Filtering capability enhancing method of three-phase LCL filter

Abstract

The invention discloses a filtering capability enhancing method of a three-phase LCL filter, and relates to the technical field of power electronic conversion. The invention aims to solve the problem that the ripple attenuation capability is reduced in the existing filter magnetic integration technology. According to the method for enhancing the filtering capacity of the three-phase LCL filter, all the filtering inductors are integrated on the same magnetic core, and compared with a traditional discrete LCL filter, the size is reduced, so that the purpose of eliminating the volume redundancy of the filtering inductors is achieved. By reasonably selecting the size parameters of the magnetic core, compared with the existing filter magnetic integration technology, the ripple attenuation capability is increased, and the purpose of enhancing the ripple attenuation capability is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic conversion. Background Art

[0002] In order to vigorously develop clean energy power generation mainly based on photovoltaic and wind power, and continuously improve its consumption rate, so as to gradually replace traditional fossil energy power generation. Photovoltaic and wind power generation both output electrical energy directly or indirectly in the form of direct current, while the current transmission network and distribution network transmit electrical energy in the form of alternating current, so the inverter is one of the key equipment for clean energy grid connection. As a typical power electronic converter, the promotion and application of grid-connected inverters need to meet the development trend of high efficiency and high power density of power electronic converters. However, the LCL filter consists of two independent inductors and capacitors, which reduces the utilization rate of the magnetic core and has a large volume redundancy.

[0003] The proportion of magnetic components in the volume of the converter cannot be ignored. According to statistics, the volume of magnetic components generally accounts for 20% to 30% of the total volume of the converter. For converters working at high frequencies or modular designs, the volume proportion will be even greater. Therefore, it is of great significance to conduct integrated research on magnetic components. The main purpose of using magnetic integration technology is to reduce the number of magnetic cores and thus reduce the volume. In addition, the existing filter magnetic integration technology will weaken the ripple attenuation capability of the filter to varying degrees. Summary of the invention

[0004] The present invention aims to solve the problem of reduced ripple attenuation capability in the existing filter magnetic integration technology, and provides a method for enhancing the filtering capability of a three-phase LCL filter.

[0005] The method for enhancing the filtering capability of the three-phase LCL filter is as follows:

[0006] Arrange six magnetic columns in parallel with their two ends aligned, and fix two rectangular magnetic blocks at the two ends of the magnetic columns respectively, so that the connection points of the six magnetic columns and the magnetic blocks are arranged in a 2×3 rectangular array to form a magnetic core structure of the filter;

[0007] The three magnetic columns in one row of the rectangular array are all wound with inverter-side self-inductance coils in the same winding direction, and the three magnetic columns in another row of the rectangular array are all wound with grid-side self-inductance coils in the same winding direction;

[0008] The ratio of the cross-sectional area of ​​the magnetic column corresponding to the self-inductance on the inverter side to the cross-sectional area of ​​the magnetic column corresponding to the self-inductance on the grid side is adjusted to be the largest, so as to maximize the filtering capacity of the filter.

[0009] Furthermore, the three magnetic columns corresponding to the inverter-side self-inductance have the same cross-sectional area and the same length; the three magnetic columns corresponding to the grid-side self-inductance have the same cross-sectional area and the same length.

[0010] Furthermore, the magnetic blocks and magnetic columns are made of high magnetic permeability soft magnetic material.

[0011] Furthermore, the magnetic block is made of nanocrystalline material, and the magnetic column is made of iron-aluminum-silicon material.

[0012] Furthermore, the inverter-side self-inductance coil and the grid-side self-inductance coil are both wound with multiple strands of Litz wires of the same diameter.

[0013] The present invention proposes a method for enhancing the filtering capability of a three-phase LCL filter, integrating all filter inductors on the same magnetic core, reducing the volume compared to the traditional discrete LCL filter, so as to achieve the purpose of eliminating the volume redundancy of the filter inductor. By reasonably selecting the size parameters of the magnetic core, the ripple attenuation capability is increased compared to the existing filter magnetic integration technology, thereby achieving the purpose of enhancing the ripple attenuation capability. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a three-dimensional diagram of the magnetic core structure;

[0015] Figure 2 It is a circuit diagram when the magnetic core structure is applied to a three-phase grid-connected inverter;

[0016] Figure 3 It is a schematic diagram of the coil winding position of the magnetic core structure and a schematic diagram of the equivalent magnetic circuit;

[0017] Figure 4 It is a schematic diagram of the modulation mode of the three-phase grid-connected inverter;

[0018] Figure 5 This is the grid-connected current simulation result diagram of the three-phase grid-connected inverter with magnetic integrated filter;

[0019] Figure 6 The grid-connected current simulation result diagram of the three-phase grid-connected inverter when using the traditional LCL filter;

[0020] Figure 7 This is the grid-connected current harmonic spectrum diagram of the three-phase grid-connected inverter with magnetic integrated filter;

[0021] Figure 8 This is the grid-connected current harmonic spectrum diagram of a three-phase grid-connected inverter with a traditional LCL filter;

[0022] Fig. 9 The inverter side inductor size diagram of the traditional LCL filter;

[0023] Fig.10 The grid-side inductor size diagram of the traditional LCL filter.

[0024] Fig.11It is a curve diagram of the change of magnetic induction intensity under different proportional relationships between the cross-sectional area of ​​the self-inductance magnetic column core on the inverter side and the cross-sectional area of ​​the self-inductance magnetic column core on the grid side;

[0025] Fig.12 It is a curve diagram showing the change of loss under different proportional relationships between the cross-sectional area of ​​the self-inductance magnetic column core on the inverter side and the cross-sectional area of ​​the self-inductance magnetic column core on the grid side;

[0026] Fig.13 It is a curve diagram of the change of the core volume under different proportional relationships between the cross-sectional area of ​​the self-inductance magnetic column core on the inverter side and the cross-sectional area of ​​the self-inductance magnetic column core on the grid side. DETAILED DESCRIPTION

[0027] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0028] Specific implementation method 1: This implementation method is specifically described with reference to the figure. The filtering capability enhancement method of the three-phase LCL filter described in this implementation method is specifically as follows:

[0029] First, a magnetic core structure is prepared, which includes two rectangular magnetic blocks and six cylindrical magnetic columns. The six magnetic columns are arranged in parallel and aligned at both ends, and two rectangular magnetic blocks are fixed at both ends of the magnetic columns, so that the connection points of the six magnetic columns and the magnetic blocks are arranged in a 2×3 rectangular array, forming the magnetic core structure of the filter.

[0030] The three magnetic columns in one row of the rectangular array are all wound with inverter-side self-inductance coils in the same winding direction, and the three magnetic columns in another row of the rectangular array are all wound with grid-side self-inductance coils in the same winding direction. The inverter-side self-inductance coil and the grid-side self-inductance coil are both wound with multiple strands of Litz wires of the same diameter.

[0031] The three magnetic columns corresponding to the self-inductance on the inverter side have the same cross-sectional area and the same length; the three magnetic columns corresponding to the self-inductance on the grid side have the same cross-sectional area and the same length. The magnetic blocks and magnetic columns are both made of high magnetic permeability soft magnetic materials, specifically, the magnetic blocks are made of nanocrystalline materials, and the magnetic columns are made of iron aluminum silicon materials.

[0032] Afterwards, the ratio of the cross-sectional area of ​​the magnetic column corresponding to the self-inductance on the inverter side to the cross-sectional area of ​​the magnetic column corresponding to the self-inductance on the grid side is adjusted to be the largest, so as to maximize the filtering capacity of the filter.

[0033] The magnetic core structure of the method for enhancing the filtering capability of the three-phase LCL filter provided in this embodiment is as follows: Figure 1 As shown, the topological structure diagram when this embodiment is applied to a three-phase grid-connected inverter is as shown in Figure 2 As shown. The self-inductance of the three inverter sides is L a1 , L b1 and L c1 , the three grid-side self-inductances are L a2 , L b2 and L c2 , L a1 , L b1 and L c1 Wound in Figure 1 On the three magnetic columns in the first row of the magnetic core shown in the figure, L a2 , L b2 and L c2 Wound in Figure 1 The six self-inductance coils on the three magnetic columns in the second row of the magnetic core are wound in the same direction. The three-phase inverter parameters used in this embodiment are: DC side voltage v dc is 800V; three-phase grid voltage v a 、v b and v c It is 220V effective value 50Hz alternating current, and the phase difference is 120 degrees; the filter capacitor C a , C b and C c are all 1μF; the DC side voltage stabilizing capacitors C1 and C2 are 1mF; the grid-connected current peak is 3.21A, and the power is 1.5kW. The three-phase currents of the self-inductance on the inverter side are i a 、i b and i c ; The three-phase currents of the grid-side self-inductance are i ga 、i gb and i gc ; The number of turns of the self-inductance coil on the inverter side and the self-inductance coil on the grid side are N1 and N2 respectively; R p and R s The magnetic column reluctance corresponding to the self-inductance on the inverter side and the magnetic column reluctance corresponding to the self-inductance on the grid side are defined here as R p =xR s , x is a positive real number.

[0034] Figure 3 The winding positions of the six self-inductance coils and the equivalent magnetic circuit diagram are shown. Based on the node voltage method in the circuit principle, the following can be obtained:

[0035]

[0036] Among them, F A is the magnetomotive force at point A; and They are the magnetic flux of the magnetic column corresponding to the three inverter side inductors, and They are the magnetic fluxes of the magnetic columns corresponding to the three grid-side inductors.

[0037] Based on formula (1), the magnetic flux of each cylindrical core can be derived.

[0039] The self-inductance on the inverter side and the self-inductance on the grid side are defined as L a1 =L b1 =L c1 = L1 and L a2 =L b2 =L c2 =L2, then the number of coil turns can be expressed as:

[0040]

[0041] Then we can further deduce that the mutual inductance between the respective induction coils is:

[0042]

[0043] Among them, M 12 is the inverter side inductance L a1 and L b1 The mutual inductance between 13 is the inverter side inductance L a1 and L c1 The mutual inductance between 23 is the inverter side inductance L b1 and L c1 The mutual inductance between 45 is the grid side inductance L a2 and L b2 The mutual inductance between 46 is the grid side inductance L a2 and L c2 The mutual inductance between 56 is the grid side inductance L b2 and L c2 The mutual inductance between 14 is the inverter side inductance L a1 and the grid-side inductance L a2 The mutual inductance between 15 is the inverter side inductance L a1 and the grid-side inductance L b2 The mutual inductance between 16 is the inverter side inductance L a1 and the grid-side inductance L c2 The mutual inductance between 24 is the inverter side inductance Lb1 and the grid-side inductance L a2 The mutual inductance between 25 is the inverter side inductance L b1 and the grid-side inductance L b2 The mutual inductance between 26 is the inverter side inductance L b1 and the grid-side inductance L c2 The mutual inductance between 34 is the inverter side inductance L c1 and the grid-side inductance L a2 The mutual inductance between 35 is the inverter side inductance L c1 and the grid-side inductance L b2 The mutual inductance between 36 is the inverter side inductance L c1 and the grid-side inductance L c2 The mutual inductance between 12 For L a1 and L b1 Mutual magnetic resistance, R 14 For L a1 and L a2 Mutual magnetic resistance, R 45 For L a2 and L b2 The mutual magnetic resistance between them.

[0044] This embodiment uses sine wave pulse width modulation technology to control the switch of the three-phase grid-connected inverter. Figure 4 The schematic diagram of the switching state of the three-phase grid-connected inverter in a certain switching cycle is shown. It can be seen that there are four switching states. The first is that all three bridge arms are turned on. The current slopes of the six self-inductance coils derived in this state are:

[0045]

[0046] in, and They are respectively the capacitor C a , C b and C c The voltage across the terminals, v OG is the voltage between points O and G.

[0047] The second switching state is that the upper tube of phase A is turned on, and the lower tubes of phases B and C are turned on. At this time, the current slopes of the six self-inductance coils are:

[0048]

[0049] The third switching state is that the upper tubes of phases A and B are turned on, and the lower tube of phase C is turned on. At this time, the current slopes of the six self-inductance coils are:

[0050]

[0051] The fourth switching state is when the three-phase lower tube is turned on. At this time, the current slopes of the six self-inductance coils are:

[0052]

[0053] Since the control strategy includes a control loop that makes the zero-sequence current zero, the sum of the steady-state value and the instantaneous value of the grid current under steady-state conditions is 0, that is:

[0054]

[0055] Substituting equation (9) into equation (5) yields the following equation in the first switching state:

[0056]

[0057] Then the mathematical model (5) under this state can be further derived as:

[0058]

[0059] For the second switch state:

[0060]

[0061] The mathematical model under this state can be further derived as follows:

[0062] For the third switch state:

[0063]

[0064] The mathematical model under this state can be further derived as follows:

[0065]

[0066] For the fourth switch state:

[0067]

[0068] The mathematical model under this state can be further derived as follows:

[0069]

[0070] From equations (11), (13), (15), (11), and (17), it can be found that the mathematical models of the grid-side self-inductance under the four switching states can be expressed as:

[0071]

[0072] Therefore, the ripple attenuation capability of the grid-connected current is related to 1-b, and the size of 1-b depends on x. Therefore, the ripple attenuation capability of the grid-connected current can be improved by reasonably designing x, and the size of x can be changed by changing the proportional relationship between the cross-sectional area of ​​the cylindrical magnetic core corresponding to the self-inductance on the inverter side and the self-inductance on the grid side, where the equation relationship of b is shown in equation (4).

[0073] When the value of x tends to infinity, the value of b tends to -1 / 2, and equation (18) is further simplified to

[0074]

[0075] It can be seen from the above formula that as the x value approaches infinity, the value of 1-b increases. Since the right side of the equation remains unchanged, the slopes of the three grid-side self-inductance currents decrease, and the grid-connected current ripple attenuation capability is enhanced.

[0076] Based on the above design principles, the magnetic core of the present invention is designed, and a simulation model is built for simulation verification. In this embodiment, the self-inductance of the inverter side is selected as L a1 =L b1 =L c1 =2mH, the self-inductance on the grid side is L a2 =L b2 =L c2 =1mH, the switching frequency is 20kHz; obviously, the selection of the core size is related to the grid-connected current amplitude, magnetic flux density, core loss, etc., and belongs to a multi-objective optimization process. However, in order to facilitate the understanding of ordinary technicians in this field, this implementation method does not perform complex optimization on the selection of the core size, and only selects one group of parameters that meet the enhanced ripple attenuation capability to be applied to this embodiment; the three cylindrical cores corresponding to the inductor on the inverter side are selected to have a height of 40mm and a cross-sectional radius of 5.6mm, and then the number of turns of the self-inductor coil on the inverter side is calculated to be 138; the three cylindrical cores corresponding to the inductor on the grid side are 40mm in height and 5.6mm in cross-sectional radius, and then the number of turns of the self-inductor coil on the inverter side is calculated to be 97; the length, width and height of the two rectangular cores are 64.6mm, 37.4mm and 5mm respectively.

[0077] Based on MATLAB / Simulink simulation software, a simulation model of a three-phase grid-connected inverter was built for simulation verification. Figure 5 The simulation results of the grid-connected current of the three-phase grid-connected inverter using the magnetic integrated LCL filter are shown. It can be seen that the ripple amplitude of the grid-connected current is about 150mA. Figure 6The simulation result of the grid-connected current when the traditional discrete LCL filter is used is shown, and the ripple amplitude is about 220mA, thereby verifying the effectiveness of the ripple attenuation enhanced magnetic integration technology of the three-phase LCL filter provided by the present invention; Figure 7 and Figure 8 They are Figure 5 and Figure 6 The harmonic spectrum diagram of the grid-connected current shown in the figure shows that the total harmonic distortion (THD) and the amplitude of each harmonic component of the grid-connected current when the magnetic integrated LCL filter is used are lower than those when the traditional discrete LCL filter is used, which once again proves the effectiveness of the magnetic integration technology provided by this embodiment; Fig. 9 and Fig.10 The inductance size diagram of the inverter side and the grid side when the traditional discrete LCL filter is used under the same parameter conditions is shown. After calculation, it can be known that when the magnetic integrated LCL filter provided by this embodiment is used, the volume of the magnetic core is 120.8cm3, while when the traditional discrete LCL filter is used, the total volume of the inductor element is 310.18cm3, which proves that the magnetic integration technology provided by this embodiment can also effectively reduce the volume of the filter; Figures 11 to 13 The changes in magnetic induction intensity, loss and core volume under different proportional relationships of the cross-sectional area of ​​the self-inductance magnetic core on the inverter side and the cross-sectional area of ​​the self-inductance magnetic core on the grid side are shown respectively. It can be found that when x increases, the induction intensity of the self-inductance on the inverter side and the self-inductance on the grid side both decrease to varying degrees, the loss shows a downward trend, and the core volume increases.

[0078] In summary, under the premise of ensuring low loss and compact core size, the cross-sectional area of ​​the inductor column on the inverter side is selected to be 1cm 2 , the cross-sectional area of ​​the inductor magnetic column on the grid side is 2cm 2 The cross-sectional radius is 5.6mm and 8mm respectively. The height of the three magnetic columns corresponding to the inductor on the inverter side and the three magnetic columns corresponding to the inductor on the grid side are both 50mm. The number of coil turns is 149 and 105 respectively. The length, width and height of the upper and lower rectangular cores are 71mm, 41.5mm and 5mm respectively.

[0079] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that the features described in conjunction with a single embodiment may be used in other described embodiments.

Claims

1. A method for enhancing the filtering capability of a three-phase LCL filter, characterized in that: Specifically: Arrange six magnetic columns in parallel with their two ends aligned, and fix two rectangular magnetic blocks at the two ends of the magnetic columns respectively, so that the connection points of the six magnetic columns and the magnetic blocks are arranged in a 2×3 rectangular array to form a magnetic core structure of the filter; The three magnetic columns in one row of the rectangular array are all wound with inverter-side self-inductance coils in the same winding direction, and the three magnetic columns in another row of the rectangular array are all wound with grid-side self-inductance coils in the same winding direction; The ratio of the cross-sectional area of ​​the magnetic column corresponding to the self-inductance on the inverter side to the cross-sectional area of ​​the magnetic column corresponding to the self-inductance on the grid side is adjusted to be the largest, so as to maximize the filtering capacity of the filter.

2. The method for enhancing the filtering capability of a three-phase LCL filter according to claim 1, characterized in that: The three magnetic columns corresponding to the self-inductance on the inverter side have the same cross-sectional area and the same length; The three magnetic columns corresponding to the self-inductance on the grid side have the same cross-sectional area and the same length.

3. The method for enhancing the filtering capability of a three-phase LCL filter according to claim 1, characterized in that: The magnetic blocks and magnetic columns are both made of high magnetic permeability soft magnetic material.

4. The method for enhancing the filtering capability of a three-phase LCL filter according to claim 3, characterized in that: The magnetic block is made of nanocrystalline material, and the magnetic column is made of iron-aluminum-silicon material.

5. The method for enhancing the filtering capability of a three-phase LCL filter according to claim 1, 2, 3 or 4, characterized in that: The inverter-side self-inductance coil and the grid-side self-inductance coil are both wound with multiple strands of Litz wires of the same diameter.

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