causing the unit to vibrate, and have a significant impact on the life of the blades and mechanical transmission system. The current doubly-fed converter cannot achieve a slow shutdown when the load is suddenly lost, and cannot effectively eliminate the impact of unit vibration. In addition, the DC side circuit in the traditional converter system is complex and inconvenient to control. Summary of the invention

[0004] In order to solve the above problems, the present invention proposes a doubly-fed converter with a slow shutdown function, a wind power generation system and a shutdown method. When the grid load is lost, the grid-side AC circuit is controlled to enter a grid-forming voltage source mode, and the machine-side AC circuit is controlled to enter a grid-following current source mode; the rotor's own inertia and the maximum safe speed are used as boundaries to give the maximum energy that the rotor can absorb, and while the rotor slip power increases, the whole machine power is distributed to the machine-side converter; the DC-side energy storage circuit is controlled to dynamically distribute the remaining capacity to store or consume energy, so that the fan system can be slowly shut down after losing the load, reducing the torque and power impact caused by the sudden loss of load due to a fault and the sudden disconnection of the fan system from the grid, protecting the transmission chain system, and reducing the fan load.

[0005] In order to achieve the above-mentioned object, in a first aspect, the present invention provides a doubly-fed converter with a slow shutdown function, which adopts the following technical solution:

[0006] A doubly-fed converter with a slow shutdown function, comprising a grid-side AC circuit, a machine-side AC circuit, and a DC-side energy storage circuit arranged between the grid-side AC circuit and the machine-side AC circuit;

[0007] The grid-side AC circuit, the machine-side AC circuit and the DC-side energy storage circuit are connected to a converter controller; the converter controller is configured to: when the grid load is lost, control the grid-side AC circuit to enter a grid-forming voltage source mode, and control the machine-side AC circuit to enter a grid-following current source mode; use the rotor's own inertia and the maximum safe speed as boundaries to give the maximum energy that the rotor can absorb, and while the rotor slip power increases, distribute the entire machine power to the machine-side converter; control the DC-side energy storage circuit to dynamically distribute the remaining capacity to store or consume energy.

[0008] Furthermore, the grid-side AC circuit is also connected to a grid-connected functional circuit, and the grid-connected functional circuit includes a main transformer and a ring main cabinet.

[0009] Furthermore, the DC side energy storage circuit includes a filter reactor, a contactor, a fuse, a first power unit, a second power unit, a detection unit resistor and an energy storage super capacitor.

[0010] Furthermore, the generator-side AC circuit and the grid-side AC circuit are respectively connected to the stator and rotor sides of the doubly-fed generator.

[0011] In order to achieve the above object, in a second aspect, the present invention further provides a wind power generation system, which adopts the following technical solution:

[0012] A method for shutting down a wind power generation system, using the doubly-fed converter with a slow shutdown function as described in the first aspect, comprising:

[0013] When the grid load is lost, the grid-side AC circuit is controlled to enter the grid-forming voltage source mode, and the machine-side AC circuit is controlled to enter the grid-following current source mode; the rotor's own inertia and the maximum safe speed are used as boundaries to give the maximum energy that the rotor can absorb, and while the rotor slip power increases, the whole machine power is distributed to the machine-side converter; the DC-side energy storage circuit is controlled to dynamically distribute the remaining capacity to store or consume energy.

[0014] Furthermore, the DC side voltage control is designed by piecewise function; the DC side voltage U dc for:

[0015]

[0016] Assume T ready The generator rotor accelerates from the pre-fault speed W to the safe speed W max The time for the converter system mode switching to enter the slow stop mode can be obtained by:

[0017]

[0018] DC side reference voltage command Udc_ref for:

[0019] ΔU grid =U grid -U n ,

[0020]

[0021] Among them, W rotor is the kinetic energy stored in the rotor; J is the generator rotor inertia; W max is the maximum speed of the generator; W cut is the cut-out speed of the wind turbine; W R is the energy consumed by the resistor; R is the resistance value; U dc is the DC side voltage; ΔT is the downtime; k1 and k2 are the distribution coefficients; P super P is the super electric power; R is the resistance power; P n is the rated power of the unit; E super The supercapacitor can absorb energy; E R The energy that the resistor can absorb; E rotor is the energy that the generator rotor can absorb, w0 is the rated speed of the generator; ΔU grid is the voltage difference between the detection voltage and the rated voltage; U grid is the grid-side converter voltage; U n is the rated voltage; ΔU pu is the voltage difference per unit value; U dc_ref It is the DC side reference voltage instruction.

[0022] Furthermore, when the wind turbine generator set suddenly loses the grid load, it manifests as the main circuit breaker in the converter cabinet being disconnected; the wind turbine controller and the converter controller respectively set the slow shutdown flag position and transmit the slow shutdown flag position to each other. When the wind turbine controller receives the setting signal from the converter controller, it starts to control the blade speed, which does not exceed the maximum safe speed, and the rotor starts to absorb energy using its own inertia; when it is detected that the main circuit breaker is in the disconnected state without giving a disconnection command, the generator stator grid-connected contactor is disconnected;

[0023] The converter controller detects whether a fault occurs. If there is no fault, the grid-side AC circuit operates in the grid-forming voltage source mode, showing voltage source characteristics to the outside, and the generator-side AC circuit continues to operate in the grid-following current source mode, establishing the generator stator voltage by giving the rotor no-load excitation current; at the same time, the converter controller controls the DC-side energy storage circuit to operate in the boost mode to provide DC voltage for the DC side;

[0024] Determine whether the stator voltage is synchronized with the voltage established on the grid side. After synchronization, issue a grid-connected command.

[0025] Furthermore, the DC side energy storage circuit operates in a constant current charging mode; the converter and the fan cooperate to slowly reduce the torque and speed according to the inertia shutdown curve until the feedback torque is detected to be 0 and the shutdown ends.

[0026] Furthermore, the ring main unit switch is disconnected, the DC side energy storage circuit starts to work in the boost mode, the DC side contactor is closed, and the second power unit is forced to be in the disconnected state; the DC side voltage is established, and when the grid side AC circuit detects that the DC side voltage is stable, it starts to start and work in the inverter state, and at the same time locks the machine side AC circuit; power is provided to the load until the button is detected to be restored and the fan mode is entered.

[0027] In order to achieve the above-mentioned purpose, in a third aspect, the present invention further provides a method for shutting down a wind power generation system, which adopts the following technical solution:

[0028] A wind power generation system uses the doubly-fed converter with a slow shutdown function as described in the first aspect.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. In the present invention, a DC side energy storage circuit is arranged between the grid side AC circuit and the machine side AC circuit; when the grid load is lost, the grid side AC circuit is controlled to enter the grid voltage source mode, and the machine side AC circuit is controlled to enter the grid current source mode; the rotor's own inertia and the maximum safe speed are used as boundaries to give the maximum energy that the rotor can absorb, and while the rotor slip power increases, the whole machine power is distributed to the machine side converter; the DC side energy storage circuit is controlled to dynamically distribute the remaining capacity to store or consume energy, so that the fan system can be shut down slowly after losing the load, reducing the torque and power impact caused by the sudden loss of load due to a fault and the sudden disconnection of the fan system from the grid, protecting the transmission chain system, and reducing the fan load.

[0031] 2. The present invention can assist the design of a converter system for the whole machine to perform a slow shutdown when the fan suddenly loses its load; and by optimizing the functions of the current traditional converter system, the operation is convenient and safer.

[0032] 3. In the present invention, when the grid load is lost, the grid-side converter enters the grid voltage source mode to establish a stable voltage, and the machine-side converter enters the grid current source mode to provide torque that meets the demand. The DC side serves as an energy consumption and storage functional unit, and the elastic distribution design of the chopper resistor and the energy storage supercapacitor is carried out according to the actual situation. Through the DC-DC design of the DC side, it is composed of a filter reactor, a contactor, a fuse, a power unit and a detection unit. The DC side circuit is simple, easy to control and stable. At the same time, a calculation method for the DC side components is provided to facilitate the selection of device parameters. The converter function of the current high-power top-mounted wind turbine generator set is optimized to solve the complex operation and safety problems of shutdown and maintenance. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings in the specification that constitute a part of this embodiment are used to provide a further understanding of this embodiment. The schematic embodiments of this embodiment and their descriptions are used to explain this embodiment and do not constitute improper limitations on this embodiment.

[0034] Figure 1 is a schematic diagram of a system circuit of Embodiment 1 of the present invention;

[0035] Figure 2 This is a logic diagram of the control method of Embodiment 1 of the present invention;

[0036] Figure 3 This is a timing diagram of the function optimization operation of Embodiment 1 of the present invention;

[0037] Figure 4 This is a logic diagram of the control method of Example 2 of the present invention. DETAILED DESCRIPTION

[0038] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0039] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0040] Wind power converter is an excitation device added to the rotor side of a doubly-fed wind turbine generator. Its main function is to control the amplitude, phase, frequency, etc. of the excitation through the converter when the rotor speed changes, so that the stator side can input constant frequency electricity to the power grid.

[0041] Single-machine disconnection from the grid: Single-machine disconnection from the grid is mainly caused by the wind turbine unit’s lack of low voltage ride-through capability, insufficient reactive power regulation capability, and the grid’s adaptability not meeting normal operation requirements.

[0042] Rotor slip power, in an induction motor, refers to the electromagnetic power transmitted to the rotor, part of which becomes rotor copper loss and the other part is converted into mechanical power.

[0043] Embodiment 1:

[0044] When a wind turbine is suddenly disconnected from the grid due to an external fault, the loss of load will cause a huge torque shock to the wind turbine, causing the unit to vibrate. This has a significant impact on the life of the blades and mechanical transmission system. The current design of high-power wind turbines generally requires the transformer and converter to be placed in the cabin. When the cabin is inspected, the 35kv ring main cabinet needs to be closed, and the 35kv / 1140 / 400V transformer needs to be energized and the converter needs to be working; and when the maintenance personnel go to the cabin and start to inspect the equipment, the above system must be powered off again, which is cumbersome and unsafe.

[0045] In order to solve at least one of the above problems, the present embodiment provides a doubly-fed converter with a slow shutdown function, which is a converter system design that can assist the entire machine in slow shutdown when the wind turbine suddenly loses load; and by optimizing the functions of the current traditional converter system, it makes operation convenient and safer.

[0046] The doubly-fed converter provided in this embodiment reduces the torque and power impact caused by the sudden loss of load due to a fault and the sudden disconnection of the wind turbine system from the grid, protects the transmission chain system, and reduces the wind turbine load. This embodiment mainly includes the topology design of the converter circuit, the control strategy and the calculation method, which can achieve a slow shutdown of the wind turbine system after the wind turbine system loses its load, optimizes the functions of the current traditional large-megawatt wind turbine set, and reduces the operational complexity of auxiliary power consumption.

[0047] like Figure 1 As shown, the doubly-fed converter with slow shutdown function includes a grid-side AC circuit, a DC-side energy storage circuit, a DC-side energy storage circuit and a grid-connected function circuit. The grid-side AC circuit is the grid-side conversion part of the converter, and the DC-side energy storage circuit is the machine-side conversion part of the converter. Optionally, the grid is connected to a 35kv ring main unit, the ring main unit is connected to a 35kv / 1140V / 400V main transformer, the main transformer is connected to the converter, and the machine-side conversion and grid-side conversion parts of the converter are respectively connected to the stator and rotor sides of the doubly-fed generator.

[0048] Specifically, when the grid load is lost, the grid-side converter enters the grid voltage source mode to establish a stable voltage; the machine-side converter enters the grid current source mode to provide torque that meets the demand. The DC side energy storage circuit serves as an energy consumption and storage functional unit, and the chopper resistor (R) and the energy storage supercapacitor (C_sp) are flexibly allocated and designed according to actual conditions. The DC side energy storage circuit provided in this embodiment has a simple structure, convenient control, and stability.

[0049] Optionally, the DC side energy storage circuit includes a filter reactor (L_dc), a contactor (K_dc), a fuse (FU), a first power unit (G1_dc), a second power unit (G2_dc) and a detection unit (TA / TV), etc. The circuit is simple and easy to control. A calculation method for DC side components is provided to facilitate the selection of device parameters. The energy consumption channel is mainly divided into three parts, and the rotor's own inertia and the maximum safe speed are used as boundaries to give the maximum energy that the rotor can absorb. While the rotor slip power increases, the backup capacity of the machine-side converter can be fully tapped. The machine-side converter allocates as much power as possible to the whole machine, which is conducive to mitigating the transient current and power impact of the fault. The remaining capacity is dynamically allocated by the chopper resistor and the supercapacitor (C_sp). Whether the energy is stored or consumed, the ratio between them will affect the selection of parameters. At the same time, considering the instability of transient power, voltage and current, in order to stabilize the control system and ensure the voltage modulation capability of the machine-grid side, the DC side voltage control is designed in a piecewise function manner.

[0050] like Figure 2 As shown, when the wind turbine generator set suddenly loses the grid load due to a fault or other reasons, the main circuit breaker Q1 in the converter cabinet is disconnected. From then on, the entire system enters the slow shutdown process. First, the wind turbine controller and the converter controller respectively set the slow shutdown flag position, and transmit the slow shutdown flag wind_Offload_StopMode=1, Dfig_Offload_StopMode=1 to each other. When the wind turbine controller receives the set signal Dfig_Offload_StopMode=1 from the converter controller, it starts to control the blade speed, and the maximum does not exceed the safe speed ω max , the rotor uses its own inertia to start absorbing energy, and this provides a certain buffer time for the converter control system to switch modes, which facilitates smooth switching of the system. When it is detected that Q1 is in a disconnected state without giving a disconnection command, the converter system first disconnects the generator stator grid-connected contactor K1 to prepare for the subsequent automatic establishment of an energy circulation channel. The converter controller detects whether a fault occurs. In the absence of a fault, the grid-side converter operates in the grid-building voltage source mode, builds a 1140V voltage, and presents a voltage source characteristic to the outside. The machine-side converter continues to operate in the grid-following current source mode, and establishes the generator stator voltage by giving the rotor no-load excitation current. At the same time, the converter controller controls the DC-DC circuit on the DC side and operates in the Boost boost mode, with the aim of providing a stable DC voltage for the DC side. And through DC-DC control, Udc is stabilized at 1800V. The next step is to enter pre-synchronization, and the system determines whether the stator voltage is synchronized with the voltage established on the grid side. After synchronization, a grid-connected command is issued to make K1_cmd=1.

[0051] The entire generator power (the sum of stator power and rotor power) enters the DC side. The DC side uses a hysteresis loop to stabilize the DC side voltage, and the upper and lower voltage limits are set in a segmented design. At the same time, the DC-DC works in buck step-down mode. The DC-DC circuit works in constant current charging mode to facilitate fast charging. Next, the converter and the fan cooperate to slowly reduce the torque and speed according to the inertia shutdown curve until the feedback torque is detected to be T_fd=0 and the shutdown ends.

[0052] Optionally, the DC side uses a hysteresis loop to stabilize the DC side voltage, and the upper and lower limit voltages are set using a segmented design. The calculation and setting methods are as follows:

[0053] Definition: The rated power of the unit is: Pn, the resistance of the Chooper resistor is: R, and the energy that the Chopper resistor can absorb is: E R , the energy that the supercapacitor can absorb is: E supper , the energy that the generator rotor can absorb is: ΔE rotor The downtime is: ΔT, where J is the generator rotor inertia, the maximum speed of the generator, the rated speed (rad / s), and the cut-out speed of the wind turbine are: W max , W0, W cut .

[0054] In order to dynamically allocate capacity and facilitate subsequent expansion design, the allocation coefficients are set as K1 and K2. The DC side voltage U dc for:

[0055]

[0056] Assume T ready The generator rotor accelerates from the pre-fault speed W to the safe speed W max The time for the converter system mode switching to enter the slow stop mode can be obtained by:

[0057]

[0058] In order to stabilize control during transient process and improve the voltage modulation capability of the converter, the DC side command voltage is designed in a segmented manner, where U grid is the grid-side converter voltage, U dc_ref is the hysteresis center command value, and the hysteresis width is flexibly set according to the control capability. n : Rated voltage, ΔU grid : The voltage difference between the detection voltage and the rated voltage, ΔU pu Voltage drop per unit value, U dc_ref : DC side reference voltage command:

[0059] ΔU grid =Ugrid -U n ,

[0060]

[0061] In some embodiments, Figure 3 As shown, when the entire fan system is shut down and there are tasks such as cabin maintenance, the auxiliary button is pressed and the system enters the auxiliary mode. To avoid the grid side voltage from entering by mistake, disconnect the ring main unit switch K2_cmd=0. First, the DC-DC starts working in the Boost boost mode, closes the DC side contactor KM1_cmd=1, and forces G2_dc to be in the disconnected state; establishes a stable DC side voltage, and when the grid side converter detects that the DC side voltage is stable, it starts and works in the inverter state, and at the same time locks the machine side converter Conv_rotor_start=0. Provide a stable AC1140V power supply for the load. Until the button recovery is detected, the system enters the fan mode, Dfig_mode=1. The fan is always in standby mode and ready to start at any time.

[0062] Embodiment 2:

[0063] This embodiment provides a doubly-fed converter with a slow shutdown function, which is different from the first embodiment in that: Figure 4 As shown, when the wind turbine generator set suddenly loses the grid load due to a fault or other reasons, the main circuit breaker Q1 in the converter cabinet is disconnected. From then on, the entire system enters the slow shutdown process. First, the wind turbine controller and the converter controller respectively set the slow shutdown flag position, and transmit the slow shutdown flag wind_Offload_StopMode=1, Dfig_Offload_StopMode=1 to each other. When the wind turbine controller receives the set signal Dfig_Offload_StopMode=1 from the converter controller, it starts to control the blade speed, and the maximum does not exceed the safe speed ω max , the rotor uses its own inertia to absorb energy, and provides a certain buffer time for the converter control system to switch modes, which facilitates smooth switching of the system. When it is detected that Q1 is in a disconnected state without giving a disconnection command, the machine-side converter operates in the grid voltage source mode, builds a 1140V voltage, and presents voltage source characteristics to the outside. The grid-side converter continues to operate in the grid current source mode. At the same time, the converter controller controls the DC-DC circuit on the DC side and operates in the Boost mode to provide a stable DC voltage for the DC side. And through DC-DC control, Udc is stabilized at 1800V.

[0064] The entire generator power (the sum of stator power and rotor power) enters the DC side. The DC side uses a hysteresis loop to stabilize the DC side voltage, and the upper and lower voltage limits are set in a segmented design. At the same time, the DC-DC works in buck step-down mode. The DC-DC circuit works in constant current charging mode to facilitate fast charging. Next, the converter and the fan cooperate to slowly reduce the torque and speed according to the inertia shutdown curve until the feedback torque is detected to be T_fd=0 and the shutdown ends.

[0065] Embodiment 3:

[0066] A method for shutting down a wind power generation system, using the doubly-fed converter with a slow shutdown function as described in Embodiment 1 or Embodiment 2, comprising:

[0067] When the grid load is lost, the grid-side AC circuit is controlled to enter the grid-forming voltage source mode, and the machine-side AC circuit is controlled to enter the grid-following current source mode; the rotor's own inertia and the maximum safe speed are used as boundaries to give the maximum energy that the rotor can absorb, and while the rotor slip power increases, the whole machine power is distributed to the machine-side converter; the DC-side energy storage circuit is controlled to dynamically distribute the remaining capacity to store or consume energy.

[0068] Optionally, the DC side voltage control is designed using a piecewise function method; the DC side voltage U dc for:

[0069]

[0070] Assume T ready The generator rotor accelerates from the pre-fault speed W to the safe speed W max The time for the converter system mode switching to enter the slow stop mode can be obtained by:

[0071]

[0072] DC side reference voltage command U dc_ref for:

[0073] ΔU grid =U grid -U n ,

[0074]

[0075] Among them, W rotor is the kinetic energy stored in the rotor; J is the generator rotor inertia; W max is the maximum speed of the generator; W cut is the cut-out speed of the wind turbine; W R is the energy consumed by the resistor; R is the resistance value; Udc is the DC side voltage; ΔT is the downtime; k1 and k2 are the distribution coefficients; P super P is the super electric power; R is the resistance power; P n is the rated power of the unit; E super The supercapacitor can absorb energy; E R The energy that the resistor can absorb; E rotor is the energy that the generator rotor can absorb, w0 is the rated speed of the generator; ΔU grid is the voltage difference between the detection voltage and the rated voltage; U grid is the grid-side converter voltage; U n is the rated voltage; ΔU pu is the voltage difference per unit value; U dc_ref It is the DC side reference voltage instruction.

[0076] Optionally, when the wind turbine generator set suddenly loses the grid load, it manifests as the main circuit breaker in the converter cabinet being disconnected; the wind turbine controller and the converter controller respectively set the slow stop flag position, and transmit the slow stop flag position to each other. When the wind turbine controller receives the setting signal from the converter controller, it starts to control the blade speed, which does not exceed the maximum safe speed, and the rotor starts to absorb energy using its own inertia; when it is detected that the main circuit breaker is in the disconnected state without giving a disconnection command, the generator stator grid-connected contactor is disconnected;

[0077] The converter controller detects whether a fault occurs. If there is no fault, the grid-side AC circuit operates in the grid-forming voltage source mode, showing voltage source characteristics to the outside, and the generator-side AC circuit continues to operate in the grid-following current source mode, establishing the generator stator voltage by giving the rotor no-load excitation current; at the same time, the converter controller controls the DC-side energy storage circuit to operate in the boost mode to provide DC voltage for the DC side;

[0078] Determine whether the stator voltage is synchronized with the voltage established on the grid side. After synchronization, issue a grid-connected command.

[0079] Optionally, the DC side energy storage circuit operates in a constant current charging mode; the converter and the fan cooperate to slowly reduce the torque and speed according to the inertia shutdown curve until the feedback torque is detected to be 0 and the shutdown ends.

[0080] Optionally, disconnect the ring main unit switch, start the DC side energy storage circuit to work in the boost mode, close the DC side contactor, and force the second power unit to be in the disconnected state; establish the DC side voltage, and when the grid side AC circuit detects that the DC side voltage is stable, start and work in the inverter state, and lock the machine side AC circuit; provide power to the load until the button is detected to be restored and enter the fan mode.

[0081] Embodiment 4:

[0082] This embodiment provides a wind power generation system, which uses a doubly-fed converter with a slow shutdown function as described in Embodiment 1 or Embodiment 2; other settings of the wind power generation system can be implemented by conventional technologies and will not be described in detail here.

[0083] The above description is only a preferred embodiment of the present embodiment and is not intended to limit the present embodiment. For those skilled in the art, the present embodiment may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present embodiment shall be included in the protection scope of the present embodiment.

Claims

1. A doubly-fed converter with a slow shutdown function, characterized in that: It includes a grid-side AC circuit, a machine-side AC circuit, and a DC-side energy storage circuit arranged between the grid-side AC circuit and the machine-side AC circuit; The grid-side AC circuit, the machine-side AC circuit and the DC-side energy storage circuit are connected to a converter controller; the converter controller is configured to: when the grid load is lost, control the grid-side AC circuit to enter a grid-forming voltage source mode, and control the machine-side AC circuit to enter a grid-following current source mode; use the rotor's own inertia and the maximum safe speed as boundaries to give the maximum energy that the rotor can absorb, and while the rotor slip power increases, distribute the entire machine power to the machine-side converter; control the DC-side energy storage circuit to dynamically distribute the remaining capacity to store or consume energy.

2. The doubly-fed converter with slow shutdown function according to claim 1, characterized in that: The grid-side AC circuit is also connected to a grid-connected functional circuit, and the grid-connected functional circuit includes a main transformer and a ring main cabinet.

3. The doubly-fed converter with slow shutdown function according to claim 1, characterized in that: The DC side energy storage circuit includes a filter reactor, a contactor, a fuse, a first power unit, a second power unit, a detection unit resistor and an energy storage super capacitor.

4. The doubly-fed converter with slow shutdown function according to claim 1, characterized in that: The generator-side AC circuit and the grid-side AC circuit are respectively connected to the stator and rotor sides of the doubly-fed generator.

5. A method for shutting down a wind power generation system, characterized in that: A doubly-fed converter with a slow shutdown function as claimed in any one of claims 1 to 4 is used, comprising: When the grid load is lost, the grid-side AC circuit is controlled to enter the grid-forming voltage source mode, and the machine-side AC circuit is controlled to enter the grid-following current source mode; the rotor's own inertia and the maximum safe speed are used as boundaries to give the maximum energy that the rotor can absorb, and while the rotor slip power increases, the whole machine power is distributed to the machine-side converter; the DC-side energy storage circuit is controlled to dynamically distribute the remaining capacity to store or consume energy.

6. The method for shutting down a wind power generation system according to claim 5, characterized in that: The DC side voltage control is designed by piecewise function; the DC side voltage U dc for: Assume T ready The generator rotor accelerates from the pre-fault speed W to the safe speed W max The time for the converter system mode switching to enter the slow stop mode can be obtained by: DC side reference voltage command U dc_ref for: Among them, W rotor is the kinetic energy stored in the rotor; J is the generator rotor inertia; W max is the maximum speed of the generator; W cut is the cut-out speed of the wind turbine; W R is the energy consumed by the resistor; R is the resistance value; U dc is the DC side voltage; ΔT is the downtime; k1 and k2 are the distribution coefficients; P super is the electric power; P R is the resistance power; P n is the rated power of the unit; E super The supercapacitor can absorb energy; E R The energy that the resistor can absorb; E rotor is the energy that the generator rotor can absorb, w0 is the rated speed of the generator; ΔU grid is the voltage difference between the detection voltage and the rated voltage; U grid is the grid-side converter voltage; U n is the rated voltage; ΔU pu is the voltage difference per unit value; U dc_ref It is the DC side reference voltage instruction.

7. The method for shutting down a wind power generation system according to claim 5, characterized in that: When the wind turbine generator set suddenly loses the grid load, the main circuit breaker in the converter cabinet is disconnected; the wind turbine controller and the converter controller respectively set the slow stop flag position and transmit the slow stop flag position to each other. When the wind turbine controller receives the setting signal of the converter controller, it starts to control the blade speed, which does not exceed the maximum safe speed, and the rotor starts to absorb energy by its own inertia; when it is detected that the main circuit breaker is in the disconnected state without giving a disconnection command, the generator stator grid-connected contactor is disconnected; The converter controller detects whether a fault occurs. If there is no fault, the grid-side AC circuit operates in the grid-forming voltage source mode, showing voltage source characteristics to the outside, and the generator-side AC circuit continues to operate in the grid-following current source mode, establishing the generator stator voltage by giving the rotor no-load excitation current; at the same time, the converter controller controls the DC-side energy storage circuit to operate in the boost mode to provide DC voltage for the DC side; Determine whether the stator voltage is synchronized with the voltage established on the grid side. After synchronization, issue a grid-connected command.

8. The method for shutting down a wind power generation system according to claim 7, characterized in that: The DC side energy storage circuit operates in a constant current charging mode; the converter and the fan cooperate to slowly reduce the torque and speed according to the inertia shutdown curve until the feedback torque is detected to be 0 and the shutdown ends.

9. The method for shutting down a wind power generation system according to claim 5, characterized in that: Disconnect the ring main unit switch, start the DC side energy storage circuit to work in the boost mode, close the DC side contactor, and force the second power unit to be in the disconnected state; establish the DC side voltage, and when the grid side AC circuit detects that the DC side voltage is stable, it starts to start and work in the inverter state, while locking the machine side AC circuit; provide power to the load until the button is detected to be restored and enter the fan mode.

10. A wind power generation system, characterized in that: A doubly-fed converter with a slow shutdown function as described in any one of claims 1 to 4 is used.

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