A Method for Suppressing DC Bus Voltage Ripple of Three-Phase PWM Rectifier by Reconstructing Three-Dimensional Negative-Sequence Current

Abstract

The present invention designs a method for suppressing the DC bus voltage ripple of a three-phase PWM rectifier by three-dimensional negative-sequence current reconstruction. The present invention belongs to the field of rectifier control technology in power electronics. In the prior art, when the grid-side parameters of a three-phase PWM rectifier cannot be accurately obtained, the suppression performance of grid-side current harmonics and DC bus voltage ripple will decline. The method described in the present invention is based on a three-dimensional space cone model of negative-sequence active and reactive currents and the amplitude of the DC bus voltage ripple. By controlling the change of negative-sequence current, the amplitude data of the DC bus voltage ripple is collected, and the negative-sequence current coordinates at the cone vertex are reconstructed. By using the method described in the present invention, the DC bus voltage ripple and grid-side current harmonics can be effectively suppressed, and parameters such as grid-side voltage and inductance are not required, and it has good robustness.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics control, and particularly relates to a method for suppressing the DC bus voltage ripple of a three-phase PWM rectifier by three-dimensional negative-sequence current reconstruction. Background Art

[0002] Rectifiers can provide stable and reliable DC power supplies and are widely used in the industrial field. However, due to the characteristics of the converter itself or some special working conditions, the DC power supply may have low-frequency fluctuations. The input power of a single-phase rectifier contains a pulsating component with twice the fundamental angular frequency of the grid voltage, so the DC bus voltage contains a second-order ripple. For a three-phase PWM rectifier, under symmetric working conditions, the pulsating power generated by each phase is equal in magnitude and 120° out of phase with each other, so the pulsating power can cancel each other out, making the output power and voltage contain only DC components. However, when three-phase asymmetric voltages appear, this balance will be disrupted, resulting in an AC component in the DC bus voltage. This AC component will cause the grid current to contain odd harmonics through voltage negative feedback, thereby deteriorating the power quality of the AC and DC sides of the rectifier. Therefore, it is of great significance to effectively suppress the DC bus voltage ripple and grid-side current harmonics.

[0003] Reference to related patent applications:

[0004] Existing methods control the grid-side current based on the grid-side power model to suppress the DC bus voltage ripple. A virtual phase current compensation control strategy is given in [1], and the current compensation value is related to the degree of voltage asymmetry, thereby effectively suppressing the pulsating component in the output power. [2] gives a method for reconstructing the phase voltage based on line voltage sampling, and controls the grid current according to the reconstructed phase voltage to reduce the current asymmetry. These two methods are simple in calculation and easy to implement, but they are both limited by grid-side voltage sampling. Once the grid-side parameters cannot be accurately obtained, the performance of suppressing the DC bus voltage ripple will decline.

[0005] In order to get rid of the bondage of accurately obtaining parameters such as grid-side voltage and inductance, [3] proposes a virtual impedance control strategy without a grid-side power model, which realizes a virtual capacitor at a specific frequency through DC bus voltage negative feedback to suppress the DC bus voltage pulsation and grid-side current harmonics. However, this method requires a filter to be connected in series in the control closed-loop, which has a negative impact on the system stability margin.

[0006] [1]C.-Y.Tang and J.-H.Jheng,"An active power ripple mitigationstrategy for three-phase grid-tied inverters under unbalanced grid voltages,"IEEE IEEE Trans.Power Electron.,vol.38,no.1,pp.27-33,Jan.2023

[0007] [2]R.Huang,J.Xu,Q.Chen,X.Guo and H.Cao,"Reconstructed phase voltagesbased power following control for three-phase buck rectifier under unbalancedphase voltages and wide ac input frequency,"IEEE Trans.Power Electron.,vol.38,no.2,pp.2022-2031,Feb.2023.

[0008] [3]T.Song,Y.Zhang,F.Gao,X.Zhu,J.Shan and Z.Kong,"Power model freevoltage ripple suppression method of three-phase PWM rectifier underunbalanced grid,"IEEE Trans.Power Electron.,vol.37,no.11,pp.13799-13807,Nov.2022. Summary of the Invention

[0009] Aiming at the defects existing in the prior art, the purpose of the present invention is to provide a method for suppressing the DC bus voltage ripple of a three-phase PWM rectifier by reconstructing the three-dimensional negative-sequence current. This method can reconstruct the negative-sequence active and reactive currents at the minimum DC bus voltage amplitude based on the online observation data of the negative-sequence active and reactive currents and the DC bus voltage ripple amplitude, thereby effectively suppressing the DC bus voltage ripple and grid current harmonics.

[0010] To achieve the above object, the technical solution adopted by the present invention is: a method for suppressing the DC bus voltage ripple of a three-phase PWM rectifier for three-dimensional negative-sequence current reconstruction, wherein the input of the three-phase PWM rectifier is a three-phase power grid. One end of the filter inductor is connected to the three-phase power grid, and the other end of the filter inductor is connected to the midpoint of the bridge arm of the three-phase full-bridge rectifier. The upper and lower ends of the three-phase full-bridge rectifier are the positive and negative poles of the DC bus output, which are respectively connected to the positive and negative ends of the DC bus capacitor, and the voltage between its two ends is the DC bus voltage. The method includes the following steps:

[0011] (1) Initialization of the negative-sequence current given value during data acquisition: Design the given values of the negative-sequence active and reactive currents during three data acquisitions according to a certain proportion of the positive-sequence active current;

[0012] (2) Data acquisition of the DC bus voltage ripple amplitude when there is no negative-sequence current given: Obtain the DC bus voltage ripple amplitude when the negative-sequence active and reactive currents are zero through calculation with fast Fourier transform (FFT);

[0013] (3) Data acquisition of the DC bus voltage ripple amplitude when a specific negative-sequence current is given: Control the rectifier in turn with the negative-sequence current according to the initialized given value, and calculate the DC bus voltage ripple amplitudes corresponding to three groups of negative-sequence active and reactive currents through FFT;

[0014] (4) Three-dimensional negative-sequence active and reactive current reconstruction: Reconstruct the negative-sequence current at the place where the DC bus voltage ripple amplitude is the smallest according to the aforementioned negative-sequence active and reactive currents and the corresponding DC bus voltage ripple amplitude data, and use it as the given value to control the three-phase PWM rectifier to complete the suppression of the DC bus voltage ripple;

[0015] Further, in step (1), under the condition of ensuring system stability, the negative-sequence current reference value can be given according to a certain proportion of the positive-sequence active current reference value :

[0016]

[0017] where z ∈ {1, 2, 3} represents the z-th negative-sequence current control, are the given values of the z-th negative-sequence active and reactive currents respectively, and λ dz , λ qz are the current coefficients corresponding to the z-th negative-sequence current control respectively. Let one of them be zero and the other be a set value.

[0018] Further, in steps (2) and (3), the negative-sequence current at the place where the DC bus voltage ripple amplitude is the smallest and are determined according to the following relationship:

[0019]

[0020] Among them

[0021]

[0022] Among them, U dc_ac0 , U dc_ac1 , U dc_ac3 are respectively the DC bus voltage ripple amplitudes under three times negative sequence current control.

[0023] Furthermore, in step (4), the negative sequence active and reactive currents at the minimum DC bus voltage ripple amplitude are determined according to the following relationship:

[0024]

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] 1. The proposed method can obtain the DC bus voltage ripple amplitude without the aid of a band-pass filter, and has a faster negative sequence current reconstruction speed.

[0027] 2. The proposed method does not require parameters such as grid-side voltage and inductance, gets rid of the bondage of the grid-side power model, and has good robustness to voltage sampling errors and inductance parameter offsets.

[0028] 3. The proposed method does not require additional auxiliary hardware circuits, thus can reduce the development cost and has a more flexible application scenario. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is the control block diagram of the present invention;

[0030] Figure 2 is the working flow chart of the three-dimensional negative sequence current controller;

[0031] Figure 3 is the plane relationship between negative sequence current and DC bus voltage ripple amplitude. (a) Horizontal distance between negative sequence currents. (b) Similar right triangles in the same plane;

[0032] Figure 4 is the waveform of the present invention method for suppressing the dynamic process of the DC bus voltage, (a) Three-phase voltage, DC bus voltage and DC bus voltage ripple amplitude, (b) Negative sequence command current in the d-q coordinate system;

[0033] Explanation of the symbols in the drawings: e a , e b , e c , i a , i b , i care the three-phase voltages and currents in the three-phase stationary coordinate system, e α and e β and i α and i β are the voltage and current in the two-phase stationary coordinate system, respectively, are the command currents in the two-phase stationary coordinate system, respectively, are the positive and negative, negative-sequence command currents in the two-phase stationary coordinate system, respectively, are the positive-sequence and negative-sequence command currents in the two-phase rotating coordinate system, respectively, are the command voltages in the two-phase stationary coordinate system, L a and L b and L c and R a and R b and R c are the three-phase inductances and their equivalent series resistances, U ref is the DC bus command voltage, U dc is the DC bus voltage, and U dc_acz are the negative-sequence active, reactive currents and the DC bus voltage ripple amplitude under the z-th negative-sequence current control, respectively. Specific implementation manners

[0034] A specific embodiment of the present invention will be described below in conjunction with the drawings, and the principle and effect of the technical solution of the present invention will be further explained.

[0035] A method for suppressing the DC bus voltage ripple of a three-phase PWM rectifier with three-dimensional negative-sequence current reconstruction, the control block diagram of which is as Figure 1 shown. FFT is the fast Fourier analysis, and the amplitude U dc_ac of the AC component with twice the fundamental angular frequency of the grid voltage in the DC bus voltage can be calculated through the sampled data within one period. When U dc_ac is greater than the set range U dc_max , the three-dimensional negative-sequence current controller starts to operate. Its operation flow chart is as Figure 2 shown.

[0036] The steps of the embodiment of the present invention will be specifically described below in conjunction with the Figure 1 shown control block diagram and the Figure 2 shown operation flow chart of the three-dimensional negative-sequence current controller.

[0037] According to step (1), assume that the rated power of the three-phase PWM rectifier is 3 kW, and the positive-sequence active current is about 16 A under the condition that the three-phase voltages are symmetrical and the effective value is 110 V, and the negative-sequence current coefficients λ dz and λ qzThey are (10%, 0%), (5%, 0%), (0%, 5%) respectively, and the corresponding actual negative-sequence current reference values are (1.6 A, 0 A), (0.8 A, 0 A), (0 A, 0.8 A).

[0038] According to step (2), the ripple amplitude of the DC bus voltage without negative-sequence current control obtained by sampling is (0 A, 0 A, U dc_ac0 ) is (0 A, 0 A, 6.98 V).

[0039] According to step (3), the ripple amplitudes of the DC bus voltage under three times of negative-sequence current control are (1.6 A, 0 A, 2.89 V), (0.8 A, 0 A, 3.55 V), (0 A, 0.8 A, 6.71 V) respectively.

[0040] The principle of step (3) is as follows:

[0041] In the unbalanced operating condition of the three-phase PWM rectifier, the positive-sequence and negative-sequence voltages and currents will cause reactive power pulsation, which can be recorded as:

[0042]

[0043] Among them, the AC component Q in the output power out can be deduced as:

[0044]

[0045] Among them

[0046]

[0047] If the initial phase of the AC component of the DC bus voltage is then the DC bus voltage U dc can be recorded as

[0048]

[0049] Among them, U dc_dc and U dc_ac are the amplitudes of the DC and AC components of the DC bus voltage respectively. Since the DC component of the DC bus voltage is much larger than the amplitude of the AC component, according to (8), the pulsating power Q of the DC bus capacitor c can be expressed as

[0050]

[0051] And according to the law of conservation of power, the pulsating power Q output by the rectifier out is equal to the pulsating power Q of the DC bus capacitor c From (6) and (9), the AC component U of the DC bus voltage can be obtaineddc_ac For

[0052]

[0053] wherein

[0054]

[0055] According to (10), when the grid voltage is asymmetric and the output power is constant, the rectifier AC voltage and positive-sequence current are also constant. At this time, the DC bus voltage ripple amplitude is determined by the negative-sequence active and reactive currents. It can be seen that the negative-sequence active and reactive currents and the DC bus voltage ripple amplitude present a conical surface relationship. In particular, at the vertex of the cone, that is, when the negative-sequence active and reactive currents are respectively and respectively, the DC bus voltage ripple amplitude is 0.

[0056] According to step (4), substituting the data obtained in steps (2) and (3) into equation (4), the negative-sequence current at the vertex of the cone can be obtained as

[0057]

[0058] The principle of step (4) is as follows:

[0059] According to the properties of the cone, any observation point on the conical surface and the point on the cone axis and the vertex of the cone z O z T form a right triangle Rt△S and The horizontal distance of is the controlled negative-sequence current Figure 3 As shown in (a); the other right side is the DC bus voltage ripple amplitude. Rotating these right triangles to the same plane, a set of similar triangles is obtained, as Figure 3 shown in (b), so as to obtain the system of equations of negative-sequence active and reactive currents and DC bus voltage ripple amplitude

[0060]

[0061] wherein

[0062]

[0063] For the convenience of simplification and calculation, let one of be 0. Here, taking equal to 0 as an example, substituting (14) into (13) and arranging, we can get

[0064]

[0065] Among them

[0066]

[0067] Taking the difference between (15) pairwise, we can obtain

[0068]

[0069] Solving for the negative-sequence current at the cone vertex coordinates is

[0070]

[0071] Respectively taking as the reference values of negative-sequence active and reactive currents, the ripple suppression of the DC bus voltage can be achieved.

[0072] In this embodiment, the specific process of the DC bus voltage ripple suppression method based on three-dimensional negative-sequence current reconstruction is given. When the rated power and current change, this process can be followed.

[0073] In order to show the remarkable effects of the present invention, some experimental results obtained from the embodiments are given in this embodiment. Figure 4 The waveform of the DC bus voltage during the dynamic process suppressed by the method of the present invention is given. From 0 to 1 s, without adding the control method proposed in the present invention, the peak-to-peak value of the DC bus voltage ripple is 6.98 V, and there is obvious double-frequency power frequency ripple fluctuation; the effective values of the three-phase voltages are 50 V, 120 V, and 110 V respectively, and the DC component of the DC bus voltage is 300 V. The effective values of the three-phase currents on the grid side are 11.23 A, 10.56 A, and 10.95 A respectively, and the total harmonic distortion rates of the currents are 3.9%, 3.7%, and 3.8% respectively, and the current distortion is serious. After 1.05 s, the calculation of the negative-sequence command current of this method is completed. Thereafter, the peak-to-peak value of the DC bus voltage ripple is 0.19 V, and the double-frequency power frequency ripple fluctuation is effectively suppressed; the amplitudes of the third-harmonic currents in the three-phase currents are 0.0026 A, 0.0025 A, and 0.0025 A respectively, and the THDs of the three-phase currents are 1.2%, 1.1%, and 1.1% respectively, and the current harmonics are significantly reduced.

[0074] The experimental results obtained in this embodiment show that using the method of the present invention can not only effectively suppress the DC bus voltage ripple, but also effectively eliminate the grid-side current harmonics, and no parameters such as grid-side voltage and inductance are required in this process, and it has strong robustness.

[0075] The present invention can be implemented in other specific forms without departing from its spirit or essential characteristics. The described embodiments are considered to be illustrative rather than restrictive in all respects. For example:

[0076] 1) The application topology includes various three-phase rectifiers;

[0077] 2) Design of the negative-sequence disturbance current disturbance coefficient;

[0078] 3) The rated operating current of the rectifier;

[0079] 4) Selection of various parameters, etc.

[0080] Therefore, the scope of the present invention is indicated by the appended claims rather than the above description. All changes that fall within the meaning and scope of equivalent technical solutions of the claims are included in its scope.

Claims

1. A method for suppressing the DC bus voltage ripple of a three-phase PWM rectifier by reconstructing the three-dimensional negative-sequence current. The input of the three-phase PWM rectifier is a three-phase power grid. One end of the filter inductor is connected to the three-phase power grid, and the other end of the filter inductor is connected to the midpoint of the bridge arm of the three-phase full-bridge rectifier. The upper and lower ends of the three-phase full-bridge rectifier are the positive and negative poles of the DC bus output, which are respectively connected to the positive and negative ends of the DC bus capacitor. The voltage between its two ends is the DC bus voltage, and the method includes the following steps: (1) Initialization of the negative-sequence current given value during data acquisition: Design the given values of the negative-sequence active and reactive currents during three data acquisition processes according to a certain proportion of the positive-sequence active current; (2) Data acquisition of the DC bus voltage ripple amplitude when there is no negative-sequence current given: Calculate and obtain the DC bus voltage ripple amplitude when the negative-sequence active and reactive currents are zero through fast Fourier transform; (3) Data acquisition of the DC bus voltage ripple amplitude when a specific negative-sequence current is given: Control the rectifier in sequence according to the initialized given value of the negative-sequence current, and calculate the DC bus voltage ripple amplitudes corresponding to three groups of negative-sequence active and reactive currents through FFT; (4) Reconstruction of three-dimensional negative-sequence active and reactive currents: Reconstruct the negative-sequence current at the minimum DC bus voltage ripple amplitude according to the aforementioned negative-sequence active and reactive currents and the corresponding DC bus voltage ripple amplitude data, and use it as the given value to control the three-phase PWM rectifier to complete the suppression of the DC bus voltage ripple; In step (1), under the condition of ensuring system stability, the negative-sequence current reference value is given according to a certain proportion of the positive-sequence active current reference value : Among them, z ∈ {1, 2, 3} represents the z-th negative sequence current control, which are the given values of the z-th negative sequence active and reactive currents, respectively, λ dz , λ qz which are the current coefficients corresponding to the z-th negative sequence current control, respectively. Let one of them be zero and the other be the set value; In steps (2) and (3), the negative-sequence current at the minimum value of the DC bus voltage ripple amplitude and is determined according to the following relationship: Wherein where U dc_ac0 , U dc_ac1 , U dc_ac3 are the DC bus voltage ripple amplitudes under three - phase negative - sequence current controls respectively; In step (4), the negative-sequence active and reactive currents at the point where the amplitude of the DC bus voltage ripple is minimum are determined according to the following relationship:

Images