WO2020026069A1

WO2020026069A1 - Traveling wave based auto-reclosing and fault location method for multi-terminal mixed lines

Info

Publication number: WO2020026069A1
Application number: PCT/IB2019/056264
Authority: WO
Inventors: Obbalareddi Demudu NAIDU
Original assignee: Abb Schweiz Ag
Priority date: 2018-07-30
Filing date: 2019-07-23
Publication date: 2020-02-06

Abstract

The invention relates to fault section and location identification in a multi-terminal mixed line, which connects three or more terminals with at least one overhead line and at least one cable. Here, the mixed line has three or more line sections connected at one or more junctions. The method comprises obtaining arrival times of travelling waves detected from measurements at the three or more terminals. Further, at least two values are estimated for the fault location, based on the arrival times, length of two or more line sections, and wave propagation velocity. One of the line sections is determined as having the fault based on a comparison of the at least two values with length of corresponding line sections. The corresponding value is determined as the actual fault location. According to the fault section identification, a trip signal is generated for controlling a switching device connected with the mixed line.

Description

TRAVELING WAVE BASED AUTO-RECLOSING AND FAULT LOCATION METHOD FOR MULTI-TERMINAL MIXED LINES

FIELD OF THE INVENTION

[001] The present invention generally relates to power transmission lines, and more specifically to travelling wave based fault section and location identification in multi terminal mixed lines.

BACKGROUND OF THE INVENTION

[002] Auto-reclosure (or auto-reclose) is referred to as the automatic closing of a switching device (e.g. a circuit breaker) in order to restore service in a power transmission line. Certain considerations are to be taken with respect to the auto reclosing function, especially in case of lines having cables (e.g. underground cables). As faults in underground cables tend to be permanent, it is important that auto-reclosure is not performed for cables. Special consideration has to be taken for auto-reclosure in mixed lines.

[003] A mixed power transmission line (mixed line) is a transmission line with non- uniform line impedance characteristics. In other words, a mixed line has at least two line sections, wherein impedance characteristic (e.g. surge impedance) of one section is different from impedance characteristic of the other section(s). Selective auto reclosure can be done for such lines only if it can be determined if the fault is on the cable portion, or on the overhead line portion. This makes fault section identification (FSI) critical in these systems. FSI becomes even more relevant and critical for multi terminal mixed lines (e.g. a three-terminal or tapped line).

[004] A mutli-terminal mixed line can have three or more terminals. Accordingly, there can be one or more junctions (or tap points). In such lines, electrical faults can happen in an overhead line, a cable, or at a junction (or tap) between overhead line(s) and / or cables. As the number of overhead line sections is typically more than the number of cables in these lines, auto-reclosing is desired in these lines. However, due to the complexity in accurately determining the section with the fault, it is in general difficult to implement auto-reclosing in such lines.

[005] Accuracy of traveling wave based methods is in general better than impedance / phasor based methods. In multi-terminal mixed lines, there is an existing single ended method for fault section and location identification. This method is based on machine learning (neural network) principles. Single ended methods have lower accuracy as compared to two ended methods. Also, it is difficult to create / maintain good models for the learning (e.g. training the neural network).

[006] In view of the above, it is desired to have improved method for fault section identification in multi-terminal mixed lines. Further, fault location is an important parameter for reducing maintenance time and increasing system availability. In multi - terminal lines, the problem of accurate fault location becomes multi-fold due to incorrect FSI. Thus, FSI becomes a mandatory input for getting reliable fault location in multi-terminal mixed lines.

SUMMARY OF THE INVENTION

[007] Various aspects of the invention relate to fault section identification and fault location determination in a multi -terminal mixed line. The mixed line connects three or more terminals (multi-terminal mixed line). For example, the mixed line can be a three terminal line connecting a first terminal, a second terminal and a third terminal. In such a three terminal line, there can be three sections and a junction (e.g. common junction (or tap) for the three sections), with a first section between the first terminal and the junction, a second section between the second terminal and the junction, and a third section between the third terminal and the junction. [008] The invention provides a method for fault section identification and fault location determination in the multi-terminal mixed line. For example, there may be an electrical fault in the multi-terminal mixed line due to disturbances like as bad weather, wind-borne debris etc. In such a case, a portion of the line or the entire line may be disconnected from the rest of power transmission network to prevent propagation of faults / damage to electrical equipment. The protection is enabled by controlling a switching device connected to the mixed line (e.g. through an Intelligent Electronic Device (IED)). Controlling the switching device includes operating the switching device to connect or disconnect the line. The switching device may be a circuit breaker, and the switching can be controlled with an auto-recloser.

[009] The method is performed with one or more processors associated with controlling the switching device. For example, the method can be implemented by an IED having a processor. This may be an IED associated with one of a line section, a junction, a terminal etc. The IED receives one or more signals from one or more measurement equipment connected to the mixed line. For example, the IED can be connected with a current transformer, and receives current signals corresponding to current as sensed from the mixed line from the current transformer. In an embodiment, the IED receives a signal(s) from the measurement equipment, and obtain measurements therefrom. In another embodiment, the measurement equipment publishes the measurements over a bus (e.g. process bus), and the IED (e.g. subscribed to receive data from such bus) receives the measurements over the bus.

[0010] In an embodiment, the method is implemented with a device associated with the multi-terminal mixed line, wherein the device has a plurality of modules. The plurality of modules can include, but need not be limited to, a travelling wave module, a fault section identification module and a trip module. The device can optionally include an input interface for obtaining measurements carried out at the three or more terminals. Each module carries out one or more steps of the method. Further, the plurality of modules may be implemented using one or more processors (e.g. of an IED). In one embodiment, the device is an IED associated with a terminal, and communicates with IEDs of other terminals of the mixed line. Thus, it has local measurements / travelling wave parameters, and receives measurements / travelling wave parameters as identified by other IEDs.

[0011] The method comprises obtaining arrival times of a first peak of travelling waves detected from measurements carried out at the three or more terminals. Travelling waves are generated when there are faults in the line. A travelling wave, and parameters thereof (e.g. arrival time, peak width, rise time etc.), can be detected from the measurements carried out at a terminal(s). For example, a current signal may be digitized and processed to detect a travelling wave. In accordance with an embodiment of the invention, the measurements are synchronized measurements of electrical parameters such as synchronized voltage / current measurements. For example, the measurements are synchronized measurements of currents carried out at the three or more terminals.

[0012] The method further comprises estimating at least two values for a fault location. Here, the at least two values are estimated based on the arrival times, length of two or more line sections of the three or more line sections, and propagation velocity of the travelling wave in one or more of the overhead line and the cable.

[0013] Consider a three terminal mixed line with an overhead line between a first terminal and a second terminal, and a cable connected at a tap point (or junction) on the overhead line and a third terminal. Thus, the three terminal mixed line has a first section between the first terminal and the junction, a second section between the second terminal and the junction, and a third section between the third terminal and the junction. Here, the first and second sections are overhead line sections while the third section is a cable. In accordance with the method, for such a three terminal line two values - a first value and a second value can be estimated for the fault location. [0014] The first value of the fault location is estimated based on the arrival times of the first peak of the travelling waves at the first and second terminals respectively, the length of the first and second sections, and the velocity of propagation of the travelling wave in the overhead line. For example, the first value can be estimated as:

[0015] In the above, LOHLI refers to length of the first section and LOHL2 refers to length of the second section, T_m and T_n refers to arrival times of first peak of the travelling waves at the first and second terminals, and VOHL refers to the wave propagation velocity in the overhead line. Similarly, a second value of the fault location is estimated from based on the arrival times of the first peak of the travelling waves at the first and third terminals respectively, the length of the first and third sections, and the velocities of propagation of the travelling wave in the overhead line and the cable respectively.

[0016] The method further comprises determining a line section of the three or more line sections as having the fault, based on a comparison of the at least two values of the fault location with the length of corresponding line sections. Considering the three terminal mixed line case as described above, the first and second values can be compared with the length of the first section to determine the faulted section. For example, the first section is determined as the faulted section if the first and second values are less than LOHLI. Similarly, the second section can be determined as the faulted section if the first value is greater than LOHLI and the second value is less than LOHLI, or the third section can be determined as the faulted section if the first value is less than LOHLI and the second value is greater than LOHLI. In this example, the two values may be quite close. Accordingly, before comparing with LOHLI, the difference between the two values can be compared with a threshold (e.g. close to zero value). If the difference is less than the threshold, then the fault is determined to be at the junction (or tap). Otherwise, as described, the faulted section may be one of the three sections. [0017] The method can also determine the fault location. A value of the at least two values estimated for the fault location is determined as the actual value for the fault location. This is according to the comparison of the at least two values with the length of the corresponding line sections. For instance, in the example wherein the fault is in the first section, then the first value as below can be determined as the fault location:

[0018] According to the fault section, a switching device may need to be operated. Accordingly, the method comprises controlling the switching device with a trip signal for protecting the multi-terminal mixed line. The trip signal is generated based on the determination of the section with the fault. The trip signal is associated with an auto reclose function, and the trip signal is generated in response to determining that the section with the fault is an overhead line section. In other words, the auto-reclose function is enabled only when it is determined that the fault is in the overhead line section, and not enabled (blocked) when it is determined that the fault is in the cable. This trip signal may be generated with an IED (e.g. IED at the first terminal) for controlling the associated switching device (e.g. circuit breaker).

BRIEF DESCRIPTION OF DRAWINGS

[0019] The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in attached drawings in which:

[0020] Fig. 1 is a simplified representation of a multi-terminal mixed line, in accordance with an embodiment of the invention; [0021] Fig. 2 is a simplified representation of a connection of an intelligent electronic device for obtaining measurements from the mixed line, in accordance with an embodiment of the invention;

[0022] Fig. 3 is a simplified block diagram of a device for fault section identification, in accordance with an embodiment of the invention;

[0023] Fig. 4 is a flowchart of a method for fault section identification, in accordance with an embodiment of the invention;

[0024] Fig. 5 is a simplified representation of a fault in a section MJ of the multi terminal mixed line, in accordance with an embodiment of the invention; and

[0025] Fig. 6 shows time space relationship in a Bewley diagram corresponding to the fault shown in Fig. 5, in accordance with the embodiment of the invention.

DETAILED DESCRIPTION

[0026] Various aspects of the present invention relate to fault section identification and fault location determination in a mixed line. Here, the mixed line is a multi-terminal line (multi-terminal mixed line), wherein the mixed line has three or more terminals. Referring to Fig. 1, which illustrates a multi -terminal mixed line (also referred as tapped line) connecting three terminals in accordance with an embodiment of the invention. The line shown in Fig. 1 is a three terminal line, having three terminals and one junction. Bus A (first terminal), Bus B (second terminal) and Bus C (third terminal) are the three terminals and J is the junction. There could also be more than one junction (e.g. two or more) in case of multiple line sections between the terminals (not shown in Fig. 1). For example, there could be an overhead line section, followed by a cable and another overhead line section between two terminals. In such a case, there could be two junctions. [0027] The invention provides a method for fault section identification and fault location determination for such mixed lines. The identification is performed in response to a fault in the multi -terminal mixed line. The section identification information is used for controlling a switching device connected to the multi-terminal mixed line. Controlling the switching device includes operating the switching device to connect or disconnect the line. The switching device may be a circuit breaker, and the switching can be controlled with an auto-recloser. For example, the switching device may be a circuit breaker such as CB 1 or CB2 connected with overhead line sections as shown in Fig. 1.

[0028] The method is performed with one or more processors associated with controlling the switching device. For example, the method can be implemented by an IED with a processor. This may be an IED associated with one of a line section, a junction, a terminal etc. An example is illustrated in Fig. 2, wherein the IED (202) is associated with Bus A. The IED receives one or more signals from one or more measurement equipment connected to the multi-terminal mixed line. In the example of Fig. 2, a current transformer (CT) provides single/multiple phase current signal to the IED.

[0029] In an embodiment the IED receives a signal(s) from the measurement equipment, and obtain measurements therefrom. In another embodiment, the measurement equipment publishes the measurements over a bus (e.g. process bus), and the IED (e.g. subscribed to receive data from such bus) receives the measurements over the bus.

[0030] In an embodiment, the method is implemented with a device associated with the multi-terminal mixed line, wherein the device has a plurality of modules. Fig. 3 is a simplified block diagram of the device. In accordance with the embodiment illustrated in Fig. 3, the plurality of modules include an input interface (302), a travelling wave module (304), a fault section identification module (306), a trip module (308) and a memory (310).

[0031] In accordance with various embodiments of the invention, each module of the device carries out one or more steps of the method (described herein after in conjunction with description of Fig. 4). Further, the plurality of modules may be implemented using one or more processors. For instance, the one or more processors may be a processor of an IED (e.g. IED 202).

[0032] Referring to Fig. 4, which illustrates a flowchart of the method for fault section identification, in accordance with an embodiment. Consider the transmission line shown in Fig.l. As shown, M, N and P are the three terminals and J is the junction. Assume, section MJ as shown in overhead line section (OHL) of length LOHLI km, section NJ is also OHL of length LOHL2 km and section PJ is a cable (UGC) of length Lu_Gc km. Assume IED1 is associated with Bus M, IED2 is associated with Bus N, and IED 3 is associated with Bus P. The three IEDs can be connected over a communication channel (e.g. optical fiber channel). Further, each IED can have a GPS clock for synchronization. Thus, when fault occurs, synchronized measurement (e.g. current) data can be recorded at the three terminals of the mixed line. These synchronized disturbance recorder (DR) data is available with the IEDs (e.g. performed by IED1, 2 or 3, or received as input from disturbance recorders).

[0033] Travelling waves can be extracted from the measurements. For example, the extraction of traveling waves can be done by decomposing the phase currents into ground and aerial modes using a transformation matrix such as Clark’s transformation matrix shown in (1).

[0034] In the above matrix, Io is the ground mode and I_a, Ip are the aerial modes. As the ground mode signal has more attenuation than the aerial modes, due to greater losses in the earth, the aerial modes signal are more suitable for analysis.

[0035] Fig. 5 shows a case where a fault (Fl) occurs on the first OHL section MJ. Let d_m, d_n and d_p be fault distances from buses M, N, and P respectively. Let the propagation velocity of the travelling waves in sections MJ and NJ be VOHL, and in section PJ be VUGC respectively. In other words, VOHL represents wave propagation velocity in overhead line, and VUGC represents wave propagation velocity in cable. Let T_m, T_n and T_p be the first traveling wave arrival times recoded at buses M, N and P respectively and to be the fault initiated time. First travelling wave arrival time can be assumed to refer to arrival time of first peak of the travelling wave.

[0036] To determine the faulted section (section having the fault), and the fault location, the arrival times are required. At 402, the method comprises obtaining arrival times of a first peak of travelling waves detected from measurements carried out at the three or more terminals. In accordance with the example of Fig. 5, the arrival times T_m, T_n and T_p are obtained.

[0037] Consider the case where the method is performed with the IED (e.g. IED1 at bus M). The signal(s) of the measured parameters (e.g. currents) is digitized and processed by the IED, to detect a travelling wave therefrom. For example, at the IED1, Clark transformation may be applied to the current signal. Subsequently aerial mode (alpha component or beta component) and ground mode components of the (current or voltage) signal are obtained. The alpha component or beta component is input for signal processing, to filter out unwanted noise, and extract the travelling waves within a predetermined frequency band. [0038] The method can be applied for 3-phase A/C line. The three phase signals are decoupled into two aerial and one ground mode signals. One of the aerial mode signal can be analyzed. This method is also applicable for D/C lines.

[0039] The arrival times (T_m, T_n and T_p) can be used for estimating at least two values for fault location at 404. The at least two values are estimated based on the arrival times, length of two or more line sections of the three or more line sections, and propagation velocity of the travelling wave in one or more of the overhead line and the cable.

[0040] Considering the example of Fig. 5, from the time space relation shown in Bewley diagram of Fig. 6, we can write (2).

[0041] As mentioned above, T_m, T„ and T_p are the first traveling wave arrival times recoded at buses M, N and P respectively and to is the fault initiation time, LOHLI is the length of section MJ, LOHL2 is the length of section NJ and LUGC is the length of section PJ, VOHL is the wave propagation velocity in sections MJ, and NJ (overhead line) and VUGC is the wave propagation velocity in section PJ (cable)

[0042] The values for fault location (FSI_MN, FSI_MP) can be obtained by using (2) as in (3) and (4)

[0043] In equations (3) and (4), FSI_MN (or first value) is the value estimated using traveling wave first arrival times measured at bus M and N and OHL propagation velocity. FSI_MP (or second value) is the value estimated using traveling wave first arrival times measured at buses M and P and OHL and UGC propagation velocities. In the equation (3) and (4), buses M and N, and buses M and P are considered. One can estimate the values considering relations between T_m and T_n, T_m and T_p, and T_n and T_p. Accordingly, different values can be estimated for arriving at the faulted section / location.

[0044] At 406, the method comprises determining a line section of the three or more line sections as having the fault, based on a comparison of the at least two values of the fault location with the length of corresponding line sections. The faulted section (section having the fault) can be identified based on following relationships for the example above.

• If FSI_MN and FSI_MP are equal (or difference less than a small threshold), then the fault is at junction J

• If FSI_MN and FSI_MP are less than L_ohl1 , then the fault is on the first section (OHL section - L_ohl1 )

• If FSI_MN is greater than L_ohl1 and FSI_MP is less than L_ohl1 , then the fault is on second section (OHL section - L_ohl2 )

• If FSI_MP is greater than L_ohl1 and FSI_MN is greater than L_ohl1 , then fault is on the third section (

[0045] The method can also determine the fault location. A value of the at least two values estimated for the fault location is determined as the actual value for the fault location. This is according to the comparison of the at least two values with the length of the corresponding line sections. For instance, equations (5), (6) and (7) can be used when the fault is first section.

[0046] In the above, d_m is fault distance from bus M, d_n is the fault distance from bus N, d_p is fault distance from bus P. The IEDs at the system can show the fault location. For example, IED1 shows value d_m, IED2 shows value d_n, and IED3 shows value d_p.

[0047] The method further comprises controlling the switching device with a trip signal for protecting the mixed line at 408. The trip signal is generated for controlling the associated switching device (e.g. circuit breaker) based on the determination of the section with the fault. The trip signal is associated with an auto-reclose function, and the trip signal is generated in response to determining that the section with the fault is an overhead line section (for example, section MJ or NJ shown in Fig. 1).

[0048] In other words, the auto-reclose function is enabled only when it is determined that the fault is in an overhead line section, and not enabled (blocked) when it is determined that the fault is in an underground cable. This trip signal may be generated with an IED (e.g. IED at the first terminal) the IED associated with the terminal (e.g. with IED 202), or with the trip module (308) of the device.

[0049] Thus, the fault section identification and fault location determination can be performed with the information of propogation velocity and tarveling wave first peak arrival times. As the method can be performed with current measurements, only inputs from CTs are sufficient for having the fault section identification and the fault location determination. Using measurements at all the terminals makes the method accurate.

[0050] The following provides an example of the method disclosed herein. Consider a case where a line-to-ground fault occurs on section MJ (OHL1 of length 100 km) at a fault distance of 50 km from bus M. Here, consider section NJ (OHL2) is of length 50 Km, and section PJ (UGC) is of length 20 Km, propoagation velocity of OHL1 and OHL2 is 2.90398 x l0⁺⁰⁸ m/s, and propogation velocity of UGS is 1.94361 x l0⁺⁰⁸ m/s. The fault inception time (to) is 0.20333 seconds and estimated peak arrival times are 0.203506194 seconds (T_m), 0.203678319 seconds (T_n) and 0.203608456 seconds (T_p) at terminal M, N and P respectively. Fault location values (FSIMN and FSIMP) are estimated by using (3) and (4). They are 50.0076 Km and 50.0928 Km respectively. Referring to the relationships for fault section identification described in the example above, we can identify the faulted section as the section MJ (i.e. accurately). In this case, it can be noted that FSIMN and FSIMP is less than the length of section MJ (lOOkm). Fault location can be estimated by using the first peak arrival times and propagation velocity of the mixed lines. In this case, fault location d_m can be estimated by the equation (5) is given by 50.0076 km.

Claims

CLAIMS We Claim

1. A method for fault section identification in a multi-terminal mixed line, wherein the mixed line connects three or more terminals (M, N, P) with at least one overhead line and at least one cable, and wherein the mixed line comprises three or more line sections (MJ, NJ, PJ) connected at one or more junctions (J), the method being performed with one or more processors associated with controlling a switching device (CB1, CB2, CB3) connected to the mixed line, the method comprising:

obtaining (402) arrival times (T_m, T„, T_p) of a first peak of travelling waves detected from measurements carried out at the three or more terminals;

estimating (404) at least two values (FSI_MN, FSI_MP) for fault location, based on the arrival times, length of two or more line sections of the three or more line sections (LOHLI, LOHL2, LUGC), and propagation velocity (VOHL, VUGC) of the travelling wave in one or more of the overhead line and the cable;

determining (406) a line section of the three or more line sections as having the fault, based on a comparison of the at least two values of the fault location with the length of corresponding line sections; and

controlling (408) the switching device with a trip signal for protecting the mixed line, wherein the trip signal is generated based on the determination of the line section with the fault.

2. The method of claim 1 further comprising determining a fault location, wherein a value of the at least two values estimated for the fault location is determined as the actual value for the fault location based on the comparison of the at least two values with the length of the corresponding line sections.

3. The method of claim 1 , wherein the mixed line connects a first (M), second (N) and third (P) terminals, and comprises three sections connected at a junction (J), wherein a first section (MJ) and a second section (NJ) are overhead line sections, and a third section (PJ) is a cable, wherein the first section connects the first terminal and the junction, the second section connects the second terminal and the junction, and the third section connects the third terminal and the junction.

4. The method of claim 3, wherein a first value (FSI_MN) of the fault location is estimated based on the arrival times (T_m, T„) of the first peak of the travelling waves at the first and second terminals respectively, the length of the first and second sections (LOHLI, LOHL2), and the velocity of propagation of the travelling wave in the overhead line (VOHL), wherein a second value (FSI_MP) of the fault location is estimated from based on the arrival times (T_n, T_p) of the first peak of the travelling waves at the first and third terminals respectively, the length of the first and third sections (LOHLI, LUGC), and the velocities of propagation of the travelling wave in the overhead line and the cable respectively (VOHL, VUGC).

5. The method of claim 4, wherein the first section is determined as the line section with the fault if the first and second values are less than the length of the first section, wherein the second section is determined as the line section with the fault if the first value is greater than the length of the first section and the second value is less than the length of the first section, and wherein the third section is determined as the line section with the fault if the first value is less than the length of the first section and the second value is greater than the length of the first section.

6. The method of claim 1, wherein the fault is determined to be at the junction if the difference between the at least two values is less than a threshold.

7. The method of claim 1, wherein the measurements comprise synchronized measurements of currents carried out at the three or more terminals.

8. A device for fault section identification in a mixed line, wherein the mixed line connects three or more terminals with at least one overhead line and at least one cable, and wherein the mixed line comprises three or more line sections connected at one or more junctions, the device comprising:

a travelling wave module for obtaining arrival times ( T_m , T_n and T_p ) of a first peak of travelling waves detected from measurements carried out at the three or more terminals;

a fault section identification module for:

estimating at least two values for a fault location, based on the arrival times, length of two or more line sections of the three or more line sections, and propagation velocity of the travelling wave in one or more of the overhead line and the underground cable; and

determining a line section of the three or more line sections as having the fault, based on a comparison of the at least two values of the fault location with the length of corresponding line sections; and

a trip module for controlling a switching device connected to the mixed line with a trip signal, wherein the trip signal is generated based on the determination with the fault section identification module, for protecting the mixed line.

9. The device of claim 8, wherein the device comprises an input interface for obtaining synchronized measurements of currents carried out at the three or more terminals.

10. The device of claim 8, wherein the device is an Intelligent Electronic Device associated with a terminal, and communicates with Intelligent Electronic Devices of other terminals of the mixed line.