Patent Publication Number: US-11038342-B2

Title: Traveling wave identification using distortions for electric power system protection

Description:
RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application No. 62/562,274, titled TRAVELING WAVE IDENTIFICATION USING DISTORTIONS FOR ELECTRIC POWER SYSTEM PROTECTION filed Sep. 22, 2017, which is incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to traveling wave identification using distortions for electric power system protection. This disclosure also relates to determining whether a fault is within a zone of protection using properly identified instances of traveling waves launched by the fault. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the disclosure are described, including various embodiments of the disclosure with reference to the figures, in which: 
         FIG. 1  shows a Bewley diagram for a fault at a location on an electric power system line consistent with embodiments of the present disclosure. 
         FIG. 2  illustrates a simplified logic diagram of a traveling wave distance element consistent with embodiments of the present disclosure. 
         FIG. 3  illustrates a Bewley diagram illustrating a pattern of reflections of a traveling wave at two different terminals consistent with embodiments of the present disclosure. 
         FIG. 4  illustrates a conceptual representation of bus-specific traveling wave distortions overlaid on a Bewley diagram that may be used to identify and distinguish a return from a fault and a return from other reflections consistent with embodiments of the present disclosure. 
         FIG. 5  illustrates a conceptual representation of bus-specific traveling wave distortions overlaid on a Bewley diagram that may be used to identify a distinguish a return from a fault in a system with multiple buses consistent with embodiments of the present disclosure. 
         FIG. 6  illustrates a flow chart of a method for identifying traveling waves using distortions in embodiments consistent with the present disclosure. 
         FIG. 7  illustrates a functional block diagram of a system for detecting and locating faults using distortions in traveling waves consistent with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Traveling waves (“TWs”) are surges of electricity resulting from sudden changes in voltage that propagate at a speed near the speed of light along overhead power lines. When launched by a line fault, these TWs carry information about the fault location and type. Furthermore, this information arrives at the line terminals within 1 to 2 ms depending on the line length and fault location. Relative arrival times and polarities of TWs allow for location of faults with accuracy on the order of a single tower span, as well as to protect the line with a permissive over-reaching transfer trip (POTT) scheme using TW-based directional elements (TW32) and with a TW-based line current differential scheme (TW87). These TW-based line protections utilize a communication channel, which may be either a standard pilot channel for the POTT scheme or a direct fiber-optic channel for the TW87 scheme; and may use current TWs, taking advantage of the adequate frequency response of current measurement devices. In various embodiments, line protection systems consistent with the present disclosure may operate on the order of 1 to 2 ms without a communication channel. 
       FIG. 1  shows a Bewley diagram for a fault at location F on an electric power system line of length LL consistent with embodiments of the present disclosure. The fault is M (km or mi) away from the local terminal (S) and LL−M (km or mi) away from the remote terminal (R). Consider another terminal (B) behind the local terminal. A TW line propagation time (TWLPT) is the time it takes for a TW to travel from one line terminal to the opposite terminal. 
     A TW launched at the fault point (F) arrives at the local terminal (S) at t 1 . Part of the TW reflects, travels back toward the fault, reflects back from the fault, and then returns to the local terminal (S) at t 4 . During the t 4 −t 1  time interval, the TW travels a distance of 2·M. We write the distance-velocity-time equation as follows:
 
2· M =( t   4   −t   1 )·PV  Eq. 1
 
where the propagation velocity, PV, is:
 
                   PV   =     LL   TWLPT             Eq   .           ⁢   2               
Substituting Eq. 2 into Eq. 1 and solving for M yields Eq. 3, which may be used to calculate the distance-to-fault value:
 
                   M   =       LL   2     ⁢         t   4     -     t   1       TWLPT               Eq   .           ⁢   3               
Introducing a per-unit reach, TW21M, we use Eq. 3 to express an operating equation for the TW21 underreaching distance element:
 
     
       
         
           
             
               
                 
                   
                     
                       
                         t 
                         F 
                       
                       - 
                       
                         t 
                         1 
                       
                     
                     
                       2 
                       · 
                       TWLPT 
                     
                   
                   &lt; 
                   
                     TW 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     21 
                     ⁢ 
                     M 
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
         
         
           
             where:
           t 1  is the arrival time of the very first TW, and   t F  is the arrival time of the first return from the fault (t 4  in  FIG. 1 ).   
         
           
         
       
    
     To emphasize reliance of the TW21 on the measurement of time, we rewrite the TW21 operating equation as follows:
 
( t   F   −t   1 )&lt;2·TW21 M ·TWLPT  Eq. 5
 
The left-hand side of Eq. 5 is the relay measurement. Responding only to TW arrival times, this measurement is not affected by CT and PT ratio errors, transients, and signal distortions in the lower (kHz) frequency band. The TW21 element does not use line impedance data when calculating the operating signal in Eq. 5, and therefore the operating signal is not affected by the finite accuracy of such line data.
 
     The right-hand side of Eq. 5 is a threshold fixed for any given application—twice the product of the line length expressed in the TW line propagation time, TWLPT, and the user-preferred per-unit reach setting, TW21M. When used in the single-ended TW-based fault locator, a fault location may be determined with the accuracy of 1 or 2 tower spans. Because of this high accuracy, the TW21M reach may be set assertively, such as at 0.95 per unit, to cover 95 percent of the line length without a communication channel (compared with a typical reach setting of 80 percent for an impedance-based distance element). 
       FIG. 2  illustrates a simplified logic diagram of a traveling wave distance element  200  consistent with embodiments of the present disclosure. A difference between the arrival time of the first return from the fault (t F ) and the arrival time of the first TW associated with the fault (t 1 ) at the terminal (t F −t 1 ) is determined by a TW detection and time-stamping subsystem  202 . The difference is representative of a distance to the fault as per Eq. 3. The difference is compared against a reach setting  204 . The reach setting  204  may be a setting related to the zone of protection of this particular protection element, such as given by the right-hand side of Eq. 5. If the difference is within the zone of protection (i.e., is less than the reach setting  204 ), then an output  212  of traveling wave distance element  200  may be asserted. The assertion of the output  212  may be used to apply a protective action such as, for example, tripping to open a line. 
     According to the illustrated embodiment, the traveling wave distance element  200  may be made more secure by requiring other conditions to exist or be determined before the output  212  is asserted. As illustrated, a directional supervision (TW32) input  208  input may indicate that the fault is in a direction of the TW21 element, typically forward. Furthermore, other security conditions  206  may be required in addition to the directional supervision  208  for the traveling wave distance element  200  to solve challenges related to protection security as described below. 
     Several challenges remain in a successful implementation of the TW21 element. For example, the distance-to-fault calculation shown in Eq. 3 works well if the element correctly identifies the first return from the fault; however, if another TW is mistaken for the first return from the fault, the calculation of the distance-to-fault calculation will be inaccurate. As illustrated in  FIG. 1 , the first TW that arrived at t 1  continued toward Terminal B, reflected from the terminal, and then returned to the local terminal (S) at time t 2 . If a TW-based system mistook t 2  for the first return from the fault, the distance-to-fault calculation result would be incorrect. Similarly, the first TW that arrived at the remote terminal (R) reflects from Terminal R and returns to the local terminal (S), propagating through the fault point (F). This TW arrives at the local terminal at t 3 . If the TW21 algorithm mistook t 3  for the first return from the fault, the distance-to-fault calculation result would also be incorrect. 
       FIG. 3  illustrates yet another challenge by showing a Bewley diagram illustrating a pattern of reflections of a traveling wave at two different terminals (Terminals S and B) consistent with embodiments of the present disclosure. The fault launches TWs toward both Terminals S and R. These TWs propagate, reflect, and transmit at various discontinuities, and as a result, a train of TWs is measured at Terminal S. Each incident TW that arrived at Terminal S from the direction of the S-R line would partially transmit through Terminal S and arrive at Terminal B. Measuring TWs of the same timing and polarity pattern, a TW21 algorithm at Terminal B would see a fictitious fault location (F*). 
     The TW21 installed at Terminal S should operate, but the TW21 installed at Terminal B should restrain. Yet, TW21 algorithms at both Terminal B and Terminal S see the same TW timing and polarity pattern. The magnitude of the TWs measured at Terminal B will be lower than the magnitude of TWs measured at Terminal S; however, these low magnitudes may be a result of TW magnitude reduction when transmitting through Terminal S for a fault at F (in which case Terminal B should restrain), or they may be a result of a fault at F* that occurred at the low point-on-wave voltage (in which case Terminal B should operate). Various embodiments disclosed herein may aid in differentiating between a fault at F and F* by using bus-specific TW distortions to identify a return from the fault. 
       FIG. 4  illustrates a conceptual representation of bus-specific TW distortions overlaid on a Bewley diagram that may be used to identify and distinguish a return from a fault and a return from other reflections consistent with embodiments of the present disclosure. When a traveling wave arrives at a discontinuity in the characteristic impedance, such as a busbar connecting multiple lines and other power system elements, part of the wave reflects back in the direction of arrival, and part of the wave continues in the original direction. These waves are separately referred to as an incident wave (the wave that arrived at the discontinuity), a reflected wave (the wave that reflected back), and a transmitted wave (the wave that continued in the original direction). 
     When launched by a fault, the incident TW is a relatively clean step in current and voltage, as indicated by signals  402   a  and  402   b . Signals  402   a  and  402   b  propagate toward buses S and R, respectively. When the sharp step in the current and voltage of signals  402   a  and  402   b  arrive at buses S and R, the signals excite the parasitic high-frequency resonant circuits present at the buses. The lumped-parameter circuits are composed of the inductance and capacitance of the buswork, parasitic capacitance of transformer windings, capacitance-coupled voltage transformer (CCVTs), surge arresters, and so on. As a result, the reflected TWs are distorted in a way that they “encode” the unique characteristics of the bus. As illustrated, the reflection from bus S  404   a  differs from the reflection from bus R  406 . These uniquely encoded TWs  404   a  and  406  reflect back from buses S and R toward the fault. 
     Signal  404   a  reflects again from fault F without significant distortion (i.e., the unique encoding from the reflection from bus S is retained) as signal  404   b . An IED implementing a TW21 algorithm at bus S may compare signals  404   a  and  404   b , determine that the shapes are similar, and thus identify the wave  404   b  as a reflection from the fault of the wave  404   a , and by doing so provide additional security associated with the determination of the distance M to the fault F. 
     Signal  406  is encoded with the unique shape associated with bus R. When signal  406  arrives at bus S, an IED implementing a TW21 algorithm may determine that the shape of signal  406  differs from the shape of signals  404   a . Based on this difference, the algorithm may correctly determine that signal  406  is not a reflection from fault F of the wave  404   a , but rather is a reflection from a different bus with a unique encoding. It shall be noted that TWs when they reflect and transmit at various discontinuities, they may change polarity. For simplicity, the explanation above does not consider polarity changes. Similarly, one must consider the three-conductor nature of power lines. A TW in one conductor couples to the other two conductors as it travels. TWs when they reflect and transmit at various discontinuities, they couple anew after reflection or transmission. A practical implementation shall take these observations into account. 
     In similar manner, TWs that come from behind the relay and are transmitted through the local bus toward the fault on the protected line are also encoded with the circuits present on the bus. If their energy is high enough to travel to the fault and back, they will return with a similar shape. In general, a return from the fault is any incident wave that traveled toward the bus from the forward direction and has a shape similar to the shape of the wave that traveled away from the bus in the forward direction. This principle can be summarized as follows: if the shapes of the TWs sent toward the fault (reflected from the bus after arriving from discontinuities in front of the bus, or transmitted through the bus after originating behind the bus) and the incident TWs arriving after 2·m·TWLPT are similar, then m is a true per-unit distance to the fault. 
       FIG. 5  illustrates a conceptual representation of bus-specific TW distortions overlaid on a Bewley diagram that may be used to identify and distinguish a return from a fault in a system with multiple buses consistent with embodiments of the present disclosure. Identification of a return from a fault may allow a relay associated with a faulted line to trip and allow a relay associated with a healthy line to restrain. Signal  502   a  first encounters bus S at time t 1 . A portion of signal propagates beyond bus S and arrives at bus B (signal  506 ), and a portion reflects from bus S (signal  504   a ) toward the fault F. The reflected signal  504   a  may be “encoded” with certain unique characteristics of bus S. Signal  504   a  reflects again from fault F, resulting in signal  504   b , which returns to bus S. The reflection of signal  504   a  from fault F does not introduce significant distortion, and accordingly the shape of signals  504   a  and  504   b  match. An IED at bus S receiving signals  504   a  and  504   b  may compare the shape of the signals and determine that the shapes match in connection with a TW21 protection system. Based on the determination that the shape of signals  504   a  and  504   b  match, an IED at bus S may operate—assuming other conditions match, such as those depicted in  FIG. 2 —to trip the line and de-energize fault F. 
     In contrast to the matching signals  504   a ,  504   b  at bus S, the signals received by an IED at bus B will not match due to the encoding of these signals resulting from different interactions with bus S and bus B. More specifically, signals  506  and  508  may arrive at bus B at t 4  and t 6 , respectively. Signal  504   a  comprises an “encoded” reflection of the signal  502   a , while signal  504   b  comprises an “encoded” representation of the signal  502   b  that passes through bus S. Stated in other words, signal  506  is equal to the difference between signal  502   a  and  504   a . The interaction with the lumped-parameter circuits associated with bus S may cause signal  504   a  and signal  506  to have differences in shape that may be analyzed by various embodiments consistent with the present disclosure. 
     An IED analyzing the signals shown in  FIG. 5  may recognize the different “encoding” of various signals as a result of reflection from or passing through various buses. Accordingly, an IED may determine that the signal  508  is not a reflection of the signal  506  at t 4  of a fault at F*, and that arrives back at bus B at t 6 . Based on such a determination, the IED at bus B may properly restrain a protective action for the fault at F, which is external to the B-S line that IED protects. 
     The term “similar shape” used above can be implemented in a number of ways. In one possible implementation, a correlation can be used between short data windows of one wave with a wave suspected to be the return of the first wave reflected from the fault. If the two shapes are similar a correlation would yield a large value relative to product of autocorrelations for each wave. 
       FIG. 6  illustrates a flow chart of a method  600  for identifying traveling waves using distortions in embodiments consistent with the present disclosure. At  602 , method  600  may determine whether a first traveling wave is detected. Upon detection of the first traveling wave at  602 , method  600  may determine a first distortion of the first traveling wave at  604 . As described above, reflection of a traveling wave from a bus in an electric power system encodes the traveling wave with a unique pattern determined by the electrical characteristics of the bus, such as the inductance and capacitance of the buswork, parasitic capacitance of transformer windings, CCVTs, surge arresters, and so on. 
     A plurality of traveling waves may be received at  606 . The plurality of traveling waves may be created by virtue of reflections from discontinuities within an electric power system. In various embodiments, each reflection may be processed one at a time. The plurality of traveling waves may comprise, for example, a plurality of reflections arriving from both the forward and reverse directions, and some of the reflections coming from the forward direction may be reflections from the fault on the protected line, and each resulting traveling wave may be analyzed. Referring to  FIG. 1 , for example, a first traveling wave arrives at t 1 . After the arrival of the first traveling wave, a plurality of traveling waves arrive at t 2 , t 3 , t 4 , and t 5 . 
     Returning to a discussion of  FIG. 6 , at  608 , a distortion associated with each of the plurality of subsequent traveling wave may be determined. The distortion associated with each of the plurality of traveling waves may be used to determine which of the plurality of subsequent traveling waves corresponds to a reflection of the first traveling wave from the fault. Each of the plurality of distortions may be compared to the first distortion to determine whether any of the plurality of distortions match the first distortion at  610 . If a match is not detected, the compared wave is not considered to be the return of the first wave from the fault and a protective action may be restrained at  612 . In various embodiments, each of the plurality traveling waves may be separately analyzed. For purposes of simplicity, method  600  analyzes the plurality of waves together at  606 ,  608 , and  610 . 
     Elements  606 ,  608 , and  610  may be explained with reference to  FIG. 1 . A first distortion associated with the traveling wave that arrived at t 1  may be compared to the distortions associated with each of the traveling waves that arrived at t 2 , t 3 , t 4 , and t 5 . The traveling wave that arrived at t 4  is a reflection from the fault, and as such, the distortion of the traveling wave at t 4  will match the distortion of the traveling wave at t 1 . 
     Returning to a discussion of  FIG. 6 , at  614 , a distance to the fault may be determined based on the arrival time of the first traveling wave and the traveling wave with a matching distortion identified from among the plurality of traveling waves. In various embodiments, the determination of the distance to the fault may be made using Eq. 3. Further, in some embodiments, the distance to the fault may be expressed in a unit ratio of the distance to the fault to the length of the line using Eq. 4. 
     At  616 , method  600  may determine whether the fault is within a reach setting based on the distance to the fault determined at  614 . In various embodiments, the distance to the fault may be compared to a specific value (e.g., a value expressed in meters) or to a per unit value (e.g., a threshold representing a certain portion of the line) to determine if the fault is within a zone of protection. If the fault is outside of the zone of protection, protective action may be restrained at  612 . 
     If the fault is within a reach setting based on the determination at  616 , a protective action may be enabled at  618 . The protective action may be initiated based on the single measurement as described above, or for enhanced security, using multiple measurements. Referring to  FIG. 1 , the wave that arrived from behind the bus at t 2  will continue toward the fault and return back to the bus S. The method may identify that return as a reflection of the wave at t 2 , and by doing so, may determine the distance to fault for a second time for the same fault. Similarly, the wave that arrived as a reflection from the remote bus at t 3  will reflect of the bus S, travel to the fault and return back. This reflection may allow a determination of the distance to the fault for a third time. In certain embodiments, the protective action may comprise actuating a breaker to de-energize a portion of an electric power system. In other embodiments, enabling a protective action at  618  may comprise a supervisory action. In one specific embodiment, a protective action may comprise asserting a signal, such as signal  206  in  FIG. 2 , that may be one condition necessary to assert output  212  of traveling wave distance element  200 . 
       FIG. 7  illustrates a functional block diagram of a system  700  for detecting and locating faults using distortions in traveling waves consistent with embodiments of the present disclosure. In certain embodiments, the system  700  may comprise an IED system to, among other things, obtain and calculate traveling wave signals and analyze distortions in traveling waves. Traveling waves may be measured by current transformers, which may be installed at the ends of transmission lines in substations. Current transformers typically have enough fidelity to measure current traveling waves with adequate accuracy for practical protection and fault locating applications. However, a current transformer measures the current at the point of its installation at the line terminal which is always a discontinuity in the characteristic impedance, and therefore it measures the sum of the incident and reflected current traveling waves. It does not measure the incident wave separately and it does not allow separating of the waves into incident, reflected, and transmitted waves. 
     The voltage and current traveling waves are linked with the characteristic impedance of the line, and according, may be separated into the incident, reflected and transmitted components. This separation may be performed using Eq. 6 for voltage traveling waves or Eq. 7 for current traveling waves. 
     
       
         
           
             
               
                 
                   
                     
                       v 
                       incident 
                     
                     = 
                     
                       
                         
                           V 
                           TW 
                         
                         - 
                         
                           
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                           ⁢ 
                           
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                       reflected 
                     
                     = 
                     
                       
                         
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                         + 
                         
                           
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     Accurate measurements of both the total current traveling wave (i TW ) and the voltage traveling wave (V TW ) are needed to separate incident, reflected, and transmitted components. Current transformers may be used in some embodiments because they provide sufficiently accurate current traveling wave measurements. High-fidelity voltage measurements may be obtained in various embodiments using the systems and methods disclosed in U.S. patent application Ser. No. 16/137,186, titled HIGH-FIDELITY VOLTAGE MEASUREMENT USING A CAPACITANCE-COUPLED VOLTAGE TRANSFORMER, and filed on Sep. 20, 2018, which is incorporated herein by reference. 
     Wave separation into the incident, reflected, and transmitted traveling waves may allow better utilization of the traveling wave information as compared with using just traveling wave measurements from current transformers, which are the sums of the incident and reflected waves. 
     System  700  may be implemented using hardware, software, firmware, and/or any combination thereof. In some embodiments, system  700  may be embodied as an IED, while in other embodiments, certain components or functions described herein may be associated with other devices or performed by other devices. The specifically illustrated configuration is merely representative of one embodiment consistent with the present disclosure. 
     System  700  includes a communications interface  716  to communicate with devices and/or IEDs. In certain embodiments, the communications interface  716  may facilitate direct communication with other IEDs or communicate with systems over a communications network. System  700  may further include a time input  712 , which may be used to receive a time signal (e.g., a common time reference) allowing system  700  to apply a time-stamp to the acquired samples. In certain embodiments, a common time reference may be received via communications interface  716 , and accordingly, a separate time input may not be required for time-stamping and/or synchronization operations. One such embodiment may employ the IEEE 1588 protocol. A monitored equipment interface  708  may receive status information from, and issue control instructions to, a piece of monitored equipment (such as a circuit breaker, conductor, transformer, or the like). 
     Processor  724  processes communications received via communications interface  716 , time input  712 , and/or monitored equipment interface  708 . Processor  724  may operate using any number of processing rates and architectures. Processor  724  may perform various algorithms and calculations described herein. Processor  724  may be embodied as a general-purpose integrated circuit, an application-specific integrated circuit, a field-programmable gate array, and/or any other suitable programmable logic device. 
     In certain embodiments, system  700  may include a sensor component  710 . In the illustrated embodiment, sensor component  710  may receive high-fidelity current measurements  702  and/or high-fidelity voltage measurements  714 . The sensor component  710  may use comprise A/D converters  718  that sample and/or digitize filtered waveforms to form corresponding digitized current and voltage signals provided to data bus  722 . High-fidelity current measurements  702  and/or high-fidelity voltage measurements  714  may include separate signals from each phase of a three-phase electric power system. A/D converters  718  may be connected to processor  724  by way of data bus  722 , through which digitized representations of current and voltage signals may be transmitted to processor  724 . 
     A non-transitory computer-readable storage medium  730  may be the repository of various software modules that perform the methods, calculations, and determinations described herein. A data bus  742  may link monitored equipment interface  708 , time input  712 , communications interface  716 , and computer-readable storage medium  730  to processor  724 . 
     Communications module  732  may allow system  700  to communicate with any of a variety of external devices via communications interface  716 . Communications module  732  may communicate using a variety of data communication protocols. 
     Data acquisition module  740  may collect data samples, such as current and voltage measurements associated with traveling waves. The data samples may be associated with a timestamp and made available for retrieval and/or transmission to a remote IED via communications interface  716 . Traveling waves may be measured and recorded in real-time, since they are transient signals that dissipate rapidly in an electric power delivery system. Data acquisition module  740  may operate in conjunction with fault detector module  734 . Data acquisition module  740  may control recording of data used by the fault detector module  734 . According to one embodiment, data acquisition module  740  may selectively store and retrieve data and may make the data available for further processing. Such processing may include processing by fault detector module  734 , which may identify distortions in traveling waves. 
     Traveling wave identification module  744  may compare distortions of traveling waves. As described and illustrated herein, traveling wave distortions may be used to determine the origin or reflection of traveling waves. Traveling waves with similar distortions may be determined to have similar origins or reflections. Traveling wave identification module  744  may identify traveling waves based on detected distortions. The fault detector module  734  may determine a fault according to several traveling wave protection algorithms using the identifications of the traveling waves according to several embodiments herein. 
     A protective action module  752  may implement a protective action based on a declaration of a fault by the fault detector module  734 . In various embodiments, a protective action may include tripping a breaker, selectively isolating a portion of the electric power system, etc. In various embodiments, the protective action module  752  may coordinate protective actions with other devices in communication with system  700 . 
     While specific embodiments and applications of the disclosure have been illustrated and described, it is to be understood that the disclosure is not limited to the precise configurations and components disclosed herein. Accordingly, many changes may be made to the details of the above-described embodiments without departing from the underlying principles of this disclosure. The scope of the present invention should, therefore, be determined only by the following claims.