WO2005119871A1

WO2005119871A1 - Directionnal and differential ground fault protective relay system for ungrounded dc traction power feed system and ground fault protective relay apparatus for detecting ground fault current

Info

Publication number: WO2005119871A1
Application number: PCT/KR2004/003369
Authority: WO
Inventors: Sang-Gi Chung; Rag-Gyo Jeong; An-Ho Lee; Yeon-Soo Kim
Original assignee: Korea Railroad Research Institute
Priority date: 2004-06-03
Filing date: 2004-12-21
Publication date: 2005-12-15
Also published as: JP2008502291A; KR100582214B1; KR20050115146A

Abstract

The present invention relates to a ground fault protective relay apparatus for an electric railroad. In the ground fault protective relay apparatus, the negative bus of a rectifier is grounded through a resistor and a current limiting device to detect a ground fault current for operating the ground fault protective relay apparatus in an ungrounded Direct Current (DC) traction power feed system. The current limiting device exhibits a high resistance characteristic during normal operation, thus limiting the leakage current to a reference value or less, whereas the current limiting device exhibits a low resistance characteristic at the time of occurrence of a ground fault, thus allowing a ground fault current, whose amount is sufficient to allow a ground fault to be detected, to flow.

Description

Description DIRECTIONNAL AND DIFFERENTIAL GROUND FAULT

PROTECTIVE RELAY SYSTEM FOR UNGROUNDED DC TRACTION POWER FEED SYSTEM AND GROUND FAULT PROTECTIVE RELAY APPARATUS FOR DETECTING GROUND FAULT CURRENT Technical Field

[1] The present invention relates to a ground fault protective relay apparatus for an electric railroad in which the negative bus of a rectifier is grounded through a resistor and a current limiting device to detect a ground fault current for operating the ground fault protective relay apparatus in an ungrounded Direct Current (DC) traction power feed system. In this case, the current limiting device exhibits a high resistance characteristic during normal operation, thus limiting the leakage current to a reference value or less, whereas the current limiting device exhibits a low resistance characteristic at the time of occurrence of a ground fault, thus allowing a ground fault current, whose amount is sufficient to allow a ground fault to be detected, to flow.

[2] Furthermore, in the present invention, whether a ground fault has occurred is determined by comparing a ground fault current value, which is detected with respect to a substation, with a reference current value. The side and section in which the ground fault has occurred are determined by comparing the sum of current values flowing through feeders and return lines on the right side and the sum of current values flowing through feeders and return lines on the left side with respect to the substation, and determining whether a ground fault sensing signal has been received from a neighboring substation.

[3] Background Art

[4] Generally, a traction power feed system for an electric railroad is installed along a railroad line to supply power to the train. In order to stably supply a required voltage to the end of a long railroad line, the traction power feed system is equipped with substations in appropriate respective sections. Each of the substations rectifies Alternating Current (AC) power to DC power and provides the DC power to the train via a DC distribution panel and a feeder.

[5] In the above-described traction power feed system for an electric railroad, when a ground fault occurs due to an unexpected accident or the like during normal operation, a ground fault protective relay apparatus detects the accident and quickly cuts off the supply of the power to protect various equipment and secure public safety.

[6] In a patent application entitled "Directional and Differential Ground Fault Protective Relay System for Electric Railroad and Method of Controlling the Same" and filed on March 19, 2004, the applicant of the present application proposed a method of detecting a ground fault and detecting the side in which the ground fault has occurred in such a way as to dispose a resistor between the earth and the negative bus of a rectifier, to allow a ground fault current to flow through the circuit, and to calculate the ground current between the negative bus of the rectifier and the earth from the sum of the current values of feeders and return lines.

[7] In order to allow such a ground fault protective relay apparatus to operate, a large amount of ground fault current must flow at the time of the occurrence of a ground fault, so that there is difficulty in selecting the resistance value of the ground resistor disposed between the earth and the negative bus of the rectifier. That is, if the resistance value of the ground resistor is high, it is impossible to obtain a ground fault current value sufficient to operate the ground fault protective relay apparatus. In contrast, if the resistance value is excessively low, a problem arises in that the rated load of the ground resistor increases.

[8] Furthermore, in regard to such a DC traction power feed system, an ungrounded system is used to prevent electro-chemical corrosion of an underground metallic object. When the resistor is disposed between the earth and the negative bus of the rectifier and the resistance value of the resistor is low, the corrosion of the underground metallic object, i.e., electro-chemical corrosion, may occur. Particularly, in the case where a ranning rail is used as a return circuit, a leakage current becomes high, so that a problem arises in that serious electro-chemical corrosion may occur.

[9] Disclosure of Invention Technical Problem

[10] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a ground fault protective relay apparatus for a directional and differential ground fault protective relay system, in which a ground resistor of the ground fault protective relay apparatus is disposed between the negative bus of a rectifier and the earth, and a current limiting device is connected in series to the ground resistor, thus preventing electro-chemical corrosion attributable to the leakage current that may occur when a ground resistance value is low. In this case, the current limiting device exhibits a high resistance characteristic during normal operation, thus limiting the leakage current to a reference value or less, whereas the current limiting device exhibits a low resistance characteristic at the time of occurrence of a ground fault, thus allowing a ground fault current, whose amount is sufficient to allow the ground fault to be detected, to flow. The present invention is advantageous in that the rated load of the ground resistor can be reduced using the current limiting device.

[11] Another object of the present invention is to provide a directional and differential ground fault protective relay system and method of detecting a ground fault, in which whether the ground fault has occurred is determined by comparing a current value, which enters through a ground resistor, with a reference current value, and the section in which the ground fault has occurred is determined by comparing the sum of the values of currents flowing through feeder and a return line on the left side with the sum of the values of currents flowing through feeders and return lines on the right side with respect to the substation, and determining whether a ground fault sensing signal has been received from a neighboring substation.

[12] Technical Solution

[13] In order to accomplish the above object, the present invention provides a ground fault protective relay apparatus for an ungrounded DC traction power feed system, wherein a negative bus of a rectifier is grounded via a current limiting device to detect a ground fault current.

[14] In addition, the present invention provides a directional and differential ground fault protective relay system, including a ground fault protective relay apparatus in which a negative bus of a rectifier is grounded via a current limiting device to detect a ground fault current; a detection unit for detecting current values flowing through feeders and return lines connected to substations, and the value of a leakage current flowing through a current limiting device; a detenriination unit for determining whether a ground fault has occurred by comparing the value of the leakage current flowing through the current limiting device with a reference current value, and determining the side and section in which the ground fault has occurred by comparing the sum of the values of currents flowing through the feeders and return lines on a left side and the sum of the values of currents flowing through the feeders and return lines on a right side, and using a ground fault sensing signal received from a neighboring substation; a transceiver unit for transmitting a ground fault sensing signal to a substation adjacent to the side in which the ground fault has occurred, according to a determination result of the determination unit, or receiving a ground fault sensing signal from a neighboring substation; and a feeder breaker controller for tripping feeder breakers on the side in which the ground fault has occurred, based on the side and section in which the ground fault has occurred and which are determined by the determination unit.

[15] In addition, the present invention provides a ground fault protection method, including the steps of detecting the value of a leakage current flowing through a current limiting device of a ground fault protective relay apparatus of a substation, and the values of currents flowing through feeders and return lines on right and left sides; determining the occurrence of a ground fault by comparing the detected leakage current value with a reference current value; detenmning the side in which the ground fault has occurred by comparing the sum of the values of currents flowing through feeders and return lines on a left side and the sum of the values of currents flowing through feeders and return lines on the right side; determining the section where the ground fault has occurred by transmitting a ground fault sensing signal to a substation adjacent to the side in which the ground fault has occurred, and determining whether the ground fault sensing signal has been received from a substation in the corresponding side; and tripping feeder breakers on the side in which the ground fault has occurred.

[16] Advantageous Effects

[17] As described above, in accordance with the present invention, a current limiting device is connected to a directional and differential ground fault protective relay system, so that the current limiting device lowers a leakage earth current to a reference value or less during normal operation, whereas the current limiting device allows a ground fault current, whose amount is sufficient to allow a ground fault current to be detected, to flow at the time of occurrence of a ground fault.

[18] Furthermore, if a relay system employing the above-described ground fault protective relay system is utilized, not only the precise and reliable detection of a ground fault but also the determination of the section where the ground fault has occurred can be performed. Accordingly, at the time of occurrence of the ground fault, only a faulty section can be isolated from the system. [19] Brief Description of the Drawings

[20] The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [21] FIG. 1 is a view illustrating the connection between the negative bus of a rectifier and a ground resistor; [22] FIG. 2 is a block diagram of a directional and differential ground fault protective relay system according to the present invention; [23] FIG. 3 is a diagram showing the locations of the current sensors of the directional and differential ground fault protective relay system in an electric railroad DC traction power feed system in accordance with the present invention; [24] FIG. 4 is a circuit diagram illustrating the ground configuration of the directional and differential ground fault protective relay system according to the present invention; [25] FIG. 5 is a characteristic curve of a current limiting device of the directional and differential ground fault protective relay system according to the present invention, which shows an example in which the current limiting device can be used in a DC 750V system; and [26] FIG. 6 is a flowchart illustrating the operation of the directional and differential ground fault protective relay system according to the present invention. [27] Best Mode for Carrying Out the Invention [28] Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. [29] As shown in FIG. 4, the ground resistor of a ground fault protective relay apparatus is disposed between the earth and the negative bus of a rectifier to detect a ground fault current, and a current limiting device 80 is connected in series to the ground resistor. [30] The resistance value of the ground resistor shown in FIG. 4 acts as a factor to limit the amount of the ground fault current at the time of the occurrence of a ground fault. Therefore, the resistance value of the ground resistor must be selected so that the amount of ground fault current detected by a current sensor is sufficient to determine the occurrence of the ground fault and the side in which the ground fault has occurred. If the resistance value is excessively low, the rated load of the resistor become excessively high and thus become inefficient. Generally, although it is determined that it is appropriate to set the resistance value of the ground resistor so that a maximum ground fault current value can be about 0.5-1.0 % of a maximum feeder load current, the resistance value may vary with the situation of an actual system, the detailed algorithm of a relay, the configuration of hardware, the accuracy of a current sensor or the like. A current limiting device 'X' which is newly added in the present invention, exhibits a high resistance characteristic during normal operation, thus limiting a leakage current to a reference value or less; whereas the current limiting device 'X' exhibits a low resistance characteristic at the time of the occurrence of a ground fault, thus allowing ground fault current, whose amount is sufficient to allow a ground fault to be detected, to flow.

[31] When the case of a 750 V DC light rail transit system whose maximum load current is 2000 A is taken as an example, a relay resistor must have a resistance value of 75 ohms or less to allow a ground fault current of a minimum 10 A to flow. In this case, when the resistor is directly connected between the DC negative bus and the earth, the leakage current becomes high and, thus, an electro-chemical corrosion problem may occur. However, when a current limiting device having a characteristic shown in FIG. 5 is connected in series to the relay resistor, the leakage current is limited to 10 mA or less during normal operation, and the ground fault current of 10A flows at the time of occurrence of a ground fault. It is also possible to lower the resistance value and the rated load of the relay resistor.

[32] FIG. 2 is a block diagram of a directional and differential ground fault protective relay system according to the present invention. FIG. 3 is a diagram showing the locations of the current sensors of the directional and differential ground fault protective relay system in an electric railroad DC traction power feed system according to the present invention.

[33] Referring to FIGS. 2 and 3, the directional and differential ground fault protective relay system includes a detection unit 10 for detecting the values of currents I to I and 1 8 a ground fault current I that flow through feeders and return lines connected to g substations, a determination unit 20 for determining whether a ground fault has occurred based on the ground fault current value I detected by the detection unit 10, g comparing the sum of the values of currents flowing through feeders and return lines on a left side and the sum of the values of currents flowing through feeders and return lines on a right side, and determining the side and section where the ground fault has occurred based on the ground fault sensing signal of a neighboring substation that is received from a transceiver unit 30, the transceiver unit 30 for transmitting the ground fault sensing signal to a substation adjacent to the location where the ground fault has occurred, or receiving a ground fault sensing signal from a neighboring substation in accordance with the determination result of the determination unit 20, a feeder breaker controller 40 for controlling feeder breakers, which are located on the side in which a ground fault has occurred, based on the side and section in which the ground fault has occurred and which are identified by the determination unit 20, a reclosing logic unit 50 for selecting one from among an up line and a down line and performing a reclosing logic after the feeder breakers have been tripped, and a reclosing determination unit 60 for determining whether the reclosing logic has been successfully performed.

[34] The operation of the directional and differential ground fault protective relay system constructed as described above is described below.

[35] The currents I to I , which flow through the feeders and the return lines that are 1 8 located on right and left sides with respect to the substations, and the ground fault currents I , which flow through the ground resistors, are detected by the detection unit g 10.

[36] The determination unit 20 compares the ground fault current I , which is detected g by the detection unit 10, with a reference current I to determine whether a ground SET fault has occurred, and compares the sum I +1 +1 +1 of the values of currents, which 1 2 3 4 flow through the feeders and the return lines on the right side, and the sum I +1 +1 +1 5 6 7 8 of the values of currents, which flow through the feeders and the return lines on the left side, to determine the side in which the ground fault has occurred.

[37] If it is determined that the ground fault has occurred on the left side with respect to the substation, the directional and differential ground fault protective relay system transmits a ground fault sensing signal through the transceiver unit 30 to the directional and differential ground fault protective relay system of a substation adjacent to the left side identical to the side in which the fault has occurred.

[38] That is, since the ground fault has occurred on the right side with respect to the substation located on the left side that received the ground fault sensing signal, the substation located on the left side transmits the ground fault sensing signal to the directional and differential ground fault protective relay system of the substation located on the right side.

[39] Through the above-described method, the directional and differential ground fault protective relay system installed in the substation determines that the ground fault has occurred in a neighboring section on the left side, based on the current value detected in its location, the ground fault sensed on the left side, and the ground fault sensing signal received from the substation adjacent to the left side.

[40] Furthermore, if the ground fault was detected but the ground fault sensing signal has not been received from the neighboring substation, the directional and differential ground fault protective relay system determines that the ground fault did not occur in a neighboring section. In this case, an optional time delayed backup trip routine installed in the system to prepare for a possible ground fault in the system itself is executed.

[41] Meanwhile, if, as the result of the determination, it is determined that the ground fault has occurred in the neighboring section, the directional and differential ground fault protective relay system separates a ground fault point from the system by tripping the feeder breakers, which supply power from the neighboring substation toward the side in which the ground fault has occurred, through the feeder breaker controller 40.

[42] That is, a faulty section is isolated from a sound section.

[43] However, in the above description, since it is impossible to determine which of the up and down lines has the ground fault, the reclosing logic unit 50 performs a reclosing logic and the reclosing determination unit 60 determines a correct line in which the ground fault has occurred.

[44] That is, if the feeder breaker for the up line is successfully reclosed, it means that the ground fault did not occur in the feeder for the up line. If the reclosing of the feeder breaker for the up line fails, it means that that the ground fault has occurred in the feeder for the up line.

[45] Such a reclosing process can be applied to the down line in the same manner.

[46] Accordingly, only a faulty section is separated from the system and the remaining sound section is kept supplied with power.

[47] This is described in more detail below.

[48] As shown in FIG. 3, the traction power feed system can be divided into a right traction power feed system and a left traction power feed system with respect to a substation B. The total amount of current that flow in all the directions is always 0 according to Kirchhoff s First Law.

[49] However, if a ground fault occurs on the right side, the total amount of current on the right side is I +1 +1 +1 =1 and the total amount of current on the left side is I +1 +1 1 2 3 4 g 5 6 7 +1 = 0, so that I +1 +1 +1 > I +1 +1 +1 . If the ground fault occurs on the left side, the 8 1 2 3 ₄ 5 6 7 8 result is the opposite. I is the value of a ground fault current that flows through the g ground resistor, as shown in FIG. 3. [50] Furthermore, if the ground fault occurs, the ground fault current value I becomes g high. Accordingly, whether a ground fault has occurred is determined by comparing the ground fault current value I with the reference current value I . g SET

[51] Moreover, the side in which the ground fault has occurred can be determined by comparing the sum I +1 +1 +1 of the current values on the right side and the sum I +1 1 2 3 4 5 6 +1 +1 of the current values on the left side. 7 8

[52] That is, if the sum I +1 +1 +1 of the current values on the right side is greater than 1 2 3 4 the sum I +1 +1 +1 of the current values on the left side, it can be determined that the 5 6 7 8 ground fault has occurred on the right side. In contrast, if the sum I +1 +1 +1 of the 5 6 7 8 current values on the left side is greater than the sum I +1 +1 +1 of the current values 1 2 3 4 on the right side, it can be determined that the ground fault has occurred on the left side. [53] If, as a result of the determination, it is determined that the ground fault has occurred on the right side with respect to the substation B, a ground fault sensing signal is transmitted to the directional and differential ground fault protective relay system of a substation C adjacent to the right side. [54] At the same time, since a ground fault point detected by the substation C is on the left side with respect to the substation C, the ground fault sensing signal is transmitted to the substation B adjacent to the left side. [55] Therefore, the substation B adjacent to the left side determines whether the ground fault has occurred in the section between the substation B and the substation C, based on both the ground fault sensing signal locally sensed and the ground fault sensing signal received from the neighboring substation C on the right side, and then trips both the feeder breakers B and B on the right side. 1 2

[56] Meanwhile, the substation C adjacent to the right side determines whether the ground fault has occurred between the substation B and the substation C, based on both the ground fault sensing signal locally sensed and the ground fault sensing signal received from the neighboring substation B on the left side, and then trips both the feeder breakers B and B on the left side to separate the faulty section from the 3 4 system. [57] Through the above-described method, the section in which the ground fault has occurred can be precisely separated from the system. [53] Reference numeral 100 designates a DC feeding substation, and reference numeral 200 designates a train traveling along the electric railroad. [59] FIG. 6 is a flowchart illustrating the operation of the directional and differential ground fault protective relay system according to the present invention.

[60] The operation of the directional and differential ground fault protective relay system includes the steps of detecting the value of a ground fault current flowing through the ground resistor of the relay system of a substation, and the values of currents flowing through feeders and return lines on right and left sides, comparing the ground fault current value and a reference current value to determine whether a ground fault has occurred, comparing the sum of the current values flowing through the feeders and the return lines on the left side and the sum of the current values flowing through the feeders and the return lines on the right side to determine the side in which the ground fault has occurred, transmitting a ground fault sensing signal to a substation adjacent to the side in which the ground fault has occurred based on the determined side, and deteπriining a faulty section by determining whether the ground fault sensing signal has been received from a substation in a corresponding side, and then tripping feeder breakers on the side of the ground faulty section.

[61] This is described in more detail below.

[62] The directional and differential ground fault protective relay system continuously detects current values I to I flowing through the feeders and the return lines on the 1 8 right and left sides with respect to substations and also detects earth leakage current value I flowing through the ground resistor at step S100. g [63] The system determines whether a ground fault has occurred by comparing the ground fault current value I with the reference current value I at step S200. If it is g SET determined that the ground fault current value I is lower than the reference current g value I , the system determines that it is in a normal state and continues to measure SET the current values I to I and the earth leakage current values I . 1 8 g

[64] Meanwhile, if it is determined that the ground fault current value I is greater than g the reference current value I , the system determines that a ground fault has occurred. SET

[65] In this case, the directional and differential ground fault protective relay system compares the sum I +1 +1 +1 of the values of currents flowing through the feeders and 1 2 3 4 the return lines on the right side and the sum I +1 +1 +1 of the values of currents 5 6 7 8 flowing through the feeders and the return lines on the left side at step S300. [66] If, as a result of the comparison, the sum I +1 +1 +1 of the current values on the 1 2 3 4 right side is greater than the sum I +1 +1 +1 of the current values on the left side, the 5 6 7 8 directional and differential ground fault protective relay system determines that the ground fault has occurred on the right side. [67] Therefore, the directional and differential ground fault protective relay system transmits a ground fault sensing signal to the directional and differential ground fault protective relay system of a substation adjacent to the right side at step S400. [68] In this case, the substation adjacent to the right side also determines whether a ground fault sensing signal has been received at step S410. If the ground fault sensing signal has been received from the neighboring substation on the right side, the directional and differential ground fault protective relay system determines that the ground fault has occurred in a neighboring section on the right side of the substation and thus trips both feeder breakers B and B on the right side at step S420. 1 2

[69] However, if the ground fault sensing signal has not been received from the substation adjacent to the right side, the directional and differential ground fault protective relay system executes the optional time delayed backup trip routine installed in the system for the purpose of back-up protection.

[70] After the feeder breakers B and B on the right side are all tripped, the directional 1 2 and differential ground fault protective relay system performs the reclosing logic of one of the up and down lines to determine which of the two railroad lines has the ground fault at step S600.

[71] Then, the directional and differential ground fault protective relay system determines whether the feeder breakers for the up line have been successfully reclosed by performing the reclosing logic at step S700. If it is determined that the feeder breakers for the up line have been successfully reclosed, the directional and differential ground fault protective relay system determines that a ground fault did not occur in the feeder for the up line at step S800.

[72] In contrast, if the feeder breakers for the up line have not been successfully reclosed, the directional and differential ground fault protective relay system determines that a ground fault has occurred in the feeder for the up line at step S900.

[73] Meanwhile, if the sum I +1 +1 +1 of the current values on the right side is smaller 1 2 3 4 than the sum I +1 +1 +1 of the current values on the left side at step S300, the di- 5 6 7 8 rectional and differential ground fault protective relay system determines that the ground fault has occurred on the left side.

[74] Therefore, the directional and differential ground fault protective relay system transmits a ground fault sensing signal to the directional and differential ground fault protective relay system of a substation adjacent to the left side at step S500.

[75] In this case, the directional and differential ground fault protective relay system determines whether a ground fault sensing signal has been received from the substation adjacent to the left side at step S510. If the ground fault sensing signal has been received from the substation adjacent to the left side, the directional and differential ground fault protective relay system determines that a ground fault has occurred in a neighboring section on the left side of the substation, and then trips both feeder breakers B and B on the right side at step S520. 3 4

[76] Meanwhile, if the ground fault sensing signal has not been received from the substation adjacent to the left side at step S510, the directional and differential ground fault protective relay system executes the optional time delayed backup trip routine installed in the system for the purpose of back-up protection. After both the feeder breakers B and B for the left side are tripped, the directional and differential ground 3 4 fault protective relay system performs a reclosing logic for one of the up and down lines to determine which of the two lines has the ground fault at step S600.

[77] The directional and differential ground fault protective relay system determines whether the feeder breakers for the up line have been successfully reclosed by performing the reclosing logic at step S700. If the feeder breakers for the up line have not been successfully reclosed, the directional and differential ground fault protective relay system determines that a ground fault did not occur in the feeder for the up line at step S800.

[78] In contrast, if the feeder breakers for the up line have not been successfully reclosed, the directional and differential ground fault protective relay system determines that a ground fault has occurred in the feeder for the up line at step S900.

[79] In the reclosing logic, it is important not to make an attempt to reclose a corresponding line after the operation of a breaker in the case where the reclosing of the up line or the down line failed.

[80] As described above, in accordance with the present invention, a current limiting device is connected to a directional and differential ground fault protective relay system, so that the current limiting device lowers a leakage earth current to a reference value or less during normal operation, whereas the current limiting device allows a ground fault current, whose amount is sufficient to allow a ground fault current to be detected, to flow at the time of occurrence of a ground fault.

[81] Furthermore, if a relay system employing the above-described ground fault protective relay system is utilized, not only the precise and reliable detection of a ground fault but also the determination of the section where the ground fault has occurred can be performed. Accordingly, at the time of occurrence of the ground fault, only a faulty section can be isolated from the system.

[82] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Industrial Applicability [83] the present invention provides a ground fault protective relay apparatus for an ungrounded DC traction power feed system, wherein a negative bus of a rectifier is grounded via a current limiting device to detect a ground fault current.

Claims

[1] A ground fault protective relay apparatus for an ungrounded Direct Current (DC) traction power feed system, wherein a negative bus of a rectifier is grounded via a current limiting device to detect a ground fault current.

[2] The ground fault protective relay apparatus as set forth in claim 1, wherein a ground resistor is connected in series to the current limiting device.

[3] The ground fault protective relay apparatus as set forth in claim 1 or 2, wherein the current limiting device has high resistance when a terminal voltage is low, and has low resistance when the terminal voltage is high.

[4] A directional and differential ground fault protective relay system, comprising: a ground fault protective relay apparatus in which a negative bus of a rectifier is grounded via a current limiting device to detect a ground fault current; a detection unit for detecting values of currents flowing through feeders and return lines connected to substations, and a value of a leakage current flowing through a current limiting device; a determination unit for determining whether a ground fault has occurred by comparing the value of the leakage current flowing through the current limiting device with a reference current value, and determining a side and a section in which the ground fault has occurred by comparing the sum of the values of currents flowing through the feeders and the return lines on a left side and the sum of the values of currents flowing through the feeders and the return lines on a right side, and using a ground fault sensing signal received from a neighboring substation; a transceiver unit for transmitting a ground fault sensing signal to a substation adjacent to a side in which the ground fault has occurred, according to a determination result of the determination unit, or receiving a ground fault sensing signal from a neighboring substation; and a feeder breaker controller for tripping feeder breakers on the side in which the ground fault has occurred, based on the side and section in which the ground fault has occurred and which are determined by the deteπriination unit.

[5] The directional and differential ground fault protective relay system as set forth in claim 4, wherein the feeder breaker controller comprises: a reclosing logic unit for, after the feeder breakers have been tripped, selecting one from among an up line and a down line and performing a reclosing logic; and a reclosing determination unit for determining whether the reclosing logic has been successfully performed.

[6] A ground fault protection method, comprising the steps of: detecting the value of a leakage current flowing through a current limiting device of a ground fault protective relay apparatus of a substation, and the values of currents flowing through feeders and return lines on right and left sides; determining the occurrence of a ground fault by comparing the detected leakage current value with a reference current value; determining a side in which the ground fault has occurred by comparing the sum of the values of currents flowing through the feeders and the return lines on the left side and the sum of the values of currents flowing through the feeders and the return lines on the right side; determining a section where the ground fault has occurred by transmitting a ground fault sensing signal to a substation adjacent to the side in which the ground fault has occurred, and determining whether the ground fault sensing signal has been received from a substation in the corresponding side; and tripping feeder breakers on the side in which the ground fault has occurred.

[7] The ground fault protection method as set forth in claim 6, further comprising the step of, after the feeder breakers have been tripped, determining a faulty section (up line or down line) through performance of a reclosing logic and separating the faulty section from a system.