Electric meter and method for measuring electric power

Abstract

An electric meter (100) comprising a non-invasive voltage measurement sensor (12), one or more non-invasive current measurement sensors (14A-14C) for measuring one or more phase currents, and a digital processing unit (10). The voltage measurement sensor (12) and the current measurement sensors (14A-14C) are connected or connectable to the processing unit (10). The processing unit (10) is configured, in a calibration mode, to determine phase shift values between a first voltage sensor signal and first current sensor signals, and to store the phase shift values into a memory. The processing unit (10) is configured, in an operation mode, to determine a second voltage sensor signal, determine voltage estimates based on the second voltage sensor signal and the stored phase shifts, and determine, based on the voltage estimates and second current sensor signals determined by the current measurement sensors (14A-14C), respectively, one or more electric power values.

Description

[0001] ELECTRIC METER AND METHOD FOR MEASURING ELECTRIC POWER

[0002] FIELD OF THE INVENTION

[0003] The present invention relates in general to electric meters.

[0004] BACKGROUND

[0005] Known electric meters typically require an electrician to install the meter into its intended position. This often, if not always, requires switching off electricity in order to ensure safety. Furthermore, the installing of the electric meter requires manual labor so that the sensors of the meter can be galvanically connected to right positions or portions of the electric system.

[0006] SUMMARY

[0007] An objective of the present invention is to provide an electric meter and a method for measuring electric. Another objective of the present invention is that the electric meter and the method provide an easy and convenient, yet accurate enough solution for measuring electric power without the need to invade to electrical portions of the target system or device.

[0008] The objectives of the invention are reached by an electric meter and a method as defined by the respective independent claims.

[0009] According to a first aspect, an electric meter is provided. The electric meter comprises a non-invasive voltage measurement sensor, one or more (at least one) non-invasive current measurement sensors for measuring one or more phase currents, and a digital processing unit. The non-invasive voltage measurement sensor and the one or more non- invasive current measurement sensors are connected or connectable, such as removable, to the processing unit, such as via input ports thereof, for providing voltage and current sensor signals to the processing unit.

[0010] The processing unit is configured, in a calibration mode, to determine one or more phase shifts between a first voltage sensor signal and one or more first current sensor signals, respectively, and to store the phase shifts into a memory. One phase shift may be determined for each phase current represented by the one or more first current sensor signals. In the calibration mode, the determination of the one or more phase shift values includes utilizing a known, estimated, or received power factor value for an electric load or source, for example, but not limited to, a resistive heater or a photovoltaic inverter. The processing unit is configured in the calibration mode: to execute a zero crossing detection with respect to the first voltage sensor signal and the one or more first current sensor signals for determining the phase shifts, or to execute a zero crossing detection with respect to the first voltage sensor signal and to determine a voltage estimate, such as a voltage vector, a voltage waveform, or a voltage time series, based on the zero crossing detection, such as an ideal voltage sinewave^) that is( / are) phase-locked relative to the first voltage sensor signal, and to determine the one or more phase shift values by calculating a plurality of electric power values based on the voltage estimate and one or more first current sensor signals, such as by a dot product therebetween, relative to a plurality of different simulated phase shifts between the voltage estimate and the one or more first current sensor signals, and to select a phase shift or shifts providing the highest electric power value or values (such as one per phase) as the one or more phase shift values.

[0011] Regarding the simulated phase shifts between the voltage estimate and the one or more first current sensor signals, the processing unit may be configured to vary a phase angle of the current or currents relative to that of the voltage estimate, such as by steps of one, five or ten degrees, for instance. For every simulated phase shift, that is an angle difference, the electric power is then calculated by the dot product of the voltage estimate and the current or currents, for instance. Alternatively, the electric power may be calculated by varying the phase angle of the voltage estimate relative to the current(s). As a result, a plurality of electric power values is obtained for each simulated phase shift. The phase shift with the maximum power value is then selected.

[0012] Furthermore, in the calibration mode, the processing unit may be configured to adapt, such as subtract a further phase shift from or add a further phase shift to, the selected phase shift or shifts providing the highest electric power value or values based on the known, estimated, or received power factor of the load or source used during the calibration mode.

[0013] The processing unit may be configured to determine, in the calibration mode, which of the first current sensor signals corresponds to the determined voltage sensor signal in the sense that they are determined of the same phase. For example, the phase shift between the first voltage sensor signal and the first current sensor signal of the same phase should be either close to zero or close to 180 degrees (such as within an angle of 0-10 or 0-20 degrees thereof), depending which direction is the current sensor measuring positive current. On the other hand, in case of a three-phase system, the phase shift with respect to the other two phases should be close to 120 and 240 degrees, respectively, within the angle of 0-10 or 0-20 degrees. Therefore, the processing unit may be configured to determine in which way the current sensors are measuring currents, that is with respect to the direction of positive current / power.

[0014] In the calibration mode, assuming (close to) resistive load, the determined phase shift thus represents phase shifts caused by the non-invasive sensor arrangement, since the actual currents and voltages should be substantially in phase (the phase shift being zero or 180 degrees).

[0015] Furthermore, the processing unit is configured, in an operation mode, to determine, by the non-invasive voltage measurement sensor, a second voltage sensor signal, determine one or more voltage estimates based on the second voltage sensor signal and the stored phase shifts, and to determine, based on the voltage estimates and one or more second current sensor signals determined by the one or more current non-invasive current measurement sensors, one or more electric power values. One voltage estimate may be determined for each phase current represented by the one or more second current sensor signals.

[0016] Furthermore, the processing unit may generate a common clock signal for the first voltage sensor signal and the one or more first current sensor signals, wherein the phase shifts are determined with respect to the common clock signal. The phase shifts may be determined as units of time or calculated as angles.

[0017] In various preferable embodiments, the processing unit may be configured to execute a zero crossing detection with respect to the first voltage sensor signal and the one or more first current sensor signals for determining the phase shifts.

[0018] The processing unit may, in addition, be configured to execute a phase-locked loop to determine a voltage clock signal and to synchronize the voltage clock signal with respect to the second voltage sensor signal. In addition, the processing unit may be configured to generate the voltage estimate based on the voltage clock signal and the stored phase shift for corresponding phase current.

[0019] In various embodiments, the processing unit may be configured to accumulate determined electric power values over a measurement period. The length of the measurement period is, preferably, at least one fundamental period of the electric supply or electrical grid, such as at least 20 milliseconds in case of 50 Hz grid, in order to get accurate information regarding the phase shift. In some embodiments, the length of the measurement period may be in the range of 0.05 to 60 seconds, such as from one to 5 or 10 seconds, for instance.

[0020] Furthermore, optionally, the processing unit may be configured to determine one or more of the following during the measurement period: a real power value, a reactive power value, an apparent power value, one or more root-mean-square current values, a power factor value.

[0021] In various embodiments, the voltage estimates, the one or more second current sensor signals, and the electric power values may represent instantaneous voltage and power values, respectively.

[0022] In various other embodiments, the voltage estimates, the one or more second current sensor signals, and the electric power values may represent average voltage and power values, respectively, over an averaging time period, such as the measurement period.

[0023] The non-invasive voltage measurement sensor may, preferable, be based on capacitive coupling, such as being a capacitive sensor. In addition, the non-invasive voltage measurement sensor may include a conductor including at least a measurement portion without a grounding shield layer, wherein the measurement portion is, preferably, adapted to be arranged adjacent or even in contact with an outer layer of a phase line.

[0024] Alternatively or in addition, the one or more current non-invasive current measurement sensors may include one or more current clamp sensors, such as one or more split core sensors or one or more Rogowski coil sensors.

[0025] In some alternative embodiments, the one or more current non-invasive current measurement sensors may include one or more Hall-effect current sensors.

[0026] In various embodiments, the processing unit may be configured to sample the voltage and current values by analog-to-digital converters at a sampling rate of at least 1000 Hz, preferably at least 3000 Hz, most preferably at least 6000 Hz per measurement.

[0027] In addition, the processing unit may be configured to detect if, in the calibration mode, a power factor value is higher than 0.95 and direction of real power is towards an electric load, the electric power of which is being determined. The electric meter may comprise an additional input for receiving power factor measurement, such as from an external device. Alternatively, the electric meter may be configured to compare the determined phase shift to a phase shift threshold of the same phase, such as 25, 30, 45, or 60 degrees, and in case it is determined that the phase shift is higher, it is determined that the power factor during the calibration mode is too low. Relative to other phases, the phase difference of 120 or 240 need to be taken into account.

[0028] Alternatively or in addition, the electric meter may be configured to det

[0029] According to a second aspect, a method for measuring electric power is provided. The method comprises, in a calibration mode, determining one or more phase shifts between a first voltage sensor signal determined by a non-invasive voltage measurement sensor and one or more first current sensor signals determined by one or more non-invasive current measurement sensors, and to store the phase shifts into a memory of a processing unit. In the calibration mode, the determination of the one or more phase shift values includes utilizing a known, estimated, or received power factor value for an electric load or source. Furthermore, the method comprises in the calibration mode: executing, by the processing unit, a zero crossing detection with respect to the first voltage sensor signal and the one or more first current sensor signals for determining the phase shifts, or executing, by the processing unit, a zero crossing detection with respect to the first voltage sensor signal and to determine a voltage estimate based on the zero crossing detection, and determining the one or more phase shift values by calculating a plurality of electric power values based on the voltage estimate and the one or more first current sensor signals, such as by a dot product therebetween, relative to a plurality of different simulated phase shifts between the voltage estimate and the one or more first current sensor signals, and to select a phase shift or shifts providing the highest electric power value or values as the one or more phase shift values.

[0030] In addition, optionally, in the calibration mode, the method may comprise, by the processing unit, adapting, such as subtracting from or adding a further phase shift to, the selected phase shift or shifts providing the highest electric power value or values based on the known, estimated, or received power factor of the load or source used during the calibration mode.

[0031] The method further comprises, in an operation mode, determining, by the non-invasive voltage measurement sensor, a second voltage sensor signal, determining one or more voltage estimates based on the second voltage sensor signal and the stored phase shift, and determining, based on the voltage estimate and one or more second current sensor signals determined by the one or more current non-invasive current measurement sensors, an electric power value.

[0032] The present invention provides an electric meter and a method for measuring electric power. The present invention provides advantages over known solutions in that the electric meter can easily and safely be arranged to determine electric power due to non- invasive voltage and current sensors, and still determine the electric power with good accuracy.

[0033] Various other advantages will become clear to a skilled person based on the following detailed description.

[0034] The expression "a number of’ may herein refer to any positive integer starting from one, such as being one, two, three, etc.

[0035] The expression "a plurality of’ may refer to any positive integer starting from two (2), respectively.

[0036] The terms “first” and “second” are herein used to distinguish one element from other element, and not to specially prioritize or order them, if not otherwise explicitly stated.

[0037] The exemplary embodiments of the present invention presented herein are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used herein as an open limitation that does not exclude the existence of also unrecited features. The features recited in the appended patent claims are mutually freely combinable unless otherwise explicitly stated.

[0038] The novel features which are considered as characteristic of the present invention are set forth in particular in the appended claims. The present invention itself, however, both as to its construction and its method of operation, together with additional objectives and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

[0039] BRIEF DESCRIPTION OF FIGURES

[0040] Some embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings.

[0041] Figure 1 illustrates schematically an electric meter.

[0042] Figure 2 illustrates schematically an electric meter. Figure 3 shows a flow diagram of a method for measuring electric power.

[0043] Figure 4 shows a voltage sensor signal.

[0044] DETAILED DESCRIPTION OF SOME EMBODIMENTS

[0045] Figure 1 illustrates schematically an electric meter 100. The electric meter 100 comprises a non-invasive voltage measurement sensor 12, such as based on capacitive coupling, arranged adjacent or even in contact with a phase line 21-23. The electric meter 100 also comprises one or more non-invasive current measurement sensors 14A-14C for measuring one or more phase currents. The electric meter 100 also comprises a digital processing unit 10, such as comprises one or more processors and memories. Preferably, the sensors 12, 14A-14C are connected to the processing unit 10 via analog-to-digital (AD) converters (not shown) which may be separate converters or comprised in or in connection with the processors.

[0046] Further illustrated in Fig. 1 are an electrical grid 20, having, in this non-limiting example, three phase lines, that is being a three-phase electrical grid. In some other non-limiting examples, the grid 20 may be, for example, a single -phase electrical grid. The electric meter 100 may also be utilized in two-phase grids.

[0047] In case there are less current measurement sensors 14A-14C than the number of phase lines 21-23, the voltage measurement sensor 12 may be arranged adjacent or contact with a phase line 21 which current is measured by a current measurement sensor 14A- 14C, or adjacent or contact with a phase line 21 which current is not measured by a current measurement sensor 14A-14C.

[0048] Figure 1 also illustrates an electric load 30. The electric meter 100 is, in Fig. 1, arranged to measure electric power flowing between the electrical grid 20 and the electric load 30. However, it is to be understood that the electric meter 100 can be arranged to measure electric power flowing in any point of an electric arrangement. There may be electric sources and / or loads on both sides of the measurement point.

[0049] The electric meter 100 may be a bidirectional electric meter, that is configured to detect in which direction is the electric power, specifically real power portion thereof, is flowing.

[0050] The non-invasive voltage measurement sensor 12 and the one or more non-invasive current measurement sensors 14A- 14C are connected or connectable, such as via input ports 16, 18A-18C, to the processing unit 10 for providing voltage and current sensor signals to the processing unit 10.

[0051] In addition, the processing unit 10 is configured, in a calibration mode, to determine one or more phase shifts between a first voltage sensor signal, measured by the non-invasive voltage measurement sensor 12, and one or more first current sensor signals, measured by the current measurement sensors 14A-14C, and to store the phase shifts into a memory of the processing unit 10. In this case, there would be three phase shifts determined and stored since there are three phase currents being measured.

[0052] In the calibration mode, the determination of the one or more phase shift values includes utilizing a known, estimated, or received power factor value for an electric load 30 or source, for example, but not limited to, a resistive heater or a photovoltaic inverter.

[0053] The processing unit 10 is configured, in an operation mode, to determine, by the non- invasive voltage measurement sensor 12, a second voltage sensor signal. Furthermore, the processing unit 10 is configured determine one or more voltage estimates based on the second voltage sensor signal and the stored phase shifts. In addition, the processing unit 10 is configured to determine, based on the voltage estimates and one or more second current sensor signals determined by the one or more current non-invasive current measurement sensors, one or more electric power values.

[0054] The processing unit 10 may generate a common clock signal for the first voltage sensor signal and the one or more first current sensor signals, wherein the phase shift is determined relative to the common clock signal. By having the common clock signal, comparable time stamps may be assigned to the first voltage sensor signal and the one or more first current sensor signals so that the phase shift therebetween can be determined.

[0055] The processing unit 10 may be configured to execute a zero crossing detection with respect to the first voltage sensor signal and the one or more first current sensor signals for determining the phase shift.

[0056] Alternatively, the processing unit 10 may be configured to execute a zero crossing detection with respect to the first voltage sensor signal and to determine a voltage estimate, such as a voltage vector, a voltage waveform, or a voltage timeseries, based on the zero crossing detection. The voltage estimate may be an ideal sine wave, being in the same phase (based on the zero crossing detection) as the first voltage sensor signal. The processing unit 10 may be further configured to determine the one or more phase shift values by calculating a plurality of electric power values based on the voltage estimate and the one or more first current sensor signals, such as by a dot product therebetween, relative to a plurality of different simulated phase shifts between the voltage estimate and the one or more first current sensor signals. The plurality of electric power values refers to power values determined with respect to the plurality of different simulated phase shifts, not per se to the many values based on the dot product, for instance, in case of a single simulated phase shift. Thus, the plurality of electric power values may refer to average power values or amplitude values for the different simulated phase shifts. Still further, the processing unit 10 may be configured to select a phase shift or shifts providing the highest electric power value or values as the one or more phase shift values.

[0057] Furthermore, in the calibration mode, the processing unit 10 may be configured to adapt, such as subtract a further phase shift from or add a further phase shift to, the selected phase shift or shifts providing the highest electric power value or values based on the known, estimated, or received power factor of the load or source used during the calibration mode. For example, if the known, estimated, or received power factor is 0.5, 60 degrees may be subtracted from or added to the selected phase shift or shifts, depending on whether the power factor is leading or lagging.

[0058] The processing unit 10 may be configured to execute a phase-locked loop to determine a voltage clock signal and to synchronize the voltage clock signal with respect to the second voltage sensor signal. In addition, optionally, the processing unit 10 may be configured to generate the voltage estimate based on the voltage clock signal and the stored phase shift for corresponding phase current.

[0059] The voltage estimate may be regarded as an ideal voltage signal which would ideally exist between the phase line 21-23 and the reference voltage, such as ground potential. The voltage estimate or estimates may, thus, be calculated for each phase line 21-23 based on the result of the zero crossing detection (which indicates frequency of the signal, for instance) and a nominal voltage peak value, a nominal RMS value or a nominal average value of the voltage being measured, that is an assumption of what it should be.

[0060] The processing unit 10 may be configured to accumulate determined electric power values over a measurement period. The length of the measurement period is, preferably, at least one fundamental period of the electric supply or electrical grid, such as at least 20 milliseconds in case of 50 Hz grid, in order to get accurate information regarding the phase shift. In some embodiments, the length of the measurement period may be in the range of 0.05 to 60 seconds, such as from one to 5 or 10 seconds, for instance. Furthermore, the measurement period may be in the range of 2-5 seconds or so, for example, about 3 seconds. In addition, the processing unit 10 may be configured to determine one or more of the following during the measurement period: a real power value, a reactive power value, an apparent power value, one or more root-mean- square current values, a power factor value.

[0061] The voltage estimates, the one or more second current sensor signals, and the electric power values may represent instantaneous voltage and power values, respectively. Alternatively, the voltage estimates, the one or more second current sensor signals, and the electric power values may represent average voltage and power values, respectively, over an averaging time period, such as the measurement period as mentioned hereinbefore.

[0062] The processing unit 10 may be configured to sample the voltage and first and second current values by analog-to-digital converters at a sampling rate of at least 1000 Hz, preferably at least 3000 Hz, most preferably at least 6000 Hz per measurement in order to include enough samples for analysis of the signals. For example, in order to detect with suitable accuracy, the zero crossings.

[0063] The processing unit 10 may be configured to detect if, in the calibration mode, a power factor value is higher than 0.95 and if the direction of real power is towards an electric load of which, the electric power is being determined.

[0064] Figure 2 illustrates schematically an electric meter 100. The electric meter 100 is being supplied with electric power from an electric supply 110 of the meter 100. Furthermore, the electric meter 100 comprises a housing 50 inside of which at least the processing unit 10, including processors 11A and memories 11B, are located. Furthermore, the input ports 16, 18A-18C may be at least partially inside the housing 50. At least the processing unit 10 may be arranged on a substrate, such as a printed circuit board or the like. The input ports, or connectors, may also be arranged on the same substrate.

[0065] Figure 2 also illustrates one type of non-invasive current sensor 14A, namely a split core sensor. As understood, there may be one or more currents sensor 14A-14C comprised in or connectable to the electric meter 100.

[0066] Figure 2 also illustrates one type of non-invasive voltage sensor 12. The non-invasive voltage measurement sensor 12 may be based on capacitive coupling, such as being a capacitive sensor. In an advantageous, yet non-limiting, example, the non-invasive voltage measurement sensor 12 includes a conductor, for example an audio cable or the like, including at least a measurement portion 13 without a grounding shield layer 103, wherein the measurement portion 13 being adapted to be arranged adjacent or even in contact with an outer layer of a phase line 21-23 to form the capacitive coupling. Even if there is no grounding shield layer 103 on the measurement portion 13, there may be additional layers, such as electrical insulations, on the conductor at the measurement portion 13.

[0067] The one or more current non-invasive current measurement sensors 14A-14C may include one or more current clamp sensors, such as one or more split core sensors as described hereinabove, or one or more Rogowski coil sensors (not shown).

[0068] Alternatively, the one or more current non-invasive current measurement sensors 14A- 14C may include one or more Hall-effect current sensors.

[0069] Figure 3 shows a flow diagram of a method for measuring electric power. Item 300 refers to a start-up phase of the method. Suitable equipment and components are obtained and systems assembled and configured for operation. For example, an electric meter 100 such as described hereinabove is obtained and the sensors included therein or therewith are arranged into proper positions for providing measurements.

[0070] Item, or method step, 301 refers to the calibration mode. The calibration mode 301 includes at least determining 310 one or more phase shifts between a first voltage sensor signal determined by a non-invasive voltage measurement sensor 12 and one or more first current sensor signals determined by one or more non-invasive current measurement sensors 14A-14C, respectively, and to store the phase shifts into a memory 1 IB of a processing unit 10.

[0071] In the calibration mode 301, the determination 310 of the one or more phase shift values includes utilizing a known, estimated, or received, such as received from an additional input for receiving power factor measurement, such as from an external device, power factor value for an electric load 30 or source.

[0072] In the calibration mode 301, the method comprises executing, by the processing unit 10, a zero crossing detection with respect to the first voltage sensor signal and the one or more first current sensor signals for determining the phase shifts. The method may alternatively comprise executing, by the processing unit 10, a zero crossing detection with respect to the first voltage sensor signal and to determine a voltage estimate, such as a voltage vector, a voltage waveform, or a voltage time series, based on the zero crossing detection, and determining the one or more phase shift values by calculating a plurality of electric power values based on the voltage estimate and one or more first current sensor signals, such as by a dot product therebetween, relative to a plurality of different simulated phase shifts between the voltage estimate and the one or more first current sensor signals, and to select a phase shift or shifts providing the highest electric power value or values as the one or more phase shift values.

[0073] Regarding the simulated phase shifts between the voltage estimate and the one or more first current sensor signals, the processing unit 10 may be configured to vary a phase angle of the current or currents relative to that of the voltage estimate, such as by steps of one, five or ten degrees, for instance. For every simulated phase shift, that is an angle difference, the electric power is then calculated by the dot product of the voltage estimate and the current or currents, for instance. Alternatively, the electric power may be calculated by varying the phase angle of the voltage estimate relative to the current(s). As a result, a plurality of electric power values is obtained for each simulated phase shift. The phase shift with the maximum power value is then selected.

[0074] In addition, optionally, in the calibration mode, the method may comprise, by the processing unit, adapting, such as subtracting a further phase shift from or adding a further phase shift to, the selected phase shift or shifts providing the highest electric power value or values the known, estimated, or received power factor of the load or source used during the calibration mode 301.

[0075] Optionally, the method comprises arranging the electric load 30 to be a resistive load during performing the calibration mode 301. Thus, the voltage signal measured by the voltage sensor 12 and the current signal measured in the same phase as the voltage should approximately be in the same phase. The phase shift(s) then determined include phase shifts caused by the measurement arrangement, namely, the non-invasive voltage measurement. Thus, the phase shifts determined can be used in the operation mode 302 to compensate the phase shifts caused by the measurement arrangement. Alternatively or in addition, there may be an electric source, such as a photovoltaic generator, connected to the electric meter 10.

[0076] Item, or method step, 302 refers to the operation mode, preferably execute after the calibration mode. The operation mode 302 includes at least determining 320, by the non- invasive voltage measurement sensor 12, a second voltage sensor signal. The operation mode 302 also comprises determining 330 one or more voltage estimates based on the second voltage sensor signal and the stored phase shifts. Furthermore, the operation mode 302 comprises determining 340, based on the voltage estimates and one or more second current sensor signals determined by the one or more current non-invasive current measurement sensors, one or more electric power values.

[0077] Method execution may be stopped at step 399.

[0078] Figure 4 shows a voltage sensor signal. The vertical axis 102 represent the voltage and the horizontal axis 101 time. The voltage sensor signal 111 is shown with solid curve and includes noise and other distortions, mainly due to the non-invasive measurement, such as based on capacitive coupling. Curve with dashed line represents the zero crossing detection signal 121. The time instances of the zero crossing detection signal 121 are stored into memory relative to the common clock signal generated by the processing unit 10. The same procedure may be used for the voltage sensor signal 111 in both the calibration mode for determining the phase shift(s) and in the operation mode to determine the voltage estimates.

[0079] In the calibration mode 301, the similar procedure may be performed for each of the first current sensor signals, although not shown in Fig. 4. Based on the zero crossing time instances in the voltage and current sensor signals, relative to the common clock signal, the phase shifts can be determined for each phase line. Since, advantageously, the voltage and the current of the same phase are in the same phase in the calibration mode (due to resistive or almost resistive load (power factor more than 0.95), the determined phase shifts thus represent the phase shifts due to the measurement arrangement.

[0080] In the operation mode 302, it is then assumed that the measurement arrangement remains the same, and that the phase shifts also remain the same. Thus, in the operation mode 302, the phase shifts are not required to be determined anymore.

[0081] In the operation mode 302, only the voltage sensor signal is analyzed with respect to zero crossings or other such means / methods to determine the phase thereof. After that voltage estimates for each line may be calculated, for example, as follows: wherein ViDEAL,N is the voltage estimate for phase line N (one of 21-23, for instance), VNOM,PEAK the nominal peak voltage value in the phase line N, ZCTv the zero crossing time of the second voltage signal, Tv the period of the voltage (such as close to 20 milliseconds in a 50 Hz grid), CAL(IN) is the phase shift determined and stored in the calibration mode 301 relative the current in phase N. For example, in the operation mode 302, a phase-locked loop may be performed in the processing unit 10, to create reference voltage clock signal, which is synchronized to the second voltage sensor signal. Then, for each phase, an ideal sine wave voltage waveform (the voltage estimate) is generated in the processing unit 10, which the voltage estimate is synchronized to said reference voltage clock signal, taking into account the determined phase shift as a calibration angle (CAL(IN)) for that phase.

[0082] Furthermore, in the operation mode 302, the electric power values may be determined by calculating the product of the VIDEAL,N and the measured instantaneous value of the phase current of phase N, that is IN, for the time instance corresponding to ZCTv. For each set of AD converted samples, instantaneous power P = VIDEAL • IN can then be calculated. The calculated power values may be accumulated for an averaging time period, for example, being in the range of 1-10 seconds, or 2-5 seconds or so, such as, about 3 seconds.

[0083] As stated hereinbefore, other parameters can also be determined, such as, for each phase, for each input current sample, accumulated square of current. Alternatively or in addition, for each phase, real power may be calculated by dividing accumulated power by number of accumulated samples. Alternatively or in addition, for each phase, RMS current may be calculated by sqrt(accumulated square of current / N), where N is the number of samples in the time period in question. Alternatively or in addition, for each phase, apparent power may be calculated by VNOM * IRMS. Alternatively or in addition, for each phase, power factor may be calculated as PF = real power / apparent power. Furthermore, alternatively or in addition, total real power may be determined as sum of phase real powers.

[0084] Still further, the electric meter 100 may be configured or the method may comprise summing total real powers • accumulation time during the observed period so as to get information about the total real energy being consumed and / or generated.

[0085] Hereinabove have been described some of the embodiments and alternative features of the present invention. However, as is understood by a skilled person, the list is not exhaustive, but the scope defined by the appended claims can also include other embodiments. For example, the electric meter 100 may comprise as plurality of non-invasive voltage sensors 12 to increase the accuracy. On the other hand, the electric meter 100 may comprise other sensors, such as an optical pulse sensor, for reading electric power usage related data from other sources. Furthermore, the electric meter 100 may include communication devices, such as operation by wireless or wired manner, in order to remotely read and / or maintain the device.

Claims

CLAIMS1. An electric meter (100), comprising: a non-invasive voltage measurement sensor (12), one or more non-invasive current measurement sensors (14A-14C) for measuring one or more phase currents, and a digital processing unit (10), wherein the non-invasive voltage measurement sensor (12) and the one or more non-invasive current measurement sensors (14A-14C) are connected or connectable to the processing unit (10) for providing voltage and current sensor signals to the processing unit (10); wherein the processing unit (10) is configured, in a calibration mode, to determine one or more phase shift values between a first voltage sensor signal and one or more first current sensor signals, respectively, and to store the phase shift values into a memory, and wherein the processing unit (10) is configured, in an operation mode, to: determine, by the non-invasive voltage measurement sensor (12), a second voltage sensor signal, determine one or more voltage estimates based on the second voltage sensor signal and the stored phase shifts, and determine, based on the one or more voltage estimates and one or more second current sensor signals determined by the one or more current non-invasive current measurement sensors (14A-14C), respectively, one or more electric power values; wherein, in the calibration mode, the determination of the one or more phase shift values includes utilizing a known, estimated, or received power factor value for an electric load (30) or source, and the processing unit (10) is configured in the calibration mode: to execute a zero crossing detection with respect to the first voltage sensor signal and the one or more first current sensor signals for determining the phase shifts, orto execute a zero crossing detection with respect to the first voltage sensor signal and to determine a voltage estimate based on the zero crossing detection, and to determine the one or more phase shift values by calculating a plurality of electric power values based on the voltage estimate and the one or more first current sensor signals, such as by a dot product therebetween, relative to a plurality of different simulated phase shifts between the voltage estimate and the one or more first current sensor signals, and to select a phase shift or shifts providing the highest electric power value or values as the one or more phase shift values.

2. The electric meter (100) of claim 1, wherein the processing unit (10) generates a common clock signal for the first voltage sensor signal and the one or more first current sensor signals, wherein the phase shifts are determined with respect to the common clock signal.

3. The electric meter (100) of claim 1 or 2, wherein the processing unit (10) is configured, in the calibration mode, to adapt the selected phase shift or shifts providing the highest electric power value or values based on the known, estimated, or received power factor of the load or source used during the calibration mode.

4. The electric meter (100) of claims 1-3, wherein the processing unit (10) is configured to execute a phase-locked loop to determine a voltage clock signal and to synchronize the voltage clock signal with respect to the second voltage sensor signal.

5. The electric meter (100) of claim 4, wherein the processing unit (10) is configured to generate the one or more voltage estimates based on the voltage clock signal and the stored phase shifts for corresponding phase current.

6. The electric meter (100) of any of claim 1-5, wherein the processing unit (10) is configured to accumulate determined electric power values over a measurement period.

7. The electric meter (100) of claim 6, wherein the processing unit (10) is configured to determine one or more of the following during the measurement period: a real power value, a reactive power value, an apparent power value, one or more root-mean-square current values, a power factor value.

8. The electric meter (100) of any of claims 1-7, whereinthe voltage estimates, the one or more second current sensor signals, and the electric power values represent instantaneous voltage and power values, respectively; or the voltage estimates, the one or more second current sensor signals, and the electric power values represent average voltage and power values, respectively, over an averaging time period, such as the measurement period.

9. The electric meter (100) of any one of claims 1-8, wherein the non-invasive voltage measurement sensor (12) is based on capacitive coupling, such as being a capacitive sensor.

10. The electric meter (100) of claim 9, wherein the non-invasive voltage measurement sensor (12) includes a conductor including at least a measurement portion (13) without a grounding shield layer (103), wherein the measurement portion (13) is adapted to be arranged adjacent or even in contact with an outer layer of a phase line (21-23).

11. The electric meter (100) of any one of claims 1-10, wherein the one or more current non-invasive current measurement sensors (14A-14C) include one or more current clamp sensors, such as one or more split core sensors or one or more Rogowski coil sensors.

12. The electric meter (100) of any one of claims 1-11, wherein the one or more current non-invasive current measurement sensors (14A-14C) include one or more Halleffect current sensors.

13. The electric meter (100) of any one of claim 1-12, wherein the processing unit (10) is configured to sample the voltage and current values by analog-to-digital converters at a sampling rate of at least 1000 Hz, preferably at least 3000 Hz, most preferably at least 6000 Hz per measurement.

14. The electric meter (100) of any one of claim 1-13, wherein the processing unit (10) is configured to detect if, in the calibration mode, a power factor value is higher than 0.95 and direction of real power is towards an electric load, the electric power of which is being determined.

15. A method for measuring electric power, the method comprising, in a calibration mode (301), determining (310) one or more phase shifts between a first voltage sensor signal determined by a non-invasive voltage measurement sensor (12) and one or more first current sensor signals determinedby one or more non-invasive current measurement sensors (14A-14C), respectively, and to store the phase shifts into a memory of a processing unit (10), wherein, in the calibration mode, the determination (310) of the one or more phase shift values includes utilizing a known, estimated, or received power factor value for an electric load (30) or source, and: executing, by the processing unit (10), a zero crossing detection with respect to the first voltage sensor signal and the one or more first current sensor signals for determining the phase shifts, or executing, by the processing unit (10), a zero crossing detection with respect to the first voltage sensor signal and to determine a voltage estimate based on the zero crossing detection, and determining the one or more phase shift values by calculating a plurality of electric power values based on the voltage estimate and the one or more first current sensor signals, such as by a dot product therebetween, relative to a plurality of different simulated phase shifts between the voltage estimate and the one or more first current sensor signals, and to select a phase shift or shifts providing the highest electric power value or values as the one or more phase shift values; and wherein the method further comprises, in an operation mode (302), determining (320), by the non-invasive voltage measurement sensor, a second voltage sensor signal, determining (330) one or more voltage estimates based on the second voltage sensor signal and the stored phase shifts, and determining (340), based on the voltage estimates and one or more second current sensor signals determined by the one or more current non-invasive current measurement sensors, one or more electric power values.

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