KR101379246B1 - Grid-connected filter and design method thereof - Google Patents

Abstract

A system-associated filter and a design method thereof are disclosed. A grid-connected filter connected between a converter connected to a wind generator and converting AC power generated by wind power to DC power and then to AC power of a predetermined frequency and a transformer connected to a grid input terminal includes the transformer and the converter. And a filter capacitor connected between the converter-side inductor connected between the converter-side inductor and the node connecting the transformer and the ground, wherein the grid-associated filter includes an equivalent inductor for the transformer in the node and the grid input stage. It may be an LCL filter used as a grid side inductor interposed therebetween.

Description

Grid-connected filter and design method

The present invention relates to a system-associated filter and a method of designing the same.

Wind power generation, which has been attracting attention as a next generation power source, is growing in size and marketability around the world. Generally, wind power generation is a power generation system that uses wind turbines to convert wind into electric energy. Wind power plays a major role as a next generation power source as it occupies a larger portion in the power grid.

Because wind power is clean, renewable and does not cause greenhouse effect, it is traditionally regarded as eco-friendly energy replacing fossil fuel. However, wind power generation system has a large influence on the surrounding power network due to its variable characteristics, It can hurt. Therefore, in order to ensure the stability of the entire grid, grid codes are established for each country to regulate grid linkage of wind power generation systems.

In driving a grid-coupled pulse width modulation (PWM) converter, it is also important to meet harmonic limiting specifications as well as low voltage ride through (LVRT) or power factor control. If a current containing harmonics enters the grid, it will cause a disturbance to sensitive loads or power supplies and will require restrictions. Representative harmonic limits are IEEE 519-1992, IEC 61000-3-2 (Harmonics), and IEC 61000-. International standards such as 3-4 are established.

The biggest source of harmonics in the converter system is the PWM switching component and the disturbance current component of the power stage. Among them, the low harmonic component can be effectively limited in size by using active damping of the converter current controller. However, harmonic components such as switching frequency have a problem in that active control is difficult due to the limitation of the controller bandwidth.

Therefore, in order to effectively attenuate harmonic components, a filter is installed at the system input stage. The form of the filter may be an L filter, an LC filter or an LCL filter. 1 shows a typical three-phase Y-connected LC filter and an LCL filter.

The L filter is simple in configuration and easy to control, but a large capacity inductor is required to obtain harmonic reduction effects that meet the criteria. As a result, the volume and manufacturing cost of the filter are increased, standalone operation is impossible, and the dynamic characteristics of the system are deteriorated.

In the case of the LC filter (see (a) of FIG. 1), the filter capacitor C is added to the filter inductor L, and the harmonic reduction effect is better than that of the L filter. However, in a large-capacity system, the inductor rating must be increased like the L filter, and resonance caused by the LC filter has a problem.

In the case of the LCL filter (refer to (b) of FIG. 1), the filter capacitor (C) is connected between the node and the ground between the series connected filter inductors (L1 and L2). It has a lower filter capacity than the L filter or LC filter, resulting in higher harmonic attenuation. A system-linked inverter system including such an LCL filter unit is disclosed in Korean Patent Publication No. 10-2012-0058833.

In general, most wind generators consist of low level (± 700V) or medium level (± 3000V or ± 6000V) devices due to the voltage level limitations of internal switching devices. However, since the grid-side voltage is 22.9 kV or more at grid linkage, a transformer is indispensable between the filter and the grid. Here, the transformer can also be seen as a coiled inductor, which requires the design of a grid-tied filter considering this.

Korean Laid-Open Patent Publication No. 10-2012-0058833

The present invention utilizes the equivalent inductor of a transformer as a filter inductor or includes it in a filter inductor, thereby making it more suitable for a power generation system while reducing current total harmonic distortion (THD) and conduction noise and improving system control performance. And a design method thereof.

The present invention provides a system-associated filter and a method of designing the same, which can reduce the overall size and reduce the cost by using an equivalent inductor of a transformer as a filter inductor or by including a filter inductor.

Other objects of the present invention will become readily apparent from the following description.

According to an aspect of the present invention, the grid-connected type is connected between the converter connected to the wind generator and converts the AC power generated by the wind power generation to the DC power and then to the AC power of a predetermined frequency and a transformer connected to the grid input terminal A filter, comprising: a converter side inductor coupled between the transformer and the converter; And a filter capacitor connected between the converter-side inductor and the node connecting the transformer and the ground, wherein the grid-associated filter includes a grid-side inductor having an equivalent inductor for the transformer disposed between the node and the grid input terminal. Provides a grid-connected filter, an LCL filter used as a system.

Meanwhile, according to another aspect of the present invention, a grid connection is connected between a converter connected to a wind generator and converting AC power generated by wind power into DC power and then converting into AC power of a predetermined frequency and a transformer connected to the grid input terminal. A filter comprising: a grid side sub inductor and a converter side inductor connected in series between the transformer and the converter; And a filter capacitor connected between a node connecting the grid side sub inductor and the converter side inductor and a ground, wherein the grid link filter includes adding an equivalent inductor for the transformer to the grid side sub inductor. And an LCL filter used as a grid-side inductor disposed between the grid input terminals.

On the other hand, according to another aspect of the present invention, between the converter connected to the wind generator in the computing device converts the AC power generated by the wind power generation to the DC power and then to the AC power of a predetermined frequency and a transformer connected to the grid input stage CLAIMS What is claimed is: 1. A method of designing a grid-connected filter to be connected, comprising: (a) calculating an equivalent inductor for the transformer; (b) using the equivalent inductor as a grid side inductor in an LCL filter; And (c) calculating a converter-side inductor and a filter capacitor among the LCL filters.

On the other hand, according to another aspect of the present invention, between the converter connected to the wind generator in the computing device converts the AC power generated by the wind power generation to the DC power and then to the AC power of a predetermined frequency and a transformer connected to the grid input stage CLAIMS What is claimed is: 1. A method of designing a grid-connected filter to be connected, comprising: (a) calculating an equivalent inductor for the transformer; (b) using the equivalent inductor as a system side sub inductor summed with the system side inductor in the LCL filter; And (c) calculating another system side sub inductor, a converter side inductor, and a filter capacitor among the LCL filters.

Step (c) may include: (c1) calculating base impedance, base inductance and base capacitance of the wind power generation system; (c2) selecting a capacitance of the converter-side inductor to have a current total harmonic distortion (THD) of a predetermined value; (c3) selecting a capacity of the filter capacitor to be less than a predetermined percentage of the base capacitance; (c4) checking whether the sum of the capacitance of the grid-side inductor and the capacitance of the converter-side inductor is less than a predetermined percentage of the base inductance; (c5) calculating a resonant frequency of the LCL filter when the sum is less than a predetermined percentage of the base inductance, and if the sum is greater than a predetermined percentage of the base inductance in step (c5); In step (a), the total current harmonic distortion may be reset, and steps (c2) to (c5) may be repeated.

(c6) further comprising checking whether the resonant frequency is within a predetermined range, and applying a damping method for removing the resonant frequency if the resonant frequency is within a predetermined range, and if the resonant frequency is not within a predetermined range, Returning to (c2), it is possible to reset the current total harmonic distortion and repeat the steps (c2) and below.

Other aspects, features, and advantages will become apparent from the following drawings, claims, and detailed description of the invention.

According to an embodiment of the present invention, by using the equivalent inductor of the transformer as a filter inductor or in the filter inductor, it is more suitable for the power generation system, reducing current total harmonic distortion (THD) and conduction noise, and improving system control performance. have.

In addition, by using the equivalent inductor of the transformer as a filter inductor or included in the filter inductor, there is an effect that can reduce the overall size and cost.

1 is a view showing a typical three-phase Y-connected LCL filter and LC filter,
2 is a view showing a power generation system structure including a system-associated filter according to an embodiment of the present invention;
3 is an equivalent circuit diagram to which an equivalent inductor of a transformer is applied in a system-associated filter included in the power generation system structure shown in FIG. 2;
4 is a flowchart of a method of designing an LCL filter;
5 is a view showing a power generation system structure including a system-associated filter according to another embodiment of the present invention;
FIG. 6 is an equivalent circuit diagram to which an equivalent inductor of a transformer is applied in a system-associated filter included in the power generation system structure shown in FIG. 5;
7 is a flowchart of a method of designing a system-associated filter according to an embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It is to be understood, however, that the invention is not to be limited to the specific embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

When a component is referred to as being "connected" or "connected" to another component, it may be directly connected to or connected to that other component, but it may be understood that other components may be present in between. Should be. On the other hand, when an element is referred to as being "directly connected" or "directly connected" to another element, it should be understood that there are no other elements in between.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, the terms "comprises" or "having" and the like refer to the presence of stated features, integers, steps, operations, elements, components, or combinations thereof, But do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof.

The terms first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another.

Also, the term "part" or the like, as described in the specification, means a unit for processing at least one function or operation, and may be implemented by hardware, software, or a combination of hardware and software.

It is to be understood that the components of the embodiments described with reference to the drawings are not limited to the embodiments and may be embodied in other embodiments without departing from the spirit of the invention. It is to be understood that although the description is omitted, multiple embodiments may be implemented again in one integrated embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

2 is a view showing a power generation system structure including a grid-associated filter according to an embodiment of the present invention, Figure 3 is an equivalent inductor of a transformer in the grid-associated filter included in the power generation system structure shown in FIG. Fig. 4 is a flow chart of the design method of the LCL filter.

The grid-associated filter according to an embodiment of the present invention is disposed between the converter and the transformer, and it is possible to reduce the overall size by using an equivalent inductor for the transformer as one of the filter inductors to function as an LCL filter.

2, the power generation system 100 includes a generator 110, a converter 130, a grid-associated filter 150, a transformer 140, and a grid 120.

The generator 110 generates three phase AC power of variable frequency from the rotation of the turbine. The three phase AC power generated by the generator 110 may have a voltage magnitude of 3.3 kV, for example. For example, the generator 110 may be a renewable energy generator that rotates a turbine using a renewable energy source such as wind power.

The grid 120 is also called a grid, and is an AC power system provided by a power company or a power generation company. For example, grid 120 is an electrical linkage over a wide area, including power plants, substations, and power lines.

The converter 130 performs a function of stably transferring power generated by the generator 110 to the system 120. The converter 130 may be one of a structure for converting AC power into DC power, a structure for converting DC power to AC power, or a structure for converting AC power to DC power and then back to AC power. In particular, the converter 130 converts AC power of a random frequency generated by the generator 110 into DC power and then converts the AC power into AC power of a predetermined frequency.

The transformer 140 boosts the voltage level of the power generated by the generator 110 and transfers it to the system 120. The transformer 140 is connected to the grid input terminal. When the voltage transmitted from the converter 130 is a low level or a medium level due to the voltage level limit of the internal switching device, the transformer 140 boosts the voltage to a high level grid side voltage.

The grid-associated filter 150 is connected between the transformer 140 and the converter 130 to attenuate harmonic components generated in the power generation process.

In the case of the LC filter or the LCL filter, elements such as filter inductor and filter capacitor generate high resonance in a specific frequency band. In order to eliminate this, an active damping method or a passive damping method is used. Passive damping method is to connect the actual resistance, and active damping method is to remove the signal of the resonant frequency band by using a band stop filter such as a notch filter (Notch Filter). In order to use this damping method, it is important to know the resonance frequency and impedance accurately.

However, in the related art, the entire system is designed by considering only the filter part without considering the impedance of the transformer 140 in the filter control. However, since the transformer 140 may also be assumed to be a wide inductor because the coil is wound, when the system is designed in consideration of the impedance of the transformer 140, the performance of the filter is designed to be more suitable for the entire system, so that the current total harmonics. Distortion and conduction noise will be reduced and system control performance will be improved.

To this end, the grid-associated filter 150 according to the present exemplary embodiment calculates an equivalent inductor L _T from the impedance of the transformer 140 and uses the grid inductor Lg as the grid-side inductor Lg. The components will be reflected in the filter design.

The grid-associated filter 150 includes a converter-side inductor Lc connected between the transformer 140 and the converter 130, and a node N1 and ground connecting the converter-side inductor Lc and the transformer 140. It includes a filter capacitor (C) connected between.

Basically it appears to be an LC filter, but since the equivalent inductor L _T of transformer 140 functions as a grid side inductor Lg, grid-associated filter 150 will be an LCL filter (see FIG. 3). .

Therefore, in calculating the resonant frequency of the system, the equivalent inductor L _T of the transformer 140 is taken into consideration, and the LCL in the design of the converter side inductor L2 and the filter capacitor C of the system-associated filter 150 is also considered. The inductance and capacitance can be calculated according to the filter design method.

An example of the design method of the LCL filter is shown in FIG. 4.

Referring to FIG. 4, the base impedance values Z _b , L _b , and C _b of the system are calculated before selecting the parameters of the LCL filter (step S210). The base inductance Lb and the base capacitance Cb can be calculated from the base impedance Zb (see Equations 1 to 3 below).

Where P is the rated power of the converter, E is the line voltage, and ω ₁ is 2πf ₁ . The total harmonic distortion (THD) injected into the system can be selected within a certain percentage (eg within 5%), depending on government regulations or the characteristics of the power generation system.

The capacitance of the converter-side inductor Lc may be selected by setting THD to an appropriate value in consideration of the switching frequency of the fundamental frequency (step S220). For example, it is appropriate to select a THD on the order of 5% to 30% so that the resonant frequency lies properly between the bandwidth and switching frequency of the current controller.

The capacitance of the filter capacitor C is selected to be less than a predetermined percentage (for example, less than 5%) of the base capacitor C _b (step S230).

Next, after selecting the current ripple attenuation ratio RAF _sw , the capacity of the grid-side inductor Lg is calculated (step S240).

It is checked whether the capacity of the entire inductor, which is the sum of the capacity of the inverter side inductor and the system side inductor, is less than a predetermined percentage (for example, less than 10%) of the base inductance L _b (step S250). If the capacity of the entire inductor is greater than a predetermined percentage of the base inductance, the above-described process is repeated by resetting the THD _i or the current ripple attenuation rate RAF _sw of the converter output current.

When all the parameters of the LCL filter are determined, the resonance frequency f _rea is calculated as shown in Equation 4 below (step S260). In general, the resonant frequency should be between 1/2 times the switching frequency f _sw or 10 times the grid frequency f ₁ at the controller bandwidth in consideration of the dynamic characteristics of the system (step S270). If the resonant frequency is out of this condition, the above-described process is repeated by resetting the THD _i or the current ripple attenuation rate RAF _sw of the converter output current.

If the resonant frequency is between 1/2 times the switching frequency f _sw or 10 times the system frequency f ₁ at the controller bandwidth, the flow proceeds to step S280 to apply the damping method determined for the system.

5 is a diagram illustrating a power generation system structure including a grid-associated filter according to another embodiment of the present invention, and FIG. 6 is an equivalent inductor of a transformer in the grid-associated filter included in the power generation system structure shown in FIG. 5. It is an equivalent circuit diagram to which is applied.

A grid-tied filter according to another embodiment of the present invention is disposed between the converter and the transformer, and it is possible to reduce the overall size by functioning as an LCL filter by using an equivalent inductor for the transformer as part of one of the filter inductors. It is done.

Referring to FIG. 5, the power generation system 100 includes a generator 110, a converter 130, a system-associated filter 250, a transformer 140, and a system 120. Since the generator 110, the converter 130, the transformer 140, and the system 120 are the same components as those shown in FIG. 2, the description thereof will be omitted, and the system-associated filter 250 will be described. .

The grid-associated filter 250 is connected between the transformer 140 and the converter 130 to attenuate harmonic components generated during the power generation process.

The grid-associated filter 250 according to the present exemplary embodiment calculates an equivalent inductor L _T from the impedance of the transformer 140 and uses it as a part of the grid-side inductor Lg (that is, the grid-side sub-inductor). The inductance component of the impedance of the transformer 140 is reflected in the filter design.

The grid-associated filter 250 includes a grid side sub inductor L1 and a converter side inductor Lc, a grid side sub inductor L1 and a converter side inductor connected in series between the transformer 140 and the converter 130. It includes a filter capacitor (C) connected between the node (N2) and the ground between (Lc).

This configuration alone is an LCL filter, but since the equivalent inductor L _T of the transformer 140 participates as part of the grid side inductor Lg, the grid-associated filter 250 would be a more suitable LCL filter for the system.

That is, in the present embodiment, the grid-side inductor Lg may be calculated as shown in Equation 5 below.

That is, in calculating the resonance frequency of the system, the equivalent inductor L _T of the transformer 140 is considered together with the grid side sub inductor L1 and the converter side inductor Lc and the filter of the grid-associated filter 250 are included. When designing the capacitor C, the inductance and capacitance may be calculated according to the design method of the LCL filter. The design method of the LCL filter has been described above with reference to FIG. 4.

In high-capacity converters used in wind power systems, grid-connected filters are typically manufactured in one independent cabinet due to their large volume and heat output, and are installed next to cabinets containing power semiconductor devices such as IGBTs.

In the present invention, the LCL filter is configured by using an equivalent inductor of a transformer as one of the filter inductors of the grid-associated filter in the existing IGBT cabinet and the grid-associated filter cabinet, or by including it in one of the filter inductors in series as a sub-inductor. It is possible to reduce the size of the whole part.

7 is a flowchart illustrating a method of designing a system-associated filter according to an embodiment of the present invention. Each step of FIG. 7 may be performed by a computing device used in the design process of a power generation system.

In step S410, an equivalent inductor for the transformer is calculated.

For example, the calculation of the equivalent inductor for a transformer is as follows.

In general, the impedance of a transformer is listed in the transformer catalog. Generally, it is 6-7%, for example, and the unit price can be made higher by the demand of the user. Impedance is the sum of the resistance (R) and inductance (L) components. The transformer efficiencies listed in the catalog can be used to distinguish this.

For example, suppose a transformer has a capacity of 3.3 MVA, a voltage of 690 V / 22900 V, an impedance of 7%, an efficiency of 99%, and a frequency of 60 Hz. Given this transformer specification, the reference impedance is about 0.14 ohms, and the impedance is about 0.01 ohms because the impedance is 7%.

Since the efficiency is 99%, 1% is lost, but the loss is largely divided into iron loss and copper loss, and if the harmonic copper loss or surface effect is ignored, the copper loss can be regarded as pure conduction loss. In general, assuming that the ratio of iron loss and copper loss is the same when designing an electric device, it is 1/2 of 1%, so 0.5% can be regarded as copper loss, which is about 721 u ohms. Dividing this by each coil results in 360u ohms. If we subtract the resistive component from the 7% impedance, about 6.75% impedance can be considered as the inductance component, which is about 26uH. Since only one coil (converter side) is converted, the other coil is about 26 uH, so the equivalent inductor and equivalent resistance of the transformer are 52 uH and 721 u ohms.

In step S420, the equivalent inductor of the transformer is used as the grid side inductor in the LCL filter. When used as a grid-side inductor will be designed as shown in FIG.

In operation S430, the remaining parameters of the LCL filter, that is, the converter side inductor and the filter capacitor, are calculated according to the equivalent inductor (system side inductor). The method of calculating the parameters of the LCL filter has been described with reference to FIG. 5.

In this case, even if the filter having the same performance, by using the transformer as a grid-side inductor, the filter part is used as the LCL filter, not the LC filter, thereby reducing the overall size of the filter part.

Alternatively, in step S440, the equivalent inductor of the transformer is used as one component (system side sub inductor) of the grid side inductor in the LCL filter. When used as a component of the grid-side inductor will be designed as shown in FIG.

In step S450, the remaining system side sub inductor is calculated corresponding to the equivalent inductor, and the remaining parameters of the LCL filter, that is, the converter side inductor and the filter capacitor are calculated. The method of calculating the parameters of the LCL filter has been described with reference to FIG. 5.

In this case, even if the filter having the same performance, by using the transformer as part of the grid-side inductor can reduce the size of the grid-side inductor can reduce the overall size of the filter.

In addition, by adding the grid-side sub-inductor to the grid-side inductor for the equivalent inductor of the transformer, there is also an effect of increasing the design freedom of the LCL filter.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as defined in the following claims And changes may be made without departing from the spirit and scope of the invention.

100, 200: power generation system 110: generator
120: system 130: converter
140: transformer 150, 250: grid-associated filter

Claims (6)

delete delete In the design method of a grid-connected filter connected between a converter connected to a wind generator in a computing device and converts AC power generated by wind power into DC power and then to AC power of a predetermined frequency and a transformer connected to a grid input terminal. In
(a) calculating an equivalent inductor for the transformer;
(b) using the equivalent inductor as a grid side inductor in an LCL filter; And
(c) calculating a converter side inductor and a filter capacitor among the LCL filters,
The step (c)
(c1) calculating base impedance, base inductance and base capacitance of the wind power generation system;
(c2) selecting a capacitance of the converter-side inductor to have a current total harmonic distortion (THD) of a predetermined value;
(c3) selecting a capacity of the filter capacitor to be less than a predetermined percentage of the base capacitance;
(c4) checking whether the sum of the capacitance of the grid-side inductor and the capacitance of the converter-side inductor is less than a predetermined percentage of the base inductance;
(c5) calculating a resonance frequency of the LCL filter when the sum is less than a predetermined percentage of the base inductance;
And returning to step (c2) to reset the current total harmonic distortion and repeating step (c2) or less if the sum is greater than a predetermined percentage of the base inductance in step (c5). In the design method of a grid-connected filter connected between a converter connected to a wind generator in a computing device and converts AC power generated by wind power into DC power and then to AC power of a predetermined frequency and a transformer connected to a grid input terminal. In
(a) calculating an equivalent inductor for the transformer;
(b) using the equivalent inductor as a system side sub inductor summed with the system side inductor in the LCL filter; And
(c) calculating other grid side inductors, converter side inductors, and filter capacitors of the LCL filters,
The step (c)
(c1) calculating base impedance, base inductance and base capacitance of the wind power generation system;
(c2) selecting a capacitance of the converter-side inductor to have a current total harmonic distortion (THD) of a predetermined value;
(c3) selecting a capacity of the filter capacitor to be less than a predetermined percentage of the base capacitance;
(c4) checking whether the sum of the capacitance of the grid-side inductor and the capacitance of the converter-side inductor is less than a predetermined percentage of the base inductance;
(c5) calculating a resonance frequency of the LCL filter when the sum is less than a predetermined percentage of the base inductance;
And returning to step (c2) to reset the current total harmonic distortion and repeating step (c2) or less if the sum is greater than a predetermined percentage of the base inductance in step (c5). delete The method according to claim 3 or 4,
(c6) further comprising checking whether the resonant frequency is within a predetermined range,
(c7) If it is confirmed that the resonant frequency is within a predetermined range, a damping method for removing the resonant frequency is applied. If the resonant frequency is not within the predetermined range, the method returns to step (c2) to reset the current total harmonic distortion and the step ( c2) A method of designing a system-driven filter which repeats the following.

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