Patent Description:
Transmission lines are a vital part of the electrical distribution system, as they provide the path to transfer power between generation and load. Transmission lines are ideally tightly interconnected for reliable operation. Factors like de-regulated market environment, economics, right-of-way clearance and environmental requirements have pushed utilities to operate transmission lines close to their operating limits. Any fault, if not detected and isolated quickly will cascade into a system wide disturbance causing widespread outages for a tightly interconnected system operating close to its limits. Transmission protection systems are designed to identify the location of faults and isolate only the faulted section. The key challenge to the transmission line protection lies in reliably detecting and isolating faults compromising the security of the system.

Phase selection is the determination of which phases are involved in a fault. Phase selection functionality is one of the important protection functions for selective tripping in case of transmission line faults. In many implementations, single phase tripping may be highly advantageous in case of phase to ground faults, which represent more than <NUM>% of transmission line faults. By using phase selection logic, it is possible to trip only faulty phases so that the healthy phases are connected in the remaining system. In this way, the overall power system stability can be maintained.

In wind farm connected power grids, the distance protection located in the wind farm side has met new challenges both in phase selection and line fault identification due to the non-linear behaviors of the system. As a weak system source with non-linear behaviors during fault conditions, it is desired to provide new solutions for reliable phase selection.

Earlier solutions provide phase selection based on change of phase currents or change of phase voltages. In wind farm connected power grids, it has been found that the method based on change of phase currents could not provide full selectivity. On the other hand, it has also been found that the phase selection method based on change of phase voltages might not work well in case of strong system conditions, although it might function for wind farm connected grids. The prior art paper "<NPL>, discloses an arrangement for determining if a fault has occurred in a three phase power line, whether the fault is a phase/phase or a phase/ ground loop, and which phases are involved. Currents on the three phases are sampled, incremental values detected and compared with thresholds, wherein the ratios of the incremental values are also formed, and their values compared with thresholds.

It has therefore become a desire to develop a more generalized phase selection solution using single end measurements, which can meet both the weak end system and strong source system conditions.

It is therefore an object of the present invention to overcome at least some of the above-mentioned drawbacks, and to provide improved fault determination and phase selection for power lines.

This and other objects are achieved by means of two methods, a device and a computer readable medium comprising instructions as defined in the appended independent claims. Other embodiments are defined by the dependent claims.

According to a first aspect of the present disclosure, a method for determining a type of fault in a power line is provided. The method comprises obtaining voltage and current measurements at a measurement point of the power line. The method further comprises determining transient energies over a first time period for a number of phase-to-ground loops and phase-to-phase loops of the power line based on the obtained voltage and current measurements. The method further comprises determining transient energy ratios for the phase-to-ground loops and the phase-to-phase loops. The method further comprises comparing the transient energy ratios with thresholds corresponding to the phase-to-ground loops or phase-to-phase loops. The method further comprises determining a type of fault based on the comparison.

It is common that power lines have three phases. For such three-phase power lines, there are three phase-to-phase loops and three phase-to-ground loops.

According to some embodiments, the determining the transient energy ratios may comprise dividing a phase-to-ground loop transient energy by a lowest one of the phase-to-ground loop transient energies. The determining the transient energy ratios may further comprise dividing a phase-to-phase loop transient energy by a lowest one of the phase-to-phase loop transient energies.

The method may comprise dividing each phase-to-phase loop transient energy by the lowest one of the phase-to-phase loop transient energies.

The method may comprise dividing each phase-to-ground loop transient energy by the lowest one of the phase-to-ground loop transient energy.

Thereby, the transient energies of the loops of a type (meaning phase-to-phase type and/or phase-to-ground type) may be normalized relative to the other loops of the same type. Normalizing the transient energies of the loops of a type may facilitate comparison of the relative sizes of the transient energies of the loop type. Specifically, if the transient energy ratios are based on normalizing the transient energies, at least one transient energy of each loop type, i.e., the lowest one, will be equal to one.

According to some embodiments, determining the transient energy ratios may comprise, if the absolute value of the lowest one of the phase-to-ground loop transient energies is below a first predetermined minimal value, each phase-to-ground loop transient energy ratio may be replaced by a first replacement value. Further, if the absolute value of the lowest one of the phase-to-phase loop transient energies is below a second predetermined minimal value, each phase-to-phase loop transient energy ratio may be replaced by a second replacement value.

If the lowest transient energy of a loop type (i.e., the lowest phase-to-phase loop transient energy or the lowest phase-to-ground loop transient energy) is too close to zero, the algorithm may become unstable. Therefore, if the divisor is too close to zero (i.e., absolute value below a predetermined minimal value/threshold), the related ratios may be replaced by a large number, such as <NUM>.

According to some embodiments, the method may further comprise obtaining further voltage and current measurements at the measurement point during a second time period. The method may further comprise determining further transient energies over the second time period for the number of phase-to-ground loops and the number of phase-to-phase loops. The method may further comprise determining further transient energy ratios for the phase-to-ground loops and the phase-to-phase loops. The method may further comprise comparing the further transient energy ratios with the thresholds corresponding to the phase-to-ground loops or phase-to-phase loops. The determining a type of fault may be based on the comparison of the transient energy ratios and the comparison of the further transient energy ratios.

In this case, the determination of the fault is based on measurements taken during at least two time periods. Therefore, the determination of the fault may be confirmed at least once before a final determination is made. Such embodiments may provide a more reliable method for determining a fault.

According to some embodiments, the types of faults to be determined may include phase-to-ground faults and phase-to-phase faults.

Different types of faults may occur in a power line. For example, a single-phase to ground fault may occur if one conductor drops to the ground or comes in contact with the neutral conductor of the power line. In such a fault, there is an insulation breakdown between one of the phases and earth. Similarly, in a two-phase-to ground fault, two phases are connected to ground. In a phase-to-phase fault, two phases are connected together, while in a three-phase fault, all three phases are connected together.

The method may be (or include) a phase selection method. The method may determine which phases of a power line are involved in a fault.

According to some embodiments, the method may further comprise obtaining at least one fault candidate loop among the number of phase-to-ground loops and phase-to-phase loops. The determining a type of fault may further be based on the at least one obtained fault candidate loop.

The at least one fault candidate loop may be a list of loops in which it has previously been determined that a fault may have occurred. Such a determination may for example be based on measurements made at the power line. The method for determining a type of fault in a power line may be triggered by the existence of at least one fault candidate loop.

There are different methods for determining fault candidate loops. For example, the determination of fault candidate loops may be performed as part of a start-up functionality. Some methods for determining fault candidate loops may comprise determining a fault indication quantity for each of the number of phase-to-ground loops and phase-to-phase loops of the power line based on the obtained voltage and current measurements. Obtaining the at least one fault candidate loop may further comprise comparing each fault indication quantity with a dedicated interval and, if any one of the fault indication quantities is outside its dedicated interval, identifying each phase-to-ground loop and/or phase-to-phase loop which related fault indication quantity exceeds its dedicated interval as a fault candidate loop.

A fault indication quantity may be a value obtained or calculated from the obtained voltage and current measurements. A fault indication quantity may for example be one of a current, incremental current, voltage or incremental voltage. The fault indication quantity may be compared with a dedicated interval, to see whether the fault indication quantity is within normal operating conditions. A fault indication quantity being outside of its dedicated interval may be an indication that a fault may have occurred in the loop related to the fault indication quantity.

According to some embodiments, a three-phase fault may be determined if: a zero-mode voltage, obtained from the obtained voltage and current measurements, is below a first zero-mode voltage threshold, a zero-mode current, obtained from the voltage and current measurements, is below a first zero-mode current threshold, all the determined phase-to-ground loop transient energy ratios are below a first threshold level, and the at least one fault candidate loop comprises all three phase-to-ground loops and all three phase-to-phase loops.

In a three-phase fault, a connection is made between the three phases. In theory, such a fault may not lead to any zero-mode current or voltage. In practice, for a three-phase fault the zero-mode current may therefore be below a first zero-mode current threshold. Further, in practice and for a three-phase fault, the zero-mode voltage may be below a first zero-mode voltage threshold.

Further, in theory and for a three-phase fault, the absolute values of the transient energies of each of the phase-to-ground loops may be substantially equal. Therefore, the difference between the transient energy ratios of the phase-to-ground loops may be small, or below a first threshold level.

Furthermore, in a three-phase fault for a three-phase power line, all the loops (phase-to-ground and phase-to-phase) may involve a faulty phase. Therefore, a determination of fault candidate loops may have flagged/identified all the loops as fault candidate loops.

According to some embodiments, a single-phase-to-ground fault may be determined for a given phase, if: the phase-to-ground loop transient energy ratio for the given phase is higher than the other phase-to-ground loop transient energy ratios, the phase-to-phase loop transient energy ratios involving the given phase are higher than the phase-to-phase loop transient energy ratios not involving the given phase, and the phase-to-ground loop for the given phase is a fault candidate loop.

In a single phase to ground fault, a connection is made between one phase and ground. In such a fault, the transient energy of the phase-to-ground loop involving the faulty phase may be larger than the transient energies of the other phase-to-ground loops. The transient energy ratio of the phase-to-ground loop involving the faulty phase may be higher than a threshold, and the transient energy ratios of the other phase-to-ground loops may be lower than the same threshold (e.g., a first single-phase-to-ground threshold).

Further, the transient energies for the phase-to-phase loops involving the faulty phase may be higher than the transient energy for the phase-to-phase loop not involving the faulty phase. The transient energy ratio of the phase-to-phase loops involving the faulty phase may be higher than a threshold, and the transient energy ratio of the other phase-to-phase loop may be lower than the same threshold (e.g., a second single-phase-to-ground threshold).

Further, a determination of fault candidate loops may have flagged/identified all the loops involving the faulty phase as fault candidate loops.

According to some embodiments, a two-phase-to-ground fault for two given phases is determined if: an incremental zero mode voltage, obtained from the obtained voltage and current measurements, is above a first incremental zero mode voltage threshold level, the phase-to-ground loop transient energy ratios for the two given phases are higher than the other phase-to-ground loop transient energy ratio, and all the phase-to-phase loop transient energy ratios are lower than a second threshold level, and the phase-to-ground loops involving the given phases and the phase-to-phase loop involving the given phases are fault candidate loops.

In a two-phase-to-ground fault, a connection is made between two phases and ground. For grounded faults, zero-mode values (current or voltage) may be large. Therefore, a two-phase-to-ground fault may be determined if an incremental zero mode voltage is above a first incremental zero-mode voltage threshold.

Further, the absolute values of the phase-to-ground-loop transient energies of the involved phases may be higher than the absolute value of the other phase-to-ground loop transient energy. Therefore, for a two-phase-to-ground fault, the phase-to-ground loop transient energy ratios for the two given phases may be higher than a threshold, and the other phase-to-ground loop transient energy ratio may be lower than the same threshold (e.g., a two-phase-to-ground threshold).

Further, for a two-phase-to-ground fault, the difference between the phase-to-phase loop transient energies may be rather small, such that all the phase-to-phase loop transient energy ratios are below a second threshold.

Further, a determination of fault candidate loops may have flagged/identified all the loops involving the faulty phases as fault candidate loops.

According to some embodiments, a two-phase fault may be determined for two given phases if: the phase-to-ground loop transient energy ratios involving the given phases are higher than the other phase-to-ground loop transient energy ratio, all the phase-to-phase-loop transient energy ratios are lower than a third threshold, and the phase-to-ground loops involving the given phases and the phase-to-phase loop involving the given phases are fault candidate loops.

In a two-phase fault, a connection is made between two phases of the power line. For a two-phase fault, the transient energy of the phase-to-ground loops involving the faulty phases may be higher than the transient energy of the other phase-to-ground loop. The transient energy ratio of the phase-to-ground loops involving the faulty phases may be higher than a threshold, and the transient energy ratio of the other phase-to-ground loop may be lower than the same threshold (e.g., a two-phase fault threshold).

Further, for a two-phase fault, the difference between the phase-to-phase loop transient energies may be rather small, such that all the phase-to-phase loop transient energy ratios are below a third threshold.

Further, an earlier determination of fault candidate loops may have flagged/identified all the loops involving the faulty phases as fault candidate loops.

According to a second aspect of the present disclosure, a (second) method for determining a type of fault in a power line is provided. The (second) method comprises obtaining current measurements at a first measurement point of the power line and a second measurement point of the power line. The (second) method further comprises determining incremental differential currents over a first time period for a number of phase-to-ground loops and/or phase-to-phase loops of the power line, based on the obtained current measurements. The (second) method further comprises determining incremental differential current ratios for the phase-to-ground loops and/or phase-to-phase loops. The (second) method further comprises comparing the incremental differential current with thresholds corresponding to the phase-to-ground loops or phase-to-phase loops and determining (<NUM>) a type of fault based on the comparison. According to the (second) method, the type of fault is determined to be a single-phase-to-ground fault for a given phase if the phase-to-ground loop incremental differential current ratio for the given phase is larger than the other phase-to-ground loop ratios and if the phase-to-phase loop incremental differential current ratios involving the given phase are larger than the other phase-to-phase loop incremental differential current ratio.

According to a third aspect of the present disclosure, a device for determining a type of fault in a power line is provided. The device is configured to obtain voltage and current measurements from a measurement point of the power line. The device is further configured to determine transient energies over a first time period for the number of phase-to-ground loops and phase-to-phase loops, based on the obtained voltage and current measurements. The device is further configured to determine transient energy ratios for the phase-to-ground loops and phase-to-phase loops. The device is further configured to compare the transient energy ratios with thresholds corresponding to the phase-to-ground loops or phase-to-phase loops and determine a type of fault based on the comparison.

The device may be configured to cooperate with other devices/units/elements in a fault detection/isolation system. For example, the device may be (or comprise) a phase selection unit for determining a type of fault in a power line.

According to some embodiments, the device may further comprise, or communicate/cooperate with, a start-up element or fault candidate loop determination element. The start-up element may be configured to determine, based on voltage and current measurements from a measurement point of the power line, at least one fault candidate loop among the number of phase-to-ground loops and phase-to-phase loops. The device may further be configured to determine the type of fault based on the at least one fault candidate loop.

Further embodiments of the device may be configured to perform methods in accordance with embodiments of the first and/or second aspect of the disclosure.

According to a fourth aspect of the present disclosure, a computer readable medium comprising instructions is provided. The instructions will, when executed by a processor, bring the processor to obtain voltage and current measurements at a measurement point of the power line. The instructions will further, when executed by a processor, bring the processor to obtain at least one fault candidate loop of a number of phase-to-ground loops and phase-to-phase loops of the power line. The instructions will further, when executed by a processor, bring the processor to determine transient energies over a first time period for the number of phase-to-ground loops and phase-to-phase loops, based on the obtained voltage and current measurements. The instructions will further, when executed by a processor, bring the processor to determine transient energy ratios for the phase-to-ground loops and phase-to-phase loops. The instructions will further, when executed by a processor, bring the processor to compare the transient energy ratios with thresholds corresponding to the phase-to-ground loops or phase-to-phase loops, and determine a type of fault based on the comparison and, optionally, on the at least one fault candidate loop.

The processor executing the instructions may, for example, be part of a device in accordance with the third aspect of the disclosure.

Other embodiments of the third aspect may comprise instruction that, when being executed by a processor, may bring the processor to perform the methods described above with reference to embodiments of the first aspect of the disclosure.

It is noted that other embodiments using all possible combinations of features recited in the above-described embodiments may be envisaged. Thus, the present disclosure also relates to all possible combinations of features mentioned herein.

Exemplifying embodiments will now be described in more detail, with reference to the following appended drawings:.

As illustrated in the figures, the sizes of the elements and regions may be exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of the embodiments.

Exemplifying embodiments will now be described more fully hereinafter with reference to the accompanying drawings in which currently preferred embodiments are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

In the Figures, the features are identified by reference numbers made up of the number of the figure to which it relates followed by the number of the feature, which are equivalents for all exemplifying embodiments, e.g., the common feature "<NUM>" is indicated by "<NUM>" in <FIG> while the corresponding feature is indicated by "<NUM>" in <FIG>.

In order to facilitate understanding of the methods and devices provided by the present disclosure, the methods and devices are presented in the context of a fault determination/handling system for a power line.

<FIG> is an illustration of a fault detection system <NUM> of a power line. <FIG> shows a power line <NUM> connected between two sources, a first local source SL at a local point L and a second remote source SR at a remote point R. The sources may for example be voltage sources or current sources. The power line <NUM> may for example be a three-phase power line. The phases may for example be denoted A, B and C. At the local point L there is also a device for determining a power line fault. The device is in this example implemented as a protection relay <NUM> connected to the power line <NUM> via a current transformer <NUM> and a voltage transformer <NUM> at a measurement point at one end of the power line, which measurement point is the above-mentioned local point L. The protection relay <NUM> measures the current i(t) and voltage v(t) of the power line <NUM> at discrete points in time or time instances, often denoted sampling times. In the figure there is also a first fault F<NUM> on the power line between the local point L and the remote point R. The fault F<NUM> is modelled as a resistance RF and a voltage source UF connected to ground. When a fault occurs in the power line, the current and or voltage of the power line is affected, which can be detected in the measurements made by the protection relay <NUM>. In <FIG>, the first fault F<NUM> is located between the relay <NUM> and the remote point R. The first fault F<NUM> will therefore cause negative transient energy E<NUM> measured at measurement point L. If the protection relay <NUM> detects negative transient energy E<NUM>, a forward fault F<NUM> may be determined.

<FIG> is an illustration of the same fault detection system <NUM>. All features in <FIG> are equivalent to the features of <FIG>, except in that, in <FIG>, a second fault F<NUM> is shown on the power line <NUM> between the local source SL and the local point L. The second fault F<NUM> will cause positive transient energy E<NUM> to be detected by the protection relay <NUM>. If the protection relay <NUM> detects negative transient energy E<NUM>, a backward fault F<NUM> may be determined.

<FIG> is a block diagram illustrating a system <NUM> for determining a type of fault in a power line, in accordance with some embodiments. The system <NUM> may for example be implemented in a processor/controller. The methods of the present disclosure may be implemented in the system <NUM>. Specifically, at least parts of the method may be implemented in the phase calculation/selection module <NUM>. The device for determining a type of fault in a power line, in accordance with some embodiments, may comprise at least the phase calculation/selection module <NUM>.

The system <NUM> comprises a measurement obtaining module <NUM>. The measurement obtaining module <NUM> is configured to obtain voltage and current measurements from a measurement point of a power line, such as the local point L of power line <NUM>, <NUM> described above with reference to <FIG>. The voltage and current measurements comprise phase-to-ground loop currents and voltages, and phase-to-phase loop currents and voltages. For a three-phase power line, the phase-to-ground loops may be denoted A-G, B-G, and C-G, and the phase-to-phase-loops may be denoted A-B, B-C, and C-A. The phase-to-ground loop currents and voltages may be denoted iA(t), iB(t), iC(t), uA(t), uB(t), and uC(t). The phase-to-phase loop currents and voltages may be denoted iAB(t), iBC(t), iCA(t), uAB(t), uBC(t), and uCA(t).

The measurement obtaining module <NUM> is further configured to provide/output the obtained measurements to a start-up module <NUM>, a phase selection module <NUM>, and a reach calculation module <NUM>.

The method of the present disclosure may be mainly implemented in the phase selection module <NUM>. Certain embodiments of the method may be partially implemented in the start-up module <NUM>.

The fault determination method may include or be a phase selection method. The phase selection method may be preceded by a start-up function, where a certain outcome of processing in the start-up function may trigger the carrying out of the phase selection method. For example, if the start-up function operates to indicate that there is a fault or disturbance on the power line, the phase selection scheme may be started for determining the fault types.

The start-up module <NUM> is configured to obtain/receive, from the measurement obtaining module <NUM>, current and voltage measurements from a measurement point of the power line <NUM>, <NUM>. The start-up module <NUM> is further configured to determine which phases may be involved in a fault, if any. The start-up module <NUM> is further configured to provide/output the resulting at least one fault candidate loop to the phase selection module <NUM> and the reach calculation module <NUM>.

The phase calculation module <NUM> obtains voltage and current measurements from a measurement point of the power line <NUM>, <NUM> via the measurement obtaining module <NUM> and may obtain at least one fault candidate loop from the start-up module <NUM>. The phase calculation module <NUM> determines transient energy, and transient energy ratios for each of the phase-to-ground loops and the phase-to-phase loops based on the obtained voltage and current measurements. The phase calculation module <NUM> is further configured to determine a type of fault based on the transient energy ratios. Optionally, the determination may further be based on the obtained fault candidate loop(s).

The phase calculation module <NUM> is further configured to provide/output the determined type of fault to the trip decision module <NUM>.

The system <NUM> further, optionally, comprises a reach calculation module <NUM>. The reach calculation module <NUM> is configured to receive/obtain voltage and current measurements from a measurement point of the power line <NUM>, <NUM> via the measurement obtaining module <NUM>, and to obtain at least one fault candidate loop from the start-up module <NUM>. Based on the obtained measurements and fault candidate loop(s), the reach calculation module <NUM> may determine if the fault is to be handled by the present fault handling system <NUM>. Along a power line <NUM>, <NUM>, there may be more than one fault handling systems <NUM>. In such cases, each fault handling system <NUM> may be responsible for faults occurring along a specific section of the power line. The reach calculation module <NUM> may for example determine a location of the fault, or a distance between the measurement point L and the fault based on reach calculations.

The reach calculation module <NUM> may output/provide its results to the trip decision module <NUM>.

The trip decision module <NUM> is configured to receive a determined type of fault from the phase calculation module <NUM>. The trip decision module <NUM> may further receive/obtain an input relating to the position/location of the fault from the reach calculation module <NUM>. The trip decision module <NUM> may, based on the information received from the calculation module <NUM> and/or the reach calculation module <NUM>, decide to trip one of more phases of the power line based on the fault determination. The trip decision module <NUM> may output a general trip decision Tgen, or trip decisions for one or more individual phases TA, TB, and TC.

The functionality of the fault determination/handling system <NUM>, including one or more of the measurement obtaining module <NUM>, start-up module <NUM>, phase selection module <NUM>, reach calculation module <NUM> and tripping module <NUM> may be implemented in at least one integrated circuit such as an Application-Specific Integrated Circuit (ASIC) or Field-Programmable Gate Arrays (FPGA). As an alternative, the modules may be realized as one or more processors acting on computer program instructions in one or more computer memories implementing the module functionality.

<FIG> is a flowchart illustrating an embodiment of the method for determining a type of fault in accordance with the first aspect of the present disclosure. The method may for example be implemented in a device or a system, such as the system <NUM> described above with reference to <FIG>.

The method <NUM> comprises, in step <NUM>, obtaining voltage and current measurements at a measurement point of a power line, such as the power line <NUM>, <NUM> described above with reference to the preceding figures.

The voltage and current measurements comprise phase-to-ground loop currents denoted iA(t), iB(t), iC(t) and voltages uA(t), uB(t), uC(t), and phase-to-phase loop currents iAB(t), iBC(t), iCA(t) and voltages uAB(t), uBC(t), uCA(t).

At step <NUM>, the method <NUM> may optionally comprise receiving/obtaining at least one fault candidate loop among the phase-to-ground loops A-G, B-G, C-G, and the phase-to-phase loops A-B, B-C, C-A.

At step <NUM>, the method <NUM> further comprises determining (<NUM>) transient energies over a first time period for a number of phase-to-ground loops and phase-to-phase loops of the power line, based on the obtained voltage and current measurements.

The transient energy may be calculated as follows: <MAT>.

Here, Δu is an incremental voltage of a loop, and Δi is an incremental current of the same loop.

The incremental current Δi(t) of a loop is the difference between the loop current at a present point in time (or time instance) t and an earlier point in time (or time instance) (t - T). The earlier point in time may be a corresponding point in time in a previous fundamental frequency (<NUM>/<NUM>) period (T) of the loop current: <MAT>.

The incremental voltage Δu(t) of a loop is the difference between the loop voltage at a present point in time (or time instance) t and an earlier point in time (or time instance) (t - T). The earlier point in time may be a corresponding point in time in a previous fundamental frequency (<NUM>/<NUM>) period (T) of the measured loop voltage: <MAT>.

For a three-phase power line having phases A, B and C, the transient energy may be calculated for three phase-to-ground loops, (A-G, B-G, and C-G), and three phase-to-phase loops (A-B, B-C, and C-A), such that in total <NUM> transient energy indices, ΔEA, ΔEB, ΔEC, ΔEAB, ΔEBC, ΔECA, may be calculated.

At step <NUM>, the method <NUM> comprises determining transient energy ratios for the phase-to-ground loops and phase-to-phase loops of the power line.

At this step, a phase-to-ground loop transient energy ratio RA, RB, RC may be determined for each phase, as well as a phase-to-phase loop transient energy ratio RAB, RBC, RCA between each phase.

A transient energy ratio may optionally be determined as a phase-to-ground loop transient energy or phase-to-phase loop transient energy divided by the lowest of the phase-to-ground loop transient energies or phase-to-phase loop transient energies, respectively.

The transient energy ratios may therefore be normalized, which may make the calculation both more stable and the settings easier. At the same time, it is also helpful to improve the sensitivity and operation speed. For a three-phase powerline, the phase-to-ground loop transient energy ratios may be determined as: <MAT> and the phase-to-phase loop transient energy ratios may be determined as: <MAT> wherein, all the transient energies ΔEX are determined using equation (<NUM>). It is noted that, when using equations <NUM> and <NUM> to determine the transient energy ratios, at least one phase-to-ground loop transient energy ratio, and as least on phase-to-phase loop transient energy ratio is always equal to one.

In some practical applications, the transient energy in the ratio calculations may be very small. If a transient energy is very small, the divisor min(ΔEA, ΔEB, ΔEC) or min(ΔEAB, ΔEBC , ΔECA) in the equations <NUM> and <NUM> may also become small. In order to make the calculation of ratios (division algorithm) stable, for such cases, the divisor may first be compared with a threshold representing a minimal acceptable divisor value. If a divisor is smaller than the threshold (e.g., zero or near zero), the ratios to be determined using that divisor may be replaced by a pre-set (large) value, such as <NUM>, to avoid the problem of division by zero.

At step <NUM>, the method <NUM> comprises comparing the determined transient energy ratios RA, RB, RC, RAB, RBC, RCA with thresholds corresponding to the phase-to-ground loops or phase-to-phase loops. Each transient energy ratio may be compared with its own dedicated threshold or thresholds. Alternatively, there may be a first set of thresholds for the phase-to-ground loop transient energy ratios and a second set of thresholds for the phase-to-phase loop transient energy ratios.

At step <NUM>, the method comprises, determining a type of fault based on the comparison, and optionally based on the (list of) at least one fault candidate loop. The determination of different types of faults will be further discussed below, with reference to <FIG>.

If, based on the comparison, a fault type can be determined, the method may proceed to signalling the fault at step <NUM>. If no fault can be determined, the method may return to obtaining (new) measurements at step <NUM>. New measurements may for example be made during a second, different, time period.

<FIG> is a flowchart illustrating an embodiment of at least part of the method for determining a type of fault in accordance with the first aspect of the present disclosure. Specifically, the method <NUM> shows an embodiment of step <NUM>, determining a type of fault based on the comparison, of method <NUM>.

In step <NUM>, the method <NUM> comprises identifying a type of fault based on the comparison and (optionally) based on the at least one fault candidate loop.

For example, a three-phase fault may be identified if the following criteria are met:.

A single-phase-to-ground fault for a given phase (using A as an example) may be identified if the following criteria are met:.

A two-phase to ground fault for two given phases (using B-C as an example) may be identified if the following criteria are met:.

A two-phase fault for two given phases (using B-C as an example) may be identified if the following criteria are met:.

If no fault has been determined in step <NUM>, the method may return to obtaining (new) measurements at step <NUM>.

After a fault has been determined in step <NUM>, the method <NUM> may optionally increase a counter related to the type of fault at step <NUM>.

At step <NUM>, the method may optionally verify whether the counter relating to a type of fault has reached a value above a predetermined number, i.e., if the type of fault has been identified a predetermined number of times. If the type of fault has not been identified the predetermined number of times, the method may return to obtaining (new) measurements at step <NUM>. If the type of fault has been identified the predetermined number of times, the type of fault may be determined, and the method may proceed to signalling the fault at step <NUM>.

The steps <NUM>, <NUM> may comprise submitting information relating to a determined fault to a tripping module which may go ahead and trip one or more of the power line phases based (at least in part) on the information.

<FIG> is a flowchart illustrating an embodiment of the method for determining a type of fault in accordance with the first aspect of the present disclosure.

In <FIG>, the method <NUM> starts by obtaining <NUM> measurements as described above with reference to <FIG>.

However, in the method <NUM>, the phase selection may be preceded by a start-up function at steps <NUM>-<NUM>, for determining whether there are any fault candidate loops.

At step <NUM>, the method may comprise determining a fault indication quantity for each of the number of phase-to-ground loops and phase-to-phase loops of the power line based on the obtained voltage and current measurements.

For example, the fault indication quantity may be an incremental current Δi(t). The incremental current Δi(t) for each loop may be the difference between the loop current at a present point in time or time instance t and an earlier point in time or time instance (t - T), where the earlier point in time is a corresponding point in time in a previous fundamental frequency period (T) of the loop current: <MAT>.

At step <NUM>, the method may comprise comparing each fault indication quantity with a dedicated interval and, if any one of the fault indication quantities is outside its dedicated interval, identifying each phase-to-ground loop and/or phase-to-phase loop which related fault indication quantity exceeds its dedicated interval as a fault candidate loop.

When the fault indication quantity is incremental current Δi(t), the comparison with a threshold may be as follows: <MAT>.

Where, Δi(t) is the incremental loop current. IN is the rated current of the current transformer <NUM>. IRMS is the RMS value for the given loop current. K<NUM> and K<NUM> are the settings for the fixed threshold and the floating threshold.

A start-up element/method based on incremental currents may for example work well for power lines in windfarm applications, despite windfarm control possibly bringing non-linear phenomena to the fault currents.

At step <NUM>, the method may comprise determining whether there are any fault candidate loops. If there are not any fault candidate loops, the method may return to obtaining measurements at step <NUM>. If there are fault candidate loops, the method may proceed to determining transient energies <NUM>, determining transient energy ratios <NUM> (not depicted) and comparing the transient energy ratios with corresponding thresholds <NUM> (not depicted) as described above with reference to steps <NUM>, <NUM> and <NUM> of method <NUM> in <FIG>.

In some embodiments of the methods described in this disclosure, only potential faults involving fault candidate loops are investigated further. For example, if the (list of) at least one fault candidate loop does not comprise any loop involving phase A, the method may comprise deciding not to investigate A-G (phase A to ground) faults.

Further, the method may comprise determining whether any zero-mode quantities obtained from the measured currents and voltages have crossed a corresponding zero-mode threshold. The presence of zero-mode quantities above the thresholds may be an indication that there is a grounded fault. Therefore, if there are zero-mode quantities above the thresholds ("high" zero mode quantities), grounded faults may be investigated. On the other hand, if there are no zero-mode quantities above the thresholds ("low" zero mode quantities), phase-to-phase faults may be further investigated.

Referring back to <FIG>, in another embodiment, the phase selection may be based on differential currents. The method <NUM> may be equivalent, except in the following:.

Where the IL,A(t) is the phase A current measured at the local measurement point L, at the local source SL, and IR,A(t) is the phase A current measured at the remote measurement point R, at the remote source SR. Often, the local source SL and the remote source SR provide like currents, but in opposite directions.

The method may further comprise determining, incremental differential currents ΔIdiffA, ΔIdiffB, ΔIdiffC, and/or ΔIdiffAB , ΔIdiffBC, ΔIdiffCA, for each of the phase-to-ground loops and/or phase-to-phase loops, based on the determined differential currents.

The incremental differential currents in a phase-to-ground loop (e.g., A-G) and a phase-to-phase loop (e.g., A-B) may be calculated using the following equations for a time instance t, where time point (t-T) is the time point in a previous fundamental frequency period (T): <MAT> <MAT>.

At step <NUM>, the method may instead of determining transient energy ratios, comprise determining incremental differential current ratios: <MAT> <MAT>.

At step <NUM>, the method may instead of comparing the transient energy ratios with corresponding threshold comprise comparing the incremental differential current ratios with corresponding thresholds.

At step <NUM>, the method may comprise determining a type of fault based on the comparison of the incremental differential current ratios with the corresponding thresholds.

A three-phase fault may be determined based on incremental differential current ratios if:.

A single-phase-to-ground fault for a given phase (using A as an example) may be determined based on incremental differential current ratios if:.

A two-phase to ground fault for two given phases (using B-C as an example) may be determined based on incremental differential current ratios if:.

A two-phase fault for two given phases (using B-C as an example) may be determined based on incremental differential current ratios if:.

It is noted that, while the method in the above description is based on calculation of current, voltage and energy in the time domain, the method may also be implemented in the phasor domain.

Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements.

Claim 1:
A method (<NUM>) for determining a type of fault in a power line, the method comprising:
obtaining (<NUM>) voltage and current measurements at a measurement point of the power line;
determining (<NUM>) transient energies over a first time period for a number of phase-to-ground loops and phase-to-phase loops of the power line, based on the obtained voltage and current measurements;
determining (<NUM>) transient energy ratios for the phase-to-ground loops and phase-to-phase loops;
comparing (<NUM>) the transient energy ratios with thresholds corresponding to the phase-to-ground loops or phase-to-phase loops; and
determining (<NUM>) a type of fault based on the comparison.