A three-phase asymmetric grid-connected filter

Abstract

The present invention discloses a three-phase asymmetric grid-connected filter, comprising: the first end of the first inductor is respectively connected to the first end of the second inductor, the first end of the first resonant circuit and the first end of the third capacitor; the first end of the third inductor is connected to the first end of the fourth inductor, the first end of the second resonant circuit and the second end of the first resonant circuit; the first end of the fifth inductor is connected to the second end of the second resonant circuit, the second end of the sixth inductor and the second end of the third capacitor; the first resonant circuit is composed of the first capacitor and the seventh inductor; the second resonant circuit is composed of the second capacitor and the eighth inductor. The application of the embodiments of the present invention aims to solve the problem that the existing trap-type high-order filter has insufficient anti-parameter drift capability and cannot really save inductance.

Description

Technical Field

[0001] The invention relates to a PWM output filter, in particular to a three-phase asymmetric grid-connected filter with a simple structure and strong resistance to parameter drift. Background Art

[0002] In the field of new energy power generation, various renewable energy DC power sources often need inverters to convert into sinusoidal AC power sources to achieve grid-connected power generation. The inverter uses PWM modulation to generate high-frequency current harmonics, which brings harmonic pollution to the power grid. Therefore, a low-pass filter needs to be connected in series between the inverter and the power grid to filter out high-frequency switching harmonics. Compared with traditional L-type filters, high-order filters have stronger high-frequency attenuation capabilities due to the existence of current bypass, use less inductance under the same working conditions, are smaller in size, and have lower costs, so they are widely used in inverter output filtering.

[0003] At present, the widely used high-order grid-connected filters mainly include three categories: LCL filters, trap filters represented by LLCL filters, and trap filters represented by LCL-2T filters. Saving volume and material costs are the main reasons for choosing high-order filters. Among the above three filters, the LCL-2T filter uses the smallest inductance. However, the more complex the filter structure, the worse its ability to resist parameter drift. Under actual working conditions, the second inductor must be selected according to the worst parameter drift situation, which in turn offsets the advantage of LCL-2T filters in saving inductance. How to balance the relationship between multiple harmonic filtering and complex structure and weak ability to resist parameter drift, and truly reduce the use of inductors, is the focus of research by scholars at home and abroad. Summary of the invention

[0004] The object of the present invention is to provide a three-phase asymmetric grid-connected filter, aiming to solve the problem that the existing trap-type high-order filter has insufficient anti-parameter drift capability and cannot truly save inductance.

[0005] In order to achieve the above object, the present invention provides a three-phase asymmetric grid-connected filter, comprising: a first inductor, a second inductor; a third inductor, a fourth inductor; a fifth inductor, a sixth inductor; a first resonant circuit; a second resonant circuit; a third capacitor;

[0006] The first end of the first inductor is respectively connected to the first end of the second inductor, the first end of the first resonant circuit and the first end of the third capacitor;

[0007] The first end of the third inductor is connected to the first end of the fourth inductor, the first end of the second resonant circuit, and the second end of the first resonant circuit;

[0008] The first end of the fifth inductor is connected to the second end of the second resonant circuit, the second end of the sixth inductor and the second end of the third capacitor;

[0009] The first resonant circuit is composed of the first capacitor and the seventh inductor; the second resonant circuit is composed of the second capacitor and the eighth inductor.

[0010] In a specific implementation, the first end of the first inductor forms a node a; the first end of the third inductor forms a node b; the first end of the fifth inductor forms a node c; and the capacitance between the node a and the node c is greater than the first capacitance and the second capacitance.

[0011] In a specific implementation, the functions of any two circuits of the first resonant circuit, the second resonant circuit and the third capacitor are swapped without changing.

[0012] In a specific implementation, the seventh inductor and the eighth inductor are coupled inductors.

[0013] In a specific implementation, the second end of the first inductor, the second end of the third inductor, and the second end of the fifth inductor are respectively connected to the converter;

[0014] The second end of the second inductor, the second end of the fourth inductor and the second end of the sixth inductor are respectively connected to a public AC power grid.

[0015] In a specific implementation, the first end of the first inductor is connected to the first end of the first capacitor, the second end of the first inductor is connected to the first end of the seventh inductor, and the second end of the seventh inductor is connected to the first end of the fourth inductor; the first end of the fourth inductor is connected to the first end of the second capacitor, the second end of the second capacitor is connected to the first end of the eighth inductor, and the second segment of the eighth inductor is connected to the first end of the fifth inductor.

[0016] In a specific implementation, the resonant frequency of the first resonant circuit is the switching frequency, and the resonant frequency of the second resonant circuit is twice the switching frequency.

[0017] In a specific implementation, the second inductor is a transformer leakage inductance or a line inductance.

[0018] A three-phase asymmetric grid-connected filter provided by an embodiment of the present invention can provide bypass for multiple switching frequency harmonics generated in the modulation process by adding series resonant circuits of different frequencies on the current bypass. The three-phase asymmetric grid-connected filter obtained according to the proposed scheme can achieve the following advantages:

[0019] 1. Compared with the LCL-2T filter which also provides multiple switching frequency harmonic bypass, the structure is relatively simple.

[0020] 2. Compared with LCL-2T filter, it has strong resistance to parameter drift and can truly save inductance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic circuit diagram of an embodiment of a three-phase asymmetric grid-connected filter of the present invention.

[0022] Figure 2 It is the main circuit diagram of the three-phase asymmetric grid-connected filter in the new energy conversion system of the present invention.

[0023] Figure 3 is a schematic circuit diagram of yet another embodiment of the present invention.

[0024] Figure 4 is a schematic circuit diagram of a fourth embodiment of the present invention.

[0025] Figure 5 is a schematic circuit diagram of a fifth embodiment of the present invention. DETAILED DESCRIPTION

[0026] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0027] LCL filter. This filter adds a current bypass consisting of a single capacitor in the middle of the inductor. The impedance of the capacitor is very small at high frequencies, so the high-frequency harmonic current flows back to the inverter through the current bypass instead of passing through the grid. Compared with the L-type filter, the LCL filter can save a lot of inductance, but its inherent resonance spike increases the difficulty of controller design.

[0028] The trap filter represented by LLCL filter makes the impedance of current bypass at a specific frequency very small through the series resonant circuit. Its attenuation ability at the resonant frequency is much greater than that of LCL filter, so it can save more inductance than LCL filter. However, it sacrifices the attenuation ability at non-specific frequencies. Although multiple frequency harmonics can be filtered out by connecting multiple resonant circuits in parallel, this makes the structure complicated. In addition, the parameter drift of the filter trap branch will also affect the actual filtering effect.

[0029] The trap type filter represented by LCL-2T filter not only contains the resonant circuit in the current bypass, but also has a single capacitor branch in parallel to ensure the attenuation capability at non-specific frequencies. However, the capacitor and the resonant branch will produce parallel resonance, and the resonance spike will make the filter more sensitive to parameter drift.

[0030] like Figure 1 The present invention provides an embodiment of a three-phase asymmetric grid-connected filter, comprising: a first inductor L11, a second inductor L12; a third inductor L21, a fourth inductor L22; a fifth inductor L31, a sixth inductor L32; a first resonant circuit; a second resonant circuit; a third capacitor C3; the first end of the first inductor L11 is respectively connected to the first end of the second inductor L12, the first end of the first resonant circuit and the first end of the third capacitor C3; the first end of the third inductor L21 is connected to the first end of the fourth inductor L22, the first end of the second resonant circuit and the second end of the first resonant circuit; the first end of the fifth inductor L31 is connected to the second end of the second resonant circuit, the second end of the sixth inductor L32 and the second end of the third capacitor C3; the first resonant circuit is composed of a first capacitor Cf1 and a seventh inductor Lf1; the second resonant circuit is composed of a second capacitor Cf2 and an eighth inductor Lf2.

[0031] Figure 1 It is a typical control block diagram of the filter. The PWM converter can be regarded as a specific voltage source, and the first inductor L11, the second inductor L12; the third inductor L21, the fourth inductor L22; the fifth inductor L31, the sixth inductor L32, two resonant branches connected in a triangle and a single capacitor C3 constitute the proposed three-phase grid-connected filter. The present invention uses feedback control of the second inductor L12, the fourth inductor L22, and the sixth inductor L32 and current regulation of the proportional-resonant controller, and the voltage source acts on the first inductor L11, the third inductor L21, and the fifth inductor L31 to form a current source, which includes a power frequency sinusoidal current and a high-frequency switching harmonic current. The switching frequency harmonic current of phase A and phase B is bypassed by resonant branch 1 (Lf1Cf1), the twice switching frequency harmonics of phase B and phase C are bypassed by resonant branch 2 (Lf2Cf2), and the twice switching frequency harmonics of phase A and the switching frequency harmonics of phase C are bypassed by a single capacitor (C3).

[0032] like Figure 1 As shown, the first end of the first inductor L11 forms a node a; the first end of the third inductor L21 forms a node b; the first end of the fifth inductor L31 forms a node c; and the capacitance between the node a and the node c is greater than the first capacitor Cf1 and the second capacitor Cf2.

[0033] like Figure 2As shown, a renewable energy power generation device such as (wind energy, fuel cell, solar energy, etc.) supplies power to a three-phase power grid through a converter grid-connected filter and an isolation transformer, and a second end of the first inductor L11, a second end of the third inductor L21, and a second end of the fifth inductor L31 are respectively connected to the converter; a second end of the second inductor L12, a second end of the fourth inductor L22, and a second end of the sixth inductor L32 are respectively connected to a public AC power grid.

[0034] Figure 3 is a schematic circuit diagram of another embodiment of the present invention. Figure 1 Compared with the embodiment shown in the figure, the difference is that L f2 C f2 The resonant branch and capacitor C3 have exchanged positions. Other similar position exchanges are also within the protection scope of this patent.

[0035] The grid-connected filter includes a series filter inductor and a current bypass. The output of the three-phase converter mainly contains two harmonics - switching frequency harmonics and twice the switching frequency harmonics. The switching frequency harmonics of phase A and phase B are bypassed by resonant branch 1 (Lf1Cf1), the twice switching frequency harmonics of phase B and phase C are bypassed by resonant branch 2 (Lf2Cf2), and the twice switching frequency harmonics of phase A and phase C are bypassed by a single capacitor (C3). Through theoretical and simulation analysis, it can be seen that the current switching harmonics of phase B are the least, and the current switching harmonics of phase C are the most.

[0036] In the embodiment of the present invention, the first capacitor Cf1 and the seventh inductor Lf1 form a first resonant circuit; the second capacitor Cf2 and the eighth inductor Lf2 form a second resonant circuit; Figure 4 As shown, the functions of any two circuits of the first resonant circuit, the second resonant circuit and the third capacitor C3 remain unchanged. f1 and resonant inductor L f2 is a set of coupled inductors. The coupled inductors may also be coupled with the first inductor.

[0037] like Figure 5 As shown, no additional inductance is used, and informal inductances such as line inductance and transformer leakage inductance are used as the second inductance L2.

[0038] The inventor of the present invention has found through research that parameter asymmetry to a certain extent can not only filter out multiple switching harmonics, but also has no significant impact on control and does not increase the difficulty of controller design. Comparison and analysis of simulation and experimental results of various filters show that under the worst parameter drift conditions, the harmonic attenuation capability of the asymmetric filter is similar to that of the LCL-2T filter, but the structure is much simpler.

[0039] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A three-phase asymmetric grid-connected filter, characterized in that: include: A first inductor (L11), a second inductor (L12), a third inductor (L21), a fourth inductor (L22), a fifth inductor (L31), a sixth inductor (L32), a first resonant circuit, a second resonant circuit, and a third capacitor (C3); The first end of the first inductor (L11) is respectively connected to the first end of the second inductor (L12), the first end of the first resonant circuit and the first end of the third capacitor (C3); The first end of the third inductor (L21) is connected to the first end of the fourth inductor (L22), the first end of the second resonant circuit and the second end of the first resonant circuit; The first end of the fifth inductor (L31) is connected to the second end of the second resonant circuit, the second end of the sixth inductor (L32) and the second end of the third capacitor (C3); The first resonant circuit is composed of a first capacitor (Cf1) and a seventh inductor (Lf1); the second resonant circuit is composed of a second capacitor (Cf2) and an eighth inductor (Lf2); The first end of the first inductor (L11) is connected to the first end of the first capacitor (Cf1), the second end of the first capacitor (Cf1) is connected to the first end of the seventh inductor (Lf1), and the second end of the seventh inductor (Lf1) is connected to the first end of the fourth inductor (L22); the first end of the fourth inductor (L22) is connected to the first end of the second capacitor (Cf2), the second end of the second capacitor (Cf2) is connected to the first end of the eighth inductor (Lf2), and the second end of the eighth inductor (Lf2) is connected to the first end of the fifth inductor (L31).

2. The three-phase asymmetric grid-connected filter according to claim 1, characterized in that: The first end of the first inductor (L11) forms a node a; the first end of the third inductor (L21) forms a node b; the first end of the fifth inductor (L31) forms a node c; and the capacitance between the node a and the node c is greater than that between the first capacitor (Cf1) and the second capacitor (Cf2).

3. The three-phase asymmetric grid-connected filter according to claim 1 or 2, characterized in that: The functions of any two circuits of the first resonant circuit, the second resonant circuit and the third capacitor (C3) remain unchanged when they are swapped.

4. The three-phase asymmetric grid-connected filter according to claim 1, characterized in that: The seventh inductor (Lf1) and the eighth inductor (Lf2) are coupled inductors.

5. The three-phase asymmetric grid-connected filter according to any one of claims 1 to 2 or 4, characterized in that: The second end of the first inductor (L11), the second end of the third inductor (L21), and the second end of the fifth inductor (L31) are respectively connected to the converter; The second end of the second inductor (L12), the second end of the fourth inductor (L22) and the second end of the sixth inductor (L32) are respectively connected to a public AC power grid.

6. The three-phase asymmetric grid-connected filter according to any one of claims 1 to 2 or 4, characterized in that: The resonant frequency of the first resonant circuit is the equivalent switching frequency of one phase bridge arm circuit in the three-phase main circuit bridge arm, and the resonant frequency of the second resonant circuit is twice the equivalent switching frequency of one phase bridge arm circuit in the three-phase main circuit bridge arm.

7. The three-phase asymmetric grid-connected filter according to claim 1 is characterized in that: The second inductor is transformer leakage inductance or line inductance.

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