KR20180037450A - Anti-islanding method of grid connected inverter - Google Patents

Abstract

Disclosed is a method for detecting islanding of a grid-connected inverter. A grid-connected inverter sets a reactive power command value, controls to output reactive power according to the reactive power command value, and detects occurrence of islanding depending on whether the reactive power deviates from the reactive power command value.

Description

TECHNICAL FIELD [0001] The present invention relates to a method of detecting an independent operation of a grid-

The present invention relates to a method of detecting a single operation of a grid-connected inverter, and more particularly, to a method of detecting a single operation of a grid-connected inverter having a significant load.

Recently, there is a growing need for a hybrid ESS (Energy Storage System) to efficiently use energy generated from irregularly generated renewable energy sources such as solar and wind power according to electric power demand. In this regard, the description will be made with reference to Fig.

1 is a conceptual diagram of a hybrid ESS.

Referring to FIG. 1, the hybrid ESS may include a bi-directional converter for bidirectional converters dedicated to ESS batteries, a bi-directional converter for generating renewable energy sources such as solar PV, and a grid-connected inverter. These hybrid ESSs store a certain amount of power generated from renewable energy sources such as solar power (PV) in the ESS battery and supply them to the system, and load leveling and peak load dispersion through charging and discharging in the system is also possible Thereby maximizing the efficiency of power use.

In particular, as shown in FIG. 1, in the case of an ESS having a critical load, the grid-connected inverter can operate so as to supply stable power without interruption to a critical load even if an abnormal system such as a power failure occurs.

Specifically, the grid-connected inverter can operate in a grid-connected mode in which power is generated from the ESS to the grid or power is supplied from the grid to the load when the recloser is closed.

In addition, when the reclosing circuit breaker is shut off due to a system such as a power failure, the grid-connected inverter can operate in the single operation mode. That is, the circuit breaker is shut off due to an abnormality in the system, but the inverter switch SW is closed due to the inability to recognize it in the grid-connected inverter. In such a case, the load voltage fluctuates according to the output power output from the grid-connected inverter and the load condition associated with the ESS, so that it is impossible to guarantee a stable voltage supply to the critical load. In addition, when the system is turned back on, the phase error of the output voltage of the system and the inverter can cause significant damage to the critical load.

Therefore, it is preferable that the grid interconnected inverter rapidly switches to a stand-alone operation mode in which it recognizes a single operation or system abnormality and disconnects the inverter switch SW so as to be separated from the system and supply power only to the load.

As a control method of the conventional grid interconnected inverter, current control is performed when operating in the grid connection mode and voltage control is performed when operating in the independent operation mode. Accordingly, when the mode is switched, State, which may lead to damage and malfunction of the load.

In this way, in order to prevent damage and malfunction of the critical load, the ESS grid-connected inverters having important loads must quickly recognize the stand-alone operation and switch to the independent operation mode. .

To this end, an indirect current control technique has been proposed that enables mode switching of the grid-connected inverter without transient state. Since the grid-connected inverters using the indirect current control method always perform the voltage control, the transient state of the critical load can be minimized because the load voltage does not fluctuate.

On the other hand, in the case of the grid-connected inverter using the indirect current control technique, since the output voltage does not fluctuate even if the inverter operates in the single operation mode due to a system abnormality, Conventional techniques to do this are not appropriate. In addition, the harmonic injection technique, which has been studied as an independent operation detection technique of the indirect current control based inverter, increases the THD (Total Harmonics Distortion) of the inverter output current, resulting in a deterioration of the power quality, .

In the case of the grid-connected inverter using the indirect current control technique, it is possible to minimize the transient state of the critical load, but the single operation can not be detected with the existing single operation detection technique, and the single operation detection using the harmonic injection technique It is necessary to propose a new single operation detection method suitable for the grid-connected inverters to which the indirect current control technique is applied.

According to an aspect of the present invention, there is provided a method for detecting a single operation of a grid-connected inverter using an indirect current control technique for detecting an independent operation of an inverter based on reactive power output from the inverter.

According to one aspect of the present invention, there is provided a method of detecting a stand-alone operation of a grid interconnected inverter, comprising: setting a reactive power command value and controlling reactive power according to the reactive power command value to be outputted; determining whether the reactive power is out of the reactive power command value The detection of the occurrence of the independent operation can be controlled to switch to the independent operation mode for shutting off the inverter switch connected to the system.

The setting of the reactive power command value may be to set a reactive power command value periodically fluctuating to a value of Positive, 0, or Negative based on the active power of the grid interconnected inverter.

The detection of the occurrence of the independent operation depending on whether the reactive power deviates from the reactive power command value includes counting the number of times the reactive power deviates from the reactive power command value and if the counted number exceeds the predetermined threshold , And may detect the occurrence of the single operation.

In addition, the indirect current control method of performing the voltage control in the grid-connected operation mode in which the inverter switch connected to the system is closed and the independent operation mode in which the inverter switch is in the off state can be followed.

According to one aspect of the present invention, by applying the indirect current control method, it is possible to supply a stable voltage to the load in the event of a stand-alone operation, and to switch to the independent operation mode without transient state. It is also confirmed that the single operation is detected quickly by detecting the single operation based on the reactive power of the inverter.

1 is a conceptual diagram of a hybrid ESS.
2 is a schematic circuit diagram of a grid-connected inverter having an important load.
3 is a control block diagram of a grid-connected inverter according to an embodiment of the present invention.
4 and 5 are graphs for explaining a single operation detection method in the grid interconnect type inverter according to an embodiment of the present invention.
6 is a flowchart illustrating a method of controlling a grid interconnected inverter according to an embodiment of the present invention.
FIGS. 7 to 11 are diagrams for explaining advantageous effects of the grid-connected inverter according to the embodiment of the present invention.

The following detailed description of the invention refers to the accompanying drawings, which illustrate, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It should be understood that the various embodiments of the present invention are different, but need not be mutually exclusive. For example, certain features, structures, and characteristics described herein may be implemented in other embodiments without departing from the spirit and scope of the invention in connection with an embodiment. It is also to be understood that the position or arrangement of the individual components within each disclosed embodiment may be varied without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is to be limited only by the appended claims, along with the full scope of equivalents to which such claims are entitled, if properly explained. In the drawings, like reference numerals refer to the same or similar functions throughout the several views.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the drawings.

FIG. 2 is a schematic circuit diagram of a grid-connected inverter having an important load, and FIG. 3 is a schematic control block diagram of a grid-connected inverter according to an embodiment of the present invention.

2, the grid-connected inverter according to an embodiment of the present invention receives a direct-current voltage from a direct-current energy source (V _dc ) and converts the direct-current voltage into an alternating-current voltage, _g . < / RTI >

To this end, the grid-connected inverter according to the embodiment of the present invention can generate an AC voltage from a DC voltage by a plurality of switches operating according to a PWM (Pulse Width Modulation) control signal, and the filter inductor L _i and a filter capacitor C _f to make the AC voltage more sinusoidal. It is also possible to control the current (i _Lg ) injected into the system, including the line inductor (L _g ).

Here, in the grid-connected inverter according to the embodiment of the present invention, the inverter switch SW is also closed in the grid-connected operation mode in the steady state where the recloser is closed, Can supply.

2, when the recloser fails to detect a system abnormal state in which the recloser is interrupted, the system interlinked inverter operates in the single operation mode in which the inverter switch SW is closed In this case, since a stable voltage can not be supplied to the critical load, it is preferable to operate in a stand-alone operation mode in which the system abnormal state or single operation is detected quickly and the inverter switch SW is shut off .

In general, the inverter includes a drive and a control unit for giving signals to the switching elements, and the grid-connected inverter according to an embodiment of the present invention may also include such components. In particular, the grid-connected inverter according to an embodiment of the present invention performs not only a control program for controlling general operation of an inverter but also an operation according to a control program stored in a memory and a memory in which a control program for detecting a single operation is stored, And a control unit capable of detecting an operation.

Referring to FIG. 3, the controller may control the voltage to be controlled in both the grid-connected mode and the independent operation mode by applying the indirect current control method. At this time, the control unit limits the output of the PLL (Phase Locked Loop) controller so that the size and frequency of the load voltage do not fluctuate even when the inverter is operated alone.

Further, the control unit may apply the reactive power fluctuation technique to detect the sole operation of the inverter. The reactive power fluctuation technique is a method in which the inverter periodically varies the reactive power to generate a change in the frequency of the load voltage due to the difference between the reactive power output by the inverter and the reactive power consumed by the load at the occurrence of the independent operation, It is a technique to detect a single operation from a frequency change. However, in the case of applying the indirect current control method like the grid-connected inverter according to the embodiment of the present invention, since the voltage control is always performed, the magnitude and frequency of the load voltage do not fluctuate even when the inverter alone operates. Therefore, in order to detect the single operation, the control unit controls the inverter to output the reactive power by applying the reactive power variation technique, but can detect the independent operation based on the reactive power, not the frequency of the load voltage. In this regard, a detailed description will be made with reference to FIG. 4 and FIG.

4 and 5 are graphs for explaining a single operation detection method in the grid interconnect type inverter according to an embodiment of the present invention.

4 is a flowchart illustrating a method of operating a grid-connected inverter according to an embodiment of the present invention in a Grid connected mode, a Standalone mode when operating in a clearing time, The fluctuation of the load voltage (V _{Load, a} ) is small at the time of occurrence of stand-alone operation, that is, at the clearing time, so that stable power supply to the critical load It is possible to confirm that it is possible.

Referring to FIG. 4, since there is a limitation on the variation of the frequency f of the load voltage at the time of occurrence of stand-alone operation in the grid-connected inverter according to the embodiment of the present invention, that is, in the clearing time, When single operation is detected on the basis of the frequency of the load voltage, it can not be out of NDZ (Non Detection Zone). Therefore, the grid-connected inverter according to the embodiment of the present invention can detect the single operation based on the reactive power Q output from the inverter.

Specifically, the control unit can control the inverter to output reactive power (Q) periodically fluctuating in the values of Positive, 0, and Negative as shown in FIG. This is to consider the load conditions. At this time, the reactive power Q output from the inverter may be set to 5% or more of the active power P, for example.

When a single operation occurs, there is becomes equal to the reactive power (Q _inv) and reactive power (Q _Load) of the load output by the inverter, at this time, the reactive power (Q _Load) of the load is expressed as shown in Equation (1) below, .

[Equation 1]

In Equation (1), Q _load represents the reactive power of the load, P _load represents the effective power of the load, Q _f represents the quality factor of the RLC parallel load, f _res represents the resonance frequency of the load, and f represents the inverter output voltage frequency.

Since the reactive power Q output from the inverter periodically fluctuates as shown in the graph of FIG. 5, according to Equation (1), it is general that the frequency f of the load voltage fluctuates when single operation occurs. However, in the grid-connected inverter according to the embodiment of the present invention, the indirect current control scheme is applied to limit the variation of the frequency f of the load voltage, so that the reactive power Q output from the inverter changes. That is, the grid interconnected inverter according to the embodiment of the present invention can not output the reactive power (Q) corresponding to the command value of the control unit when the single operation is performed.

From this point of view, the control unit can set the reactive power command value as shown in FIG. 5, and can detect the sole operation of the inverter by counting the number of times the reactive power (Q) output from the inverter deviates from the reactive power command value.

Hereinafter, a method of controlling the grid-connected inverter according to an embodiment of the present invention will be described with reference to FIG.

The grid interconnection inverter according to an embodiment of the present invention can receive a DC voltage from a renewable energy source such as an ESS battery or solar light, convert it into an AC voltage, and supply it to an important load or system. In this case, the grid-connected inverter according to the embodiment of the present invention can be controlled according to the indirect current control method for stable power supply to the critical load. In addition, the grid-connected inverter according to the embodiment of the present invention can detect the independent operation of the inverter by applying the reactive power variation technique.

Specifically, referring to FIG. 6, the grid-connected inverter can set a reactive power command value (600). At this time, the reactive power command value can be set to fluctuate periodically with the values of Positive, 0, and Negative based on the active power.

In addition, the grid-connected inverter may control the reactive power to be periodically varied according to the reactive power command value (610).

In addition, the grid-connected inverter can check whether the reactive power output from the inverter is out of the reactive power setpoint (620).

In the grid-connected inverter, when the reactive power output from the inverter is out of the reactive power setpoint, it is determined that the stand-alone operation has occurred and the stand-alone operation can be detected (630). The grid interconnected inverter counts the number of times the reactive power deviates from the reactive power command value and can detect the occurrence of the independent operation when the counted number exceeds the predetermined threshold value.

In addition, the grid-connected inverter can be switched to the independent operation mode when independent operation is detected (640). That is, the grid-connected inverter can switch the inverter switch SW to the independent operation mode when the stand-alone operation is detected.

Hereinafter, advantageous effects of the grid-connected inverter according to the embodiment of the present invention will be described with reference to FIGS. 7 to 11. FIG.

FIGS. 7 to 11 are diagrams for explaining advantageous effects of the grid-connected inverter according to the embodiment of the present invention.

First, as shown in FIG. 7, a grid-connected inverter connected to an RLC parallel load (q-factor = 2.5) and having the specifications shown in Table 1 below was designed to simulate the operation of the grid-connected inverter alone.

[Table 1]

Here, the grid-connected inverter according to an embodiment of the present invention can perform the constant voltage control by applying the indirect current control method, and can output the reactive power according to the reactive power command value by applying the reactive power variation technique.

Referring to FIG. 8, the grid-connected inverter according to the embodiment of the present invention uses an indirect current control method, so that even when islanding occurs, the load voltage and frequency do not change, And it can be confirmed that even when the single operation is detected and the mode is switched to the independent operation mode, the mode is switched without the transient state. In addition, when the single operation is generated, it can be confirmed that the reactive power (Q) output from the inverter deviates from the reactive power command value, from which the single operation can be detected.

9 (a) shows a result of detection of the inverter alone by the conventional reactive power fluctuation technique in the grid-connected inverter to which the indirect current control method is applied. In the conventional reactive power fluctuation technique The single operation is detected based on the frequency of the load voltage. However, according to the indirect current control method, it can be confirmed that the single operation is not detected because there is no frequency change of the load voltage.

In contrast, FIG. 9B shows the result of detecting the inverter alone in the grid-connected inverter according to the embodiment of the present invention. Even if the indirect current control scheme is applied, in the conventional reactive power variation technique, It can be confirmed that the inverter can detect the stand-alone operation based on the reactive power, and it can be confirmed that the stand-alone operation of the inverter is detected and the mode is switched to the independent operation mode.

Also, as shown in FIG. 7, a grid-connected inverter connected to an RLC parallel load (q-factor = 2.5) and having the following Table 2 specifications was designed to simulate the operation of the grid-connected inverter alone.

[Table 2]

Fig. 10 shows the result of the single operation detection when the grid interconnected inverter supplies all the load electric power (DELTA P = 0W), Fig. 11 shows the case where the grid interconnected inverter supplies electric power to the load and the grid -500W). Fig. Referring to FIGS. 10 and 11, the grid-connected inverter according to an embodiment of the present invention is capable of supplying stable voltage to a load when an independent operation is generated by applying an indirect current control method, It can be confirmed that the conversion is possible. In addition, the grid-connected inverter according to an embodiment of the present invention detects single operation based on the reactive power of the inverter, so that it can be confirmed that the single operation is detected quickly within the time limit.

Such a control method of the grid-connected inverter may be implemented in an application or may be implemented in the form of program instructions that can be executed through various computer components and recorded in a computer-readable recording medium. The computer-readable recording medium may include program commands, data files, data structures, and the like, alone or in combination.

The program instructions recorded on the computer-readable recording medium may be ones that are specially designed and configured for the present invention and are known and available to those skilled in the art of computer software.

Examples of computer-readable media include magnetic media such as hard disks, floppy disks and magnetic tape, optical recording media such as CD-ROMs and DVDs, magneto-optical media such as floptical disks, media, and hardware devices specifically configured to store and execute program instructions such as ROM, RAM, flash memory, and the like.

Examples of program instructions include machine language code such as those generated by a compiler, as well as high-level language code that can be executed by a computer using an interpreter or the like. The hardware device may be configured to operate as one or more software modules for performing the processing according to the present invention, and vice versa.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (5)

A reactive power command value is set,
Controls the reactive power according to the reactive power command value to be outputted,
And detecting occurrence of the independent operation depending on whether the reactive power deviates from the reactive power command value. The method according to claim 1,
Further comprising switching to a stand-alone operation mode for shutting off an inverter switch connected to the system when the occurrence of the stand-alone operation is detected. The method according to claim 1,
Setting the reactive power command value includes:
And setting a reactive power command value periodically changing to a value of Positive, 0, or Negative based on the active power of the grid interconnected inverter. The method according to claim 1,
Detecting the occurrence of the independent operation according to whether or not the reactive power deviates from the reactive power command value,
And counts the number of times the reactive power deviates from the reactive power command value, and detects the occurrence of the independent operation when the counted number exceeds a predetermined threshold value. The method according to claim 1,
Connected inverter according to an indirect current control method in which voltage control is performed both in a grid-connected operation mode in which an inverter switch connected to the system is closed and in an independent operation mode in which the inverter switch is cut off.

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