Power flow direction measurement apparatus

Abstract

An electrical apparatus forming part of an AC power supply network comprises a conductor, sensors for measuring current and voltage on the conductor and a control unit. The control unit receives, from the sensors, measurements of the current and the voltage at the conductor and calculates a phase angle between them. Whether a short circuit current is occurring in the conductor is determined and a set of predetermined phase angle ranges is selected depending on whether a fault is present. Which of the ranges contains the calculated phase angle is used to determine if the power flow is incoming or outgoing. First and second phase angle ranges are used when no short circuit fault is present; third and fourth ranges are used when a fault is present. The directions of power flow may be measured at different points in the network and the location of a fault determined as lying between two of these points.

Description

Field The present disclosure relates to an apparatus for measuring voltages and currents in a conductor and determining a direction of power flow. In particular, the disclosure relates to an apparatus comprising an electrical switching device including sensors for voltage and current measurement for use in medium voltage AC power distribution network. Background The direction of power flow through a conductor can be determined by measuring a phase angle between the current and the local voltage at the conductor. Methods for determining the direction of power flow can be highly sensitive to inaccuracies in determining the phase angle of the voltage at site, especially in case of large fault currents, where the phase angle of the measured voltage in general will be shifted compared to the driving voltage upstream in the network. Inaccuracy in measuring the phase angle of the current can be very significant in the determination of power flow direction. This is especially the case in almost entirely inductive and almost entirely capacitive currents. Small miscalibrations in the measurement devices can lead to errors in measurements of the phase angle and result in a determination of the direction of power flow that is opposite to the actual direction. In view of the above, there is a need for means for performing robust determination of the direction of power flow in power distribution networks. Summary of invention Disclosed herein is an electrical apparatus forming part of an AC power supply network. The electrical apparatus comprises a conductor; one or more sensors for measuring a current in the conductor and a voltage on the conductor; and a control unit. The control unit is configured to: receive, from the one or more sensors, measurements of the value of the current in the conductor and the voltage at the conductor; calculate a phase angle between the measured voltage and the measured current or receive a calculated phase angle between the measured voltage and the measured current from the one or more sensors; determine whether a short circuit current is occurring in the conductor; based on determining that no short circuit current is occurring in the conductor, determine which of a first predetermined range and a second predetermined range contains the calculated phase angle; based on determining that a short circuit current is occurring in the conductor, determine which of a third predetermined range and a fourth predetermined range contains the calculated phase angle; based on determining that the calculated phase angle is contained in the first or the third predetermined range, provide as an output an indication that a direction of power flow in the conductor is outgoing; and based on determining that the calculated phase angle is contained in the second or fourth predetermined range, provide as an output an indication that the direction of power flow in the conductor is incoming, wherein the first predetermined range and the second predetermined range do not overlap each other, and wherein the third predetermined range and the fourth predetermined range do not overlap each other. By using different predetermined phase angle ranges between current and corresponding driving voltage for determination of the power flow direction under different operating conditions, the determination of direction of power flow can be performed robustly in the presence of timing errors in voltage- and current measurements and in different operating conditions of the network. In one aspect of the invention, an electrical apparatus forming part of an AC power supply network is provided. The electrical apparatus comprises: a conductor; one or more sensors for measuring a current in the conductor and a voltage on the conductor; and a control unit , the control unit being configured to: receive, from the one or more sensors, measurements of the value of the current in the conductor and the voltage at the conductor; calculate a phase angle between the measured voltage and the measured current or receive a calculated phase angle between the measured voltage and the measured current from the one or more sensors; determine whether a short circuit current is occurring in the conductor; based on determining whether or not a short circuit current is present in the conductor, select a set of predetermined ranges from a plurality of sets of predetermined ranges, determine which predetermined range from the selected set of predetermined ranges contains the calculated phase angle, based on determining the predetermined range of the selected set of predetermined ranges that contains the calculated phase angle, provide as an output an indication of whether a direction of power flow in the conductor is outgoing or incoming. In some examples, based on determining that no short circuit current is present in the conductor, a first set of predetermined ranges is selected, wherein the first set of predetermined ranges comprises a first predetermined range and a second predetermined range; based on determining that a short circuit current is occurring in the conductor, a second set of predetermined ranges is selected, wherein the second set of predetermined ranges comprises a third predetermined range and a fourth predetermined range; and wherein the control unit is further configured to: based on determining that the calculated phase angle is contained in the first or the third predetermined range, provide as an output an indication that a direction of power flow in the conductor is outgoing; and based on determining that the calculated phase angle is contained in the second or fourth predetermined range, provide as an output an indication that the direction of power flow in the conductor is incoming, wherein the first predetermined range and the second predetermined range do not overlap each other, and wherein the third predetermined range and the fourth predetermined range do not overlap each other. In some examples, the first predetermined range is related to the third predetermined range by a constant phase angle offset, and wherein the second predetermined range is related to the fourth predetermined range by the same constant phase angle offset. By relating the predetermined ranges in the normal operating conditions to the predetermined ranges under fault conditions in terms of a constant phase angle offset, determination of the direction of power flow in the two conditions can be implemented simply by performing a rotation of the measured phase angles. In some examples, the first predetermined range and the second predetermined range cover together a phase angle range of 360°, and the third predetermined range and the fourth predetermined range also cover together a phase angle range of 360°. In some examples, wherein each of the first, second, third and fourth predetermined ranges cover a phase angle range of 180°. In some examples, the first predetermined range is entirely contained in the range of -70° to 110°, and the second predetermined range is entirely contained in the range of 110° to 290°. In some examples, the first predetermined range is from -70° to 110°, and the second predetermined range is from 110° to 290° In some examples, the third predetermined range is entirely contained in the range -150° to 30°, and the fourth predetermined range is entirely contained in the range 30° to 210°. In some examples, the third predetermined range is from -150° to 30°, and the fourth predetermined range is from 30° to 210°. In some examples, the end points of the first predetermined range and the second predetermined range are chosen based on a minimum permitted power factor of the power supply network such that the end points of the first and second predetermined ranges correspond to phase angles that are inconsistent with the minimum permitted power factor of the power supply network in the absence of a fault condition. In some examples, the end points of the third predetermined range and the fourth predetermined range are chosen based on the number of affected phases in the power supply such that the end points of the third and fourth predetermined ranges correspond to phase angles that are inconsistent with a measured phase angle in a short circuit condition between any of the phases or with ground. By choosing endpoints of the ranges for classification of the power flow direction in phase angle ranges that are inconsistent with actual measurements, small errors in the measured values will not lead to changes in the determined direction of power flow. In some examples, the conductor is electrically connected to a load and to a shared power supply busbar, and the outgoing power flow is a power flow towards the load and an incoming power flow is a power flow towards the shared busbar. In some examples, the control unit determines whether a fault condition is occurring based at least in part on the measured current. In some examples, the fault condition is a short circuit condition. In some examples, the conductor forms part of one phase of a three-phase power supply system. In some examples, the control unit receives measurements of voltages and currents from a plurality of sensors at different locations in the power supply network; and wherein the control unit is configured to: determine the directions of power flow at the different locations of the power supply network; based on the determined directions of power flow at the different locations of the power supply network, identify the two adjacent measuring locations where the fault in the power supply network is located between these two adjacent measuring locations; and after interrupting power supply throughout the power supply network, reconnect healthy parts of the power supply network based on the identified location of the fault. A method performed by a control unit of an electrical apparatus forming part of an AC power supply network is provided. The power supply apparatus comprises a conductor and one or more sensors for measuring a current in the conductor and a voltage on the conductor. The method comprises: receiving, from the one or more sensors, measurements of the value of the current in the conductor and the voltage at the conductor; calculating a phase angle between the measured voltage and the measured current or receiving a calculated phase angle between the measured voltage and the measured current from the one or more sensors; determining whether a short circuit current is occurring in the conductor; based on determining that no short circuit current is present in the conductor, determining which of a first predetermined range and a second predetermined range contains the calculated phase angle; based on determining that a short circuit current is occurring in the conductor, determining which of a third predetermined range and a fourth predetermined range contains the calculated phase angle; based on determining that the calculated phase angle is contained in the first or the third predetermined range, providing as an output an indication that a direction of power flow in the conductor is outgoing; and based on determining that the calculated phase angle is contained in the second or fourth predetermined range, providing as an output an indication that the direction of power flow in the conductor is incoming, wherein the first predetermined range and the second predetermined range do not overlap each other, and wherein the third predetermined range and the fourth predetermined range do not overlap each other. In one aspect, method performed by a control unit of an electrical apparatus forming part of an AC power supply network is provided. The power supply apparatus comprises a conductor and one or more sensors for measuring a current in the conductor and a voltage on the conductor. The method comprises: receiving, from the one or more sensors, measurements of the value of the current in the conductor and the voltage at the conductor; calculating a phase angle between the measured voltage and the measured current or receiving a calculated phase angle between the measured voltage and the measured current from the one or more sensors; determining whether a short circuit current is occurring in the conductor; based on determining whether or not a short circuit current is present in the conductor, selecting a set of predetermined ranges from a plurality of sets of predetermined ranges, determine which predetermined range from the selected set of predetermined ranges contains the calculated phase angle, based on determining the predetermined range of the selected set of predetermined ranges that contains the calculated phase angle, provide as an output an indication of whether a direction of power flow in the conductor is outgoing or incoming. In some examples, based on determining that no short circuit current is present in the conductor, a first set of predetermined ranges is selected, wherein the first set of predetermined ranges comprises determining which of a first predetermined range and a second predetermined range contains the calculated phase angle; based on determining that a short circuit current is occurring in the conductor, a second set of predetermined ranges is selected, wherein the second set of predetermined ranges comprises determining which of a third predetermined range and a fourth predetermined range contains the calculated phase angle; wherein the method further comprises: based on determining that the calculated phase angle is contained in the first or the third predetermined range, providing as an output an indication that a direction of power flow in the conductor is outgoing; and based on determining that the calculated phase angle is contained in the second or fourth predetermined range, providing as an output an indication that the direction of power flow in the conductor is incoming, wherein the first predetermined range and the second predetermined range do not overlap each other, and wherein the third predetermined range and the fourth predetermined range do not overlap each other. Brief Description of Figures The detailed description is with reference to the following figures. Fig. 1 shows a schematic illustration of part of an electrical apparatus suitable for use in examples of the disclosure; Fig. 2 shows a cross-section of a switching cap of an electrical apparatus suitable for use in examples of the disclosure; Fig. 3 shows a schematic illustration of a phase diagram showing the relationship between the measured voltage and current during normal operating conditions of a power supply network in the present disclosure; Fig. 4 shows a schematic illustration of a phase diagram showing the relationship between the measured phase voltage and current during a short circuit condition in a power supply network in the present disclosure; Fig. 5 shows an example method performed by a control unit in examples of the disclosure; Fig. 6 is a schematic illustration of elements of the electrical apparatus in examples of the disclosure. Detailed Description of the Disclosure The present disclosure relates to an electrical apparatus that can be used in a power distribution network. The electrical apparatus comprises a conductor, one or more sensors for measuring a current in the conductor and a voltage in the conductor, and a control unit. The electrical apparatus may for example be part of a medium-voltage (MV) switchgear of a power distribution network. The electrical apparatus may comprise a single unit disposed in one location or may comprise elements that are distributed. For example, the control unit of the power electrical apparatus may be disposed at a remote location with respect to the conductor and sensors or the electrical apparatus or may form part of a single device comprising the sensors and the conductor. Based on measurements of a current in and a voltage on the conductor, the control unit calculates a phase angle between the voltage on and the electric current flowing through the conductor. Based on the calculated phase angle and whether the power distribution network is operating under normal operating conditions or under a fault condition, the control unit determines a direction of power flow through the conductor. By taking into account operating conditions of the power distribution network, the control unit is able to make robust determinations of the direction of the power flow. In particular, the control unit is able to choose classification criteria for determining whether a power flow direction is incoming or outgoing depending on whether or not a fault condition is occurring, where the classification criteria are robust to errors in the calculations of a phase angle, based on the measured current and corresponding voltage. In the embodiments described in detail below, one or more sensors are incorporated into an electrical switching apparatus and are used to measure the voltage and current flowing in a conductive path through the electrical switching apparatus. The sensors could, however, be provided in other AC power supply elements and at other locations in a power distribution network and used the determine the current and voltage of the current at these locations in the same way. With reference to Figs. 1 and 2, an electrical apparatus suitable for use in examples of the present disclosure is described. Fig. 1 illustrates a base body 2 comprising a plurality of input terminals 8b for receiving power from a power supply and a plurality of output terminals 8b for providing power to a load. All terminals of the same phase (in a 3-phase network) can be interconnected via the busbars 8a. For example, the conductor may form any part of a main current path through the power supply network, with the one or more sensors being arranged to measure the current or voltage at the conductor. In the case of a three-phase power supply, for example, the apparatus may comprise several units combined via busbar 8a, each having three input or output terminals 8b corresponding to separate phases of the power supply. In other power supply networks, the apparatus may comprise a different number of inputs and output terminals and may comprise, for example, only one input terminal and one output terminal. Each in-or output terminal 8b is connected to the busbar 8a by a switching cap 10. The switching cap 10 comprises a conductor 12 that is electrically connected to both an input or output terminal 8b and the busbar 8a via fixed contacts 9 so that current passes through the conductor 12 between the input or output terminal 8b and the busbar 8a. The switching cap 10 comprises sensing circuitry 22 configured to measure a voltage on and a current in the conductor 12. The measured voltage and current values may be provided to a control unit (not pictured), which may form part of an electronic processing unit of the switching cap 10 or may be remote to the switching cap 10. For example, the switching cap 10 may provide voltage and current measurements to a control unit via wireless communication. When the switching cap 10 is connected to the base body 2, the conductor 12 forms a conductive contact bridge that is connected at each end to a respective fixed electrical contact 9 such that the input or output terminal 8b of the switching cap 10 are electrically connected to the busbar 8a via the conductive contact bridge 12. When the switching cap 10 is pulled off the fixed contacts 9, the electrical connection between the terminal 8b and the busbar is interrupted, and when the switching cap 10 is placed on the fixed contacts 9, the electrical connection between the terminal 8b and the busbar is closed, and a current may flow between the input 8b and output terminals 8b via the busbar 8a. The sensing circuitry 22 may comprise one or more current sensor(s) and / or a voltage sensor. In the illustrated apparatus, the sensing circuitry 22 is separated from the conductor 12 by one or more insulating layers 21. The sensing circuitry 22 may indirectly detect a current and / or a voltage in the conductive bridge portion 12 by directly detecting a magnetic field generated in the vicinity of the sensing circuitry 22. The sensing circuitry 22 may comprise a pick-up coil with or without a ferro-magnetic core and / or a Hall sensor. In some examples, the one or more sensors may be arranged across a plurality of devices, such as a plurality of different switching caps 10. In some examples, the one or more sensors include a current transformer disposed around the conductor 12. The current flowing through the conductor 12 during operation is an alternating current that flows alternately towards both the input terminal 8b and the output terminal 8b of another panel (unit). The direction of power flow during operation is dependent on the phase angle between the current and the voltage at the conductor (i.e., the angle between the phase of the current and the phase of the driving voltage). The phase angle is defined such that a positive phase angle between the current and the voltage implies that the current leads the voltage (capacitive current) and a negative phase angle implies that the current lags the voltage (more inductive current). The phase angle between the current and the voltage in the power supply network is dependent on the complex impedance of the circuit. Depending on the operating conditions of the power supply network, power may flow from the direction of a power supply towards the central busbar 8a of a multi-panel arrangement (defined as "incoming" power flow) or from the direction of the central busbar 8a towards a load (defined as "outgoing" power flow.) It can be important in practice to understand the direction of power transmission in a power supply network for various reasons. For example, by determining the direction of power flow at several different locations, the location of a fault in the network can be identified and the fault can be isolated. The direction of power flow can also be used, for example, the determine the cause of an overcurrent condition and determine whether such an overcurrent condition is potentially dangerous in respect of possible overload of network components. Figs. 3 and 4 illustrate the relationship between the phase angle between voltage and current under different operating conditions and how these are used to determine a direction of power flow. In these diagrams, the voltage signal U is represented as lying on the positive x-axis 31. The current signal I is represented as being rotated anti clockwise with respect to the positive x-axis according to the phase angle between the voltage U and the current. Thus, when a phase angle O = 0°, the current I lies along the positive x-axis 31. When the phase d> = 90°, the current I lies along the positive y-axis 32, and so on. Fig. 3 illustrates the relationship between a calculated phase angle and the direction of power flow during normal operating conditions of the power supply network. Normal operating conditions of the power supply network are conditions in which continuous (load) currents flow within the rated current of the system. Normal operating conditions may also include overload conditions within a certain duration of time. During normal operating conditions of the power supply network, outgoing power flows are characterized by a phase angle ¢, where -90° <O <90°. Incoming power flows are characterized by a phase angle ¢, where 90° <O <180° or -180° <O <-90°. In other words, currents in which the phase angle lies on the righthand side of the y-axis 32 correspond to outgoing currents, while currents on the lefthand side of the y-axis correspond to incoming currents. During normal operating conditions, the control unit could determine whether the power flow is incoming or outgoing by determining which side of the y-axis 32 the calculated phase angle lies. However, in the region of O = 90° and O = -90°, the determination of the power flow direction is highly sensitive to errors in the calculation in the phase angle such that small errors in the calculated phase angle can result in a reversal of detected power flow direction. Errors in the calculation of the phase angle could arise as a result of inaccuracies in the timings of voltage and current measurements. As described in more detail below, by constraining the range of possible phase angles under different operating conditions of the power supply network, the classification of the power supply direction can be made robust to small errors in phase angle calculation. The power supply network is depicted in accordance with performance standards, such as IEC 62271-103, and the power supply network is therefore configured to operate in normal operating conditions with a power factor of 0.65 or higher inductive. Due to the constraints of the power factor of the power supply network, the phase angle for outgoing current cannot be more negative than -50°. In case of unloaded cable connections, the maximum phase angle can be max +90°. Accordingly, the phase angle of outgoing power is constrained to lie within the range 33 in Fig.3 of -50° <O <90°. Similarly, the phase angle for incoming current cannot be less than 130°. Accordingly, the phase angle of incoming power is constrained to lie within the range 34 in Fig.3 of 130° <0 <270°. The constraints in the range of possible phase angles are determined by the minimum power factor of the system. Therefore, there range of possible phase angles can be further constrained by increasing the minimum allowable power factor in the power supply network. Fig. 4 illustrates the relationship between a calculated phase angle and the direction of power flow during fault conditions of the power supply network. When the sensing circuitry 22 of the switching cap 10 or the control unit determines a fault condition, such a short circuit situation, it will enter a fault condition mode. The sensing of a short circuit condition could be achieved by detecting a current in the conductor 12 having a magnitude that is greater than a threshold value. The threshold value may be a preset value or could be modified dynamically by a user. As the sensing circuitry 22 cannot infer the conditions of the other phases, its calculations of the phase angle may be impaired. In the fault condition mode, the sensing circuitry 22 or control unit uses the measured phase voltage from a period prior to the fault as a timing reference for phase angle determination. In particular, the control unit may store measurements from several periods of the AC signal. On determining that a short circuit condition is taking place, the control unit may use the phase voltage from a period before the onset of the short circuit condition as a timing reference. In other words, when calculating the phase angle between the current and the voltage in short circuit conditions, the timings of the voltage signal based on measurements from periods preceding the short circuit event are used as a reference for comparing the phase of the current to the phase of the voltage. Short circuit situations with earth result in a phase current that will be between 0° and -90° in comparison to the phase voltage in normal network conditions for outgoing current faults. The range of possible phase angles in a short circuit condition depends on the particular short circuit scenario. For three-phase power supply systems, single-phase-earth, two-phase-earth, two-phase, three-phase and three-phase-earth short circuits conditions result in measured phase angles within the following ranges. For outgoing power flows, the phase angle is constrained to lie within the range 44 (shown in Fig.4) of 240° <0 <360° (or -120° <0 <0°). The outer value of 240° ( = -120°) is the outcome of a 2-phase short circuit without earth fault situation. All other short circuit fault situations have 270° (=-90°) as outer value and therefore lie within this range. For incoming power flows, the phase angle is constrained to lie within the range 43 (shown in Fig.4) of 60° <0 <180°. In other fault conditions, the phase angle may be constrained to lie within different ranges. Due to the constraints in the allowable phase angles between the current and voltage described above, the control unit can perform robust determinations of the direction of power flow by taking into account the operating conditions of the power supply network. In particular, the control unit uses a different set of predetermined ranges of phase angles to classify outgoing and incoming power flows depending on whether a fault condition is present in the power supply network or whether the power supply network is operating under normal conditions (i.e., in the absence of a fault condition). In some examples, the control unit receives a signal from a fault detection module indicating the presence of a fault condition. In the absence of a signal indicating the presence of a fault condition, the control unit may determine that no fault condition is present. In some examples, the control unit may determine whether or not a fault condition is occurring based on measurements of the current received from the sensing circuitry 22. For example, the control unit may determine that a short circuit condition is occurring based on the magnitude of the measured current in the conductor exceeding a current threshold value or based on the measured current exceeding the thresholds specified by a time-current curve. Based on determining that no fault condition is occurring, the control unit may determine the direction of the power flow by determining whether calculated phase angle between the voltage and the current lies in a first predetermined range or a second predetermined range. The first and second predetermined ranges form a first set of predetermined ranges. Instead of using the y-axis 32 to classify incoming and outgoing phase angles, the control unit uses a rotated axis 35 as boundary between the first predetermined range and the second predetermined range. The rotated axis is chosen such that the boundary between the first predetermined range and the second predetermined range lies in regions outside the expected phase angles of the incoming and outgoing currents under normal operating conditions. Because the phase angles of the actual currents are constrained to belong to a region separated from the rotated boundary axis 35, small errors in the measurement or calculation of the phase angles of the actual currents will not result in a change in the classification of the power transmission direction. Preferably, the boundary axis lies on the line of 0 = 110°; 0 = -70°. As such, the first predetermined range is the region between -70° <0 <110°, and the second predetermined range is the range between 110° <0 <290° (or -250° <0 <-70°.) In some examples, the first and / or second predetermined ranges may be smaller than 180°. For example, the first or the second region may comprise a narrower range with the above-described ranges and surrounding the expected outgoing phase angles and the expected incoming phase angles. If the control unit determines that the calculated phase angle lies within the first predetermined region, the control unit determines that the direction of power flow is outgoing. The control unit provides an indication that the direction of power flow is outgoing to a user or to further elements of the power supply network. If the control unit determines that the calculated phase angle lies within the second predetermined region, the control unit determines that the direction of power flow is incoming. In this case, the control unit provides an indication that the direction of power flow is incoming to a user or to further elements of the power supply network. Where the calculated phase angle does not lie within the expected ranges of outgoing 33 and incoming 34 power flow, it is assumed that the discrepancy is a result of a small error in the calculation / measurement of the phase angle and that the classification of the power direction is correct. Similarly, based on determining that a fault condition is occurring, the control unit may determine the direction of the power flow by determining whether the calculated phase angle between the voltage and the current lies in a third predetermined range or a fourth predetermined range. The third and fourth predetermined ranges form a second set of predetermined ranges that is different to the first set of predetermined ranges. The rotated axis 45 (shown in Fig.4) in these circumstances is chosen such that the boundary between the third predetermined range and the fourth predetermined range lies in regions outside the expected phase angles of the incoming and outgoing currents that occur in fault conditions. Preferably the phase angles are defined by the measured phase angles that occur in practice under short circuit conditions in three-phase power supply systems. Preferably, the boundary axis lies on the line of 0 = 30°; 0 = 210°. As such, the fourth predetermined range, corresponding to incoming power flow in a short circuit condition, is the region between 30° <0 <210°, and the third predetermined range, corresponding to outgoing power flow in a short circuit condition, is the range between 210° <O <390° (or -150° <0 <30°.) In some examples, the third and / or fourth predetermined ranges may be smaller than 180°. For example, the third or the fourth predetermined range may comprise a narrower range with the above-described ranges and surrounding the expected outgoing phase angles and the expected incoming phase angles. Preferably, the end points of the first predetermined range are related to the end points of the third predetermined range by a constant phase angle offset. Preferably, the end points of the second predetermined range are related to the end points of the fourth predetermined range by a constant phase angle offset. As such, the third and fourth predetermined ranges correspond to the first and second predetermined range being rotated around the phase diagram by the constant offset angle. If the control unit determines, in the case of a short circuit, that the calculated phase angle lies within the third predetermined region, the control unit determines that the direction of power flow is outgoing. The control unit provides an indication that the direction of power flow is outgoing to a user or to further elements of the power supply network. If the control unit determines that the calculated phase angle lies within the fourth predetermined region, the control unit determines that the direction of power flow is incoming. In this case, the control unit provides an indication that the direction of power flow is incoming. The indication may be provided to a user or to further elements of the power supply network. Where the calculated phase angle does not lie within the possible ranges of outgoing 44 and incoming 43 power flow, it is assumed that the discrepancy is a result of a small error in the calculation / measurement of the phase angle and that the classification of the power direction is correct. In some examples, the control unit may receive measurements regarding the phase angles at several different parts of a power supply network. For example, the control unit may receive measurements regarding the phase angles at a plurality of switching units at different locations in the power supply network, or in the same switchgear (from several functional units). In normal operating conditions of the power supply network, the control unit may use the information regarding the power flow directions at different locations of the network to optimize power flow. For example, the control unit may open or close one or more switching devices at different locations in the power supply network in based on determining the direction of power flow at different locations in the power supply network. One or more different control units may be used determine power flow directions at different locations of the power supply network. Each control unit may provide information regarding the determined power flow directions to a separate central control system that is used to control the entire power supply network. The separate central control system may open or close one or more switching devices at different locations in the power supply network in order to optimize power flow. In fault conditions, such as a short circuit condition, the control unit (or central control system) may use the information regarding the power flow directions at different locations of the network to identify an interconnection between two adjacent measuring points within which the fault in the network is located. After interrupting power flow in the network by opening one or more switching devices, the control unit or central control system may reconnect healthy parts of the power supply network while keeping the location with the fault disconnected. With reference to Figure 5, a method performed by a control unit of an electrical apparatus forming part of an AC power supply network is described. At operation S10, the control unit receives measurements of the value of the current in a conductor and the voltage at the conductor. The conductor forms part of the main current path of the power supply network. The measurements may be performed by one or more sensors of a switching cap 10 of the power supply network, as described above. At operation S20, the control unit calculates a phase angle between the measured voltage and the measured current. In some embodiments, instead of the control unit calculating the phase angle, the one or more sensors include circuitry that is used to calculate the phase angle between the measured voltage and the measured current, and the one or more sensors provide the calculated phase angle to the control unit. Calculation of the phase angle may be performed based on the timings of measurements of the voltage and the timings of the measurement of the current. At operation S30, the control unit determines whether a short circuit condition is present in the power supply network. Determining that a short circuit condition is occurring comprise determining that a measured voltage or current is indicative of a short circuit condition or receiving an indication from a fault detection device configured to detect a short circuit in the power supply network that a short circuit condition is occurring. Determining that no short circuit condition is present may comprise determining that measured current or voltages lie within normal ranges or determining that no indication of a short circuit condition has been received from a fault detection device. At operation S40, based on determining that no short circuit condition is present in the power supply network, the control unit determines which of a first predetermined range and a second predetermined range contains the calculated phase angle; and based on determining that a short circuit current is occurring in the conductor, determining which of a third predetermined range and a fourth predetermined range contains the calculated phase angle. At operation S50, based on determining that the calculated phase angle is contained in the first or the third predetermined range, provide as an output an indication that a direction of power flow in the conductor is outgoing; and based on determining that the calculated phase angle is contained in the second or fourth predetermined range, provide as an output an indication that the direction of power flow in the conductor is incoming. The classification criteria that the control unit uses to determine the direction of power flow is selected based on whether or not a short circuit condition is occurring. The classification criteria can be selected such that the boundaries between outgoing and incoming power flows lie in phase angle ranges within which the power supply network cannot have a phase angle of the current compared to the related voltage. Under normal operating conditions of the power supply network (i.e., in the absence of a fault condition), the expected range of phase angles of the current can be constrained by a minimum declared power factor in the distribution network. Therefore, the boundaries between the first and second predetermined range may be determined based at least in part on the minimum power factor of the power distribution network that can occur in practice. Under fault conditions, the allowed phase angles of the current is constrained by the type of fault condition present. The boundaries between the third and further predetermined range may be determined based on the constraints in the measured phase angle during different short circuit situations in a three-phase power supply network. Fig 6. schematically illustrates an apparatus according to the present disclosure. The control unit 601 is in communication with or connected to one or more sensors 22 that are configured to sense a voltage on and a current in a conductor 12. The conductor 12 forms part of a main current path of a power supply network and may form part of a current path through a switching device 10. The one or more sensors 22 may communicate wirelessly with the control unit 601 using a wireless communication 5 terminal 602 that is electrically connected to the one or more sensors 22 and a wireless communication terminal 603 of the control unit 601. Alternatively, the control unit 601 may be directly connected to the one or more sensors 22. In some examples, a control unit 601 that is directly connected to the one or more sensor may calculate a phase angle between a voltage and a current on a single pole basis, and the control 10 unit 601 may send the result to a central control system. The central control system may receive calculated phase angles from several different control units. The control unit may be in wireless communication with several sensors 22 at different locations in the power supply network. 15 The electrical apparatus and method of the above disclosure allows robust determination of a power flow direction in an AC power supply network where measurements of a phase angle in the current path may include inaccuracies.

Claims

1. An electrical apparatus forming part of an AC power supply network comprising:a conductor (12);one or more sensors (22) for measuring a current in the conductor and a voltage on the conductor; anda control unit (601), the control unit being configured to:receive, from the one or more sensors, measurements of the value of the current in the conductor (12) and the voltage at the conductor;calculate a phase angle between the measured voltage (U) and the measured current or receive a calculated phase angle between the measured voltage and the measured current from the one or more sensors (22);determine whether a short circuit current is occurring in the conductor;based on determining whether or not a short circuit current is present in the conductor (12), select a set of predetermined ranges from a plurality of sets of predetermined ranges,determine which predetermined range from the selected set of predetermined ranges contains the calculated phase angle,based on determining the predetermined range of the selected set of predetermined ranges that contains the calculated phase angle, provide as an output an indication of whether a direction of power flow in the conductor is outgoing or incoming.

2. The electrical apparatus of claim 1, wherein:based on determining that no short circuit current is present in the conductor (12), a first set of predetermined ranges is selected, wherein the first set of predetermined ranges comprises a first predetermined range and a second predetermined range;based on determining that a short circuit current is occurring in the conductor (12), a second set of predetermined ranges is selected, wherein the second set of predetermined ranges comprises a third predetermined range and a fourth predetermined range;and wherein the control unit is further configured to:based on determining that the calculated phase angle is contained in the first or the third predetermined range, provide as an output an indication that a direction of power flow in the conductor is outgoing; andbased on determining that the calculated phase angle is contained in the second or fourth predetermined range, provide as an output an indication that the direction of power flow in the conductor is incoming,wherein the first predetermined range and the second predetermined range do not overlap each other, and wherein the third predetermined range and the fourth predetermined range do not overlap each other.

3. The electrical apparatus of claim 2, wherein the first predetermined range is related to the third predetermined range by a constant phase angle offset, and wherein the second predetermined range is related to the fourth predetermined range by the same constant phase angle offset.

4. The electrical apparatus of claim 2 or claim 3, wherein the first predetermined range and the second predetermined range cover a phase angle range of 360°, and the third predetermined range and the fourth predetermined range cover a phase angle range of 360°.

5. The electrical apparatus of any of claims 2 to 4, wherein each of the first, second, third and fourth predetermined ranges cover a phase angle range of 180°.

6. The electrical apparatus of any of claims 2 to 5, wherein the first predetermined range is entirely contained in the range of -70° to 110°, and the second predetermined range is entirely contained in the range of 110° to 290°, and preferably, wherein the first predetermined range is from -70° to 110°, and the second predetermined range is from 110° to 290°7. The electrical apparatus of any of claims 2 to 6, wherein the third predetermined range is entirely contained in the range -150° to 30°, and the fourth predetermined range is entirely contained in the range 30° to 210°, and preferably, wherein the third predetermined range is from -150° to 30°, and the fourth predetermined range is from 30° to 210°.

8. The electrical apparatus of any of claims 2 to 7, wherein the end points (35) of the first predetermined range and the second predetermined range are chosen based on a minimum permitted power factor of the power supply network such that the end points (35) of the first and second predetermined ranges correspond to phase angles that are inconsistent with the minimum permittedpower factor of the power supply network in the absence of a detected short circuit current.

9. The electrical apparatus of any of claims 2 to 8, wherein the end points (45) of the third predetermined range and the fourth predetermined range are chosen based on the number of phases in the power supply such that the end points of the third and fourth predetermined ranges correspond to phase angles that are inconsistent with a measured phase angle in a short circuit condition between any of the phases or with ground.10.The electrical apparatus of any preceding claim, wherein the conductor (12) is electrically connected to a load and to a shared power supply busbar, and wherein the outgoing power flow is a power flow towards the load and an incoming energy flow is a power flow towards the shared power supply busbar.

11. The electrical apparatus of any preceding claim, wherein the control unit (601) determines whether a short circuit current is occurring based at least in part on the magnitude of the measured current.12.The electrical apparatus of any preceding claim, wherein the conductor (12) forms part of one phase of a three-phase power supply system.13.The electrical apparatus of any preceding claim, wherein the control unit (601) receives measurements of voltages and currents from a plurality of sensors at different locations in the power supply network; and wherein the control unit is configured to:determine the directions of power flow at the different locations of the power supply network;based on the determined directions of power flow at the different locations of the power supply network, identify two adjacent measuring locations between which the fault in the power supply network is located; andafter interrupting power supply throughout the power supply network, reconnect healthy parts of the power supply network based on the identified location of the fault.

14. A method performed by a control unit of an electrical apparatus forming part of an AC power supply network, the power supply apparatus comprising aconductor and one or more sensors for measuring a current in the conductor and a voltage on the conductor, the method comprising:receiving, from the one or more sensors (22), measurements of the value of the current in the conductor (12) and the voltage at the conductor (12);calculating a phase angle between the measured voltage and the measured current or receiving a calculated phase angle between the measured voltage and the measured current from the one or more sensors (22);determining whether a short circuit current is occurring in the conductor (12);based on determining whether or not a short circuit current is present in the conductor (12), selecting a set of predetermined ranges from a plurality of sets of predetermined ranges,determine which predetermined range from the selected set of predetermined ranges contains the calculated phase angle,based on determining the predetermined range of the selected set of predetermined ranges that contains the calculated phase angle, provide as an output an indication of whether a direction of power flow in the conductor is outgoing or incoming.

15. The method of claim 14, wherein:based on determining that no short circuit current is present in the conductor (12), a first set of predetermined ranges is selected, wherein the first set of predetermined ranges comprises a first predetermined range and a second predetermined range;based on determining that a short circuit current is occurring in the conductor, a second set of predetermined ranges is selected, wherein the second set of predetermined ranges comprises a third predetermined range and a fourth predetermined range;wherein the method further comprises:based on determining that the calculated phase angle is contained in the first or the third predetermined range, providing as an output an indication that a direction of power flow in the conductor is outgoing; andbased on determining that the calculated phase angle is contained in the second or fourth predetermined range, providing as an output an indication that the direction of power flow in the conductor is incoming,wherein the first predetermined range and the second predetermined range do not overlap each other, and wherein the third predetermined range and the fourth predetermined range do not overlap each other.23