Double frequency generation current suppression method and system based on multi-winding transformer direct current APF

Abstract

The invention relates to the technical field of micro-grids, and provides a double-frequency current suppression method and system based on a multi-winding transformer direct current APF, and the method comprises the steps: obtaining the H-bridge direct current side current of a cascaded H-bridge energy storage system according to the AC-DC side power conservation law, and obtaining a double-frequency current component through a second-order band-pass filter; a direct current APF circuit is introduced to the direct current side of each phase H bridge; the direct current APF circuit emits frequency doubling compensation current which has the same amplitude as the frequency doubling current component on the direct current side of the single H bridge and is opposite to the frequency doubling current component in phase; and the frequency doubling compensation current is introduced to counteract the frequency doubling current component of the H-bridge direct current side. According to the invention, the direct current APF circuit is introduced to the direct current side of each phase H bridge; according to the invention, reverse compensation suppression of the double frequency harmonic current of the direct current side of each H-bridge power unit is realized in a stable charging and discharging state of the cascaded H-bridge energy storage system, the ripple current of the battery module is greatly reduced, and the service life of the battery module and even the whole energy storage system is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrids, and particularly to a method and system for suppressing double-frequency current based on a multi-winding transformer DC APF. Background Art

[0002] The battery modules of a cascaded H-bridge energy storage converter are distributedly connected to the DC sides of each H-bridge power module, which is convenient for realizing the monitoring and protection of battery units. However, at the same time, due to the lack of a common DC bus, in addition to direct current, there are a large number of double-frequency harmonic current components and high-frequency harmonic current components related to the switching frequency in the DC current on the H-bridge DC side. If not suppressed, there will be a large amount of ripple current in each battery module, resulting in battery heating and aging, shortening the battery life, and affecting the stable operation of the energy storage system.

[0003] At present, the suppression of double-frequency harmonic current on the H-bridge DC side can generally be divided into passive filtering methods and active filtering methods. Regarding the passive filtering method, generally, a large electrolytic capacitor is connected in parallel on the DC side of the H-bridge to reduce the influence of ripple current. Although this method is simple to operate, if it is necessary to effectively filter the double-frequency harmonic current on the H-bridge DC side, a relatively large number of DC filter capacitors need to be connected in parallel, which will result in a large volume and uneconomicalness; in addition, on this basis, a filter inductor is connected in series in each battery module to form an LC filter circuit. This method uses both a filter capacitor and a filter inductor to form a low-pass filter, which can effectively filter most of the harmonic current components on the H-bridge DC side. However, the connection of an excessive inductor L in series with the battery will affect the charging and discharging speed of the battery, reduce the dynamic performance of the system, and the inductor continuously passes through a DC component related to the transmitted power, which not only causes power loss, but also there is a DC magnetic flux in the magnetic core, making the design difficult. Regarding the active filtering method, a bidirectional DC / DC converter is connected between the DC side of the converter H-bridge and the battery module. However, the introduced DC / DC module has a large power, resulting in an increase in the volume and cost of the equipment, and there will be certain losses during the two-stage conversion of energy, reducing the overall efficiency of the system. Some domestic and foreign scholars classify the DC APF circuit and summarize four basic DC APF circuit topologies, including Buck, Boost, Buck-Boost, and their split-capacitor circuits. However, in order to achieve accurate compensation, for a single-phase cascaded H-bridge energy storage system with N links, 3N sets of DC APF circuits, 3N band-pass filters are required to obtain double-frequency harmonic current, and 3N PI controllers are required, which places a large burden on the processor and is costly. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the related art. For this purpose, the present invention provides a method and system for suppressing double-frequency current based on a multi-winding transformer DC APF. The present invention introduces a DC APF circuit on the DC side of each phase H-bridge; and only one DC APF circuit is required for the single-phase cascaded H-bridge energy storage system to complete the secondary harmonic current compensation and suppression on the DC side of N H-bridge power units, enabling the cascaded H-bridge energy storage system to complete the reverse compensation and suppression of the double-frequency harmonic current on the DC side of each H-bridge power unit in a stable charge and discharge state, greatly reducing the ripple current of the battery module, and being beneficial to improving the service life of the battery module and even the entire energy storage system.

[0005] The present invention provides a method for suppressing double-frequency current based on a multi-winding transformer DC APF, including: S1: Obtaining the H-bridge DC side current of the cascaded H-bridge energy storage system according to the power conservation law of the AC and DC sides; S2: Obtaining the double-frequency current component on the DC side of the H-bridge according to the H-bridge DC side current; S3: Introducing a DC APF circuit on the DC side of each phase H-bridge; the DC APF circuit emits a double-frequency compensation current with the same amplitude and opposite phase as the double-frequency current component on the DC side of a single H-bridge; S4: Canceling the double-frequency current component on the DC side of the H-bridge through the double-frequency compensation current.

[0006] Further, the DC APF circuit includes a DC side voltage source, a filter inductor, a single-phase multi-winding transformer, a plurality of power switches, and a plurality of DC blocking capacitors. The power switches include a switching tube and a freewheeling diode connected in parallel, and the plurality of power switches form an H-bridge inverter unit. One end of the DC blocking capacitor is connected to the cascaded H-bridge energy storage system, the other end of the DC blocking capacitor is connected in series with the secondary side of the single-phase multi-winding transformer, the H-bridge inverter unit is connected to the primary side of the single-phase multi-winding transformer through the filter inductor, and the DC side voltage source is connected across the DC side of the H-bridge inverter unit.

[0007] Further, the number of the DC blocking capacitors is the same as the number of single-phase links of the cascaded H-bridge energy storage system.

[0008] Further, the primary side of the multi-winding transformer emits a double-frequency compensation current, and each secondary winding of the multi-winding transformer induces a corresponding amplitude of double-frequency compensation current according to the turns ratio of the primary and secondary sides.

[0009] Further, the H-bridge DC side current includes a DC component and a double-frequency current component, and the amplitude ratio of the double-frequency current component to the DC component is: , where is the phase angle of the AC voltage. is the phase angle difference between the grid-connected voltage and the output current of the converter. The amplitudes of the second-harmonic current components on the DC sides of the H-bridges in each phase are the same and the phases are consistent.

[0010] Further, the obtaining of the second-harmonic current component on the DC side of the H-bridge includes: Using the single-phase AC-side output current of the cascaded H-bridge energy storage system, multiplying it by the modulation voltage of a single H-bridge power module in the phase, dividing by the average voltage of the DC sides of N H-bridges, and then obtaining the second-harmonic current on the DC side of a single H-bridge through a second-order band-pass filter.

[0011] Further, the H-bridge inverter unit of the DC APF circuit adopts single-current-loop control.

[0012] Further, the second-harmonic compensation current output from the primary side of the multi-winding transformer is controlled by a quasi-proportional-resonant controller. The calculation expression of the transfer function is: Wherein, is the transfer function of the quasi-proportional-resonant controller, is the control proportional gain, is the second-harmonic resonant gain, is the resonant bandwidth, is the resonant frequency, is the Laplace transform complex variable.

[0013] Further, a single-phase cascaded H-bridge energy storage system only needs one DC APF circuit to complete the suppression of the second-harmonic current on the DC sides of N H-bridge power units.

[0014] The present invention also provides a second-harmonic current suppression system based on a multi-winding transformer DC APF for implementing any one of the above-mentioned second-harmonic current suppression methods based on a multi-winding transformer DC APF, including: A DC-side current acquisition module, which obtains the DC-side current of the H-bridges of the cascaded H-bridge energy storage system according to the law of conservation of power on the AC and DC sides; A second-harmonic current component acquisition module, which obtains the second-harmonic current component on the DC side of the H-bridge according to the DC-side current of the H-bridge; A DC APF circuit module, which introduces a DC APF circuit on the DC sides of the H-bridges in each phase; the DC APF circuit emits a second-harmonic compensation current with the same amplitude and opposite phase as the second-harmonic current component on the DC side of a single H-bridge; A suppression module, which cancels the second-harmonic current component on the DC side of the H-bridge through the second-harmonic compensation current.

[0015] One or more of the above technical solutions in the embodiments of the present invention have at least one of the following technical effects: In the present invention, a DC APF circuit is introduced on the DC side of each phase H-bridge; and only one DC APF circuit is required for the single-phase cascaded H-bridge energy storage system to complete the second-harmonic current compensation and suppression on the DC side of N H-bridge power units, enabling the cascaded H-bridge energy storage system to complete the reverse compensation and suppression of the second-harmonic current on the DC side of each H-bridge power unit in a stable charge and discharge state, greatly reducing the ripple current of the battery module, which is beneficial to improving the service life of the battery module and even the entire energy storage system.

[0016] Additional aspects and advantages of the present invention will be given in part in the following description, will become apparent in part from the following description, or will be understood through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0018] Figure 1 is a schematic flow chart of a method for suppressing second-harmonic current based on a multi-winding transformer DC APF provided by the present invention.

[0019] Figure 2 is a grid-connected topology diagram of a cascaded H-bridge energy storage converter in an embodiment of the present invention.

[0020] Figure 3 is a single-phase circuit topology diagram of a cascaded H-bridge energy storage system after adding a multi-winding transformer DC APF in an embodiment of the present invention.

[0021] Figure 4 is an equivalent circuit diagram of the DC side of a single H-bridge after adding a DC APF circuit in an embodiment of the present invention.

[0022] Figure 5 is a schematic diagram for obtaining the reference current of the inverter unit of the DC APF circuit in an embodiment of the present invention.

[0023] Figure 6 is a block diagram of the transfer function of the current loop of the DC APF circuit in an embodiment of the present invention.

[0024] Figure 7 is a schematic diagram of the overall control strategy principle of the DC APF circuit in an embodiment of the present invention.

[0025] Figure 8It is an effectiveness simulation experiment of a method for suppressing double-frequency current of a DC APF based on a multi-winding transformer in the present invention.

[0026] Figure 9 It is a schematic structural diagram of a system for suppressing double-frequency current of a DC APF based on a multi-winding transformer provided by the present invention.

[0027] Reference numerals: 101. DC-side current acquisition module; 102. Double-frequency current component acquisition module; 103. DC APF circuit module; 104. Suppression module. Detailed implementation manners

[0028] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without making creative efforts based on the embodiments in the present invention belong to the scope of protection of the present invention. The following embodiments are used to illustrate the present invention, but cannot be used to limit the scope of the present invention.

[0029] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the embodiments of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, without conflict, those skilled in the art can combine the different embodiments or examples described in this specification and the features of the different embodiments or examples.

[0030] The following combines Figures 1 to 9 to describe a method and a system for suppressing double-frequency current of a DC APF based on a multi-winding transformer in the present invention.

[0031] As Figure 1 shown, a method for suppressing double-frequency current of a DC APF based on a multi-winding transformer includes: S1: Obtain the DC-side current of the cascaded H-bridge energy storage system according to the power conservation law of the AC-DC side; As Figure 2 shown, it is a topology of a three-phase star-connected cascaded energy storage system grid-connected inverter. The single phase of this system is cascaded by N H-bridge power modules to output a multi-level, and the three phases are star-connected. After its output passes through a grid-connected reactor, it is respectively connected to the three-phase power grid through inductors. is phase AC power supply voltage is phase AC power supply voltage is phase AC power supply voltage. The single H-bridge power module adopts a single-stage structure, and a DC filter capacitor is connected in parallel on its DC side and then directly connected to the battery module , and point O is the neutral point of the star connection.

[0032] Taking a single H-bridge power module as an example, ignoring the power conversion loss on the AC and DC sides, according to the power conservation theorem on the AC and DC sides, the following expression can be obtained: where is the terminal voltage of the battery module at time is the output current on the DC side of a single H-bridge at time is the output voltage at time is the output current at time is the amplitude of the AC output voltage of a single H-bridge is the angular frequency of the grid voltage is the phase angle of the AC voltage is the fundamental phase angle, where is , , any one of , , , is the amplitude of the output current of the energy storage converter. Since the harmonic content of the output current of the cascaded H-bridge energy storage system is very small, it is considered that the output AC current of the converter only contains the fundamental component. Therefore is the same as the grid voltage frequency is the phase angle difference between the grid-connected voltage and the output current of the converter, that is, the power factor angle.

[0033] When the energy storage system operates in a steady state, the terminal voltage of the battery module has a long plateau period. During this period, the terminal voltage of the battery module remains basically unchanged, and the terminal voltage of the battery module is regarded as a constant value within the power frequency cycle , and the expression of the output current on the DC side of a single H-bridge can be solved from the above formula as: S2: Obtain the double-frequency current component on the DC side of the H-bridge according to the DC side current of the H-bridge; Based on the power conservation theorem, when analyzing the cascaded H-bridge energy storage system under steady-state operation, the DC side of a single H-bridge power module contains a DC component and a second-harmonic current component, and the amplitude ratio of the second-harmonic current component to the DC component is: , the amplitudes of the second-harmonic current components on the DC sides of the H-bridges within the same phase are the same and the phases are consistent; due to the existence of , the second-harmonic current components between phases 、 、 lag behind in negative sequence by 120° in turn.

[0034] Multiply the single-phase AC side output current of the cascaded H-bridge energy storage system by the modulation voltage of a single H-bridge power module within the phase; according to the power conservation theorem of the AC-DC sides, divide by the average voltage of the DC sides of N H-bridges, and then obtain the second-harmonic current component on the DC side of a single H-bridge through a second-order band-pass filter (Band Pass Filter) . The center frequency of the second-order band-pass filter is 100 Hz.

[0035] S3: Introduce a DC APF circuit on the DC side of each phase H-bridge; the DC APF circuit generates a second-harmonic compensation current with the same amplitude and opposite phase as the second-harmonic current component on the DC side of a single H-bridge; Figure 3 is the single-phase circuit topology of the cascaded H-bridge energy storage system after adding the DC APF based on the multi-winding transformer. Among them, is the output current on the DC side of a single H-bridge, is the current flowing through the battery module, is the bus voltage across the first battery module, is the bus voltage across the second battery module, is the th bus voltage across the battery module, is the primary voltage of the transformer, is the compensation current flowing through the primary side of the transformer, is the compensation current flowing through the secondary side of the transformer. The DC APF circuit includes a DC side voltage source , a filter inductor, a single-phase multi-winding transformer, multiple power switches, and multiple DC-blocking capacitors . The number of DC-blocking capacitors is equal to the number of single-phase links of the cascaded H-bridge energy storage system. The power switches include a switching tube and a freewheeling diode connected in parallel. Multiple said power switches form an H-bridge inverter unit. One end of the DC-blocking capacitor is connected to the DC side of the H-bridge power unit of the cascaded H-bridge energy storage system, and the other end of the DC-blocking capacitor is connected in series with the secondary side of the single-phase multi-winding transformer. The H-bridge inverter unit is connected to the primary side of the single-phase multi-winding transformer through the filter inductor. The DC side voltage source is connected across the DC side of the H-inverter unit.

[0036] S4: Cancel the double - frequency current component on the DC side of the H - bridge through the double - frequency compensation current.

[0037] In some specific embodiments of the present invention, the number of power switches is 4.

[0038] After adding the DC APF circuit based on the multi - winding transformer, the equivalent circuit on the DC side of a single H - bridge is as Figure 4 shown. The DC APF circuit is equivalent to a current - controlled current source . By controlling the inverter unit to control the on - off of the H - bridge switching tubes, according to the turns ratio relationship of the primary and secondary windings of the multi - winding transformer, the DC APF circuit outputs a double - frequency compensation current of corresponding magnitude on the primary side , and each secondary winding induces a compensation current , , canceling the double - frequency harmonic current components on the DC side of each H - bridge within the phase , while the high - frequency harmonic components are basically filtered out through the DC - side filter capacitor. is the same as in frequency, and the amplitude ratio satisfies the turns ratio of the primary and secondary windings of the multi - winding transformer. Figure 4 In , a filter circuit on the DC side is composed of a resistor and a DC filter capacitor is the internal resistance of the battery module.

[0039] The DC double - frequency APF circuit based on the multi - winding transformer is relatively independent of the main circuit. Due to the existence of the DC - blocking capacitor, the voltages of the primary and secondary windings of the transformer only exist in the double - frequency voltages related to the compensation current, and there is no DC component related to the transmitted power in the compensation current. The addition of the APF circuit is only to suppress the double - frequency harmonic current flowing into the battery module and will not affect the operation of the main circuit.

[0040] The DC side of a single H - bridge contains a DC component and a double - frequency current component. The amplitude ratio of the double - frequency current component to the DC component is: , where is the phase angle of the AC voltage, is the phase difference between the grid - connected voltage and the output current of the converter. The amplitudes of the double - frequency current components on the DC sides of each H - bridge within the phase are the same and the phases are consistent.

[0041] The acquisition of the double - frequency current component on the DC side of the H - bridge is as Figure 5 shown. Multiply the single-phase AC side output current of the cascaded H-bridge energy storage system by the modulation voltage of a single H-bridge power module within the phase; according to the power conservation theorem of the AC-DC sides, divide it by the average voltage of the DC sides of N H-bridges, and then obtain the double-frequency current component of the DC side of a single H-bridge through a second-order band-pass filter (Band Pass Filter). The center frequency of the second-order band-pass filter is 100 Hz.

[0042] The calculation expression is: Wherein, is the output current of the DC side of a single H-bridge, is the modulation voltage of a single H-bridge power module in the phase of the AC side, is the single-phase AC side output current of the cascaded H-bridge energy storage system in the is the average voltage of the DC sides of N H-bridges in the This invention does not need to additionally increase current sensors on the DC side of the H-bridge to collect current, thus saving costs.

[0043] The H-bridge inverter unit of the DC APF circuit adopts single current loop control, and its transfer function block diagram is as Figure 6 shown, is the transfer function of the quasi-proportional resonance (Quasi Proportional Resonance, QPR) controller. Ignoring the delay of the QPR controller, the calculation expression of the transfer function is: Wherein, is the expected value of the output voltage of the multi-winding transformer, is the output voltage value of the multi-winding transformer.

[0044] Combining Figure 5 , Figure 6 can obtain the overall control strategy of the DC APF circuit. The overall control strategy of the DC APF circuit is as Figure 7 shown. The double-frequency compensation current obtains the reference current through the gain link K. The reference current is compared with the compensation current on the primary side of the DC APF multi-winding transformer, and the difference signal is input into the QPR controller to stably track and output the double-frequency compensation current of the corresponding magnitude. According to Kirchhoff's voltage law, the voltage loop equation is obtained: Wherein, is The output voltage of the H-bridge inverter unit in the DC APF at time is The primary side voltage of the single-phase multi-winding transformer in the DC APF at time is the inductance of the primary side of the multi-winding transformer in the DC APF, and is the primary side compensation current of the multi-winding transformer in the DC APF; wherein, is the primary side compensation current of the DC APF in the frequency domain, is the output voltage of the H-bridge inverter unit in the DC APF in the frequency domain, is the primary side voltage of the single-phase multi-winding transformer in the DC APF in the frequency domain, is the complex variable of the Laplace transform.

[0045] The double-frequency compensation current output from the primary side of the multi-winding transformer is controlled by a QPR controller, and the calculation expression of the transfer function is: wherein, is the transfer function of the quasi-proportional resonant controller, is the control proportional gain, is the double-frequency resonant gain, is the resonant bandwidth, is the resonant frequency, is the complex variable of the Laplace transform.

[0046] In some specific embodiments of the present invention, .

[0047] The output of the QPR is added to the primary side voltage of the transformer to obtain the expected value of the output voltage of the multi-winding transformer. The is normalized through the DC side voltage source to obtain the modulation wave.

[0048] The modulation wave is compared with the triangular wave through bipolar PWM modulation to realize the on and off of the H-bridge switching tubes in the DC APF circuit.

[0049] The single-phase cascaded H-bridge energy storage system only needs one DC APF circuit to complete the suppression of the second harmonic current on the DC side of N H-bridge power units.

[0050] Such as Figure 9As shown in the figure, a double-frequency current suppression system based on a multi-winding transformer DC APF is used to execute a double-frequency current suppression method based on a multi-winding transformer DC APF, including: The DC-side current acquisition module 101 acquires the output current of the H-bridge DC side of the cascaded H-bridge energy storage system according to the law of conservation of AC-DC side power. The double-frequency current component acquisition module 102 acquires the double-frequency current component of the H-bridge DC side according to the output current of the H-bridge DC side. The DC APF circuit module 103 introduces a DC APF circuit on the DC side of each phase H-bridge; the DC APF circuit has a double-frequency compensation current with the same amplitude and opposite phase as the double-frequency current component of a single H-bridge DC side. The suppression module 104 cancels the double-frequency current component of the H-bridge DC side through the double-frequency compensation current.

[0051] Through the collaborative work of the above modules, a DC APF circuit is introduced on the DC side of each phase H-bridge; and a single-phase cascaded H-bridge energy storage system only needs one DC APF circuit to complete the compensation and suppression of the second harmonic current on the DC side of N H-bridge power units, enabling the cascaded H-bridge energy storage system to complete the reverse compensation and suppression of the double-frequency harmonic current on the DC side of each H-bridge power unit in a stable charge and discharge state, greatly reducing the ripple current of the battery module, and being beneficial to improving the service life of the battery module and even the entire energy storage system.

[0052] To verify the effectiveness of the invention, a simulation experiment is carried out on matlab / simulink. The system simulation parameters are shown in Table 1, and the simulation parameters of the DC APF circuit are shown in Table 2.

[0053] Table 1 System simulation parameters Table 2 DC APF circuit simulation parameters Figure 8 Analysis of the current waveforms flowing through the battery module before and after adding the DC APF circuit. Among them, the energy storage system discharges at the rated power, and by comparing Figure 8 Figure (a) in Figure 8 and Figure 8 Figure (c) in Figure 8In Figure (d), before and after adding the APF circuit, the direct current component (DC) of the battery module remains unchanged, and the battery discharges at a current of approximately 62.5 A. The content of the second-harmonic ripple current (THD) decreases from 54.48% to 0.61%, and the amplitude of the second-harmonic ripple current decreases from 34.06 A to approximately 0.38 A. The results of the simulation experiment verify the effectiveness of the second-harmonic current suppression strategy based on the multi-winding transformer DC APF. It can effectively track the second-harmonic current on the DC side of the H-bridge and suppress it in the reverse direction, and has a good suppression effect on the second-harmonic ripple current of the battery module, which helps to improve the service life and safety of the energy storage system.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. However, such modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for suppressing double-frequency current of a DC APF based on a multi-winding transformer, characterized in that Including: S1: Obtain the H-bridge DC-side current of the cascaded H-bridge energy storage system according to the AC-DC side power conservation law; S2: Obtain the second-harmonic current component of the H-bridge DC side based on the H-bridge DC-side current; S3: Introduce a DC APF circuit on the DC side of each phase H-bridge; the DC APF circuit generates a second-harmonic compensation current with the same amplitude and opposite phase as the second-harmonic current component of a single H-bridge DC side; S4: Cancel the second-harmonic current component of the H-bridge DC side through the second-harmonic compensation current.

2. A method for suppressing the second harmonic current of a multi-winding transformer-based DC APF according to claim 1, characterized in that The DC APF circuit includes a DC-side voltage source, a filter inductor, a single-phase multi-winding transformer, a plurality of power switches, and a plurality of DC-blocking capacitors. The power switches include a switching tube and a freewheeling diode connected in parallel. A plurality of the power switches form an H-bridge inverter unit. One end of the DC-blocking capacitor is connected to the cascaded H-bridge energy storage system, and the other end of the DC-blocking capacitor is connected in series with the secondary side of the single-phase multi-winding transformer. The H-bridge inverter unit is connected to the primary side of the single-phase multi-winding transformer through the filter inductor. The DC-side voltage source is connected across the DC side of the H-bridge inverter unit.

3. A method for suppressing double - frequency current of a DC APF based on a multi - winding transformer according to claim 2, characterized in that, The number of the DC-blocking capacitors is the same as the number of single-phase links of the cascaded H-bridge energy storage system.

4. A method for suppressing the double - frequency current of a DC APF based on a multi - winding transformer according to claim 2, characterized in that, The primary side of the multi-winding transformer generates a second-harmonic compensation current, and each secondary winding of the multi-winding transformer induces a second-harmonic compensation current with a corresponding amplitude according to the turns ratio relationship between the primary and secondary sides.

5. A method for suppressing double-frequency current of a DC APF based on a multi-winding transformer according to claim 1, characterized in that, The H-bridge DC-side current includes a DC component and a double-frequency current component, and the amplitude ratio of the double-frequency current component to the DC component is , where is the phase angle of the AC voltage, is the phase angle difference between the grid-connected voltage and the converter output current. The amplitudes of the double-frequency current components on the DC sides of the H-bridges within the phase are the same and the phases are consistent.

6. A method for suppressing double - frequency current of a DC APF based on a multi - winding transformer according to claim 1, characterized in that, The obtaining of the second-harmonic current component of the H-bridge DC side includes: Using the single-phase AC-side output current of the cascaded H-bridge energy storage system, multiplying it by the modulation voltage of a single H-bridge power module within a phase, dividing it by the average voltage of the N H-bridge DC sides, and then obtaining the second-harmonic current of a single H-bridge DC side through a second-order band-pass filter.

7. A method for suppressing double - frequency current of a multi - winding transformer - based DC APF according to claim 4, characterized in that, The H-bridge inverter unit of the DC APF circuit adopts single-current-loop control.

8. A method for suppressing double-frequency current of a multi-winding transformer-based DC APF according to claim 4, characterized in that The second-harmonic compensation current output by the primary side of the multi-winding transformer is controlled by a quasi-proportional-resonant controller, and the calculation expression of the transfer function is: Among them, is the transfer function of the quasi-resonant controller, is the control proportional gain, is the double-frequency resonant gain, is the resonant bandwidth, is the resonant frequency, is the Laplace transform complex variable.

9. A method for suppressing the double - frequency current of a DC APF based on a multi - winding transformer according to claim 1, characterized in that, A single-phase cascaded H-bridge energy storage system only needs one DC APF circuit to complete the suppression of the second-harmonic current of the DC sides of N H-bridge power units.

10. A double-frequency current suppression system based on a multi-winding transformer DC APF, characterized in that, Used to execute a method for suppressing second-harmonic current based on a multi-winding transformer DC APF according to any one of claims 1 to 9, including: A DC-side current acquisition module, which obtains the H-bridge DC-side current of the cascaded H-bridge energy storage system according to the AC-DC side power conservation law; A second-harmonic current component acquisition module, which obtains the second-harmonic current component of the H-bridge DC side based on the H-bridge DC-side current; A DC APF circuit module, which introduces a DC APF circuit on the DC side of each phase H-bridge; the DC APF circuit generates a second-harmonic compensation current with the same amplitude and opposite phase as the second-harmonic current component of a single H-bridge DC side; A suppression module, which cancels the second-harmonic current component of the H-bridge DC side through the second-harmonic compensation current.

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