Isolated high-leakage inductor flywheel energy storage converter and control method thereof

Abstract

This invention discloses an isolated high-leakage-inductance flywheel energy storage converter and its control method. The converter includes a DC bus module, a bidirectional inverter module, a high-leakage-inductance isolation transformer, a flywheel motor interface module, and a main control drive module. The AC output terminal of the bidirectional inverter module is directly electrically connected to the primary side of the high-leakage-inductance isolation transformer, and the secondary side of the high-leakage-inductance isolation transformer is connected to the flywheel motor interface module. The high-leakage-inductance isolation transformer consists of three independent single-phase transformer units forming a three-phase structure. The primary and secondary windings are wound on different columns of the iron core, and the axial center of the primary winding is offset from the axial center of the secondary winding in the iron core. A leakage magnetic path structure with an adjustable air gap is provided between the primary and secondary windings, giving it an adjustable leakage inductance value. The control method switches operating conditions through interlocking of charge and discharge commands and uses multi-closed-loop vector control to adjust the PWM modulation wave. This invention features a simplified topology, superior power quality, fast dynamic response, and high safety.

Description

Technical Field

[0001] This invention belongs to the field of flywheel energy storage power conversion and power electronic converter technology, specifically relating to an isolated high leakage inductance flywheel energy storage converter and its control method. Background Technology

[0002] Flywheel energy storage, as a physical energy storage technology characterized by long lifespan, high safety, and millisecond-level response, possesses significant advantages such as no chemical degradation, high-frequency charging and discharging capability, no flammability or explosion risks, and adaptability to impact conditions. It is now widely used in new power system energy storage scenarios. The flywheel energy storage converter, as the core equipment of the flywheel energy storage system, plays a crucial role in bidirectional power conversion, speed regulation, and voltage and current control between the grid and the flywheel motor. The flywheel energy storage converter needs to simultaneously meet the special requirements of high-frequency PWM modulation, wide-speed-range motor drive, bidirectional power flow, and millisecond-level dynamic switching, placing higher standards on power quality and system integration.

[0003] Currently, mainstream flywheel energy storage converters generally adopt a traditional topology of back-to-back converters, independent isolation transformers, and external filters. This structure requires separate configuration of passive components such as isolation transformers, filter inductors, and filter capacitors, resulting in a large number of components, complex assembly, large equipment size and weight, and low integration, which is not conducive to the miniaturization and modular integration of flywheel energy storage devices. At the same time, the series connection of multiple stages of components increases line impedance and power loss, and the separate arrangement of filter components and isolation transformers leads to more fault points and greater difficulty in later maintenance and repair. More importantly, conventional isolation transformers have extremely low leakage inductance and can only achieve electrical isolation without filtering, requiring the configuration of additional filter circuits. However, external filters are prone to resonance risks, leading to increased harmonic content in the output current, affecting the operational stability of the flywheel motor, and often causing current jitter and speed fluctuations under high-speed conditions. In addition, the split topology has long links, is difficult to match parameters, and has limited dynamic response speed for bidirectional charging and discharging, making it unsuitable for the millisecond-level rapid switching and high-frequency impact charging and discharging characteristics of flywheel energy storage.

[0004] In the prior art, patent CN223844122U discloses a water-cooled flywheel energy storage converter for urban rail transit. This converter includes a main control box assembly, a DC main input circuit, a DC loop, an AC output loop, and a closed-loop water-cooling system. Its AC output loop consists of a reactor, a current sensor assembly, and output wiring copper busbars connected by a large wire. An independent reactor is connected in series between the power unit and the flywheel motor for filtering. Its core improvement lies in using a water-cooled cabinet to cool the power unit, reactor, and other heat-generating components, solving the problems of low efficiency, easy dust accumulation, and high failure rate of forced air cooling. While this solution improves heat dissipation performance, it still does not depart from the traditional split-type topology. An additional independent reactor must be set up in the AC output loop to achieve the filtering function, and it does not address electrical isolation issues. There are no isolation measures between the power unit and the flywheel motor, which cannot meet the safety requirements of some application scenarios that require AC / DC electrical isolation. More importantly, the topology of this solution is designed for energy feedback scenarios in urban rail transit, and the control objective is the energy feedback during flywheel deceleration. Its working mode is fundamentally different from the bidirectional charging and discharging of flywheel energy storage and wide-speed motor drive scenarios, and it cannot be directly applied to flywheel energy storage converters that require simultaneous electrical isolation and output filtering.

[0005] Regarding the technological direction of utilizing transformer leakage inductance, there are existing technologies that attempt to use transformer leakage inductance for filtering. For example, there are schemes that use the inherent leakage inductance of the transformer to replace part of the filter inductance for new energy grid connection scenarios, and schemes that integrate resonant inductance and transformer leakage inductance in magnetically integrated high-frequency transformers. However, the above schemes are all aimed at unidirectional grid connection or DC-DC conversion conditions, the leakage inductance value is fixed and cannot be adjusted, and they are not integrated into the special operating characteristics of flywheel energy storage bidirectional charging and discharging and wide-speed motor drive. Therefore, to date, there has been no flywheel-specific converter topology that integrates electrical isolation and output filtering functions into the same transformer. Summary of the Invention

[0006] The purpose of this invention is to provide an isolated high-leakage-inductance flywheel energy storage converter and its control method, so as to solve the technical problems of redundant devices, complex structure, high loss, and inability to integrate isolation and filtering in traditional flywheel converters.

[0007] To achieve the above objectives, the present invention first provides an isolated high leakage inductance flywheel energy storage converter, including a DC bus module, a bidirectional inverter module, a high leakage inductance isolation transformer, a flywheel motor interface module, and a main control drive module;

[0008] The DC side of the bidirectional inverter module is electrically connected to the DC bus module, and the AC output terminal of the bidirectional inverter module is directly electrically connected to the primary side of the high leakage inductance isolation transformer.

[0009] The secondary side of the high leakage inductance isolation transformer is electrically connected to the flywheel motor interface module, and the flywheel motor interface module is used to connect the flywheel motor.

[0010] The main control drive module is connected to the bidirectional inverter module, the high leakage inductance isolation transformer and the flywheel motor interface module respectively;

[0011] The primary and secondary windings of the high leakage inductance isolation transformer are wound on different columns of the iron core, and the axial center of the primary winding and the axial center of the secondary winding are offset in the axial direction of the iron core. The magnetic circuit between the primary and secondary windings is provided with a leakage magnetic path structure with an adjustable air gap, so that the high leakage inductance isolation transformer has an adjustable leakage inductance value.

[0012] Furthermore, the leakage magnetic path structure is disposed in the magnetic circuit space between the primary winding and the secondary winding, and includes a fixed frame and a movable magnetic guide block. The fixed frame is made of non-magnetic material, and the movable magnetic guide block is made of magnetic material. A first air gap is formed between the first end of the movable magnetic guide block and the iron core column where the primary winding is located, and a second air gap is formed between the second end of the movable magnetic guide block and the iron core column where the secondary winding is located. The movable magnetic guide block can move relative to the fixed frame, so that the widths of the first air gap and the second air gap change synchronously in opposite directions, thereby adjusting the total magnetic reluctance of the leakage magnetic path.

[0013] Furthermore, the bidirectional inverter module adopts a three-phase full-bridge PWM bidirectional converter topology, including six IGBT power devices and freewheeling diodes connected in anti-parallel to each IGBT power device. The midpoint of the three-phase bridge arm serves as the AC output terminal and is directly connected to the primary three-phase winding of the high leakage inductance isolation transformer. The high leakage inductance isolation transformer consists of three independent single-phase transformer units. Each single-phase transformer unit includes two iron core columns, with the primary winding and secondary winding wound on the two iron core columns respectively. Each single-phase transformer unit has an independent leakage magnetic path structure.

[0014] Furthermore, the core of the high leakage inductance isolation transformer is made of stacked silicon steel sheets, and an insulating isolation layer is provided between the primary winding and the secondary winding.

[0015] Furthermore, the flywheel motor interface module includes wiring terminals and an overcurrent protection unit.

[0016] Furthermore, the main control drive module includes a DSP digital controller or an FPGA controller, and the DSP digital controller or FPGA controller has a built-in corresponding multi-closed-loop control program; the main control drive module also includes a bus voltage detection circuit, an output current detection circuit, a motor speed detection circuit, a PWM drive circuit, and a fault protection circuit.

[0017] Furthermore, the DC bus module includes a DC support capacitor; the DC bus module also includes a braking chopper unit, used to consume feedback energy when the DC bus voltage exceeds a preset threshold.

[0018] In this product's technical solution, the DC bus module provides a DC power input / output channel for the bidirectional inverter module. Its built-in DC support capacitor ensures stable bus voltage and low ripple. The DC side of the bidirectional inverter module is connected to the DC bus module, and the AC output terminal is directly connected to the primary side of the high leakage inductance isolation transformer. It operates in inverter mode during charging and in rectification mode during discharging. The high leakage inductance isolation transformer is the core innovative component, consisting of three independent single-phase transformer units forming a three-phase structure. Each single-phase transformer unit includes two core columns, with the primary and secondary windings wound on the two core columns respectively. The axial center of the primary winding and the axial center of the secondary winding are offset along the axial direction of the core, thereby lengthening the closed path of the leakage flux. In this embodiment, the offset distance is selected as 15% to 30% of the core column length.

[0019] In the leakage flux path structure of each phase, a movable magnetic guide block is installed in the core window between the primary and secondary windings. Its first end forms a first air gap with the core column containing the primary winding, and its second end forms a second air gap with the core column containing the secondary winding. Leakage flux enters the movable magnetic guide block from the primary core column through the first air gap, and then enters the secondary core column from the movable magnetic guide block through the second air gap, forming a controllable leakage flux branch. When the movable magnetic guide block moves relative to the fixed frame, the widths of the first and second air gaps change synchronously and in opposite directions; that is, one air gap increases while the other decreases, thereby changing the total magnetic reluctance of the leakage flux path. When the total air gap width decreases, the magnetic reluctance of the leakage flux path decreases, the leakage flux increases, and the corresponding leakage inductance value increases; when the total air gap width increases, the leakage inductance value decreases. This achieves precise adjustment of the transformer leakage inductance value. This leakage inductance parameter is configured to match the filtering requirements of the bidirectional inverter module's rated switching frequency and rated output power. The primary and secondary sides of the transformer maintain electrical isolation, achieving reliable insulation of the AC / DC system. At the same time, the high leakage inductance formed by the above structure is used to smooth and filter the PWM pulse waveform of the inverter output, suppressing output current harmonics, voltage spikes and high-frequency disturbances, and optimizing the sinusoidal nature of the output waveform.

[0020] In practical applications, the selection of leakage inductance value should comprehensively consider the rated power, rated voltage, fundamental frequency range, switching frequency, allowable fundamental voltage drop, and filtering requirements of the flywheel motor. Matching is achieved by adjusting the position of the movable magnetic blocks in the leakage magnetic path structure of each phase. The transformer leakage inductance presents high impedance to the switching frequency ripple, achieving smooth filtering of the PWM waveform; the leakage reactance voltage drop at the fundamental frequency is compensated by the modulation strategy of the main control drive module.

[0021] The flywheel motor interface module enables a reliable connection between the secondary side of the high leakage inductance isolation transformer and the flywheel motor, adapting to the wide speed and voltage operating range of the flywheel. The main control drive module acquires the bus voltage signal, converter output current signal, and flywheel motor speed signal in real time, precisely controlling the PWM modulation waveform to achieve flywheel speed-up energy storage, constant power discharge, and voltage and current stabilization control; it also features overvoltage protection, overcurrent protection, overheat protection, insulation abnormality detection, and overload protection functions.

[0022] In this product's technical solution, the flywheel motor is a permanent magnet synchronous motor with a damping winding on the rotor, capable of generating asynchronous torque. The operating speed range of the flywheel energy storage system is typically 45% to 100% of the rated speed. When the flywheel motor is in this operating range, the fundamental frequency of its output current is always higher than the transformer's minimum safe operating frequency, thus preventing the transformer from saturating due to volt-second imbalance. For zero-speed start-up of the flywheel motor, the main control drive module adopts a frequency-increase start-up strategy: initially, it outputs an AC voltage higher than the minimum safe operating frequency and uses constant voltage-frequency ratio control to maintain constant magnetic flux, enabling the flywheel motor to generate starting torque asynchronously and gradually accelerate; once the speed reaches the minimum operating speed, it switches to a vector control mode based on rotor position sensor detection. This frequency-increase start-up strategy ensures that the transformer operates within a safe frequency range throughout the entire operation, and the voltage amplitude to frequency ratio does not exceed the rated volt-second capacity, avoiding core saturation problems caused by near-DC current and overexcitation.

[0023] During discharge and energy release, the electrical energy fed back to the DC bus module by the flywheel motor can be absorbed in the following ways: the DC bus is connected to an external DC load or an active front-end converter on the grid side, which inverts the DC power back to the grid; or the braking chopper unit configured in the DC bus module activates when the bus voltage exceeds a preset threshold, dissipating excess energy in the braking resistor. The main control drive module monitors the DC bus voltage in real time. When the voltage exceeds the upper limit, it limits the discharge power or triggers braking protection to prevent bus overvoltage.

[0024] This invention also provides a control method for an isolated high-leakage-inductance flywheel energy storage converter, including charging energy storage mode and discharging energy release mode:

[0025] Charging and energy storage operation: The main control drive module receives the charging command and collects the DC bus voltage signal and flywheel motor speed signal. When the charging command is valid, the DC bus voltage is within the rated range, and the flywheel motor speed is lower than the maximum allowable speed, it enters the charging mode. The main control drive module outputs a PWM modulation signal to the bidirectional inverter module, which converts DC power into AC voltage and transmits it to the flywheel motor interface module via a high leakage inductance isolation transformer to drive the flywheel motor to accelerate. During charging, the main control drive module adjusts the frequency and duty cycle of the PWM modulation wave in real time according to the flywheel motor speed signal and maintains constant voltage-frequency ratio control.

[0026] Discharge and energy release mode: The main control drive module receives the discharge command and collects the DC bus voltage signal and flywheel motor speed signal; when the discharge command is valid and the flywheel motor speed is higher than the minimum discharge speed, it enters the discharge mode; the charging command and the discharge command are interlocked, and only one mode is allowed to operate at the same time; the flywheel motor outputs AC power, which is transmitted to the secondary side of the high leakage inductance isolation transformer through the flywheel motor interface module, and then rectified into DC power by the bidirectional inverter module and fed back to the DC bus module; during the discharge process, the main control drive module adjusts the duty cycle of the PWM modulation wave in real time according to the DC bus voltage signal and the flywheel motor speed signal; when the DC bus voltage exceeds the upper limit threshold, the main control drive module starts the braking chopper unit to consume the feedback energy or limits the discharge power.

[0027] Furthermore, in the charging and energy storage operation, the main control drive module adopts dual closed-loop vector control of speed outer loop and current inner loop. The current inner loop is implemented in a synchronous rotating coordinate system, controlling the d-axis and q-axis current components according to the rotor electrical angle. In the discharging and energy release operation, the main control drive module adopts dual closed-loop control of DC bus voltage outer loop and current inner loop. The current inner loop controls the d-axis and q-axis current components according to the rotor electrical angle to adjust the output active power and reactive power. The rotor electrical angle is obtained by the rotor position sensor.

[0028] In this technical solution, the dual closed-loop vector control under charging conditions specifically involves: the outer speed loop acquiring the flywheel motor speed signal, comparing it with a given speed reference value, and then outputting a q-axis current reference command via PI regulation; the inner current loop acquiring the converter output current, obtaining d-axis and q-axis current feedback values ​​through coordinate transformation, comparing them with the d-axis and q-axis current reference commands respectively, and then outputting d-axis and q-axis voltage commands via PI regulation, followed by inverse coordinate transformation to generate a PWM modulation wave. The dual closed-loop control under discharging conditions specifically involves: the outer DC bus voltage loop acquiring the DC bus voltage signal, comparing it with a given voltage reference value, and then outputting a q-axis current reference command via PI regulation; the inner current loop structure is the same as in the charging condition. Switching between motoring and generating states is achieved by controlling the sign of the q-axis current, thus ensuring a smooth transition between charging and discharging conditions. The charging and discharging commands for the flywheel energy storage system come from the host computer or the grid dispatch system. The main control drive module selects to enter charging mode or discharging mode according to the commands and operating conditions, and the two modes operate interlocked.

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

[0030] I. Integrated Isolation and Filtering, Simplified Topology: The primary and secondary windings of the high leakage inductance isolation transformer are wound on different cylinders of the iron core, and the axial centers of the primary and secondary windings are offset from each other along the axial direction of the iron core. A leakage magnetic path structure with an adjustable air gap is incorporated between them, enabling the transformer to maintain electrical isolation while possessing a significant and adjustable leakage inductance value. This leakage inductance value smooths and filters the PWM waveform at the rated switching frequency, thereby eliminating the need for external filter inductors, filter capacitors, and other passive components, significantly simplifying the main circuit topology, reducing the number of components, and lowering equipment size and cost.

[0031] II. Excellent power quality and high operational stability: The high leakage inductance of the transformer is generated by the axial offset of the windings and the adjustable air gap structure, which has an inherent ability to suppress PWM high-frequency harmonics, current spikes, and voltage disturbances, significantly reducing the harmonic content of the output current and improving the sinusoidal nature of the waveform. At the same time, this structure avoids resonance problems that may be caused by external filters, reduces speed jitter and current fluctuations when the flywheel is running at high speed, and improves the operational stability of the flywheel energy storage system under wide speed and bidirectional charging and discharging conditions.

[0032] III. Electrical Safety and Reliability: The high leakage inductance isolation transformer has an insulating isolation layer between the primary and secondary windings, which achieves complete electrical isolation. This effectively blocks the transmission of DC bus high voltage and grid disturbances to the flywheel motor side, protects the high-speed motor and control circuit, and improves the equipment's insulation safety level and anti-interference capability.

[0033] IV. High integration and fast dynamic response: By eliminating external filtering components, the energy transmission link is shortened, line impedance loss and heat generation are reduced, the overall conversion efficiency of bidirectional charging and discharging is improved, the number of fault points is reduced, and the operation and maintenance costs are reduced; the integrated topology link is short and the parameter matching degree is high, and the bidirectional operating condition switching response speed is faster, which can well adapt to the high frequency, millisecond-level switching and impact charging and discharging working characteristics of flywheel energy storage. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the system topology of the isolated high leakage inductance flywheel energy storage converter of the present invention.

[0035] Figure 2 for Figure 1 A schematic diagram of the circuit topology of the bidirectional inverter module.

[0036] Figure 3 for Figure 1 A schematic diagram of a medium-high leakage inductance isolation transformer (showing a single-phase transformer unit).

[0037] Figure 4 This is a schematic diagram of the leakage flux path of the high leakage inductance isolation transformer in this invention.

[0038] Figure 5 This is a schematic diagram of the leakage flux path of a typical isolation transformer.

[0039] Figure 6 This is a flowchart illustrating the charging and energy storage operation process of the control method of the present invention.

[0040] Figure 7 This is a flowchart illustrating the discharge and energy release process of the control method of the present invention. Detailed Implementation

[0041] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The following embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it; without departing from the concept of the present invention, those skilled in the art can make several modifications and improvements, all of which fall within the protection scope of the present invention.

[0042] Example 1: System Topology

[0043] See Figure 1 This embodiment provides an isolated high leakage inductance flywheel energy storage converter, including a DC bus module 1, a bidirectional inverter module 2, a high leakage inductance isolation transformer 3, a flywheel motor interface module 4, and a main control drive module 5.

[0044] The DC bus module 1 has a built-in DC support capacitor to provide a stable DC voltage input for the bidirectional inverter module 2, and also serves as an energy feedback receiver during discharge. The DC bus module 1 is also equipped with a braking chopper unit 11. When the DC bus voltage exceeds a preset threshold, the braking chopper unit 11 is activated to dissipate the excess feedback energy in the braking resistor.

[0045] Bidirectional inverter module 2 adopts a three-phase full-bridge PWM bidirectional converter topology, see [link / reference]. Figure 2 The system includes six IGBT power devices 21 and their anti-parallel freewheeling diodes 22. The six IGBT power devices 21 are designated Q1 to Q6, and the six anti-parallel freewheeling diodes 22 are designated D1 to D6. The positive and negative DC buses are connected to the upper and lower ends of the three-phase bridge arms, respectively. The midpoints of the three-phase bridge arms serve as AC output terminals A, B, and C, which are directly electrically connected to the primary three-phase windings of the high-leakage-inductance isolation transformer 3. The bidirectional inverter module 2 operates in inverter mode during charging and in rectification mode during discharging.

[0046] The high leakage inductance isolation transformer 3 is the core innovative component of this invention. See [link to relevant documentation]. Figure 3 The high leakage inductance isolation transformer 3 consists of three independent single-phase transformer units, corresponding to phases A, B, and C respectively. Figure 3The structure of one single-phase transformer unit is shown; the other two units have identical structures. The transformer core 31 of each single-phase transformer unit is made of high-permeability silicon steel sheets and has two parallel core columns. The primary winding 32 is wound on the left core column, and the secondary winding 33 is wound on the right core column. The axial center of the primary winding 32 and the axial center of the secondary winding 33 are offset axially from each other on the core 31 by a distance of 15% to 30% of the core column length. The primary winding 32 can be connected in a star or delta configuration, and the secondary winding 33 can be connected in a corresponding star or delta configuration. In this embodiment, the primary winding 32 is connected in a delta configuration, and the secondary winding 33 is connected in a star configuration, with a turns ratio of 1:1. The greater the offset distance, the weaker the magnetic coupling between the primary and secondary windings, and the more significant the leakage flux.

[0047] A leakage magnetic path structure 34 and an insulating layer 35 are provided in the magnetic circuit space between the primary winding 32 and the secondary winding 33. The leakage magnetic path structure 34 includes a fixed frame 39, a movable magnetic guide block 38, an adjusting screw, and a locking nut. The fixed frame 39 is made of a non-magnetic material, such as epoxy glass cloth, and serves as a support and positioning element. The movable magnetic guide block 38 is made of a magnetic material, such as laminated silicon steel sheets or ferrite. The movable magnetic guide block 38 is installed in the core window between the primary winding 32 and the secondary winding 33. Its first end forms a first air gap 36 with the core column where the primary winding 32 is located, and its second end forms a second air gap 37 with the core column where the secondary winding 33 is located. The leakage magnetic flux enters the movable magnetic guide block 38 from the primary core column through the first air gap 36, and then enters the secondary core column from the movable magnetic guide block 38 through the second air gap 37, forming a controllable leakage magnetic branch. The movable magnetic block 38 is fixedly connected to the adjusting screw. By rotating the adjusting screw, the movable magnetic block 38 moves relative to the fixed frame 39, causing the widths of the first air gap 36 and the second air gap 37 to change synchronously and in opposite directions. When the first air gap 36 decreases, the second air gap 37 increases, and vice versa. The change in the total air gap width alters the magnetic reluctance of the leakage magnetic path: the smaller the total air gap width, the lower the magnetic reluctance and the greater the leakage inductance; the larger the total air gap width, the higher the magnetic reluctance and the smaller the leakage inductance. The locking nut is used to lock the movable magnetic block 38 in the set position after adjustment.

[0048] In actual assembly, the required leakage inductance value is first calculated based on the rated power of the flywheel motor, the fundamental frequency range, and the switching frequency of the bidirectional inverter module. Then, the movable magnetic blocks 38 of each phase are adjusted to their corresponding positions by rotating the adjusting screw, and finally the locking nuts are tightened to complete the setting. The selection of the leakage inductance value should comprehensively consider the allowable range of the fundamental voltage drop and the filtering requirements: the transformer leakage inductance presents high impedance to the switching frequency ripple, realizing smooth filtering of the PWM waveform; the leakage reactance voltage drop at the fundamental frequency is compensated by the modulation strategy of the main control drive module 5.

[0049] An insulating isolation layer 35 is disposed between the primary winding 32 and the secondary winding 33. It is made of high-voltage resistant insulating material and is used to enhance the electrical isolation strength between the primary and secondary windings, ensuring reliable insulation between the DC bus high voltage and the flywheel motor side.

[0050] See Figure 4 , Figure 4 The high leakage inductance isolation transformer of this invention is shown on the left. Figure 4 The image on the right shows a standard isolation transformer. Standard isolation transformers have tightly coupled primary and secondary windings, resulting in a short leakage flux path, extremely low leakage inductance, and no filtering capability. This invention, through axial winding offset and the addition of a leakage flux path structure with an adjustable air gap, allows some flux to close via the leakage flux path instead of passing through the main magnetic circuit. This reduces the coupling between the primary and secondary windings, significantly increases the leakage inductance value, and allows for precise control, thus providing filtering capability. Since this transformer integrates the required filtering function, no additional external filter inductor or capacitor is needed between the bidirectional inverter module 2 and the high leakage inductance isolation transformer 3 in this embodiment.

[0051] In this embodiment, the flywheel motor is a permanent magnet synchronous motor with a damping winding on the rotor, capable of generating asynchronous torque. The operating speed range of the flywheel energy storage system is typically 45% to 100% of the rated speed. When the flywheel motor is in this operating range, the fundamental frequency of its output current is always higher than the minimum safe operating frequency of the transformer, thus preventing the transformer from saturating due to volt-second imbalance. For zero-speed start-up of the flywheel motor, the main control drive module 5 adopts a frequency-increase start-up strategy: initially, it outputs an AC voltage higher than the minimum safe operating frequency and uses constant voltage-frequency ratio control to maintain constant magnetic flux, enabling the flywheel motor to generate starting torque asynchronously and gradually accelerate; once the speed reaches the minimum operating speed, it switches to a vector control mode based on rotor position sensor detection. The rotor position sensor can be a rotary transformer or an encoder. This frequency-increase start-up strategy ensures that the transformer operates within a safe frequency range throughout the entire operation, and the voltage amplitude to frequency ratio does not exceed the rated volt-second capacity, avoiding core saturation problems caused by near-DC current and over-excitation.

[0052] During the discharge and energy release operation, the electrical energy fed back to the DC bus module 1 by the flywheel motor can be absorbed in the following ways: the DC bus is connected to an external DC load or an active front-end converter on the grid side, which inverts the DC power back to the grid; or the braking chopper unit 11 configured in the DC bus module 1 operates when the bus voltage exceeds a preset threshold, dissipating the excess energy in the braking resistor. The main control drive module 5 monitors the DC bus voltage in real time. When the voltage exceeds the upper limit, it limits the discharge power or triggers braking protection to prevent bus overvoltage.

[0053] The flywheel motor interface module 4 includes wiring terminals and an overcurrent protection unit to ensure the reliability and safety of the connection between the secondary side of the high leakage inductance isolation transformer 3 and the flywheel high-speed motor 6. The overcurrent protection unit can be a fuse or a circuit breaker.

[0054] The main control drive module 5 uses a DSP or FPGA digital controller as its core and incorporates a multi-closed-loop control program. During charging, it employs dual closed-loop vector control with an outer speed loop and an inner current loop. The outer speed loop acquires the flywheel motor speed signal, compares it with a given speed reference value, and outputs a q-axis current reference command via PI regulation. The inner current loop acquires the converter output current, obtains d-axis and q-axis current feedback values ​​through coordinate transformation, compares them with the reference commands, and outputs d-axis and q-axis voltage commands via PI regulation. These are then converted into PWM modulation waves through inverse coordinate transformation. During discharging, it employs dual closed-loop control with an outer DC bus voltage loop and an inner current loop. The outer DC bus voltage loop outputs a q-axis current reference command, and the inner current loop structure is the same as in the charging condition. The rotor electrical angle is obtained from a rotor position sensor. Switching between motoring and generating states is achieved by controlling the sign of the q-axis current. The main control drive module 5 also implements overvoltage, overcurrent, overheat, and overload protection through a fault protection circuit.

[0055] Example 2: Charging and Energy Storage Operating Conditions

[0056] See Figure 5 The workflow for charging and energy storage is as follows:

[0057] Step S101: The main control drive module 5 receives the charging command, detects the DC bus voltage and the flywheel motor speed, and determines whether the charging command is valid, whether the DC bus voltage is within the rated range, and whether the flywheel motor speed is lower than the maximum allowable speed.

[0058] Step S102: When the conditions are met, enter the charging mode. The main control drive module 5 outputs a PWM modulation signal to the bidirectional inverter module 2. The bidirectional inverter module 2 converts the DC bus power into AC voltage and maintains constant voltage-frequency ratio control.

[0059] Step S103: AC voltage is input to the primary side of the high leakage inductance isolation transformer 3. The transformer achieves electrical isolation through the primary and secondary insulation structure, blocking the conduction of DC high voltage to the flywheel motor side.

[0060] Step S104: The high leakage inductance of the transformer suppresses the high-frequency harmonics of the PWM in the AC voltage. The inductive characteristics of the leakage inductance prevent the current from changing abruptly, thus naturally filtering out high-frequency harmonic components and voltage spikes.

[0061] Step S105: The filtered AC power drives the high-speed flywheel motor 6 through the flywheel motor interface module 4. The main control drive module 5 collects the motor speed signal in real time and adjusts the frequency and duty cycle of the PWM modulation wave in real time according to the speed signal through dual closed-loop vector control of the speed outer loop and current inner loop to achieve smooth motor speed increase.

[0062] Step S106: After the flywheel speed reaches the set value, it enters a constant speed standby state, and electrical energy is stored in the flywheel rotor in the form of kinetic energy.

[0063] Example 3: Discharge and Energy Release Condition

[0064] See Figure 6 The working process for the discharge and energy release condition is as follows:

[0065] Step S201: The main control drive module 5 receives the discharge command, detects the DC bus voltage and flywheel motor speed, and determines whether the discharge command is valid and whether the current flywheel speed is higher than the minimum discharge speed. The charging command and the discharging command are interlocked, and only one operating condition is allowed to run at a time.

[0066] Step S202: When the conditions are met, enter the discharge mode. The flywheel high-speed motor 6 operates as a generator, outputting frequency- and voltage-converted AC power, which is input to the secondary side of the high leakage inductance isolation transformer 3 via the flywheel motor interface module 4.

[0067] Step S203: The high leakage inductance of the transformer suppresses harmonic components and disturbances in the feedback current, smoothing the feedback waveform.

[0068] Step S204: The filtered AC power is input to the AC side of the bidirectional inverter module 2, and the bidirectional inverter module 2 switches to rectification mode to rectify the AC power into stable DC power.

[0069] Step S205: The rectified DC power is fed back to the system DC bus via DC bus module 1 to achieve grid connection or power supply to the load. When the DC bus voltage exceeds the upper limit threshold, the main control drive module 5 starts the braking chopper unit 11 to consume the fed-back energy or limit the discharge power.

[0070] Step S206: The main control drive module 5 monitors the DC bus voltage and flywheel speed in real time. Through the dual closed-loop control of the DC bus voltage outer loop and the current inner loop, it adjusts the duty cycle of the PWM modulation wave in real time according to the DC bus voltage signal and the flywheel motor speed signal to ensure stable voltage and current during the discharge process.

[0071] Example 4: Alternative Solution

[0072] As an alternative embodiment of the present invention, the high leakage inductance of the high leakage inductance isolation transformer 3 can also be achieved by: increasing the physical distance between the primary and secondary sides using a split winding frame, adding an adjustable magnetic shunt structure in the magnetic circuit, or partially replacing low magnetic reluctance materials with high magnetic reluctance materials. In addition to the three-phase full-bridge topology, the bidirectional inverter module 2 can also adopt a two-level NPC topology, a T-type three-level topology, or a multi-level cascaded topology, etc., and its AC output terminals are all directly connected to the high leakage inductance isolation transformer 3. As long as the transformer maintains its electrical isolation function while possessing a preset leakage inductance value for filtering, it falls within the protection scope of the present invention.

Claims

1. An isolated high-leakage-inductance flywheel energy storage converter, characterized in that, include: DC bus module (1), bidirectional inverter module (2), high leakage inductance isolation transformer (3), flywheel motor interface module (4) and main control drive module (5); The DC side of the bidirectional inverter module (2) is electrically connected to the DC bus module (1), and the AC output terminal of the bidirectional inverter module (2) is directly electrically connected to the primary side of the high leakage inductance isolation transformer (3). The secondary side of the high leakage inductance isolation transformer (3) is electrically connected to the flywheel motor interface module (4), and the flywheel motor interface module (4) is used to connect the flywheel motor. The main control drive module (5) is connected to the bidirectional inverter module (2), the high leakage inductance isolation transformer (3) and the flywheel motor interface module (4) respectively; The primary winding (32) and secondary winding (33) of the high leakage inductance isolation transformer (3) are wound on different columns of the iron core (31), and the axial center of the primary winding (32) and the axial center of the secondary winding (33) are offset in the axial direction of the iron core (31); a leakage magnetic path structure (34) with an adjustable air gap is provided in the magnetic circuit between the primary and secondary windings, so that the high leakage inductance isolation transformer (3) has an adjustable leakage inductance value.

2. The isolated high-leakage-inductance flywheel energy storage converter according to claim 1, characterized in that, The leakage magnetic path structure (34) is set in the magnetic circuit space between the primary winding (32) and the secondary winding (33), including a fixed frame and a movable magnetic block. The fixed frame is made of non-magnetic material, and the movable magnetic block is made of magnetic material. A first air gap is formed between the first end of the movable magnetic block and the iron core column where the primary winding (32) is located, and a second air gap is formed between the second end of the movable magnetic block and the iron core column where the secondary winding (33) is located. The movable magnetic block can move relative to the fixed frame, so that the widths of the first air gap and the second air gap change synchronously in opposite directions, thereby adjusting the total magnetic resistance of the leakage magnetic path.

3. The isolated high-leakage-inductance flywheel energy storage converter according to claim 1, characterized in that, The bidirectional inverter module (2) adopts a three-phase full-bridge PWM bidirectional converter topology, including six IGBT power devices (21) and freewheeling diodes (22) connected in antiparallel to each IGBT power device. The midpoint of the three-phase bridge arm is directly connected to the primary three-phase winding of the high leakage inductance isolation transformer (3) as the AC output terminal. The high leakage inductance isolation transformer (3) is composed of three independent single-phase transformer units. Each single-phase transformer unit includes two iron core columns. The primary winding and secondary winding are respectively wound on the two iron core columns. Each single-phase transformer unit is provided with an independent leakage magnetic path structure (34).

4. The isolated high-leakage-inductance flywheel energy storage converter according to claim 1, characterized in that, The core (31) of the high leakage inductance isolation transformer (3) is made of stacked silicon steel sheets, and an insulating isolation layer (35) is provided between the primary winding (32) and the secondary winding (33).

5. The isolated high-leakage-inductance flywheel energy storage converter according to any one of claims 1 to 4, characterized in that, The flywheel motor interface module (4) includes wiring terminals and an overcurrent protection unit.

6. The isolated high-leakage-inductance flywheel energy storage converter according to any one of claims 1 to 4, characterized in that, The main control drive module (5) includes a DSP digital controller or an FPGA controller. The DSP digital controller or FPGA controller has a built-in corresponding dual closed-loop control program to form a control structure of voltage outer loop and current inner loop.

7. The isolated high-leakage-inductance flywheel energy storage converter according to claim 6, characterized in that, The main control drive module (5) also includes a bus voltage detection circuit, an output current detection circuit, a motor speed detection circuit, a PWM drive circuit, and a fault protection circuit.

8. The isolated high-leakage-inductance flywheel energy storage converter according to any one of claims 1 to 4, characterized in that, The DC bus module (1) includes a DC support capacitor; the DC bus module (1) also includes a braking chopper unit (11) for consuming feedback energy when the DC bus voltage exceeds a preset threshold.

9. A control method for an isolated high-leakage-inductance flywheel energy storage converter, applied to the isolated high-leakage-inductance flywheel energy storage converter according to any one of claims 1 to 8, characterized in that, Including charging and energy storage conditions and discharging and energy release conditions: Charging and energy storage operation: The main control drive module (5) receives the charging command and collects the DC bus voltage signal and the flywheel motor speed signal; when the charging command is valid, the DC bus voltage is within the rated range and the flywheel motor speed is lower than the maximum allowable speed, it enters the charging mode; the main control drive module (5) outputs the PWM modulation signal to the bidirectional inverter module (2), the bidirectional inverter module (2) converts the DC power into AC voltage, and transmits it to the flywheel motor interface module (4) through the high leakage inductance isolation transformer (3) to drive the flywheel motor to speed up; during the charging process, the main control drive module (5) adjusts the frequency and duty cycle of the PWM modulation wave in real time according to the flywheel motor speed signal, and maintains constant voltage-frequency ratio control; Discharge and energy release mode: The main control drive module (5) receives the discharge command and collects the DC bus voltage signal and the flywheel motor speed signal; when the discharge command is valid and the flywheel motor speed is higher than the minimum discharge speed, it enters the discharge mode; the charging command and the discharge command are interlocked, and only one mode is allowed to run at the same time; the flywheel motor outputs AC power, which is transmitted to the secondary side of the high leakage inductance isolation transformer (3) through the flywheel motor interface module (4), and then rectified into DC power by the bidirectional inverter module (2) and fed back to the DC bus module (1); during the discharge process, the main control drive module (5) adjusts the duty cycle of the PWM modulation wave in real time according to the DC bus voltage signal and the flywheel motor speed signal; when the DC bus voltage exceeds the upper limit threshold, the main control drive module (5) starts the braking chopper unit (11) to consume the feedback energy or limit the discharge power.

10. The control method according to claim 9, characterized in that, In the charging and energy storage mode, the main control drive module (5) adopts a dual closed-loop vector control of speed outer loop and current inner loop. The current inner loop is implemented in a synchronous rotating coordinate system and controls the d-axis and q-axis current components according to the rotor electrical angle. In the discharging and energy release mode, the main control drive module (5) adopts a dual closed-loop control of DC bus voltage outer loop and current inner loop. The current inner loop controls the d-axis and q-axis current components according to the rotor electrical angle to adjust the output active power and reactive power. The rotor electrical angle is obtained by the rotor position sensor.

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