A new energy grid-connected method and system based on an electromechanical interface

Abstract

The application discloses a new energy grid-connected method and system based on an electromechanical interface and belongs to the technical field of new energy power generation grid connection. The system comprises a new energy power generation unit, a new energy side converter, a synchronous motor, a synchronous generator and a high-inertia flywheel. The method comprises the following steps: under frequency disturbance, inertia time constants of the high-inertia flywheel are determined according to target support power, support duration and allowed speed deviation of the high-inertia flywheel; inertia support power provided by the joint shaft system to the power grid is determined based on the inertia time constants of the high-inertia flywheel; when power imbalance occurs in the joint shaft system, the inertia support power of the joint shaft system is corrected by using the rotational inertia of the high-inertia flywheel to absorb or release kinetic energy. The application has the effect of realizing power sending out, frequency support and fault disturbance isolation through the electromechanical coupling interface with physical rotational inertia.

Description

Technical Field

[0001] This invention relates to the technical field of grid connection of new energy power generation, and in particular to a new energy grid connection method and system based on electromechanical interface. Background Technology

[0002] With the large-scale integration of new energy power generation units such as wind power and photovoltaics into the power system, the proportion of new energy power generation in the power structure is constantly increasing. Unlike traditional synchronous generators, new energy power generation units are usually directly connected to the AC grid through power electronic converters. Their output power is mainly determined by control algorithms and current commands, lacking the rotational inertia and electromagnetic transient support capabilities that are naturally coupled with the grid frequency. As the proportion of conventional synchronous units decreases, the equivalent physical inertia and short-circuit capacity of the power system are continuously decreasing. Under conditions of large disturbances or weak grids, problems such as increased frequency change rate, insufficient voltage support, and transient overcurrent of new energy converters are more likely to occur.

[0003] To improve the frequency support and fault adaptability of renewable energy grid-connected systems, existing technologies mainly address the frequency support from the perspectives of virtual synchronous control, flywheel energy storage-assisted frequency regulation, or synchronous condenser support. However, the following shortcomings remain: First, the inertia support of virtual synchronous control technology essentially relies on power electronic control algorithms, lacking real physical kinetic energy support, and the converter still directly bears the fault impact during grid short-circuit faults. Second, supplementary energy storage technology requires independent power electronic converter access, and during faults, both the energy storage converter and the renewable energy converter are still subject to fault voltage and transient current impacts, making it difficult to structurally suppress the direct electrical impact of grid faults. Third, in synchronous condenser technology, the renewable energy generation unit is still directly connected to the AC grid, making it difficult to simultaneously achieve renewable energy power transmission, physical inertia support, and renewable energy-side fault disturbance isolation. Summary of the Invention

[0004] In order to enable new energy power generation units to no longer rely entirely on power electronic converters to be directly connected to the AC grid, but to achieve power transmission, frequency support and fault disturbance isolation through an electromechanical coupling interface with physical rotational inertia, this invention provides a new energy grid connection method and system based on an electromechanical interface.

[0005] A renewable energy grid connection method based on an electromechanical interface is applied to a renewable energy grid connection system based on an electromechanical interface. The system includes: a renewable energy power generation unit that converts renewable energy into DC power; a renewable energy-side converter connected to the DC side of the renewable energy power generation unit; an AC side of the renewable energy-side converter connected to the stator of a synchronous motor; a synchronous motor that receives AC power output from the renewable energy-side converter at its stator and converts it into mechanical energy; a synchronous generator whose stator is directly connected to the AC grid and converts mechanical energy into AC power to be sent to the grid; a high-inertia flywheel disposed between the synchronous motor and the synchronous generator; and the rotor of the synchronous motor, the central shaft of the high-inertia flywheel, and the rotor of the synchronous generator are rigidly connected coaxially via a mechanical shaft to form a combined shaft system including the synchronous motor, the high-inertia flywheel, and the synchronous generator.

[0006] The method includes: under frequency disturbance, determining the inertial time constant of the high-inertia flywheel based on the target support power, support duration, and allowable speed deviation of the high-inertia flywheel;

[0007] Based on the inertial time constant of the high-inertia flywheel, the inertial support power provided by the combined shaft system to the power grid is determined;

[0008] When the combined shaft system experiences power imbalance, the rotational inertia of the high-inertia flywheel is used to correct the inertia of the combined shaft system to support power, thereby absorbing or releasing kinetic energy.

[0009] Optionally, determining the inertial time constant of the high-inertia flywheel under frequency disturbance based on the target support power, support duration, and allowable speed deviation of the high-inertia flywheel includes:

[0010] Based on the target support power, input the allowable speed deviation and support duration, and calculate the required support energy;

[0011] Estimate the required total inertia based on the relationship between the kinetic energy changes of the shaft system;

[0012] Determine the flywheel inertia based on the required total inertia;

[0013] Based on the flywheel inertia, the design conditions for the inertial time constant of a high-inertia flywheel are obtained.

[0014] If the inertia time constant of the high-inertia flywheel can meet the design conditions, the flywheel parameter selection is completed; if the inertia time constant of the high-inertia flywheel cannot meet the design conditions, the allowable speed deviation and support duration are re-entered to design the flywheel inertia and calculate the inertia time constant of the high-inertia flywheel until the design conditions are met, and the flywheel parameter selection is completed.

[0015] Optionally, determining the inertial support power provided by the combined shaft system to the power grid based on the inertial time constant of the high-inertia flywheel includes:

[0016] Based on the inertial time constant of the high-inertia flywheel, the rotational speed change rate of the combined shaft system is determined;

[0017] Based on the rotational speed change rate of the combined shaft system, the inertial support power provided by the combined shaft system to the power grid is determined.

[0018] Optionally, determining the inertia support power provided by the combined shaft system to the power grid based on the rotational speed change rate of the combined shaft system includes:

[0019] The mechanical kinetic energy stored in the combined shaft system is determined, and in the per-unit system, the transient support power supplied to the power grid is determined based on the rate of change of the rotational speed of the combined shaft system.

[0020] Based on the transient support power, the inertia support power provided by the combined shaft system to the power grid is determined.

[0021] Optional, also includes:

[0022] Under grid voltage fault conditions, the combined shaft power imbalance is determined based on the electromagnetic power of the synchronous generator and the input power of the synchronous motor, and the target speed change rate is determined based on the combined shaft power imbalance.

[0023] Based on the synchronous generator electromagnetic power and the target speed change rate, a power reference value for the new energy side converter is generated, and the power reference value for the new energy side converter is corrected to generate a power command for the new energy side converter.

[0024] Optionally, under grid voltage fault conditions, the combined shaft power imbalance is determined based on the electromagnetic power of the synchronous generator and the input power of the synchronous motor, and the target speed change rate is determined based on the combined shaft power imbalance, including:

[0025] Calculate the electromagnetic power of the synchronous generator based on the voltage and current on the synchronous generator side;

[0026] Calculate the input power of the synchronous motor based on the voltage and current on the synchronous motor side;

[0027] The power imbalance of the combined shaft system is determined based on the electromagnetic power of the synchronous generator, the speed deviation of the combined shaft system, and the input power of the synchronous motor.

[0028] The allowable deceleration rate and allowable acceleration rate are determined based on the power imbalance.

[0029] The target speed change rate is determined based on the combined shaft speed deviation, allowable deceleration rate, and allowable acceleration rate.

[0030] Optionally, based on the synchronous generator electromagnetic power and the target speed change rate, a power reference value for the new energy side converter is generated, and the power reference value for the new energy side converter is corrected to generate a power command for the new energy side converter, including:

[0031] Based on the synchronous generator electromagnetic power, combined shaft speed deviation, combined shaft total inertia and the target speed change rate, a reference value for the power of the new energy side converter is generated.

[0032] The power reference value of the new energy side converter is corrected based on the DC bus voltage deviation, and the power command of the new energy side converter is generated in combination with the power limiting to adjust the synchronous motor side current.

[0033] Optionally, based on the synchronous generator electromagnetic power and the target speed change rate, a power reference value for the new energy side converter is generated and corrected. After generating the power command for the new energy side converter, the method further includes:

[0034] When a short-circuit fault occurs in the AC power grid, the voltage at the grid connection point of the synchronous generator drops. The d-q axis voltages of the synchronous generator are obtained, and the transient offset of the mechanical speed of the combined shaft system caused by the voltage drop at the grid connection point is calculated based on the stator voltage, stator current, electromagnetic power and combined shaft system equations on the synchronous generator side.

[0035] Based on the transient offset of the combined shaft system's mechanical rotational speed, an inertia adjustment command is generated to increase the total rotational inertia of the combined shaft system and reduce the rate of change of shaft system rotational speed during power grid faults.

[0036] Embodiments of the present invention also provide a new energy grid-connected system based on an electromechanical interface, comprising:

[0037] A new energy power generation unit that converts renewable energy into direct current (DC) power.

[0038] The DC side is connected to the new energy side converter of the new energy power generation unit;

[0039] The AC side of the new energy-side converter is connected to the stator terminal of the synchronous motor;

[0040] The stator receives the AC power output from the new energy-side converter.

[0041] A synchronous motor that converts it into mechanical energy;

[0042] A synchronous generator whose stator is directly connected to the AC power grid, converting mechanical energy into AC electrical energy and sending it into the grid.

[0043] A high-inertia flywheel is disposed between the synchronous motor and the synchronous generator;

[0044] The rotor of the synchronous motor, the central shaft of the high-inertia flywheel, and the rotor of the synchronous generator are rigidly connected coaxially via a mechanical shaft to form a combined shaft system comprising the synchronous motor, the high-inertia flywheel, and the synchronous generator.

[0045] A control device connected to the combined shaft system is used to determine the inertial time constant of the high-inertia flywheel based on the target support power, support duration, and allowable speed deviation of the high-inertia flywheel under frequency disturbances; to determine the inertial support power provided by the combined shaft system to the power grid based on the inertial time constant of the high-inertia flywheel; and to correct the inertial support power of the combined shaft system by utilizing the rotational inertia of the high-inertia flywheel when a power imbalance occurs in the combined shaft system, thereby absorbing or releasing kinetic energy.

[0046] Embodiments of the present invention also provide a computing device, including: a processor and a memory storing a computer program, wherein the computer program, when executed by the processor, performs the method described above.

[0047] Embodiments of the present invention also provide a computer-readable storage medium storing instructions that, when executed on a computer, cause the computer to perform the method described above.

[0048] The beneficial effects of the above technical solution are as follows: By significantly increasing the total rotational inertia and mechanical energy storage capacity of the combined shaft system through the high-inertia flywheel, the rate of change of shaft speed is effectively suppressed during transient processes such as grid faults. This significantly weakens the propagation intensity of fault disturbances to the new energy-side converter through the electromechanical coupling chain of "motor-flywheel-generator," resulting in much smaller fluctuation amplitudes of port voltage, current, and DC bus voltage on the converter side compared to conventional direct grid connection methods. This significantly improves the fault ride-through capability and continuous operation reliability of the new energy unit. Simultaneously, by real-time detection of the synchronous generator terminal voltage amplitude and the introduction of a limiting link to generate excitation voltage commands, the transient internal potential or excitation voltage of the generator can be quickly adjusted, enhancing the voltage support capability at the grid connection point and improving the system's transient stability and grid adaptability. Furthermore, the new energy-side converter is electrically isolated from the AC grid via a synchronous motor-synchronous generator. Voltage dips, harmonics, and other disturbances on the grid side will not directly affect the converter, further ensuring the safe operation of the converter and power quality. This provides a new grid connection solution for high-proportion new energy grid connection that combines inertia support, voltage regulation, and fault isolation functions. Attached Figure Description

[0049] Figure 1 This is a flowchart illustrating the new energy grid connection method based on electromechanical interface according to an embodiment of the present invention.

[0050] Figure 2 This is a block diagram of the modules of the new energy grid-connected system based on electromechanical interfaces according to an embodiment of the present invention.

[0051] Figure 3 This is a flowchart of the flywheel inertia selection process for the electromechanical interface in this embodiment of the invention.

[0052] Figure 4 This is a flowchart of power coordination control under grid voltage fault in an embodiment of the present invention.

[0053] Figure 5 This is a response comparison diagram between the traditional direct grid connection method and the electromechanical interface grid connection method proposed in this invention under frequency disturbance in the embodiments of the present invention.

[0054] Figure 6 This is a response comparison diagram between the traditional direct grid connection method and the electromechanical interface grid connection method proposed in this invention under grid voltage faults in this embodiment of the invention. Detailed Implementation

[0055] The present invention will be further described in detail below with reference to all the accompanying drawings.

[0056] In the following embodiments of the present invention, physical inertia support capability is a core indicator for measuring the power grid's ability to resist frequency abrupt changes. It refers to the power grid's ability to resist rapid frequency changes when a momentary imbalance occurs between power generation and consumption.

[0057] Fault ride-through capability refers to the ability of new energy power generation equipment to maintain grid-connected operation instead of immediately disconnecting from the grid to protect itself when voltage or frequency abnormal faults occur in the power grid.

[0058] like Figure 1 As shown, this embodiment of the invention discloses a new energy grid connection method based on electromechanical interfaces, applicable to, for example... Figure 2 The illustrated new energy grid-connected system based on an electromechanical interface includes a new energy power generation unit, a new energy-side converter, a synchronous motor, a high-inertia flywheel, and a synchronous generator. The DC side of the new energy-side converter is connected to the new energy power generation unit, and the AC side is connected to the stator of the synchronous motor. The converter converts the DC power output from the new energy power generation unit into AC power to drive the synchronous motor. The synchronous motor converts the AC power output from the new energy-side converter into mechanical energy, and its stator is directly connected to the AC power grid, converting the mechanical energy back into AC power and feeding it into the grid. The high-inertia flywheel is positioned between the synchronous motor and the synchronous generator to increase the equivalent rotational inertia of the combined shaft system. The energy exchange of the high-inertia flywheel is determined by the difference between the input power of the synchronous motor and the electromagnetic power of the synchronous generator. The rotor of the synchronous motor, the central shaft of the high-inertia flywheel, and the rotor of the synchronous generator are rigidly connected coaxially via a mechanical shaft, forming a combined shaft system including the synchronous motor, the high-inertia flywheel, and the synchronous generator. The method includes:

[0059] S11, the high-inertia flywheel increases the total rotational inertia and mechanical energy storage capacity of the combined shaft system through its own rotational inertia;

[0060] S12, when the combined shaft system is in steady-state operation, the high-inertia flywheel maintains the energy storage state, inputs the active power output from the new energy side converter into the synchronous motor, and the synchronous motor converts it into mechanical power and drives the combined shaft system to rotate; the combined shaft system transmits the mechanical power to the synchronous generator, and the synchronous generator converts the mechanical power into active power and outputs it to the AC grid;

[0061] S13, determine the terminal voltage amplitude of the synchronous generator, obtain the excitation voltage command through the limiting circuit, and adjust the transient internal potential or excitation voltage of the synchronous generator according to the excitation voltage command;

[0062] S21, Under frequency disturbance, determine the inertial time constant of the high-inertia flywheel based on the target support power, support duration and allowable speed deviation of the high-inertia flywheel;

[0063] S22, Based on the inertial time constant of the high-inertia flywheel, determine the inertial support power provided by the combined shaft system to the power grid;

[0064] S23, when the combined shaft system experiences power imbalance, the rotational inertia of the high-inertia flywheel is used to correct the inertia of the combined shaft system to support power, thereby absorbing or releasing kinetic energy.

[0065] In S11 of this embodiment, the combined shaft system is a system in a generator set that typically integrates different power source shaft systems via couplings to jointly drive the generator. In this embodiment, the synchronous motor M, the high-inertia flywheel FW, and the synchronous generator G constitute the combined shaft system. The high-inertia flywheel is a mechanical energy storage component located between the synchronous motor and the synchronous generator, which increases the total rotational inertia of the combined shaft system by generating rotational inertia. During the operation of the combined shaft system, the high-inertia flywheel is not equipped with an independent power electronic frequency converter drive device and is not used as an independent motor unit for speed control. Instead, it rotates together with the rotors of the synchronous motor and the synchronous generator. Under the first-order approximation that ignores the elastic torsional vibration of the shaft system, the high-inertia flywheel has the same mechanical angular velocity and the same mechanical rotation angle as the rotors of the synchronous motor and the synchronous generator. Therefore, the high-inertia flywheel does not introduce an independent speed state. However, it increases the total rotational inertia and mechanical energy storage capacity of the combined shaft system through its own rotational inertia, thereby changing the rate of change of the combined shaft system's speed under power imbalance. The combined shaft system satisfies the following relationship:

[0066]

[0067]

[0068]

[0069]

[0070] In the formula, , and These are the moments of inertia of the synchronous motor rotor, the high-inertia flywheel, and the synchronous generator rotor, respectively, referred to the same mechanical shaft. The total moment of inertia of the combined shaft system. Let be the moment of inertia of the i-th rotating component; For the mechanical kinetic energy of the combined shaft system; and These are the actual mechanical angular velocity and the rated mechanical angular velocity of the combined shaft system, respectively. To unify the power base value; For components The inertial time constant, where ; , and These are the inertial time constants of a synchronous motor, a high-inertia flywheel, and a synchronous generator, respectively. The total inertia time constant of the combined shaft system.

[0071] Under the first-order approximation that neglects the elastic torsional vibration of the shaft system, the three components have the same mechanical angular velocity and the same mechanical rotation angle, that is:

[0072]

[0073]

[0074] In the formula, , and These are the mechanical angular velocities of a synchronous motor, a high-inertia flywheel, and a synchronous generator, respectively. , and These are the mechanical rotation angles of the three components; Let be the mechanical rotation angle of the combined shaft system. Therefore, the combined shaft system can be represented by a uniform mechanical angular velocity. and unified mechanical angle Its dynamic behavior can be uniformly characterized by a single rotational equation.

[0075] Under a unified power base value and the base value of rated mechanical angular velocity Under these conditions, the total inertia time constant of the combined shaft system can be expressed as: ;

[0076] Under the aforementioned per-unit datum, the per-unit motion equation of the combined shaft system is:

[0077]

[0078] In the formula, For time; For the mechanical speed deviation of the combined shaft system; To absorb and convert electromagnetic power from the new energy side converter into mechanical shaft input; The electromagnetic power output by the synchronous generator to the AC power grid; The equivalent damping coefficient of the combined shaft system; This represents the dimensionless per-unit speed deviation. and All are based on The per-unit power is the power base value.

[0079] In S12, when the combined shaft system is in steady-state operation, the high-inertia flywheel is kept in an energy-storing state, and no significant net energy exchange occurs; under steady-state operating conditions... Therefore: That is, the active power output by the new energy converter is converted into mechanical power by the synchronous motor, and then transmitted to the synchronous generator through the mechanical shaft system, and then sent to the AC grid by the synchronous generator; at this time, the high inertia flywheel is mainly in the energy storage and maintenance state, and no significant net energy exchange occurs.

[0080] In S13, the terminal voltage amplitude of the synchronous generator is detected, an excitation voltage command is obtained through a limiting circuit, and the transient internal potential or excitation voltage of the synchronous generator is adjusted according to the excitation voltage command. An engineering model with transient potential followed by equivalent reactance can be used on the synchronous generator side. Let the transient internal potential of the synchronous generator be... The sum of the generator transient reactance and the external network equivalent reactance is The equivalent voltage of the AC power grid is Then the current on the synchronous generator side satisfies: ;

[0081] The active power at the synchronous generator port is: ;

[0082] When resistance is ignored and the classical work angle relationship is used, it can be written as: ;

[0083] The terminal voltage of a synchronous generator can be written as: ,Right now: ;

[0084] In the above formula, the underlined quantity represents the AC phasor, and the ununderlined quantity represents the corresponding phasor amplitude; This refers to the transient internal potential phasor of a synchronous generator. Its amplitude, The power angle of the transient internal potential phasor of the synchronous generator relative to the equivalent voltage phasor of the AC grid, where j is the imaginary unit. Represents the phase rotation factor, corresponding to the phasor angle. ; For AC power grid equivalent voltage phasors, Its amplitude; This refers to the current phasor flowing from the synchronous generator to the AC power grid; This refers to the phasor of the synchronous generator terminal voltage. For the transient reactance of the synchronous generator, This is the equivalent reactance from the generator terminal to the AC power grid, and ; The imaginary unit, To take the real part of a complex number, This indicates the conjugate operation.

[0085] Synchronous generators are equipped with excitation regulation devices to adjust transient internal electromotive force. or excitation voltage In one implementation, the excitation adjustment command is: ;

[0086] In the formula, This is the excitation voltage command. This indicates the input quantity. Limited to the range Inside, This is the steady-state excitation voltage. To adjust the excitation gain, This is the reference value for the generator terminal voltage. This represents the amplitude of the synchronous generator terminal voltage. and These are the lower and upper limits of the excitation voltage, respectively. The excitation regulation command regulates the excitation of the synchronous generator to maintain a stable terminal voltage and controls the input power of the synchronous motor to maintain a balance with the electromagnetic power of the synchronous generator.

[0087] In this embodiment, the combined shaft system realizes the mechanical transfer of new energy power from the new energy side converter to the AC grid under steady state, and releases or absorbs the kinetic energy of the shaft system under grid frequency disturbance, thereby providing physical inertia response for the AC grid;

[0088] There is no direct electrical connection between the stator circuit of the synchronous motor and the stator circuit of the synchronous generator. The disturbance of the AC power grid is indirectly transmitted to the new energy-side converter side through the combined shaft power imbalance caused by the electromagnetic power change of the synchronous generator.

[0089] The high-inertia flywheel can be a solid disc type, a solid cylindrical type, a rim-spoke type, or other mechanical flywheel structure capable of providing the target rotational inertia, and can be manufactured using metal or composite materials. This invention is not limited to the specific geometry and material form of the flywheel. The new energy-side converter can employ an active power outer loop, a DC bus voltage outer loop, or a current inner loop control method to provide a set electromagnetic power to the synchronous motor; the synchronous generator can be equipped with an excitation regulation device to provide voltage and reactive power support during grid disturbances; the inertia parameters of the high-inertia flywheel can be selected based on the target frequency support power, support duration, and allowable speed deviation to meet the inertia support requirements under different new energy grid connection scenarios.

[0090] In S21 of the above embodiment, under frequency disturbance, the inertial time constant of the high-inertia flywheel is determined based on the target support power, support duration, and allowable speed deviation of the high-inertia flywheel; wherein, the allowable speed deviation refers to the speed deviation of the combined shaft system composed of the synchronous motor, the high-inertia flywheel, and the synchronous generator relative to the rated mechanical angular velocity. Since the three are rigidly connected coaxially, it is also equivalent to the speed deviation of each of the three components.

[0091] like Figure 3 As shown, in an optional embodiment of the present invention, in S21, under frequency disturbance, determining the inertial time constant of the high-inertia flywheel based on the target support power, support duration, and allowable speed deviation of the high-inertia flywheel may include:

[0092] S211, based on the target support power, input the allowable speed deviation and support duration, and calculate the required support energy;

[0093] S212, estimate the required total inertia based on the relationship of shaft kinetic energy change;

[0094] S213, Determine the flywheel inertia based on the required total inertia;

[0095] S214, Based on the flywheel inertia, obtain the design conditions for the inertia time constant of the high-inertia flywheel;

[0096] S215, If the inertia time constant of the high-inertia flywheel can meet the design conditions, then the flywheel parameter selection is completed; if the inertia time constant of the high-inertia flywheel cannot meet the design conditions, then re-enter the allowable speed deviation and support duration to design the flywheel inertia and calculate the inertia time constant of the high-inertia flywheel until the design conditions are met, and the flywheel parameter selection is completed.

[0097] In S211 of this embodiment, if the combined shaft system is required to provide the target support power... Then, based on the input allowable speed deviation and support duration Design the flywheel inertia and calculate the required supporting energy. This refers to the mechanical kinetic energy that can be released during the deceleration of the combined shaft system from the rated speed to the minimum permissible speed.

[0098] In S212, the required total inertia is estimated based on the relationship between the kinetic energy changes of the shaft system. ,in, The total inertia time constant of the combined shaft system is used to characterize the equivalent inertia of the synchronous motor, high-inertia flywheel, and synchronous generator, and to establish a frequency support relationship based on the rotational speed deviation of the combined shaft system. Let be the inertial time constant of the synchronous motor rotor. The inertial time constant of a high-inertia flywheel. The inertial time constant of the synchronous generator rotor;

[0099] In S213, the inertia of the synchronous motor is deducted based on the required total inertia. and synchronous generator inertia Determine the flywheel moment of inertia ;

[0100] In S214, the frequency support capacity and rotational speed deviation constraints are checked, and the mechanical kinetic energy that the combined shaft system can release during the support time is calculated: ,in, To unify the power base value, To allow the maximum allowable speed deviation; and to ensure that the allowable released mechanical kinetic energy is not less than the releaseable mechanical kinetic energy, that is: Therefore, the design condition for the inertial time constant of a high-inertia flywheel is: ;

[0101] In step S215, if the design conditions are met, the flywheel parameter selection is completed; if the design conditions are not met, the allowable speed deviation is re-entered. and support duration The flywheel inertia is designed and calculated until the design conditions are met, thus completing the flywheel parameter selection. The design conditions provide the implementation path for the inertia time constant of the high-inertia flywheel. Based on the design conditions for the inertia time constant of the high-inertia flywheel, the known inertia time constants of the synchronous motor rotor and the synchronous generator rotor, the inertia time constant of the high-inertia flywheel that needs to be additionally configured can be determined.

[0102] In an optional embodiment of the present invention, in S22, determining the inertia support power provided by the combined shaft system to the power grid based on the inertia time constant of the high-inertia flywheel may include:

[0103] S221, Based on the inertial time constant of the high-inertia flywheel, determine the rotational speed change rate of the combined shaft system;

[0104] S222, Based on the rotational speed change rate of the combined shaft system, determine the inertia support power that the combined shaft system provides to the power grid.

[0105] In S221 of this embodiment, when an active power imbalance occurs in the AC power grid and causes a frequency shift, the synchronous motor, high-inertia flywheel, and synchronous generator are considered as a coaxially rigidly connected joint shaft system. Under a unified power base value and a unified speed base value, their speed dynamically satisfies: ;

[0106] In the formula, The total inertial time constant of the combined shaft system; , , These are the inertial time constants of a synchronous motor, a high-inertia flywheel, and a synchronous generator, respectively. This represents the per-unit speed deviation of the combined shaft system relative to the synchronous speed. For the combined shaft system mechanical angular velocity; The electromagnetic power input to the synchronous motor from the new energy side converter and converted into mechanical shaft power; The electromagnetic power output by the synchronous generator to the AC power grid; is the equivalent damping coefficient of the combined shaft system.

[0107] In S222, when the grid frequency decreases, the electromagnetic power output of the synchronous generator and the kinetic energy release of the combined shaft system together provide transient support to the grid. Under the condition of maintaining synchronous operation with small deviation, the rotational speed deviation of the combined shaft system approximately follows the change in grid frequency deviation. Therefore, the inertial support power provided by the combined shaft system to the grid satisfies: In the formula, This refers to the power grid frequency deviation. The rate of change of AC power grid frequency; This represents the incremental mechanical input power on the converter side.

[0108] In S23, when a power imbalance occurs in the combined shaft system, the inertial time constant of the high-inertia flywheel is increased. Increase the total inertial time constant of the combined shaft system By changing the inertia support power of the combined shaft system, the physical inertia response capability of the combined shaft system to frequency changes is improved, and the grid frequency change rate is reduced, which is conducive to the combined shaft system returning to a steady-state operating state.

[0109] like Figure 5As shown, in an optional embodiment of the present invention, in S222, determining the inertia support power provided by the combined shaft system to the power grid based on the rotational speed change rate of the combined shaft system may include:

[0110] S2221, determine the mechanical kinetic energy stored in the combined shaft system, and in the per-unit system, determine the transient support power supplied to the power grid based on the rotational speed change rate of the combined shaft system;

[0111] S2222, Based on the transient support power, determine the inertia support power that the combined shaft system provides to the power grid.

[0112] In S2221 of this embodiment, when an active power imbalance occurs in the AC power grid and causes a frequency shift, the mechanical kinetic energy stored in the combined shaft system is: In the formula, This refers to the total moment of inertia of the electric motor, flywheel, and generator referred to the same mechanical axis. Let be the mechanical angular velocity. In a per-unit system, let be... The rated mechanical angular velocity of the combined shaft system, Let be the mechanical kinetic energy stored in the combined shaft system at its rated mechanical angular velocity. Then, the mechanical kinetic energy and the inertial time constant satisfy the following: .

[0113] When a power grid imbalance causes a frequency drop, the electromagnetic power of the synchronous generator... When the relationship with the power angle of the power grid changes, the combined shaft system is adjusted according to the formula. During deceleration, the kinetic energy stored in the flywheel and dual rotors is released and injected into the grid through the synchronous generator. The system's additional transient support power supplied to the grid satisfies: .

[0114] In S2222, under the condition of maintaining synchronous operation and the short-term deviation of the shaft speed approximately following the deviation of the power grid frequency, there is: Then the transient support power formula is: The formula for the power of inertia support can be obtained from deformable models: In the formula, Used to represent the additional support power that the system provides to the grid relative to the pre-disturbance steady state. This is the power increment input to the synchronous motor; The per-unit frequency deviation of the power grid, where, and These are the actual frequency and the rated frequency of the power grid, respectively. The per-unit frequency change rate of the power grid. The transient support power supplied by the system to the power grid depends on the rate of change of the mechanical kinetic energy of the combined shaft system and the flywheel inertia. The larger the total inertia The larger the value, the greater the inertial support power that can be generated under the same grid frequency change rate.

[0115] In an optional embodiment of the present invention, the renewable energy grid connection method based on electromechanical interfaces may include:

[0116] S31, Under the condition of grid voltage fault, determine the combined shaft power imbalance based on the electromagnetic power of the synchronous generator and the input power of the synchronous motor, and determine the target speed change rate based on the combined shaft power imbalance;

[0117] S32, based on the electromagnetic power of the synchronous generator and the target speed change rate, generates a power reference value for the new energy side converter, corrects the power reference value for the new energy side converter, and generates a power command for the new energy side converter.

[0118] like Figure 4 As shown, in an optional embodiment of the present invention, in S31, under a grid voltage fault, determining the combined shaft power imbalance based on the electromagnetic power of the synchronous generator and the input power of the synchronous motor, and determining the target speed change rate based on the combined shaft power imbalance, may include:

[0119] S311, Real-time detection of the voltage at the grid connection point of the synchronous generator; When the voltage at the grid connection point is lower than a preset threshold, it is determined that a voltage fault has occurred in the AC power grid.

[0120] S312, Calculate the electromagnetic power of the synchronous generator based on the voltage and current on the synchronous generator side;

[0121] S313, Calculate the input power of the synchronous motor based on the voltage and current on the synchronous motor side;

[0122] S314. Determine the power imbalance of the combined shaft system based on the electromagnetic power of the synchronous generator, the speed deviation of the combined shaft system, and the input power of the synchronous motor.

[0123] The allowable deceleration rate and allowable acceleration rate are determined based on the power imbalance.

[0124] S315, determine the target speed change rate based on the combined shaft speed deviation, allowable deceleration rate of change, and allowable acceleration rate of change.

[0125] In S311 of this embodiment, the voltage at the grid connection point of the synchronous generator is detected in real time. When the grid connection point voltage is lower than a preset threshold, i.e. ,in, The threshold for determining voltage faults at the grid connection point. and Using the same actual value or per-unit value, a voltage fault is determined to have occurred in the AC power grid.

[0126] In S312, the electromagnetic power of the synchronous generator is calculated based on the voltage and current on the synchronous generator side. , ,in, , These are the d-axis and q-axis voltages of the synchronous generator stator, respectively. , These are the stator d-axis and q-axis currents of the synchronous generator, respectively.

[0127] In S313, the input power of the synchronous motor is calculated based on the voltage and current on the synchronous motor side. , ,in, , These are the stator d-axis and q-axis voltages of the synchronous motor, respectively. , These are the stator d-axis and q-axis currents of the synchronous motor, respectively.

[0128] In S314, based on the electromagnetic power of the synchronous generator Combined shaft speed deviation and synchronous motor input power Determine the power imbalance of the combined shaft system: From the axis system equations: It can be seen that when At that time, the combined shaft system absorbs energy and accelerates; when At this time, the combined shaft system releases energy and decelerates. This leads to the control objective: ,in, and These are the allowable rate of deceleration and the allowable rate of acceleration, respectively.

[0129] In S315, based on the combined shaft speed deviation Permissible rate of deceleration and allow for accelerated rate of change Generate the target rotational speed change rate: ,in, This is the speed deviation adjustment coefficient.

[0130] like Figure 4 As shown, in an optional embodiment of the present invention, in S32, based on the electromagnetic power of the synchronous generator and the target speed change rate, a power reference value for the new energy side converter is generated, and the power reference value for the new energy side converter is corrected to generate a power command for the new energy side converter. This may include:

[0131] S321, based on the electromagnetic power of the synchronous generator, the combined shaft speed deviation, the combined shaft total inertia and the target speed change rate, generate the power reference value of the new energy side converter;

[0132] S322, corrects the power reference value of the new energy side converter according to the DC bus voltage deviation, and generates a power command for the new energy side converter in combination with power limiting, and adjusts the current on the synchronous motor side.

[0133] In S321 of this embodiment, based on the electromagnetic power of the synchronous generator Combined shaft speed deviation Total inertia of the combined shaft system and the target rotational speed change rate The power reference value of the converter on the new energy side can be calculated by back-calculating the shaft system equation: ;

[0134] In S322, based on the DC bus voltage deviation The power reference value of the new energy side converter is corrected, and power limiting is applied. Generate power commands for the new energy side converter ,in, This is the DC bus voltage correction factor. For the DC bus voltage on the new energy side, This is the reference value for the DC bus voltage. This represents the upper limit of the power command for the new energy-side converter. This refers to the upper limit of the allowable current on the stator side of the new energy-side converter or synchronous motor. The lower limit of the power command for the new energy side converter can be determined based on whether the converter allows reverse power flow; if reverse power flow is not allowed, then take [the lower limit]. ; The final power command obtained after power limiting is the actual input electromagnetic power of the synchronous motor when the converter tracks the command. and the electromagnetic torque acting on the combined shaft system They are respectively: ; In the formula, The electromagnetic torque of the synchronous motor acting on the combined shaft system is determined by the new energy side converter according to... Generate a reference current for the motor side.

[0135] Based on the power command of the new energy-side converter and according to the energy balance relationship between the energy storage change of the DC bus capacitor and the input power of the new energy power generation unit, the synchronous motor side current is dynamically corrected and adjusted. The new energy-side converter uses current inner loop tracking: , ,in, This is the reference value for the d-axis current of the synchronous motor. This is the reference value for the q-axis current of the synchronous motor; to illustrate the effect of DC bus voltage correction, the DC bus energy balance relationship is as follows: In the formula, For the DC bus capacitor on the new energy side, This provides the power input to the DC bus of the new energy power generation unit. The converter power control enables the actual power... Tracking and correcting reference values Under the condition that, when At that time, improve This increases the output power of the DC bus; when When, then reduce This helps to suppress DC bus voltage deviation.

[0136] If active and reactive power decoupling control is adopted, the reference value of the d-axis current of the synchronous motor generated by the new energy side converter is... The calculation formula is: Reference value of q-axis current of synchronous motor The calculation formula is: In the formula, This is the reactive power command for the synchronous motor, and its positive direction is according to... Sure.

[0137] In an optional embodiment of the present invention, S32, after generating a power reference value for the new energy side converter based on the electromagnetic power of the synchronous generator and the target speed change rate, and correcting the power reference value for the new energy side converter, and generating a power command for the new energy side converter, the method further includes:

[0138] S33, When a short-circuit fault occurs in the AC power grid, the voltage at the grid connection point of the synchronous generator drops. Obtain the d-q axis voltage of the synchronous generator, and calculate the transient offset of the mechanical speed of the combined shaft system caused by the voltage drop at the grid connection point based on the stator voltage, stator current, electromagnetic power and combined shaft system equations on the synchronous generator side.

[0139] S34. Based on the transient offset of the mechanical rotational speed of the combined shaft system, an inertia adjustment command is generated to increase the total rotational inertia of the combined shaft system and reduce the rate of change of shaft rotational speed during power grid faults.

[0140] In S331 of this embodiment, the synchronous generator is directly connected to the power grid, and the synchronous motor is powered by the renewable energy converter. There is no direct electrical connection between the stator circuit of the synchronous motor and the stator circuit of the synchronous generator; therefore, grid faults will not be directly transmitted to the renewable energy converter side as an electrical short-circuit path. The synchronous motor and synchronous generator respectively satisfy the following d-q axis voltage equations:

[0141]

[0142] In the formula, , These are the stator d-axis and q-axis voltages of the synchronous motor, respectively. , These are the d-axis and q-axis voltages of the synchronous generator stator, respectively. , These are the stator d-axis and q-axis currents of the synchronous motor, respectively. , These are the stator d-axis and q-axis currents of the synchronous generator, respectively. , , , These are the stator d-axis and q-axis magnetic flux linkages of a synchronous motor and a synchronous generator, respectively. , Stator resistance; and The electrical angular velocities of the synchronous motor and the synchronous generator are respectively, satisfying... , ; and These represent the number of pole pairs for synchronous motors and synchronous generators, respectively.

[0143] In one implementation, if both the synchronous motor and the synchronous generator adopt an electrically excited synchronous machine model, then the stator d-axis and q-axis flux linkages can be specifically represented as follows:

[0144]

[0145] In the formula, , These are the equivalent inductances of the d and q axes of the synchronous motor, respectively. , These are the equivalent inductances of the d-axis and q-axis of the synchronous generator, respectively. , These are the mutual inductances between the excitation windings of the synchronous motor and the d-axis, respectively; and These are the excitation winding currents of the synchronous motor and the synchronous generator, respectively.

[0146] When a short-circuit fault occurs in the AC power grid, causing a drop in the grid connection point voltage of the synchronous generator, the voltage drop first affects the synchronous generator side, changing the stator voltage on the synchronous generator side. Stator current and electromagnetic power And through the joint shaft system equations Increase the mechanical speed of the combined shaft system A transient offset occurs; calculate the transient offset of the combined shaft mechanical speed caused by the voltage drop at the grid connection point. Therefore, according to the synchronous generator-grid interface equation, an AC grid voltage fault first changes... , , and It is indirectly propagated through a chain coupling of "generator electromagnetic state - combined shaft mechanical state - motor electromagnetic state".

[0147] Based on the synchronous motor's d-q axis voltage equation, the synchronous motor's d- and q-axis current reference values ​​generated by the new energy-side converter, and the current inner-loop control relationship, it can be determined that the electromagnetic power of the synchronous generator... The transient changes are transmitted through the dynamic relationship of the joint shaft system. The power imbalance affects the combined shaft system and causes changes in the combined shaft system's speed deviation; from the synchronous motor d-q equation and the converter current control equation, it can be seen that the voltage and current on the synchronous motor side are caused by... , , , , And the converter control commands determine the voltage, but do not include AC grid fault voltage. Direct algebraic input items.

[0148] In S332, an inertia adjustment command is generated based on the transient offset of the combined shaft system's mechanical speed. This increases the total rotational inertia of the combined shaft system, weakens the propagation intensity of the transient offset to the synchronous motor side and the new energy converter side via the chain coupling path, and reduces the fluctuation amplitude of the port voltage, current, and DC bus voltage on the new energy converter side. Since there is no direct electrical input channel for the synchronous motor side to the grid fault, it propagates indirectly through the chain coupling of "generator electromagnetic state - combined shaft system mechanical state - motor electromagnetic state." Because the high-inertia flywheel provides a large rotational inertia, the combined shaft system's speed change rate satisfies:

[0149]

[0150] Therefore, the moment of inertia of the combined shaft system should be increased. This invention can reduce the rate of change of shaft speed during a fault, thereby weakening the propagation intensity of fault disturbances to the synchronous motor side and the renewable energy converter side. Therefore, under the same external fault conditions, this invention can significantly reduce the amplitude of port voltage, current, and DC bus fluctuations on the renewable energy converter side compared to the direct grid connection method, thereby improving the continuous operation capability and fault ride-through capability of renewable energy units.

[0151] like Figure 6As shown, in this embodiment of the present invention, since there is no direct stator electrical connection between the synchronous motor side and the synchronous generator side, there is no direct stator electrical input channel for grid disturbances to the new energy converter side. Its impact is mainly transmitted through the combined shaft power imbalance caused by the change in electromagnetic power of the synchronous generator. By actively adjusting the power imbalance, the buffering effect of grid frequency disturbances and voltage fault disturbances can be achieved simultaneously, reducing the propagation intensity of disturbances to the new energy side, and improving the stable operation capability and fault ride-through capability of the new energy power generation unit.

[0152] The implementation principle of the new energy grid connection method based on electromechanical interface in this application embodiment is as follows: Under frequency disturbance, the synchronous motor, high inertia flywheel and synchronous generator are regarded as a coaxial rigidly connected joint shaft system. A joint shaft system dynamic model is established under a unified power base value and a unified speed base value, and the total inertia is used as the basis for the model. The equivalent inertia of synchronous motors, high-inertia flywheels, and synchronous generators is characterized, and a frequency support relationship is established based on the combined shaft speed deviation.

[0153] When an active power imbalance occurs in the AC power grid, causing a frequency shift, the imbalance between the electromagnetic power of the synchronous generator and the input mechanical power of the synchronous motor causes the combined shaft system to accelerate or decelerate. The mechanical kinetic energy stored in the combined shaft system is then released or absorbed, thus forming inertial support against changes in grid frequency. By increasing the inertial time constant of the high-inertia flywheel, the total inertia of the combined shaft system can be increased, the rate of change of grid frequency can be reduced, and the system's transient support capability against frequency disturbances can be enhanced.

[0154] Under grid voltage fault conditions, detect the voltage at the synchronous generator's grid connection point and calculate the synchronous generator's electromagnetic power. and synchronous motor input power ,according to The power imbalance of the combined shaft system is obtained, and based on the total inertia of the combined shaft system... The system allows for the generation of power commands for the new energy-side converter based on the rate of change of rotational speed and the deviation of DC bus voltage, thereby limiting the rate of change of shaft speed, synchronous motor-side current, and DC bus voltage fluctuations.

[0155] Embodiments of the present invention also provide a new energy grid-connected system based on an electromechanical interface, comprising:

[0156] New energy power generation unit, used to convert renewable energy into DC power;

[0157] The new energy side converter is connected to the new energy power generation unit on the DC side and to the stator of the synchronous motor on the AC side.

[0158] A synchronous motor, whose stator receives AC electrical energy output from the new energy-side converter and converts it into mechanical energy;

[0159] Synchronous generators have their stator terminals directly connected to the AC power grid to convert mechanical energy into AC electrical energy and send it into the grid.

[0160] A high-inertia flywheel is disposed between the synchronous motor and the synchronous generator;

[0161] The energy exchange of the high-inertia flywheel is not accomplished through an independent energy storage converter, but rather through the input power of the synchronous motor. Electromagnetic power of synchronous generator The difference between them determines the amount. When At that time, the combined shaft system absorbs energy and accelerates, increasing the mechanical kinetic energy of the flywheel; when At this time, the combined shaft system releases mechanical kinetic energy and decelerates. The new energy side converter does not directly control the flywheel speed, but rather adjusts the input power of the synchronous motor. It indirectly regulates the power imbalance of the combined shaft system and the change in flywheel kinetic energy.

[0162] The high-inertia flywheel is a mechanical energy storage component without an independent frequency converter drive; it utilizes its own rotational inertia. Increase the total moment of inertia of the combined shaft system consisting of the synchronous motor rotor, the high-inertia flywheel, and the synchronous generator rotor.

[0163] The synchronous generator has a pole pair number that matches the rated frequency of the AC power grid, and the synchronous generator operates synchronously with the AC power grid during grid connection. The high-inertia flywheel is mounted on a combined shaft system consisting of the synchronous motor rotor and the synchronous generator rotor, and its rotational speed varies with the rotational speed of the combined shaft system. It is not equipped with an independent power electronic frequency conversion drive device.

[0164] In the above embodiments of the present invention, a coaxially coupled joint shaft system is formed by a synchronous motor, a high-inertia flywheel, and a synchronous generator, enabling the output power of the new energy power generation unit to be sent to the AC grid via an energy conversion path of "electrical energy - mechanical energy - electrical energy". Compared with the direct grid connection of new energy converters, the present invention introduces a mechanical coupling link between the new energy side and the AC grid side, reducing the risk of grid short-circuit faults directly impacting the new energy-side converter through the stator electrical channel, and improving the continuous operation capability of the new energy grid-connected system during grid disturbances.

[0165] By using a high-inertia flywheel to increase the equivalent rotational inertia of the combined shaft system, when the power grid experiences active power imbalance and frequency abrupt changes, the combined shaft system can release or absorb mechanical kinetic energy through transient changes in rotational speed, forming real physical inertia support. This reduces the complete reliance on power electronic virtual inertia control, thereby enhancing the frequency transient support capability of high-proportion new energy power systems.

[0166] The combined shaft dynamics of synchronous motor, high-inertia flywheel, and synchronous generator were established. The input power of synchronous motor, electromagnetic power of synchronous generator, total inertia of combined shaft system, and equivalent damping were unified into the same dynamic model. This model can be used to describe the energy exchange process of shaft system under frequency disturbance and voltage fault, and provides a calculation basis for flywheel inertia selection, frequency support capability assessment, and new energy grid connection interface parameter design.

[0167] The electromagnetic power of the synchronous generator is calculated by detecting the voltage at the grid connection point. and synchronous motor input power and according to The power command generated by the new energy side converter can actively limit the rate of change of the combined shaft speed during grid voltage faults, and suppress excessive fluctuations in the synchronous motor side current and the DC bus voltage on the new energy side.

[0168] Since there is no direct stator electrical connection between the synchronous motor side and the synchronous generator side, the impact of grid short-circuit faults on the renewable energy-side converter is mainly propagated indirectly through electromagnetic power disturbances of the synchronous generator, combined shaft speed disturbances, and changes in the electromagnetic state of the synchronous motor. By utilizing the electromechanical buffering effect of the high-inertia flywheel and the power coordination control of the renewable energy-side converter, the speed and amplitude of fault disturbance propagation to the renewable energy side can be reduced. This balances renewable energy power output, frequency inertia support, and grid fault adaptability, demonstrating significant engineering application value.

[0169] This invention also discloses a computing device, including: a processor and a memory storing a computer program, wherein the computer program, when executed by the processor, performs the method described above. All implementations in the above method embodiments are applicable to this embodiment and can achieve the same technical effects.

[0170] This invention also discloses a computer-readable storage medium storing instructions that, when executed on a computer, cause the computer to perform the method described above. All implementations in the above method embodiments are applicable to this embodiment and achieve the same technical effect. Computer-readable storage media include, for example, various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0171] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on these embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still combine, add, delete, or otherwise adjust the features of the various embodiments of the present invention according to the circumstances without conflict or creative effort, thereby obtaining different technical solutions that do not fundamentally depart from the concept of the present invention. These technical solutions also fall within the scope of protection of the present invention.

Claims

1. A new energy grid connection method based on electromechanical interface, characterized in that, This invention relates to a new energy grid-connected system based on an electromechanical interface. The system includes: a new energy power generation unit that converts renewable energy into DC power; a new energy-side converter connected to the new energy power generation unit on its DC side; an AC side of the new energy-side converter connected to the stator of a synchronous motor; a synchronous motor that receives AC power output from the new energy-side converter at its stator and converts it into mechanical energy; a synchronous generator whose stator is directly connected to the AC grid, converting mechanical energy into AC power and feeding it into the grid; a high-inertia flywheel disposed between the synchronous motor and the synchronous generator; and a coaxial rigid connection between the rotor of the synchronous motor, the central shaft of the high-inertia flywheel, and the rotor of the synchronous generator via a mechanical shaft, forming a combined shaft system comprising the synchronous motor, the high-inertia flywheel, and the synchronous generator. The method includes: under frequency disturbance, determining the inertial time constant of the high-inertia flywheel based on the target support power, support duration, and allowable speed deviation of the high-inertia flywheel; Based on the inertial time constant of the high-inertia flywheel, the inertial support power provided by the combined shaft system to the power grid is determined; When the combined shaft system experiences power imbalance, the rotational inertia of the high-inertia flywheel is used to correct the inertia of the combined shaft system to support power, thereby absorbing or releasing kinetic energy.

2. The renewable energy grid connection method based on electromechanical interface according to claim 1, characterized in that, Based on the target support power, support duration, and allowable speed deviation of the high-inertia flywheel, the inertial time constant of the high-inertia flywheel is determined, including: Based on the target support power, input the allowable speed deviation and support duration, and calculate the required support energy; Estimate the required total inertia based on the relationship between the kinetic energy changes of the shaft system; Determine the flywheel inertia based on the required total inertia; Based on the flywheel inertia, the design conditions for the inertial time constant of a high-inertia flywheel are obtained. If the inertia time constant of the high-inertia flywheel can meet the design conditions, then the flywheel parameter selection is complete. If the inertia time constant of the high-inertia flywheel cannot meet the design conditions, then re-enter the allowable speed deviation and support duration to design the flywheel inertia and calculate the inertia time constant of the high-inertia flywheel until the design conditions are met, thus completing the flywheel parameter selection.

3. The new energy grid connection method based on electromechanical interface according to claim 1, characterized in that, The determination of the inertia support power provided by the combined shaft system to the power grid based on the inertia time constant of the high-inertia flywheel includes: Based on the inertial time constant of the high-inertia flywheel, the rotational speed change rate of the combined shaft system is determined; Based on the rotational speed change rate of the combined shaft system, the inertial support power provided by the combined shaft system to the power grid is determined.

4. The renewable energy grid connection method based on electromechanical interface according to claim 3, characterized in that, The determination of the inertia support power provided by the combined shaft system to the power grid based on the rotational speed change rate of the combined shaft system includes: The mechanical kinetic energy stored in the combined shaft system is determined, and in the per-unit system, the transient support power supplied to the power grid is determined based on the rate of change of the rotational speed of the combined shaft system. Based on the transient support power, the inertia support power provided by the combined shaft system to the power grid is determined.

5. The renewable energy grid connection method based on electromechanical interface according to claim 1, characterized in that, Also includes: Under grid voltage fault conditions, the combined shaft power imbalance is determined based on the electromagnetic power of the synchronous generator and the input power of the synchronous motor, and the target speed change rate is determined based on the combined shaft power imbalance. Based on the synchronous generator electromagnetic power and the target speed change rate, a power reference value for the new energy side converter is generated, and the power reference value for the new energy side converter is corrected to generate a power command for the new energy side converter.

6. The renewable energy grid connection method based on electromechanical interface according to claim 5, characterized in that, Under grid voltage fault conditions, the combined shaft power imbalance is determined based on the electromagnetic power of the synchronous generator and the input power of the synchronous motor, and the target speed change rate is determined based on the combined shaft power imbalance, including: Calculate the electromagnetic power of the synchronous generator based on the voltage and current on the synchronous generator side; Calculate the input power of the synchronous motor based on the voltage and current on the synchronous motor side; The power imbalance of the combined shaft system is determined based on the electromagnetic power of the synchronous generator, the speed deviation of the combined shaft system, and the input power of the synchronous motor. The allowable deceleration rate and allowable acceleration rate are determined based on the power imbalance. The target speed change rate is determined based on the combined shaft speed deviation, allowable deceleration rate, and allowable acceleration rate.

7. The renewable energy grid connection method based on electromechanical interface according to claim 6, characterized in that, Based on the synchronous generator electromagnetic power and the target speed change rate, a power reference value for the new energy side converter is generated. This new energy side converter power reference value is then corrected to generate a new energy side converter power command, including: Based on the synchronous generator electromagnetic power, combined shaft speed deviation, combined shaft total inertia and the target speed change rate, a reference value for the power of the new energy side converter is generated. The power reference value of the new energy side converter is corrected based on the DC bus voltage deviation, and the power command of the new energy side converter is generated in combination with the power limiting to adjust the synchronous motor side current.

8. The renewable energy grid connection method based on electromechanical interface according to claim 5, characterized in that, Based on the synchronous generator electromagnetic power and the target speed change rate, a power reference value for the new energy side converter is generated. This new energy side converter power reference value is then corrected, and a new energy side converter power command is generated. The process further includes: When a short-circuit fault occurs in the AC power grid, the voltage at the grid connection point of the synchronous generator drops. The d-q axis voltages of the synchronous generator are obtained, and the transient offset of the mechanical speed of the combined shaft system caused by the voltage drop at the grid connection point is calculated based on the stator voltage, stator current, electromagnetic power and combined shaft system equations on the synchronous generator side. Based on the transient offset of the combined shaft system's mechanical rotational speed, an inertia adjustment command is generated to increase the total rotational inertia of the combined shaft system and reduce the rate of change of shaft system rotational speed during power grid faults.

9. A new energy grid-connected system based on an electromechanical interface, characterized in that, include: A new energy power generation unit that converts renewable energy into direct current (DC) power. The DC side is connected to the new energy side converter of the new energy power generation unit; The AC side of the new energy-side converter is connected to the stator terminal of the synchronous motor; The stator receives the AC power output from the new energy-side converter. A synchronous motor that converts it into mechanical energy; A synchronous generator whose stator is directly connected to the AC power grid, converting mechanical energy into AC electrical energy and sending it into the grid. A high-inertia flywheel is disposed between the synchronous motor and the synchronous generator; The rotor of the synchronous motor, the central shaft of the high-inertia flywheel, and the rotor of the synchronous generator are rigidly connected coaxially via a mechanical shaft to form a combined shaft system comprising the synchronous motor, the high-inertia flywheel, and the synchronous generator. A control device connected to the combined shaft system is used to determine the inertial time constant of the high-inertia flywheel based on the target support power, support duration, and allowable speed deviation of the high-inertia flywheel under frequency disturbances; to determine the inertial support power provided by the combined shaft system to the power grid based on the inertial time constant of the high-inertia flywheel; and to correct the inertial support power of the combined shaft system by utilizing the rotational inertia of the high-inertia flywheel when a power imbalance occurs in the combined shaft system, thereby absorbing or releasing kinetic energy.

10. A computing device, characterized in that, include: A processor, a memory storing a computer program, wherein the computer program, when executed by the processor, performs the method as described in any one of claims 1 to 8.

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