Method for suppressing broadband harmonic current of two-stage cascaded H-bridge energy storage frequency converter

Abstract

The invention belongs to the field of electronic circuits, and particularly relates to a method for suppressing broadband harmonic current of a two-stage cascaded H-bridge energy storage frequency converter, which is used for solving the problem that the suppression effect is influenced when the output voltage frequency of the existing suppression method changes quickly. The method comprises the following steps: performing equivalent transformation on a state space equation to obtain an equivalent model; obtaining an expression of alternating current disturbance current based on observer gain, performing forward difference or backward difference on the expression, obtaining discrete time based on a difference equation, and observing and compensating the disturbance current based on the discrete time; performing small signal linearization on the equivalent model to obtain a voltage outer loop output transfer function corresponding to the alternating disturbance current; and calculating a voltage outer loop output alternating current fluctuation quantity based on the transfer function, and subtracting the voltage outer loop output alternating current fluctuation quantity from an inner loop input current for performing feedforward control on DC-DC. According to the invention, the problem of broadband-range harmonic current suppression is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics, and particularly relates to a method for suppressing broadband harmonic current of a two-stage cascaded H-bridge type energy storage inverter. Background Art

[0002] The two-stage cascaded H-bridge energy storage converter (CHBESC) is an advanced topology that integrates energy storage elements into a cascaded multilevel converter. It has wide applications in high-voltage direct-mounted energy storage systems, electric vehicle drive systems, and marine power systems. Within a CHBESC, energy storage integration can be achieved through direct integration or DC-DC integration. The two-stage (DC-DC + DC-AC) configuration helps reduce high-frequency currents flowing into the energy storage elements and stabilizes the DC bus voltage, offering advantages in power-oriented applications using supercapacitors. However, when a CHBESC powers a motor, the motor's rapid acceleration and deceleration can cause significant variations in the converter's output voltage frequency and amplitude, leading to wide-frequency harmonic currents (WFHCs) in each submodule. WFHCs not only shorten the supercapacitor lifespan and increase DC-DC losses, but can also cause overcurrent faults.

[0003] Existing research primarily focuses on suppressing second harmonic current (SHC) in two-stage converters, with methods falling into two main categories. One involves adding additional circuitry, such as increasing DC link capacitance or connecting an LC circuit with a parallel resonant frequency of twice the fundamental frequency on the DC link capacitor side. However, these methods require large passive components. Alternatively, a DC-DC-based SHC compensator can be used, but this increases hardware cost and system complexity. Another approach involves employing appropriate control strategies in the DC-DC stage to suppress SHC, such as dual closed-loop control with a cutoff frequency significantly lower than twice the fundamental frequency. However, this can degrade system dynamic performance. Adding a notch filter to the voltage outer loop can suppress SHC and improve dynamic response, but it introduces negative phase shift, reducing the system's phase margin. While virtual impedance methods can suppress SHC, they are less effective when the output frequency fluctuates significantly, potentially impacting system stability. In summary, existing methods are designed for fixed-frequency SHC suppression and are less effective when the output voltage frequency fluctuates rapidly, potentially degrading system stability. Summary of the Invention

[0004] In order to solve the above-mentioned problem in the prior art, namely, the existing method is designed for fixed-frequency SHC suppression, which is affected by the suppression effect when the output voltage frequency changes rapidly and reduces the stability of the system, the present invention provides a two-stage cascade H-bridge type energy storage inverter wide-band harmonic current suppression method, the suppression method comprising:

[0005] Based on the instantaneous power of the energy storage inverter, the state space equation of the DC-DC is transformed to obtain an equivalent model;

[0006] Combining the disturbance observer and the equivalent model, an expression for the AC disturbance current based on the observer gain is obtained. By converting the expression into a difference equation through forward or backward differentiation, the discrete time is obtained based on the difference equation, and the disturbance current is observed and compensated in the controller based on the discrete time.

[0007] The DC-DC is controlled by voltage and current dual closed-loop control, and the equivalent model is linearized by small signals to obtain the voltage outer loop output transfer function corresponding to the AC disturbance current.

[0008] The AC fluctuation of the voltage outer loop output is calculated based on the transfer function, and the AC fluctuation of the voltage outer loop output is subtracted from the inner loop input current for feedforward control of the DC-DC, thereby effectively suppressing the broadband harmonic current.

[0009] In a preferred embodiment, the instantaneous power of the energy storage inverter is:

[0010] Among them, V m is the amplitude of the output voltage of the energy storage inverter, I m is the amplitude of the output current of the energy storage inverter, is the output voltage v ac and the output current i ac The angle, f o (t) is the output frequency, t is the time, v ac (t) is the instantaneous voltage, i ac (t) is the instantaneous current.

[0011] In a preferred embodiment, performing an equivalent transformation on the state space equation of the DC-DC to obtain an equivalent model includes:

[0012] The DC component is represented by a resistive load, and the AC power is represented by a disturbance;

[0013] Obtaining the DC power in the instantaneous power of the energy storage inverter based on the instantaneous power of the energy storage inverter;

[0014] Calculate the equivalent resistance based on the DC power; and calculate the transformed equivalent model based on the equivalent resistance.

[0015] In a preferred embodiment, calculating the equivalent resistance based on the DC power includes:

[0016] Among them, R eq is the equivalent resistance, P dc is the DC power, V ce is the steady-state value of the DC link capacitor voltage.

[0017] In a preferred embodiment, the equivalent transformation model is:

[0018] Among them, L f is the filter inductor, C dc is the DC link capacitance, R L is the supercapacitor equivalent resistance; i Lf 、v c 、v s , α are the inductor current, capacitor voltage, supercapacitor voltage and DC–DC duty cycle respectively.

[0019] In a preferred embodiment, combining the disturbance observer and the equivalent model, obtaining the expression of the AC disturbance current based on the observer gain includes:

[0020] By introducing the intermediate variable z, the expression of the AC disturbance current based on the observer gain is expressed as:

[0021] Where λ is the observer gain, is the disturbance current, is the derivative of the disturbance current, is the derivative of the intermediate variable.

[0022] In a preferred embodiment, the equivalent model is subjected to small signal linearization to obtain the voltage outer loop output transfer function corresponding to the AC disturbance current, which includes:

[0023] The voltage outer loop output transfer function corresponding to the AC disturbance current is simplified to:

[0024] Among them, G cv is the voltage outer loop PI controller, G ci is the current inner loop PI controller, V ce , I Le are the DC link capacitor voltage and current in steady state, α′e is the reference value of the steady-state duty cycle; α′ e =1-α e ; A1, A2, A3, A4(s) are the first reference parameter, the second reference parameter, the third reference parameter, and the fourth reference parameter, respectively, i ref_ac is the AC component in the current reference value, i d is the disturbance current, s is the Laplace operator, G ci (s) is the current inner loop controller transfer function, G DCFF0 (s) is the transfer function of the current reference value corresponding to the disturbance current input, G DCFF_final (s) is the simplified transfer function of the current reference value corresponding to the disturbance current input.

[0025] In a second aspect of the present invention, a method for suppressing broadband harmonic current in a two-stage cascaded H-bridge type energy storage inverter is proposed. The suppression system includes:

[0026] An equivalent transformation module is used to perform an equivalent transformation on the state space equation of the DC-DC based on the instantaneous power of the energy storage inverter to obtain an equivalent model;

[0027] The disturbance compensation module is used to combine the disturbance observer and the equivalent model to obtain an expression for the AC disturbance current based on the observer gain. By converting the expression into a difference equation through forward or backward differentiation, the discrete time is obtained based on the difference equation, and the disturbance current is observed and compensated in the controller based on the discrete time.

[0028] The transfer function determination module is used to adopt voltage and current dual closed-loop control for DC-DC, perform small signal linearization on the equivalent model, and obtain the voltage outer loop output transfer function corresponding to the AC disturbance current;

[0029] The current suppression module is used to calculate the AC fluctuation of the voltage outer loop output based on the transfer function, and use the inner loop input current for feedforward control of the DC-DC to subtract the AC fluctuation of the voltage outer loop output to achieve effective suppression of broadband harmonic current.

[0030] Beneficial effects of the present invention:

[0031] (1) The disturbance current feedforward suppression control method of the present invention solves the problem of suppressing wide-band harmonic currents. By actively controlling the DC-DC converter, the wide-band harmonic currents are transferred to the DC link capacitor, achieving effective suppression of the wide-band harmonic currents.

[0032] (2) This paper proposes a digital method based on a nonlinear disturbance observer to eliminate the need for additional sensors when measuring load harmonic currents. Compared with traditional methods for suppressing fixed-frequency harmonic currents, the proposed method improves system stability under variable-frequency conditions.

[0033] (3) The disturbance current feedforward suppression control method provides a stable and effective solution for broadband harmonic current suppression in the two-stage cascaded H-bridge energy storage converter, improving its performance and reliability in various industrial applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Other features, objects and advantages of the present application will become more apparent upon reading the detailed description of non-limiting embodiments made with reference to the following drawings:

[0035] Figure 1 Schematic diagram of a method for suppressing broadband harmonic current in a two-stage cascaded H-bridge energy storage inverter according to an embodiment of the present invention;

[0036] Figure 2 This is a control block diagram of the disturbance current feedforward suppression method proposed in an embodiment of the present invention.

[0037] Figure 3 This is a circuit topology diagram of a two-stage cascaded H-bridge energy storage inverter according to an embodiment of the present invention;

[0038] Figure 4 is a comparison diagram of the observed value and the true value of the nonlinear disturbance observer according to an embodiment of the present invention;

[0039] Figure 5 This is the waveform of capacitor voltage and inductor current when the two-stage cascade H-bridge type energy storage inverter wide-frequency harmonic current suppression method is not used;

[0040] Figure 6 This is a waveform diagram of capacitor voltage and inductor current when using the disturbance current feedforward suppression method proposed by the present invention;

[0041] Figure 7 This is a comparison diagram of current fluctuations when the output voltage and frequency of a two-stage cascaded H-bridge energy storage inverter change rapidly after using the suppression method of the present invention. DETAILED DESCRIPTION

[0042] The present application will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are intended only to illustrate the relevant invention and are not intended to limit the invention. It should also be noted that, for ease of description, only portions relevant to the relevant invention are shown in the accompanying drawings.

[0043] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0044] The present invention provides a method for suppressing broadband harmonic current of a two-stage cascaded H-bridge type energy storage inverter, the method comprising:

[0045] Based on the instantaneous power of the energy storage inverter, the state space equation of the DC-DC is transformed to obtain an equivalent model;

[0046] Combining the disturbance observer and the equivalent model, an expression for the AC disturbance current based on the observer gain is obtained. By converting the expression into a difference equation through forward or backward differentiation, the discrete time is obtained based on the difference equation, and the disturbance current is observed and compensated in the controller based on the discrete time.

[0047] The DC-DC is controlled by voltage and current dual closed-loop control, and the equivalent model is linearized by small signals to obtain the voltage outer loop output transfer function corresponding to the AC disturbance current.

[0048] The AC fluctuation of the voltage outer loop output is calculated based on the transfer function, and the AC fluctuation of the voltage outer loop output is subtracted from the inner loop input current for feedforward control of the DC-DC, thereby effectively suppressing the broadband harmonic current.

[0049] In order to more clearly illustrate the method for suppressing broadband harmonic current of the two-stage cascade H-bridge type energy storage inverter of the present invention, the following is combined with Figure 1 Each step in the embodiment of the present invention is described in detail.

[0050] In a two-stage cascaded H-bridge energy storage inverter, harmonic currents caused by AC instantaneous power exist in the DC-DC converter. In particular, when the load frequency changes rapidly over a wide frequency range, it is extremely difficult to suppress these harmonics. To address this problem, the present invention proposes a wide-band harmonic current suppression method using disturbance current feedforward. This method uses a nonlinear disturbance observer to extract harmonic components from the DC-DC current. Then, a DC-DC controller based on disturbance current feedforward is used to suppress harmonics over a wide frequency range. The control method proposed in the present invention can suppress the harmonic currents of the DC-DC converter regardless of how the load frequency changes, while maintaining good system stability and dynamic performance, which has engineering and technical value.

[0051] The method for suppressing broadband harmonic current of a two-stage cascaded H-bridge energy storage inverter according to the first embodiment of the present invention is described in detail below:

[0052] Based on the instantaneous power of the energy storage inverter, the state space equation of the DC-DC is transformed to obtain an equivalent model;

[0053] In this embodiment of the present invention, the instantaneous power of the energy storage inverter is:

[0054] Among them, V m is the amplitude of the output voltage of the energy storage inverter, I mis the amplitude of the output current of the energy storage inverter, is the output voltage v ac and the output current i ac The angle, f o (t) is the output frequency, t is the time, v ac (t) is the instantaneous voltage, i ac (t) is the instantaneous current.

[0055] In an embodiment of the present invention, performing an equivalent transformation on the state space equation of the DC-DC to obtain an equivalent model includes:

[0056] The DC component is represented by a resistive load, and the AC power is represented by a disturbance;

[0057] Obtaining the DC power in the instantaneous power of the energy storage inverter based on the instantaneous power of the energy storage inverter;

[0058] Calculate the equivalent resistance based on the DC power; and calculate the transformed equivalent model based on the equivalent resistance.

[0059] In an embodiment of the present invention, calculating the equivalent resistance based on the DC power includes:

[0060] Among them, R eq is the equivalent resistance, P dc is the DC power, V ce is the steady-state value of the DC link capacitor voltage.

[0061] In the embodiment of the present invention, the equivalent transformation model is: L f i Lf =v s -i Lf R L -v c (1-α);

[0062] Among them, L f is the filter inductor, C dc is the DC link capacitance, R L is the supercapacitor equivalent resistance; i Lf 、v c 、v s , α are the inductor current, capacitor voltage, supercapacitor voltage and DC–DC duty cycle respectively.

[0063] Combining the disturbance observer and the equivalent model, an expression for the AC disturbance current based on the observer gain is obtained. By converting the expression into a difference equation through forward or backward differentiation, the discrete time is obtained based on the difference equation, and the disturbance current is observed and compensated in the controller based on the discrete time.

[0064] In an embodiment of the present invention, in combination with the disturbance observer and the equivalent model, the expression for obtaining the AC disturbance current based on the observer gain includes:

[0065] By introducing the intermediate variable z, the expression of the AC disturbance current based on the observer gain is expressed as:

[0066] Where λ is the observer gain, is the disturbance current, is the derivative of the disturbance current, is the derivative of the intermediate variable.

[0067] The DC-DC is controlled by voltage and current dual closed-loop control, and the equivalent model is linearized by small signals to obtain the voltage outer loop output transfer function corresponding to the AC disturbance current.

[0068] In this embodiment, the equivalent model is subjected to small signal linearization to obtain the voltage outer loop output transfer function corresponding to the AC disturbance current, which includes:

[0069] The voltage outer loop output transfer function corresponding to the AC disturbance current is simplified to:

[0070] Among them, G cv is the voltage outer loop PI controller, G ci is the current inner loop PI controller, V ce , I Le are the DC link capacitor voltage and current in steady state, α′ e is the reference value of the steady-state duty cycle; α′ e =1-α e ; A1, A2, A3, A4(s) are the first reference parameter, the second reference parameter, the third reference parameter, and the fourth reference parameter, respectively, i ref_ac is the AC component in the current reference value, i d is the disturbance current, s is the Laplace operator, G ci (s) is the current inner loop controller transfer function, G DCFF0 (s) is the transfer function of the current reference value corresponding to the disturbance current input, G DCFF_final (s) is the simplified transfer function of the current reference value corresponding to the disturbance current input.

[0071] The AC fluctuation of the voltage outer loop output is calculated based on the transfer function, and the AC fluctuation of the voltage outer loop output is subtracted from the inner loop input current for feedforward control of the DC-DC, thereby effectively suppressing the broadband harmonic current.

[0072] Feeding the calculated results forward to the inner current loop significantly suppresses fluctuations in the inner current loop reference value. Because the inner current loop's dynamic response is sufficiently fast, DC-DC current fluctuations are significantly reduced, achieving broadband harmonic current suppression.

[0073] By inputting the observed value of the nonlinear disturbance observer, the fluctuation of the voltage outer loop output can be obtained. By introducing the feedforward control method, the fluctuation of the voltage outer loop output can be significantly suppressed by subtracting the observed fluctuation of the current loop reference value, thereby achieving broadband harmonic current suppression.

[0074] Although the various steps in the above embodiment are described in the above-mentioned order, those skilled in the art will understand that in order to achieve the effect of this embodiment, different steps do not have to be executed in such an order. They can be executed simultaneously (in parallel) or in a reverse order. These simple changes are within the scope of protection of the present invention.

[0075] The method of the present invention is further described with reference to the accompanying drawings and specific embodiments. The simulation parameters of the two-stage cascade H-bridge energy storage inverter using the present invention are as follows:

[0076] Two-stage cascade H-bridge pure energy inverter module number N: 10

[0077] Submodule DC capacitance Cdc: 105.6mF

[0078] Supercapacitor value Csc: 25.38F

[0079] Supercapacitor equivalent resistance RL: 0.2Ω

[0080] DC-DC filter inductor Lf: 120mH

[0081] DC–DC switching frequency fs: 5kHz

[0082] Submodule DC capacitor voltage Vce: 800V

[0083] Supercapacitor voltage Vse: 500V

[0084] like Figure 2This is the circuit topology diagram of the two-stage cascaded H-bridge energy storage inverter used in the present invention. When the AC side is connected to the grid, the two-stage cascaded H-bridge energy storage inverter charges the supercapacitor at low power for a long period of time. When the motor is running, the two-stage cascaded H-bridge energy storage inverter disconnects from the grid, allowing the supercapacitor to release a large amount of energy in a short period of time. Because the instantaneous output power varies significantly over a wide frequency range, broadband harmonic currents are generated in the DC-DC converter. The present invention suppresses these harmonic currents.

[0085] Figure 3 This is the control block diagram of the disturbance current feedforward suppression method proposed in the present invention. The controller consists of a DC-DC controller and a disturbance current feedforward strategy. The disturbance current feedforward strategy consists of a nonlinear disturbance observer and a voltage outer loop output calculation module. By real-time acquisition of the capacitor voltage vc, the inductor current iLf, the duty cycle α output by the DC-DC controller, and real-time calculation of Req, the disturbance current id can be observed. Afterwards, id is sent as input to the voltage outer loop output calculation module to calculate the AC fluctuation in the voltage outer loop output of the DC-DC controller. Finally, the calculated voltage outer loop AC fluctuation is subtracted from the current inner loop input, which significantly reduces the fluctuation of the current reference value and effectively suppresses broadband harmonic currents.

[0086] Figure 4 Comparison between the observed value and the true value of the proposed nonlinear disturbance observer. It can be seen that the observed value is almost the same as the true value at different frequencies of 40Hz, 100Hz, and 400Hz.

[0087] Figure 5 The capacitor voltage and inductor current waveforms are shown when the method of the present invention is not used. The voltage fluctuations at 40 Hz and 100 Hz are 149 V and 55 V, respectively. The current fluctuations at 40 Hz, 100 Hz, and 400 Hz are 1639 A, 770 A, and 350 A, respectively. The current fluctuations are very large.

[0088] Figure 6 The capacitor voltage and inductor current waveforms when using the disturbance current feedforward suppression method proposed in the present invention are shown. The voltage fluctuations at 40 Hz and 100 Hz are 147 V and 59 V, respectively. The current fluctuations at 40 Hz, 100 Hz, and 400 Hz are 398 A, 235 A, and 235 A, respectively, showing that the current fluctuations are significantly suppressed.

[0089] Figure 7 This is a comparison diagram of current fluctuations when the output voltage and frequency of a two-stage cascaded H-bridge energy storage inverter change rapidly. Compared with ordinary dual closed-loop control, after using the method of the present invention, the harmonic currents in the entire wide frequency range are greatly suppressed.

[0090] A method for suppressing broadband harmonic current in a two-stage cascaded H-bridge energy storage inverter according to a second embodiment of the present invention comprises:

[0091] An equivalent transformation module is used to perform an equivalent transformation on the state space equation of the DC-DC based on the instantaneous power of the energy storage inverter to obtain an equivalent model;

[0092] The disturbance compensation module is used to combine the disturbance observer and the equivalent model to obtain an expression for the AC disturbance current based on the observer gain. By converting the expression into a difference equation through forward or backward differentiation, the discrete time is obtained based on the difference equation, and the disturbance current is observed and compensated in the controller based on the discrete time.

[0093] The transfer function determination module is used to adopt voltage and current dual closed-loop control for DC-DC, perform small signal linearization on the equivalent model, and obtain the voltage outer loop output transfer function corresponding to the AC disturbance current;

[0094] The current suppression module is used to calculate the AC fluctuation of the voltage outer loop output based on the transfer function, and use the inner loop input current for feedforward control of the DC-DC to subtract the AC fluctuation of the voltage outer loop output to achieve effective suppression of broadband harmonic current.

[0095] Those skilled in the art will clearly understand that, for the convenience and brevity of description, the specific working process and related instructions of the system described above can refer to the corresponding process in the aforementioned method embodiment and will not be repeated here.

[0096] It should be noted that the two-stage cascaded H-bridge energy storage inverter broadband harmonic current suppression system provided in the above embodiment is only illustrated by the division of the above functional modules. In actual applications, the above functions can be assigned to different functional modules as needed, that is, the modules or steps in the embodiments of the present invention can be further decomposed or combined. For example, the modules in the above embodiments can be combined into one module or further divided into multiple sub-modules to complete all or part of the functions described above. The names of the modules and steps involved in the embodiments of the present invention are only for the purpose of distinguishing the modules or steps and are not to be regarded as improper limitations of the present invention.

[0097] The flow charts and block diagrams in the accompanying drawings illustrate the possible architecture, functions and operations of the systems, methods and computer program products according to various embodiments of the present application. In this regard, each box in the flow chart or block diagram can represent a module, program segment or a part of code, and the module, program segment or a part of code contains one or more executable instructions for realizing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the box can also occur in a different order than that marked in the accompanying drawings. For example, two boxes represented in succession can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram and / or flow chart, and the combination of the boxes in the block diagram and / or flow chart can be implemented by a dedicated hardware-based system that performs the specified function or operation, or can be implemented by a combination of dedicated hardware and computer instructions.

[0098] The terms "first", "second", etc. are used to distinguish similar objects, rather than to describe or indicate a particular order or sequence.

[0099] The term "comprise" or any other similar term is intended to cover non-exclusive inclusion such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0100] Thus far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art may make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will fall within the scope of protection of the present invention.

Claims

1. A method for suppressing broadband harmonic current in a two-stage cascaded H-bridge type energy storage inverter, characterized in that: The inhibition method comprises: Based on the instantaneous power of the energy storage inverter, the state space equation of the DC-DC is transformed to obtain an equivalent model; Combining the disturbance observer and the equivalent model, an expression for the AC disturbance current based on the observer gain is obtained. By converting the expression into a difference equation through forward or backward differentiation, the discrete time is obtained based on the difference equation, and the disturbance current is observed and compensated in the controller based on the discrete time. The DC-DC is controlled by voltage and current dual closed-loop control, and the equivalent model is linearized by small signals to obtain the voltage outer loop output transfer function corresponding to the AC disturbance current. The AC fluctuation of the voltage outer loop output is calculated based on the transfer function, and the AC fluctuation of the voltage outer loop output is subtracted from the inner loop input current for feedforward control of the DC-DC, thereby effectively suppressing the broadband harmonic current.

2. The method for suppressing broadband harmonic current of a two-stage cascaded H-bridge type energy storage inverter according to claim 1, characterized in that: The instantaneous power of the energy storage inverter is: Among them, V m is the amplitude of the output voltage of the energy storage inverter, I m is the amplitude of the output current of the energy storage inverter, is the output voltage v ac and the output current i ac The angle, f o (t) is the output frequency, t is the time, v ac (t) is the instantaneous voltage, i ac (t) is the instantaneous current.

3. The method for suppressing broadband harmonic current of a two-stage cascaded H-bridge type energy storage inverter according to claim 2, characterized in that: The equivalent model obtained by performing equivalent transformation on the DC-DC state space equation includes: The DC component is represented by a resistive load, and the AC power is represented by a disturbance; Obtaining the DC power in the instantaneous power of the energy storage inverter based on the instantaneous power of the energy storage inverter; Calculate the equivalent resistance based on the DC power; and calculate the transformed equivalent model based on the equivalent resistance.

4. The method for suppressing broadband harmonic current of a two-stage cascaded H-bridge type energy storage inverter according to claim 3, characterized in that: Calculating equivalent resistance based on DC power includes: Among them, R eq is the equivalent resistance, P dc is the DC power, V ce is the steady-state value of the DC link capacitor voltage.

5. The method for suppressing broadband harmonic current of a two-stage cascaded H-bridge type energy storage inverter according to claim 4, characterized in that: The equivalent transformation model is: L f i Lf =v s -i Lf R L -v c (1-α); Among them, L f is the filter inductor, C dc is the DC link capacitance, R L is the supercapacitor equivalent resistance; i Lf 、v c 、v s , α are the inductor current, capacitor voltage, supercapacitor voltage and DC–DC duty cycle respectively.

6. The method for suppressing broadband harmonic current of a two-stage cascaded H-bridge type energy storage inverter according to claim 5, characterized in that: Combining the disturbance observer and the equivalent model, the expression for the AC disturbance current based on the observer gain is: By introducing the intermediate variable z, the expression of the AC disturbance current based on the observer gain is expressed as: Where λ is the observer gain, is the disturbance current, is the derivative of the disturbance current, is the derivative of the intermediate variable.

7. The method for suppressing broadband harmonic current of a two-stage cascaded H-bridge type energy storage inverter according to claim 6, characterized in that: Performing small signal linearization on the equivalent model, the voltage outer loop output transfer function corresponding to the AC disturbance current is obtained, including: The voltage outer loop output transfer function corresponding to the AC disturbance current is simplified to: Among them, G cv is the voltage outer loop PI controller, G ci is the current inner loop PI controller, V ce , I Le are the DC link capacitor voltage and current in steady state, α′ e is the reference value of the steady-state duty cycle; α′ e =1-α e ; A1, A2, A3, A4(s) are the first reference parameter, the second reference parameter, the third reference parameter, and the fourth reference parameter, respectively, i ref_ac is the AC component in the current reference value, i d is the disturbance current, s is the Laplace operator, G ci (s) is the current inner loop controller transfer function, G DCFF0 (s) is the transfer function of the current reference value corresponding to the disturbance current input, G DCFF_final (s) is the simplified transfer function of the current reference value corresponding to the disturbance current input.

8. A two-stage cascade H-bridge type energy storage inverter broadband harmonic current suppression system, characterized in that: The suppression system comprises: An equivalent transformation module is used to perform an equivalent transformation on the state space equation of the DC-DC based on the instantaneous power of the energy storage inverter to obtain an equivalent model; The disturbance compensation module is used to combine the disturbance observer and the equivalent model to obtain an expression for the AC disturbance current based on the observer gain. By converting the expression into a difference equation through forward or backward differentiation, the discrete time is obtained based on the difference equation, and the disturbance current is observed and compensated in the controller based on the discrete time. The transfer function determination module is used to adopt voltage and current dual closed-loop control for DC-DC, perform small signal linearization on the equivalent model, and obtain the voltage outer loop output transfer function corresponding to the AC disturbance current; The current suppression module is used to calculate the AC fluctuation of the voltage outer loop output based on the transfer function, and use the inner loop input current for feedforward control of the DC-DC to subtract the AC fluctuation of the voltage outer loop output to achieve effective suppression of broadband harmonic current.

Images