Rectifier harmonic current suppression method and system based on discrete Fourier transform

Abstract

The invention relates to the technical field of harmonic suppression, and provides a rectifier harmonic current suppression method and system based on discrete Fourier transform, and the method comprises the following steps: employing DFT to extract a higher harmonic component of a power grid current for an obtained current inputted into a rectifier; respectively setting independent current PI controllers according to corresponding orders for the harmonic waves of the negative sequence component and the positive sequence component with set orders; taking the harmonic component subjected to DFT decomposition as a feedback value, taking zero as a harmonic current reference value, and obtaining a reference voltage suppressed by the harmonic component of a corresponding order based on a PI controller; and adding the reference voltages of the harmonic components of the fundamental component, the negative sequence component and the positive sequence component to generate a reference voltage signal for modulation. The coefficient obtained through DFT is directly used as feedback, PI controller output is obtained based on proportional-integral calculation, modular design can be achieved, and portability of the control system is achieved.

Description

Technical Field

[0001] The present disclosure relates to the technical field related to harmonic suppression, and in particular to a rectifier harmonic current suppression method and system based on discrete Fourier transform. Background Art

[0002] The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art.

[0003] The control strategy of the rectifier is usually based on a synchronous rotating coordinate system, such as a three-phase Vienna rectifier. The quality of the AC side current of the rectifier controlled based on a synchronous rotating coordinate system is crucial. However, when the three-phase grid voltage is distorted, the total harmonic distortion (THD) of the rectifier current deteriorates. This is because the grid current is converted into a DC component in the rotating coordinate system, and the harmonic components in the current appear as high-frequency AC components in DC; the PI controller usually has good feedback control for low-frequency or DC components, but cannot effectively regulate high-frequency AC components, resulting in the grid current being unable to maintain an ideal sinusoidal waveform.

[0004] At present, the methods for reducing the THD of this type of rectifier current include: zero-sequence component injection modulation method, modified proportional resonance method, and hybrid space vector modulation method. However, most of these methods are very complex in calculation and control, and their universality in different systems is also limited.

[0005] First, the control strategies of these methods usually require complex calculations and signal processing, especially in multivariable control and high-order modulation algorithms, which involve a large number of real-time operations, increasing the burden on the control system and reducing the system response speed. In addition, since these methods are highly dependent on system parameters, precise parameter matching and optimization debugging must be performed to obtain the ideal control effect, which makes them less universal in different systems.

[0006] Secondly, under high power conditions, the distortion problem of grid voltage will be more serious, and conventional THD suppression methods may not be able to cope with strong voltage distortion. In this case, the rectifier current THD will increase significantly, the effectiveness of existing control methods will be greatly reduced, and may even cause system instability. Especially in the scenario of severe grid voltage distortion, these control methods cannot effectively maintain the sinusoidal nature of the current waveform, resulting in distortion of the current waveform, increasing equipment loss and electromagnetic interference, and affecting the overall efficiency and stability of the system. Summary of the invention

[0007] In order to solve the above problems, the present disclosure proposes a rectifier harmonic current suppression method and system based on discrete Fourier transform, which directly uses the coefficients obtained by DFT transformation as feedback, obtains the PI controller output based on proportional-integral calculation, and can realize modular design and portability of the control system.

[0008] In order to achieve the above objectives, the present disclosure adopts the following technical solutions:

[0009] One or more embodiments provide a rectifier harmonic current suppression method based on discrete Fourier transform, comprising the following steps:

[0010] Based on the current of the three-phase grid of the input rectifier, DFT is used to extract the high-order harmonic components of the grid current;

[0011] For the obtained high-order harmonic components, for the harmonics of the negative-sequence components and the positive-sequence components of the set order, independent current PI controllers are set according to the corresponding orders;

[0012] The harmonic components after DFT decomposition are used as feedback values, and zero is used as the harmonic current reference value, which are input into the current PI controller to obtain the reference voltage for suppressing the harmonic components of the corresponding order;

[0013] The reference voltages of the harmonic components of the fundamental component, the negative sequence component and the positive sequence component are added together to generate a reference voltage signal for modulation, and the harmonic suppression control of the rectifier is performed.

[0014] One or more embodiments provide a rectifier harmonic current suppression system based on discrete Fourier transform, comprising:

[0015] Decomposition module: configured to extract high-order harmonic components of the grid current by using DFT for the current of the three-phase grid of the input rectifier;

[0016] A controller setting module: configured to set independent current PI controllers according to corresponding orders for the obtained high-order harmonic components, and for the harmonics of the negative-sequence components and the positive-sequence components of the set order;

[0017] The first reference voltage calculation module is configured to use the harmonic component after DFT decomposition as the feedback value and zero as the harmonic current reference value, which is input into the current PI controller to obtain a reference voltage for suppressing the harmonic component of the corresponding order;

[0018] Superposition module: configured to add the reference voltages of the harmonic components of the fundamental component, the negative sequence component and the positive sequence component to generate a reference voltage signal for modulation and perform harmonic suppression control on the rectifier.

[0019] One or more embodiments provide a rectifier harmonic current suppression system based on discrete Fourier transform, including a fundamental wave control unit and a harmonic control unit;

[0020] The harmonic control unit includes a PI controller and a voltage converter set for high-order harmonic components; the PI controller uses the amplitudes of the cosine component and the sine component in the high-order harmonic components as feedback, takes zero as the harmonic current reference value, and outputs a reference voltage for suppressing the corresponding order harmonic components.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] In the present disclosure, a current harmonic suppression strategy of real-time discrete Fourier transform (DFT) is proposed; DFT is used to extract the sine and cosine components of the higher harmonics of the grid current; and the higher harmonic components are directly applied to the feedback loop of the dq rotating coordinate system, and the decomposed coefficients are directly used as the feedback values ​​of the corresponding PI controller, as the feedback of the q-axis component and the d-axis component for PI algorithm calculation to obtain the reference voltage, and the control method is simple; an independent control loop for the higher harmonic current is introduced, and the reference value is set to zero to suppress the corresponding harmonic current. The control strategy is simple, versatile and portable. In the entire load range, the total harmonic distortion (THD) of the current under grid voltage distortion is effectively improved.

[0023] The advantages of the present disclosure and the advantages of additional aspects will be described in detail in the following specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings constituting a part of the present disclosure are used to provide a further understanding of the present disclosure. The exemplary embodiments of the present disclosure and the description thereof are used to explain the present disclosure but do not constitute a limitation of the present disclosure.

[0025] Figure 1 is a typical circuit topology diagram of the Vienna rectifier of Example 1 of the present disclosure;

[0026] Figure 2 is a typical control block diagram of the Vienna rectifier in the synchronous rotating coordinate system of Example 1 of the present disclosure;

[0027] FIG3( a ) is a waveform diagram of the VIENNA rectifier in Embodiment 1 of the present disclosure when operating at unity power factor (pf=1)

[0028] FIG3( b ) is a vector diagram of the VIENNA rectifier in Embodiment 1 of the present disclosure when operating at unity power factor (pf=1);

[0029] FIG4( a ) is a schematic diagram of the rotation of the positive sequence component in Embodiment 1 of the present disclosure in a dq rotating coordinate system;

[0030] FIG4( b ) is a schematic diagram of the rotation of the negative sequence component in Embodiment 1 of the present disclosure in the dq rotating coordinate system;

[0031] Figure 5 is a vector diagram of voltage reference values ​​of different orders according to Embodiment 1 of the present disclosure;

[0032] Figure 6 is a control block diagram of a rectifier harmonic current suppression method based on discrete Fourier transform according to Embodiment 1 of the present disclosure;

[0033] FIG. 7( a ) is an experimental waveform diagram before compensation when the grid voltage contains 3% of the fifth harmonic in the experimental example of Embodiment 1 of the present disclosure;

[0034] FIG7( b ) is an experimental waveform diagram after compensation using the proposed strategy when the grid voltage contains 3% of the fifth harmonic in the experimental example of Embodiment 1 of the present disclosure;

[0035] FIG8( a ) is an experimental waveform diagram before compensation when the grid voltage contains 3% of the 5th harmonic and 1% of the 7th harmonic in the experimental example of Embodiment 1 of the present disclosure;

[0036] FIG8( b ) is an experimental waveform after compensation using the method of Embodiment 1 when the grid voltage contains 3% of the 5th harmonic and 1% of the 7th harmonic in the experimental example of Embodiment 1 of the present disclosure;

[0037] FIG9( a ) is a steady-state waveform before compensation at an output power of 10 kW when connected to a three-phase grid with a voltage THD of 4.75% in an experimental example of Embodiment 1 of the present disclosure;

[0038] FIG9( b ) is a steady-state waveform after compensation at an output power of 10 Kw when connected to a three-phase grid with a voltage THD of 4.75% in an experimental example of Embodiment 1 of the present disclosure;

[0039] FIG. 10( a ) is a steady-state waveform before compensation at an output power of 20 Kw when connected to a three-phase grid with a voltage THD of 5.172% in an experimental example of Embodiment 1 of the present disclosure;

[0040] FIG10( b ) is a steady-state waveform after compensation at an output power of 20 Kw when connected to a three-phase grid with a voltage THD of 5.172% in an experimental example of Embodiment 1 of the present disclosure;

[0041] FIG. 11( a ) is a steady-state waveform after compensation at an output power of 30 kW in an experimental example of Embodiment 1 of the present disclosure;

[0042] FIG11( b ) is a steady-state waveform after compensation at an output power of 40 Kw in an experimental example of Embodiment 1 of the present disclosure;

[0043] FIG11( c ) is a steady-state waveform after compensation at an output power of 50 kW in an experimental example of Embodiment 1 of the present disclosure;

[0044] Fig.12 4 is a comparison diagram of current THD under different load conditions in the experimental example of Embodiment 1 of the present disclosure. DETAILED DESCRIPTION

[0045] The present disclosure is further described below in conjunction with the accompanying drawings and embodiments.

[0046] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present disclosure. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present disclosure belongs.

[0047] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present disclosure. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof. It should be noted that, in the absence of conflict, the various embodiments in the present disclosure and the features in the embodiments can be combined with each other. The embodiments will be described in detail below in conjunction with the accompanying drawings.

[0048] Example 1

[0049] In the technical solutions disclosed in one or more embodiments, Figures 1 to 12 As shown, a rectifier harmonic current suppression method based on discrete Fourier transform includes the following steps:

[0050] Step 1: for the current of the three-phase grid input rectifier obtained, the high-order harmonic components of the grid current are used;

[0051] Step 2: for the obtained high-order harmonic components, for the harmonics of the negative-sequence components and the positive-sequence components of the set order, set independent current PI controllers according to the corresponding orders;

[0052] Step 3, using the harmonic component after DFT decomposition as the feedback value and zero as the harmonic current reference value, inputting them into the current PI controller to obtain the reference voltage for suppressing the harmonic component of the corresponding order;

[0053] Among them, the d-axis and q-axis components of the negative sequence component are interchanged as feedback values;

[0054] Step 4: Add the reference voltages of the harmonic components of the fundamental component, the negative sequence component and the positive sequence component to generate a reference voltage signal for modulation, and perform harmonic suppression control on the rectifier.

[0055] In this embodiment, a current harmonic suppression strategy of real-time discrete Fourier transform (DFT) is proposed; DFT is used to extract the sine and cosine components of the higher harmonics of the grid current; and the higher harmonic components are directly applied to the feedback loop of the dq rotating coordinate system, and the decomposed coefficients are directly used as the feedback values ​​of the corresponding PI controller, as the feedback of the q-axis component and the d-axis component for PI algorithm calculation to obtain the reference voltage, and the control method is simple; an independent control loop for the higher harmonic current is introduced, and the reference value is set to zero to suppress the corresponding harmonic current. The control strategy is simple, versatile and portable. In the entire load range, the total harmonic distortion (THD) of the current under grid voltage distortion is effectively improved.

[0056] In the above scheme, there is no need to design the transfer function of the controller for harmonics of different orders respectively. The coefficients obtained by DFT transformation are directly used as feedback, and the output of the PI controller is obtained based on proportional integral calculation, which can realize modular design and portability of the control device.

[0057] The above-mentioned current suppression method can be used for rectifiers that use a rotating coordinate system for harmonic suppression control.

[0058] Optionally, the rectifier may be a three-phase VIENNA rectifier, a three-phase PWM rectifier, a three-phase diode clamped (NPC) converter, a three-phase active neutral point clamped (ANPC) converter, or the like.

[0059] The three-phase Vienna rectifier, referred to as the three-phase VIENNA rectifier, is a highly efficient power factor correction (PFC) rectifier structure. As a three-level converter, it is widely used in multiple industrial fields due to its advantages of high efficiency, high power density, high reliability and high cost performance, including electric vehicle chargers, renewable energy systems, wind turbines, power conversion and other efficient energy management scenarios.

[0060] This embodiment mainly takes a three-phase VIENNA rectifier as an example to illustrate the method for suppressing the above current harmonics.

[0061] Figure 1 The topology of the VIENNA rectifier is shown. x (x=a,b,c) represents the three-phase grid voltage, i x Indicates the three-phase grid current, u x '(x=a,b,c) represents the midpoint voltage. x represents the three-phase boost inductor, u dcis the DC side output voltage, R L The DC link capacitor consists of two capacitors C connected in series. u and C d Each capacitor can withstand u dc Half the voltage.

[0062] The three-phase rectifier bridge uses diodes, and the bidirectional switch on each phase consists of two reverse series connected switching devices S x1 and S x2 The two switches share a driving signal, and no dead time is required, which is further simplified to a bidirectional switch Sx (x = a, b, c). The three bidirectional switches are all connected to the DC midpoint O to achieve a three-level output. By controlling the three bidirectional switches, the current of each phase can be adjusted to achieve power factor correction.

[0063] To facilitate derivation and simplification, the following assumptions are made for VIENNA: 1) the losses of all switching devices are not considered; 2) the three-phase grid voltage is ideal and the system operates in steady state; 3) the switching frequency is much higher than the grid frequency; 4) the output voltages of the two capacitors on the DC side are balanced.

[0064] The grid voltage can be defined as:

[0065]

[0066] Among them, U m and θ are the amplitude and phase angle of the grid voltage, respectively;

[0067] When the VIENNA rectifier operates at unity power factor, the grid current can be expressed by the following formula:

[0068]

[0069] Among them, I m is the amplitude of the grid current. When the grid voltage contains harmonics, the grid current inevitably contains harmonic components of the corresponding order.

[0070] Since the three-phase VIENNA rectifier can be decoupled, the working principle is explained by taking phase a as an example. The distorted grid current can be expressed as:

[0071]

[0072] Where n is the harmonic order, i an is the nth current harmonic, I mn is the amplitude of the nth current harmonic, is the phase, θ=ωt is the phase angle.

[0073] The grid current i aUsing Fourier series expansion:

[0074]

[0075] Among them, a0, a n and b n is the Fourier coefficient and can be expressed as:

[0076]

[0077] Where T is the period of the grid current. Since the characteristics of the power system harmonics satisfy the half-wave symmetry, there is no DC component and the even harmonics can be offset. That is, a0 = 0, n = 1, 3, 5, ... a n It is the coefficient of the cosine component corresponding to the frequency nω, indicating the amplitude of the cosine component at this frequency in the signal. n It is the coefficient of the sinusoidal component corresponding to the frequency nω, and represents the amplitude of the sinusoidal component at this frequency in the signal.

[0078] DFT appears in discrete form in the time domain and frequency domain and is suitable for digital control systems. Sample the current signal N times to generate a sampling sequence i a [N], the sampling period is Ts. a (k) is the kth sample value, i a [N] can be expressed as:

[0079]

[0080] in, is a complex exponential function used to capture the frequency components in the signal. Using the coefficients of formula (5), the amplitude and phase of each harmonic can be calculated as follows:

[0081]

[0082] The fundamental and harmonic components can be separated using DFT, and the harmonic frequencies and amplitudes in the current can be identified, thereby achieving compensation or suppression.

[0083] Furthermore, in addition to the control of higher harmonics, the control method for fundamental wave control is also included: including voltage outer loop and current inner loop control, the voltage outer loop is used to control the DC voltage u output by the rectifier dc Stability, the current inner loop controls the amplitude and phase of the input current of the rectifier;

[0084] The three-phase VIENNA rectifier is a boost PFC, and the three-phase PWM rectifier is also suitable for the above control method.

[0085] Typical control strategies in synchronous rotating coordinate systems are as follows: Figure 2As shown. The control strategy includes a voltage outer loop and a current inner loop. The voltage outer loop is used to control the DC voltage u dc Stability, its output is used as the reference i of the current loop q axis q_ref The current inner loop controls the amplitude and phase of the input current. The d-axis current reference i d_ref Set to 0 to achieve power factor correction. Current loop output u d_ref and u q_ref， After dq / abc conversion, it is used as the three-phase fundamental voltage reference u x_ref (x=a,b,c) to generate the modulated signal.

[0086] In a three-phase power system, harmonics can be divided into positive sequence uP, negative sequence uN and zero sequence uZ components according to their order. The phase of the positive sequence is the same as the fundamental wave, that is, abc. The phase of the negative sequence is opposite to the fundamental wave and rotates in the order of acb. The three-phase harmonics of the zero sequence have the same phase and will not form a rotating magnetic field. In the following equation, α = 1∠120°, α 2 =1∠240°, α is a vector operator representing the angle.

[0087] Positive sequence component u P :This is the component with the same phase as the fundamental wave, and the phase is consistent with the fundamental wave of the three-phase system. The fundamental wave rotates in the order of abc (i.e., the order of phase A, phase B, phase C), so the phase of the positive sequence harmonic is also in the order of abc, which will form a normal rotating magnetic field, which is consistent with the ideal three-phase power system. Among the odd harmonics, the 1st, 7th, 13th harmonics belong to the positive sequence component.

[0088] Negative sequence component u N :This is the component with the opposite phase to the fundamental wave. The phase of the negative sequence component rotates in the order of acb, which is opposite to the abc rotation direction of the fundamental wave; the existence of negative sequence components may cause the motor and other equipment to generate torque or imbalance in the opposite direction, affecting the normal operation of the equipment. Among the odd harmonics, the 5th, 11th and other harmonics belong to negative sequence components;

[0089] Zero sequence component u Z :This is the harmonic component that is the same in all three phases (phase A, phase B, phase C). Because their phases are exactly the same, the zero-sequence component does not produce a rotating magnetic field. Zero-sequence harmonics are common in three-phase four-wire systems and usually flow back through the neutral line. The 3rd, 9th, and other multiples of three harmonics are zero-sequence harmonics.

[0090] Furthermore, the phase sequence classification rule can be expressed as follows:

[0091]

[0092] The dq / abc transformation can be expressed as

[0093]

[0094] Figure 2 The control strategy only includes the fundamental wave. The output reference voltage can be expressed as:

[0095]

[0096] Among them, u m1 is the amplitude of the fundamental reference voltage, and δ1 is the phase shift of the fundamental reference signal relative to the grid voltage. When the VIENNA rectifier operates at unity power factor (pf = 1), the input waveform and reference signal are shown in Figure 3. L is the inductor voltage. L Ahead a At 90°, i a and u a Same phase.

[0097] In step 1, for the current of the three-phase grid input rectifier obtained, DFT is used to extract the high-order harmonic components of the grid current in the dq coordinate system, including the following steps:

[0098] Step 11: Decompose the acquired input current using DFT;

[0099] Step 12: Identify the positive sequence component and the negative sequence component in the high-order harmonics according to the set phase sequence classification rule as the extracted high-order harmonic components; the phase sequence classification rule adopts the above formula (8).

[0100] Furthermore, the control block diagram of the current harmonic suppression strategy based on DFT is as follows: Figure 6 As shown. For the positive sequence component in the high-order harmonic component, the amplitude of the sinusoidal component after DFT decomposition is n As the q-axis component feedback, it is input into the PI controller for the q-axis component to calculate the reference voltage of the q-axis component; the cosine component amplitude b after DFT decomposition is n As the d-axis component feedback, it is input into the PI controller for the d-axis component to calculate the reference voltage of the d-axis component;

[0101] For the negative sequence components in the high-order harmonic components, the amplitude of the sinusoidal component after DFT decomposition is a n As the d-axis component feedback, it is input into the PI controller for the d-axis component to calculate the reference voltage of the d-axis component; the cosine component amplitude b after DFT decomposition is n As the q-axis component feedback, it is input into the PI controller for the q-axis component to calculate the reference voltage of the q-axis component.

[0102] In this embodiment, the seventh harmonic component is taken as the positive sequence component and the fifth harmonic component is taken as the negative sequence component as an example for description.

[0103] In order to suppress harmonics of a specific order, based on Figure 2 The typical control strategy shown in Figure 1 can extract the high-order harmonic components of the grid current in the dq coordinate system through real-time DFT, and then introduce the current PI controller of the corresponding order to set the harmonic current reference value to 0. As the harmonic order increases, the amplitude gradually decreases. Since the 5th and 7th harmonics are close to the fundamental frequency, their current distortion is more obvious.

[0104] If DFT is performed on the grid current, each order of harmonics after decomposition includes sine components and cosine components; among them, a n is the amplitude of the cosine component, b n is the amplitude of the sinusoidal component;

[0105] For the fundamental wave, b1 corresponds to the sinusoidal component and can be regarded as related to the active power of the fundamental wave. d As the d-axis component feedback, the fundamental q-axis current i q As q-axis component feedback;

[0106] The impedance characteristics of the fundamental wave and higher-order waves are different. Specifically, the higher the harmonic frequency, the greater the inductor impedance and the more obvious the current phase lag. For the 5th and 7th harmonic currents, the cosine component is dominant, while the sine component is smaller. Therefore, a5 and a7 can be regarded as corresponding to the q-axis component, and b5 and b7 correspond to the d-axis component.

[0107] For the 5th harmonic current as negative sequence component, the PI controller generates a compensation voltage u on the dq axis. d5_ref and u q5_ref The corresponding fifth harmonic voltage reference signal u can be obtained through dq / abc transformation. m5_ref (m = a, b, c). It is worth noting that the 5th harmonic is a negative sequence component, so the phase information is -5θ. In addition, due to the opposite direction of rotation, the q-axis of the negative sequence is also opposite to the positive sequence. When 5θ becomes -5θ, sin(5θ) becomes -sin(5θ). In order to ensure that the output reference signal has the same phase, if the positive sequence q-axis is positive, the negative sequence q-axis will be negative (i.e. -u q5_ref ), as shown in Figure 4(a) and Figure 4(b).

[0108] u m5_ref It can be calculated as follows:

[0109]

[0110] For the 7th harmonic current, the PI controller generates compensation voltage u on the dq axis respectively.d7_ref and u q7_ref The corresponding seventh harmonic voltage reference signal u can be obtained through dq / abc transformation. m7_ref It is worth noting that the 7th harmonic is a positive sequence component, so the phase information is 7θ.

[0111] u m7_ref It can be calculated as follows:

[0112]

[0113] In this way, all harmonics and fundamental references can be expressed by the same formula, with uniform initial phase, such as Figure 5 When VIENNA operates at unity power factor, the corresponding voltage and current waveforms and vector diagrams of the 5th and 7th harmonics are also shown in Figure 3.

[0114] Finally, the fundamental component, the 5th and 7th harmonics are added together to generate a reference voltage signal for modulation.

[0115]

[0116] like Figure 6 As shown, although the negative sequence component rotates in the opposite direction, the q axis still leads the d axis by 90 degrees. The direction of the negative sequence d axis is the same as that of the positive sequence, while the direction of the q axis is opposite. This means that the negative sequence q axis current is reflected on the positive sequence d axis, while the d axis current corresponds to the positive sequence q axis. The influence of the negative sequence d axis can be regarded as the same as that of the positive sequence q axis. Therefore, for the negative sequence component (ie, the 5th harmonic), the physical meaning of the dq coordinate system is opposite, so the feedback of the d axis and the q axis are interchanged (a5, b5). By introducing independent 5th and 7th harmonic compensation loops, the corresponding harmonic currents can be suppressed.

[0117] For other orders, the control strategy is similar, and PI controllers can be directly added to independent control loops for direct control.

[0118] In order to illustrate the technical effect of the above method, a 50kW three-phase VIENNA rectifier prototype is used to verify the effectiveness of the proposed harmonic current suppression control method.

[0119] When the grid voltage contains 3% of the 5th harmonic, Figure 7(a) and 7(b) The experimental waveforms before and after compensation are shown in Figure 7(a). As shown in Figure 7(a), the grid current is obviously distorted before compensation. After FFT analysis, it can be seen that i a Contains 5th harmonic components. As shown in Figure 7(b), after compensation, aIt is a sine wave, and the 5th harmonic component is completely suppressed. When the voltage contains 3% of the 5th harmonic and 1% of the 7th harmonic, Figure 8(a) and Figure 8(b) show the experimental waveforms before and after compensation using the proposed strategy. a Contains 5th and 7th harmonic components. After compensation, i a The 5th and 7th harmonic components in the simulation are basically 0. The results show that the proposed strategy can effectively suppress the 5th and 7th harmonics.

[0120] In FIG. 7( a ), the horizontal grid and the vertical grid are the networks of the oscilloscope display interface. For different output channels of the oscilloscope, the size and unit of each grid are different. The figure shows the size and unit of the data measured by the four channels of the oscilloscope. Different colors are used to represent the measurement curves of different channel outputs. Specifically, for example, div represents each grid, 200V / div represents that the voltage of one grid is 200V, u represents voltage, and i represents current. For current i a The curve after DFT operation is a yellow line, and the corresponding horizontal grid represents frequency. The frequency change of one grid is 100HZ; 20.0A / div means that the current size of one grid is 20A; except for the curve after DFT operation, the other horizontal grids for voltage and current are time, and the time size (Time base) of each grid is 10ms;

[0121] The VIENNA rectifier was connected to an actual three-phase grid for testing, and the grid voltage THD was about 4.75%. Figures 9(a) and 9(b) respectively compare the steady-state waveforms when the output power Po is 10kW, and Figures 10(a) and 10(b) respectively compare the steady-state waveforms when the output power Po is 20kW. Before compensation, the grid current contains a large number of 5th and 7th harmonics. After compensation using the proposed strategy, the 5th and 7th harmonic currents are effectively suppressed. Figures 11(a), 11(b) and 11(c) show the compensated steady-state waveforms at Po=30kW, Po=40kW and Po=50kW, respectively. It can be seen that even when the grid voltage is severely distorted, the current THD is improved over the entire load range. Since the current harmonics are suppressed, the DC voltage fluctuates less and is more stable.

[0122] Fig.12 The THD comparison under different load conditions is shown. It can be seen that above 20kW, the THD is less than 5%, and the lowest THD is 2.501%, which proves the feasibility and superiority of the proposed harmonic current suppression method.

[0123] Example 2

[0124] Based on Example 1, this embodiment provides a rectifier harmonic current suppression system based on discrete Fourier transform, including:

[0125] Decomposition module: configured to extract high-order harmonic components of the grid current by using DFT for the current of the three-phase grid of the input rectifier;

[0126] A controller setting module: configured to set independent current PI controllers according to corresponding orders for the obtained high-order harmonic components, and for the harmonics of the negative-sequence components and the positive-sequence components of the set order;

[0127] The first reference voltage calculation module is configured to use the harmonic component after DFT decomposition as the feedback value and zero as the harmonic current reference value, which is input into the current PI controller to obtain a reference voltage for suppressing the harmonic component of the corresponding order;

[0128] Superposition module: configured to add the reference voltages of the harmonic components of the fundamental component, the negative sequence component and the positive sequence component to generate a reference voltage signal for modulation and perform harmonic suppression control on the rectifier.

[0129] Example 3

[0130] Based on Embodiment 1, this embodiment provides a rectifier harmonic current suppression system based on discrete Fourier transform, including: a fundamental wave control unit and a harmonic control unit;

[0131] The harmonic control unit includes a PI controller and a voltage converter set for high-order harmonic components; the PI controller uses the amplitudes of the cosine component and the sine component in the high-order harmonic components as feedback, takes zero as the harmonic current reference value, and outputs a reference voltage for suppressing the corresponding order harmonic components;

[0132] A voltage converter, used for performing dq / abc conversion on the obtained reference voltage;

[0133] For the positive sequence component in the high-order harmonic component, the amplitude of the sinusoidal component after DFT decomposition is a n As the q-axis component feedback, it is input into the PI controller for the q-axis component to calculate the reference voltage of the q-axis component; the cosine component amplitude b after DFT decomposition is n As the d-axis component feedback, it is input into the PI controller for the d-axis component to calculate the reference voltage of the d-axis component.

[0134] For the negative sequence components in the high-order harmonic components, the amplitude of the sinusoidal component after DFT decomposition is a n As the d-axis component feedback, it is input into the PI controller for the d-axis component to calculate the reference voltage of the d-axis component; the cosine component amplitude b after DFT decomposition is n As the q-axis component feedback, it is input into the PI controller for the q-axis component to calculate the reference voltage of the q-axis component.

[0135] Specifically, the fundamental wave control unit includes a voltage control loop and a current control loop. The voltage control loop is provided with a PI controller for controlling the DC voltage u output by the rectifier. dc stability;

[0136] The current control loop includes two PI controllers and a voltage converter. The amplitude a1 of the cosine component of the fundamental wave is fed back as the d-axis component, and the amplitude b1 of the sine component of the fundamental wave is fed back as the q-axis component, which are input into the corresponding PI controllers respectively.

[0137] Example 4

[0138] Based on Example 1, this embodiment provides an application of a rectifier harmonic current suppression method based on discrete Fourier transform, which can be applied to three-phase VIENNA rectifiers, three-phase PWM rectifiers, three-phase diode clamped (NPC) converters, three-phase active neutral point clamped (ANPC) converters, etc. rectifiers that can be controlled based on a synchronous rotating coordinate system.

[0139] The above description is only a preferred embodiment of the present disclosure and is not intended to limit the present disclosure. For those skilled in the art, the present disclosure may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure shall be included in the protection scope of the present disclosure.

[0140] Although the above describes the specific implementation methods of the present disclosure in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present disclosure. Technical personnel in the relevant field should understand that on the basis of the technical solution of the present disclosure, various modifications or variations that can be made by those skilled in the art without creative work are still within the scope of protection of the present disclosure.

Claims

1. A rectifier harmonic current suppression method based on discrete Fourier transform, characterized in that: The steps include: Based on the current of the three-phase grid of the input rectifier, DFT is used to extract the high-order harmonic components of the grid current; For the obtained high-order harmonic components, for the harmonics of the negative-sequence components and the positive-sequence components of the set order, independent current PI controllers are set according to the corresponding orders; The harmonic components after DFT decomposition are used as feedback values, and zero is used as the harmonic current reference value, which are input into the current PI controller to obtain the reference voltage for suppressing the corresponding order harmonic components; The reference voltages of the harmonic components of the fundamental component, the negative sequence component and the positive sequence component are added together to generate a reference voltage signal for modulation, and the harmonic suppression control of the rectifier is performed.

2. The method for suppressing rectifier harmonic current based on discrete Fourier transform according to claim 1, characterized in that: The control method for fundamental wave control includes voltage outer loop and current inner loop control. The voltage outer loop is used to control the DC voltage u output by the rectifier. dc Stability, the inner current loop controls the amplitude and phase of the rectifier input current.

3. The rectifier harmonic current suppression method based on discrete Fourier transform according to claim 1, characterized in that: The method for extracting high-order harmonic components comprises the following steps: Use DFT to decompose the acquired input current; According to the set phase sequence classification rules, the positive sequence components and negative sequence components in the high-order harmonics are identified as the extracted high-order harmonic components.

4. The method for suppressing rectifier harmonic current based on discrete Fourier transform according to claim 1, characterized in that: For the positive sequence component in the high-order harmonic component, the amplitude of the sinusoidal component after DFT decomposition is a n As the q-axis component feedback, it is input into the PI controller for the q-axis component to calculate the reference voltage of the q-axis component; The cosine component amplitude b after DFT decomposition n As the d-axis component feedback, it is input into the PI controller for the d-axis component to calculate the reference voltage of the d-axis component.

5. The method for suppressing rectifier harmonic current based on discrete Fourier transform according to claim 1, characterized in that: For the negative sequence components in the high-order harmonic components, the amplitude of the sinusoidal component after DFT decomposition is a n As the d-axis component feedback, it is input into the PI controller for the d-axis component to calculate the reference voltage of the d-axis component; The cosine component amplitude b after DFT decomposition n As the q-axis component feedback, it is input into the PI controller for the q-axis component to calculate the reference voltage of the q-axis component.

6. The method for suppressing rectifier harmonic current based on discrete Fourier transform according to claim 1, characterized in that: The fundamental d-axis current i d As the d-axis component feedback, the fundamental q-axis current i q As q-axis component feedback, they are input into the corresponding PI controllers respectively.

7. A rectifier harmonic current suppression system based on discrete Fourier transform, characterized in that: include: Decomposition module: configured to extract high-order harmonic components of the grid current by using DFT for the current of the three-phase grid of the input rectifier; A controller setting module: configured to set independent current PI controllers according to corresponding orders for the obtained high-order harmonic components, and for the harmonics of the negative-sequence components and the positive-sequence components of the set order; The first reference voltage calculation module is configured to use the harmonic component after DFT decomposition as the feedback value and zero as the harmonic current reference value, which is input into the current PI controller to obtain a reference voltage for suppressing the harmonic component of the corresponding order; Superposition module: configured to add the reference voltages of the harmonic components of the fundamental component, the negative sequence component and the positive sequence component to generate a reference voltage signal for modulation and perform harmonic suppression control on the rectifier.

8. A rectifier harmonic current suppression system based on discrete Fourier transform, characterized in that: It includes a fundamental wave control unit and a harmonic control unit; The harmonic control unit includes a PI controller and a voltage converter set for high-order harmonic components; the PI controller uses the amplitudes of the cosine component and the sine component in the high-order harmonic components as feedback, takes zero as the harmonic current reference value, and outputs a reference voltage for suppressing the corresponding order harmonic components.

9. The rectifier harmonic current suppression system based on discrete Fourier transform according to claim 8, characterized in that: For the positive sequence component in the high-order harmonic component, the amplitude of the sinusoidal component after DFT decomposition is a n As the q-axis component feedback, it is input into the PI controller for the q-axis component to calculate the reference voltage of the q-axis component; The cosine component amplitude b after DFT decomposition n As the d-axis component feedback, it is input into the PI controller for the d-axis component to calculate the reference voltage of the d-axis component; Or, for the negative sequence component in the high-order harmonic component, the amplitude of the sinusoidal component after DFT decomposition is a n As the d-axis component feedback, it is input into the PI controller for the d-axis component to calculate the reference voltage of the d-axis component; The cosine component amplitude b after DFT decomposition n As the q-axis component feedback, it is input into the PI controller for the q-axis component to calculate the reference voltage of the q-axis component.

10. The rectifier harmonic current suppression system based on discrete Fourier transform according to claim 8, characterized in that: The fundamental wave control unit includes a voltage control loop and a current control loop, and a PI controller is set in the voltage control loop; The current control loop includes two PI controllers and a voltage converter. The amplitude a1 of the cosine component of the fundamental wave is fed back as the d-axis component, and the amplitude b1 of the sine component of the fundamental wave is fed back as the q-axis component, which are input into the corresponding PI controllers respectively.

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