KR102276105B1 - Apparatus and method for selective harmonic compensation method for grid-connected inverter - Google Patents

Abstract

It relates to a selective harmonic compensation device for a grid-connected inverter, the selective harmonic compensation device comprising: an extraction unit for extracting a control target harmonic component to be controlled from a current of a three-phase system; a coordinate conversion unit that converts the extracted harmonic component to be controlled into a DC value, and outputs a harmonic component to be controlled after conversion into a DC value; a residual component removing unit for outputting a control target harmonic component from which the residual component is removed through the removal of residual components other than the control target harmonic component after the conversion; and a harmonic compensator transmitting the control target harmonic component from which the residual component is removed to an input of the proportional integration controller to compensate for the control target harmonic component from which the residual component is removed.

Description

Selective harmonic compensation device and method for grid-connected inverters {APPARATUS AND METHOD FOR SELECTIVE HARMONIC COMPENSATION METHOD FOR GRID-CONNECTED INVERTER}

The present application relates to a selective harmonic compensation apparatus and method for a grid-connected inverter.

A grid-connected inverter controls an output voltage when a distributed power generation system using a renewable energy source is connected to the grid and performs a function to maintain power generation efficiency to the maximum.

However, when the grid voltage is distorted by the nonlinear load, a low-order harmonic current excluding the fundamental component is generated in the output current of the grid-connected inverter. That is, the grid voltage distorted by the operation of the non-linear load causes the generation of harmonics in the grid-connected inverter output current.

If the quality of the output current of the inverter deteriorates, the inverter operates as another non-linear load and may adversely affect the grid. Therefore, the THD of the inverter output current should be limited to less than 5% according to the IEEE-1547 standard. As such, according to the IEEE-1547 standard, since the output current THD of the grid-connected inverter must be maintained within 5%, the implementation of a precise control method for low-order harmonic current control is required in the related field.

1 is a diagram schematically showing the configuration of a conventional grid-connected inverter system (1).

Referring to FIG. 1 , a conventional grid-connected inverter system 1 may include a grid 2 , a grid-connected inverter 3 , and a controller 4 for controlling the operation of the inverter 3 . have. Here, the system (2) may mean a three-phase system.

In the conventional grid-connected inverter system 1, when the grid voltage is distorted by a non-linear load such as a rectifier and an inverter system 1, a harmonic component is included in the output current of the grid-connected inverter 3 .

In order to reduce (reduce) the harmonic component in the output current generated by the grid-connected inverter 3, an additional hardware configuration or a complicated control technique (or complicated calculation process) is required in the prior art. That is, in the conventional (conventional) harmonic reduction technique, a hardware filter is added to the output of the inverter or a harmonic component is removed by using a complex control technique.

However, when the additional filter is used, the weight and size as well as the configuration cost of the entire system increase. Also, the harmonic reduction technique using the additional control technique has a disadvantage in that the control process is complicated or the performance is poor.

In other words, adding a hardware filter to the output of the inverter 3 has a problem that acts as a factor to increase the configuration cost or volume and weight of the system 1 . In addition, in the case of the method of reducing harmonics by utilizing the conventionally used software method, the control method is complicated and if the distortion of the grid (2) voltage becomes large, the magnitude of the low-order harmonic cannot be reduced to within the standard.

Meanwhile, the conventionally developed harmonic control technique reduces harmonic components by using various controllers such as a proportional resonance controller, an iterative controller, and a proportional integral controller.

The harmonic reduction technique through the proportional resonance controller is a method for directly controlling the AC component on the stationary coordinate system to reduce the harmonic component, and has a feature that the coordinate transformation process is relatively simple. This can effectively reduce harmonics, but since it has a very large controller gain in a specific frequency band, it can amplify not only the harmonic components of the corresponding frequency band but also the surrounding signals, so it has instability.

The harmonic control technique through the iterative controller refers to a control technique that has the advantage of a simple controller configuration and high-quality current output. However, the repetitive controller has a disadvantage that the dynamic response time to the step change of the reference current is 100 ms or more, which is very slow compared to other controllers, so that the dynamic response characteristics may be deteriorated.

For example, the conventional literature [Q. Yan, X. Wu, X. Yuan, and Y. Geng, "An Improved Grid-Voltage Feedforward Strategy for High-Power Three-Phase Grid-Connected Inverters Based on the Simplified Repetitive Predictor," IEEE Trans. Power Electron., vol. 31, no. 5, pp. 3880-3897, May 2016.] discloses a technique related to an iterative predictor for compensating the delay of the iterative controller used to reduce harmonics included in the output current of a grid-connected inverter.

The harmonic reduction technique through the proportional integral controller has the advantage that the DC component can be controlled without a steady-state error. Accordingly, in order to input a harmonic component to be compensated to the proportional integral controller as a DC component, a harmonic on the three-phase coordinate system must be converted into a value on the d-q-axis synchronous coordinate system. In this case, the harmonic component may be converted into a DC value, while a relatively large fundamental wave component remains as an AC component.

Therefore, the conventional harmonic control technique (particularly, the conventional harmonic reduction technique using the proportional integral controller) uses a low-pass filter to reduce the residual AC component. However, the low-pass filter applied to the conventional harmonic control technique does not completely remove the fundamental component, and because of this, it has a structure that allows the AC component to be input to the proportional integral controller, which not only reduces the harmonic reduction performance, but also There are problems that can cause other harmonics to occur.

The present application is intended to solve the problems of the prior art, and unlike the harmonic control technique through an iterative controller in which a delay occurs in the control process, there is no delay in the control process, so proportional integration does not require a separate delay compensation process An object of the present invention is to provide a selective harmonic compensation device and method for a grid-connected inverter that can reduce harmonics using a controller.

The present application is intended to solve the problems of the prior art described above, and by effectively reducing harmonic components without the need for additional hardware configuration and complicated control techniques (or complicated calculation processes), in terms of configuration cost (price) or volume and weight of the system An object of the present invention is to provide a selective harmonic compensation device and method for a grid-connected inverter that can be competitive.

However, the technical problems to be achieved by the embodiments of the present application are not limited to the technical problems as described above, and other technical problems may exist.

As a technical means for achieving the above technical problem, the selective harmonic compensation device for a grid-connected inverter according to an embodiment of the present application is an extraction unit for extracting a control target harmonic component from a current of a three-phase system ; a coordinate conversion unit that converts the extracted harmonic component to be controlled into a DC value, and outputs a harmonic component to be controlled after conversion into a DC value; a residual component removing unit for outputting a control target harmonic component from which the residual component is removed through the removal of residual components other than the control target harmonic component after the conversion; and a harmonic compensator transmitting the control target harmonic component from which the residual component is removed to an input of the proportional integration controller to compensate for the control target harmonic component from which the residual component is removed.

Also, the extractor may extract the control target harmonic component using a band-pass filter disposed between the three-phase system and the coordinate converter.

In addition, the extraction unit may selectively extract any one harmonic component from among a plurality of harmonic components included in the current of the three-phase system as the control target harmonic component.

In addition, any one harmonic component may be a 5th harmonic component or a 7th harmonic component.

In addition, the residual component removing unit may remove the residual component by passing the control target harmonic component after the transformation through a low-pass filter disposed between the coordinate converting unit and the proportional integral controller.

In addition, the harmonic compensator may include: the proportional integral controller for generating a command voltage in consideration of a control target harmonic component from which the residual component is removed; and a compensator for compensating the generated command voltage to the output of the fundamental wave current controller.

In addition, the compensator may include a command voltage coordinate converter for inversely converting the generated command voltage into an AC value on a dq-axis stationary coordinate system.

In addition, harmonics included in the output current of the grid-connected inverter may be reduced by the compensator.

On the other hand, the selective harmonic compensation method for a grid-connected inverter according to an embodiment of the present application, (a) extracting, in the extraction unit, a control target harmonic component to be controlled from the current of the three-phase system; (b) converting the extracted harmonic component to be controlled into a DC value, and outputting the harmonic component to be controlled after conversion into a DC value; (c) outputting, in the residual component removing unit, the control target harmonic component from which the residual component is removed through the removal of residual components other than the control target harmonic component after the conversion; and (d) transmitting, in the harmonic compensator, the control target harmonic component from which the residual component is removed to the input of the proportional integral controller, in order to compensate the control target harmonic component from which the residual component is removed for the proportional integral controller. may include.

In addition, in step (a), the control target harmonic component may be extracted using a band-pass filter disposed between the three-phase system and the coordinate converter.

In addition, in step (a), any one harmonic component among a plurality of harmonic components included in the current of the three-phase system may be selectively extracted as the control target harmonic component.

In addition, the step (d) may include: (d1) generating, in the proportional integral controller, a reference voltage in consideration of a control target harmonic component from which the residual component has been removed; and (d2) compensating the generated command voltage to the output of the fundamental wave current controller by the compensator.

The above-described problem solving means are merely exemplary, and should not be construed as limiting the present application. In addition to the exemplary embodiments described above, additional embodiments may exist in the drawings and detailed description.

According to the above-described problem solving means of the present application, it is possible to reduce harmonics by using a proportional integral controller that does not require a separate delay compensation process because a delay does not occur in the control process.

According to the above-described problem solving means of the present application, it is possible to selectively reduce harmonic components effectively by using a bandpass filter without the need for additional hardware configuration and complicated control techniques (or complicated calculation processes). Through this, the present application can be competitive in terms of configuration cost (price) or volume and weight of the system.

However, the effects obtainable herein are not limited to the above-described effects, and other effects may exist.

1 is a diagram schematically showing the configuration of a conventional grid-connected inverter system.
2 is a diagram schematically showing the overall configuration of a grid-connected inverter system according to an embodiment of the present application.
3 is a diagram schematically showing a schematic configuration of a selective harmonic compensation device for a grid-connected inverter according to an embodiment of the present application.
4 is a block diagram showing a schematic configuration of a selective harmonic compensation device for a grid-connected inverter according to an embodiment of the present application.
5 is a schematic diagram illustrating the configuration of a general grid-connected inverter system including a grid-connected inverter.
6 is a diagram illustrating an example of a Bode diagram according to the bandwidth of a bandpass filter considered in the selective harmonic compensation device for a grid-connected inverter according to an embodiment of the present application.
7 is an operation flowchart for a selective harmonic compensation method for a grid-connected inverter according to an embodiment of the present application.

Hereinafter, embodiments of the present application will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art to which the present application pertains can easily implement them. However, the present application may be embodied in several different forms and is not limited to the embodiments described herein. And in order to clearly explain the present application in the drawings, parts irrelevant to the description are omitted, and similar reference numerals are attached to similar parts throughout the specification.

Throughout this specification, when a part is "connected" with another part, it is not only "directly connected" but also "electrically connected" or "indirectly connected" with another element interposed therebetween. "Including cases where

Throughout this specification, when it is said that a member is positioned "on", "on", "on", "under", "under", or "under" another member, this means that a member is positioned on the other member. It includes not only the case where they are in contact, but also the case where another member exists between two members.

Throughout this specification, when a part "includes" a component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.

The present application provides selective harmonics for grid-connected inverters that can effectively reduce harmonic components without the need for additional hardware configuration and complicated control techniques (or complex calculation processes) to reduce system configuration cost (price), volume, weight, etc. A compensation device is proposed.

2 is a diagram schematically showing the overall configuration of the grid-connected inverter system 100 according to an embodiment of the present application. 3 is a diagram schematically showing a schematic configuration of a selective harmonic compensation device 10 for a grid-connected inverter according to an embodiment of the present application. 4 is a block diagram showing a schematic configuration of a selective harmonic compensation device 10 for a grid-connected inverter according to an embodiment of the present application.

Hereinafter, for convenience of description, the grid-connected inverter system 100 according to an embodiment of the present application is referred to as the present system 100, and a selective harmonic compensation device 10 for a grid-connected inverter according to an embodiment of the present application is provided. It will be referred to as the present device (10).

First, prior to a detailed description of the present system 100 and the present apparatus 10, a description of the cause of harmonic generation in a grid-connected inverter in general and its analysis will be described with reference to FIG. 5 . decide to do

5 is a schematic diagram showing the configuration of a general grid-connected inverter system 200 including the grid 21-linked inverter 22. As shown in FIG. Here, the grid-connected inverter system 200 refers to the entire system including the grid-connected inverter, and may be referred to as a grid-connected inverter system, a three-phase grid system, or the like.

Referring to FIG. 5 , the grid-connected inverter system 200 may include a grid 21 and a grid-connected inverter 22 that is an inverter connected to the grid. Here, the system 21 may be referred to as a three-phase system differently.

,

, 및

와 같이 간략화하여 표현될 수 있다. 3상 계통(21)의 전압 및 전류는 각각 후술하는 설명에서 계통 전압, 계통 전류로 지칭될 수 있다.The output voltage of the inverter 22, the current of the three-phase system 21, and the voltage of the three-phase system 21 are respectively

,

, and

It can be expressed briefly as The voltage and current of the three-phase grid 21 may be referred to as grid voltage and grid current in the description to be described later, respectively.

In the grid-connected inverter system 200 as shown in FIG. 5 , the voltage applied to the grid-side filter inductor and the resistor can be expressed as Equation 1 below.

[Equation 1]

와 3상 계통(21)의 전압

를 기본파와 고조파 성분으로 분리하여 표현하는 경우, 하기 식 2와 같이 나타낼 수 있다.Here, the output voltage of the inverter 22

and the voltage of the three-phase grid (21)

When expressed by separating the fundamental wave and the harmonic component, it can be expressed as in Equation 2 below.

[Equation 2]

In addition, in the case of expressing Equation 1 by dividing it into a fundamental component and a harmonic component, it can be expressed as Equation 3 and Equation 4 below.

[Equation 3]

[Equation 4]

)이 이상적일 경우에는 상기 식 3과 같이 계통의 전압 및 전류 측에 고조파 성분이 발생하지 않는다. 하지만, 계통 전압이 왜곡되었을 때, 인버터(22)가 계통의 고조파 성분과 동일한 크기의 고조파 성분을 출력하지 않을 경우에는 저항과 인덕터 양단에 고조파 성분에 의한 전압강하가 발생하여 인버터(22)의 출력 전류에 고조파 성분이 포함되게 된다.Voltage of the three-phase grid 21 (i.e. grid voltage) (

) is ideal, harmonic components do not occur on the voltage and current sides of the system as in Equation 3 above. However, when the grid voltage is distorted, if the inverter 22 does not output a harmonic component of the same magnitude as the harmonic component of the grid, a voltage drop occurs at both ends of the resistor and the inductor due to the harmonic component, and the output of the inverter 22 The current contains harmonic components.

At this time, as the harmonic components included in the output current of the inverter 22 due to the distortion of the grid voltage, a plurality of harmonic components (multiple harmonic components) such as the 3rd harmonic component, the 5th harmonic component, the 7th harmonic component, etc. may be included.

The characteristics of these harmonics are analyzed as follows. In order to analyze the characteristics of harmonics, the current output from the three-phase inverter (ie, three-phase grid-connected inverter) 22 can be expanded (expressed) as in Equation 5 below using a Fourier transform.

[Equation 5]

If Equation 5 expressed in the three-phase coordinate system is expressed in the synchronous coordinate system using Park's transformation, it can be expressed as Equation 6 below.

[Equation 6]

However, as can be seen from Equation 6, it can be seen that the harmonic component of the zero-phase component including the third harmonic is canceled through Equations 7 and 8 below under the three-phase current balance condition.

In other words, the third harmonic component among the plurality of harmonic components (multi-order harmonic components) included in the output current of the inverter 22 due to the distortion of the grid voltage is the three-phase current balance condition (with respect to the current of the three-phase grid). parallel condition), it can be erased through equations such as Equations 7 and 8 above. That is, the third harmonic component may act (apply) as a component that does not affect the output current of the inverter 22 through Equations 7 and 8.

[Equation 7]

[Equation 8]

In a similar manner, if the output current of the three-phase inverter 22 is expressed on the abc three-phase coordinate system in consideration of each phase sequence, the harmonic current having a positive sequence sequence and a harmonic current having a reverse phase sequence sequence are respectively shown below. It can be expressed as Equation 9 and Equation 10.

[Equation 9]

[Equation 10]

As can be seen from Equations 9 and 10, it can be confirmed that each harmonic (harmonic component) included in the output current of the inverter 22 has individual characteristics according to the order. Therefore, in order to properly compensate for harmonics of each order, it is necessary to consider their characteristics.

Accordingly, the present system 100 and the present apparatus 10 propose a selective harmonic compensation technology for a grid-connected inverter that can effectively compensate each order of harmonics in consideration of the characteristics of each harmonic. A more detailed description of the control algorithm of the present system 100 to the present apparatus 10 is as follows.

Hereinafter, the present system 100 and the present apparatus 10 will be described in detail based on the contents described with reference to FIG. 5 . In this case, the contents described with reference to FIG. 5 may be equally applied to the description of the present system 100 and the present apparatus 10 even if the contents are omitted below.

2 to 4 , the present system 100 includes a grid (three-phase grid, 21), an inverter (grid-connected inverter, 22) connected to the grid 21, the device 10, and a fundamental wave current It may include a controller 30 and a harmonic current controller (or harmonic controller) 15 . The device 10 may be arranged to be connected to the grid 21 , the grid-connected inverter 22 , the fundamental wave current controller 30 , and the harmonic current controller 15 .

The present system 100 may be expressed differently as a grid-connected (linked) inverter system, a three-phase grid system in which the apparatus 10 is considered, and the like. The device 10 is a selective harmonic compensation device 10 for the grid-connected inverter 22, and may be expressed differently as a selective harmonic reduction device, a selective harmonic controller, and the like.

In general, the fundamental wave current controller (ie, the fundamental wave component current controller) 30 performs system control on the assumption that the voltage (system voltage) of the grid 21 is not distorted. Accordingly, when the fundamental wave current controller 30 is connected to the distorted system 21 , a harmonic component is included in the output current of the inverter 22 .

The grid-connected inverter 22 serves to supply high-quality power at a common junction between the grid 21 and the power source. According to the IEEE-1547 standard, the Total Harmonics Distortion (THD) of the output current of the grid-connected inverter 22 must be within 5%, and for this purpose, it is included in the output current of the inverter 22 It is necessary to reduce the current of the harmonic component. In other words, among the harmonic components included in the output current of the inverter 22, harmonic components such as 3rd, 5th, and 7th harmonics have a large size, and thus need to be reduced.

Accordingly, the present application proposes a selective harmonic compensation device (this device, 10) for a grid-connected inverter that can effectively reduce the x-th harmonic as a harmonic component included in the output current of the inverter 22 . In particular, FIG. 3 shows an example of a control block diagram of the apparatus 10 for reducing the x-th harmonic, that is, an example of a control block diagram of a selective harmonic reduction technique using a bandpass filter by the apparatus 10. .

Here, the x-th harmonic reduced by the device 10 may preferably mean the 5th harmonic and the 7th harmonic among a plurality of harmonic components included in the output current of the inverter 22, but is not limited thereto. . Meanwhile, the third harmonic among the plurality of harmonic components included in the output current of the inverter 22 may be canceled through Equations 7 and 8 as described above.

The apparatus 10 may include an extractor 11 , a coordinate transform unit 12 , a residual component remover 13 , and a harmonic compensator 14 . The harmonic compensator 14 may include a proportional-integral controller (PI controller) 14a and a compensator 14b. The compensator 14b may include a command voltage coordinate conversion unit 14b'.

)로부터 제어 대상이 되는 제어 대상 고조파 성분(

)을 추출할 수 있다. 본원에서 고조파 성분이라 함은 특히 고조파 전류 성분을 의미할 수 있다.The extraction unit 11 is the current (

) from the control target harmonic component (

) can be extracted. As used herein, the harmonic component may mean in particular a harmonic current component.

The extraction unit 11 may extract a control target harmonic component using a band-pass filter (BPF) 11a disposed between the three-phase system 21 and the coordinate transformation unit 12 . have.

The extraction unit 11 may selectively extract any one harmonic component among a plurality of harmonic components included in the current of the three-phase system 21 as a control target harmonic component. Here, the harmonic component extracted by the extraction unit 11 may be, for example, a low-order harmonic component, and for example, the harmonic component extracted by the extraction unit 11 may be a 5th harmonic component or a 7th harmonic component. .

That is, the extraction unit 11 can pass the current of the three-phase system 21 to the BPF 11a, and the control target harmonic component that is a control target in the current of the three-phase system 21 through the BPF 11a As an x-th harmonic component can be extracted. Here, the extraction unit 11 may extract the 5th harmonic component and the 7th harmonic component as the xth harmonic component.

At this time, the current of the three-phase system 21 may include a 3rd harmonic component in addition to the 5th and 7th harmonic components, but these 3rd harmonic components are erased through Equations 7 and 8 as described above. Therefore, the extraction unit 11 may not separately extract the third harmonic component.

Specifically, when a component (for example, a harmonic component other than the 5th harmonic component or the 7th harmonic component) that is not a control target (control target) is input to the proportional integration controller 14a, the harmonic current controller 15 performance may be reduced. Therefore, in order to solve this problem, the extraction unit 11 of the apparatus 10 uses only the harmonic component (harmonic current component) to be controlled from the current of the three-phase system 21 using the BPF 11a. It can be extracted as a component.

The extraction unit 11 may extract a control target harmonic component using a transfer function of the BPF 11a, and the transfer function of the BPF 11a may be expressed as Equation 11 below.

[Equation 11]

는 각각 BPF(11a)의 대역폭과 중심 주파수를 나타낸다. BPF의 대역폭은 작게 설정될수록 고조파 성분에 대한 선별성이 증가할 수 있으며, 이는 실제 계통(21) 전압의 주파수 변화를 고려하여 적절히 선정해야 할 필요가 있다.Here, B and

denotes a bandwidth and a center frequency of the BPF 11a, respectively. As the bandwidth of the BPF is set smaller, selectivity for harmonic components may increase, which needs to be appropriately selected in consideration of the frequency change of the actual grid voltage.

6 is a diagram illustrating an example of a Bode diagram according to the bandwidth of the bandpass filter 11a considered in the selective harmonic compensation device 10 for a grid-connected inverter according to an embodiment of the present application. That is, FIG. 6 shows a Bode diagram according to the bandwidth of the BPF 11a for selective extraction of harmonics in the apparatus 10 . In particular, in FIG. 6 (a) shows a Bode diagram of a band pass filter (BPF, 11a) for 5th harmonic extraction, and in FIG. 6 (b) is a band pass filter (BPF, 11a) for 7th harmonic extraction shows the board diagram of

Referring to FIG. 6 , the extraction unit 11 may selectively extract a fifth harmonic or a seventh harmonic among a plurality of harmonics included in the system current. To this end, the extraction unit 11 selects and extracts a harmonic of which order among a plurality of harmonics, considering a change in frequency of the actual system 11 voltage, the BPF ( 11a) characteristics (eg, bandwidth, center frequency, etc.) may be determined (set).

)이 추출된 이후, 좌표 변환부(12)는 추출부(11)에서 추출된 제어 대상 고조파 성분을 직류 값으로 변환하고, 직류 값으로 변환 후 제어 대상 고조파 성분(

,

)을 출력할 수 있다.The harmonic component (

) is extracted, the coordinate conversion unit 12 converts the control target harmonic component extracted by the extraction unit 11 into a DC value, and converts it into a DC value Controlled harmonic components (

,

) can be printed.

The harmonic current extracted by using the BPF 11a (ie, the control target harmonic component, the control target harmonic current component) needs to be converted into a DC value in order to be controlled without a steady-state error through the proportional integral controller 14a. Therefore, the coordinate transformation unit 12 converts the control target harmonic component extracted by the extraction unit 11 (that is, the harmonic component, harmonic current component, harmonic current extracted using BPF) into a DC value to perform coordinate transformation. can

In this case, the coordinate transformation unit 12 may perform the coordinate transformation of each harmonic component in consideration of the phase order.

Since the coordinate transformation unit 12 typically has the 5th harmonic component inverse phase, when the control target harmonic component is the 5th harmonic component, the coordinate transformation unit 12 converts the control target harmonic component to 5 times the angular velocity of the grid 21 voltage. It can be converted to a DC value on a rotating synchronous coordinate system.

On the other hand, since the 7th harmonic component is a normal component, the coordinate conversion unit 12 rotates the control target harmonic component through coordinate transformation at 7 times the angular velocity of the voltage of the system 21 when the control target harmonic component is the 7th harmonic component It can be converted into a DC value on the synchronous coordinate system.

The coordinate transformation process by the coordinate transformation unit 12 can be expressed as Equations 12 and 13 below.

In other words, when the control target harmonic component extracted by the extraction unit 11 is the 5th harmonic component, the coordinate transformation unit 12 may perform coordinate transformation so that the extracted 5th harmonic component satisfies the following Equation 12. have. Through this, the coordinate converter 12 may output a fifth harmonic component (ie, a fifth harmonic component after conversion) converted into a DC value as a control target harmonic component after conversion into a DC value.

Similarly, when the control target harmonic component extracted by the extraction unit 11 is the 7th harmonic component, the coordinate transformation unit 12 may perform coordinate transformation so that the extracted 7th harmonic component satisfies Equation 13 below. Through this, the coordinate converter 12 may output the 7th harmonic component (ie, the 7th harmonic component after conversion) converted into a DC value as a control target harmonic component after conversion into a DC value.

[Equation 12]

[Equation 13]

,

)을 출력할 수 있다. 여기서, 잔여 성분이라 함은 잔여 교류 성분을 의미할 수 있다.After the conversion to a DC value is performed through the coordinate conversion unit 12, the residual component removing unit 13 removes the residual component other than the control target harmonic component after the conversion to remove the control target harmonic component (

,

) can be printed. Here, the residual component may mean a residual alternating current component.

The residual component removing unit 13 outputs to a low-pass filter (LPF) 13a disposed between the coordinate converting unit 12 and the proportional integral controller 14 through the coordinate converting unit 12 . After the conversion, the residual component can be removed by passing the harmonic component to be controlled.

That is, the residual component removal unit 13 is a control target harmonic component (ie, 5th harmonic component and 7th harmonic component) that is a control target within the control target harmonic component after transformation output through the coordinate transformation unit 12. The harmonic component to be controlled after conversion may be passed through the LPF 13a so that other harmonic components do not exist. Through this, the residual component removing unit 13 uses the LPF 13a to control the control target harmonic component (that is, the fifth order) as the residual component (residual AC component) included in the control target harmonic component after conversion. It can be removed so that other harmonic components other than the harmonic component and the 7th harmonic component) do not remain.

In other words, the residual component removal unit 13 removes the residual components other than the harmonic components converted into DC values by the coordinate conversion unit 12 (ie, the harmonic components to be controlled after conversion), the LPF 13a. After the transformation output from the coordinate transformation unit 12, it may be applied to the harmonic component to be controlled.

The low-pass filter used in the general harmonic controller 15 to which the band-pass filter (BPF, 11a) is not applied is used with a very small cut-off frequency in order to remove a fundamental component having a large magnitude compared to the harmonic component. As such, when the cut-off frequency of the filter is small, the time constant of the filter has a large value in inverse proportion to this. As such, the filter having a large time constant has a very slow response characteristic, and when applied to a system (grid-connected inverter system), control performance is deteriorated.

On the other hand, according to the device 10 performing the selective harmonic control technique to which the bandpass filter (BPF, 11a) is applied, the device 10 removes most of the fundamental wave component having a large magnitude compared to the harmonic component in the low pass. Since the input is made to the filter (LPF, 13a), the use of a low-pass filter having an excessively small cut-off frequency in the present device 10 is not forced.

In other words, the apparatus 10 allows the value (ie, only the 5th harmonic component and the 7th harmonic component) to be input to the low-pass filter (LPF, 13a) from which most of the fundamental wave component of a relatively large size has been removed. It is possible to eliminate the need for the use of a low-pass filter having an excessively small cut-off frequency as in the technology, and as a result, factors that deteriorate the control performance of the system (grid-connected inverter system) (that is, create distortion of the grid voltage) factors) can be reduced.

After the control target harmonic component from which the residual component is removed is outputted through the residual component removal unit 13, the harmonic compensator 14 compensates the control target harmonic component from which the residual component is removed for the proportional integral controller 14a. For this purpose, the control target harmonic component from which the residual component has been removed may be transmitted to the input of the proportional integral controller 14a. Through this, the harmonic compensator 14 may compensate the control target harmonic component selectively extracted by the extraction unit 11 by the proportional integral controller 14a in a state in which the residual component has been removed.

The harmonic compensating unit 14 may include a proportional integral controller 14a and a compensating unit 14b, and the compensating unit 14b may include a reference voltage coordinate converting unit 14b'.

The proportional integral controller 14a may generate a command voltage in consideration of the control target harmonic component from which the residual component output from the residual component removal unit 13 is removed. In this case, the proportional integral controller 14a may generate a command voltage (voltage command) for controlling the difference between the command value and the actual harmonic component to be zero.

The compensator 14b may compensate the command voltage generated by the proportional integral controller 14a to the output of the fundamental wave current controller 30 . The command voltage coordinate conversion unit 14b' in the compensator 14b inversely converts the command voltage generated by the proportional integral controller 14a into an AC value on the dq-axis stationary coordinate system, and then compensates the output of the fundamental wave current controller 30 can be provided as much as possible. Harmonics included in the output current of the grid-connected inverter 22 included in the device 10 may be reduced by the compensation unit 14b. A more detailed description is as follows.

,

)는 직류 값에 근사하게 됨에 따라, 비례 적분 제어기(14a)를 이용하여 오차가 거의 없이 지령 값으로 제어할 수 있다.A signal that has passed through the band-pass filter 11a, the coordinate transformation by the coordinate transformation unit 12, and the low-pass filter 13a (that is, the control target harmonic component from which the residual component outputted through the residual component removal unit is removed) (

,

) is approximated to the DC value, so it can be controlled as a command value with little error by using the proportional integral controller 14a.

,

)과 실제 고조파 성분(

,

)의 차이를 비례 적분 제어기(14a)로 입력시킬 수 있다.The harmonic compensator 14 outputs the control target harmonic component from which the residual component is removed from the residual component removal unit 13, and then receives all of the dq-axis current commands of each harmonic component with respect to the control target harmonic component from which the residual component is removed. set to 0, and the reference value (

,

) and the actual harmonic component (

,

) can be input to the proportional integral controller 14a.

The proportional integral controller 14a generates a reference voltage for controlling the difference between the reference value and the actual harmonic component to be zero. The command voltage generated by the proportional integral controller 14a may be inversely converted into an AC value on the dq-axis stationary coordinate system by the command voltage coordinate conversion unit 14b ′, and then compensated for the output of the fundamental wave current controller 30 .

,

)을 기본파 전류 제어기(30)의 출력에 보상되도록 제공할 수 있다.In other words, the command voltage coordinate conversion unit 14b' may receive the command voltage generated by the proportional integral controller 14a as an input, and inversely convert the input command voltage into an AC value on the dq-axis stationary coordinate system. Thereafter, the command voltage coordinate conversion unit 14b 'is the inversely transformed voltage value (

,

) may be provided to compensate the output of the fundamental wave current controller 30 .

The command voltage coordinate conversion unit 14b ′ converts the command voltage generated by the proportional integral controller 14a so that the output voltage, which is the output value of the device 10 (selective harmonic controller), is expressed on the dq-axis stationary coordinate system. A coordinate transformation process of inverse transformation into an alternating value on the dq-axis stationary coordinate system may be performed. In this case, the command voltage coordinate conversion unit 14b ′ may perform this coordinate conversion process in consideration of the respective phase order as in Equation 12 and Equation 13 described above.

Representatively, the coordinate conversion equations of the reference voltage (voltage reference) for reducing the 5th harmonic of the inverse component and the 7th harmonic of the normal component can be expressed as Equations 14 and 15 below, respectively. In other words, the command voltage coordinate conversion unit 14b ′ converts the command voltage generated by the proportional integral controller 14a into an AC value on the dq-axis stationary coordinate system (inverse conversion) using Equations 14 and 15 below. have.

[Equation 14]

[Equation 15]

Thereafter, the output value output through the device 10 (that is, the coordinate-converted voltage value by the command voltage coordinate conversion unit) is the output value output through the fundamental wave current controller 4 (in other words, the fundamental wave current command voltage for generation) and may be provided as an input of the grid-connected inverter 22 . Through this, the grid-connected inverter 22 may output an output current having an improved THD based on the input value.

That is, the grid-connected inverter 22 can improve the THD of the output current by using (comparing) the corresponding command voltage (a value generated by the proportional integral controller and coordinate-converted) and the command voltage for generating the fundamental wave current. .

The grid-connected inverter 22 may perform a switching operation to output an output current with reduced (reduced, removed, erased) harmonic components according to the comparison of the corresponding command voltage and the command voltage for generating the fundamental wave current. .

The device 10 uses a band pass filter (BPF) 11a to selectively control and compensate only specific harmonics in order to improve the performance of the grid-connected inverter system 100. suggest The device 10 extracts a specific harmonic component (in particular, the 5th harmonic component and the 7th harmonic component) using the BPF 11a, and then reduces harmonics by allowing only the extracted corresponding harmonic component to be input to the proportional integration controller 14a Performance can be increased (improved).

By providing a selective harmonic reduction technique using the bandpass filter 11a, the apparatus 10 can effectively reduce the harmonic component of the grid current only through a relatively simple calculation process.

The present application proposes a selective harmonic reduction technique using the band-pass filter 11a through the device 10 . The apparatus 10 can selectively reduce specific harmonic components by using a band pass filter (BPF) without the need for additional hardware configuration and complicated control techniques.

Also, the device 10 may selectively extract harmonic component currents from the output current of the grid-connected inverter 22 using a digital filter implemented in software. This device 10 can maintain the THD of the output current to 5% or less even under a distorted system voltage condition through the design of an individual controller according to the characteristics of the extracted corresponding harmonic component current.

Accordingly, the band-pass filter BPF 11a applied to the apparatus 10 may not be a passive filter, but may be a digital filter implemented by software. This device 10 can solve the problem of increasing the price and the overall system volume when using the passive filter through the application of the digital filter.

The device 10 is the output current of the power conversion system (ie, the grid-connected inverter system, 100) through the selective harmonic reduction technique using the BPF (11a) to improve the performance of the grid-connected inverter 22 It is possible to effectively reduce (reduce) the harmonics included in the

Since the device 10 uses only the harmonic component extracted through the BPF 11a as an input of each harmonic current controller 15, excellent harmonic control performance can be provided.

In addition, the apparatus 10 can selectively reduce harmonics without additional hardware, thereby providing competitiveness in terms of production cost, weight, and volume of the system.

Since the apparatus 10 uses a proportional integral controller, there is an effect superior in stability compared to a resonant controller having a very large gain for a signal of a specific frequency band. In addition, the apparatus 10 can more effectively reduce (reduce) harmonic components in the output current generated by the grid-connected inverter 3 through the extraction of harmonic components using the band-pass filter 11a.

The present application proposes a selective harmonic reduction (compensation) technique for improving the output current THD of the grid-connected inverter 22 through the device 10 .

In the grid-connected inverter, when the grid voltage is distorted by a non-linear load, the output current THD may be distorted. This distorted output current can not only reduce the efficiency of the overall system, but also promote premature aging of system components.

In order to solve this problem, the device 10 uses components (ie, 5th order, 5th order) that increase the THD of the output current of the grid-connected inverter 22 through selective harmonic reduction (compensation) using a bandpass filter. 7th harmonic component), as a result, the performance of the grid-connected inverter 22 and the present system 100 can be improved.

Conventional harmonic reduction technique reduces harmonic components by designing an additional filter at the output stage of the grid-connected inverter or using a complex control technique.

On the other hand, the harmonic reduction technique proposed in the present application (ie, the harmonic reduction technique by this device) uses a bandpass filter without additional hardware components to extract only a specific harmonic current (only a specific harmonic current component) to extract the corresponding component. The THD of the output current can be improved by reducing it.

That is, the device 10 extracts only a specific harmonic component to be controlled using a bandpass filter (that is, the 5th and 7th harmonic components that are harmonic components to be reduced among harmonic components included in the output current) And, by allowing only the extracted harmonic component to be input to the proportional integration controller 14a in a state that approximates the DC value, the specific harmonic component can be accurately controlled with little error by the proportional integration controller 14a. . The command voltage generated by the proportional integral controller 14a is a specific harmonic component (5th and 7th harmonic components) among harmonic components included in the output current in the output current of the grid-connected inverter 22 It can be applied (applied) as a control signal to reduce (clear, reduce).

That is, in the device 10, with respect to the command voltage for generating a fundamental wave current, the command voltage generated by the proportional integral controller 14a in the device 10 corresponds to the harmonics (5th and 7th harmonics) in the output current. ) is added little by little as a command voltage generated to cancel, so that harmonics in the output current can be effectively reduced.

The device 10 may improve the THD of the output current by extracting only a specific harmonic current by using a band-pass filter and reducing a corresponding component. This device 10 can be effectively applied to improve the performance of the power conversion device for the grid-connected inverter that is currently in the spotlight. In addition, since the device 10 does not require additional hardware configuration and complicated calculation processes, price competitiveness can be increased in system design and production.

The device 10 can be effectively applied to the field of grid-connected distributed power generation systems that must meet the IEEE-1547 standard in which the THD of the output current must be maintained within 5%. In addition, the apparatus 10 can improve the grid-side current control performance of the electric motor driving system using the Back-to-Back system by reducing EMI and common voltage noise.

When the output current of the grid-connected inverter is distorted due to harmonics, the device 10 can effectively improve the THD of the output current by reducing harmonics without additional hardware and complicated calculation process.

The device 10 can be applied to grid-connected inverters for renewable energy and energy storage devices, and can be applied to a motor drive system using a Back-to-Back system with high utilization.

The existing harmonic reduction technique for improving the output current THD of the power converter has a problem in that it increases the volume and weight as well as the manufacturing cost of the entire system due to the additional hardware configuration, and involves a complicated calculation process. On the other hand, the present application can selectively reduce harmonics through a relatively simple calculation process without a complicated control method or additional hardware, thereby contributing to improving the price competitiveness of the system.

The device 10 may contribute to performance improvement and competitiveness enhancement of a power conversion device used in solar power generation and energy storage devices.

Grid-connected inverters are widely used as inverters for renewable energy sources and energy storage devices. The output current of the grid-connected inverter must satisfy THD within 5% according to the IEEE-1547 standard.

Compared to the two-level inverter, the multi-level inverter has a large number of switch elements and the control technique is complicated, but the THD of the output current can be improved. However, under the condition that the grid voltage is already distorted, there is a problem that it is difficult to satisfy the THD of the output current to the IEEE-1547 standard only by changing the topology.

Accordingly, even when the output current of the grid-connected inverter is distorted due to the harmonic component, the device 10 can selectively reduce a specific harmonic component by using a bandpass filter without a complicated calculation process. Unlike the conventional harmonic reduction technique that requires additional hardware configuration or a complex control technique, the present application can reduce harmonic currents other than the fundamental wave only through a simple calculation process.

The selective harmonic reduction technique proposed herein (ie, the selective harmonic reduction/compensation technique by the device) can improve the THD of the output current of a multi-level converter including a 2-level inverter, and thus the grid-connected inverter currently attracting attention It can contribute to increasing the competitiveness of the system.

The device 10 may improve the output current THD by using a selective harmonic reduction technique using a bandpass filter when the output current of the grid-connected inverter is distorted.

After selectively extracting only harmonic components of a specific order, the apparatus 10 may generate a voltage command for reducing harmonics for each order by using a low-pass filter and a proportional integral controller. Based on the reference voltage generated by the proportional integral controller 14a in the device 10, the grid-connected inverter 22 provides the corresponding voltage reference (ie, the reference voltage generated by the proportional integral controller) and the fundamental wave current. It is possible to improve the THD of the output current (ie, output the output current with the THD improved) by using the reference voltage for generation.

Hereinafter, an operation flow of the present application will be briefly reviewed based on the details described above.

7 is an operation flowchart for a selective harmonic compensation method for a grid-connected inverter according to an embodiment of the present application.

The selective harmonic compensation method for the grid-connected inverter shown in FIG. 7 may be performed by the apparatus 10 described above. Accordingly, even if omitted below, the description of the device 10 may be equally applied to the description of the selective harmonic compensation method for the grid-connected inverter.

Referring to FIG. 7 , in step S11, the extraction unit may extract a control target harmonic component to be controlled from the current of the three-phase system.

In this case, in step S11, the extraction unit may extract the control target harmonic component using a band-pass filter disposed between the three-phase system and the coordinate conversion unit.

In addition, in step S11, the extraction unit may selectively extract any one harmonic component from among a plurality of harmonic components included in the current of the three-phase system as a control target harmonic component.

Here, any one harmonic component to be extracted may be a 5th harmonic component or a 7th harmonic component.

Next, in step S12, the coordinate converter may convert the control target harmonic component extracted in step S11 into a DC value, and output the control target harmonic component after conversion into a DC value.

Next, in step S13 , the residual component removing unit may output the control target harmonic component from which the residual component is removed through the removal of residual components other than the control target harmonic component after conversion output in step S12 .

In addition, in step S13, the residual component removing unit removes the residual component by passing the control target harmonic component output in step S12 through a low-pass filter disposed between the coordinate conversion unit and the proportional integral controller. can

Next, in step S14, the harmonic compensator receives the control target harmonic component from which the residual component is removed from the residual component output in step S13 as an input of the proportional integration controller in order to compensate the control target harmonic component from which the residual component is removed. can transmit

In addition, step S14 may include generating the command voltage in consideration of the control target harmonic component from which the residual component is removed by the proportional integral controller. In addition, step S14 may include, after generating the command voltage, compensating the generated command voltage to the output of the fundamental wave current controller by the compensator.

In this case, the compensating may include inversely converting the command voltage generated in the step of generating the command voltage by the command voltage coordinate converter into an AC value on the dq-axis stationary coordinate system. Through this, in the compensating step, the command voltage inversely transformed by the command voltage coordinate converting unit may be compensated to the output of the fundamental wave current controller.

By compensating, harmonics included in the output current of the grid-connected inverter may be reduced.

In the above description, steps S11 to S14 may be further divided into additional steps or combined into fewer steps, according to an embodiment of the present application. In addition, some steps may be omitted as necessary, and the order between steps may be changed.

The selective harmonic compensation method for a grid-connected inverter according to an embodiment of the present application may be implemented in the form of a program command that can be executed through various computer means and recorded in a computer-readable medium. The computer-readable medium may include program instructions, data files, data structures, etc. alone or in combination. The program instructions recorded on the medium may be specially designed and configured for the present invention, or may be known and available to those skilled in the art of computer software. Examples of the computer-readable recording medium include magnetic media such as hard disks, floppy disks and magnetic tapes, optical media such as CD-ROMs and DVDs, and magnetic such as floppy disks. - includes magneto-optical media, and hardware devices specially configured to store and execute program instructions, such as ROM, RAM, flash memory, and the like. Examples of program instructions include not only machine language codes such as those generated by a compiler, but also high-level language codes that can be executed by a computer using an interpreter or the like. The hardware devices described above may be configured to operate as one or more software modules to perform the operations of the present invention, and vice versa.

In addition, the selective harmonic compensation method for the above-described grid-connected inverter may be implemented in the form of a computer program or application executed by a computer stored in a recording medium.

The above description of the present application is for illustration, and those of ordinary skill in the art to which the present application pertains will understand that it can be easily modified into other specific forms without changing the technical spirit or essential features of the present application. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. For example, each component described as a single type may be implemented in a dispersed form, and likewise components described as distributed may be implemented in a combined form.

The scope of the present application is indicated by the following claims rather than the above detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present application.

100: grid-connected inverter system
10: Optional harmonic compensation device for grid-tied inverter
11: Extraction part
12: coordinate conversion unit
13: Residual component removal unit
14: harmonic compensation unit
14a: proportional integral controller
14b: Compensation Department
14b': command voltage coordinate conversion unit

Claims (11)

An optional harmonic compensation device for a grid-connected inverter, comprising:
an extraction unit for extracting a control target harmonic component to be controlled from the current of the three-phase system;
a coordinate conversion unit that converts the extracted harmonic component to be controlled into a DC value, and outputs a harmonic component to be controlled after conversion into a DC value;
a residual component removing unit for outputting a control target harmonic component from which the residual component is removed through the removal of residual components other than the control target harmonic component after the conversion; and
a harmonic compensator transmitting the control target harmonic component from which the residual component is removed to an input of the proportional integral controller for compensation of the control target harmonic component from which the residual component is removed to the proportional integral controller;
including,
The extraction unit,
The selective harmonic compensating apparatus of claim 1, wherein the control target harmonic component is extracted using a band-pass filter disposed between the three-phase system and the coordinate conversion unit. delete According to claim 1,
The extraction unit,
Selective harmonic compensation device that selectively extracts any one harmonic component among a plurality of harmonic components included in the current of the three-phase system as the control target harmonic component. 4. The method of claim 3,
Any one of the harmonic components is a 5th harmonic component or a 7th harmonic component, the selective harmonic compensation device. According to claim 1,
The residual component removal unit,
The selective harmonic compensation apparatus of claim 1, wherein the residual component is removed by passing the control target harmonic component after the transformation through a low-pass filter disposed between the coordinate transformation unit and the proportional integral controller. According to claim 1,
The harmonic compensator,
the proportional integral controller for generating a command voltage in consideration of a control target harmonic component from which the residual component has been removed; and
a compensating unit compensating the generated command voltage to the output of the fundamental wave current controller;
A selective harmonic compensation device comprising a. 7. The method of claim 6,
The compensation unit,
and a command voltage coordinate converter for inversely converting the generated command voltage into an AC value on a dq-axis stationary coordinate system. 8. The method of claim 7,
The harmonics included in the output current of the grid-connected inverter by the compensator is reduced, the selective harmonic compensation device. A selective harmonic compensation method for a grid-connected inverter, comprising:
(a) in the extraction unit, extracting a control target harmonic component to be controlled from the current of the three-phase system;
(b) converting the extracted harmonic component to be controlled into a DC value, and outputting the harmonic component to be controlled after conversion into a DC value;
(c) outputting, in the residual component removing unit, the control target harmonic component from which the residual component is removed through the removal of residual components other than the control target harmonic component after the conversion; and
(d) in the harmonic compensator, for compensating the control target harmonic component from which the residual component is removed to the proportional integral controller, transferring the control target harmonic component from which the residual component is removed to the input of the proportional integral controller;
including,
The step (a) is,
The selective harmonic compensation method of extracting the control target harmonic component using a band-pass filter disposed between the three-phase system and the coordinate converter. delete 10. The method of claim 9,
Step (d) is,
(d1) generating, in the proportional integral controller, a reference voltage in consideration of a control target harmonic component from which the residual component is removed; and
(d2) compensating, in the compensating unit, the generated command voltage to the output of the fundamental wave current controller;
Including, a selective harmonic compensation method.

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