SYSTEM AND METHOD FOR SWITCHING LINE SHORT-CIRCUIT FAULT SECTION IN INVERTER-BASED STAND-ALONE MICROGRID

Abstract

The present invention provides a system and a method for switching a line short-circuit fault section in an inverter-based stand-alone microgrid, the system comprising: multiple intelligent switchgears disposed on a line in a stand-alone microgrid so as to control opening or closing of the line; a battery inverter having a built-in current limiter so as to limit a current output therefrom; and an operating system for receiving short-circuit fault information of the line from the multiple intelligent switchgears, determining a fault section, and controlling the battery inverter to limit the output current. The present invention can quickly and accurately switch a line short-circuit fault section occurring in a stand-alone microgrid which does not include separate cutoff equipment such as a recloser.

Claims

1. A system for switching a line short-circuit fault section in an inverter-based stand-alone microgrid, the system comprising: multiple intelligent switchgears disposed on a line in the stand-alone microgrid so as to control opening or closing of the line; a battery inverter having a current limiter built therein so as to limit an output current; and an operating system receiving information on a short-circuit fault of the line from the multiple intelligent switchgears, determining a fault section, and controlling the battery inverter to limit the output current.

2. The system of claim 1, wherein the operating system determines the fault section on the basis of information received from the intelligent switchgears and performs control to open an intelligent switchgear disposed in front of the determined fault section.

3. The system of claim 2, wherein the operating system checks whether or not the battery inverter limits the output current and performs control to open the intelligent switchgear.

4. The system of claim 3, wherein the operating system checks whether a magnitude of a short-circuit fault current limited by the battery inverter is smaller than a magnitude of a minimum current that is capable of opening the intelligent switchgear and performs control to open the intelligent switchgear.

5. The system of claim 3, wherein the operating system checks whether a magnitude of a short-circuit fault current limited by the battery inverter is smaller than a magnitude of a minimum current that is capable of opening the intelligent switchgear and performs control to open a circuit breaker disposed on the line when the current is not limited by the battery inverter.

6. The system of claim 4, wherein when the short-circuit fault occurs in a section between a circuit breaker disposed on the line and a first intelligent switchgear from the circuit breaker, the operating system performs control to open the circuit breaker.

7. The system of claim 4, wherein when the intelligent switchgear is opened and the failure section is switched, the operating system performs control to terminate the current limiting of the battery inverter.

8. The system of claim 7, wherein the operating system checks whether or not the fault section is switched and performs control to open the circuit breaker disposed on the to line when the switching of the fault section is impossible.

9. The system of claim 8, wherein the operating system checks whether or not normal current flows in an intelligent switchgear disposed in front of the opened intelligent switchgear to check whether or not the fault section is switched.

10. A system for switching a line short-circuit fault section in an inverter-based stand-alone microgrid, the system comprising: multiple intelligent switchgears disposed on a line in the stand-alone microgrid so as to control opening or closing of the line; an output limiting means limiting an output current on the line; and an operating system controlling the output limiting means to limit the output current, when a short-circuit fault is determined by receiving information on the short-circuit fault of the line from the multiple intelligent switchgears.

11. The system of claim 10, wherein the operating system opens an intelligent switchgear disposed in front of a point of the short-circuit fault when the output current is limited by the output limiting means.

12. The system of claim 11, wherein the output limiting means limits the output current to be less than a magnitude of a minimum current that is capable of opening the intelligent switchgear.

13. A method of switching a line short-circuit fault section in an inverter-based stand-alone microgrid, the method comprising: limiting an output current in a battery inverter having a current limiter built therein to limit the output current, when a short-circuit fault occurs on a line of the stand-alone microgrid; and opening an intelligent switchgear disposed in front of a point of the short-circuit fault when the output current is limited in the limiting of the output current.

14. The method of claim 13, further comprising: measuring, by the intelligent switchgear, a fault current to transmit fault information to an operating system; determining whether or not the fault information received by the operating system is on the short-circuit fault; and limiting the output current in the battery inverter when the short-circuit fault is determined.

15. The method of claim 14, wherein the determining of whether or not the fault information is on the short-circuit fault determines that there is the short-circuit fault when a magnitude of the fault current received according to the fault information is larger than an operating current of an over current relay.

16. The method of claim 13, further comprising: determining whether a fault section due to the short-circuit fault is a section between a circuit breaker on the line and a first intelligent switchgear; and opening the circuit breaker on the line when the fault section is the section between the circuit breaker on the line and the first intelligent switchgear.

17. The method of claim 13, wherein the limiting of the output current limits the output current to be less than a magnitude of a minimum current that is capable of opening the intelligent switchgear.

18. The method of claim 17, further comprising: checking whether the output current is limited to less than the magnitude of the minimum current that is capable of opening the intelligent switchgear in the limiting of the output current, wherein the opening of the intelligent switchgear is performed by checking whether the output current is limited.

19. The method of claim 18, further comprising: opening a circuit breaker disposed on the line when the output current is not limited, as a result of checking the limiting of the output current.

20. The method of claim 18, further comprising: terminating the limiting of the output current, when the intelligent switchgear is opened to switch the fault section in the opening of the intelligent switchgear.

21. The method of claim 20, further comprising: opening a circuit breaker disposed on the line when the switching of the fault section is impossible.

22. The method of claim 21, further comprising checking whether or not normal current flows in an intelligent switchgear disposed in front of the opened intelligent switch, thereby checking whether or not the fault section is switched by checking whether or not the normal current flows in the intelligent switchgear disposed in front of the opened intelligent switchgear.

Description

DESCRIPTION OF DRAWINGS

[0056] FIG. 1 is a view conceptually illustrating a situation due to a line fault in a stand-alone microgrid in the related art.

[0057] FIG. 2 is a view illustrating switching of a fault section by an intelligent switchgear.

[0058] FIG. 3 is a view illustrating an example of a fault section according to a short-circuit fault.

[0059] FIG. 4 is a view illustrating a system for switching a line short-circuit fault section in an inverter-based stand-alone microgrid according to the present invention.

[0060] FIGS. 5A and 5B are a view illustrating an output current limit performed by an inverter current limiter.

[0061] FIG. 6 is a flow diagram illustrating a method of switching a line short-circuit fault section in an inverter-based stand-alone microgrid according to the present invention.

BEST MODE

[0062] In order to fully understand operational advantages of the present invention and objects achieved by the practice of the present invention, reference should be made to the accompanying drawings which illustrate preferred embodiments of the present invention and contents described in the accompanying drawings.

[0063] In describing the preferred embodiments of the present invention, a description of known or repeated descriptions that may unnecessarily obscure the gist of the present invention will be reduced or omitted.

[0064] As intelligent distribution systems have been distributed to island areas, intelligent switchgears have been built in many island areas. However, it is possible only to provide and receive fault information through communication, but impossible to switch the fault section at the time of the short-circuit fault.

[0065] However, even when a short-circuit fault occurs, when the magnitude of the fixed current becomes small enough that the intelligent switchgear may open, it is possible to switch the fault section using only switchgear without a need to open the circuit breaker of the line.

[0066] For this purpose, a device for reducing the magnitude of the fault current is required, which may be realized by a battery inverter which is the main power source of the stand-alone microgrid.

[0067] That is, it is possible to instantaneously limit the magnitude of the fault current using a current limiter built in the inverter when the short-circuit fault occurs.

[0068] In other words, since the battery inverter limits the magnitude of the short-circuit to fault current, and the intelligent switchgear is opened to switch only the fault section without a power failure of the entire line, it is possible to supply the power to the normal sections.

[0069] In consideration of this, a method of switching a fault section without a power failure when the line short-circuit fault occurs in the inverter-based stand-alone microgrid according to the present invention is configured of detecting the short-circuit fault by an operating system, switching the fault section via the intelligent switchgear, and limiting the magnitude of the fault current by using the battery inverter.

[0070] When the short-circuit fault occurs in the line, the fault current flows, the fault information is transferred to a stand-alone microgrid operating system (hereinafter referred to as operating system) by the intelligent switchgear.

[0071] The operating system determines whether the short-circuit fault occurs through the received fault information. In addition, fault sections are determined by the operating system according to the fault information received from each intelligent switchgear.

[0072] Next, the operating system instructs the battery inverter to perform the output current limiting function, so that the magnitude of the short-circuit current may be made small enough that the switchgear may open through the output current limiting function of the inverter.

[0073] Generally, when the short-circuit fault occurs, a short-circuit current of a few to several tens of times of rated current flows, whereby a switchgear without a fault current shut-off function cannot interrupt the short-circuit current. However, when the magnitude of the current becomes smaller through the output current limiting function, it is possible to switch the fault section because the switchgear may be opened.

[0074] When the magnitude of the short-circuit current is limited to be equal or less than a predetermined value (the magnitude of the current that the switchgear may open), the operating system instructs to open the intelligent switchgear disposed just before the fault section.

[0075] In this case, as shown in FIG. 2, only the fault section is switched and the power may be normally supplied to the normal sections in front of the opened intelligent switchgear without a power failure.

[0076] However, when the point where the fault occurs is immediately behind the circuit breaker (between the high voltage circuit breaker and the first circuit breaker on the resin line), the circuit breaker must be opened because it is not impossible to switch the fault section using the breaker.

[0077] Multiple intelligent switchgears capable of detecting the short-circuit fault are disposed on the line, and fault sections are classified according to the point where the fault occurs on the basis of the intelligent switches.

[0078] FIG. 3 illustrates an example having three switchgears on the line, in which the fault sections are shown according to the point where the short-circuit fault occurs on the basis of the three switchgears. In the case of the example having three switchgears, it will be appreciated that it is possible to be classified into fault sections of four cases.

[0079] FIG. 4 illustrates a system for switching a line short-circuit fault section in an inverter-based stand-alone microgrid according to an embodiment of the present invention.

[0080] Referring to FIG. 4, a system for switching a line short-circuit fault section in an inverter-based stand-alone microgrid according to an embodiment of the present invention includes multiple intelligent switchgears, a battery inverter, and an operating system.

[0081] The intelligent switchgear includes a switchgear and a feeder remote terminal unit (FRTU).

[0082] The switchgear has a PT and a CT built therein so that the failure information may be acquired by measuring the voltage, the current, and the like, and the switchgear may be opened and closed depending on the situation.

[0083] The terminal unit is connected to the operating system via communication to transmit the fault information, receive the instruction, and control the opening and closing of the switchgears.

[0084] In the present invention, multiple intelligent switchgears are disposed on the line, and the fault sections may be classified on the basis of the intelligent switchgears.

[0085] When the short-circuit fault occurs, the fault is detected by the switchgear, and the terminal unit is connected to the operating system via communication to transfer the corresponding information thereto.

[0086] When the short circuit current is limited by the operating system and the battery inverter, the specific switchgear is opened according to the fault section so as to switch the fault section.

[0087] The battery inverter serves to convert the DC voltage of the battery into the AC voltage of the system using the power electronic-based switching element.

[0088] In the stand-alone microgrid, the battery inverter becomes the main power source and the voltage and frequency of the whole system is maintained.

[0089] The battery inverter has a current limiter built therein, thereby making it possible to limit the output current.

[0090] According to the present invention, the operating system instructs to operate the output current limiting function according to the occurrence of the fault, the magnitude of the short-circuit current is limited to the magnitude that the switchgear may be opened and closed

[0091] The operating system implements and performs the entire logic of the present invention.

[0092] The operating system is connected to the terminal unit and the battery inverter of the multiple intelligent switchgears via communication and transmits/receives failure information and instructions.

[0093] The operating system determines the fault section according to the detection result of the short-circuit fault received from the terminal unit of the intelligent switchgear and instructs the battery inverter to perform the output current limiting function.

[0094] Accordingly, when the short-circuit current is limited, the terminal unit is instructed to open the switchgear according to the fault section.

[0095]

[0096] FIG. 6 shows a flow diagram illustrating a method of switching a line short-circuit fault section in an inverter-based stand-alone microgrid according to an embodiment of the present invention.

[0097] Hereinafter, a method of switching a line short-circuit fault section in an inverter-based stand-alone microgrid according to an embodiment will be sequentially described with reference to FIG. 6.

Short-Circuit Fault Detection and Fault Section DeterminationS10, S20

[0098] When the short-circuit fault occurs in the line, a fault current flows. The fault current is measured by the intelligent switchgear, and the terminal unit determines that the fault has occurred through the fault current.

[0099] Next, the corresponding fault information (three-phase voltage, current) is to transmitted to the operating system in order to determine a type of the fault.

[0100] The operating system determines whether or not the short-circuit fault has occurred through the received information. When no voltage is present and the magnitude of the detected fault current is greater than the operating current of an over current relay (OCR), it is determined that the fault is a short-circuit fault.

[0101] When the operating system checks the occurrence of the short-circuit fault, the operating system receives information from multiple intelligent switchgears disposed on the line to determine the fault section. The fault section is determined by checking no-voltage and a direction of the fault current measured by the switchgear.

[0102]

Checking Whether There is Fault in First Fault SectionS30

[0103] When the fault section is the first section immediately after the breaker (between the high-voltage breaker and the first breaker on the resin line), it is impossible to switch the fault section because there is no switchgear in front of the fault section.

[0104] In this case, the breaker must be opened, and the entire line becomes a fault section, whereby a power failure occurs.

[0105]

Limiting Short-Circuit Fault Current by Battery InverterS40

[0106] When the fault occurrence point is not the first section, the operating system instructs the battery inverter to perform the output current limiting function.

[0107] Upon receiving the instruction, the inverter performs the current limiting function using the built-in current limiter to limit the magnitude of the short-circuit fault current to a current magnitude enough that the switchgear may open/close.

[0108] As shown in FIGS. 5A and 5B, the magnitude of current instantaneously sharply increases due to the occurrence of the short-circuit fault, but the current is immediately limited by the battery inverter.

[0109] As shown, the magnitude of the current is limited to approximately the same magnitude as the load current before the short-circuit fault.

[0110]

Determining Whether the Intelligent Switchgear May Be OpenedS50

[0111] The operating system determines whether the magnitude of the fault current becomes small enough that the switchgear may open or close through the output current limiting function of the battery inverter.

I.sub.FI.sub.SW [Equation 1]

[0112] Herein, I.sub.F: magnitude of short-circuit fault current limited by the inverter [A]

[0113] I.sub.SW: magnitude of minimum current that the switchgear may open [A]

[0114] When it is impossible to limit the short-circuit current to less than I.sub.SW due to a fault situation, a malfunction of the current limiting function, or the like (S51), the high-voltage circuit breaker on the line is opened to prevent the fault from being propagated (S90).

[0115]

Switching Fault Section Via Operation of Intelligent SwitchgearS60

[0116] When the condition of S50 is satisfied, the operating system instructs to open the intelligent switchgear provided immediately before the fault section, and accordingly the intelligent switchgear is opened so that the fault section is switched and the normal section is normally supplied with power without a power failure. The opened intelligent switchgear transfers the corresponding information to the operating system.

Checking Whether Fault Section is Normally Switched and Terminating Current Limiting FunctionS70, S80

[0117] The operating system receives information on the opened switchgear from the terminal unit of the opened switchgear and checks whether or not the fault is detected in an intelligent switchgear (located in the normal section) disposed in front of the opened intelligent switchgear so as to check whether the fault section is normally switched.

[0118] When the switchgear is normally opened to switch the fault section, the operating system instructs the battery inverter to terminate the current limiting function.

[0119] When the switchgear cannot be opened due to a malfunction, a communication error, or the like so that the fault section is not normally switched (S71), the high voltage circuit breaker on the line is opened to prevent the fault from being propagated (S90).

[0120]

[0121] As described above, the system and method of switching a line short-circuit fault section in the inverter-based stand-alone microgrid according to the present invention enables the intelligent switchgear to be opened by controlling a battery inverter when a short-circuit fault occurs in a stand-alone microgrid which does not include separate cutoff equipment such as a recloser, whereby it is possible to quickly and accurately identify the fault section and switch the fault line, and to quickly cope with the short-circuit fault and prevent the normal section from being shutdown, thereby making it possible to maintain the power supply more stably.

[0122] While the present invention has been described with reference to exemplary embodiments, it will be obvious to those of ordinary skill that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, is intended to cover various modifications and alternative arrangements included within the spirit and scope of the invention. Accordingly, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

[0123]

DESCRIPTION OF THE REFERENCE NUMERALS

[0124] S10: detecting short-circuit fault

[0125] S20: determining fault section

[0126] S30: checking whether there is fault in first fault section?

[0127] S40: limiting fault current by battery inverter

[0128] S50: determining whether it is possible to open intelligent switchgear

[0129] S51: limiting of fault current is impossible?

[0130] S60: switching fault section through operation of intelligent switchgear

[0131] S70: checking whether fault section is normally switched

[0132] S71: checking whether opening of switchgear is impossible

[0133] S80: terminating current limiting

[0134] S90: opening high voltage circuit breaker