TRANSMISSION SYSTEM

TECHNICAL FIELD

The present disclosure relates in general to fault location in power transmission systems. More specifically, the present invention relates to fault location in a two -terminal transmission system.

BACKGROUND

Accurate fault location in transmission 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 save the revenue loss for the utilities.

In case of two-terminal lines, there are certain prior art methods for fault location. Depending on the type of line (e.g. distribution, non -homogenous etc.), the methods use electrical measurements at different terminals / points of the line(s), typically along with fault type or fault loop information.

For example, there is a method for fault location using only selected negative sequence quantities at all terminals. Such solution does not work for all fault cases. For example, balanced faults do not produce any negative- sequence signals rendering such a method not suitable. The method also requires a setting of source impedance magnitude and angle at all terminals, which may not always be practically available.

Another fault location method for two-terminal lines using all symmetrical components (positive, negative, zero-sequence quantities) of the voltages and currents measured at each terminal is known. A limitation of this method is that the method requires a fault type (fault loop) as an input. Also, since the method uses zero-sequence quantities to locate the fault, mutual coupling is considered for double-circuit lines. Thus, this method may fail in cases where the second circuit is open and grounded since there will not be any zero sequence current measurements available from a healthy line.

Other prior art fault location methods using fault type or fault loop impedance (e.g. for distribution lines), all sequence impedances, negative sequence voltage profiles (e.g. for non- homogenous lines) etc. are also known. Accordingly, in the prior art methods, there is a need to have all sequence impedances, and fault type information. Such information may not always be available, or even if available, it may not be accurate.

In view of the above, there is need for improved fault location method for two-terminal lines, and in particular for two-terminal double circuit or parallel lines.

SUMMARY

Various aspects of the invention relate to a device and method for fault location in a power transmission system. The power transmission system is a two-terminal system. For instance, the power transmission system has a first terminal and a second terminal connected by a pair of transmission lines that are parallel to each other. Such lines are commonly referred to as double-circuit lines and may have either overhead lines or cables.

There may be an electrical fault (or disturbance) at a particular location in a first transmission line (i.e. one of the two transmission lines). Such faults can happen due to temporary disturbances (e.g. due to bad weather conditions), due to insulation failure and so forth.

The device of the present invention, locates the fault in the first transmission line. The device can be an Intelligent Electronic Device (IED). The device may be an IED associated with a terminal (or a bus, or an end). The IED obtains 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 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 and the second terminal and receives data from other devices (that is associated with the other terminal/end of the corresponding line).

Where the device is an IED associated with a terminal, the IED receives a signal (s) from the corresponding measurement equipment and obtains 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 (or other estimated values) obtained by the corresponding IEDs. Here, the measurements are synchronized at the two terminals.

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, or at a server, or distributed between a server and other devices associated with the power transmission system (e.g. server and IED).

In one embodiment, the device has an interface, a phasor estimator, a memory or storage and a fault locator. The interface obtains measurements of voltages and currents carried out at the first terminal and the second terminal. For example, the IED gets the measurements at the terminal it is associated with, and the measurements of the other terminal from the other IED (via communication).

The phasor estimator of the device is for estimating positive sequence voltage and current phasors from the corresponding voltage / current measurements (i.e. of the first and second terminals of the first transmission line). For instance, the IED can calculate 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. Positive sequence quantities can be derived by using methods such as symmetrical component analysis etc.

The memory / storage of the device has line parameters. For example, the device can have the line parameters of the corresponding line. Alternately, the device may have line parameters of both the lines (e.g. in case the device is the server receiving measurements from other IEDs). The line parameters include, but need not be limited to, surge impedance and propagation constant. Such parameter information can be stored in the device beforehand (e.g. by operating personnel).

The fault locator obtains the fault location based on the positive sequence voltage and current phasors obtained for the corresponding terminals, and the surge impedance and propagation constant of the corresponding transmission line. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

Figure 1 is a simplified representation of a two-terminal power transmission system with two parallel transmission lines, in accordance with an embodiment of the present invention;

Figure 2 is a simplified representation of a connection of an IED for obtaining measurements from a line, in accordance with an embodiment of the invention;

Figure 3 is a simplified block diagram of a device for fault location in the two-terminal power transmission system, in accordance with an embodiment if the invention;

Figure 4 is a simplified representation of a fault in a two-terminal power transmission system, in accordance with an embodiment of the present invention; and

Figure 5 is a representation of a two-terminal line positive sequence network after fault, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Various embodiments of the present invention relate to fault location in a two-terminal power transmission system.

Referring to Figure 1, a two-terminal power transmission system is illustrated. The transmission system is a two-terminal system. Bus 1 (first terminal), and Bus 2 (second terminal) are the two terminals of the transmission system. The electrical bus 1 is connected to a source M (101), the source can be a substation 1 (or generating station). Likewise, the electrical bus 2 is connected to a source N (102), the source can be a substation 2. The bus 1 and the bus 2 are connected by two parallel transmission lines. Each of the parallel lines may comprise at least one Circuit Breaker (CB) to break the circuit when needed. In an embodiment, each of the two transmission lines may carry a three-phase current. In an embodiment, the two transmission lines may be referred as a double-circuit line. The invention discloses 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 a terminal. An example is illustrated in Figure 2, wherein the IED (201) is associated with Bus 1. The IED (201) receives one or more signals from one or more measurement equipment connected to the line. In the example of Figure 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 (201).

In an embodiment, the IED (201) receives a signal(s) from the measurement equipment and obtain measurements therefrom. In another embodiment, the measurement equipment publishes the measurements over a communication bus (e.g. process bus) or in a communication channel or through suitable interface (e.g., input/output modules), and the IED (201) (e.g. subscribed to receive data from such bus/communication channel) receives the measurements over the communication bus.

The IED (201) also communicates with IEDs associated with other electrical buses (i.e. Bus 2). Here, the IED (201) at bus 1 may receive measurements, or phasors obtained at other IEDs. Similarly, the IED at bus 2 would receive information from IED (201) at buses 1.

In an embodiment, the device has a plurality of modules. Figure 3 is a simplified block diagram of the device (300). In accordance with the embodiment illustrated in Figure 3, the plurality of modules includes an interface (301), a memory (302), a phasor estimator (303), and a fault locator (304). 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 201).

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. interface for voltage / current measurements, phasor estimator etc.). Alternately, the method may be implemented at the server, and the fault location communicated to the IED (201). Here, the server has also information about the line that has the fault (e.g. communicated to the server from IED or other fault line detector), and line parameters of the two lines.

The interface (301) obtains measurements of voltages and currents, that are measured at the two ends (or terminals) of the line with the fault. Consider that the device (300) is the IED (201) at bus 1. In this case, the IED (201) receives the measurements obtained from the measurement equipment at Bus 1. Alternately, the interface (301) 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 (e.g. other IEDs or server). For instance, the measurements may be published over the process bus, and the IED subscribes to the same. Taking another example, the IED at bus 1 can receive information from IED at buses 2 or other modules (e.g. a phasor estimator (303)) of other devices (e.g. on the server or other power system devices).

The phasor estimator (303) estimates positive sequence voltage and current phasors from the measurements of voltages and currents carried out at the corresponding terminals. The phasor estimator (303) can estimate different phasors from measured data. For example, the phasor estimator (303) can estimate positive sequence voltage and current phasors from measurements at one terminal (or bus). For instance, the IED can calculate 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. Positive sequence quantities can be derived by using methods such as symmetrical component analysis etc.

The memory (302) (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 such as surge impedance of the first and / or second transmission lines and propagation constant of the first and / or second transmission lines etc. Such parameter information can be stored in the device (300) beforehand (e.g. by an operating personnel). This may also be stored in the server or other device and communicated to the IED (201) for fault location purposes.

The fault locator (304) obtains the fault location based on the positive sequence voltage and current phasors obtained for each terminal of the line with the fault, and the positive sequence line impedance parameters of the faulted transmission line. In an embodiment, the fault locator (304) obtains the fault location by calculating one or more parameters based on the positive sequence phasors, and the line parameters. For example, an inverse hyperbolic tangent of K1/K2 may be calculated. Here, Kl and K2 can be calculated from one or more of the positive sequence voltage and current phasors of each terminal of the faulted line, and the positive sequence line impedance parameters of the corresponding line.

The following describes the method of the invention, various steps of which are implemented using the device (or modules) (300) described hereinabove.

In accordance with the method, the device (300) obtains current and voltage measurements measured at one of a first terminal and a second terminal. These terminals are the terminals of the line with the fault. The exemplary embodiment illustrated in Fig. 4, the device (103 a) is obtaining the current and voltage measurements measured at the first terminal and receiving the current and voltage measurements over communication from other terminal (l03b).

Now referring to Figure 5, the fault point is denoted as“F”. Let the distance of the“F” be“d” from the first terminal. Let the length of the transmission line be“1”. Thus, the distance of the fault point from the second terminal is“l-d”.

The fault point voltage V _F based on the current and voltage measurements measured at the first terminal can be calculated as follows:

In the above equations, the positive sequences are defined up to the fault point“F”.

Similarly, the fault point voltage V _F based on the current and voltage measurements measured at the second terminal can be calculated as follows:

Further, the cosh and sinh terms are expanded.

The equation (3) and (8) are equated (i.e. the fault point voltages calculated for both ends is assumed to be equal) and simplified to determine the location of the fault point.

Where,

R = Resistance per unit length of the transmission line;

L = Inductance per unit length of the transmission line;

C = Capacitance per unit length of the transmission line;

Thus, in view of equation (12) the value of“d” is obtained to determine the fault location in the transmission line. Thus, the device (IED or other) can compute fault location using positive sequence phasors and line parameters in accordance with equation 12.

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.

The proposed method is a non-iterative and the accuracy is not affected by fault resistance, fault loop information, loading conditions, mutual coupling and line configuration.

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