WO2006001566A1

WO2006001566A1 - Ground overcurrent protection relay system for ungrounded dc power feed system and method of controlling the same

Info

Publication number: WO2006001566A1
Application number: PCT/KR2004/003370
Authority: WO
Inventors: Sang-Gi Chung; Sock-Yeun Han; Rag-Gyo Jeong; Yeon-Soo Kim
Original assignee: Korea Railroad Research Institute
Priority date: 2004-06-26
Filing date: 2004-12-21
Publication date: 2006-01-05
Also published as: KR100602914B1; JP2008504795A; KR20050122978A

Abstract

The present invention provides a ground protection relay device, in which a current limiting means having different resistance characteristics according to voltage differences between both ends thereof is constructed in a ground protection relay, and particularly placed between the negative bus of a rectifier and the ground. Therefore, in a normal operating state, the current limiting means exhibits high resistance characteristics, thus limiting a leakage current to a reference level or less, while, at the time of a ground fault, the current limiting means exhibits small resistance characteristics, thus enabling a ground fault current having sufficient value for measurement to flow through the current limiting means. Further, the present invention provides a ground protection relay system and method of controlling the same, which determines the occurrence of a ground fault by measuring the value of ground fault current flowing through the current limiting means, and determines a faulty feeder by comparing variations in currents flowing through feeders with variation in ground fault current for the same period of time.

Description

GROUND OVERCURRENT PROTECTION RELAY SYSTEM FOR UNGROUNDED DC POWER FEED SYSTEM AND METHOD OF CONTROLLING THE SAME Technical Field [1] The present invention relates to a ground protection relay system, which determines the occurrence of a ground fault (detection of a ground faulty section) and a ground faulty line in an ungrounded direct current power feed system for trains. Background Art [2] Generally, a power feed system for an electric railway is installed along a railroad track so as to supply a train with power. In order to stably supply a defined voltage to the end of a long railroad track, substation equipment is provided at regular intervals. The substation equipment rectifies Alternating Current (AC) power into Direct Current (DC) power and supplies the DC power to the train through a DC distributing panel and a feeder. [3] In such a power feed system for an electric railway, a ground protection relay is installed to protect various equipment and guarantee public safety when a ground fault occurs due to unexpected accidents, etc. during normal operation, thus preventing damage caused by a ground fault and promptly cutting off the supply of power. [4] A DC power feed system for a metropolitan railway is operated in an ungrounded manner to prevent electrolytic corrosion. [5] Therefore, since a ground fault current does not flow even if a ground fault occurs, a voltage-type relay, not a current-type relay, is used as a ground protection relay for the DC power feed system. [6] The voltage-type relay is problematic in that it can only determine whether a fault occurs or not, but cannot determine the location where the fault occurs. [7] Therefore, there may occur a problem in that power is interrupted even to a sound section as well as a faulty section and a risk to the safety of passengers may be caused according to circumstances. [8] In the case of FIG. 1, if a ground fault occurs on a location of a first train, feeder breakers B B , B and B of all substations A, B, C and D are tripped to cut off the 1, 2 3 4 supply of power to a second train, operating on a sound section, thus causing a risk to the safety of passengers. [9] Reference numerals RLl to RL4 that have not been described are conventional ground protection relays, which are installed as shown in FIG. 2. [10] The conventional ground protection relay is constructed in such a way that a resistor is placed between the ground and a negative bus of a rectifier to allow a ground fault current to flow through the resistor, a voltage at both ends of the resistor is measured, the measured voltage is compared to a preset value to determine the occurrence of a ground fault, and the feeder breakers B , B , B and B for power 1 2 3 4 supply installed in substations are operated depending on the determination results to cut off the supply of power. [11] Fbwever, the conventional ground protection relay is problematic in that, since a ground fault current flows into other substations in addition to a substation on a faulty section, it is difficult to determine a faulty section. [12] Disclosure of Invention Technical Problem [13] 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 protection relay device, in which a current limiting means having different resistance characteristics according to voltage differences between both ends thereof is installed in the ground protection relay and, in detail, placed between a negative bus of a rectifier and the ground, so that the current limiting means exhibits high resistance characteristics during normal operation, thus limiting a leakage current to a reference value or less, while the current limiting means exhibits very low resistance charac¬ teristics at the time of a ground fault, thus enabling a ground fault current having a sufficient value for measurement to flow through the current limiting means. [14] Another object of the present invention is to provide a ground protection relay system and method of controlling the same, which determines the occurrence of a ground fault (with a faulty section) by measuring the value of a ground fault current flowing through the current limiting means, and determines a faulty feeder by comparing the variations (increment) in currents flowing through feeders with the variation in a ground fault current for the same period of time. Technical Solution [15] In order to accomplish the above object, the present invention provides a ground protection relay device, comprising current limiting means connected to a negative bus of a rectifier to measure a ground fault current, the negative bus being grounded through the current limiting means. [16] Preferably, the current limiting means may be implemented with a device having different resistance characteristics according to voltage differences between both ends thereof, and is constructed so that the current limiting means exhibits high resistance characteristics when a terminal voltage thereof is low (in a normal operating state), thus limiting a leakage current to a reference value or less, while it exhibits low resistance characteristics when the terminal voltage thereof is high (at the time of a ground fault), thus preventing the amount of ground fault current from being limited by the current limiting means. [17] Further, the present invention provides a ground protection relay system using the above-described ground protection relay device, comprising current limiting means connected to a negative bus of a rectifier to measure a ground fault current, the negative bus being grounded through the current limiting means, current measurement means for measuring the ground fault current flowing through the current limiting means placed between the negative bus and the ground, feeder current measurement means for measuring currents flowing through up, down, left and right track feeders extending from each substation, and control means for comparing the ground fault current that flows through the current limiting means and is measured by the current measurement means with time-current characteristics of a relay to determine occurrence of a ground fault and a faulty section, for comparing the ground fault current that flows through the current limiting means and is measured by the current measurement means with the currents that flow through the feeders and are measured by the feeder current measurement means to determine a faulty line, and for outputting a trip control signal to a corresponding breaker. [18] Preferably, the control means may be operated so that it compares the ground fault current measured by the current measurement means with a set current value to determine occurrence of a ground fault, compares variation or waveform of the ground fault current, measured by the current measurement means, with variations or waveforms of the currents that flow through the feeders and are measured by the feeder current measurement means for a preset period of time to determine a faulty line, and outputs a trip control signal to a breaker corresponding to the faulty line after a predetermined time delay. Advantageous Effects [19] As described above, the present invention provides a ground overcurrent protection relay system, which does not require a sensor for measuring a current flowing through a feedback circuit to determine a faulty section or faulty line, and a device for exchanging a fault sensing signal with neighboring substations. [20] Further, the present invention is advantageous in that the relay system can perform together the detection of a ground fault and the determination of a faulty electric car line, unlike a conventional relay of determining a faulty electric car line in a reclosing process, so that only a breaker on the faulty electric car line is tripped in the present invention, thus improving the reliability of a power feed system. Brief Description of the Drawings [21] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [22] FIG. 1 is a view showing the flow of a ground fault current and the problem of a conventional ground protection relay; [23] FIG. 2 is a view showing the installation of a conventional ground protection relay; [24] FIG. 3 is a view of a schematic power feed system to show the variation in a ground fault current flowing into a substation; [25] FIG. 4 is a graph showing a time delay element of a relay device in a system according to the present invention; [26] FIG. 5 is a circuit diagram of an equivalent circuit for calculating a fault current in the system of the present invention; [27] FIG. 6 is a graph showing the variation in a current flowing through a current limiting means and variations in currents flowing through feeders to describe a method of determining a faulty line in the system of the present invention; [28] FIG. 7 is a view showing the construction of the ground of the current limiting means of the ground protection relay device in the system of the present invention; [29] FIG. 8 is a view showing feeders to describe the locations of current sensors in a ground protection relay system of the present invention; [30] FIG. 9 is a block diagram showing the construction of the ground protection relay system of the present invention; [31] FIG. 10 is a flowchart showing an operating flow of the system of the present invention; and [32] FIG. 11 is a view showing typical current- voltage characteristics of the current limiting means in the system of the present invention. [33] Best Mode for Carrying Out the Invention [34] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. [35] * Determination of faulty section (determination of occurrence of ground fault) [36] The determination of a faulty section is performed by measuring the value of a current flowing into a substation through a current limiting means of the substation. [37] That is, a high ground fault current always flows through a substation placed within a faulty section compared to substations placed outside of the faulty section. [38] FIG. 3 is a view of a schematic power feed system, which shows only a system placed on the left side of a faulty location for the simplification of circuit analysis. [39] FIG. 3 shows that a ground fault current I-b flowing into a substation B close to a faulty location is always higher than a ground fault current Ia flowing into a substation A relatively far from the faulty location. [40] In this case, since a load current does not flow through a current limiting means, it is omitted. [41] As shown in FIG. 3, a loop equation of the power feed system can be established as in the following Equations [1] and [2], [42]

V= I_a( R + R_a+ R_b)+I_b R_b

[i] [43]

V= I_b { R + R _b)+I_a I_b

[2] [44] where R is a source resistance, R and R are resistance components of feeders, and s a b I and I are loop currents. A load current does not flow through the current limiting a b means, so that it is omitted. [45] If Equation [2] is subtracted from Equation [1], subtraction results are arranged as in the following Equation [3]. If Equation [3] is arranged again, the following Equation [4] is obtained. [46] I_a( R + R _a)- I_b R =0

[3] [47]

[4] [48] As described above, the ground fault current I is always higher than the ground b fault current I , which proves that the ground fault current I flowing into the substation a b B close to the faulty location is always higher than the ground fault current I flowing a into the substation A relatively far from the faulty location. [49] Further, FIG. 4 shows the characteristics of time delay elements of a relay. [50] When a ground fault occurs on the right side of the substation B, the substation A always has a ground fault current lower than that of the substation B, that is, I < I , so a b that the relay of the substation B always operates earlier than that of the substation A, thus cutting off a fault current. [51] Referring to FIG. 4, t > t is satisfied, and t > t is satisfied. al bl a2 b2 [52] Further, if the relay of the substation B cannot normally operate due to failure, the relay of the substation A operates after a time delay, so that a function of a backup breaker can be performed. [53] Accordingly, a faulty section (the occurrence of a ground fault) is determined using the amount of current flowing through the current limiting means. [54] [55] *Determination of faulty line [56] The determination of which feeder is a faulty electric car line, on which a ground fault occurs, among the feeders for left, right, up and down tracks is performed, by a substation in a faulty section, by comparing the variation or waveform of a ground fault current with the variations or waveforms of currents flowing through respective feeders. [57] FIG. 5 is a circuit diagram of an equivalent circuit for calculating a ground fault current, which shows a simplified power feed system. [58] In this case, R is the resistance component of a source, R is the resistance s 1 component of a feeder, L is the inductance component of the source, and L is the s 1 inductance component of the feeder. [59] In FIG. 5, a ground fault current can be expressed by the following Equation [5]. [60]

[5] [61] In Equation [5], the ground fault current is the value of a ground fault current flowing from a faulty electric car line into the ground. [62] The value of the ground fault current flowing into a substation through the current limiting means of the substation may be lower than the value of the ground fault current of Equation [5]. The reason for this is that the ground fault current is distributed to the current limiting means of respective substations and flows through the current limiting means. [63] On the faulty feeder, since the fault current of Equation [5] may be added to a load current and increased, a faulty feeder can be determined by comparing the amount of current flowing through the current limiting means with the amount of current flowing through the feeder. [64] FIG. 6 shows the current variations on the faulty feeder and the current limiting means of FIG. 3 on the same time axis. [65] In this case, the size of a window t required to compare current variations should be noted. [66] The size of the comparison window is "t" of Equation [5]. [67] That is, the value of current variation is a function of "t". In order to obtain current variation having a sufficient value for comparison, "t" having a sufficient size is required, which would be the size of a minimum comparison window. [68] For example, if a comparison window t is set to 5ms in a system having a time constant of 20ms, the current variation is 22% of the ground fault current value

E R in a steady state according to Equation [5]. [69] Whether this value is sufficiently high to measure a current increment due to a ground fault must be investigated at the time of setting a relay. [70] The system of the present invention considering this fact can be constructed using the following embodiment. [71] FIG. 7 is a view showing the construction of the ground of a ground protection relay device according to the present invention, FIG. 8 is a view of feeders of a train to show the locations of current sensors of a ground protection relay system, and FIG. 9 is a block diagram showing the construction of the ground protection relay system of the present invention. [72] The ground protection relay system of the present invention includes a current limiting means 10 placed between a negative bus of a rectifier and the ground, a ground current sensor I for measuring a current flowing through the current limiting g means 10, current sensors I to I installed on left, right, up and down tracks, extending 1 4 from each substation, to measure currents flowing through feeders, and a control unit 20 for determining the occurrence of a ground fault depending on the value of the current that flows through the current limiting means 10 and is measured by the ground current sensor I , comparing the variation or waveform of the current measured by the g ground current sensor I with the variations or waveforms of the currents measured by g the feeder current sensors I to I to determine a faulty line, and outputting a trip 1 4 control signal to a corresponding breaker of breakers B to B . 1 4 [73] The control unit 20 includes a ground fault occurrence determination unit 21 for comparing a ground fault current measured by the ground current sensor I with a set g current to determine the occurrence of a ground fault, a faulty line determination unit 22 for comparing the variation or waveform of the ground fault current measured by the ground current sensor I with the variations or waveforms of currents that flow g through feeders and are measured by the feeder current sensors I to I for a preset 1 4 period of time to determine a faulty line, and a breaker control unit 23 for outputting a trip control signal to a corresponding breaker of the line, on which the ground fault occurs, among the breakers B to B . 1 4 [74] In the ground protection relay system of the present invention, the current sensors I to I for measuring currents flowing through the feeders for all right, left, up and down 4 tracks extending from each substation are constructed. [75] Further, the ground current sensor I for grounding the negative bus of a DC dis- g tributing panel of each substation through the current limiting means 10 and measuring the current flowing through the current limiting means 10 is constructed. [76] As described above, the current limiting means 10 is characterized in that it exhibits high resistance characteristics in a normal operating state, thus limiting a leakage current of a negative line to a reference value or less, while it exhibits low resistance characteristics at the time of a ground fault, that is, when the terminal voltage of the current limiting means 10 is high, thus preventing the amount of ground fault current from being limited by the current limiting means 10. [77] The typical voltage-current characteristics of the current limiting means 10 are shown in FIG. 11. [78] The current limiting means 10 has different resistance characteristics according to voltage differences between both ends thereof, and may be an arrestor or constructed using a power semiconductor device. [79] The ground protection relay system having the above construction executes the following operating control process. [80] The operating control process includes the steps of determining the occurrence of a ground fault by comparing the ground fault current, measured by the ground current sensor I , with a set current I ; determining a faulty line by comparing the variation or g set waveform of the ground fault current, measured by the ground current sensor I , with g the variations or waveforms of the currents that flow through the feeders and are measured by the feeder current sensors I to I for a preset period of time; and 1 4 outputting a trip control signal to a breaker corresponding to the faulty line after a pre¬ determined time delay has elapsed from the determination of the occurrence of the ground fault. [81] The operating process of controlling the breaker through the determination of a faulty section and a faulty line by the system of the present invention is described. [82] FIG. 10 is a flowchart showing an operating flow of the ground protection relay system of the present invention. [83] The currents I to I flowing through the feeders of individual substations and the 1 4 current I flowing through the current limiting means 10 are always measured. g [84] The measured current I flowing through the current limiting means 10 is compared g to a set current Iset. [85] If the measured current I flowing through the current limiting means 10 is higher g than the set current Iset, it is determined that a ground fault occurs. [86] If it is determined that a ground fault occurs in this way, a set time

At starts to be accumulated from that time. [87] An accumulated time T is compared to a set time T that is defined on a time d set delay characteristic curve shown in FIG. 4 and is set in the relay. [88] The set time T represents a time delay defined to correspond to the ground fault set current value on the time delay characteristic curve. [89] If the accumulated time T becomes higher than the set time T , a trip signal is d set output to a corresponding breaker of the breakers B to B . 1 4 [90] At this time, before the trip signal is output, a faulty line, that is, one of up, down, left and right track lines extending from each substation, is selected, and the trip signal is output to a breaker (any one of the breakers B to B ) corresponding to the faulty 1 4 electric car line. [91] As described above, the determination of a faulty electric car line is performed by comparing the variation or waveform of the ground fault current I with the variations g or waveforms of currents I to I flowing through individual feeders through the use of 1 4 variation curves obtained for the same period of time, as shown in FIG. 6. [92] The system of the present invention does not require a reclosing operation, and may further include a reclosing circuit control unit in the control unit 20 to selectively determine whether to perform the reclosing operation. [93] As described above, the present invention provides a ground overcurrent protection relay system, which does not require a sensor for measuring a current flowing through a feedback circuit to determine a faulty section or faulty line, and a device for exchanging a fault sensing signal with neighboring substations. [94] Further, the present invention is advantageous in that the relay system can perform together the measurement of a ground fault and the determination of a faulty line, unlike a conventional relay of determining a faulty line in a reclosing process, so that only a breaker on the faulty line is tripped in the present invention, thus improving the reliability of a power feed system. [95] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modi¬ fications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. [96] Industrial Applicability [97] The present invention relates to a ground protection relay system for determining the occurrence of a ground fault (detecting a ground faulty section) and detecting a ground faulty line in an ungrounded DC power feed system for trains.

Claims

Claims [1] A ground overcurrent protection relay device, comprising: current limiting means connected to a negative bus of a rectifier to measure a ground fault current, the negative bus being grounded through the current limiting means. [2] The ground overcurrent protection relay device according to claim 1, wherein the current limiting means is implemented with a device having different resistance characteristics according to voltage differences between both ends thereof, and is constructed so that the current limiting means exhibits high resistance charac¬ teristics when a terminal voltage thereof is low (in a normal operating state), thus limiting a leakage current to a reference value or less, while it exhibits low resistance characteristics when the terminal voltage thereof is high (at the time of a ground fault), thus preventing the amount of ground fault current from being limited by the current limiting means. [3] A ground overcurrent protection relay system for an ungrounded Direct Current (DC) power feed system, comprising: current limiting means connected to a negative bus of a rectifier to measure a ground fault current, the negative bus being grounded through the current limiting means; current measurement means for measuring the ground fault current flowing through the current limiting means placed between the negative bus and the ground; feeder current measurement means for measuring currents flowing through up, down, left and right track feeders extending from each substation; and control means for comparing the ground fault current that flows through the current limiting means and is measured by the current measurement means with time-current characteristics of a relay to determine occurrence of a ground fault and a faulty section, for comparing the ground fault current that flows through the current limiting means and is measured by the current measurement means with the currents that flow through the feeders and are measured by the feeder current measurement means to determine a faulty line, and for outputting a trip control signal to a corresponding breaker. [4] The ground overcurrent protection relay system according to claim 3, wherein the control means comprises: a ground fault occurrence determination unit for comparing the ground fault current measured by the current measurement means with a set current value, thus determining occurrence of a ground fault; a faulty line determination unit for comparing variation in the ground fault current, measured by the current measurement means, with variations in the currents that flow through the feeders and are measured by the feeder current measurement means for a preset period of time, thus determining a faulty line; and a breaker control unit for outputting a trip control signal to a breaker cor¬ responding to the faulty line in which the ground fault occurs. [5] The ground overcurrent protection relay system according to claim 3, wherein the control means comprises: a ground fault occurrence determination unit for comparing the ground fault current measured by the current measurement means with a set current value, thus determining occurrence of a ground fault; a faulty line determination unit for comparing a waveform of the ground fault current, measured by the current measurement means, with waveforms of the currents that flow through the feeders and are measured by the feeder current measurement means for a preset period of time, thus determining a faulty line; and a breaker control unit for outputting a trip control signal to a breaker cor¬ responding to the faulty line in which the ground fault occurs. [6] A method of controlling a ground overcurrent protection relay system for an ungrounded DC power feed system, comprising the steps of: determining occurrence of a ground fault by comparing a ground fault current that flows through current limiting means and is measured by current measurement means with a set current; determining a faulty line by comparing variation in ground fault current, measured by the current measurement means, with variations in currents that flow through feeders and are measured by feeder current measurement means for a preset period of time; and outputting a trip control signal to a breaker corresponding to the faulty line. [7] The control method according to claim 6, wherein the faulty line determining step is performed by comparing waveform of the ground fault current, measured by the current measurement means, with waveforms of the currents that flow through the feeders and are measured by the feeder current measurement means for a preset period of time.