DC side battery impedance identification method and system based on FC-NPC inverter

Abstract

The invention belongs to the technical field of battery state monitoring, particularly discloses a DC side battery impedance identification method and system based on an FC-NPC inverter, and aims to overcome the defects of high cost and difficulty in online application of a traditional electrochemical impedance spectroscopy detection method of a battery by multiplexing flying capacitor bridge arm hardware resources of the FC-NPC inverter. Superimposing an alternating current disturbance signal of which the amplitude is inversely proportional to the target frequency in the capacitor voltage reference value; a PI / PR dual-mode decoupling controller is adopted, a PI controller maintains the stability of a direct current component, and a PR controller accurately tracks alternating current disturbance; battery impedance response is excited based on charging and discharging current generated by disturbance, a target frequency component of battery end voltage / current is extracted through fast Fourier transform, an impedance value is calculated, and frequency sweeping is carried out on a fixed frequency range to obtain an electrochemical impedance spectrum. According to the invention, the online monitoring of the battery impedance can be realized without increasing extra equipment cost.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery status monitoring, and more specifically, relates to a method and system for identifying DC side battery impedance based on an FC-NPC inverter. Background Art

[0002] Electrochemical impedance spectroscopy (EIS), also known as AC impedance spectroscopy, is one of the most widely used non-destructive and effective electrochemical detection methods in the field of electrochemical systems. It has been widely used in lithium-ion battery research and application over the past two decades.

[0003] There are two main methods for identifying battery impedance spectra: offline and online. Offline identification equipment is relatively mature and is primarily performed in a laboratory setting using precision instruments such as electrochemical workstations, frequency response analyzers, and specialized electrochemical impedance analysis chips. Among online identification methods, those without additional excitation circuits hold greater research potential than those with additional excitation circuits due to their reduced hardware requirements and portability.

[0004] Traditional electrochemical impedance spectroscopy (EIS) testing technology relies on specialized equipment such as electrochemical workstations, which have the following drawbacks: The equipment is expensive and cannot be integrated into a battery management system for real-time monitoring; offline testing cannot reflect the dynamic operating conditions of the battery. Existing improvement schemes have limitations: Some studies have attempted to reuse DC / DC converters to inject disturbance signals, but this has struggled to balance wideband response and system compatibility; inverter-based solutions are mostly limited to two-level topologies, and the disturbance injection mechanism is unclear, resulting in significant harmonic interference on the AC side. Summary of the Invention

[0005] In response to the above defects or improvement needs of the prior art, the present invention provides a DC side battery impedance identification method and system based on FC-NPC inverter, the purpose of which is to realize online monitoring of battery impedance without increasing additional equipment costs.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for identifying the DC side battery impedance based on an FC-NPC inverter is proposed, comprising the following steps:

[0007] During the operation of the battery-inverter system, a voltage V is applied to the flying capacitor in the FC-NPC inverter. cref (t), generates charge and discharge current, stimulating the impedance response of the battery;

[0008] The voltage V cref (t) includes the flying capacitor voltage reference value V dref And superimposed on V dref The AC disturbance signal V href (t); the AC disturbance signal Vhref (t) According to the target frequency f t Sure;

[0009] Get the battery terminal voltage V in real time bat (t) and current I bat (t), calculate the battery impedance Z at the target frequency bat (f t ).

[0010] As a further preferred embodiment, the AC disturbance signal V href (t) = A h sin 2πf t t, where A h is the disturbance amplitude, which is determined by the target frequency f t Determine, t is the signal injection time.

[0011] As a further preferred embodiment, the disturbance amplitude A h With the target frequency f t Satisfies the inverse relationship: A h =k / f t , where k is the adjustable gain coefficient, and k remains unchanged during the identification process.

[0012] As a further preferred embodiment, the flying capacitor voltage reference value V dref =V dc / 2, where V dc The voltage value provided by the DC side battery.

[0013] As a further preferred method, the battery impedance is calculated as follows:

[0014] Based on the battery terminal voltage V bat (t) and current I bat (t), extract the target frequency f through fast Fourier transform t The voltage component U(f t ) and the current component I(f t ); According to the voltage component U(f t ) and the current component I(f t ) Calculate the battery impedance Z bat (f t ):

[0015] As a further preferred embodiment, during the operation of the battery-inverter system, a PI controller is used to stabilize the flying capacitor DC component to V dref , a PR controller is used to track the AC disturbance signal.

[0016] As a further preferred embodiment, during the operation of the battery-inverter system, SVPWM is used to control the NPC three-phase bridge arm.

[0017] As a further preferred embodiment, a series of target frequencies f are preset. t value, and calculate the battery impedance Z accordingly bat (f t ), thereby obtaining the battery impedance spectrum.

[0018] As a further preferred embodiment, the target frequency f t The range is 1Hz~1kHz.

[0019] According to another aspect of the present invention, a DC side battery impedance identification system based on an FC-NPC inverter is provided, comprising a processor configured to execute the above-mentioned DC side battery impedance identification method based on an FC-NPC inverter.

[0020] In general, the above technical solutions conceived by the present invention have the following technical advantages compared with the existing technology:

[0021] 1. By reusing the degrees of freedom of control of the flying capacitor bridge arm of the FC-NPC inverter and superimposing a small AC disturbance signal on the flying capacitor voltage reference signal, the present invention can generate charge and discharge currents with frequencies within the required identification range, thereby stimulating the battery impedance response. This enables online monitoring of battery impedance without increasing additional equipment costs and can be used for battery safety monitoring in battery-inverter-load or grid-connected systems.

[0022] 2. This method modulates the frequency and amplitude of the injected signal to determine the relationship between battery voltage and current at different frequencies. Fast Fourier analysis then yields the battery impedance spectrum. For inverter systems, obtaining the battery impedance spectrum facilitates real-time monitoring of battery health and reduces the impact of disturbance injection on AC-side operating conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flow chart of a DC side battery impedance identification method based on an FC-NPC inverter according to an embodiment of the present invention;

[0024] Figure 2 Schematic diagram of a battery-inverter system according to an embodiment of the present invention;

[0025] Figure 3 This is a schematic diagram of a disturbance injection control system according to an embodiment of the present invention;

[0026] Figure 4 A comparison chart of the actual and identified values ​​of battery impedance according to an embodiment of the present invention;

[0027] Figure 5Schematic diagram of the error between the actual value and the identified value of battery impedance according to an embodiment of the present invention. DETAILED DESCRIPTION

[0028] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0029] The embodiment of the present invention provides a DC side battery impedance identification method based on FC-NPC inverter, which is aimed at battery-inverter system, such as Figure 2 As shown, the hardware part of the system includes: a DC side battery, a flying capacitor-neutral point clamped (FC-NPC) inverter, and an AC side (filter, load, etc.); the software part includes: a 3-phase stationary coordinate / 2-phase stationary coordinate transformation module, a 2-phase stationary coordinate / 2-phase rotating coordinate transformation module, a voltage space vector pulse width modulation module, a current PI regulation module, an FC bridge arm control and disturbance injection module. The present invention mainly relates to the disturbance injection module, and the other modules are functional modules for the operation of the inverter.

[0030] The present invention reuses the degree of freedom of the flying capacitor bridge arm control of the FC-NPC inverter to perform disturbance injection, so as to realize the online monitoring of the battery impedance without increasing the cost of additional equipment. Figure 1 As shown, the battery impedance identification method specifically includes:

[0031] (1) During the operation of the battery-inverter system, a voltage V is applied to the flying capacitor of the FC-NPC inverter. cref (t), generates a charge and discharge current with a frequency within the range to be identified, and stimulates the impedance response of the battery.

[0032] Specifically, during the operation of the battery-inverter system, the flying capacitor voltage reference value V dref The AC disturbance signal V is superimposed on href (t) = A h sinω h t:

[0033] V cref (t) = V dref +V href (t) = V dc / 2+A h sin 2πf t t

[0034] Among them, V dcThe voltage value provided by the DC side battery, A h is the disturbance amplitude, which is determined by the target frequency f t OK, ω h is the frequency of the injected signal, i.e. the frequency of the identified impedance, and t is the signal injection time.

[0035] Then the charging and discharging current of the flying capacitor is:

[0036]

[0037] Among them C F is the flying capacitance value, then according to Kirchhoff's current law:

[0038] I bat (t) = I FC (t)+I in (t) = C F A h ω h cosω h t+I in (t)

[0039] The battery is injected with a frequency of ω h The current component forms a loop through the polarization impedance of the battery, thereby generating a voltage response corresponding to the frequency component.

[0040] Furthermore, the disturbance amplitude A h With the target frequency f t Satisfies the inverse relationship: A h =k / f t , where k is the adjustable gain coefficient; after the coefficient k is determined, it exists as a constant during the identification process, and its value does not change with changes in system parameters. Target frequency f t The scanning range is 1Hz~1kHz.

[0041] (2) Real-time acquisition of battery terminal voltage V bat (t) and current I bat (t), extract the frequency f by Fast Fourier Transform (FFT) t The voltage component U(f t ) and the current component I(f t ), calculate the battery impedance Z bat (f t ):

[0042]

[0043] (3) By inputting different target frequencies f t Repeat the above process to get Z within a certain range. bat (f t) value, and thus the battery impedance spectrum is obtained.

[0044] Furthermore, a PI / PR dual-mode decoupling controller is used, in which the PI controller maintains the stability of the DC component and the PR controller accurately tracks the AC disturbance. Figure 3 As shown, a proportional-integral (PI) controller is used to stabilize the flying capacitor DC component to V dref , using a proportional resonant (PR) controller to track AC disturbance signals. The NPC three-phase bridge arm uses SVPWM control, which improves DC voltage utilization while reducing switching losses and electromagnetic interference to a certain extent. It also improves the current waveform and power factor, making the operation more stable.

[0045] The transfer function of the PR controller is:

[0046]

[0047] where K pr is the proportionality coefficient, K r is the resonance coefficient, ω c The switching signals of the four switching tubes in the FC bridge arm can be obtained by superimposing the modulation signals of the DC and AC parts obtained by the controller and performing triangular carrier comparison.

[0048] Specifically, the injection of disturbance has little effect on the AC side operating conditions, which is reflected in the fact that the line voltage does not contain disturbance components. Assuming that the voltage on the flying capacitor is controlled without difference, that is:

[0049]

[0050] Then the expression of each phase output voltage is:

[0051]

[0052] Among them, S x is a switching function with values ​​of +1, 0, and -1, corresponding to the upper tube conducting, neutral point connected, and lower tube conducting states, respectively. Therefore, the phase voltage will contain an AC component A h sinω h t, taking phases a and b as an example, the line voltage is:

[0053]

[0054] It can be seen that for the line voltage, the AC disturbance terms injected into the neutral point voltage cancel each other out, and are determined only by the switching function and the DC bus voltage. This shows that the line voltage is not affected by the neutral point AC disturbance and remains consistent with the state before the disturbance was injected.

[0055] The following are specific embodiments:

[0056] The simulation results of the battery impedance identification method of the present invention are as follows: Figure 4 、 Figure 5 As shown, the battery uses the second-order polarization model parameters, the inverter operates normally, and the AC current is maintained at 50Hz. Then, a target frequency of 1Hz to 1kHz is given, and 9 target points are selected logarithmically every decade of frequency (a total of 27 target points) for frequency sweep identification. Figure 4 The solid line is the actual calculated impedance value, and the dots are the impedance identification values; Figure 5 is the error between the actual calculated value and the identified value. The results show that in the 1Hz-1kHz frequency range, the frequency response characteristics of the system identification impedance value are highly consistent with the measured impedance value, and the relative error of impedance identification at each frequency is less than 0.25%. This method can achieve battery impedance identification in a wide frequency range by adjusting the frequency and amplitude of the injected signal. At the same time, the results clearly reflect the charge transfer process and the solid electrolyte interface, proving that the online identification results can be used for the study and analysis of related electrochemical mechanisms, thereby providing relevant assistance for battery management systems.

[0057] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A DC side battery impedance identification method based on FC-NPC inverter, characterized in that: The steps include: During the operation of the battery-inverter system, a voltage V is applied to the flying capacitor in the FC-NPC inverter. cref (t), generates charge and discharge current, stimulating the impedance response of the battery; The voltage V cref (t) includes the flying capacitor voltage reference value V dref And superimposed on V dref The AC disturbance signal V href (t); the AC disturbance signal V href (t) According to the target frequency f t Sure; Get the battery terminal voltage V in real time bat (t) and current I bat (t), calculate the battery impedance Z at the target frequency bat (f t ).

2. The DC side battery impedance identification method based on the FC-NPC inverter according to claim 1, characterized in that: The AC disturbance signal V href (t) = A h sin 2πf t t, where A h is the disturbance amplitude, which is determined by the target frequency f t Determine, t is the signal injection time.

3. The DC side battery impedance identification method based on the FC-NPC inverter according to claim 2, characterized in that: The disturbance amplitude A h With the target frequency f t Satisfies the inverse relationship: A h =k / f t , where k is the adjustable gain coefficient, and k remains unchanged during the identification process.

4. The DC side battery impedance identification method based on the FC-NPC inverter according to claim 2, characterized in that: The flying capacitor voltage reference value V dref =V dc / 2, where V dc The voltage value provided by the DC side battery.

5. The DC side battery impedance identification method based on FC-NPC inverter according to claim 1, characterized in that: The battery impedance is calculated as follows: Based on the battery terminal voltage V bat (t) and current I bat (t), extract the target frequency f through fast Fourier transform t The voltage component U(f t ) and the current component I(f t ); According to the voltage component U(f t ) and the current component I(f t ) Calculate the battery impedance Z bat (f t ):

6. The DC side battery impedance identification method based on FC-NPC inverter according to claim 1, characterized in that: During the operation of the battery-inverter system, a PI controller is used to stabilize the DC component of the flying capacitor to V dref , a PR controller is used to track the AC disturbance signal.

7. The DC side battery impedance identification method based on FC-NPC inverter according to claim 1, characterized in that: During the operation of the battery-inverter system, SVPWM is used to control the NPC three-phase bridge arm.

8. The DC side battery impedance identification method based on the FC-NPC inverter according to any one of claims 1 to 7, characterized in that: Preset a series of target frequencies f t value, and calculate the battery impedance Z accordingly bat (f t ), thereby obtaining the battery impedance spectrum.

9. The DC side battery impedance identification method based on the FC-NPC inverter according to claim 8, characterized in that: The target frequency f t The range is 1Hz~1kHz.

10. A DC side battery impedance identification system based on FC-NPC inverter, characterized in that: The method comprises a processor configured to execute the DC side battery impedance identification method based on the FC-NPC inverter according to any one of claims 1 to 9.

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