Electrical apparatus for phase angle calculation

Abstract

An electrical apparatus (figure 1) forms part of an AC power supply network and comprises one or more measurement devices (22, figure 2) to measure a voltage and current on a conductor (12, figure 1) forming part of a power line of the AC network. A control unit receives current and voltage measurements from the measurement device. The controller determines the presence of a short circuit condition at time T1 and identifies a first voltage measurement recorded at time T2, prior to onset of the short circuit. Based on the first voltage measurement, a phase of the voltage signal at a first time T1 during which the short circuit condition is occurring can be determined. From the determined phase of the voltage signal and a measured phase of the current signal at the first time, a phase angle between the current signal and the voltage signal can be calculated, even though the voltage at this time cannot accurately be recorded.

Description

Field The present disclosure relates to an electrical apparatus forming part of an AC power supply network. In particular, the disclosure relates to an electrical apparatus that is capable of measuring voltages and currents in a power line of the AC power flow network and calculating a phase angle between a voltage signal and a current signal. Background It is important to be able to determine the phase angle between a driving voltage and a current in an AC power supply network. The phase angle can, for example, be used to determine the direction of power flow at a given location in the power supply network. Calculation of the phase angle may be disrupted during fault conditions of the power supply network due to loss of information regarding the state of the network. During a short circuit condition, it may no longer be possible to measure the phase of the driving voltage and, hence, the phase angle to the short circuit current. The inability to calculate a phase angle during a short circuit condition can prevent accurate determination of a direction of power flow, preventing location of the source of a fault and delaying rectification of the underlying issue. In view of the above, there is a need for improved methods for determining a phase angle between a voltage signal and a current signal in an AC power supply network during a short circuit condition. Summary One aspect of the disclosure provides an electrical apparatus forming part of an AC power supply network. The electrical apparatus comprises: one or more measurement devices being configured to measure a voltage signal on a conductor forming part of a power line of the AC power supply network and a current signal in the conductor; a control unit configured to receive current measurements and voltage measurements from the at least one measurement device, wherein the controller is configured to: determine the presence of a short circuit condition in the power supply network; identify a first voltage signal measurement recorded prior to onset of the short circuit condition; based on the identified first voltage signal measurement, determine a phase of the voltage signal at a first time during which the short circuit condition is occurring; based on the determined phase of the voltage signal at the first time and a measured phase of the current signal at the first time, calculate a phase angle between the current signal and the voltage signal at the first time. By using voltage signal measurements recorded prior to onset of a short circuit condition to infer the phase of a voltage during the short circuit condition, an accurate determination of a phase angle between the supply voltage and the current at a location in the power supply network can be achieved without the need to accurately measure the supply voltage during the short circuit condition. In some examples, the control unit is further configured to record a series of entries in a register based on voltage signal measurements received from the one or more measurement devices. In some examples, the register is a first-in-first-out, FIFO, array providing a moving window of voltage signal measurements. In some examples, the moving window of the FIFO array records voltage signal measurements corresponding to a predetermined time period or a predetermined number of voltage signal cycles. In some examples, each entry of the series of entries indicates a zero-crossing of the voltage signal or each entry of the series of entries indicates a peak of the voltage signal. In some examples, the series of entries corresponds to an analogue to digital conversion of the voltage signal. In some examples, identifying the first voltage signal measurement occurring prior to onset of the short circuit condition comprises: identifying a second time that the short circuit condition was detected; identifying a third time before the short circuit condition was detected by subtracting a predetermined offset from the second time; and identifying an entry in the register having a timestamp in the vicinity of the identified third time. In some examples, determining the phase of the voltage signal at the first time comprises: determining the phase of the identified first voltage signal measurement; determining the period, P, of the voltage signal of the power supply network; determining the elapsed time difference between the time of the first voltage signal measurement and the first time; and, based on the phase of the identified first voltage signal measurement and the elapsed time difference, infer the phase of the voltage signal at the first time. In some examples, the control unit is further configured to perform the steps of: determine, based on the calculated phase angle between the voltage signal and the current signal at the first time, a direction of power flow in the conductor at the at least one measurement device. In some examples, the control unit is further configured to select a switching device in the power supply network based on the direction of power flow and cause the switching device to be opened in order to disconnect a region of the power supply network including the location of the short circuit event. In some examples, determining the presence of the short circuit condition in the power supply network is performed based on the magnitude and / or duration of the received voltage signal measurements and / or current measurements. In some examples, the power line is one phase line of a three-phase power distribution network. In some examples, the one or more measurement devices comprises a voltage measurement device that uses a star point of the three phases of the power distribution network as a reference for voltage measurements of the voltage signal at the power line. In some examples, the star point of the three phases of the power distribution network is not connected to local earth. Another aspect of the disclosure provides a method performed by a control unit of an electrical apparatus forming part of an AC power supply network. The method comprises: determining the presence of a short circuit condition in the power supply network; identifying a first voltage signal measurement recorded prior to onset of the short circuit condition; based on the identified first voltage signal measurement, determining a phase of the voltage signal at a first time during which the short circuit condition is occurring; and based on the determined phase of the voltage signal at the first time and a measured phase of the current signal at the first time, calculate a phase angle between the current signal and the voltage signal at the first time. Brief Description of the 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 schematically illustrates a suitable circuit for use in measurements of the voltage signal in embodiments of the disclosure; Fig. 4 schematically illustrates the changes in voltage signal measurements at a conductor of a power line before and after onset of a short circuit condition in embodiments of the disclosure; Fig. 5 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. 6 shows an example method performed by a control unit in examples of the disclosure; Fig. 7 is a schematic illustration of elements of the electrical apparatus in examples of the disclosure. Detailed Description The present disclosure relates to an electrical apparatus that can be used in an AC power distribution network. The electrical apparatus comprises a control unit and one or more measurement devices for locally measuring a current and a voltage in power lines of the AC power distribution network. A power source may provide a driving voltage (or "supply voltage") to the power lines of the power distribution network. The electrical apparatus may include conductive elements forming parts of a main current path of the AC power distribution network, or the electrical apparatus may be located in the vicinity of the conductive path an AC power distribution network and thereby measure the voltage and current at that location. The electrical apparatus may, for example, form part of a medium-voltage (MV) switchgear in an AC power distribution network. The electrical apparatus may be presented as a single unit disposed in one location or may comprise elements that are distributed. For example, the control unit of the electrical apparatus may be disposed at a remote location with respect to the power lines and measurement devices of the electrical apparatus, or the control unit may form part of single unit along with the power lines and measurement devices, or the control unit may include distinct elements of processing circuitry distributed across several locations in communication with each other. Based on measurements of a current in a power line and a voltage on the power line, the control unit calculates a phase angle between the voltage and the current at that location. When the power distribution network is a three-phase power distribution network, current and voltage measurements may be performed on a single-phase basis, and a separate measurement device may measure the voltage and currents of each phase separately. The voltage on each phase is measured with reference to the common star point of the three-phase measurement circuit. The common star point may acts as a virtual earth ("ground") with respect to which the voltage of each phase is compared without requiring physical connection of the star point to the local earth. In other examples, a physical connection to the local ground may be provided. The phase angle between the driving voltage of each individual phase and the current of each respective phase of the power system is used to determine the direction of power flow at the measurement location. In normal operation conditions of a healthy three-phase circuit, the on site voltage level and symmetry between the phases of the circuit allows robust voltage measurements to be performed at a single phase of the power supply network with respect to the common star point. However, during short circuit conditions, the voltage level and / or the symmetry of the three phase voltages may be impaired and the floating star point no longer provides a suitable reference for the measurement of a voltage signal. In a three-phase power supply network, a short circuit may be single-phase to ground, two-phase with or without ground, or three-phase with or without ground. Even in short circuit conditions where one or more phases remain healthy, the impairment of the symmetry between the phases prevents the use of the common star point as a reference for the measurement of a voltage signal on the healthy phases. In embodiments of the present disclosure, an electrical apparatus calculates a phase angle between a current and the driving voltage in a power line during a short circuit condition by using voltage measurements from a time before onset of the short circuit condition as a reference for the voltage phase during the short circuit condition. During normal operation of the power supply network, in the absence of a short circuit condition, the electrical apparatus takes measurements of the voltage signal at one or more phases of the power line. The measurements of the voltage signal are recorded as entries in a register. On detection of a short circuit condition, previously recorded measurements from before the onset of the short circuit condition are used to infer a phase of the voltage signal at one or more times after onset of the short circuit condition. The inferred voltage phase is used to calculate a phase angle between the current signal and the driving voltage signal (or "supply voltage"). The calculated phase angle can be used to determine a direction of power flow at the location of the measurements. With reference to Figs. 1 and 2, an electrical apparatus suitable for use in examples of the present disclosure is described. While the detailed description in relation to Fig.l and 2 describes a particular arrangement of the electrical apparatus as forming part of a switching cap 10, the technical teaching herein is applicable to different arrangements. For example, in other embodiments the electrical apparatus may not include part of a main current path of the AC power distribution network and may instead comprise measurement devices and sensing circuitry that monitor signals in a power line of the AC power distribution network that is distinct from the electrical apparatus. 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 power line. 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 in this example, or, in general, to a switching device or even a direct connection. 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. Busbars 8a and 8b along with a conductor 12 may form part of a power line connecting a power source and a load. The switching cap 10 comprises a measurement device 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. The switching cap 10 may, for example, 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 measurement device 22 may comprise one or more current sensor(s) and one or more voltage sensor(s). In the illustrated apparatus, at least part of the measurement device 22 is separated from the conductor 12 by one or more insulating layers 21. The current sensor of the measurement device 22 may comprise a Rogowski coil or pickup coil with or without a ferro-magnetic core. The voltage sensor of the measurement device 22 may comprise a capacitive voltage divider for downscaling the voltage signal in the conductor 12 of the power line. Fig. 3 illustrates a suitable circuit for use in measurements of the voltage in embodiments of the disclosure. A first capacitor Cl is disposed between the conductor 12 and a voltage measurement sensor comprising an analogue to digital converter (ADC). The first capacitor Cl may be formed of the insulating layer(s) 21 of the switching cap 10 between the conductor 12 and a conductive or semiconductive layer near a PCB of the measurement device. The first capacitor Cl may have a capacitance between lOpF or lOOpF and, preferably, about 15pF. A second capacitor C2 is disposed between the conductive or semiconductive layer as voltage measurement point and ground. The second capacitor C2 may have a capacitance of about 200nF. The current flowing through the conductor 12 during operation is an alternating current that flows between 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, for an "outgoing" power flow, 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. With respect to Fig. 4, aspects relating to the calculation of a phase angle between a voltage signal and a current signal in embodiments of the disclosure are described. Voltage signal 41 of a single phase of the electrical apparatus is measured by the measurement device 22 with reference to the common star point of the three phases during normal operating conditions of the power supply network and provided to the control until. Measurements of the voltage signal 41 are stored by the control unit in a register. The register may comprise a first-in-first-out (FIFO) array comprising voltage signal measurements corresponding to a moving window. For example, the FIFO array may include voltage measurements for a window comprising a predetermined time period (e.g. 100ms) or a predetermined number of cycles of the voltage signal (e.g. 5 periods). In some examples, the FIFO array may comprise the entire ADC voltage signal during the window defined by the size of the FIFO array and the sampling rate at which the voltage signal is sampled. In other examples, the FIFO array may include an indication of the zero-crossing point or a signal peak for each cycle of the voltage signal. For example, the FIFO array may include a timestamp for each zero-crossing or power frequency signal maxima or minima during the moving window. At time TO, a short circuit occurs at one or more phases of the power supply network. Loss of symmetry between the three phases of the power supply prevents the common star point acting as a reliable reference for voltage signal measurement after time TO. In a short circuit condition, the primary voltage (or "supply voltage" or "driving voltage") may become too low to measure the reference accurately, depending on the distance from the measuring devices to the short circuit location. At time Tl, the control unit detects the presence of a short circuit in the power supply network. Detection of a short circuit is not generally instantaneous, and there is a lag between onset of the short circuit condition at TO and detection of the short circuit condition atTl. The control unit may, in some examples, receive a signal from another element of the power supply network (such as the control unit of a switching cap 10 of another phase of the electrical apparatus ora detection device at another location in the power supply network) indicating the detection of a short circuit condition. In another example, the control unit may determine the presence of a short circuit condition based on measurements of the voltage and / or current. For example, the control unit may determine the presence of a short circuit condition based on measurements of the current exceeding limits specified by a predetermined timecurrent curve. On detecting the presence of a short circuit condition at Tl, the control unit accesses the register to retrieve measurements of the voltage signal 41 at time T2, which is a predetermined time, D, prior to Tl. The predetermined time D is configured such that T2 occurs before TO. For example, D may be chosen to be greater than a time required to detect a short circuit condition in the power supply network. Based on the measurements of the voltage signal at T2, the control unit infers the phase of the driving voltage signal at a first time after onset of the short circuit condition. The first time may be Tl or another time after onset of the short circuit condition. The power supply network provides an AC driving voltage with a predefined period P. The first time may be chosen such that it occurs an integer number of periods P after time T2. In this case, the control unit may infer that the phase of the driving voltage signal 41 at the first time is the same as the phase of the voltage signal at T2. The first time may also be an integer number of half-periods, an integer number of quarter-periods, or another offset after T2. Generally, the driving voltage at an arbitrary first time after T2 can be inferred based on the phase of the driving voltage signal at T2 and information regarding the predefined period P of the driving voltage signal. In some examples, the control unit may comprise memory having a prestored value of P corresponding to the supply frequency of the power supply network. In other examples, the control unit may determine the value of P from the measured voltage signal. In some examples, the FIFO array records only a single value for each period of the voltage signal 41. For example, the FIFO array may record a timestamp corresponding to a zero-crossing or maximum of the voltage signal 41. In these examples, the control unit may identify an entry of the FIFO array occurring in the vicinity of T2. For example, the control unit may identify the entry of the FIFO having a timestamp occurring closest to T2, immediately after T2, or immediately before T2. The control unit may infer the phase of the driving voltage signal 41 at a time after TO based on the identified entry of the FIFO array. For example, when the FIFO array records zerocrossing of peaks of the voltage signal 41, the control unit may infer the phase of the voltage signal occurring an integer multiple of P, or half-P, after time TO. In general, the control unit may infer the phase of the driving voltage signal at an arbitrary first time after onset of the short circuit condition based on the phase of the voltage signal at a time and information regarding the predefined period P of the driving voltage signal. Measurement of the current in the power line can be performed by the measurement device 22 during a short circuit condition. The control unit can calculate a phase angle between the inferred phase of the voltage signal 41 at a first time and the measured phase of the current signal at the first time. Based on the calculated phase angle, the control unit can determine a direction of power flow at the measurement device. The control unit can also determine the type of short circuit fault based on the calculated phase angle at the measurement device. For example, the control unit may determine that the short circuit is 1-phase, 2-phase with or without ground, or three-phase with or without ground. Fig. 5 illustrates the relationship between a calculated phase angle and the direction of power flow during fault conditions of the power supply network and how these are used to determine a direction of power flow. 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 0 = 0°, the current I lies along the positive x-axis 31. When the phase 0 = 90°, the current I lies along the positive y-axis 32, and so on. 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. When 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 the first predetermined range 44 or the second predetermined range 43. After determining the direction of power flow, the control unit (or a 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, the fault location may be isolated by opening one or more switching devices. Then, healthy parts of the power supply network may be energised again while keeping the location with the fault disconnected. With reference to Figure 6, 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 determines the presence of a short circuit condition in the power supply network. The presence of a short circuit condition may, for example, be determined using measurements received by the control unit, or the control unit may receive an indication of the presence of a short circuit condition by a separate entity. At operation S20, the control unit identifies a first voltage signal measurement recorded prior to onset of the short circuit condition. The first voltage signal measurement may be retrieved from a register of previously recorded voltage measurements. The register may be a FIFO array comprising a moving window of voltage (reference) measurements received from a measurement device. The identified voltage measurement may be a measurement corresponding to a predetermined time period prior to the time of detection of the short circuit condition or a predetermined number of voltage signal periods prior to the time of detection of the voltage signal. At operation S30, the control unit determines a phase of the voltage signal 41 at a first time during which the short circuit condition is occurring based on the identified first voltage measurement. The control unit may determine a phase of the voltage signal 41 at the time of the first voltage measurement. The control unit may then determine the phase of the voltage signal 41 at a later time based on the period P of the voltage signal 41. In particular, the control unit may infer that the supply voltage has the same phase as the phase of the first voltage measurement at a time that is an integer number of periods after the time of the first voltage measurement. In other words, the voltage measurements recorded prior to the onset of the short circuit condition are used as a timing reference for the determination of the voltage signal phase after the onset of the short circuit condition. At operation S40, based on the determined phase of the voltage signal at the first time and a measured phase of the current signal at the first time, the control unit calculates a phase angle between the current signal and the voltage signal at the first time. The calculated phase angle can be used to determine the type of short-circuit and the power flow direction at the measurement device of the power supply network. The determined power flow direction can be used to identify the location of a short circuit fault, and control of circuit breaking switches can be performed to isolate parts of the power supply network including the short circuit fault. Fig 7. schematically illustrates an apparatus according to the present disclosure. The control unit 601 is in communication with or connected to measurement devices 22 that are configured to sense a voltage on and a current in a conductor 12. The conductor 12 forms part of a power line of a power supply network and may form part of a current path through a switching device 10. The one or more measurement devices 22 may communicate wirelessly with the control unit 601 using a wireless communication terminal 602 that is electrically connected to the one or more measurement devices 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 measurement devices 22. In some examples, a control unit 601 that is directly connected to the one or more sensors may calculate a phase angle between a voltage and a current on a single pole basis, and the control 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 measurement devices 22 at different locations in the power supply network. One or more processors 604 of the control unit may perform processing operations according to the methods described above. The electrical apparatus and method of the above disclosure allows a phase angle between a voltage signal and a current signal to be calculated during a short circuit condition in which the reference point for measurement of a voltage is no longer reliable. The measurement of the phase angle may be used to determine a direction of power flow in the power supply network.

Claims

1. An electrical apparatus forming part of an AC power supply network, the electrical apparatus comprising:one or more measurement devices (22) being configured to measure a voltage signal (41) on a conductor (12) forming part of a power line of the AC power supply network and a current signal in the conductor;a control unit (601) configured to receive current measurements and voltage measurements from the at least one measurement device, wherein the controller is configured to:determine the presence of a short circuit condition in the power supply network;identify a first voltage signal measurement recorded prior to onset of the short circuit condition;based on the identified first voltage signal measurement, determine a phase of the voltage signal at a first time during which the short circuit condition is occurring;based on the determined phase of the voltage signal (41) at the first time and a measured phase of the current signal at the first time, calculate a phase angle between the current signal and the voltage signal at the first time.

2. The electrical apparatus of claim 1, wherein the control unit (601) is further configured to record a series of entries in a register based on voltage signal measurements received from the one or more measurement devices (22).

3. The electrical apparatus of claim 1, wherein the register is a first-in-first-out, FIFO, array providing a moving window of voltage signal measurements.

4. The electrical apparatus of claim 3, wherein the moving window of the FIFO array records voltage signal measurements corresponding to a predetermined time period or a predetermined number of voltage signal cycles.

5. The electrical apparatus of any of claims 2 to 4, wherein each entry of the series of entries indicates a zero-crossing of the voltage signal (41) or each entry of the series of entries indicates a peak of the voltage signal (41).

6. The electrical apparatus of any of claims 2 to 4, wherein the series of entries corresponds to an analogue to digital conversion of the voltage signal (41).

7. The electrical apparatus of any of claims 2 to 6, wherein identifying the first voltage signal measurement occurring prior to onset of the short circuit condition comprises:identifying a second time (Tl) that the short circuit condition was detected;identifying a third time (T2) before the short circuit condition was detected by subtracting a predetermined offset (D) from the second time; and identifying an entry in the register having a timestamp in the vicinity of the identified third time.

8. The electrical apparatus of any preceding claim, wherein determining the phase of the voltage signal at the first time comprises:determining the phase of the identified first voltage signal measurement;determining the period, P, of the voltage signal (41) of the power supply network;determining the elapsed time difference between the time of the first voltage signal measurement and the first time; andbased on the phase of the identified first voltage signal measurement and the elapsed time difference, infer the phase of the voltage signal (41) at the first time.

9. The electrical apparatus of any preceding claim, wherein the control unit is further configured to perform the steps of:determine, based on the calculated phase angle between the voltage signal (41) and the current signal at the first time, a direction of power flow in the conductor (12) at the one or more measurement devices (22).10.The electrical apparatus of claim 7, wherein the control unit (601) is further configured to select a switching device in the power supply network based on the direction of power flow and cause the switching device to be opened in order to disconnect a region of the power supply network including the location of the short circuit event.11.The electrical apparatus of any preceding claim, wherein determining the presence of the short circuit condition in the power supply network is performed based on the magnitude and / or duration of the received voltage signal measurements and / or current measurements.

12. The electrical apparatus of any preceding claim, wherein the power line is one phase line of a three-phase power distribution network.

13. The electrical apparatus of claim 12, wherein the one or more measurement devices (22) comprises a voltage measurement device that uses a star point of the three phases of the power distribution network as a reference for voltage measurements of the voltage signal (41) at the conductor (12).

14. The electrical apparatus of claim 13, wherein the star point of the three phases of the power distribution network is not connected to local earth.

15. A method performed by a control unit (601) of an electrical apparatus forming part of an AC power supply network, the method comprising:determining the presence of a short circuit condition in the power supply network;identifying a first voltage signal measurement recorded prior to onset of the short circuit condition;based on the identified first voltage signal measurement, determining a phase of the voltage signal at a first time during which the short circuit condition is occurring; andbased on the determined phase of the voltage signal at the first time and a measured phase of the current signal at the first time, calculate a phase angle between the current signal and the voltage signal at the first time.17

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