FIELD OF THE INVENTION

[001] The present invention generally relates to fault location in power transmission systems. More specifically, the present invention relates to fault location in multiterminal tapped lines.

BACKGROUND OF THE INVENTION

[002] In order to evacuate the renewable power or supply power to bulk industrial loads, multi-terminal or tapped lines provides an attractive solution to provide power at minimum cost. Using multi-terminal/tapped lines saves the costs associated with building substation and installing measurement transformers at tap/junction point. There has been a huge growth in the number of multi-terminal/tapped lines mainly due to increased renewable integration to the grid. This is true in cases of solar parks and wind farms where the power evacuation to the main grid is done through short lines. Introducing tap in the main line complicates the protection and fault location.

[003] Fast restoration of outages is very important to utilities for maintaining the thermal loading of the transmission lines and stability of the power system. Thus, accurate fault location in multi-terminal lines is very important for maintenance crew to reach the fault point and undertake repair quickly. Quick identification of fault location improves the reliability, availability and assists in reducing revenue losses.

[004] There are numerous techniques available for the transmission lines, but accurate fault location for multi-terminal lines (different configurations) and multi-terminal mixed lines still remains a challenge as each section have different X/R ratio.

[005] A fault location method for multi-terminal lines using only selected negative sequence quantities at all terminals is known. Though the communication requirement between relays is reduced in this method, the solution does not work for all fault cases. Balanced faults do not produce any negative-sequence signals rendering this method not suitable for these fault cases. The method also requires the knowledge of source impedance magnitude and angle at all terminals, which might not always be available. This method does not consider capacitive current compensation and it may introduce more error in case of fault in cable section.

[006] A fault location method for multi-terminal lines using voltage and current quantities at all terminal is also known. In this method, a solution for two-terminal lines is formulated. Further, for the multi-terminal line, the faulted section is identified; and the fault location is calculated by considering the faulted section as a two-terminal line, and using the solution formulated for two-terminal lines.

[007] In the existing solutions, the calculation of junction current on a faulted line is performed using current distribution factors. These current distribution factors are dependent on various factors like source impedances, and may not compensate for line capacitance effects, which can introduce considerable error. The calculation of junction current from the currents measured at the terminal for the faulted line is challenging. This is because the fault resistance and the currents through the fault resistance, both of which are unknown, has to be accounted for during this calculation.

[008] There is accordingly a need for alternate means for obtaining the fault location in case of multi-terminal tapped lines.

SUMMARY OF THE INVENTION

[009] Various aspects of the invention relate to a device and method for fault location in a power transmission system. The power transmission system can be a multiterminal tapped line. For instance, the power transmission system can be three -terminal system, with three line sections (e.g. overhead line sections) that are connected at a junction or a tap. [0010] In accordance with various embodiments, the power transmission system connects three or more terminals. Accordingly, the system has at least a first terminal, a second terminal and a third terminal. In such a scenario, the power transmission system comprises three or more sections, with at least a first section, a second section and a third section. Here, the first, second and third sections are connected at a junction(s), wherein the first section is from the first terminal to the junction, the second section is from the second terminal to the junction, and the third section is from the third terminal to the junction.

[0011] There may be an electrical fault (or disturbance) at a particular location in the power transmission system. Faults can happen in an overhead line, or at a junction between overhead lines. The fault may be located at the junction or, in a particular section of the power transmission system. For example, the fault can be located in one of the first section, the second section and the third section.

[0012] The device of the present invention, locates the fault in the power transmission system. The device can be an Intelligent Electronic Device (IED). 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 line. For example, the measurement equipment can include a current transformer, a potential transformer, a sensor-based measurement equipment (e.g. Rogowski coils, non- conventional instrument transformers etc.) and/or the like, which provides a signal corresponding to current, voltage or other information as sensed from the line. For example, a current transformer provides single/multiple phase current signal and a potential transformer can provide single/multiple phase voltage signal to the IED. In an embodiment, the device is associated with one of the first terminal, the second terminal and the third terminal, and receives data from other devices (that are associated with the other terminals). [0013] The device has one or more modules for performing the fault location and other functions of the device. Such modules may be implemented with a processor(s) of the corresponding device. In one embodiment, the device has an input interface, a phasor calculation module, a storage and a fault locator. The input interface receives one or more of measurements of voltages and currents carried out at one or more of the first terminal, the second terminal and the third terminal; and positive sequence voltage and current phasors obtained from the measurements of voltages and currents carried out at one or more of the first terminal, the second terminal and the third terminal. In the embodiment wherein the device is an IED associated with a terminal, the IED receives a signal(s) from the corresponding measurement equipment, and obtain measurements therefrom. Alternately, 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. Here, the IED can receive voltage / current measurements from other IEDs (associated with other terminals), or phasors obtained by the corresponding IEDs. Here, the measurements are synchronized at the three terminals.

[0014] The phasor calculation module of the device is for calculating positive sequence voltage and current phasors from the measurements of voltages and currents carried out at a terminal of the power transmission system. For instance, the IED can calculate positive sequence voltage and current phasors (e.g. using suitable phasor calculation such as Fourier calculations etc.), from the voltage / current measurements carried out at the associated terminal. The storage of the device has positive sequence line impedance parameters of each section of the power transmission system. Such parameter information can be stored in the device beforehand (e.g. by operating personnel).

[0015] The device can optionally have a fault section identification module for determining the section having the fault. In an embodiment, the section with the fault is determined by calculating a value for a voltage phasor for each terminal, and comparing that calculated value with the voltage phasor value (i.e. obtained from the voltage measurements carried at the terminal). Here, the voltage phasor is calculated for each terminal of the power transmission system. The calculation of the voltage phasor for a terminal is performed using the current phasor of the corresponding terminal, and at least the voltage and current phasors obtained for at least one other terminal of the power transmission system.

[0016] In an embodiment, determining the section with the fault comprises comparing the difference between the calculated and obtained voltage phasors for each terminal. Consider a case where the difference for each terminal is less than a threshold. In such a case, the fault may be determined to be at the junction. In an embodiment, the terminal for which the difference between the calculated and obtained voltage phasors is the lowest among all, can be identified as healthy or non-faulty sections, and conclude that other section as fault. For example, if the terminal A voltage calculated from terminal B is almost equal to the measured voltage at terminal A (difference is lowest), the fault can be identified on section CJ (that is between terminal C and junction). In another embodiment, the section associated with the terminal for which the difference between the calculated and obtained voltage phasors is the highest among all, can be identified as the section with the fault.

[0017] The fault locator obtains the fault location based on the section with the fault, the positive sequence voltage and current phasors obtained for each terminal, and the positive sequence line impedance parameters of each section. In an embodiment, the fault locator obtains the fault location by calculating an inverse hyperbolic tangent of K1/K2. Here, Kl and K2 are calculated from one or more of the positive sequence voltage and current phasors of each terminal, and the positive sequence line impedance parameters of each section.

[0018] Consider a case, wherein the fault is in the first section (or AJ). In such a case, the fault location can be derived from:

[0019] Here, is the propagation constant of section AJ, and Kl and K2 can be calculated from positive sequence voltage and current phasors and positive sequence line impedance parameters of each terminal (A, B, and C and corresponding sections

AJ, BJ and CJ) using:

[0020] The inverse hyperbolic tangent may be calculated by making certain assumptions or taking only a limited number of terms in the expansion series for the hyperbolic.

[0021] The parameters Kl and K2 are obtained by equating the current flowing at the junction end in the first, second and third sections. More specifically, the fault location is obtained by equating the sum of the three currents at the junction to zero.

[0022] In the sections other than the section with the fault, the current flowing into the junction is calculated with at least the positive sequence voltage and current phasors obtained from measurements carried out at the corresponding terminals, and the positive sequence line impedance parameters of the corresponding sections.

[0023] For the section with the fault, the current at the junction end is calculated as a function of the fault location (or fault distance). The current is calculated with at least the positive sequence voltage and current phasors obtained from measurements carried out at the corresponding terminal, and the positive sequence line impedance parameters of the corresponding section. [0024] For calculating the current at the junction end, first the voltage at the fault location is calculated. This is calculated from a two port network between the corresponding terminal and the fault location. Using the voltage at the fault location, the current at the junction end is calculated from a two port network between the fault location and the junction.

BRIEF DESCRIPTION OF DRAWINGS

[0025] 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:

[0026] Fig. 1 is a simplified representation of a power transmission system, in accordance with an embodiment of the invention;

[0027] Fig. 2 is a simplified representation of a connection of an intelligent electronic device for obtaining measurements from a line, in accordance with an embodiment of the invention;

[0028] Fig. 3 is a simplified block diagram of a device for fault location, in accordance with an embodiment of the invention;

[0029] Fig. 4 is a flowchart of a method for fault location, in accordance with an embodiment of the invention; and

[0030] Fig. 5 is a simplified representation of a fault on section AJ of the power transmission system, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

[001] Various aspects of the present invention relate to fault location in a power transmission system. The power transmission system comprises a multi-terminal tapped line (multi-terminal line), which has three or more terminals. [002] Referring to Fig. 1 , which illustrates a multi-terminal tapped line (also referred as tapped line) connecting three terminals in accordance with an embodiment of the invention. The transmission system shown in Fig. 1 is a three terminal system. 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). There are three sections in the system shown in Fig. 1. The first section ( AJ) is from bus A and the junction, the second section is from bus B to the junction, and the third section is from bus C to the junction.

[0031] The invention provides a device and method for fault location in such a power transmission system. The fault location is performed in response to a fault in the system. The method is performed with one or more processors associated with the device. For example, the method can be implemented by an Intelligent Electronic Device (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 line. In the example of Fig. 2, a current transformer (CT) provides single/multiple phase current signal and a potential transformer (PT) provides single/multiple phase voltage signal to the IED.

[0032] 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.

[0033] The IED also communicates with IEDs associated with other buses (i.e. Bus B, C). Here, the IED at bus A may receive measurements, or phasors obtained at other IEDs. Similarly, the IED at bus B would receive information from IED at buses A and / or C, and IED at bus C would receive information from IED at buses A and / or B.

[0034] In an embodiment, 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 phasor calculation module (304), a fault locator (306), a fault section identification module (308) and a memory (310). 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).

[0035] The method may also be implemented with communication between a device associated with the line, and a server. Here, some of the modules may be implemented with one or more processors of the server (e.g. calculations or use of models using measurements from various measurement equipment at various terminals of the line), while the others are performed with one or more processors of the device (e.g. input interface, fault locator etc.). Alternately, the method may be implemented at the server, and the fault location communicated to the IED or other power system device.

[0036] The input interface receives one or more of measurements of voltages and currents. Consider that the device is the IED at bus A. In this case, the IED receives the measurements obtained from the measurement equipment at Bus A. Alternately, the input interface can receive a signal(s) from the measurement equipment and obtain measurements therefrom. The input interface also acts as a communication interface for receiving information from other devices. For instance, the measurements may be published over the bus, and the IED subscribes to the same. Taking another example, the IED at bus A can receive information from IED at buses B and C, or other modules (e.g. a phasor calculation module) of other devices (e.g. on the server or other power system devices).

[0037] The phasor calculation module calculates positive sequence voltage and current phasors from the measurements of voltages and currents. The phasor calculation module can calculate different phasors from measured data. For example, the phasor calculation module can calculate positive sequence voltage and current phasors from measurements at one terminal (or bus). Such phasors may be obtained using a suitable phasor calculation such as, but not limited to, Fourier calculations. For example, from the calculated voltage phasors, positive sequence quantities are computed using following transformation matrix:

where,

[0038] The calculation of the voltage phasor for a terminal may also be performed for fault section identification, using voltage and current phasors obtained for another terminal, and the current phasor obtained for the same terminal. This is another / second voltage phasor (different from the first phasor that can be calculated from the voltage phasor from the measurement at the same terminal). For example, in case of the three terminal system with, the voltage phasor for the first terminal can be calculated using the voltage and current phasors obtained for the second terminal, and the current phasor obtained the first terminal. Here, in place of using the voltage and current phasors obtained for the second terminal, voltage and current phasors obtained for the third terminal may be used. Alternately, two voltage phasors can be calculated using the second and third terminal measurements along with the first terminal measurements, and one of the two selected.

[0039] The storage (of the device or on server) can be any suitable storage for storing different information such as, but not limited to, disturbance records, line parameters etc. Positive sequence line impedance parameters of each section of the power transmission system, is available in the storage. Such parameter information can be stored in the device beforehand (e.g. by operating personnel). This may also be stored in the server or other device, and communicated to the IED for fault location purposes.

[0040] The fault locator obtains the fault location based on the section with the fault, the positive sequence voltage and current phasors obtained for each terminal, and the positive sequence line impedance parameters of each section. The fault section identification may optionally be performed with the fault section identification module.

[0041] In an embodiment, the section with the fault is determined by calculating a value for a voltage phasor (second phasor mentioned above) for each terminal, and comparing that calculated value with the voltage phasor value (i.e. first phasor obtained from the voltage measurements carried at the terminal). Here, the voltage phasor is calculated for each terminal of the power transmission system. The calculation of the voltage phasor for a terminal is performed using the current phasor of the corresponding terminal and at least the voltage and current phasors obtained for at least one other terminal of the power transmission system.

[0042] The fault section information is used for fault location calculations. In an embodiment, the fault locator obtains the fault location by calculating an inverse hyperbolic tangent of K1/K2. Here, Kl and K2 are calculated from one or more of the positive sequence voltage and current phasors of each terminal, and the positive sequence line impedance parameters of each section.

[0043] The following describes the method of the invention, various steps of which are implemented using the device (or modules) described hereinabove.

[0044] The three-terminal system as shown in Fig. 1 is used for describing the fault location solution explained below. As shown, terminals A, B and C are the first, second and third terminals; sections AJ, BJ and CJ are the first, second and third sections and J is the junction common to all three sections. The respective line lengths of the sections AJ, BJ and CJ are denoted by L _a , L _b and L _c (not shown in Fig.l). [0045] Positive sequence line impedance parameters of the three sections AJ, BJ and CJ are represented using ABCD parameters. Here, AAJ ¹ , BAJ ¹ , CAJ ¹ and DAJ ¹ are respectively A, B, C and D parameters of section AJ. Similarly, ABJ ¹ , BBJ ¹ , CBJ ¹ and DBJ ¹ are respectively A, B, C and D parameters of section BJ, and, ACJ ¹ , BCJ ¹ , CCJ ¹ and DCJ ¹ are respectively A, B, C and D parameters of section CJ. The line parameters may be obtained and stored beforehand, at 402.

[0046] Voltage and current measurements are available from the disturbance recorders (IEDs) at all the three terminals. Voltage and current measurements are recorded at terminal A and post fault positive sequence voltage and current phasors are calculated from these measurements. These are denoted by VA ¹ and IA ¹ respectively (i.e. phasors obtained from measurements at terminal A). Pre-fault positive sequence voltage and current phasors may also be calculated and are denoted by V _Apre ¹ and l _Apre ¹ respectively. Similarly, post-fault positive sequence voltage and current phasors calculated from measurements at terminal B and C are denoted by VB ¹ , IB ¹ and Vc ¹ , T ¹ respectively. Pre-fault positive sequence voltage and current phasors may be calculated from measurements at terminal B and C, and are denoted by VBpre ¹ ,lBpf ¹ and Vcpre ¹ , Tpre ¹ respectively. The phasors can be calculated using the phasor calculation module.

[0047] At 404, the section with the fault may be determined. In order to obtain the fault location, it is required to determine which section has the fault. Such determination may be performed using the steps described below, or using other fault section identification methods. These steps are performed with the fault section identification module. It is assumed that the measurements at the three terminals are synchronized.

[0048] Step I in fault section identification: Calculate the voltages at all three terminals using measurements from the other terminals:

[0049] The post-fault positive sequence voltage at terminal C is calculated using voltage and current measured at terminal B, and current measured at terminal C, using equation (1)

[0050] Similarly, the post-fault voltage at terminal A is calculated using voltage and current measured at terminal C, and current measured at terminal A, using equation (2)

[0051] In a similar manner, the post-fault voltage at terminal B is calculated using voltage and current measured at terminal A, and current measured at terminal B, using equation (3)

[0052] For calculating post-fault voltage of terminal C, measurements at terminal A may be used in place of measurements of terminal B. Alternately, two phasors can be calculated for comparison purposes (described below). Similar calculations can be performed for other terminals.

[0053] Step II in fault section identification: Compare the calculated voltage at each terminal to the measured voltage at the respective terminals:

[0054] The comparison between the measured and calculated voltages for each terminal can be done using equations (4), (5) and (6):

[0055] Step III: Identification of faulted section:

[0056] Case I: Fault at the junction [0057] If the fault is at the junction, ideally, the calculated voltages at all three terminals should be equal to the respective measured voltages. That is, A

and AV _C2B should be ideally equal to zero. Hence, if and are less

than a small threshold value, e, the fault is identified to be at the junction.

[0058] Case II: Fault in section AJ

[0059] If the fault is in section AJ, calculated from Bus B measurements

will be ideally equal to the measured value, That is, will be ideally zero.

[0060] Case III: Fault in section BJ

[0061] Similarly, if fault is in section calculated from Bus C

measurements will be ideally equal to the measured value, That is, will be

ideally zero.

[0062] Case IV: Fault in section CJ

[0063] Similarly, if fault is in section calculated from Bus A

measurements will be ideally equal to the measured value, That is, will be

ideally zero.

[0064] Based on the above, we can conclude the following conditions:

[0065] After faulted section is identified, the fault location can be calculated using the appropriate one among the following sub-routines.

[0066] Case I: Fault is at the junction [0067] If fault is at junction,

• IED A will display length of the section AJ as fault location

• IED B will display length of the section BJ as fault location

• IED C will display length of the section CJ as fault location

[0068] Case II: Fault in section AJ

[0069] For fault in section AJ, the unknown fault location from terminal A is denoted by d _A - For calculating the fault location, the current flowing into the junction from the terminals corresponding to the healthy sections (406), and at the junction end in the section with the fault (408) needs to be calculated.

[0070] Calculate the current flowing into the junction from one of the two terminals corresponding to one of the two healthy sections: Let us take terminal B here. From the two port network between terminal B and the junction J, the current flowing into the junction, is calculated as,

[0071] Where,

[0072] Here, the positive sequence ABCB parameters comprises parameters associated with charging currents (i.e. propagation constant). [0073] Calculate the current flowing into the junction from second of the two terminals corresponding to second of the two healthy sections: Let us take terminal C here. From the two port network between terminal C and the junction J, the current flowing into the junction, Lc, is calculated as,

[0074] Where,

[0075] With the knowledge that fault is on section AJ, calculate the current flowing at the junction end on section AJ, namely, I _fA . From the two port network between terminal A and the fault point F (see Fig. 5), calculate the voltage at receiving end F using voltage and current at terminal A. This will be a function of the unknown fault distance.

[0076] Where,

[0077] From two-port networks between F and J; and J and B, and using the voltage VF calculated in equation (9), the current F _A can be calculated as in (10). This will be a function of the unknown fault distance.

[0078] Substituting (9) in (10), we get,

[0079] Where,

[0080] The sum of all the three currents at the junction is equated to zero to obtain the fault location at 410. Accordingly, equate the sum of all three currents, and I _fC to zero, since the sum of currents at any node is equal to zero. Using this relation, solve for the unknown fault location.

[0081] Substituting (7), (8) and (11) in (12), we get,

[0082] Simplifying equation (13) further gives,

[0083] Substituting for equation (14), gives equation (15)

as below,

[0084] The cosine and sine hyperbolic terms can be expanded such as,

[0085] Expanding, the cosine and sine hyperbolic terms in equation (15) and simplifying, we get equation (16),

[0087] The unknown fault location CIA can be derived from the above equation as,

[0088] Where,

[0089] The fault locator can obtain the fault location by calculating the inverse hyperbolic tangent. This may be calculated by making certain assumptions or taking only a limited number of terms in the expansion series for the hyperbolic.

[0090] Case III: Fault in section BJ

[0091] For fault in section BJ, the unknown fault location from terminal B is denoted by d _B . Like in case of section AJ, calculate currents at the junctions on all three sections. Equate the sum of all three currents to zero and solve for unknown fault distance as in equation (18)

[0092] Where,

[0093] Case IV: Fault in section CJ

[0094] For fault in section CJ, the unknown fault location from terminal C is denoted by dC. Fike the above, equate the sum of all three currents at the junction to zero and solve for unknown fault distance as in equation (19)

[0095] Where,

[0096] The invention provides an accurate fault location device and method for multiterminal/tapped lines using synchronized data from all terminals. The solution identifies the faulted section first, and then points the fault location on the identified section. This invention requires post fault positive sequence voltage and current measurements and positive sequence line parameters only. This invention is independent of source impedances, fault resistance, loading information, phase unbalance, zero sequence parameters, mutual compensation, fault loop information etc.