Network type doubly-fed flywheel energy storage unit control loop starting and grid-connection method and system

Abstract

The present application relates to the technical field of energy storage, in particular to a network-constructed double-fed flywheel energy storage unit control loop starting and grid-connection method and system, the method comprising: starting a grid-side converter, including pre-charging a DC capacitor, and starting the grid-side converter control loop in turn according to a preset timing sequence: first starting a phase-locked loop, then starting a current loop, and finally synchronously starting a DC voltage loop and a reactive power loop; three-phase short-circuiting a double-fed motor stator, starting the flywheel and accelerating to a set speed by using a machine-side converter phase-locked loop and a network-constructed current control loop; opening the double-fed motor stator, starting the machine-side network-constructed control loop according to the timing sequence, and connecting to an AC power grid. Through the method and system, the flywheel can be directly started under network-constructed control and connected to the power grid, which is suitable for operation under weak power grid conditions and is more conducive to stable operation of the system.

Description

Technical Field

[0001] This invention relates to the field of energy storage technology, and in particular to a method and system for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit to the grid. Background Technology

[0002] Doubly-fed induction generator (DFIG) wind turbine energy storage is widely used in power systems due to its advantages such as low cost, high safety, and low losses. It employs a DFIG converter to control grid connection and charging / discharging, making it a high-quality grid-connecting source in power systems. Grid-connecting control further improves the system's equivalent short-circuit ratio, which is of great significance for the safe and stable operation of the power system.

[0003] Unlike conventional doubly-fed converter-controlled units, doubly-fed flywheel energy storage systems have a unique startup process. The stator must first be short-circuited, and the flywheel is accelerated to a specific speed by the generator-side converter. Only after the stator short-circuit is released and the flywheel is opened, and then synchronized with the grid through pre-synchronization control, can it be connected and put into operation. This special startup process means that conventional grid-connected startup strategies used in doubly-fed wind power cannot be directly applied to doubly-fed flywheel energy storage systems.

[0004] Currently, mainstream doubly-fed flywheel energy storage grid connection schemes typically employ grid-connected control for initial grid connection, followed by a switching strategy to grid-connected control. While feasible in strong grid environments, in scenarios with high requirements for system inertia and voltage support, such as weak grids or isolated grids, it is desirable for flywheel energy storage to connect to the grid in grid-connected mode from the startup phase to actively build system voltage and enhance dynamic stability. However, the existing grid-connected-then-switched strategy struggles to meet these needs, exhibiting technical bottlenecks such as reliance on grid support during startup, susceptibility to instability during switching, and inability to independently build voltage in weak grid conditions. Summary of the Invention

[0005] To address the aforementioned technical issues, this invention proposes a method and system for starting and connecting the control loop of a grid-connected doubly-fed induction generator (DFIG) to the grid. This method enables the flywheel to be started and connected to the grid directly under grid control, making it suitable for operation under weak grid conditions and facilitating stable system operation.

[0006] This invention is achieved by adopting the following technical solution:

[0007] The method for starting up and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit includes the following steps:

[0008] Step S1. Start-up of the grid-side converter, including pre-charging of the DC capacitor and starting the grid-side converter control loop in sequence according to the preset timing: first start the phase-locked loop, then start the current loop, and finally start the DC voltage loop and reactive power loop simultaneously.

[0009] Step S2. Short-circuit the three phases of the stator of the doubly fed motor, and start the flywheel and accelerate it to the set speed using the phase-locked loop of the generator-side converter and the grid current control loop;

[0010] Step S3. Open the stator of the doubly-fed motor, start the grid-type control loop on the motor side according to the timing sequence, and connect it to the AC power grid.

[0011] Step S1 specifically includes the following steps:

[0012] Step S 11 Determine if a start signal from the grid-connected converter has been received. If so, proceed to step S. 12 ;

[0013] Step S 12 Precharge the DC capacitor;

[0014] Step S 13 Determine if the pre-charge voltage value has been reached; if so, proceed to step S. 14 ;

[0015] Step S 14 Disconnect the pre-charging circuit to connect the grid-connected converter to the AC grid. According to the preset timing sequence, first start the phase-locked loop, then start the current loop, and finally start the DC voltage loop and reactive power loop simultaneously.

[0016] Step S 15 Determine if the DC voltage is stable at the rated value. If so, the grid-side converter has completed its startup.

[0017] The current loop, DC voltage loop, and reactive power loop are each controlled by a PI controller in a closed-loop manner, specifically as follows:

[0018] ,

[0019] In the formula, V dc_ref and V dc_meas These are the given and measured values ​​of the DC voltage, respectively. Q grid_ref and Q grid_meas These are the given and measured values ​​of the reactive power of the grid-side converter, respectively. K Vdc_p , K Vdc_i and K Qgrid_p , K Qgrid_i These are the PI parameters for the DC voltage loop and the grid-side converter reactive power loop, respectively. I gd_ref , I gd_meas andI gq_ref and I gq_meas Let the given and measured values ​​of the dq-axis current be separate. K id_p , K id_i and K iq_p , K iq_i These are the PI parameters for the dq-axis current loop, respectively. U gd_PWM and U gq_PWM These are the reference values ​​for the dq axis modulation voltage of the grid-side converter, respectively. This indicates integration.

[0020] When the DC voltage loop is closed-loop regulated using a PI controller, the PI parameter of the DC voltage loop is initially 0, and increases to the set value using a linear rolling growth method.

[0021] ,

[0022] In the formula, K Vdc_p_set , K Vdc_i_set These are the PI parameter settings for the DC voltage loop. k Vdc This is the gradual increase coefficient. K Vdc_p ( t +1) K Vdc_i ( t +1) and K Vdc_p ( t ), K Vdc_i ( t ) represent the PI parameters of the DC voltage loop at the next and current times, respectively.

[0023] Step S2 specifically refers to: after the grid-side converter starts up, short-circuiting the three phases of the doubly-fed motor stator, enabling the phase-locked loop and grid current control loop of the generator-side converter to run, and accelerating the flywheel by increasing the d-axis rotor current.

[0024] ,

[0025] In the formula, I rd_ref , I rd_meas and I rq_ref , I rq_measThese are the given and measured values ​​of the rotor dq-axis current, respectively; K Ird_p , K Ird_i and K Irq_p , K Irq_i These are the PI parameters of the rotor dq shaft network current control loop; and These are the setpoint and measured speed values ​​for the doubly-fed motor, respectively. k w This is the proportionality coefficient; U rd_PWM and U rq_PWM These are the reference values ​​for the dq-axis modulation voltage of the machine-side converter, respectively. Indicates integration operation;

[0026] Once the flywheel reaches the preset speed, the grid current control loop is disconnected, and the current command is switched.

[0027] The machine-side grid-type control loop includes the grid-type active power control loop K. s_enable_1 , K-network reactive power control loop s_enable_2 , grid voltage control loop K s_enable_3 , grid current control loop K s_enable_4 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 The machine-side grid-type control loop is equipped with a dual-input single-output selection switch KG, including: a selection switch KG1 for selecting the voltage reference command source under different operating modes, a selection switch KG2 for switching the control mode of the grid-type reactive power control loop, a selection switch KG3 or KG4 for selecting the machine-side rotor current command generation path, and a KG5 for selecting the coordinate transformation angle under different operating modes.

[0028] Step S3 involves starting the generator-side grid-type control loop according to the timing sequence and connecting it to the AC power grid. Specifically, this includes the following steps:

[0029] Step S 31 Start the active power control loop K of the network. s_enable_1 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 Enabling pre-synchronization ensures that the mesh generation angle is in phase with the phase-locked loop angle;

[0030] Step S 32 The d-axis voltage reference value of the grid voltage loop is directly given, and the grid current control loop K is run. s_enable_4 And the grid voltage control loop K s_enable_3Start by setting the stator voltage, where the stator d-axis voltage setpoint is gradually increased from 0 to the grid d-axis voltage, and the q-axis voltage setpoint is set to 0;

[0031] Step S 33 Detect grid connection conditions and determine whether the voltage deviation and phase deviation are simultaneously less than the set values. If so, exit the grid pre-synchronization loop K. s_enable_5 This allows the stator of the doubly fed motor to be connected to the power grid.

[0032] Step S 34 Start the reactive power control loop K of the network. s_enable_2 Switch the stator d-axis voltage reference command to the grid reactive power control loop K. s_enable_2 The output completes the loop start-up and grid connection process of the entire grid-type controlled doubly-fed flywheel energy storage system.

[0033] The step S 31 In this context, enabling pre-synchronization to ensure that the network generation angle is in phase with the phase-locked loop angle specifically refers to:

[0034] ,

[0035] In the above formula, and These represent the active power control loop K of the network at the current time and the next time, respectively. s_enable_1 angle; k θerror This is the pre-synchronization ratio coefficient. This represents the current angle relative to the network-locked loop, calculated as follows:

[0036] ,

[0037] In the above formula, P ref , P meas and P droop These represent the power setpoint, the power measurement, and the droop power increment, respectively. J This is virtual inertia; U gq_meas1 The q-axis grid voltage, K, is obtained by transforming the grid voltage collected from the generator side using the phase-locked loop of the generator-side converter. pll_p1 and K pll_i1 These are the PI parameters of the phase-locked loop of the machine-side converter; w 0 represents the rated value of the power grid angular frequency, taken as a per-unit value. w 0 = 1.

[0038] The step S 32 In the middle, the grid current control loop K is running. s_enable_4 And the grid voltage control loop K s_enable_3The initial given sub-build compression specifically includes:

[0039] Calculate the given values ​​for the stator d-axis and q-axis voltages:

[0040] ,

[0041] In the formula, and These represent the given values ​​of the stator d-axis voltage at the next and current moments, respectively. k Usd This is the d-axis voltage ramp-up coefficient; U sq_ref This is the given value for the stator q-axis voltage; U gd_meas1 The d-axis grid voltage is obtained by transforming the grid voltage collected from the generator side through the phase-locked loop of the generator-side converter into angular coordinates.

[0042] The calculated values ​​of the stator d-axis and q-axis voltages are sent to the grid current control loop K. s_enable_4 After passing through the grid voltage control loop K s_enable_3 and the grid current control loop K s_enable_4 After dual closed-loop operation, the reference value for the dq-axis modulated voltage of the generator-side converter is:

[0043] ,

[0044] In the formula, I rd_ref , I rd_meas and I rq_ref , I rq_meas These are the given and measured values ​​of the rotor dq-axis current, respectively; K Usd_p , K Usd_i and K Usq_p , K Usq_i The stator dq axis structure voltage control loop K is respectively s_enable_3 PI parameters; U sd_meas This represents the feedback value of the stator d-axis voltage; U sq_ref , U sq_meas These are the given and feedback values ​​of the stator q-axis voltage, respectively; U rd_PWM and U rq_PWM These are the reference values ​​for the dq-axis modulation voltage of the machine-side converter, respectively. K Ird_p ,K Ird_i and K Irq_p , K Irq_i These are the PI parameters of the rotor dq shaft network current control loop; This indicates integration.

[0045] The step S 33 The judgment method in is:

[0046] ,

[0047] In the above formula, θ error and U error These are the absolute values ​​of phase deviation and voltage deviation, respectively. θ error_max and U error_max These are the maximum values ​​of the set phase and voltage deviations, respectively. KM4=1 indicates that contactor KM4 is closed. θ GFM and θ PLL These are the active power control loops K and K of the network. s_enable_1 And the angle of the phase lock loop, U sq_meas The stator q-axis voltage is obtained after coordinate transformation of the stator voltage and current. U sd_meas The stator d-axis voltage is obtained after coordinate transformation of the stator voltage and current. U gd_meas1 The grid voltage d-axis voltage is obtained by transforming the grid voltage collected from the generator side through the phase-locked loop of the generator-side converter.

[0048] Switch the stator d-axis voltage reference command to the grid reactive power control loop K. s_enable_2 Output specifically refers to:

[0049] ,

[0050] or,

[0051] ,

[0052] In the formula, Q s_ref , Q s_meas These are the given and measured values ​​of the stator reactive power, respectively. K Qs_p and K Qs_i These are the reactive power control loops K and K of the network. s_enable_2PI control parameters; K droop_q This is the reactive power droop coefficient; U gd_0 This is the rated value of the d-axis voltage of the power grid. U sd_ref This is the reference value for the stator d-axis voltage.

[0053] The grid-connected doubly-fed induction generator (DFIG) control loop start-up and grid-connection system includes a flywheel, a DFIG motor, a grid-side converter, a generator-side converter, a DC capacitor, a three-phase uncontrolled rectifier bridge, four contactors, and a control and grid-connection switching device for operating modes. The control and grid-connection switching device includes a grid-side converter control loop, a generator-side grid-connected control loop, and a central controller for coordinating the enable timing, mode switching, and grid-connection operation of each control loop. The AC side of the grid-side converter is connected to the AC grid via contactors, and the DC side is connected to the DC capacitor. The two ends of the DC capacitor are connected to the three-phase uncontrolled rectifier bridge, which is connected to the AC grid via contactors. The contactor connects to the generator-side converter, which connects to the rotor winding of the doubly-fed motor. The grid-side converter control loop is used to achieve DC capacitor voltage pre-charging and slow-start voltage build-up through contactor closure. After pre-charging, the grid-side converter control loop is activated according to a predetermined sequence to complete the grid-side converter voltage build-up and start-up. The generator-side grid-type control loop is equipped with a dual-input single-output selection switch KG for selecting the loop input and output. The generator-side grid-type control loop is used to achieve three-phase short circuit of the doubly-fed motor stator through the contactor, start the flywheel to accelerate to the set speed, and achieve stator open circuit of the doubly-fed motor through the contactor, start the motor in a time sequence, and connect to the AC grid after achieving stator voltage amplitude-frequency synchronization.

[0054] The four contactors are KM1 to KM4. The AC side of the grid-side converter is connected to the AC grid via contactor KM1, and the three-phase uncontrolled rectifier bridge is connected to the AC grid via contactor KM2. The stator of the doubly fed motor is connected in parallel with contactors KM3 and KM4. The other end of contactor KM3 is short-circuited, and the other end of contactor KM4 is connected to the AC grid. The DC capacitor voltage pre-charging and slow-start voltage build-up achieved by contactor closure specifically means: closing contactor KM2, using the three-phase uncontrolled rectifier bridge to pre-charge the DC capacitor, and after pre-charging is completed, opening contactor KM2 and closing contactor KM1 to connect the grid-side converter to the AC grid.

[0055] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0056] 1. This method avoids overshoot and overshoot during the DC voltage build-up process of the grid-side converter and the start-up of the flywheel to rated speed of the generator-side converter. After the generator-side grid-connected control loop is started and the voltage loop is enabled, the stator voltage gradually increases to the same amplitude and phase as the grid voltage before connecting to the grid. The current transient impact is small throughout the process. The direct start-up of the flywheel and connection to the grid under grid-connected control proposed in this invention is more suitable for operation under weak grid conditions and is more conducive to stable system operation.

[0057] 2. The present invention sets parameter slow start or slow setting at the PI control parameters of the grid-side DC voltage loop and the current setting at the machine side, which can effectively reduce the impact caused by sudden loop connection.

[0058] 3. In this invention, by pre-charging the DC capacitor and designing the phased PI regulation timing of the grid-side converter control loop, dynamic conflicts and system oscillations caused by the simultaneous start-up of multiple loops can be effectively avoided, thus achieving a smooth and safe start-up of the grid-side converter.

[0059] 4. In this invention, the flywheel is rapidly and smoothly accelerated to the preset speed through closed-loop control of the d-axis rotor current, without relying on external drive equipment, making startup flexible and reliable. After acceleration, the grid current control loop is promptly disconnected and control commands are switched, preparing for subsequent grid-connected applications.

[0060] 5. In the process of connecting to the AC power grid, this invention adopts a phased activation strategy for the control loops to avoid dynamic oscillations caused by the simultaneous activation of multiple loops. This method enables the doubly-fed induction generator (DFIG) to upgrade its ability from passively following the grid to actively connecting to it.

[0061] 6. This invention constructs a pre-synchronization ring K. s_enable_5 Actively adjust the phase of the grid voltage to accurately track the grid phase, achieving quasi-synchronous matching of voltage amplitude and phase before grid connection, significantly reducing closing inrush current and improving grid connection safety.

[0062] 7. This system has a simple structure and can achieve active voltage build-up and smooth grid connection through a grid-based control strategy and time-sequential loop startup, significantly improving grid connection reliability and system stability. Combined with flywheel acceleration and multi-mode switching design, it supports autonomous startup and black start in weak grid and even off-grid scenarios, greatly enhancing the grid support capability and operational resilience of the new energy system. Attached Figure Description

[0063] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments, wherein:

[0064] Figure 1 This is a schematic diagram of the system structure of the present invention;

[0065] Figure 2This is a schematic diagram of the startup logic of the grid-side converter control loop in this invention;

[0066] Figure 3 This is a schematic diagram of the startup logic of the machine-side network control loop in this invention;

[0067] Figure 4 This is a schematic diagram of the startup process of the grid-side converter control loop in this invention;

[0068] Figure 5 This is a schematic diagram of the startup process of the machine-side network control loop in this invention;

[0069] Figure 6 This is a waveform diagram of the DC voltage slow-start on the grid side in this invention;

[0070] Figure 7 This is a waveform diagram of the starting speed of the flywheel in the current loop of the machine-side converter in this invention;

[0071] Figure 8 This is a waveform diagram of the voltage and current during the start-up and grid connection experiment of the machine-side grid-type control loop in this invention. Detailed Implementation

[0072] Example 1

[0073] As a basic embodiment of the present invention, the present invention includes a method for starting up and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit to the grid, comprising the following steps:

[0074] Step S1. Start-up of the grid-side converter, including pre-charging of the DC capacitor and starting the grid-side converter control loops sequentially according to a preset timing sequence: first, start the phase-locked loop, then start the current loop, and finally start the DC voltage loop and reactive power loop simultaneously.

[0075] Step S2. Short-circuit the three phases of the stator of the doubly fed motor, and start the flywheel and accelerate it to the set speed using the phase-locked loop of the generator-side converter and the grid current control loop.

[0076] Step S3. Open the stator of the doubly-fed motor, start the grid-type control loop on the motor side according to the timing sequence, and connect it to the AC power grid.

[0077] Example 2

[0078] As a preferred embodiment of the present invention, the present invention includes a method for starting up and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit to the grid, comprising the following steps:

[0079] Step S1. Start-up of the grid-side converter, including pre-charging of the DC capacitor and sequentially starting the grid-side converter control loop according to a preset timing sequence: first, start the phase-locked loop, then start the current loop, and finally simultaneously start the DC voltage loop and reactive power loop. Specifically, this includes the following steps:

[0080] Step S 11 Determine if a start signal from the grid-connected converter has been received. If so, proceed to step S. 12 .

[0081] Step S 12 Precharge the DC capacitor.

[0082] Step S 13 Determine if the pre-charge voltage value has been reached; if so, proceed to step S. 14 .

[0083] Step S 14 Disconnect the pre-charging circuit to connect the grid-connected converter to the AC grid. According to the preset timing sequence, first start the phase-locked loop, then start the current loop, and finally start the DC voltage loop and reactive power loop simultaneously.

[0084] Step S 15 Determine if the DC voltage is stable at the rated value. If so, the grid-side converter has completed its startup.

[0085] The current loop, DC voltage loop, and reactive power loop are each controlled by a PI controller in a closed-loop manner, specifically as follows:

[0086] ,

[0087] In the formula, V dc_ref and V dc_meas These are the given and measured values ​​of the DC voltage, respectively. Q grid_ref and Q grid_meas These are the given and measured values ​​of the reactive power of the grid-side converter, respectively. K Vdc_p , K Vdc_i and K Qgrid_p , K Qgrid_i These are the PI parameters for the DC voltage loop and the grid-side converter reactive power loop, respectively. I gd_ref , I gd_meas and I gq_ref and I gq_meas Let the given and measured values ​​of the dq-axis current be separate. K id_p , K id_i and K iq_p , Kiq_i These are the PI parameters for the dq-axis current loop, respectively. U gd_PWM and U gq_PWM These are the reference values ​​for the dq axis modulation voltage of the grid-side converter, which drive the switching action of the machine-side converter after being modulated by SPWM or SVPWM. This indicates integration.

[0088] Step S2. The stator three phases of the doubly fed motor are short-circuited, and the flywheel is started and accelerated to the set speed using the phase-locked loop of the machine-side converter and the grid current control loop.

[0089] Step S3. Open the stator of the doubly-fed motor, start the grid-type control loop on the motor side according to the timing sequence, and connect it to the AC power grid.

[0090] Example 3

[0091] In another preferred embodiment of the present invention, the present invention includes a method for starting up and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit to the grid, comprising the following steps:

[0092] Step S1. Start-up of the grid-side converter, including pre-charging of the DC capacitor and starting the grid-side converter control loops sequentially according to a preset timing sequence: first, start the phase-locked loop, then start the current loop, and finally start the DC voltage loop and reactive power loop simultaneously.

[0093] Step S2. Short-circuit the three phases of the stator of the doubly fed motor, and start the flywheel and accelerate it to the set speed using the phase-locked loop of the generator-side converter and the grid current control loop.

[0094] Step S3. Open the stator of the doubly-fed motor, start the grid-type control loop on the motor side according to the timing sequence, and connect it to the AC power grid.

[0095] The machine-side grid-type control loop includes a grid-type active power control loop K. s_enable_1 , K-network reactive power control loop s_enable_2 , grid voltage control loop K s_enable_3 , grid current control loop K s_enable_4 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 Each machine-side grid-type control loop is equipped with a dual-input single-output selection switch KG, including: a selection switch KG1 for selecting the voltage reference command source under different operating modes, a selection switch KG2 for switching the control mode of the grid-type reactive power control loop, a selection switch KG3 or KG4 for selecting the machine-side rotor current command generation path, and a KG5 for selecting the coordinate transformation angle under different operating modes.

[0096] Based on this, step S2 specifically includes the following steps:

[0097] After the grid-side converter finishes startup, short-circuit the three phases of the doubly-fed motor stator and activate the grid current control loop K. s_enable_4 and the phase-locked loop K of the machine-side converter s_enable_6 The phase-locked loop and grid current control loop of the machine-side converter are enabled to operate, accelerating the flywheel by increasing the d-axis rotor current.

[0098] ,

[0099] In the formula, I rd_ref , I rd_meas and I rq_ref , I rq_meas These are the given and measured values ​​of the rotor dq-axis current, respectively; K Ird_p , K Ird_i and K Irq_p , K Irq_i These are the PI parameters of the rotor dq shaft network current control loop; and These are the setpoint and measured speed values ​​for the doubly-fed motor, respectively. k w This is the proportionality coefficient; U rd_PWM and U rq_PWM These are the reference values ​​for the dq-axis modulation voltage of the machine-side converter, respectively. This indicates integration.

[0100] Once the flywheel reaches the preset speed, the grid current control loop is disconnected, and the current command is switched.

[0101] Step S3, which involves starting the machine-side grid-type control loop according to the timing sequence and connecting it to the AC power grid, specifically includes the following steps:

[0102] Step S 31 Start the active power control loop K of the network. s_enable_1 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 Enabling pre-synchronization ensures that the mesh generation angle is in phase with the phase-locked loop angle.

[0103] Step S 32 The d-axis voltage reference value of the grid voltage loop is directly given, and the grid current control loop K is run. s_enable_4 And the grid voltage control loop K s_enable_3Start by setting the stator voltage, where the stator d-axis voltage setpoint is gradually increased from 0 to the grid d-axis voltage, and the q-axis voltage setpoint is set to 0.

[0104] Step S 33 Detect grid connection conditions and determine whether the voltage deviation and phase deviation are simultaneously less than the set values. If so, exit the grid pre-synchronization loop K. s_enable_5 This allows the stator of the doubly fed motor to be connected to the power grid.

[0105] Step S 34 Start the reactive power control loop K of the network. s_enable_2 Switch the stator d-axis voltage reference command to the grid reactive power control loop K. s_enable_2 The output completes the loop start-up and grid connection process of the entire grid-type controlled doubly-fed flywheel energy storage system.

[0106] Example 4

[0107] In another preferred embodiment of the present invention, the present invention includes a grid-connected doubly-fed induction generator (DFIG) flywheel energy storage unit control loop start-up and grid connection system, comprising a flywheel, a DFIG motor, a grid-side converter, a generator-side converter, a DC capacitor, a three-phase uncontrolled rectifier bridge, four contactors, and a control and grid connection switching device for operating modes. The control and grid connection switching device for operating modes includes a grid-side converter control loop, a generator-side grid-connected control loop, and a central controller for coordinating the enable timing, mode switching, and grid connection operation of each control loop. The AC side of the grid-side converter is connected to the AC grid via contactors, and the DC side is connected to the DC capacitor. The two ends of the DC capacitor are connected to the three-phase uncontrolled rectifier bridge, which is connected to the AC grid via contactors. The generator-side converter is connected to the DFIG motor rotor winding. The grid-side converter control loop is used to achieve pre-charging and slow-start voltage build-up of the DC capacitor through contactor closure, and after pre-charging, the grid-side converter control loop is engaged according to a predetermined sequence to complete the voltage build-up and start-up of the grid-side converter. Each machine-side grid-type control loop is equipped with a dual-input single-output selection switch (KG) for selecting the loop input and output. The machine-side grid-type control loop is used to achieve a three-phase short circuit of the doubly-fed motor stator via a contactor, then start the flywheel to accelerate to a set speed. After the doubly-fed motor stator is opened via a contactor, it starts in a time-sequenced manner, achieving stator voltage amplitude-frequency synchronization before connecting to the AC power grid.

[0108] Example 5

[0109] As another preferred embodiment of the present invention, the present invention includes a grid-connected doubly-fed flywheel energy storage unit control loop start-up and grid connection system, as described in the appendix to the specification. Figure 1It includes a flywheel, a doubly-fed induction generator (DFIG), a grid-side converter, a machine-side converter, a DC capacitor, a three-phase uncontrolled rectifier bridge, four three-phase contactors, and a control and grid-connection switching device for operating modes. The control and grid-connection switching device for operating modes includes a grid-side converter control loop, a machine-side grid-connection control loop, and a central controller for coordinating the enable timing, mode switching, and grid-connection operation of each control loop.

[0110] Four three-phase contactors, KM1 to KM4, are used. The AC side of the grid-side converter is connected to the AC grid via three-phase contactor KM1, and the DC side is connected to a DC capacitor. The two ends of the DC capacitor are connected to a three-phase uncontrolled rectifier bridge, which is connected to the AC grid via three-phase contactor KM2. The generator-side converter is connected to the rotor winding of the doubly-fed induction generator (DFIG). Three-phase contactors KM3 and KM4 are connected in parallel to the stator of the DFIG. The other end of contactor KM3 is short-circuited, and the other end of contactor KM4 is connected to the AC grid. The trigger signal of contactor KM uses high and low levels: KM=1 indicates closed, and KM=0 indicates open.

[0111] Refer to the instruction manual appendix Figure 2 The software enable signal set in the grid-side converter control loop is denoted as K. g_enable K g_enable =1 indicates loop enable, K g_enable =0 indicates loop exit, including the loop enable signal K located in the DC voltage loop and reactive power loop. g_enable_1 The loop enable signal K located in the current loop g_enable_2 The loop enable signal K located in the phase-locked loop g_enable_3 .

[0112] Refer to the instruction manual appendix Figure 3 The machine-side grid-type control loop includes a grid-type active power control loop K. s_enable_1 , K-network reactive power control loop s_enable_2 , grid voltage control loop K s_enable_3 , grid current control loop K s_enable_4 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 The enable signal K located in the machine-side network control loop. s_enable K s_enable =1 indicates loop enable, K s_enable=0 indicates loop exit. The machine-side grid-type control loop includes a dual-input, single-output selection switch KG, comprising: switch KG1 for selecting the voltage reference command source under different operating modes; switch KG2 for switching the grid-type reactive power control loop control mode; switch KG3 or KG4 for selecting the machine-side rotor current command generation path; and switch KG5 for selecting the coordinate transformation angle under different operating modes. Setting KG=0 or KG=1 enables a two-way selection of the two inputs. KG1 selects the voltage reference command, and KG1=1 indicates selection of the grid-type reactive power control loop KG1. s_enable_2 The output serves as the d-axis voltage reference value for the grid voltage loop. KG1=0 indicates that the d-axis voltage reference value for the grid voltage loop is directly given. KG2 selects the control mode of the grid reactive power control loop. KG2=1 indicates that the grid reactive power control loop uses PI control, and KG2=0 indicates that the grid reactive power control loop uses droop control. KG3 and KG4 select the rotor current command on the machine side. KG3=0 / KG4=0 indicates that the rotor d / q axis current command is directly given, and KG3=1 / KG4=1 indicates that the rotor d / q axis current is the output of the grid stator q / d axis voltage loop. KG5 is used to indicate the angle selection for coordinate transformation. KG5=0 indicates the selection of the machine side phase-locked loop angle, and KG5=1 indicates the selection of the machine side grid active power loop output angle.

[0113] The grid-side converter control loop is used to pre-charge and slowly build up the DC capacitor voltage by closing the three-phase contactor KM1. After pre-charging, the grid-side converter control loop is activated according to a predetermined sequence to complete the grid-side converter voltage build-up and startup. The machine-side grid-type control loop is used to start the flywheel to accelerate to a set speed after short-circuiting the three phases of the doubly-fed motor stator by closing the three-phase contactor KM3. After opening the stator of the doubly-fed motor by disconnecting the three-phase contactor KM3, the motor starts in a time-sequenced manner, achieving stator voltage amplitude-frequency synchronization before connecting to the AC grid.

[0114] Based on the above system, a method for starting up and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit to the grid can be implemented, including the following steps:

[0115] Step S1. Start-up of the grid-side converter, including pre-charging of the DC capacitor and sequentially starting the grid-side converter control loops according to a preset timing sequence: first, the phase-locked loop is started, followed by the current loop, and finally the DC voltage loop and reactive power loop are started synchronously. Refer to the attached instruction manual. Figure 4 Specifically, it includes the following steps:

[0116] Step S 11 Initialize, set KM2=0, KM1=0, K g_enable_3 =0,K g_enable_2 =0,K g_enable_1 =0. Determine if a start signal from the grid-connected converter has been received. If yes, proceed to step S.12 .

[0117] Step S 12 Set KM2=1 and use an uncontrolled rectifier bridge to precharge the DC capacitor.

[0118] Step S 13 Determine if the pre-charge voltage value has been reached; if so, proceed to step S. 14 .

[0119] Step S 14 Set KM2=0 to disconnect the pre-charging circuit, and set KM1=1 to connect the grid-connected converter to the AC grid. Sequentially set KM2 at 1ms intervals. g_enable_3 =1, start the phase-locked loop; set K g_enable_2 =1, start the current loop, set K g_enable_1 =1, start the DC voltage loop and reactive power loop.

[0120] Step S 15 Determine if the DC voltage is stable at the rated value. If so, the grid-side converter has completed its startup.

[0121] Among them, the grid voltage collected by the grid side U gabc and grid current I gabc After phase-locked loop coordinate transformation, the grid voltage and current components along the dq axis are obtained, which are: grid d-axis voltage U gd_meas q-axis voltage of the power grid U gq_meas d-axis current of the power grid I gd_meas q-axis current of the power grid I gq_meas The reactive power of the grid-side converter was obtained through dq-axis power theory calculations. Q grid_meas The coordinate transformation angle is obtained using a phase-locked loop (PLL), specifically as follows:

[0122] ,

[0123] In the above formula, θ PLL_g Indicates the grid locking angle; K PLL_p and K PLL_i These represent the PI parameters of the grid-side phase-locked loop; Indicates integration operation; ω 0 represents the rated angular frequency of the power grid.

[0124] The current loop, DC voltage loop, and reactive power loop are each controlled by a PI controller in a closed-loop manner, specifically as follows:

[0125] ,

[0126] In the formula, V dc_ref and V dc_meas These are the given and measured values ​​of the DC voltage, respectively. Q grid_ref and Q grid_meas These are the given and measured values ​​of the reactive power of the grid-side converter, respectively. K Vdc_p , K Vdc_i and K Qgrid_p , K Qgrid_i These are the PI parameters for the DC voltage loop and the grid-side converter reactive power loop, respectively. I gd_ref , I gd_meas and I gq_ref and I gq_meas Let the given and measured values ​​of the dq-axis current be separate. K id_p , K id_i and K iq_p , K iq_i These are the PI parameters for the dq-axis current loop, respectively. U gd_PWM and U gq_PWM These are the reference values ​​for the dq axis modulation voltage of the grid-side converter, respectively. After being modulated by SPWM or SVPWM, these reference values ​​generate the switching signal PWM1 to drive the grid-side converter to switch.

[0127] Furthermore, to prevent DC voltage overshoot, when the DC voltage loop uses a PI controller for closed-loop regulation, the PI parameter of the DC voltage loop is initially 0. When K is set... g_enable_1 When =1, the value is increased linearly and continuously until the set value is reached.

[0128] ,

[0129] In the formula, K Vdc_p_set , K Vdc_i_set These are the PI parameter settings for the DC voltage loop.k Vdc This is the gradual increase coefficient. K Vdc_p ( t +1) K Vdc_i ( t +1) and K Vdc_p ( t ), K Vdc_i ( t ) represent the PI parameters of the DC voltage loop at the next and current times, respectively.

[0130] Step S2. The three phases of the doubly-fed motor stator are short-circuited. The flywheel is started and accelerated to the set speed using the phase-locked loop of the generator-side converter and the grid current control loop. In this embodiment, the dq-axis voltage and current components used in the step of starting the flywheel to the set speed are obtained based on the angle of the phase-locked loop; in other states, the grid angle is used for coordinate transformation.

[0131] Specifically, the grid voltage collected by the machine side U gabc After coordinate transformation via the phase-locked loop of the generator-side converter, the d-axis grid voltage is obtained. U gd_meas1 and q-axis grid voltage U gq_meas1 The angles used for the above coordinate transformations should be selected from the phase-locked loop angles as needed. θ PLL Or construct a network active power loop output angle θ GFM The rotor current is obtained by coordinate transformation to obtain the rotor d-axis current. I rd_meas Rotor q-axis current I rq_meas The angle used for coordinate transformation is the rotor slip angle. θ slip ( θ slip= θ PLL / θ GFM - θ r , θ r For the rotor angle, by adjusting the rotational speed ω meas (obtained by integration); the stator output power is calculated from its dq-axis voltage and current.

[0132] Refer to the instruction manual appendix Figure 5 This step specifically includes the following steps:

[0133] In the initial state, KM3=0, KM4=0, K s_enable_1 ~K s_enable_6 =0, KG1=KG2=KG3=KG4=KG5=0. After the grid-side converter starts up, set KM3=1, KM4=0 to short-circuit the three phases of the doubly-fed motor stator, and set K... s_enable_4 =K s_enable_6 =1, enable the machine-side converter phase-locked loop and grid current control loop, set KG3=KG4=KG5=0, and provide a current command. Accelerate the flywheel by increasing the d-axis rotor current:

[0134] ,

[0135] In the formula, I rd_ref , I rd_meas and I rq_ref , I rq_meas These are the given and measured values ​​of the rotor dq-axis current, respectively; K Ird_p , K Ird_i and K Irq_p , K Irq_i These are the PI parameters of the rotor dq shaft network current control loop; and These are the setpoint and measured speed values ​​for the doubly-fed motor, respectively. k w This is the proportionality coefficient; U rd_PWM and U rq_PWM These are the reference values ​​for the dq axis modulation voltage of the generator-side converter, respectively. After being modulated by SPWM or SVPWM, these reference values ​​generate the switching signal PWM2 to drive the switching action of the grid-side converter. This indicates integration.

[0136] Once the flywheel reaches the preset speed, set K. s_enable_4 =0, disconnect the grid current control loop, and simultaneously set KG3=KG4=KG5=1 to switch the current command and coordinate transformation angle.

[0137] Step S3. Set KM3=0 and KM4=0 to open the stator of the doubly-fed induction generator, start the grid-type control loop on the generator side according to the timing sequence, and connect to the AC power grid. Specifically, this includes the following steps:

[0138] Step S 31 Set K s_enable_1 =K s_enable_5 =Ks_enable_6 =1, initiate the network active power control loop K s_enable_1 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 Enable pre-synchronization to ensure that the mesh generation angle is in phase with the phase-locked loop angle:

[0139] ,

[0140] In the above formula, and These represent the active power control loop K of the network at the current time and the next time, respectively. s_enable_1 angle; k θerror This is the pre-synchronization ratio coefficient. This represents the current angle relative to the network-locked loop, calculated as follows:

[0141] ,

[0142] In the above formula, P ref , P meas and P droop These represent the power setpoint, the power measurement, and the droop power increment, respectively. J This is virtual inertia; U gq_meas1 The q-axis grid voltage, K, is obtained by transforming the grid voltage collected from the generator side through the phase-locked loop of the generator-side converter. pll_p1 and K pll_i1 These are the PI parameters of the phase-locked loop of the machine-side converter; w 0 represents the rated value of the power grid angular frequency, taken as a per-unit value. w 0=1. This needs to be set during the startup phase. P ref =0.

[0143] Step S 32 Set K s_enable_3 =K s_enable_4 =1, set KG1=0, the reference value of the d-axis voltage of the grid voltage loop is directly given, and the grid current control loop K is run. s_enable_4 And the grid voltage control loop K s_enable_3 Start setting up the stator voltage. Specifically, the stator d-axis voltage setpoint is gradually increased from 0 to the grid d-axis voltage, while the q-axis voltage setpoint is set to 0. Specifically, the grid current control loop K is run. s_enable_4 And the grid voltage control loop K s_enable_3 The initial given sub-build compression specifically includes:

[0144] Calculate the given values ​​for the stator d-axis and q-axis voltages:

[0145] ,

[0146] In the formula, and These represent the given values ​​of the stator d-axis voltage at the next and current moments, respectively. k Usd This is the d-axis voltage ramp-up coefficient; U sq_ref This is the given value for the stator q-axis voltage; U gd_meas1 The d-axis grid voltage is obtained by transforming the grid voltage collected from the generator side through the phase-locked loop of the generator-side converter into angular coordinates.

[0147] The calculated values ​​of the stator d-axis and q-axis voltages are sent to the grid current control loop K. s_enable_4 After passing through the grid voltage control loop K s_enable_3 and the grid current control loop K s_enable_4 After dual closed-loop operation, the reference value for the dq-axis modulated voltage of the generator-side converter is:

[0148] ,

[0149] In the formula, I rd_ref , I rd_meas and I rq_ref , I rq_meas These are the given and measured values ​​of the rotor dq-axis current, respectively; K Usd_p , K Usd_i and K Usq_p , K Usq_i The stator dq axis structure voltage control loop K is respectively s_enable_3 PI parameters; U sd_meas This represents the feedback value of the stator d-axis voltage; U sq_ref , U sq_meas These are the given and feedback values ​​of the stator q-axis voltage, respectively; U rd_PWM and U rq_PWM These are the reference values ​​for the dq-axis modulation voltage of the machine-side converter, respectively. K Ird_p , K Ird_i and K Irq_p ,K Irq_i These are the PI parameters of the rotor dq shaft network current control loop; This indicates integration.

[0150] Step S 33 Detect grid connection conditions and determine whether both voltage deviation and phase deviation are simultaneously less than set values. The determination method is as follows:

[0151] ,

[0152] In the above formula, θ error and U error These are the absolute values ​​of phase deviation and voltage deviation, respectively. θ error_max and U error_max These are the maximum values ​​of the set phase and voltage deviations, respectively. KM4=1 indicates that the three-phase contactor KM4 is closed. θ GFM and θ PLL These are the active power control loops K and K of the network. s_enable_1 And the angle of the phase lock loop, U sq_meas The stator q-axis voltage is obtained after coordinate transformation of the stator voltage and current. U sd_meas The stator d-axis voltage is obtained after coordinate transformation of the stator voltage and current. U gd_meas1 The d-axis voltage is the grid voltage obtained by transforming the grid voltage collected from the machine side into angular coordinates using a phase-locked loop.

[0153] If both voltage deviation and phase deviation are less than the set value, set K. s_enable_5 =0, exit the network pre-synchronization ring K s_enable_5 And set KM4=1 and KM3=0 to connect the stator of the doubly fed motor to the power grid.

[0154] Step S 34 Set KG2=0 or 1, K s_enable_2 =1, activate the reactive power control loop K in the network. s_enable_2 Set KG1=1 to switch the stator d-axis voltage reference command to the grid reactive power control loop K. s_enable_2 Output:

[0155] ,

[0156] In the formula, Q s_ref , Q s_measThese are the given and measured values ​​of the stator reactive power, respectively. K Qs_p and K Qs_i These are the reactive power control loops K and K of the network. s_enable_2 PI control parameters; K droop_q This is the reactive power droop coefficient; U gd_0 This is the rated value of the d-axis voltage of the power grid. U sd_ref This is the reference value for the stator d-axis voltage.

[0157] This completes the loop startup and grid connection process of the entire grid-connected doubly-fed flywheel energy storage system.

[0158] Example 6

[0159] As another specific embodiment of the present invention, taking a single 100KW doubly-fed flywheel energy storage unit as an example, the main electrical parameters include grid-side voltage: 380V, 50Hz; DC bus voltage: 700V; the main parameters of the doubly-fed motor are stator resistance: 0.03pu, rotor resistance: 0.006pu, stator leakage inductance: 0.04pu, rotor leakage inductance: 0.005pu; the grid-side converter adopts an RL filter circuit with parameters of R=1mΩ and L=3mH. The grid-side converter is connected to the grid via AC contactors (KM, KM=1 indicates the contactor is closed, KM=0 indicates the contactor is open), numbered as follows: KM1 is located at both ends of the AC side of the grid-side converter and the AC grid; KM2 is located at both ends of the AC side of the three-phase uncontrolled rectifier bridge and the AC grid, with the DC side of the uncontrolled rectifier bridge connected to both ends of the DC capacitor of the converter; one end of KM3 is connected to the stator of the doubly-fed motor, and the other end is short-circuited; KM4 is connected to both ends of the doubly-fed motor stator and the AC grid.

[0160] The main control loop parameters are as follows: ① The grid-side converter control parameters include the DC voltage loop and the reactive power loop PI control parameters, which are respectively taken as follows: K Vdc_p_set = K Qgrid_p =21、 K Vdc_i_set = K Qgrid_i =7; Gradual increase coefficient k Vdc =0.005; Current loop control parameters are taken as follows: K Ird_p = K Irq_p =5.3, K Ird_i and, K Irq_i=2.7; the phase-locked loop (PLL) PI parameters are taken as 15 and 3 respectively; ② the machine-side converter control parameters include the virtual inertia parameter J=3kg*m of the grid active power loop. 2 Pre-synchronization ratio coefficient k θerror =0.01, PI parameter of the phase-locked loop of the machine-side converter K pll_p1 =15 and K pll_i1 =3; the reactive power PI parameter or droop coefficient are respectively taken as 3; K Qs_p =7, K Qs_i =1.2, K droop_q =0.04; voltage loop parameters are respectively taken as K Usd_p = K Usq_p =20、 K Usd_i = K Usq_i =1; Current loop parameters are respectively taken as... K Ird_p = K Irq_p =5, K Ird_i = K Irq_i =2.5; the grid connection condition judgment parameter is set to 2.5. θ error_max =0.005 and U error_max =0.05.

[0161] The control loop is equipped with a loop enable switch / signal (K) enable =1 indicates loop operation, K enable=0 (Indicates the loop is not running), the network-side loop enable switch number is K. g_enable_1 (Control loops for grid-side DC voltage loop and reactive power loop), K g_enable_2 (Grid-side current loop), located in the loop enable signal K of the phase-locked loop. g_enable_3 (Grid-side phase-locked loop); The machine-side loop enable switch is numbered K. s_enable_1 (Network active power control loop), K s_enable_2 (Network reactive power control loop), K s_enable_3 (Grid voltage control loop), K s_enable_4 (Network current control loop), K s_enable_5 (Network pre-synchronization ring), K s_enable_6 (Phase-locked loop of the machine-side converter)

[0162] The machine-side grid-type control loop includes a 2-to-1 selection switch. KG1 selects the voltage reference command: KG1=0 indicates that the grid-type reactive power loop output is selected as the d-axis voltage reference value for the grid-type voltage loop; KG1=1 indicates that the d-axis voltage reference value for the grid-type voltage loop is directly given. KG2 selects the reactive power loop control mode: KG2=0 indicates that the grid-type reactive power loop uses PI control; KG2=1 indicates that the grid-type reactive power loop uses droop control. KG3 and KG4 select the machine-side rotor current command: KG3=0 / KG4=0 indicates that the rotor d / q-axis current command is directly given; KG3=1 / KG4=1 indicates that the rotor d / q-axis current is the output of the grid-type stator q / d-axis voltage loop. KG5 selects the angle for coordinate transformation: KG5=0 indicates that the phase-locked loop angle is used; KG5=1 indicates that the grid-type angle is used.

[0163] Based on this, this embodiment includes a method for starting up and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit to the grid, comprising the following steps:

[0164] Step S1. Slow start-up and grid-connected voltage building of the grid-side converter. Specifically, set KM2=1 to charge the DC capacitor through the uncontrolled rectifier bridge. After the DC voltage is pre-charged and stabilized, set KM2=0 and KM1=1, then set K... g_enable_3 =1,K g_enable_2 =1,K g_enable_1 =1 enables the control loop. Once the DC voltage stabilizes at the given value, the grid-side converter completes DC voltage build-up.

[0165] Step S2. Start grid connection of the generator-side converter.

[0166] Set KM3=1 to short-circuit the stator of the doubly-fed motor, and set K... s_enable_6 =K s_enable_4 =1, KG3=KG4=KG5=0. The starter-side converter phase-locked loop and grid current control loop cause the doubly-fed motor to drive the flywheel to rotate and accelerate. After reaching the set speed, K is set to... s_enable_4 =0 Exit current loop, set KG3=KG4=KG5=1 to switch current command and coordinate transformation angle; set K s_enable_1 =K s_enable_5 =K s_enable_6 =1 Start the grid-connected active power control loop, the generator-side converter phase-locked loop, and the grid-connected pre-synchronization loop; Set K s_enable_3 =K s_enable_4 =1, KG1=0, the grid current control loop and grid voltage control loop start to establish voltage for the feeder; determine the grid connection conditions, and set KG1 when the grid connection conditions are met. s_enable_5 =0, KM3=0, KM4=1 disconnect the pre-synchronization loop of the grid, and connect the stator of the doubly-fed motor to the grid; set K s_enable_2=1, KG2=1 to run the grid reactive power control loop, set KG1=1 to switch the stator d-axis voltage setpoint to the grid reactive power control loop output value.

[0167] A grid-connection and startup experiment of a doubly-fed induction generator (DFIG) grid-connected energy storage unit was conducted, and simulation waveforms of a 10kW DFIG flywheel energy storage unit were obtained. (Instruction manual attached.) Figure 6 During the DC voltage build-up process of the grid-side converter, after the DC voltage pre-charge is completed, the control loops, including the outer voltage loop and inner current loop of the grid side, are enabled, and the DC voltage is buffered to the rated value. The entire process proceeds without overshoot or overcurrent, verifying the effectiveness of the method. (See attached instruction manual.) Figure 7 The waveform shows the process of the flywheel of the generator-side converter starting up to rated speed. There is no overshoot during the entire startup process, and the speed stabilizes to approximately the set value after startup. (Instruction manual attached) Figure 8 The diagram shows the stator voltage, grid voltage, and three-phase stator current waveforms during the startup and grid connection of the machine-side grid-type control loop. It can be seen that after the enable voltage loop is activated, the stator voltage gradually increases to the same amplitude and phase as the grid voltage before being connected to the grid. The transient current impact is small throughout the process, which verifies the effectiveness of the method of the present invention.

[0168] In summary, any other corresponding modifications made by those skilled in the art after reading this invention document, without requiring creative mental effort, based on the technical solutions and concepts of this invention, are all within the scope of protection of this invention.

Claims

1. A method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit to the grid, characterized in that: Includes the following steps: Step S1. Start-up of the grid-side converter, including pre-charging of the DC capacitor and starting the grid-side converter control loop in sequence according to the preset timing: first start the phase-locked loop, then start the current loop, and finally start the DC voltage loop and reactive power loop simultaneously. Step S2. Short-circuit the three phases of the stator of the doubly fed motor, and start the flywheel and accelerate it to the set speed using the phase-locked loop of the generator-side converter and the grid current control loop; Step S3. Open the stator of the doubly-fed induction generator and start the generator-side grid-connected control loop according to the timing sequence to connect to the AC power grid; the generator-side grid-connected control loop includes the grid-connected active power control loop K. s_enable_1 , K-network reactive power control loop s_enable_2 , grid voltage control loop K s_enable_3 , grid current control loop K s_enable_4 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 Step S3 involves starting the machine-side grid-type control loop according to the timing sequence and connecting it to the AC power grid, specifically including the following steps: Step S 31 Start the active power control loop K of the network. s_enable_1 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 Enabling pre-synchronization ensures that the mesh generation angle is in phase with the phase-locked loop angle; Step S 32 The d-axis voltage reference value of the grid voltage loop is directly given, and the grid current control loop K is run. s_enable_4 And the grid voltage control loop K s_enable_3 Start by setting the stator voltage, where the stator d-axis voltage setpoint is gradually increased from 0 to the grid d-axis voltage, and the q-axis voltage setpoint is set to 0; Step S 33 Detect grid connection conditions and determine whether the voltage deviation and phase deviation are simultaneously less than the set values. If so, exit the grid pre-synchronization loop K. s_enable_5 This allows the stator of the doubly fed motor to be connected to the power grid. Step S 34 Start the reactive power control loop K of the network. s_enable_2 Switch the stator d-axis voltage reference command to the grid reactive power control loop K. s_enable_2 The output completes the loop start-up and grid connection process of the entire grid-type controlled doubly-fed flywheel energy storage system.

2. The method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit according to claim 1, characterized in that: Step S1 specifically includes the following steps: Step S 11 Determine if a start signal from the grid-connected converter has been received. If so, proceed to step S. 12 ; Step S 12 Precharge the DC capacitor; Step S 13 Determine if the pre-charge voltage value has been reached; if so, proceed to step S. 14 ; Step S 14 Disconnect the pre-charging circuit to connect the grid-connected converter to the AC grid. According to the preset timing sequence, first start the phase-locked loop, then start the current loop, and finally start the DC voltage loop and reactive power loop simultaneously. Step S 15 Determine if the DC voltage is stable at the rated value. If so, the grid-side converter has completed its startup.

3. The method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit according to claim 1, characterized in that: The current loop, DC voltage loop, and reactive power loop are each controlled by a PI controller in a closed-loop manner, specifically as follows: ， In the formula, V dc_ref and V dc_meas These are the given and measured values ​​of the DC voltage, respectively. Q grid_ref and Q grid_meas These are the given and measured values ​​of the reactive power of the grid-side converter, respectively. K Vdc_p , K Vdc_i and K Qgrid_p , K Qgrid_i These are the PI parameters for the DC voltage loop and the grid-side converter reactive power loop, respectively. I gd_ref , I gd_meas and I gq_ref and I gq_meas Let the given and measured values ​​of the dq-axis current be separate. K id_p , K id_i and K iq_p , K iq_i These are the PI parameters for the dq-axis current loop, respectively. U gd_PWM and U gq_PWM These are the reference values ​​for the dq axis modulation voltage of the grid-side converter, respectively. This indicates integration.

4. The method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit according to claim 3, characterized in that: When the DC voltage loop is closed-loop regulated using a PI controller, the PI parameter of the DC voltage loop is initially 0, and increases to the set value using a linear rolling growth method. ， In the formula, K Vdc_p_set , K Vdc_i_set These are the PI parameter settings for the DC voltage loop. k Vdc This is the gradual increase coefficient. K Vdc_p ( t +1) K Vdc_i ( t +1) and K Vdc_p ( t ), K Vdc_i ( t ) represent the PI parameters of the DC voltage loop at the next and current times, respectively.

5. The method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit according to claim 1, characterized in that: Step S2 specifically refers to: after the grid-side converter starts up, short-circuiting the three phases of the doubly-fed motor stator, enabling the phase-locked loop and grid current control loop of the generator-side converter to run, and accelerating the flywheel by increasing the d-axis rotor current. ， In the formula, I rd_ref , I rd_meas and I rq_ref , I rq_meas These are the given and measured values ​​of the rotor dq-axis current, respectively; K Ird_p , K Ird_i and K Irq_p , K Irq_i These are the PI parameters of the rotor dq shaft network current control loop; and These are the setpoint and measured speed values ​​for the doubly-fed motor, respectively. k w This is the proportionality coefficient; U rd_PWM and U rq_PWM These are the reference values ​​for the dq-axis modulation voltage of the machine-side converter, respectively. Indicates integration operation; Once the flywheel reaches the preset speed, the grid current control loop is disconnected, and the current command is switched.

6. The method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit according to claim 1, characterized in that: The machine-side grid-type control loop is equipped with a dual-input single-output selection switch KG, including: a selection switch KG1 for selecting the voltage reference command source under different operating modes, a selection switch KG2 for switching the control mode of the grid-type reactive power control loop, a selection switch KG3 or KG4 for selecting the machine-side rotor current command generation path, and a KG5 for selecting the coordinate transformation angle under different operating modes.

7. The method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit according to claim 1, characterized in that: Step S 31 In this context, enabling pre-synchronization to ensure that the network generation angle is in phase with the phase-locked loop angle specifically refers to: ， In the above formula, and These represent the active power control loop K of the network at the current time and the next time, respectively. s_enable_1 angle; k θerror This is the pre-synchronization ratio coefficient. This represents the current angle relative to the network-locked loop, calculated as follows: ， In the above formula, P ref , P meas and P droop These represent the power setpoint, the power measurement, and the droop power increment, respectively. J This is virtual inertia; U gq_meas1 The q-axis grid voltage, K, is obtained by transforming the grid voltage collected from the generator side using the phase-locked loop of the generator-side converter. pll_p1 and K pll_i1 These are the PI parameters of the phase-locked loop of the machine-side converter; w 0 represents the rated value of the power grid angular frequency, taken as a per-unit value. w 0 = 1.

8. The method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit according to claim 1, characterized in that: Step S 32 In the middle, the grid current control loop K is running. s_enable_4 And the grid voltage control loop K s_enable_3 The initial given sub-build compression specifically includes: Calculate the given values ​​for the stator d-axis and q-axis voltages: ， In the formula, and These represent the given values ​​of the stator d-axis voltage at the next and current moments, respectively. k Usd This is the d-axis voltage ramp-up coefficient; U sq_ref This is the given value for the stator q-axis voltage; U gd_meas1 The d-axis grid voltage is obtained by transforming the grid voltage collected from the generator side through the phase-locked loop of the generator-side converter into angular coordinates. The calculated values ​​of the stator d-axis and q-axis voltages are sent to the grid current control loop K. s_enable_4 After passing through the grid voltage control loop K s_enable_3 and the grid current control loop K s_enable_4 After dual closed-loop operation, the reference value for the dq-axis modulated voltage of the generator-side converter is: ， In the formula, I rd_ref , I rd_meas and I rq_ref , I rq_meas These are the given and measured values ​​of the rotor dq-axis current, respectively; K Usd_p , K Usd_i and K Usq_p , K Usq_i The stator dq axis structure voltage control loop K is respectively s_enable_3 PI parameters; U sd_meas This represents the feedback value of the stator d-axis voltage; U sq_ref , U sq_meas These are the given and feedback values ​​of the stator q-axis voltage, respectively; U rd_PWM and U rq_PWM These are the reference values ​​for the dq-axis modulation voltage of the machine-side converter, respectively. K Ird_p , K Ird_i and K Irq_p , K Irq_i These are the PI parameters of the rotor dq shaft network current control loop; This indicates integration.

9. The method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit according to claim 1, characterized in that: Step S 33 The judgment method in is: ， In the above formula, θ error and U error These are the absolute values ​​of the phase deviation and voltage deviation, respectively. θ error_max and U error_max These are the maximum values ​​of the set phase and voltage deviations, respectively. KM4=1 indicates that contactor KM4 is closed. θ GFM and θ PLL These are the active power control loops K and K of the network. s_enable_1 And the angle of the phase lock loop, U sq_meas The stator q-axis voltage is obtained after coordinate transformation of the stator voltage and current. U sd_meas The stator d-axis voltage is obtained after coordinate transformation of the stator voltage and current. U gd_meas1 The grid voltage d-axis voltage is obtained by transforming the grid voltage collected from the generator side through the phase-locked loop of the generator-side converter.

10. The method for starting and connecting the control loop of a grid-connected doubly-fed flywheel energy storage unit according to claim 1, characterized in that: Switch the stator d-axis voltage reference command to the grid reactive power control loop K. s_enable_2 Output specifically refers to: ， or, ， In the formula, Q s_ref , Q s_meas These are the given and measured values ​​of the stator reactive power, respectively. K Qs_p and K Qs_i These are the reactive power control loops K and K of the network. s_enable_2 PI control parameters; K droop_q This is the reactive power droop coefficient; U gd_0 This is the rated value of the d-axis voltage of the power grid. U sd_ref This is the reference value for the stator d-axis voltage.

11. A grid-connected doubly-fed induction generator control loop start-up and grid connection system, characterized in that: The system includes a flywheel, a doubly-fed induction generator (DFIG), a grid-side converter, a generator-side converter, a DC capacitor, a three-phase uncontrolled rectifier bridge, four contactors, and a control and grid-connection switching device for operating modes. The control and grid-connection switching device includes a grid-side converter control loop, a generator-side grid configuration control loop, and a central controller for coordinating the enable timing, mode switching, and grid-connection operation of each control loop. The AC side of the grid-side converter is connected to the AC grid via contactors, and the DC side is connected to a DC capacitor. The two ends of the DC capacitor are connected to a three-phase uncontrolled rectifier bridge, which is connected to the AC grid via contactors. The generator-side converter... The doubly-fed induction generator (DFIG) rotor winding is connected. The grid-side converter control loop is used to achieve DC capacitor voltage pre-charging and slow-start voltage building through contactor closure, and after pre-charging, the grid-side converter control loop is activated according to a predetermined sequence to complete the grid-side converter voltage building and start-up. The machine-side grid-type control loop is equipped with a dual-input single-output selection switch KG for selecting the loop input and output. The machine-side grid-type control loop is used to achieve three-phase short circuit of the DFIG stator through contactor, start the flywheel to accelerate to the set speed, and achieve stator open circuit of the DFIG stator through contactor, start in sequence, and connect to the AC grid after stator voltage amplitude and frequency synchronization is achieved. The predetermined timing sequence for activating the grid-side converter control loop specifically refers to: first, starting the phase-locked loop, then starting the current loop, and finally simultaneously starting the DC voltage loop and reactive power loop; the generator-side grid-type control loop includes the grid-type active power control loop K. s_enable_1 , K-network reactive power control loop s_enable_2 , grid voltage control loop K s_enable_3 , grid current control loop K s_enable_4 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 The time-sequential startup specifically includes the following steps: Step S 31 Start the active power control loop K of the network. s_enable_1 , network pre-synchronization ring K s_enable_5 and the phase-locked loop K of the machine-side converter s_enable_6 Enabling pre-synchronization ensures that the mesh generation angle is in phase with the phase-locked loop angle; Step S 32 The d-axis voltage reference value of the grid voltage loop is directly given, and the grid current control loop K is run. s_enable_4 And the grid voltage control loop K s_enable_3 Start by setting the stator voltage, where the stator d-axis voltage setpoint is gradually increased from 0 to the grid d-axis voltage, and the q-axis voltage setpoint is set to 0; Step S 33 Detect grid connection conditions and determine whether the voltage deviation and phase deviation are simultaneously less than the set values. If so, exit the grid pre-synchronization loop K. s_enable_5 This allows the stator of the doubly fed motor to be connected to the power grid. Step S 34 Start the reactive power control loop K of the network. s_enable_2 Switch the stator d-axis voltage reference command to the grid reactive power control loop K. s_enable_2 The output completes the loop start-up and grid connection process of the entire grid-type controlled doubly-fed flywheel energy storage system.

12. The grid-connected doubly-fed flywheel energy storage unit control loop start-up and grid connection system according to claim 11, characterized in that: The four contactors are KM1 to KM4. The AC side of the grid-side converter is connected to the AC grid via contactor KM1, and the three-phase uncontrolled rectifier bridge is connected to the AC grid via contactor KM2. The stator of the doubly fed motor is connected in parallel with contactors KM3 and KM4. The other end of contactor KM3 is short-circuited, and the other end of contactor KM4 is connected to the AC grid. The DC capacitor voltage pre-charging and slow-start voltage build-up achieved by contactor closure specifically means: closing contactor KM2, using the three-phase uncontrolled rectifier bridge to pre-charge the DC capacitor, and after pre-charging is completed, opening contactor KM2 and closing contactor KM1 to connect the grid-side converter to the AC grid.

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