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

Abstract

The present invention relates to the field of microgrid technology, and provides a method and system for suppressing double-frequency current based on a multi-winding transformer DC APF. The method comprises obtaining the DC-side current of a cascaded H-bridge energy storage system H-bridge according to the AC / DC power conservation law and obtaining its double-frequency current component through a second-order bandpass filter; 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; and offsetting the double-frequency current component on the DC side of the H-bridge by introducing the double-frequency compensation current. The present invention introduces a DC APF circuit on the DC side of each phase H-bridge; and achieves 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 of the cascaded H-bridge energy storage system, thereby greatly reducing the ripple current of the battery module and improving the service life of the battery module and even the entire energy storage system.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrids, and in particular 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 the cascaded H-bridge energy storage converter are connected to the DC side of each H-bridge power module in a distributed manner, facilitating monitoring and protection of the battery units. However, due to the lack of a common DC bus, the current on the DC side of the H-bridge contains not only DC power but also a large amount of double-frequency harmonic current components and high-frequency harmonic current components related to the switching frequency. If these are not suppressed, each battery module will have a large amount of ripple current, causing battery heating and aging, shortening the battery life, and affecting the stable operation of the energy storage system.

[0003] Currently, the suppression of the double-frequency harmonic current on the DC side of an H-bridge can generally be divided into passive filtering and active filtering methods. Regarding the passive filtering method, a large electrolytic capacitor is generally connected in parallel to the DC side of the H-bridge to reduce the impact of ripple current. Although this method is simple to operate, if you want to effectively filter the double-frequency harmonic current on the DC side of the H-bridge, you need to connect more DC filter capacitors in parallel, which will make it larger and uneconomical. In addition, on this basis, an additional filter inductor is connected in series with each battery module to form an LC filter circuit. This method uses filter capacitors and filter inductors to form a low-pass filter, which can effectively filter out most of the harmonic current components on the DC side of the H-bridge. However, the excessively large inductance L connected in series with the battery will affect the battery's charging and discharging speed, reducing the system's dynamic performance. In addition, the inductor continuously flows through the DC component related to the transmitted power, causing power loss. In addition, there is a DC magnetic flux in the magnetic core, which makes the design more difficult. Regarding active filtering methods, a bidirectional DC / DC converter is connected between the DC side of the converter's H-bridge and the battery module. However, the high power of the introduced DC / DC module increases the size and cost of the equipment, and the two-stage energy conversion generates certain losses, reducing the overall efficiency of the system. Domestic and foreign scholars have classified DC APF circuits and summarized four basic DC APF circuit topologies: Buck, Boost, Buck-Boost, and split-capacitor circuits. However, to achieve precise compensation, these methods require 3N DC APF circuits, 3N bandpass filters to obtain the second-harmonic current, and 3N PI controllers for a single-phase N-link cascaded H-bridge energy storage system, which places a heavy 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. To this end, the present invention provides a double frequency current suppression method and system 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 a single-phase cascade H-bridge energy storage system only requires one DC APF circuit to complete the second harmonic current compensation and suppression on the DC side of N H-bridge power units, so that the cascade H-bridge energy storage system can 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 charging and discharging 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.

[0005] The present invention provides a method for suppressing double frequency current based on a DC APF of a multi-winding transformer, comprising:

[0006] S1: Obtain the H-bridge DC side current of the cascaded H-bridge energy storage system according to the AC and DC power conservation law;

[0007] S2: Obtain the double frequency current component of the H-bridge DC side according to the H-bridge DC side current;

[0008] 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;

[0009] S4: The double frequency current component on the DC side of the H-bridge is offset by the double frequency compensation current.

[0010] Furthermore, 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 power switch includes a switch tube and a freewheeling diode connected in parallel. The multiple power switches form an H-bridge inverter unit.

[0011] 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 to both ends of the DC side of the H-bridge inverter unit.

[0012] Furthermore, the number of the DC blocking capacitors is the same as the number of single-phase links in the cascaded H-bridge energy storage system.

[0013] Furthermore, 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 double frequency compensation current of corresponding amplitude according to the primary-to-secondary turns ratio.

[0014] Furthermore, 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: ,in, is the phase angle of the AC voltage, is the phase angle difference between the grid-connected voltage and the converter output current. The double frequency current components on the DC side of each H-bridge in the phase have the same amplitude and consistent phase.

[0015] Furthermore, obtaining the double frequency current component of the H-bridge DC side includes:

[0016] The single-phase AC side output current of the cascaded H-bridge energy storage system is multiplied by the modulation voltage of a single H-bridge power module in the phase, divided by the average voltage of the DC side of N H-bridges, and then passed through a second-order bandpass filter to obtain the doubled frequency current of the DC side of a single H-bridge.

[0017] Furthermore, the H-bridge inverter unit of the DC APF circuit adopts single current loop control.

[0018] Furthermore, the double frequency compensation current outputted from 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:

[0019]

[0020] in, is the transfer function of the quasi-proportional resonant controller, To control the proportional gain, is the double frequency resonance gain, is the resonant bandwidth, is the resonant frequency, is the Laplace transform of the complex variable.

[0021] Furthermore, the single-phase cascade H-bridge energy storage system only requires one DC APF circuit to complete the second harmonic current compensation and suppression on the DC side of N H-bridge power units.

[0022] The present invention also provides a double frequency current suppression system based on a multi-winding transformer DC APF, which is used to execute any of the above-mentioned double frequency current suppression methods based on a multi-winding transformer DC APF, comprising:

[0023] A DC side current acquisition module, which obtains the DC side current of the H-bridge of the cascaded H-bridge energy storage system according to the AC and DC power conservation law;

[0024] A double frequency current component acquisition module, which acquires the double frequency current component of the H-bridge DC side according to the H-bridge DC side current;

[0025] A DC APF circuit module, wherein the DC APF circuit is introduced 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;

[0026] The suppression module offsets the double frequency current component on the H-bridge DC side through a double frequency compensation current.

[0027] The above one or more technical solutions in the embodiments of the present invention have at least one of the following technical effects:

[0028] The present invention introduces a DC APF circuit on the DC side of each phase H-bridge; and a single-phase cascade H-bridge energy storage system only requires 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. This allows the cascade 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 charging and discharging 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.

[0029] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 The present invention provides a flow chart of a method for suppressing double frequency current based on a DC APF of a multi-winding transformer.

[0032] Figure 2 This is a grid-connected topology diagram of the cascaded H-bridge energy storage converter according to an embodiment of the present invention.

[0033] Figure 3 This is a single-phase circuit topology diagram of an H-bridge energy storage system cascaded after adding a multi-winding transformer DC APF according to an embodiment of the present invention.

[0034] Figure 4 This is the 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.

[0035] Figure 5 Schematic diagram of obtaining the reference current of the inverter unit of the DC APF circuit according to an embodiment of the present invention.

[0036] Figure 6 It is a transfer function block diagram of the current loop of the DC APF circuit according to an embodiment of the present invention.

[0037] Figure 7 This is a schematic diagram of the overall control strategy of the DC APF circuit according to an embodiment of the present invention.

[0038] Figure 8 This is a simulation experiment on the effectiveness of the double frequency current suppression method based on the DC APF of a multi-winding transformer of the present invention.

[0039] Figure 9 The present invention provides a schematic structural diagram of a double frequency current suppression system based on a multi-winding transformer DC APF.

[0040] Reference numerals:

[0041] 101. DC side current acquisition module; 102. Double frequency current component acquisition module; 103. DC APF circuit module; 104. Suppression module. DETAILED DESCRIPTION

[0042] To make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below. Obviously, the embodiments described are part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. The following embodiments are used to illustrate the present invention, but are not used to limit the scope of the present invention.

[0043] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the embodiment 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, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0044] The following combination Figures 1 to 9 The present invention describes a method and system for suppressing double frequency current based on a DC APF of a multi-winding transformer.

[0045] like Figure 1As shown, a method for suppressing double frequency current based on a DC APF of a multi-winding transformer includes:

[0046] S1: Obtain the H-bridge DC side current of the cascaded H-bridge energy storage system according to the AC and DC power conservation law;

[0047] like Figure 2 The figure shows the grid-connected inverter topology of a three-phase star-connected cascade energy storage system. The system outputs multiple levels by cascading N H-bridge power modules in a single phase. The three phases are connected in a star shape. The output passes through the grid-connected reactor and is connected to the three-phase grid through inductance. for Phase AC power supply voltage, for Phase AC power supply voltage, for Phase AC power supply voltage, a single H-bridge power module adopts a single-stage structure, and a DC filter capacitor is connected in parallel on its DC side Then directly connect to the battery module , point O is the neutral point of star connection.

[0048] Taking a single H-bridge power module as an example, ignoring the AC / DC side power conversion loss, according to the AC / DC side power conservation theorem, the following expression can be obtained:

[0049]

[0050] in, for The terminal voltage of the battery module at this moment, for The DC side output current of a single H-bridge at the moment, for Output voltage at all times, for Output current at all times, is the AC output voltage amplitude of a single H-bridge, is the grid voltage angular frequency, is the phase angle of the AC voltage, is the fundamental phase angle, where for 、 、 Any one of 、 、 , is the output current amplitude of the energy storage converter. Since the output current harmonic content of the cascaded H-bridge energy storage system is very small, it is considered that the converter output AC current only contains the fundamental component. Therefore Same as the grid voltage frequency, It is the phase angle difference between the grid voltage and the converter output current, that is, the power factor angle.

[0051] When the energy storage system is in steady-state operation, 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. The terminal voltage of the battery module is regarded as a constant value within the power frequency cycle. , the expression of the output current of a single H-bridge DC side can be solved from the above formula:

[0052]

[0053] S2: Obtain the double frequency current component of the H-bridge DC side according to the H-bridge DC side current;

[0054] Based on the power conservation theorem, the cascaded H-bridge energy storage system is analyzed in steady-state operation. The DC side of a single H-bridge power module contains a DC component and a double-frequency current component, and the amplitude ratio of the double-frequency current component to the DC component is: , the double frequency current components of each H-bridge DC side in the phase have the same amplitude and phase; due to the existence of , the double frequency current component between phases is in phase 、 、 In negative order, they lag 120°.

[0055] The single-phase AC output current of the cascaded H-bridge energy storage system is multiplied by the modulation voltage of a single H-bridge power module in the phase; according to the AC and DC power conservation theorem, it is divided by the average voltage of the DC side of N H-bridges, and then the doubled frequency current component of the DC side of a single H-bridge is obtained through a second-order bandpass filter. The center frequency of the second-order bandpass filter is 100Hz.

[0056] 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;

[0057] Figure 3 To add a single-phase circuit topology based on a multi-winding transformer DC APF followed by a cascaded H-bridge energy storage system, is the DC side output current 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, For the The 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, To compensate the current flowing through the secondary side of the transformer, the DC APF circuit includes a DC side voltage source , filter inductors, single-phase multi-winding transformers, multiple power switches, and multiple DC blocking capacitors The number of DC blocking capacitors is equal to the number of single-phase links in the cascaded H-bridge energy storage system. The power switch includes a parallel switch tube and a freewheeling diode. A plurality of the power switches form an H-bridge inverter unit.

[0058] 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 a filter inductor, and the DC side voltage source is connected to both ends of the DC side of the H inverter unit.

[0059] S4: The double frequency current component on the DC side of the H-bridge is offset by the double frequency compensation current.

[0060] In some specific embodiments of the present invention, the number of power switches is four.

[0061] The equivalent circuit of the DC side of a single H-bridge after adding the DC APF circuit based on a multi-winding transformer is as follows: Figure 4 As shown, the DC APF circuit is equivalent to a current-controlled current source By controlling the inverter unit to control the on and off of the H-bridge switch tube, according to the turns ratio of the primary and secondary windings of the multi-winding transformer, the primary side of the DC APF circuit outputs a corresponding double frequency compensation current. , each secondary winding induces a compensation current , , offsetting the double frequency harmonic current component of each H-bridge DC side in the phase , and the high-frequency harmonic components Basic filtering is achieved through the DC side filter capacitor. and The frequency is the same, and the amplitude ratio satisfies the turns ratio of the primary and secondary windings of the multi-winding transformer. Figure 4 In the resistor and DC filter capacitors The filter circuit on the DC side is composed of is the internal resistance of the battery module.

[0062] The DC double frequency APF circuit based on a multi-winding transformer is relatively independent of the main circuit. Due to the presence of the DC blocking capacitor, the voltage of the primary and secondary windings of the transformer only exists in the double frequency voltage related to the compensation current. There is no DC component related to the transferred 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.

[0063] 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: ,in, is the phase angle of the AC voltage, is the phase angle difference between the grid-connected voltage and the converter output current. The double frequency current components on the DC side of each H-bridge in the phase have the same amplitude and consistent phase.

[0064] The double frequency current component of the H-bridge DC side is obtained as follows: Figure 5 As shown,

[0065] The single-phase AC output current of the cascaded H-bridge energy storage system is multiplied by the modulation voltage of a single H-bridge power module in the phase; according to the AC and DC power conservation theorem, it is divided by the average voltage of the DC side of N H-bridges, and then the doubled frequency current component of the DC side of a single H-bridge is obtained through a second-order bandpass filter. The center frequency of the second-order bandpass filter is 100Hz.

[0066] The calculation expression is:

[0067]

[0068] in, is the DC side output current of a single H-bridge, For AC side Phase modulation voltage of a single H-bridge power module, for Phase cascade H-bridge energy storage system single-phase AC side output current, for The average voltage of the DC side of the N-phase H-bridge,

[0069] The present invention does not require an additional current sensor to be added on the DC side of the H-bridge to collect current, thus saving costs.

[0070] The H-bridge inverter unit of the DC APF circuit adopts single current loop control, and its transfer function block diagram is as follows: Figure 6 As shown, is the transfer function of the Quasi Proportional Resonance (QPR) controller. Ignoring the delay of the QPR controller, the transfer function is calculated as:

[0071]

[0072] in, is the expected output voltage of the multi-winding transformer, is the output voltage value of the multi-winding transformer.

[0073] Combine Figure 5 、 Figure 6 The overall control strategy of the DC APF circuit can be obtained. The overall control strategy of the DC APF circuit is as follows: Figure 7 As shown, the double frequency compensation current The reference current is obtained through the gain link K , reference current By comparing the difference signal with the primary-side compensation current of the DC APF multi-winding transformer, the QPR controller is input to stably track and output the corresponding double frequency compensation current. The voltage loop equation is obtained according to Kirchhoff's voltage law:

[0074]

[0075] in, for The output voltage of the H-bridge inverter unit in the DC APF at this moment, for The primary side voltage of the single-phase multi-winding transformer in the DC APF at the moment, is the primary side inductance of the multi-winding transformer in the DC APF, It is the primary side compensation current of DC APF multi-winding transformer;

[0076] Performing Laplace transform on the above formula yields the mathematical model in the frequency domain:

[0077]

[0078] in, is the DC APF primary side compensation current 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 Laplace transform of the complex variable.

[0079] The double frequency compensation current output on the primary side of the multi-winding transformer is controlled by a QPR controller, and the calculation expression of the transfer function is:

[0080]

[0081] in, is the transfer function of the quasi-proportional resonant controller, To control the proportional gain, is the double frequency resonance gain, is the resonant bandwidth, is the resonant frequency, is the Laplace transform of the complex variable.

[0082] In some specific embodiments of the present invention, .

[0083] QPR output and transformer primary voltage After adding, we get the expected output voltage of the multi-winding transformer , through the DC side voltage source Will Normalize and obtain the modulated wave.

[0084] The comparison between the modulation wave and the triangle wave is completed through bipolar PWM modulation to realize the on and off of the H-bridge switch tube of the DC APF circuit.

[0085] The single-phase cascade H-bridge energy storage system only requires one DC APF circuit to complete the second harmonic current compensation and suppression on the DC side of N H-bridge power units.

[0086] like Figure 9 As shown, a double frequency current suppression system based on a multi-winding transformer DC APF is used to perform a double frequency current suppression method based on a multi-winding transformer DC APF, including:

[0087] The DC side current acquisition module 101 acquires the DC side output current of the H-bridge of the cascaded H-bridge energy storage system according to the AC and DC power conservation law;

[0088] The double frequency current component acquisition module 102 acquires the double frequency current component of the H-bridge DC side according to the H-bridge DC side output current;

[0089] 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 on the DC side of a single H-bridge;

[0090] The suppression module 104 offsets the double frequency current component on the H-bridge DC side through the double frequency compensation current.

[0091] Through the coordinated operation of the above modules, a DC APF circuit is introduced on the DC side of each phase H-bridge. Moreover, the single-phase cascaded H-bridge energy storage system only requires 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. This allows 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 charging and discharging state, greatly reducing the ripple current of the battery module, which is beneficial to extending the service life of the battery module and even the entire energy storage system.

[0092] To verify the effectiveness of the invention, simulation experiments were conducted on Matlab / Simulink. The system simulation parameters are shown in Table 1, and the DC APF circuit simulation parameters are shown in Table 2.

[0093] Table 1 System simulation parameters

[0094]

[0095] Table 2 DC APF circuit simulation parameters

[0096]

[0097] Figure 8 This is an analysis of the current waveform flowing through the battery module before and after adding the DC APF circuit. Figure 8 Figure (a) and Figure 8 As shown in Figure (c), after adding the DC APF circuit, the ripple current of the battery module is significantly reduced; FFT analysis is performed on the current waveform of the battery module before and after adding the DC APF circuit. Figure 8 Figure (b) and Figure 8 In Figure (d), the DC component of the battery module remains unchanged before and after the addition of the APF circuit, discharging at approximately 62.5A. The THD (threshold harmonic distortion) decreases from 54.48% to 0.61%, and the amplitude of the THD decreases from 34.06A to approximately 0.38A. Simulation results validate the effectiveness of the THD current suppression strategy based on a multi-winding transformer DC APF. It effectively tracks and reversely suppresses the THD harmonic current on the DC side of the H-bridge, effectively suppressing the THD ripple current in the battery module and helping to improve the lifespan and safety of the energy storage system.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for suppressing double frequency current based on DC APF of multi-winding transformer, characterized in that: include: S1: Obtain the H-bridge DC side current of the cascaded H-bridge energy storage system according to the AC and DC power conservation law; S2: Obtain the double frequency current component of the H-bridge DC side 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: The double frequency current component on the DC side of the H-bridge is offset by the double frequency compensation current.

2. The method for suppressing double frequency current based on DC APF of a multi-winding transformer 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, multiple power switches and multiple DC blocking capacitors. The power switch includes a parallel switch tube and a freewheeling diode. The multiple 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 to both ends of the DC side of the H-bridge inverter unit.

3. The method for suppressing double frequency current based on DC APF of 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 in the cascaded H-bridge energy storage system.

4. The method for suppressing double frequency current based on DC APF of a multi-winding transformer according to claim 2, characterized in that: The primary side of the multi-winding transformer emits a double frequency compensation current, and each secondary winding of the multi-winding transformer induces and emits a double frequency compensation current of corresponding amplitude according to the primary-secondary turns ratio.

5. The method for suppressing double frequency current based on DC APF of 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 ,in, is the phase angle of the AC voltage, is the phase angle difference between the grid-connected voltage and the converter output current. The double frequency current components on the DC side of each H-bridge in the phase have the same amplitude and consistent phase.

6. The method for suppressing double frequency current based on DC APF of a multi-winding transformer according to claim 1, characterized in that: The acquisition of the double frequency current component on the DC side of the H-bridge includes: The single-phase AC side output current of the cascaded H-bridge energy storage system is multiplied by the modulation voltage of a single H-bridge power module in the phase, divided by the average voltage of the DC side of N H-bridges, and then passed through a second-order bandpass filter to obtain the doubled frequency current of the DC side of a single H-bridge.

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

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

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

10. A double frequency current suppression system based on a multi-winding transformer DC APF, characterized in that: The method for suppressing double frequency current based on a DC APF of a multi-winding transformer as claimed in any one of claims 1 to 9 comprises: A DC side current acquisition module, which obtains the DC side current of the H-bridge of the cascaded H-bridge energy storage system according to the AC and DC power conservation law; A double frequency current component acquisition module, which acquires the double frequency current component of the H-bridge DC side according to the H-bridge DC side current; A DC APF circuit module, wherein the DC APF circuit is introduced 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; The suppression module offsets the double frequency current component on the H-bridge DC side through a double frequency compensation current.

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