Method for restraining and modulating loop current of network-forming inverter for heavy truck battery swap station

Abstract

The invention discloses a loop current suppression and modulation method for a network-forming inverter of a heavy truck battery swap station, which comprises the following steps: after harmonic waves are suppressed by adopting a reduced-order resonance controller, converting a negative-sequence double frequency, a positive-sequence quadruple component and a frequency loop current harmonic wave component into third-order loop current harmonic waves, and considering a primary frequency modulation characteristic, carrying out secondary frequency modulation on the third-order loop current harmonic waves, and carrying out secondary frequency modulation on the third-order loop current harmonic waves; and a proportional resonance integral controller is connected in parallel to ensure harmonic suppression during frequency deviation, and meanwhile, suppressed harmonic is introduced into a hysteresis controller to correct a nearest level approximation modulation wave. Through a combined control method of direct inhibition and indirect inhibition of the circulating current, the purpose of inhibiting the circulating current is achieved. According to the invention, harmonic suppression after power frequency and frequency deviation is guaranteed, the influence of capacitor voltage ripples of the sub-modules is improved while modulation waves are corrected, and the control mode is simplified.

Description

Technical Field

[0001] The present invention belongs to the field of control technology, and specifically relates to a method for suppressing and modulating circulating current of a grid-type inverter used in a heavy truck battery swap station. Background Art

[0002] The increase in demand for short-distance transportation has driven the growth of the power battery market. Battery swapping can, to a certain extent, solve the problems of high purchase cost and long charging time of early power batteries. Heavy truck swapping stations can provide support for the power grid through vehicle-to-grid interaction (V2G) technology. However, a large number of renewable energy and energy storage systems are connected to the power system through inverters, making the power system present a "high proportion of renewable energy and high proportion of power electronic equipment" double-high form. The high penetration rate of renewable energy and the large investment in power electronic equipment have led to a decrease in system inertia and damping, resulting in insufficient voltage and frequency regulation capabilities of the power grid. The primary frequency regulation characteristics of the grid-type converter using a virtual synchronous generator (VSG) can provide inertia and damping for the power grid, becoming an effective control method for the interaction of "grid, load, and storage". The VSG-based modular multilevel converter battery energy storage system (MMC-BESS) is suitable for high-voltage and high-power applications. It has the advantages of modularity and easy cascading, as well as the advantages of synchronous generators such as inertia and damping. However, it also has the inherent defects of ordinary MMCs, namely, the relative independence of the sub-module capacitors will cause capacitor voltage fluctuations and imbalances, thereby generating arm circulation, causing problems such as bridge arm current waveform distortion and battery state of charge (SOC) imbalance. Summary of the invention

[0003] The purpose of the present invention is to overcome the above-mentioned shortcomings and to propose a method for suppressing the circulating current of a grid-type inverter used in a heavy-duty truck battery swap station, which ensures the suppression of harmonics at the power frequency and after frequency offset, corrects the modulation wave, and improves the influence of the sub-module capacitor voltage ripple, simplifies the control method.

[0004] Based on the above purpose, the present invention proposes a method for suppressing and modulating circulating current of a grid-connected inverter for a heavy truck swap station, comprising the following steps:

[0005] (1) Analyze the submodule capacitor current and the circulating current between the upper and lower bridge arms of the grid-type modular multi-level battery energy storage system used for heavy truck swap stations. The current values ​​in the upper and lower bridge arms of the MMC are derived using KCL and KVL to obtain the circulating current model.

[0006] (2) extracting typical negative sequence double frequency components and positive sequence quadruple frequency components according to the circulating current model, and obtaining a circulating current suppressor, wherein the circulating current suppressor includes a reduced-order proportional resonant controller and a proportional resonant integral controller connected in parallel;

[0007] (3) The harmonics suppressed by the circulating current suppressor are introduced into the hysteresis controller to perform current hysteresis correction on the nearest level approaching modulation wave;

[0008] (4) In order to solve the problem of battery state of charge imbalance in the grid-connected modular multi-level battery energy storage system used for heavy truck swap stations, battery state of charge balancing control is introduced, and the nearest level approximation modulation wave correction is performed according to the method in step (3).

[0009] Preferably, the circulating current model in step (1) causes even-order harmonics through the interaction between the submodule capacitor voltage and the double-frequency circulating current.

[0010] Preferably, the negative-sequence 2-fold frequency and positive-sequence 4-fold frequency components of the circulating current suppressor in step (3) are converted into 3-fold frequency circulating current in the dq coordinate system by coordinate transformation.

[0011] Preferably, in step (3), the current error correction reference value is 0.5. When the current error correction reference value is 0.5, the modulated wave output is one level higher. When the reference value is -0.5, the modulated wave output is one level lower.

[0012] Preferably, the battery state of charge balance control described in step (4) is achieved by controlling the AC and DC components of the circulating current.

[0013] Compared with the prior art, the present invention has obvious beneficial effects. It can be seen from the above technical solutions that the present invention adopts a meshing control in the MMC-BESS control mode, which can provide support for the power grid in the V2G scenario. According to the characteristic that the PR controller has infinite gain at a specific frequency, the ROR controller after the PR controller is reduced in order can uniformly control the conversion of the 2- and 4-fold frequency circulating currents of the MMC sub-modules into 3-fold frequency. At the same time, the circulating current suppressor composed of the parallel PIR controller ensures the suppression of harmonics after the power frequency and frequency offset. When dealing with the influence of the circulating current on the modulated wave, the hysteresis controller is used to correct the modulated wave, and at the same time, the influence of the sub-module capacitor voltage ripple is improved. In the power battery SOC balancing method of the MMC-BESS system, the AC and DC components of the circulating current are controlled, and no additional components are added, which simplifies the control method. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is the structure diagram of the MMC-BESS system;

[0015] Figure 2 It is a flow chart of the control method;

[0016] Figure 3 Phase a upper arm circulating current waveform and FFT analysis;

[0017] Figure 4 System frequency and power dynamic response waveform. DETAILED DESCRIPTION

[0018] In combination with the accompanying drawings and preferred embodiments, the specific implementation method, characteristics and effects of a grid-type inverter circulating current suppression and modulation method for a heavy truck battery swap station proposed in accordance with the present invention are described in detail as follows.

[0019] Embodiment 1:

[0020] like Figure 1 As shown, a method for suppressing and modulating circulating current of a grid-type inverter for a heavy truck swap station of the present invention (the system parameters used are shown in Table 1) comprises the following steps:

[0021] Table 1 System parameters

[0022]

[0023] (1) The submodule capacitor current and the circulating current between the upper and lower bridge arms of the grid-connected modular multi-level battery energy storage system used for heavy truck swap stations are analyzed to obtain a circulating current model. The double frequency circulating current will cause the SM capacitor voltage to fluctuate at a triple frequency, thereby generating a 4th frequency harmonic. Similarly, the MMC circulating current only contains even-order harmonic components. Therefore, the modified a-phase circulating current is:

[0024]

[0025] (2) The circulating current suppression mainly considers suppressing the 2-fold and 4-fold circulating currents. According to the circulating current model, the typical negative-sequence 2-fold and positive-sequence 4-fold components are extracted to obtain the circulating current suppressor. The circulating current suppressor includes a parallel reduced-order proportional resonant controller and a proportional resonant integral controller. The ROR controller has a narrow bandwidth problem and cannot meet the suppression requirements when dealing with primary frequency modulation conditions. Therefore, a quasi-proportional integral resonant controller is connected in parallel on the basis of the ROR controller to meet the circulating current suppression requirements when the system frequency offsets. The transfer function of the ROR-PIR controller is:

[0026]

[0027] In the formula, K p is the proportionality coefficient; K i is the integral coefficient; K r is the resonance coefficient, ω c is the cut-off frequency;

[0028] (3) The circulating current model described in step (1) introduces the harmonics suppressed by the circulating current suppressor into the hysteresis controller to perform current hysteresis correction on the nearest level approximation modulation wave through the interaction between the submodule capacitor voltage and the frequency-doubled circulating current, such as Figure 2As shown, in step (2), the negative sequence 2 times frequency and positive sequence 4 times frequency components of the circulating current suppressor are converted into 3 times frequency circulating current in the dq coordinate system through coordinate transformation;

[0029]

[0030] In the formula, Δu c is the reference voltage correction value of the circulating current suppressor output; when the current error is greater than δ, the controller outputs the correction value Δu = 0.5, and when the current error is less than δ, the controller outputs the correction value Δu = -0.5. The correction value output by the controller enters the NLM to generate the output voltage waveform. If i diffj At the current sampling time, it is just at the bottom of the hysteresis loop, that is, i diffj =-δ, the modulation output increases by one level. Similarly, i diffj When =δ, the output is one level lower.

[0031] (4) The battery state of charge balance control strategy is implemented by controlling the AC and DC components of the circulating current. For the battery state of charge imbalance phenomenon in the grid-type modular multi-level battery energy storage system used for heavy truck swap stations, the battery state of charge balance control is introduced, and the nearest level approximation modulation wave correction is performed according to the method in step (3).

[0032] Comparative Example 1: Comparison of the effects of the circulation suppression methods according to the parameters in Table 1

[0033] Phase a upper arm circulating current waveform and FFT analysis (such as Figure 3 As shown in the figures, the circulation suppressors were put into operation at 0.5s. (a) is the circulation waveform without the circulation suppressor. After the system reached stability, the circulation fluctuation range was about 15A-55A; (b) is the circulation waveform with the QPR circulation suppressor. After reaching steady state, the circulation fluctuation range was about 28.5A-33A, which was reduced by about 88.75% compared with the system without the circulation suppressor; (c) is the circulation combined suppression method using the ROR-PIR circulation suppressor and the improved NLM. After the circulation suppressor was put into operation at 0.5s, the system reached stability about 0.2s slower than the QPR circulation suppression method, but after reaching stability, the circulation fluctuation range was about 29A-32A, which was reduced by about 92.5% compared with the system without the circulation suppressor.

[0034] System frequency and power dynamic response waveform (such as Figure 4 As shown in the figure, the droop characteristic and damping characteristic are combined to make the oscillation range within 0.05Hz. The system power dynamic response waveform shows that the grid-connected active power oscillates; the reactive power starts to increase when the system frequency decreases, maintaining the grid-connected voltage amplitude without fluctuations. At the same time, the system has damping and inertia.

[0035] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for suppressing and modulating circulating current of a grid-connected inverter for a heavy truck swap station, comprising the following steps: (1) Analyze the submodule capacitor current and the circulating current between the upper and lower bridge arms of the grid-connected modular multi-level battery energy storage system used for heavy truck swap stations. The current values ​​in the upper and lower bridge arms of the MMC are derived using KCL and KVL to obtain the circulating current model. (2) According to the circulating current model, typical negative sequence double frequency components and positive sequence quadruple frequency components are extracted to obtain a circulating current suppressor, wherein the circulating current suppressor includes a reduced-order proportional resonant controller and a proportional resonant integral controller connected in parallel; (3) Introducing the harmonics suppressed by the circulating current suppressor into the hysteresis controller to perform current hysteresis correction on the nearest level approaching modulation wave; (4) In order to solve the problem of battery state of charge imbalance in the grid-connected modular multi-level battery energy storage system used for heavy truck swap stations, battery state of charge balancing control is introduced, and the nearest level approximation modulation wave correction is performed according to the method in step (3).

2. A method for suppressing and modulating circulating current of a grid-connected inverter for a heavy truck swap station according to claim 1, wherein: The circulating current model in step (1) causes even-order harmonics through the interaction between the submodule capacitor voltage and the double-frequency circulating current.

3. A method for suppressing and modulating circulating current of a grid-connected inverter for a heavy truck swap station according to claim 1, wherein: The negative-sequence 2-fold frequency component and the positive-sequence 4-fold frequency component of the circulating current suppressor in step (3) are converted into the 3-fold frequency circulating current in the dq coordinate system through coordinate transformation.

4. A method for suppressing and modulating circulating current of a grid-connected inverter for a heavy truck swap station according to claim 1, wherein: In step (3), the current error correction reference is 0.

5. When the current error correction reference is 0.5, the modulation wave output is one level higher. When the reference is -0.5, the modulation wave output is one level lower.

5. A method for suppressing and modulating circulating current of a grid-connected inverter for a heavy truck swap station according to claim 1, wherein: The battery state of charge balance control described in step (4) is achieved by controlling the AC and DC components of the circulating current.

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