CN104466969A - LCL type filter based on three-winding transformer - Google Patents

Abstract

The embodiment of the invention discloses an LCL type filter based on a three-winding transformer. The LCL type filter is used for filtering higher harmonic current of a grid-connected inverter and comprises a filter capacitor and a first winding, a second winding and a third winding of the three-winding transformer. The first winding is connected with the output end of the grid-connected inverter, the second winding is connected with a power grid, and the third winding is connected with the filter capacitor. The windings of the three-winding transformer are used for replacing an independent electric reactor and are matched with the filter capacitor to form the LCL type filter, the structure of the grid-connected inverter system is simplified, the size of the LCL type filter is reduced, and the manufacturing cost and the running cost are reduced.

Description

A kind of LCL type filter based on three-winding transformer

Technical field

The present invention relates to electrical technology field, particularly relate to a kind of LCL type filter based on three-winding transformer.

Background technology

In photovoltaic generating system, utilize combining inverter, with transformer, the direct current of photovoltaic array is converted to the alternating current identical with line voltage and frequency.Combining inverter adopts high-frequency PWM to modulate usually, except first-harmonic, also produces a large amount of higher harmonic currents and enters electrical network, have impact on the electrical network quality of power supply.Grid-connection standards proposes strict restriction to grid-connected current harmonic content.

In generating electricity by way of merging two or more grid systems, in order to reduce loss and the high-frequency electromagnetic interference of switching device, usually adopting lower switching frequency, making harmonic voltage component larger.In order to reduce the higher harmonic current flowing into electrical network, in grid-connected inverter system, the current LCL type filter that generally adopts is connected with electrical network, need two independently reactor and one group of filtering capacitor, adopt other transformer boost again, independently LCL type filter bulky, complex structure and cost is higher.

Thus, how to improve the filter in grid-connected inverter system, realizing simplifying grid-connected inverter system structure, reduce floor space, reduce costs and improve the reliability of system, is the current technical issues that need to address of those skilled in the art.

Summary of the invention

The object of this invention is to provide the LCL type filter based on three-winding transformer, grid-connected inverter system structure can be simplified, reduce floor space, reduce costs and improve the reliability of system.

For solving the problems of the technologies described above, the invention provides following technical scheme:

Based on a LCL type filter for three-winding transformer, for the higher harmonic current filtering of combining inverter, comprising: the first winding of filter capacitor and three-winding transformer, the second winding and the tertiary winding;

Wherein, described first winding is connected with the output of combining inverter, and described second winding is connected with electrical network, and the described tertiary winding is connected with described filter capacitor.

Preferably, the equivalent leakage inductance of the described tertiary winding is in preset range.

Preferably, the described tertiary winding is the middle winding in three-winding transformer coils arrangement.

Preferably, the rated voltage of described first winding is identical with the rated voltage that combining inverter exports, and the rated voltage of described second winding is identical with the rated voltage of electrical network.

Preferably, the rated voltage computational methods of the described tertiary winding are:

$U_{3N}=\sqrt{3}|\frac{U_{1N}}{\sqrt{3}}-\mathrm{j\ω}{L}_1{I}_{1N}|,$

Wherein, U _3Nfor the rated voltage of the tertiary winding, U _1Nbe the rated voltage of the first winding, L ₁be the equivalent inductance of the first winding, I _1Nfor the rated current that combining inverter exports, ω=2 π f ₁, f ₁for mains frequency.

Preferably, the rated capacity of described first winding is identical with the rated capacity of combining inverter, and the rated capacity of described second winding is identical with the rated capacity of the first winding.

Preferably, the rated capacity computational methods of the described tertiary winding are:

P _3N＝2πf ₁C(U _3N) ²，

Wherein, P _3Nfor the rated capacity of the tertiary winding, f ₁for mains frequency, C is the capacitance of filter capacitor, U _3Nfor the rated voltage of the tertiary winding.

Compared with prior art, technique scheme has the following advantages:

A kind of LCL type filter based on three-winding transformer that the embodiment of the present invention provides, for the higher harmonic current filtering of combining inverter, comprising: the first winding of filter capacitor and three-winding transformer, the second winding and the tertiary winding; Wherein, described first winding is connected with the output of combining inverter, and described second winding is connected with electrical network, and the described tertiary winding is connected with described filter capacitor.The winding of three-winding transformer is utilized to replace independently reactor, match with filter capacitor and form LCL type filter, simplify the structure of grid-connected inverter system, due to without the need to independently reactor, reduce the volume of LCL type filter, reduce manufacturing cost and operating cost simultaneously.

Accompanying drawing explanation

In order to be illustrated more clearly in the embodiment of the present invention or technical scheme of the prior art, be briefly described to the accompanying drawing used required in embodiment or description of the prior art below, apparently, accompanying drawing in the following describes is some embodiments of the present invention, for those of ordinary skills, under the prerequisite not paying creative work, other accompanying drawing can also be obtained according to these accompanying drawings.

Fig. 1 is applied to the circuit diagram of the higher harmonic current filtering of combining inverter for the LCL type filter based on three-winding transformer that one embodiment of the invention provides;

Fig. 2 is the LCL type filter one phase equivalent circuit schematic diagram of traditional desirable employing independently reactor and capacitor composition;

Fig. 3 is the LCL type filter one phase equivalent circuit schematic diagram based on three-winding transformer provided by the present invention;

Fig. 4 is the schematic equivalent circuit of three-winding transformer;

Fig. 5 is three-winding transformer winding position schematic diagram.

Embodiment

Just as described in the background section, bulky, the complex structure of current LCL type filter and cost is higher.

On basis based on above-mentioned research, embodiments provide a kind of LCL type filter based on three-winding transformer, for the higher harmonic current filtering of combining inverter, comprising: the first winding of filter capacitor and three-winding transformer, the second winding and the tertiary winding;

Wherein, described first winding is connected with the output of combining inverter, and described second winding is connected with electrical network, and the described tertiary winding is connected with described filter capacitor.

The scheme that the embodiment of the present invention provides, the winding of three-winding transformer is utilized to replace independently reactor, match with filter capacitor and form LCL type filter, simplify the structure of grid-connected inverter system, due to without the need to independently reactor, reduce the volume of LCL type filter, reduce manufacturing cost and operating cost simultaneously.

In order to enable above-mentioned purpose of the present invention, feature and advantage more become apparent, and are described in detail the specific embodiment of the present invention below in conjunction with accompanying drawing.

Set forth detail in the following description so that fully understand the present invention.But the present invention can be different from alternate manner described here to implement with multiple, those skilled in the art can when without prejudice to doing similar popularization when intension of the present invention.Therefore the present invention is not by the restriction of following public embodiment.

Please refer to Fig. 1, Fig. 1 is applied to the circuit diagram of the higher harmonic current filtering of combining inverter for the LCL type filter based on three-winding transformer that one embodiment of the invention provides.

One embodiment of the invention provides a kind of LCL type filter 1 based on three-winding transformer, for combining inverter, comprising: first winding 11, second winding 12 and the tertiary winding 13 of filter capacitor 10 and three-winding transformer; Wherein, described first winding 11 is connected with the output of combining inverter 2, and described second winding 12 is connected with electrical network 3, and the described tertiary winding 13 is connected with described filter capacitor 10.

In the present embodiment, the described LCL type filter based on three-winding transformer can be applied in single-phase or three-phase grid-connected inverter, the leakage inductance of the first winding of three-winding transformer and these two windings of the second winding is utilized to replace the inductance of two independent reactors in traditional LCL type filter, the tertiary winding is connected with filter capacitor, first winding, the second winding, the tertiary winding and filter capacitor cooperatively interact and form LCL type filter, and the alternating voltage that combining inverter exports is transformed to the alternating voltage matched with line voltage by three-winding transformer simultaneously.Three-winding transformer is made to have the double action of filtering and transformation concurrently, with reference to figure 1.

On the basis of above-described embodiment, the LCL type filter that one embodiment of the invention provides, the equivalent leakage inductance of the preferred described tertiary winding is in preset range.

In the present embodiment, the equivalent leakage inductance of the first winding is L ₁, the equivalent leakage inductance of the second winding is L ₂, the equivalent leakage inductance of the tertiary winding is L ₃, the LCL type filter that the first winding, the second winding, the tertiary winding and filtering capacitor form jointly, compared with L-type filter, can decay with higher slope when high frequency, suppresses the high order harmonic component in combining inverter output current.

Wherein, according to existing empirical formula, the equivalent leakage inductance of the first winding is:

$L_1\≥\frac{U_{\mathrm{dc}}}{4\sqrt{3}\×\mathrm{\Δ}{i}_{\max }{f}_K},$

The equivalent leakage inductance of the second winding is:

L ₂＝0.2L ₁，

The capacitance of filter capacitor is:

$C\≤\frac{0.05\×P_N}{3\×2\mathrm{\π}\×{\left(\frac{U_{1N}}{\sqrt{3}}\right)}^2\×f_1},$

Wherein, U _dcfor combining inverter DC voltage, Δ i _maxbe current ripples in the first winding, f _kfor combining inverter switching frequency, P _nfor combining inverter rated power, U _1Nfor combining inverter output AC side voltage, f ₁for fundamental frequency.

In order to make the frequency characteristic based on the LCL type filter of three-winding transformer consistent with the frequency characteristic of desirable LCL type filter, need to make the leakage inductance of the tertiary winding close to zero, namely described preset range is enough little, and it is close to zero.

The natural resonance frequency of LCL type filter is:

$f_{\mathrm{res}}=\frac{\sqrt{\frac{(L_1+L_2)}{L_1{L}_{2}C}}}{2\mathrm{\π}},$

For avoiding LCL type filter to produce resonance in lower and higher frequency spectrum part, its natural resonance frequency f should be controlled _resscope at the fundamental frequency f of 10 times _nwith the switching frequency f of 0.5 times _sbetween, i.e. 10 × f _n< f _res< 0.5f _s.

Desirable use independently reactor and capacitor composition LCL type filter, its one phase equivalent circuit as shown in Figure 2, first reactor is connected with combining inverter output, and the second reactor is connected with electrical network, and the transfer function between its grid-connected current and combining inverter output voltage is:

$G_1\left(S\right)=\frac{i_s}{u_0}=\frac{1}{L_1{L}_1{\mathrm{CS}}^3+(L_1+L_2)S^2},$

Wherein, u _ofor combining inverter output voltage, L ₁be the first reactor inductance, L ₂be the second reactor inductance, C is condenser capacity.

LCL type filter based on three-winding transformer provided by the present invention, as shown in Figure 3, the transfer function between its grid-connected current and combining inverter output voltage is its one phase equivalent circuit:

$G_3\left(S\right)=\frac{i_s}{u_0}=\frac{L_3{\mathrm{CS}}^2+1}{(L_1{L}_{2}C+(L_1+L_2)L_{3}c)S^3+(L_1+L_2)S^2},$

Wherein, u _ofor combining inverter output voltage, L ₁be the equivalent leakage inductance of the first winding, L ₂be the equivalent leakage inductance of the second winding, L ₃for the equivalent leakage inductance of the tertiary winding, C is the capacitance of filter capacitor.

Based on above-mentioned traditional desirable LCL type filter and the transfer function that the present invention is based between the grid-connected current of LCL type filter of three-winding transformer and combining inverter output voltage known, want to make the LCL type filter based on three-winding transformer have identical frequency characteristic with traditional desirable LCL type filter that independently reactor and capacitor form, the equivalent leakage inductance of the tertiary winding of three-winding transformer will be made to be zero, thus, for making the close as much as possible frequency characteristic adopting traditional desirable LCL type filter of independently reactor and capacitor composition of frequency characteristic based on the LCL type filter of three-winding transformer, the equivalent leakage inductance of the tertiary winding of three-winding transformer just must be made as much as possible close to zero.

On the basis of above-mentioned any embodiment, the LCL type filter that one embodiment of the invention provides, preferably the tertiary winding described in it is the middle winding in three-winding transformer coils arrangement.

If ignore the resistance of each winding, by two of three-winding transformer, each amount of tertiary winding converts a winding, and can obtain the equivalent electric circuit of three-winding transformer as shown in Figure 4, the equivalent reactance of each winding is respectively:

X ₁＝ωL ₁＝ω(L ₁₁-M′ ₁₂-M′ ₁₃+M' ₂₃)，

X' ₂＝ωL ₂＝ω(L' ₂₂-M′ ₁₂-M' ₂₃+M′ ₁₃)，

X ₃'＝ωL ₃＝ω(L' ₃₃-M ₁' ₃-M' ₂₃+M′ ₁₂)，

Wherein, L ₁₁, L' ₂₂, L' ₃₃the complete inductance of each winding, M ' ₁₂, M ' ₁₃, M' ₂₃for the mutual inductance of winding.This shows, equivalent reactance X ₁, X' ₂, X ₃' not only L ₁₁, L' ₂₂, L' ₃₃size relevant, also with M ' ₁₂, M ' ₁₃, M ' ₂₃size and combine relevant.

According to electromagnetic principles, when winding height is identical, the distance between two windings is larger, and magnetic coupling is each other more weak, and mutual inductance is less.For three-winding transformer, its winding is arranged as shown in Figure 5, according to Electrical Motor principle, is positioned at middle winding equivalent reactance minimum, and thus, for making equivalent leakage inductance close to zero, preferably, winding is connected with filter capacitor.

Based on transformer operation logic, the rated voltage of described first winding is identical with the rated voltage that combining inverter exports, and the rated voltage of described second winding is identical with the rated voltage of electrical network.

Wherein, the rated voltage computational methods of the described tertiary winding are:

$U_{3N}=\sqrt{3}|\frac{U_{1N}}{\sqrt{3}}-\mathrm{j\&omega;}{L}_1{I}_{1N}|,$

In formula, U _3Nfor the rated voltage of the tertiary winding, U _1Nbe the rated voltage of the first winding, L ₁be the equivalent leakage inductance of the first winding, I _1Nfor the rated current that combining inverter exports, ω=2 π f ₁, f ₁for mains frequency.

The LCL type filter that one embodiment of the invention provides, based on transformer principle, the rated capacity of the first winding described in it is identical with the rated capacity of combining inverter, and the rated capacity of described second winding is identical with the rated capacity of the first winding.

Wherein, the rated capacity computational methods of the tertiary winding are:

P _3N＝2πf ₁C(U _3N) ²，

In formula, P _3Nfor the rated capacity of the tertiary winding, f ₁for mains frequency, C is the capacitance of filter capacitor, U _3Nfor the rated voltage of the tertiary winding.

In sum, a kind of LCL type filter based on three-winding transformer that the embodiment of the present invention provides, utilize the independently reactor that the winding of three-winding transformer replaces in traditional LCL type filter, match with filter capacitor and form LCL type filter, simplify the structure of grid-connected inverter system, due to without the need to independently reactor, reduce the volume of LCL type filter, reduce manufacturing cost and operating cost simultaneously, the alternating voltage that three-winding transformer is had combining inverter exports is transformed to the alternating voltage matched with line voltage and the dual-use function suppressing the high order harmonic component in combining inverter output current, improve the reliability of system.

Above the LCL type filter based on three-winding transformer provided by the present invention is described in detail.Apply specific case herein to set forth principle of the present invention and execution mode, the explanation of above embodiment just understands the present invention and core concept thereof for helping.It should be pointed out that for those skilled in the art, under the premise without departing from the principles of the invention, can also carry out some improvement and modification to the present invention, these improve and modify and also fall in the protection range of the claims in the present invention.

Claims (7)

1. based on a LCL type filter for three-winding transformer, for the higher harmonic current filtering of combining inverter, it is characterized in that, comprising: the first winding of filter capacitor and three-winding transformer, the second winding and the tertiary winding;

Wherein, described first winding is connected with the output of combining inverter, and described second winding is connected with electrical network, and the described tertiary winding is connected with described filter capacitor.

2. LCL type filter according to claim 1, is characterized in that, the equivalent leakage inductance of the described tertiary winding is in preset range.

3. LCL type filter according to claim 1 and 2, is characterized in that, the described tertiary winding is the middle winding in three-winding transformer coils arrangement.

4. LCL type filter according to claim 1, is characterized in that, the rated voltage of described first winding is identical with the rated voltage that combining inverter exports, and the rated voltage of described second winding is identical with the rated voltage of electrical network.

5. LCL type filter according to claim 4, is characterized in that, the rated voltage computational methods of the described tertiary winding are:

$U_{3N}=\sqrt{3}|\frac{U_{1N}}{\sqrt{3}}-\mathrm{j\&omega;}{L}_1{I}_{1N}|,$

Wherein, U _3Nfor the rated voltage of the tertiary winding, U _1Nbe the rated voltage of the first winding, L ₁be the equivalent inductance of the first winding, I _1Nfor the rated current that combining inverter exports, ω=2 π f ₁, f ₁for mains frequency.

6. LCL type filter according to claim 5, is characterized in that, the rated capacity of described first winding is identical with the rated capacity of combining inverter, and the rated capacity of described second winding is identical with the rated capacity of the first winding.

7. LCL type filter according to claim 6, is characterized in that, the rated capacity computational methods of the described tertiary winding are:

P _3N＝2πf ₁C(U _3N) ²，

Wherein, P _3Nfor the rated capacity of the tertiary winding, f ₁for mains frequency, C is the capacitance of filter capacitor, U _3Nfor the rated voltage of the tertiary winding.