KR20110115945A - Apparatus and method for controling power quality of power generation system - Google Patents

Abstract

The present invention relates to an apparatus and method for controlling power quality of a power generation system. The apparatus for controlling power quality of the present invention includes a power system including a DC / AC inverter for converting a DC voltage into an AC voltage to supply inverter current to a system. An applied power quality control apparatus, comprising: a system voltage phase follower for generating a system signal having a phase of the system voltage; A fundamental wave extracting unit extracting a magnitude of a fundamental wave of a load current flowing into a nonlinear load connected between the DC / AC inverter and a grid; A first calculating unit which subtracts a preset current compensation value from the magnitude of the fundamental wave from the fundamental wave extracting unit; And a second calculator configured to generate the inverter current command value for the DC / AC inverter using the output value of the first calculator, the system signal from the grid voltage phase follower, and the load current.

Description

Power quality control device and method of power generation system {APPARATUS AND METHOD FOR CONTROLING POWER QUALITY OF POWER GENERATION SYSTEM}

The present invention relates to a power quality control apparatus and method that can be applied to a power conditioning system, such as a solar power generation system, and in particular, by performing an active power filter function regardless of whether power generation is performed, harmonics caused by nonlinear loads An apparatus and method for controlling power quality of a power generation system capable of compensating for distortion and thus improving power quality.

Typically, electronic products such as computers, communication equipment, medical equipment, military equipment, and the like are load-sensitive loads. Since these industrial, commercial and military loads are interconnected in an automated process, the power system to which these loads are connected is very sensitive to power quality (PQ).

In general, a product having a nonlinear load characteristic that adversely affects the power quality of a power system includes an adjustable speed drive (ASD), a switching mode power supply, an arc furnace, and the like. Products.

On the other hand, power quality is divided into voltage quality and current quality. First, voltage quality includes voltage sags / swells, under / over voltage, interruption, and noise. It handles the status of other factors (DC Offset, Harmonics, lnterharmonics, Notching), Voltage Imbalance, Voltage Fluctuation and Flicker (Voltage Fuctuation / Flicker). Conditions that degrade this voltage quality can cause malfunction or failure of electrical and electronic equipment. This condition can cause insulation breakdown of various equipment, loss of equipment, deletion of memory data of controller, change of lighting brightness, shutdown or reset of equipment, overheating of motor, transformer and wiring.

Next, as the nonlinear load increases, the current quality is treated with power failure problems such as voltage distortion due to current distortion.

On the other hand, in the conventional power generation system, there is no countermeasure for compensating for distortion generated when the nonlinear load is connected. Accordingly, when the nonlinear load is connected, a pulse current flows through the AC power system and the pulse impedance is caused by the line impedance of the distribution network. There is a problem that causes tripping or malfunction of a sensitive load by generating a voltage drop of.

Accordingly, the distribution transformer has an increase in operating loss, thereby overheating the transformer and shortening its lifespan, or the fifth and seventh voltage harmonics cause resonance in neighboring distribution systems, thereby destroying the distribution transformer.

An object of the present invention is to solve the above problems of the prior art, the present invention, by performing an active power filter (active power filter) function regardless of whether or not to perform the power generation, to compensate for harmonic distortion caused by non-linear load The present invention provides a power quality control apparatus and method of a power generation system that can improve power quality.

The first technical aspect of the present invention for solving the above problems of the present invention, power quality control applied to a power system including a DC / AC inverter for supplying an inverter current to the system by converting a DC voltage into an AC voltage An apparatus comprising: a system voltage phase follower for generating a system signal having a phase of said system voltage; A fundamental wave extracting unit extracting a magnitude of a fundamental wave of a load current flowing into a nonlinear load connected between the DC / AC inverter and a grid; A first calculating unit which subtracts a preset current compensation value from the magnitude of the fundamental wave from the fundamental wave extracting unit; And a second calculator configured to generate the inverter current command value for the DC / AC inverter using the output value of the first calculator, the system signal from the grid voltage phase follower, and the load current. Suggest.

In addition, a second technical aspect of the present invention is a power quality control device applied to a power system including a DC / AC inverter for supplying an inverter current to a system by converting a DC voltage into an AC voltage, the system voltage of the A grid voltage phase follower for generating a grid signal having a phase; A fundamental wave extracting unit extracting a magnitude of a fundamental wave of a load current flowing into a nonlinear load connected between the DC / AC inverter and a grid; A first calculating unit which subtracts a preset current compensation value from the magnitude of the fundamental wave from the fundamental wave extracting unit; A second calculator configured to generate the inverter current command value for the DC / AC inverter using the output value of the first calculator, the system signal from the grid voltage phase follower, and the load current; And a control unit for controlling the DC / AC inverter based on the inverter current command value from the second calculating unit.

In the first and second technical aspects of the present invention, the system voltage phase follower includes: a phase detector for detecting a phase of the system voltage; And a signal generator based on the phase from the phase detector and having the same phase as the phase of the system voltage and generating the system signal having a predetermined unit size.

The fundamental wave extracting unit is implemented by one of a Goertzel algorithm using a Discrete Fourier Transform (DFT), a Fast Fourier Transform (FFT), and a Discrete Fourier Transform, and extracts the fundamental wave from the load current. It is characterized by detecting the size of.

The second calculator may include: a multiplier that multiplies an output value of the first calculator by a system signal from the system voltage phase follower; And an adder configured to generate the inverter current command value by subtracting the output signal of the multiplier from the load current.

In addition, a third technical aspect of the present invention is a power quality control method applied to a power system including a DC / AC inverter for supplying an inverter current to a system by converting a DC voltage into an alternating voltage, wherein A system signal generation step of generating a system signal having a phase; A fundamental wave extraction step of extracting a magnitude of a fundamental wave of a load current flowing into a nonlinear load connected between the DC / AC inverter and a grid; And a first calculating step of subtracting a predetermined current compensation value from the magnitude of the fundamental wave from the fundamental wave extracting step. And a second calculation step of generating the inverter current command value for the DC / AC inverter using the output value of the first calculation step, the system signal from the system signal generation step, and the load current. Suggest a method.

The system signal generating step may include: a phase detecting step of detecting a phase of the system voltage; And a signal generation step of generating the system signal having a phase equal to the phase of the system voltage and having a predetermined unit size based on the phase from the phase detection step.

The fundamental wave extracting step is implemented by one of the Goertzel algorithms using the Discrete Fourier Transform (DFT), Fast Fourier Transform (FFT) and Discrete Fourier Transform, extracting the fundamental wave from the load current, It is characterized by detecting the size of the wave.

The second operation step may include: a multiplication step of multiplying an output value of the first operation step by a system signal from the system signal generation step; And an addition step of generating the inverter current command value by subtracting the output signal of the multiplication step from the load current.

Subsequently, the power quality control method of the power generation system of the present invention further includes a control step of controlling the DC / AC inverter based on the inverter current command value from the second calculation step.

The control step includes: a current control step of detecting an inverter current error between the inverter current command value from the second calculation step and the inverter current; And a PWM control step of providing a PWM inverter signal for controlling the DC / AC inverter based on the inverter current error from the current control step.

In addition, the power quality control method of the power generation system of the present invention, the voltage control step of obtaining a voltage error between the input DC voltage and the predetermined DC voltage of the DC / AC inverter, and providing the voltage error as the current compensation value It further comprises.

According to the present invention, it is possible to compensate for harmonic distortion caused by nonlinear load by performing an active power filter function regardless of whether power generation is performed, thereby improving power quality.

1 is a block diagram of a power quality control apparatus of a power generation system according to the present invention;
2 is a block diagram of a system voltage phase follower of the present invention;
3 is an exemplary view of a fundamental wave extracting unit implemented by a Goertzel filter of the present invention.
4 is a timing waveform diagram of the Goertzel filter of FIG. 3;
5 is a zero and pole position diagram in the Z-region for the Goertzel filter of FIG.
6 is a frequency magnitude response graph for the Goertzel filter of FIG.
7 is a flowchart of a power quality control method of a power generation system according to the present invention;
8 is a flow chart of the system signal generation step of the present invention.
9 is a flow chart of the fundamental wave extraction step of the present invention.
10 is a flowchart of a second operation step of the present invention.
11 is a main waveform diagram when connecting a non-linear load to an existing power generation system.
12 is a graph of a frequency component analysis result of each current in the existing power generation system.
13 is a main waveform diagram of a power generation system of the present invention.
14 is a graph of a frequency component analysis result of each current of the power generation system of the present invention.
15 is a main waveform diagram of a power generation system of the present invention;
16 is a graph of a frequency component analysis result of each current of the power generation system of the present invention.
17 is a flow chart of the FFT operation.
Fig. 18 is a flowchart of frequency sound operation of the FFT.
19 is a block diagram of a fundamental wave extracting unit using a DFT.
20 is a block diagram of a fundamental wave extracting unit using an FFT.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

The present invention is not limited to the embodiments described, and the embodiments of the present invention are used to assist in understanding the technical spirit of the present invention. In the drawings referred to in the present invention, components having substantially the same configuration and function will use the same reference numerals.

1 is a block diagram of a power quality control apparatus of a power generation system according to the present invention.

Referring to FIG. 1, a power generation system to which the power quality control apparatus of the present invention is applied includes a DC / AC inverter 30 which converts a DC voltage into an AC voltage and supplies an inverter current Iinv to the system 40. can do.

In addition, the power generation system to which the power quality control apparatus of the present invention is applied includes a DC which boosts the DC voltage Vpv from the solar cell 10 to a predetermined voltage level and follows the maximum power point emitted from the solar cell 10. / DC converter 20 may be included.

The power quality control apparatus of the present invention applicable to the power generation system includes a system voltage phase follower 100 for generating a system signal Sin (wt) having a phase of the system voltage Vg, and the DC / A fundamental wave extracting unit 200 for extracting a magnitude (│IL (1) │) of the fundamental wave of the load current IL flowing into the nonlinear load 50 connected between the AC inverter 30 and the grid 40); The first calculator 300 subtracts a predetermined current compensation value ICV from the magnitude of the fundamental wave from the fundamental wave extracting unit 200 (IL (1)), and the first calculating unit 300. A second calculator configured to generate an inverter current command value I * inv for the DC / AC inverter 30 by using the output value of the output signal, the system signal from the system voltage phase follower 100, and the load current IL; 400).

2 is a block diagram of a system voltage phase follower according to the present invention.

Referring to FIG. 2, the grid voltage phase follower 100 includes a phase detector 110 for detecting a phase PH of the grid voltage Vg, and a phase PH from the phase detector 110. The signal generator 120 may include a signal generator 120 having a phase equal to that of the grid voltage Vg and generating the grid signal Sin (wt) having a predetermined unit size.

Referring to FIG. 1, the fundamental wave extracting unit 200 may be implemented as one of a Goertzel algorithm using a Discrete Fourier Transform (DFT), a Fast Fourier Transform (FFT), and a Discrete Fourier Transform. A fundamental wave may be extracted from the load current IL, and the magnitude of the extracted fundamental wave (IL (1)) may be extracted.

3 is an exemplary view of a fundamental wave extracting unit implemented with a Goertzel filter.

Referring to FIG. 3, the fundamental wave extracting unit 200 is configured to have a transfer function H (z) including a feedback part (FBP) and a feed forward part (FFP). Can be.

More specifically, the fundamental wave extracting unit 200 may include a predetermined feedback part (FBP) 210, a predetermined feed forward part (FFP) 220, and a fundamental wave (IL (1) of the load current IL). It may include a calculation unit 230 for calculating the size of).

The transfer function H (z) may be expressed by Equation 1 below. Equation 2 shows a relationship between a discrete frequency k to be extracted, a sampling frequency fs, and a frequency to be extracted. Equations 3, 4, and 5 show Equations 1 below as difference equations.

In Equations 1 to 5, x [n] is an input signal sample, v [n] is an intermediate result from the current calculation, v [n-1] is an output from a previous sampling, v [n-2] Indicates where the output from the calculation was stored before the two samplings. Y [n] represents the result of the Goertzel filter.

As described above, the following Equation 3, Equation 4 and Equation 5 may be represented as a block diagram as shown in FIG.

4 is a timing waveform diagram of the Goertzel filter of FIG. 3. In the timing waveform of FIG. 4, if N samplings are performed during one period of the grid voltage Vg, a feedback part is calculated for each sampling, and the feed forward part is calculated after the Nth feedback number calculation is completed, the frequency of the frequency to be extracted is calculated. I can calculate the size

And, as shown in Equation 1, the zero point and the pole of the Goertzel filter cancel the pole and only one pole exists, so that only the signal corresponding to the discrete frequency k corresponding to this pole passes through the Goertzel filter. do.

5 is a zero and pole position diagram in the Z-region for the Goertzel filter of FIG. 3, and FIG. 6 is a frequency magnitude response graph for the Goertzel filter of FIG.

5 and 6, the magnitude of the frequency that may be extracted by the Goertzel filter may be obtained as in Equation 6 below.

Referring to FIG. 1, the second calculator 400 may include a multiplier 410 multiplying an output value of the first calculator 300 and a system signal from the system voltage phase follower 100, and the multiplication unit. And an adder 420 for generating the inverter current command value I * inv by subtracting the output signal of the unit 410 from the load current IL.

In addition, the power quality control device of the present invention includes a control unit 500 for controlling the DC / AC inverter 30 based on the inverter current command value I * inv from the second calculating unit 400, and the The voltage controller 600 which obtains a voltage error between the input DC voltage Vdc of the DC / AC inverter 30 and the predetermined DC voltage command value V * dc and provides the voltage error as the current compensation value ICV. It may further include. In this case, the voltage controller 600 controls a DC-LINK voltage.

The controller 500 includes a current controller 510 for detecting an inverter current error between the inverter current command value I * inv and the inverter current Iinv from the second calculator 400, and the current controller 510. And a PWM controller 520 for providing a PWM inverter signal PWMmin for controlling the DC / AC inverter 30 based on the inverter current error.

7 is a flowchart of a power quality control method of a power generation system according to the present invention.

1 and 7, the power generation system to which the power quality control method of the present invention is applied includes a DC / AC inverter 30 which converts a DC voltage into an AC voltage and supplies an inverter current Iinv to the system 40. ) May be included.

In addition, the power generation system to which the power quality control method of the present invention is applied includes a DC which boosts the DC voltage Vpv from the solar cell 00 to a predetermined voltage level and follows the maximum power point emitted from the solar cell 10. / DC converter 20 may be included.

The power quality control method of the present invention applied to the power system includes a system signal generation step S100 for generating a system signal Sin (wt) having a phase of the system voltage Vg, and the DC / AC inverter. A fundamental wave extraction step 200 for extracting the magnitude (│IL (1) │) of the fundamental wave of the load current IL flowing into the nonlinear load 50 connected between the system 30 and the system 40; A first calculation step S300 for subtracting a predetermined current compensation value ICV from the magnitude of the fundamental wave from the fundamental wave extraction step S200 (│IL (1) │), and the first calculation step (S300) A second calculation step of generating an inverter current command value I * inv for the DC / AC inverter 30 by using the output value of the system signal and the system signal from the system signal generating step S100 and the load current IL; S400) may be included.

8 is a flowchart of a system signal generation step of the present invention.

Referring to FIG. 8, the system signal generation step S100 may include a phase detection step S110 for detecting a phase PH of the system voltage Vg, and a phase PH from the phase detection step S110. ) May include a signal generation step (S120) having the same phase as that of the grid voltage (Vg) and generating the grid signal Sin (wt) having a predetermined unit size.

9 is a flow chart of the fundamental wave extraction step of the present invention.

Referring to FIG. 9, the fundamental wave extracting step S200 may be implemented by a Goertzel algorithm using a discrete Fourier transform (DFT), extracting the fundamental wave from the load current IL, and extracting the fundamental wave. It can be made to extract the size of | (IL (1) │) (S210).

The fundamental wave extraction step S200 may be performed to have a transfer function H (z) as shown in Equation 1. In this case, the fundamental wave extracting step S200 may be performed to detect the magnitude of the fundamental wave IL (1) of the load current IL.

10 is a flowchart of a second operation step of the present invention.

Referring to FIG. 10, the second operation step S400 may include a multiplication step S410 of multiplying an output value of the first operation step S00 by a system signal from the system signal generation step S100, and And an addition step 420 of generating the inverter current command value I * inv by subtracting the output signal of the multiplication step S410 from the load current IL.

In addition, the power quality control method of the present invention, the control step (S500) for controlling the DC / AC inverter 30 based on the inverter current command value (I * inv) from the second calculation step (S400) and And obtaining a voltage error between the input DC voltage Vdc of the DC / AC inverter 30 and a predetermined DC voltage command value V * dc, and providing the voltage error as the current compensation value ICV. It may include (S600).

The control step (S500), the current control step (S510) for detecting an inverter current error between the inverter current command value (I * inv) and the inverter current (Iinv) from the second calculation step (S400), and the current On the basis of the inverter current error from the control step (S510), it may include a PWM control step 500 for providing a PWM inverter signal (PWMinv) for controlling the DC / AC inverter (30).

11 is a main waveform diagram when a nonlinear load is connected to an existing power generation system in the presence of solar radiation, and FIG.

11 and 12, Vg is a grid voltage, Ig is a grid current, Iload is a load current, Iinv is an inverter current, Vdc is an inverter input voltage, Pg is a grid power, Pload is a load power, and Ppv is a solar cell power.

11 and 12, when the non-linear load is connected, distortion occurs in the grid current and the load current, thereby increasing total harmonic distortion (THD), thereby deteriorating power quality.

13 is a main waveform diagram of the power generation system of the present invention in the presence of solar radiation, Figure 14 is a graph of the frequency component analysis results of each current of the power generation system of the present invention in the presence of solar radiation, Figure 15 is a graph of the power generation system of the present invention in the absence of solar radiation Main waveform diagram. And, Figure 16 is a graph of the frequency component analysis results of each current of the power generation system of the present invention in the absence of solar radiation.

13 to 16, Vg is a grid voltage, Ig is a grid current, Iload is a load current, Iinv is an inverter current, Vdc is an inverter input voltage, Pg is a grid power, Pload is a load power, and Ppv is a solar cell power.

13 to 16, according to the present invention, even when the nonlinear load is connected, distortion does not occur in the system current, and thus power quality is not deteriorated.

17 shows a flow chart of the FFT operation. Fig. 18 shows a flowchart of the frequency noise operation of the FFT.

19 is a block diagram of a fundamental wave extraction unit using a DFT.

Referring to FIG. 19, the fundamental wave extracting unit 200 may include a DFT unit 211 for extracting a fundamental wave from the load current IL, and a magnitude of the fundamental wave extracted from the DFT unit 211. It may include a first calculation unit 212 for detecting the IL (1) |

20 is a block diagram of a fundamental wave extracting unit using an FFT.

Referring to FIG. 20, the fundamental wave extracting unit 200 may include an FFT unit 221 extracting a fundamental wave from the load current IL, and a magnitude of the fundamental wave extracted from the FFT unit 221. It may include a second calculation unit 222 for detecting the IL (1) |

Hereinafter, the operation and effects of the present invention will be described with reference to the accompanying drawings.

First, referring to FIGS. 1 to 6, a power quality control apparatus of a power generation system according to the present invention will be described. In FIG. 1, the power quality control apparatus of the present invention is a direct current voltage from the solar cell 10. Vpv) is boosted to a predetermined voltage level and follows the maximum power point from the solar cell 10 and the DC / DC converter 20 converts a DC voltage into an AC voltage to convert the inverter current (Iinv) into the system 40. It can be applied to a power generation system including a DC / AC inverter 30 for supplying.

Here, the role of the DC / DC converter 20 is to find the maximum power point (MPPT (Maximum Power Point Tracking) algorithm) at the output of the solar cell 10 that changes according to the amount of solar radiation and temperature, and extract the maximum power to DC- It accumulates in the Link stage capacitor. The DC-Link terminal voltage (for example, 380 [V]) is controlled by the DC / AC inverter 30. When the maximum power of the solar cell 10 is extracted from the DC / DC converter 20, the DC-Link stage voltage increases, and the DC / AC inverter 30 stores the energy accumulated in the DC-Link stage. By injecting into the DC-Link stage voltage is controlled while the DC-Link stage voltage is reduced.

The DC / AC inverter 30 converts the voltage from the DC / DC converter 20 into an AC voltage synchronized with the phase of the system and supplies it to the system.

In this case, the system voltage phase follower 100 included in the power quality control apparatus of the present invention generates a system signal Sin (wt) having a phase of the system voltage Vg and provides the system signal Sin (wt) to the second calculator 400. do.

In addition, the fundamental wave extraction unit 200 included in the power quality control apparatus of the present invention, the load current (IL) flowing into the non-linear load 50 connected between the DC / AC inverter 30 and the system 40 The magnitude | size of the fundamental wave of (ISIL (1) |) is extracted, and the magnitude | size of the fundamental wave (│IL (1) |) of the load current IL is provided to the first calculation unit 300.

The first calculation unit 300 subtracts a current compensation value ICV preset from the magnitude of the fundamental wave from the fundamental wave extracting unit 200 (IL (1)), and then calculates a system current command value I * g. ) Is provided, and the system current command value I * g is provided to the second calculation unit 400.

The second calculator 400 is a grid current command value I * g, which is an output value of the first calculator 300, a grid signal sin (wt) from the grid voltage phase follower 100, and the load. The inverter IL command value I * inv for the DC / AC inverter 30 is generated using the current IL.

Meanwhile, the power quality control apparatus of the present invention may further include a controller 500 and a voltage controller 600.

In this case, the controller 500 may control the DC / AC inverter 30 based on the inverter current command value I * inv from the second calculator 400.

For example, when the controller 500 includes the current controller 510 and the PWM controller 520, the current controller 510 may be an inverter current command value I * inv from the second calculator 400. ) And the inverter current error between the inverter current Iinv can be detected. The PWM controller 520 generates a PWM inverter signal PWMmin based on the inverter current error from the current controller 510, and uses the PWM inverter signal PWMmin to generate the PWM / AC inverter 30. Can be controlled.

In addition, the voltage controller 600 obtains a voltage error between the input DC voltage Vdc of the DC / AC inverter 30 and a predetermined DC voltage command value V * dc, and calculates the voltage error as the current compensation value. Provided by (ICV).

Referring to FIG. 2, the grid voltage phase follower 100 may include a phase detector 110 and a signal generator 120.

In this case, the phase detector 110 detects the phase PH of the grid voltage Vg and provides it to the signal generator 120.

The signal generator 120 has the same phase as the phase of the system voltage Vg based on the phase PH from the phase detector 110 and has the predetermined unit size. wt)).

Referring to FIG. 1, the fundamental wave extracting unit 200 may be implemented as one of a Goertzel algorithm using a Discrete Fourier Transform (DFT), a Fast Fourier Transform (FFT), and a Discrete Fourier Transform. The fundamental wave may be extracted from the load current IL, and the magnitude of the extracted fundamental wave (IL (1)) may be detected.

First, a case in which the fundamental wave extracting unit 200 is implemented with a Goertzel algorithm (Goertzel filter) will be described with reference to FIGS. 3 to 6.

As shown in FIG. 3, when the fundamental wave extracting unit 200 is implemented with a Goertzel filter, the fundamental wave extracting unit 200 includes the fundamental wave IL (1) of the detected load current IL. The size of can be detected.

In addition, the Goertzel filter shown in FIG. 3 receives a signal x [t] of discrete time as a digital filter and outputs a discrete frequency spectrum x [k]. Where k denotes a discrete frequency point, which is the only k integer multiple of the fundamental wave frequency ko. Goertzel algorithm applied to the present invention is based on the N-point DFT, it can reduce the amount of calculation than the N-point DFT when detecting the magnitude and phase of the frequency component to extract.

[Equation 1]

[Equation 2]

&Quot; (3) "

&Quot; (4) "

[Equation 5]

&Quot; (6) "

Referring to the timing waveform of the Goertzel filter shown in FIG. 4, the Goertzel filter samples N input signals x [n] during one period of the system voltage Vg, and performs a feedback part FBP for each sampling. After calculating the N-th feedback part (FBP) and calculating the feed forward part (FFP), the magnitude of the frequency to be extracted can be calculated.

5 and 6, the zero of one of the two poles of the Goertzel filter cancels the pole so that only one pole exists, so that only the signal corresponding to the discrete frequency k corresponding to this pole passes through the Goertzel filter. do. The result can be obtained as shown in Equation 6.

Next, as shown in FIGS. 3 and 19, the fundamental wave extracting unit 200 may be implemented by a DFT. In this case, the fundamental wave extracting unit 200 may include a DFT unit 211 and a first unit. The calculation unit 212 may be included.

The DFT unit 211 may extract a fundamental wave from the load current IL, and detect the magnitude of the extracted fundamental wave (IL (1)).

If N discrete signals are given, a Discrete Fouirer Transform (DFT) is defined as in Equation 7 below.

[Equation 7]

On the other hand, a point that moves the unit circle circumference of the complex plane by 1 / N circumference can be defined as Equation 8 below as a twiddle factor.

[Equation 8]

에서, N은 샘플링 총 수, n은 현재 샘플링, k는 이산주파수를 나타낸다.
Here, the rotation factor

Where N is the total number of sampling, n is the current sampling, and k is the discrete frequency.

The discrete Fourier transform for the rotation factor of Equation 8 may be expressed as Equation 8 below.

[Equation 9]

는 입력신호

의 고조파 성분의 크기와 위상을 나타내는 복소수이다.In Equation 9,

Is the input signal

Is a complex number representing the magnitude and phase of the harmonic component of.

Accordingly, Equation 9 may be represented by a real part Re and an imaginary part Im as shown in Equations 10 and 11 below.

[Equation 10]

[Equation 11]

As shown in Equations 10 and 11, the first calculating unit 212 shown in FIG. 19 calculates the magnitudes of the harmonic components extracted using the real part Re and the imaginary part Im. Can be calculated as

[Equation 12]

As described above, the fundamental wave size of the load current to be extracted using the DFT may be extracted through Equation 13.

Next, as shown in FIGS. 3 and 17, 18 and 20, the fundamental wave extracting unit 200 may be implemented as an FFT. In this case, as illustrated in FIG. 20, the fundamental wave The extractor 200 may include an FFT unit 221 and a second calculator 222.

The FFT unit 221 may extract the fundamental wave from the load current IL, and extract the magnitude of the extracted fundamental wave (IL (1)).

이 주어질 때,

의 이산 푸리에 변환(DFT : Discrete Fouirer Transform)은 하기 수학식 13과 같이 정의 될 수 있다.
If, N discrete signals

Given this,

Discrete Fouirer transform (DFT) of may be defined as in Equation 13.

[Equation 13]

Using the Discrete Fourier Transform shown in Equation 13, a point that moves the unit circle circumference of the complex plane by 1 / N circumference is a twiddle factor, which can be defined as Equation 14 below. .

[Equation 14]

는 회전인자이고, N은 샘플링 총 수, n은 현재 샘플링, k는 이산주파수를 나타낸다.
In Equation 14,

Is the rotation factor, N is the total number of sampling, n is the current sampling, and k is the discrete frequency.

The discrete Fourier transform for the rotation factor of Equation 14 may be expressed as Equation 15 below.

[Equation 15]

는 입력신호

의 고조파 성분의 크기와 위상을 나타내는 복소수이다. 따라서, 복소수는 하기 수학식 16 및 17과 같이 실수부(Re)와 허수부(Im)으로 나타낼 수 있다. 이때, 회전인자

는 반복되는 특정 주기성을 가진다.

Is the input signal

Is a complex number representing the magnitude and phase of the harmonic component of. Therefore, the complex number may be represented by a real part Re and an imaginary part Im as shown in Equations 16 and 17. At this time, the rotation factor

Has a specific periodicity that is repeated.

[Equation 16]

For example, odd numbers of the rotation factors may be repeated between odd numbers and even numbers of the rotation factors. Therefore, it can be seen that the FFT can speed up the calculation by reducing the calculations that are repeated in the rotation factor of the DFT. Accordingly, the rotation factor of Equation 16 may be expressed as Equation 17 below.

[Equation 17]

In Equation 17, when n is even, n = 2r, and when n is odd, n = 2r + 1, the following Equations 18, 19 and 20 are given.

Equation 18

[Equation 19]

[Equation 20]

Meanwhile, referring to FIGS. 17 and 18, there are two types of fast forward transforms (FFTs), time-out (Decimation-in-time) and frequency-out (Decimation-in-frequency). It is sorting and frequency subtraction is sorting with respect to frequency, and each calculation amount is equal to each other.

Typically, FFT reduces computation by using what is called a butterfly operation. Figure 17 shows a butterfly operation flow chart of a timed FFT. Referring to FIG. 17, it can be seen that the extracted frequency components are listed in order. 18 shows a butterfly operation flow chart of a frequency noise FFT. Referring to FIG. 18, it can be seen that the components of the time domain are arranged in order. The frequency component extracted in this form may be represented by a real part Re and an imaginary part Im as shown in Equations 21 and 22.

[Equation 21]

[Equation 22]

As shown in Equations 21 and 22, the second calculator 222 shown in FIG. 19 calculates the magnitude and phase of the harmonic components extracted by using the real part Re and the imaginary part Im. It can be calculated as shown in Equation 23.

&Quot; (23) "

As described above, the fundamental wave size of the load current to be extracted using the FFT may be extracted through Equation 23.

를 구하기 위해 복소 계산은 N × N = N² 회의 곱셈과 N(N-1)회의 덧셈이 필요하다. 여기서, N이 커질수록 그 계산은 비약적으로 증가한다. On the other hand, if you calculate the DFT yourself,

The complex computation requires N × N = N ² multiplications and N (N-1) additions. Here, as N increases, the calculation increases remarkably.

Alternatively, the FFT (Nlog ₂ N) / ₂ and multiply if 2log ₂ N conference addition is complete, the calculation.

는 실수부와 허수부로 나뉘어져서 추출하고자 하는 주파수의 크기와 위상을 구할 수 있다.
Especially, in computer calculation, the calculation time depends on the number of multiplications, so the larger the value of N, the more effective the use of FFT than DFT is. This output

Is divided into a real part and an imaginary part to obtain a magnitude and phase of a frequency to be extracted.

Again, referring to FIG. 1, when the second calculator 400 includes a multiplier 410 and an adder 420, the multiplier 410 may include an output value of the first calculator 300. The system signal from the system voltage phase follower 100 is multiplied and provided to the adder 420.

The adder 420 subtracts the output signal of the multiplier 410 from the load current IL to generate the inverter current command value I * inv to generate the current controller 510 of the controller 500. To provide.

Hereinafter, a power quality control method of a power generation system according to the present invention will be described with reference to FIGS. 1 and 7 to 10.

1 and 6, the power generation system to which the power quality control method of the present invention is applied includes boosting the DC voltage Vpv from the solar cell 10 to a predetermined voltage level and outputting the maximum power from the solar cell 10. A point-to-point DC / DC converter 20 and a DC / AC inverter 30 for converting a DC voltage into an AC voltage and supplying an inverter current Iinv to the system 40 may be included.

Referring to the power quality control method of the present invention applied to such a power system, first, the system signal generation step (S100) of the present invention, the system signal (Sin (wt)) having a phase of the system voltage (Vg). It generates and provides to the second operation step (S400).

Next, the fundamental wave extraction step 200 of the present invention, the magnitude of the fundamental wave of the load current (IL) flowing into the non-linear load 50 connected between the DC / AC inverter 30 and the grid 40 (│ IL (1) │) is extracted and provided to the first calculation step S300.

The first operation step S300 may be performed by subtracting the current compensation value ICV preset from the magnitude of the fundamental wave from the fundamental wave extraction step S200 (│IL (1) │). To provide.

The second operation step S400 is performed by using the output value of the first operation step S300, the system signal from the system signal generation step S100, and the load current IL. Generate the inverter current command value I * inv for (30).

In addition, referring to FIG. 7, when the power quality control method of the present invention includes a control step S500 and a voltage control step S600, the control step S500 may include the second calculation step S400. The DC / AC inverter 30 is controlled on the basis of the inverter current command value I * inv from.

In the voltage control step S600, the voltage error between the input DC voltage Vdc of the DC / AC inverter 30 and a predetermined DC voltage command value V * dc is calculated, and the voltage error is compensated for by the current. Provided as a value (ICV).

More specifically, when the control step (S500) includes a current control step (S510) and the PWM control step 500, the current control step (S510), the inverter from the second operation step (S400) The inverter current error between the current command value I * inv and the inverter current Iinv is detected and provided to the PWM control step 500.

The PWM control step 500 provides a PWM inverter signal PWMmin for controlling the DC / AC inverter 30 based on the inverter current error from the current control step S510.

Referring to FIG. 8, when the branch signal generation step S100 includes a detection step S110 and a signal generation step S120, the detection step S110 includes a phase PH of the branch voltage Vg. Is detected and provided to the signal generation step (S120).

The signal generation step S120 may be based on the phase PH from the phase detection step S110, and have the same phase as that of the grid voltage Vg and have the predetermined unit size. Sin (wt)).

Referring to FIG. 9, when the fundamental wave extraction step S200 is implemented by a Goertzel algorithm using a discrete Fourier transform (DFT), the fundamental wave is extracted from the load current IL, and the extracted fundamental wave The magnitude (IL (1)) may be detected (S210).

At this time, the fundamental wave extraction step (S200) may be made to have a transfer function (H (z)) as shown in Equation (1).

More specifically, the fundamental wave extracting step S200 includes a feedback part FBP and a feedforward part FFP, and uses the feedback part FBP and the feedforward part FFP. The magnitude of the fundamental wave IL (1) of the load current IL can be detected.

Referring to FIG. 10, when the second operation step S400 includes a multiplication step S410 and an addition step 420, the multiplication step S410 may include an output value of the first operation step S00 and the operation value. The system signal from the system signal generation step S100 is multiplied and provided to the addition step 420.

The addition step 420 subtracts the output signal of the multiplication step S410 from the load current IL to generate the inverter current command value I * inv.

Hereinafter, with reference to FIGS. 11 to 16, of the existing power generation system and the power generation system of the present invention will be described.

First, referring to FIGS. 11 and 12, when the non-linear load is connected to the existing power generation system, distortion may occur in the grid current Ig and the load current Iload, thereby deteriorating power quality.

That is, as shown in Figure 11, referring to the waveform diagram of the main signal of the existing power generation system without the active power filter (APF) function, when the non-linear load is connected at 0.6 [S] during power generation, Referring to the grid voltage (Vg) and the grid current (Ig) shown in (a), it can be seen that distortion occurs in the grid current due to the influence of the nonlinear load after 0.6 [s]. The waveform of the grid current is shown in (b) of FIG. 11, and the load current (Iload) flowing through the nonlinear load is shown in (c) of FIG. 11, and the inverter current (Iinv) shown in (d) of FIG. ), The inverter current Iinv is synchronized with the phase of the grid voltage at a power factor of 1. Referring to the input DC voltage (DC-Link voltage) of the DC / AC inverter 30 shown in FIG. 11E, the DC voltage Vdc has a ripple of 120 Hz and indicates that the DC voltage is 380 [V]. have. In FIG. 11F, the PV voltage Vpv of the solar cell 10 is output at 3 [kW], and the grid power Pg is −3 [kW]. Referring to (f) of FIG. 11, it is shown that the load power Pload consumed by the nonlinear load at 0.6 [S] is generated by 2 [kW] by 1 [kW].

In addition, referring to the frequency component of the system current Ig shown in FIG. 12A in the graph of the frequency component of each current shown in FIG. 12, the system current Ig includes a fundamental wave. In addition, since a number of harmonic components are included, it can be seen that the grid current is contaminated by a nonlinear load, thereby degrading power quality. Referring to the frequency component of the load current Iload shown in FIG. 12B, it can be seen that a plurality of harmonic components are also included in the load current to adversely affect the system. Referring to the frequency component of the inverter current Iinv shown in FIG. 12C, it can be seen that the inverter current Iinv outputs only the current of the fundamental wave component.

On the contrary, referring to FIGS. 13 and 14, it can be seen that in the power generation system to which the present invention is applied, distortion does not occur in the grid current even when a nonlinear load is connected, and thus power quality is not deteriorated.

That is, Figure 13 shows the main waveform of the power generation system to which the present invention is applied in the presence of solar radiation.

Referring to the grid voltage Vg and the grid current Ig shown in FIG. 13A, it can be seen that the grid current Ig is not distorted even when the nonlinear load is connected. In FIG. 13B, the system current Ig which is not distorted is illustrated. FIG. 13C shows a waveform of the load current Iload flowing through the nonlinear load. Referring to the inverter current Iinv shown in FIG. 13D, it can be seen that a component for compensating for distortion caused by nonlinear load is added to the fundamental wave after 0.6 [s]. have. Referring to FIG. 13E, the output power Ppv of the solar cell 10 is 3 [kW], and the grid power Pg generated into the grid is −3 [kW], and at 0.6 [s]. It can be seen that the power generation to 2 [kW] by the load power (Pload) consumed in the nonlinear load.

In addition, as shown in FIG. 14, referring to the graph of the result of analyzing the frequency component of each current, referring to the frequency component of the grid current shown in FIG. Because the distortion is compensated for, the grid current shows no distortion due to the nonlinear load and only the fundamental wave component. Referring to the frequency component of the load current shown in (b) of FIG. 14, it can be seen that the harmonic component is largely distorted. Referring to the inverter current Iinv and the distortion current compensation harmonics shown in FIG. 14C, it can be seen that the harmonic components included in the grid current Ig are transferred to the inverter current Iinv. Therefore, the grid current does not cause distortion.

In addition, referring to FIG. 15 and FIG. 16, in the power generation system to which the present invention is applied when power generation is not performed in a night mode without solar radiation, distortion does not occur in the grid current even when a nonlinear load is connected. It turns out that this does not fall.

That is, FIG. 15 shows waveforms in the case where solar radiation does not exist in the power generation system to which the present invention is applied. If there is no solar radiation, such as at night, the power generation system of the present invention does not generate power, but performs only the APF function. The power generation system uses a DC link capacitor connected to the input terminal of the DC / AC inverter to repeat the charging and discharging to compensate for the current distortion caused by the nonlinear load. In the end, the grid current Ig is not distorted.

That is, referring to the grid voltage Vg and the grid current Ig in FIG. 15A, since the grid voltage Vg is supplied to the nonlinear load, the grid voltage and the grid current appear in reverse phase with each other. With the APF function of the present invention, distortion of the system current is not shown. In FIG. 15B, the waveform of the grid current is shown. 15C shows current waveforms of the nonlinear load. 15 (d) shows that the output current Iinv of the inverter compensates for the distortion of the system current. 15E illustrates an input DC link voltage of the DC / AC inverter. Referring to the respective powers shown in FIG. 15F, the output power Ppv of the solar cell 10 does not appear in the night mode, and only the grid power Pg is supplied to the nonlinear load of 1 [kW]. .

In addition, as shown in FIG. 16, referring to the graph of the result of analyzing the frequency component of each current, only the fundamental wave component of the system current is shown in FIG. Therefore, power quality is not deteriorated. In FIG. 16B, harmonic components according to the nonlinear load are shown in the current flowing through the nonlinear load. 16 (c) shows harmonic components that compensate for distortion of the system current. Since it is a night mode and is not generating power, the frequency of the fundamental wave component is not shown.

In the present invention as described above, by adding a load current detection circuit to the existing grid-connected system, and adds a power quality control (PQC) algorithm to perform the APF function, an active power filter (APF) that is a power quality device in addition to the distributed power supply There is no need to install additional, economical and space advantages.

In addition, since the solar power generation system is used day and night, the utilization rate may be operated at 100 [%].

In the present invention, since only the load current detection circuit is required, the calculation operation is considerably less than the conventional method of obtaining RMS (RMS) values of voltage and current by performing a simple calculation using the Goertzel algorithm. .

30: DC / AC inverter 40: grid
50: nonlinear load 100: grid voltage phase follower
110: phase detector 120: signal generator
200: fundamental wave extraction unit 210: feedback circuit unit
220; Feed forward circuit 230: arithmetic unit
300: first operation unit 400: second operation unit
410; Multiplication Unit 420: Adder
500 control unit 510 current control unit
520: PWM control unit 600: voltage control unit

Claims (20)

In the power quality control device applied to a power system including a DC / AC inverter for supplying an inverter current to the system by converting a DC voltage into an AC voltage,
A system voltage phase follower for generating a system signal having a phase of the system voltage;
A fundamental wave extracting unit extracting a magnitude of a fundamental wave of a load current flowing into a nonlinear load connected between the DC / AC inverter and a grid;
A first calculating unit which subtracts a preset current compensation value from the magnitude of the fundamental wave from the fundamental wave extracting unit; And
A second calculator configured to generate the inverter current command value for the DC / AC inverter using the output value of the first calculator, the system signal from the grid voltage phase follower, and the load current;
Power quality control device of the power generation system comprising a. The method of claim 1, wherein the grid voltage phase tracking unit
A phase detector for detecting a phase of the grid voltage; And
A signal generator for generating the systematic signal having a phase equal to the phase of the systematic voltage and having a predetermined unit size based on the phase from the phase detector
Power quality control device of a power generation system comprising a. The method of claim 1, wherein the fundamental wave extraction unit,
Implemented by one of the Goertzel algorithms using the Discrete Fourier Transform (DFT), Fast Fourier Transform (FFT) and Discrete Fourier Transform, extracting the fundamental wave from the load current and detecting the magnitude of the extracted fundamental wave A power quality control device for a power generation system. The method of claim 3, wherein the fundamental wave extraction unit,
Having a transfer function H (z) such as
[Equation]

Where k is the discrete frequency and N is the number of samples during one cycle of the grid voltage.
Power quality control device of the power generation system, characterized in that. The method of claim 1, wherein the second operation unit,
A multiplier for multiplying an output value of the first calculator by a system signal from the system voltage phase follower; And
An adder configured to generate the inverter current command value by subtracting an output signal of the multiplier from the load current
Power quality control device of a power generation system comprising a. The method of claim 1,
And a voltage controller which obtains a voltage error between the input DC voltage of the DC / AC inverter and a predetermined DC voltage and provides the voltage error as the compensation value. In the power quality control device applied to a power system including a DC / AC inverter for supplying an inverter current to the system by converting a DC voltage into an AC voltage,
A system voltage phase follower for generating a system signal having a phase of the system voltage;
A fundamental wave extracting unit extracting a magnitude of a fundamental wave of a load current flowing into a nonlinear load connected between the DC / AC inverter and a grid;
A first calculating unit which subtracts a preset current compensation value from the magnitude of the fundamental wave from the fundamental wave extracting unit;
A second calculator configured to generate the inverter current command value for the DC / AC inverter using the output value of the first calculator, the system signal from the grid voltage phase follower, and the load current; And
A control unit that controls the DC / AC inverter based on the inverter current command value from the second calculating unit
Power quality control device of the power generation system comprising a. The method of claim 7, wherein the grid voltage phase tracking unit
A phase detector for detecting a phase of the grid voltage; And
A signal generator for generating the systematic signal having a phase equal to the phase of the systematic voltage and having a predetermined unit size based on the phase from the phase detector
Power quality control device of a power generation system comprising a. The method of claim 7, wherein the fundamental wave extraction unit,
Implementing one of the Goertzel algorithms using the Discrete Fourier Transform (DFT), Fast Fourier Transform (FFT) and Discrete Fourier Transform, extracting the fundamental wave from the load current and detecting the magnitude of the extracted fundamental wave
Power quality control device of the power generation system, characterized in that. The method of claim 9, wherein the fundamental wave extraction unit,
Having a transfer function H (z) such as
[Equation]

Where k is the discrete frequency and N is the number of samples during one cycle of the grid voltage.
Power quality control device of the power generation system, characterized in that. The method of claim 7, wherein the second operation unit,
A multiplier for multiplying an output value of the first calculator by a system signal from the system voltage phase follower; And
An adder configured to generate the inverter current command value by subtracting an output signal of the multiplier from the load current
Power quality control device of a power generation system comprising a. The method of claim 7, wherein
And a voltage controller which obtains a voltage error between the input DC voltage of the DC / AC inverter and a predetermined DC voltage and provides the voltage error as the compensation value. In the power quality control method applied to a power system including a DC / AC inverter for supplying the inverter current to the system by converting a DC voltage into an AC voltage,
A system signal generation step of generating a system signal having a phase of the system voltage;
A fundamental wave extraction step of extracting a magnitude of a fundamental wave of a load current flowing into a nonlinear load connected between the DC / AC inverter and a grid; And
A first calculating step of subtracting a preset current compensation value from the magnitude of the fundamental wave from the fundamental wave extracting step; And
A second calculation step of generating the inverter current command value for the DC / AC inverter using the output value of the first calculation step, the system signal from the system signal generation step, and the load current;
Power quality control method of the power generation system comprising a. The method of claim 13, wherein the generating the system signal,
A phase detecting step of detecting a phase of the grid voltage; And
A signal generation step of generating the system signal having a phase equal to the phase of the system voltage and having a predetermined unit size based on the phase from the phase detection step
Power quality control method of a power generation system comprising a. The method of claim 13, wherein the fundamental wave extraction step,
Implementing one of the Goertzel algorithms using the Discrete Fourier Transform (DFT), Fast Fourier Transform (FFT) and Discrete Fourier Transform, extracting the fundamental wave from the load current and detecting the magnitude of the extracted fundamental wave
Power quality control method for a power generation system, characterized in that. The method of claim 15, wherein the fundamental wave extraction step,
Having a transfer function H (z) as
[Equation]

Where k is the discrete frequency and N is the number of samples during one cycle of the grid voltage.
Power quality control method for a power generation system, characterized in that. The method of claim 13, wherein the second operation step,
A multiplication step of multiplying the output value of the first calculation step and the system signal from the system signal generation step; And
An addition step of generating the inverter current command value by subtracting the output signal of the multiplication step from the load current
Power quality control method of a power generation system comprising a. The method of claim 13,
And a control step of controlling the DC / AC inverter based on the inverter current command value from the second calculation step. The method of claim 18, wherein the control step,
A current control step of detecting an inverter current error between the inverter current command value from the second calculation step and the inverter current; And
A PWM control step of providing a PWM inverter signal for controlling the DC / AC inverter based on the inverter current error from the current control step
Power quality control method of a power generation system comprising a. The method of claim 13,
And a voltage control step of obtaining a voltage error between an input side DC voltage of the DC / AC inverter and a predetermined DC voltage and providing the voltage error as the compensation value.

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