KR101072016B1

KR101072016B1 - Device and method for detecting islanding of distributed generation system interconnected with utility grid

Info

Publication number: KR101072016B1
Application number: KR1020100025144A
Authority: KR
Inventors: 김일송
Original assignee: 충주대학교 산학협력단
Priority date: 2010-03-22
Filing date: 2010-03-22
Publication date: 2011-10-10
Also published as: KR20110105981A

Abstract

The independent operation detection apparatus of the grid-connected distributed power supply system according to an embodiment of the present invention receives a voltage and a current from a distributed power supply as a first input signal, and receives a voltage and a current from the power system as a second input signal. A fast Fourier transform unit for outputting first and second harmonic frequency components by performing a Fast Fourier Transform (FFT) on each of the first input signal and the second input signal; A harmonic extracting unit extracting pure harmonic components by removing switching frequency components and fundamental frequency components from each of the first and second harmonic frequency components; A state equation calculator configured to construct a state system voltage component by constructing a state equation using the extracted harmonic components; A time correlation unit extracting a power spectral density by performing time correlation on the virtual system voltage component; And a peak detector configured to detect the maximum value of the power spectral density and compare it with a preset threshold level, and if the maximum value is less than or equal to the reference value, recognize that the distributed power supply is in an independent operation state.

Description

DEVICE AND METHOD FOR DETECTING ISLANDING OF DISTRIBUTED GENERATION SYSTEM INTERCONNECTED WITH UTILITY GRID}

Embodiments of the present invention relate to an apparatus and method for detecting standalone operation of a distributed system.

Recently, due to the fossil energy depletion and environmental pollution, much attention has been focused on power generation using alternative energy. Generation through alternative energy is called distributed power since it has smaller capacity and distributed around the demand area than large capacity generation. Early distributed power supplies were operated in a state separated from the existing power system due to their small capacity. In recent years, they have been operating in connection with the power system as their capacity increases.

Distributed power supply is generally connected to the power system through a grid linkage device having a structure of a circuit breaker, and is always connected together and separated from the power system when a problem occurs in the power system.

In other words, if an accident such as a ground fault or short circuit occurs on the side where commercial AC power is supplied during operation, the grid linkage device is opened to separate the commercial power system and the distributed power system, thereby overcurrent generated in the power system It does not flow to the load, protecting the load and grid-connected distributed power system.

At this time, when the power system is separated, only the distributed power supply is connected to the load to supply power. This state is called an islanding state. However, in such a single operation state, frequency fluctuation occurs depending on the size of the active power and the load of the distributed power source, so that it is difficult to supply a stable power. There is a possibility that an accident may occur due to a phase difference between both voltages, which causes a short circuit or a step out. Therefore, it is necessary to quickly detect such a single operation state when linking distributed power sources.

An embodiment of the present invention provides an apparatus and method for detecting standalone operation of a system-linked distributed power supply system capable of detecting a standalone operation even in the event of disturbance and instantaneous power failure of a power system such as sag and swell. .

An embodiment of the present invention provides an apparatus and method for independently operating detection of a system-linked distributed power supply system capable of properly recognizing a momentary power failure by distinguishing a momentary power failure from a permanent power failure through a time correlation operation, and minimizing a non-detection area. to provide.

The problem to be solved by the present invention is not limited to the problem (s) mentioned above, and other object (s) not mentioned will be clearly understood by those skilled in the art from the following description.

According to an embodiment of the present invention, a method for detecting standalone operation of a system-linked distributed power supply system receives a voltage and a current from a distributed power supply as a first input signal, and receives a voltage and a current from a power system as a second input signal. Outputting first and second harmonic frequency components by performing a Fast Fourier Transform (FFT) on each of the first input signal and the second input signal; Extracting pure harmonic components by removing switching frequency components and fundamental frequency components from each of the first and second harmonic frequency components; Constructing a state equation using the extracted harmonic components to generate a virtual system voltage component; Extracting a power spectral density by time-correlating the virtual system voltage component; Detecting a maximum value of the power spectral density and comparing the maximum value with a predetermined threshold level, and recognizing that the distributed power supply is in an independent operation state when the maximum value is less than or equal to the reference value as a result of the comparison.

Specific details of other embodiments are included in the detailed description and the accompanying drawings.

Advantages and / or features of the present invention and methods for achieving them will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, only the present embodiments to make the disclosure of the present invention complete, and common knowledge in the art to which the present invention pertains. It is provided to fully inform the person having the scope of the invention, which is defined only by the scope of the claims. Like reference numerals refer to like elements throughout.

According to an embodiment of the present invention, it is possible to provide a robust single operation detection technique that can detect single operation normally even in the event of disturbance and instantaneous power failure of a power system such as Sag and Swell.

According to an embodiment of the present invention, the instantaneous power failure and the permanent power failure may be distinguished through time correlation calculation to properly recognize the instantaneous power failure, and the non-detection area may be minimized.

1 is a diagram illustrating a system-linked distributed power supply system according to an embodiment of the present invention.
FIG. 2 is a block diagram illustrating the configuration of the single driving detection device of FIG. 1.
3 is a flowchart illustrating a single operation detection method of a system-linked distributed power supply system according to an exemplary embodiment of the present invention.

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

1 is a diagram illustrating a system-linked distributed power supply system according to an embodiment of the present invention.

In an embodiment of the present invention will be described with an example that the photovoltaic power generation as a distributed power source is associated with the power system. However, an embodiment of the present invention is not limited thereto, and alternative power such as wind power generation and hydro power generation, as well as small scale generation such as turbine power generation, may be associated with the power system as the distributed power source.

Referring to FIG. 1, a system-linked distributed power supply system according to an embodiment of the present invention may include a solar cell 110, a standalone operation detecting device 120, a power system 130, and a circuit breaker (CB) 140. ), And load 150.

The solar cell 110 generates electrical energy using sunlight. The distributed power source such as the solar cell 110 is connected to the power system 130 through a grid linkage device having a structure of a circuit breaker. Therefore, the solar cell 110 is always connected to the power system 130, but has a structure that is separated from the power system 130 when a problem occurs in the power system 130.

That is, when an accident such as a ground fault or a short circuit occurs in the power system 130 while the solar cell 110 is operated in connection with the power system 130, the system linkage device (circuit breaker 140) is opened. Thus, the power system 130 which is a commercial power source and the solar cell 110 which is a distributed power system are separated. Accordingly, the overcurrent generated in the power system 130 does not flow to the load 150 to protect the entire system-linked distributed power supply system including the load 150.

In this case, when the power system 130 is separated, only the distributed power source, that is, the solar cell 110 is connected to the load 150 to supply power. This state is referred to as an islanding state.

For reference, the independent operation means that the feeder circuit breaker 140 is disconnected due to an inspection or an accident of the power system 130, and the operation is continued without stopping the photovoltaic power generation facility in the state separated from the power system 130. I mean. In this case, there is a risk of electric shock, equipment damage, worker electric shock, etc., due to charging of the electrostatic wiring line.

In order to detect the single driving condition as described above, the single driving detection device 120 may be mounted in the solar power inverter (PSC) 100. For reference, the solar inverter 100 is a function of linking a photovoltaic power generation system which is an example of a distributed power system to the power system 130, and converts the power generated from the solar cell 110 from AC to DC. Do it.

The single operation detection device 120 receives a voltage and a current from the solar cell 110 and the power system 130 to perform fast Fourier transform, and as a result of the switching frequency component, fundamental frequency component, and DC component Pure harmonics from which the back is removed are extracted.

The single operation detection device 120 constructs a state equation using the pure harmonic component, and based on the state equation, a value corresponding to a magnitude of a virtual system voltage component or a virtual system current component, that is, a system voltage. Outputs the value corresponding to the magnitude of the grid current.

The single operation detection device 120 inputs the magnitude of the virtual system voltage / current component (harmonic component) to a correlator for time to obtain a power spectrum for a period. The single operation detection device 120 may obtain the power spectral density by dividing the power spectral component with respect to a period.

The single operation detection device 120 detects the maximum value of the power spectral density and generates an alarm by recognizing that the single operation state is when the incoming power density is lower than or equal to a preset threshold level.

The power system 130 is connected to the solar cell 110 through a system linkage device having a structure of a circuit breaker. Therefore, the power system 130 is always connected to the solar cell 110, but when a problem (an accident such as ground, short circuit, etc.) occurs in the power system 130 is separated from the solar cell 110 structure Has

The circuit breaker 140 is a switch that automatically cuts off the overheated circuit in the electrical circuit. When the circuit breaker 140 has a problem such as an inspection or an accident in the power system 130, the circuit breaker 140 opens the switch to separate the power system 130 from the photovoltaic power generation facility, thereby generating the power system 130. Overcurrent can be prevented from flowing to the load 150.

The load 150 is commonly connected to the power system 130 and the solar cell 110 to share power generated from the solar cell 110 and power transmitted through the power system 130.

Therefore, when the amount of power generated by the solar cell 10 is greater than the power consumed by the load 150, the remaining power consumed by the load 150 is a grid that is a supply unit of the power system 130. Is delivered. On the other hand, when the amount of power generated in the solar cell 10 is less than the power consumed by the load 150, the load 150 receives the insufficient power from the power system 130.

Hereinafter, the configuration of the single driving detection apparatus 120 will be described in detail with reference to FIG. 2.

FIG. 2 is a block diagram illustrating the configuration of the single driving detection device 120 of FIG. 1.

Referring to FIG. 2, the single operation detection apparatus 120 includes a fast Fourier transform unit 210, a harmonic extractor 220, a state equation calculator 230, a time correlation unit 240, a peak detector 250, The alarm unit 260 may include a control unit 270.

The fast Fourier transform unit 210 receives a voltage and a current from a solar cell (see “110” of FIG. 1), which is an example of a distributed power supply, as a first input signal, and a power system (see “130” of FIG. 1). The voltage and current from are received as the second input signal.

The fast Fourier transform unit 210 performs a Fast Fourier Transform (FFT) on each of the first input signal and the second input signal, and outputs first and second harmonic frequency components. Here, the first and second harmonic frequency components represent frequency components corresponding to integer multiples of 60 Hz when the system rated frequency is 60 Hz.

In this case, the fast Fourier transform unit 210 may perform fast Fourier transform on each of the first and second input signals by using an N-point Discrete Fourier Transform (DFT) algorithm. Here, N may be set to an appropriate value in consideration of computational complexity and computational speed. For example, in the present embodiment, the N may be set to 256.

The first and second harmonic frequency components include switching frequency components, fundamental frequency components, and direct current (DC) components in addition to pure harmonic components.

The harmonic extracting unit 220 removes the switching frequency component, the fundamental frequency component, the direct current component, and the like from each of the first and second harmonic frequency components. As a result, the harmonic extracting unit 220 extracts pure harmonic components from each of the first and second harmonic frequency components.

The state equation calculator 230 constructs a state space equation using the extracted pure harmonic components. In this case, the state equation calculator 230 may sequentially receive the harmonic components (first and second harmonic frequency components) input during one cycle (for example, 16.67 ms) to configure the state equation.

The mathematical expression for the state equation calculator 230 to construct the state equation is as follows.

The equation below includes a signal including sinwt of the first harmonic extracted by the harmonic extracting unit x1 (k), a signal including coswt of the first harmonic x2 (k) and sin2wt of the second harmonic A signal containing x3 (k), the second harmonic cos2wt, a signal containing x2 (k), ..., the nth harmonic sin2nwt, and a signal containing x2n-1 (k), the nth harmonic If the signal containing cos2nwt is x2n (k), the state equation (Equation 1) and the output equation (Equation 3) using the state variable x (k) (Equation 2) can be constructed. A virtual system voltage (Equation 4) can be obtained from the output thus obtained.

[Equation 1]

w (k) = random walk process whose average is zero,

[Equation 2]

,

,

N ₁ = Number of samplings during the grid voltage cycle.

[Equation 3]

,

v (k): Random measurement noise.

[Equation 4]

: Phase angle obtained as a result of FFT (Fast Fourier Transform).

The state equation calculating unit 230 is based on the above state equation

By outputting the value of, a virtual system voltage / current component is produced.

When the state equation calculator 230 constructs the state equation as described above, when only the system voltage is input as the pure harmonic component, the state equation calculation unit constructs the second order of the state equation, and the harmonic of the system voltage and current is used as the harmonic component. Upon receipt of the input, the order of the state equation can be configured in quadratic order.

The time correlator 240 extracts power spectral density by performing time correlation on the virtual system voltage / current component.

That is, the time correlation unit 240 is the virtual system voltage component, that is,

The power spectrum for each period is measured by integrating with respect to time, and the power spectrum density is extracted by dividing the measured power spectrum by the period.

As a result, the time correlation unit 240 may distinguish between instantaneous power failure and permanent power failure, thereby preventing malfunction of the single operation detection device 120 caused by transient such as transient power failure. . In addition, the time correlation unit 240 may increase the stability of the single operation detection device 120 by providing a time-gap at an output.

The time correlation operation may be expressed by Equation 5 below.

[Equation 5]

The peak detector 250 detects a maximum value of the power spectral density and compares the detected maximum value with a preset threshold level.

The peak detector 250 recognizes that the distributed power supply (solar cell) is in a standalone operation state when the maximum value is less than or equal to the reference value as a result of the comparison, and when the maximum value exceeds the reference value, the distributed power supply operates alone. Recognize that it is not a state.

When the peak detector 250 recognizes that the distributed power source is in the standalone operation state, the peak detector 250 may transmit a single drive related signal to the controller 270. Accordingly, the controller 270 may receive the single driving related signal from the peak detector 250 to generate an alarm signal, and transmit the generated alarm signal to the alarm unit 260.

The alarm unit 260 may receive the alarm signal from the control unit 270 and output the alarm signal, thereby notifying a system administrator that the distributed power source is in a single operation state.

In addition, the control unit 270 receives the single driving related signal and generates a cutoff signal, and provides the generated breaker signal to a circuit breaker (see “140” in FIG. 1), thereby causing the circuit breaker to stand alone. The distributed power supply being tripped can be tripped.

3 is a flowchart illustrating a single operation detection method of a system-linked distributed power supply system according to an exemplary embodiment of the present invention. The single driving detection method may be performed by the single driving detection device 120 of FIGS. 1 and 2.

Referring to FIG. 3, in operation 310, the single driving detection apparatus receives first and second input signals and performs fast Fourier transform (FFT) on each of them. As a result, the single operation detection device outputs first and second harmonic frequency components corresponding to integer multiples of the system rated frequency, for example, 60 Hz, as a result of the execution.

In this case, the single operation detection apparatus may perform fast Fourier transform on each of the first and second input signals by using an N-point DFT algorithm. Here, N may be set to an appropriate value in consideration of computational complexity and computational speed. For example, in the present embodiment, the N may be set to 256.

Here, the first input signal represents a voltage and current signal input from a solar cell, and the second input signal represents a grid voltage and grid current signal input from a power system.

Next, in operation 320, the single operation detection apparatus extracts pure harmonic components by removing switching frequency components, fundamental frequency components, DC components, and the like from each of the first and second harmonic frequency components.

Next, in step 330, the single operation detection device constructs a state equation using the extracted pure harmonic components. In this case, the single operation detection device may configure the state equation by sequentially receiving the harmonic components (first and second harmonic frequency components) input during one cycle (for example, 16.67 ms).

Thus, the single operation detection device generates a virtual system voltage / current component based on the state equation.

When the single operation detection device constructs the state equation as described above, when only the system voltage is input as the pure harmonic component, the order of the state equation is second-order, and when the harmonic of the system voltage and current is input as the harmonic component, The order of the state equation can be configured as 4th order.

Next, in operation 340, the single operation detection apparatus extracts power spectral density by performing time correlation on the virtual system voltage / current component.

That is, the single operation detection device may integrate the virtual system voltage / current component over time to measure the power spectrum for each period, and divide the measured power spectrum by the period to extract the power spectral density.

As a result, the single operation detection device can distinguish between instantaneous power failure and permanent power failure, thereby preventing malfunctions caused by transient events such as instantaneous power failure. In addition, the single operation detection device may increase stability by providing a time-gap at the output.

Next, in operation 350, the single operation detection device detects a maximum value of the power spectral density and compares the maximum value with a preset threshold level.

As a result of the comparison, if the maximum value of the power spectral density is less than or equal to the reference value (YES direction 350), the single operation detection device recognizes that the solar cell is in the single operation state in step 360.

In this case, the single operation detection device may generate an alarm signal to alert the system administrator of the single operation state, and generate a cutoff signal to cause the circuit breaker to trip the solar cell during single operation.

On the other hand, if the maximum value of the power spectral density is less than the reference value ("No" direction of 350) as a result of the comparison, the single operation detection device recognizes that the solar cell is not in the single operation state, and proceeds to step 350 Return.

As such, according to an embodiment of the present invention, a robust single operation detection technique capable of detecting single operation normally even in the event of disturbance and instantaneous power failure of a power system such as Sag and Swell may be provided. .

In addition, according to an embodiment of the present invention, the instantaneous power outage and the permanent power outage may be distinguished through time correlation calculation to properly recognize the instantaneous power failure and minimize the non-detection area.

While specific embodiments of the present invention have been described so far, various modifications are possible without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the claims below, but also by the equivalents of the claims.

As described above, the present invention has been described by way of limited embodiments and drawings, but the present invention is not limited to the above-described embodiments, which can be variously modified and modified by those skilled in the art to which the present invention pertains. Modifications are possible. Accordingly, the spirit of the present invention should be understood only by the claims set forth below, and all equivalent or equivalent modifications thereof will belong to the scope of the present invention.

100: solar inverter
110: solar cell
120: single operation detection device
130: power system
140: circuit breaker
150: load
210: fast Fourier transform unit
220: harmonic extraction unit
230: state equation calculation unit
240: time correlation
250: peak detector
260: alarm unit
270: control unit

Claims

Receives a voltage and current from a distributed power supply as a first input signal, receives a voltage and current from a power system as a second input signal, and performs a fast Fourier transform on each of the first and second input signals. A fast Fourier transform unit for outputting first and second harmonic frequency components by performing a Fast Fourier Transform (FFT);
A harmonic extracting unit extracting pure harmonic components by removing switching frequency components and fundamental frequency components from each of the first and second harmonic frequency components;
A state equation calculator configured to construct a state system voltage component by constructing a state equation using the extracted harmonic components;
A time correlation unit extracting a power spectral density by performing time correlation on the virtual system voltage component;
A peak detector that detects the maximum value of the power spectral density and compares it with a preset threshold level, and recognizes that the distributed power supply is in a single operation state when the maximum value is less than or equal to the reference value as a result of the comparison.
Single operation detection device of a system-linked distributed power supply system comprising a.

The method of claim 1,
The time correlation unit
Integrating the virtual system voltage component over time to measure the power spectrum for each cycle, and dividing the measured power spectrum by the period to extract the power spectral density. Device.

The method of claim 1,
The fast Fourier transform unit
And an N-point DFT algorithm to perform the fast Fourier transform.

The method of claim 1,
A control unit for generating an alarm signal and providing the generated alarm signal when the distributed power supply is recognized as the independent operation state by the peak detector;
Single operation detection device of the system-linked distributed power supply, characterized in that it further comprises.

The method of claim 1,
A control unit for generating a blocking signal and providing the circuit breaker to the circuit breaker
Single operation detection device of the system-linked distributed power supply, characterized in that it further comprises.

The method of claim 1,
The state equation calculation unit
When the extracted harmonic components are sequentially input to form the state equation, when only the system voltage is input as the harmonic component, the order of the state equation is second-order, and the harmonics of the system voltage and current are input as the harmonic components. The independent operation detection apparatus of the grid-connected distributed power supply system, characterized in that comprises the order of the state equation to the fourth order.

In the single operation detection method of the grid-connected distributed power supply system,
In the system-linked distributed power supply system, a voltage and a current from a distributed power supply are input as a first input signal, a voltage and a current from a power system are input as a second input signal, and the first input signal and the second input are input. Performing a Fast Fourier Transform (FFT) on each of the signals to output first and second harmonic frequency components;
Extracting pure harmonic components by removing switching frequency components and fundamental frequency components from each of the first and second harmonic frequency components in the system-linked distributed power supply system;
Constructing a state equation using the extracted harmonic components to generate a virtual grid voltage component in the grid-connected distributed power supply system;
Extracting a power spectral density by performing a time correlation operation on the virtual grid voltage component in the grid-connected distributed power supply system;
In the system-linked distributed power supply system, the maximum value of the power spectral density is detected and compared with a preset threshold level. Recognition step
Single operation detection method of a system-linked distributed power supply system comprising a.

The method of claim 7, wherein
Extracting the power spectral density
Integrating the virtual system voltage component over time to measure a power spectrum for each period; And
Dividing the measured power spectrum by the period to extract the power spectral density
Single operation detection method of a system-linked distributed power supply system comprising a.

The method of claim 7, wherein
Comprising the state equation
Constructing the order of the state equation in quadratic when only a system voltage is input as the harmonic component; And
When the harmonics of the system voltage and current are input as the harmonic component, constructing the order of the state equation in quadratic order
Single operation detection method of a system-linked distributed power supply system comprising a.