An autonomous power harvesting system and current measurement system

Abstract

An autonomous power harvesting system (100) for harvesting power from an alternating current, and an autonomous current measurement system (112) for measuring the alternating current, in a primary wire (105), both systems comprise a multi-ratio transformer (126) comprising a plurality of sets of secondary windings (101, 102, 103, 104) configured to be applied to the primary wire (105), the sets of secondary windings being connected in series, and a plurality of taps (106, 107, 108, 109) connecting the sets of secondary windings to the power harvesting unit (111) or current measurement unit (120), wherein each plurality of taps (106, 107, 108, 109) are arranged so that when the Mth tap (where M is an integer and M³1) is closed and M+1th and higher taps are open, the sets of secondary windings up to and including the Mth set of secondary windings form one or more closed circuits with the power harvesting unit (111) or current measurement unit (120) and the remaining sets of secondary windings form an open circuit. The windings which are not required can be completely isolated from the circuit and so do not draw power.

Description

[0001] An autonomous power harvesting system and Current Measurement System

[0002] Technical Field of the Invention

[0003] The present invention relates to an autonomous power harvesting system and an autonomous current measurement system. In particular, but not exclusively, it relates to an autonomous power harvesting system and an autonomous current measurement system.

[0004] Background to the Invention

[0005] It is beneficial to power devices directly from conductor lines through induction. As an example, in remote area where monitoring of the environment and / or a power line is desired or required, it is preferable to power the monitoring unit directly from the power line. Otherwise, a separate power source such as a diesel generator or solar panels is required. However, this is challenging given power lines typically are at high voltage levels and carry large, alternating currents which can vary across a broad range of current levels. In comparison lower, direct currents and voltages levels are required to power devices, and it is difficult to obtain such voltage and currents levels from the varying, large, alternating currents of a power line.

[0006] WO2019030781 A1 discloses a known solution which allows power to be drawn indirectly from a conductor line by induction. In the document an apparatus is disclosed in which a number of sets of windings 11 , 12 draw power from a power line 2 via exposure to the alternating magnetic field 1 generated by the alternating current in the line 2 (shown in figure 2). As can be seen from figure 1 and figure 2, the current in each set of windings 11 , 12 is rectified via a respective rectifier 70, before being combined across the element to be powered 90 (i.e. the burden). Each set of windings 12 can be cut off between it and the respective rectifier 70 via a respective MOSFET current shunt 60, shown in figure 2. The separated sets of windings 11 , 12 result in increased harvesting efficiency and higher power generation. Further, the shunting of sets of windings 12 when less power generation is required prevents heat generation and overvoltage transients, and results in more stable power generation. This allows the apparatus to power devices directly from power lines.

[0007] Given the fact that maximum and minimum currents through a conductor line can vary significantly, the apparatus of WO2019030781 A1 needs to be specially customized to the conductor line 2 in question for best use. For example, the number of windings in each set of windings 11 , 12 needs to be chosen based on the current range of the conductor line 2 which the apparatus will be used on. This means the apparatus must be specially customizes each time for the particular conductor line in question, increasing the cost of implementing the apparatus.

[0008] If, alternatively, the apparatus of WO2019030781 A1 is designed to work for all conductor lines 2 and so the largest number of windings in each set 1 1 , 12 are chosen to deal with the maximum current, due to how much the current can vary the resulting apparatus will operate at reduced capacity when lower levels of current run through the conductor line 2. If a lower number of turns is chosen such that the apparatus operates at full capacity at the lower current levels, the apparatus risks damage when the current in the secondary winding and connected circuits reaches or passes overloading conditions.

[0009] Current transformers are used as instrument transformers in utility power grids and industrial power applications. The conductor to be measured is passed through a window in the centre of the transformer core and acts as a single turn primary winding, inducing current in the secondary winding by the aid of electromagnetic field. The number of turns on the secondary side of the current transformer can vary depending on application and current levels to measure. When the current transformer is manufactured it has a fixed transfer ratio between the primary and the secondary side.

[0010] The current transformer transfer ratio is chosen with respect to the highest expected current on the primary side. So, for a primary winding of a particular current transformer having a certain highest expected current, the appropriate core material and cross section is selected, then the number of turns in the secondary winding and the appropriate resistance of the burden on the seconder side of the current transformer to accurately measure the primary currents between zero and the highest expected current. If for some reason the primary current reaches levels that are above the highest expected current, that is the max expected current, the current transformer can reach knee-point voltage level and core saturation resulting in errors in the current measurements. If the max current levels are surpassed for a prolonged time the secondary winding and the measurement circuitry can be permanently damaged or can create hazardous fire in the electric system that it is utilized in.

[0011] Most self-powered line sensors that are clamped on conductors of high voltage power lines, use current transformers for harvesting power from the electromagnetic field surrounding the conductors. The harvested power is used for powering their measurement circuitry and communication devices in the sensors. As by nature of current transformers, the fluctuating conductor current on the primary side of a current transformer, greatly affects the current and voltage levels on the secondary side and therefore the power harvesting capabilities of the power harvesting circuitry.

[0012] Current solutions of inductive power harvesting circuits try to tackle this by using current transformers with fixed current transfer ratio that can handle wide ranges of primary currents. However, fluctuation in the energy demand on the user side can be large, both on daily and seasonal bases so every current transformer used in power harvesting circuit of an inductive based power supply of a line sensor device must be carefully selected to adapt to the multiple current profiles in any given power grid.

[0013] US2021 / 0257158 A1 discloses a current transformer powered controller. As shown in figure 3, the controller is configured to adjust the turns ratio selection of the multi-tap current transformer 310 based on an amount of current flowing in the secondary coil 309 of the current transformer 310. The set-up of this apparatus means that, regardless of the switches 332_2, 332_4 for adjusting the turns ratio selection which are selected, some current will be able to run through all the windings during the periods when the alternating runs downwards through the windings, in the orientation of figure 3. As such, when the current through the primary coil 308 is low the non-tapped sets of windings would still draw some of the power, and so result in the transformer working poorly if applied to a conductor of high current overhead power line.

[0014] Furtherto this, the set-up means the ‘top’ section of windings (in the orientation of figure 3) always has its own connection to the power supply 340 to be powered and ground. Accordingly, when the one and / or both switches 332_2, 332_4 are closed the windings will at least partially act as two separate sets of windings in parallel, rather than one whole set of individual windings. Each of these two separate sets of windings will have fewer turns than required for the present current level, risking damage to the apparatus.

[0015] This being the case, the current transformer powered controller of US2021 / 0257158 A1 is ill-suited for use on a conductor of high current overhead line.

[0016] CN110783076 A discloses a high-precision current transformer designed to accurately measure electrical currents across a wide range in electrical monitoring for better adaptability to changing current without overloading the system. The transformer system includes connecting multiple secondary windings, where each winding is wound on it‘s respective core, and where the outlet terminal of two adjacent secondary windings are commonly connected to the moving terminal of the temperature-controlled circuit breakers to change the ratio of the secondary side. To increase or decrease the transfer ratio at least two temperature control switches must change their state. As their operation is dependent on slowly changing transformer core temperature, they cannot operate instantaneously, nor can they change states synchronously.

[0017] Embodiments of the present invention seek to overcome / ameliorate these or other disadvantages and provide an improved apparatus for extracting power from an alternating current carrying conductor’s magnetic field.

[0018] Summary of the Invention

[0019] The present invention discloses a fully autonomous ratio controller for current transformers including the function and electronic circuitry of an autonomous dynamic ratio selection circuitry on the secondary side of a current transformer, or multiple current transformers if that’s the case, in the power harvesting circuitry. The solution disclosed herein is based on an electronic circuitry that measures current and voltage levels of a multi tap secondary winding side of a current transformer and provides instantaneous or immediate switching between taps in that opening open and closing of taps is based on the detected current and / or voltage.

[0020] The solution presented herein provides an improved and novel solution over prior art techniques in that it provides instantaneous switching between taps in that opening and closing of taps is based on the detected current and / or voltage while in operation without requiring access to the device or mechanical re-configuration. Opening un-used taps enables the use of a single core compared to multiple specialized cores for each secondary winding as disclosed in CN110783076 A. Additionally, by instantaneously switching between taps, inefficient heating in the core is prevented whereas this heating is necessary in CN110783076 A as the solution is based on thermal switching.

[0021] In some embodiments autonomous multi-ratio current transformer solution incorporated with inductive power harvesting circuitry, provides a solution for selecting the number of windings on the secondary side as fits the design, even when the number of turns between each tap may be different. The autonomous multi-ratio current transformer provides advantages such as i) a much wider range of electronic line sensors and other measurement devices with regards to the irregularity in phase wire currents in high voltage powerlines, where the autonomous multiratio current transformer adapts the number of turns in the secondary winding to achieve the most efficient current transfer ratio between the primary and the secondary winding of the current transformer, ii) securing safe operation of electronic line sensors and devises even though the current in the phase wire surpasses max rated current for an extended time, iii) allowing operators of power grids to apply excessive currents to the powerline for the purpose of heating the powerline phase wires in case of efforts to melt ice accretion of the phase wires, iv) allowing transfer of a power harvesting apparatus powered by inductive power harvesting circuits from one powerline to another with totally different current operational profile, without minding the max current profile at the new location, and v) simplifying stocking of current transformer types forthe manufacturer and therefore greatly reducing and effort in the assembly process of line sensors and measurement devices.

[0022] In some embodiments an autonomous multi-ratio current transformer for use in current measurement circuits for utility power grids and industrial power applications, provides a solution for widening the measurement range of a metering circuit based on current transformer and thereby increase the measurement accuracy overthe whole current spectrum. By changing the transfer ratio in an autonomous manner without any human intervention this solution is beneficial for both protecting the metering circuitry and increasing the accuracy of the current measurement circuit over a wide range of current levels. The use of autonomous multi-ratio current transformers in current measurements circuits, provides advantages such as i) the accuracy of the measurements can be increase significantly, especially at maximum and minimum levels of the current spectrum measured, and ii) providing measurement devices which can be used in a wide variety of current measurement circuits without the user having to select current transformers with tailored and fixed measurement range.

[0023] According to a first aspect of the present invention there is provided an autonomous multi-ratio transformer system for drawing current from an alternating current in a primary wire, the autonomous multi-ratio transformer system comprising a multi-ratio transformer, an application circuit, and a tap control unit, wherein: (a) the multi-ratio transformer comprises a plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings being connected in series, and a plurality of taps connecting the sets of secondary windings to the application circuit, wherein the plurality of sets of secondary windings are arranged around a single core, and wherein the core is configured to be arranged around the primary wire;

[0024] (b) a first set of secondary windings of the plurality of sets of secondary windings is connected between a common reference and a first tap of the plurality of taps, and each Nth set of secondary windings (where N is an integer and N>1) of the plurality of sets of secondary windings is connected between an Nth tap and an N-1th tap of the plurality of taps;

[0025] (c) the application circuit is connected between the plurality of taps and the common reference, the application circuit being operable in use to obtain the current by induction from the primary wire via the one or more of the sets of secondary windings when one of the taps is closed;

[0026] (d) the tap control unit being operable to detect the current and / or voltage of the active set of secondary windings and close and / or open one or more of the taps based on the detected values; and

[0027] (e) the plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and all other taps are open, the sets of secondary windings up to and including the Mth set of secondary windings form one closed circuit with the application circuit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

[0028] The position of the taps means each section of windings of the multi-tap transformer does not form part of a complete circuit unless the associated tap or higher is closed. As such, the windings which are not required can be completely isolated from the circuit and so not draw power.

[0029] The multi-ratio transformer may comprise a plurality of sets of secondary windings arranged around a core, and where the core is configured to be applied to or arranged around the primary wire.

[0030] The autonomous multi-ratio transformer system may comprise a power harvesting unit operable in use to draw power by induction from the primary wire. The application circuit may be the power harvesting unit operable in use to draw power by induction from the primary wire via the one or more of the sets of windings when one of the taps is closed. The application circuit may be a metering unit operable in use to measure the alternative current in the primary wire via the one or more of the sets of windings when one of the taps is closed. The autonomous multi-ratio transformer system may be a power harvesting system for harvesting power from the alternating current in the primary wire. The autonomous multi-ratio transformer system may be an autonomous current measurement system for measuring the alternating current in a primary wire.

[0031] In the present context the terms “primary wire” an “primary winding” are used equally.

[0032] According to a second aspect of the present invention there is provided an autonomous power harvesting system for harvesting power from an alternating current in a primary wire, the autonomous power harvesting system comprising a multi-ratio transformer, a power harvesting unit, and a tap control unit, wherein:

[0033] (a) the multi-ratio transformer comprises a plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings being connected in series, and a plurality of taps connecting the sets of secondary windings to the power harvesting unit, wherein the plurality of sets of secondary windings are arranged around a single core, and wherein the core is configured to be arranged around the primary wire;

[0034] (b) a first set of secondary windings of the plurality of sets of secondary windings is connected between a common reference and a first tap of the plurality of taps, and each Nth set of secondary windings (where N is an integer and N>1) of the plurality of sets of secondary windings is connected between an Nth tap and an N-1th tap of the plurality of taps;

[0035] (c) the power harvesting unit is connected between the plurality of taps and the common reference, the power harvesting unit being operable in use to draw power by induction from the primary wire via the one or more of the sets of secondary windings when one of the taps is closed;

[0036] (d) the tap control unit being operable to detect the current and / or voltage of the active set of secondary windings and close and / or open one or more of the taps based on the detected values; and

[0037] (e) the plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and M+1th and higher taps are open, the sets of secondary windings up to and including the Mth set of secondary windings form one closed circuit with the power harvesting unit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

[0038] The position of the taps means each section of windings of the multi-tap transformer does not form part of a complete circuit unless the associated tap or higher is closed. As such, the windings which are not required can be completely isolated from the circuit and so not draw power.

[0039] The tap control unit may be operable to close the Mth tap and open all other taps based on the detected current.

[0040] The tap control unit may be operable to close the Mth tap and open the remaining taps of the plurality of taps. In such cases, the plurality of taps may be arranged so that when the Mth tap (where M is an integer and M31) is closed, the sets of secondary windings up to and including the Mth set of secondary windings form one closed circuit with the application circuit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

[0041] By only having one tap closed at a time, it can be ensured that the windings all act as one set of windings and not as two or more separate sets of windings in parallel. This reduces the risk of damage at higher current levels due to separate sets having an inadequate number of windings.

[0042] The tap control unit may be operable to determine the maximum of an alternating current through the primary wire as the current. The maximum may be the current RMS value. Alternatively, the maximum may be the current peak value. The tap control unit may be operable to determine the maximum of an alternating current through the sets of secondary windings as the suitable current at each time.

[0043] The tap control unit may be operable to have a respective range of currents associated with each tap of the plurality of taps. The first range of currents, associated with the first tap, may run from zero to a first level of current. The second range of currents, associated with the second tap, may run from the first level of current to a second level of current. The Mth range of currents, associated with the Mth tap, may run from an M-1th level of current to an Mth level of current. The Mth level of current may be higher than the M-1th level of current. Each range of currents may be the same size. Each range of currents may be proportional in size to the number of windings in a respective Mth set of windings.

[0044] The tap control unit may be operable to detect the current through the plurality of sets of secondary windings to detect the current through the primary wire. The power harvesting system may comprise an ammeter to which the tap control unit is connected to measure the current. The ammeter may be positioned to measure the current through the common reference and the tap or each closed tap. The ammeter may be connected in series with the sets of secondary windings.

[0045] The autonomous power harvesting system may comprise a zero-crossing detection unit operable to determine the zero crossing detection points of the current through the sets of secondary windings. The zero-crossing detection unit may be operable to receive the reading from the ammeter. The zero-crossing detection unit may be operable to determine the zerocrossing points of the reading of the ammeter. The zero-crossing detection unit may be connected between the ammeter and the tap control unit. The zero-crossing detection unit may be directly connected to the ammeter. The zero-crossing detection unit may be directly connected to the tap control unit.

[0046] The tap control unit may be operable to close one and open all other taps when a zerocrossing point is detected. The tap control unit may be operable to open and / or close one or more of the taps only when a zero-crossing point is detected. The tap control unit may be operable to open and / or close one or more of the taps at a leading edge of a zero-cross detection pulse. The tap control unit may be operable to open and / or close one or more of the taps only at a leading edge of a zero-cross detection pulse.

[0047] The tap control unit may comprise multi-ratio controller. The tap control unit may be a multi-ratio controller. The multi-ratio controller may be a microcontroller. The power harvesting unit may comprise a rectifier. The power harvesting unit may comprise a shunt.

[0048] The tap control unit may be operable to detect the voltage across the plurality of sets of secondary windings and close and / or open one or more of the taps based on the detected voltage. The tap control unit may be operable to close the Mth tap and open M+1th and higher taps based on the detected voltage. The tap control unit may be operable to open the currently closed Mth tap and close M+1th tap when the voltage is above a set danger voltage level. The tap control unit may be operable to open the currently closed Mth tap and close M+1th tap when the voltage is above a set danger voltage level, regardless of the current level. The tap control unit may be operable to open and close taps so as to increase the number of sets of windings in the closed circuit when the voltage is above a set danger voltage level. The tap control unit may be operable to open and close taps so as to increase the number of sets of windings in the closed circuit when the voltage is above a set danger voltage level, regardless of the current level.

[0049] The tap control unit may be operable to detect knee-point distortions in the voltage. The tap control unit may be operable to open and / or close taps when a knee-point distortion is detected in the voltage. The tap control unit may be operable to close a higher Mth tap when a knee-point distortion is detected in the voltage. The tap control unit may be operable to open the currently closed Mth tap and close M+1th tap when a knee-point distortion is detected in the voltage. The tap control unit may be operable to open the currently closed Mth tap and close M+1th tap when a knee-point distortion is detected in the voltage, regardless of the current level. The tap control unit may be operable to open and close taps so as to increase the number of sets of windings in the closed circuit when a knee-point distortion is detected in the voltage. The tap control unit may be operable to open and close taps so as to increase the number of sets of windings in the closed circuit when a knee-point distortion is detected in the voltage, regardless of the current level.

[0050] The autonomous multi-ratio transformer system may comprise a transformer voltmeter operable to detect the voltage output to the application circuit. The transformer voltmeter may be connected in parallel with the application circuit. The transformer voltmeter may be connected between the plurality of taps and the common reference. The transformer voltmeter may be directly connected to each of the plurality of taps. The transformer voltmeter may be directly connected to the common reference. The transformer voltmeter may be operable to analyse the AC waveform of the voltage. The zero-crossing detection unit may be operable to receive the reading from the voltmeter. The zero-crossing detection unit may be directly connected to the voltmeter. The zero-crossing detection unit may be operable to receive the analysis of the AC waveform and / or a voltage reading.

[0051] The tap control unit may be operable to have a respective range of voltages associated with each tap of the plurality of taps. The first range of voltages, associated with the first tap, may run from zero to a first level of voltage. The second range of voltages, associated with the second tap, may run from the first level of voltage to a second level of voltage. The Mth range of voltages, associated with the Mth tap, may run from an M-1th level of voltage to an Mth level of voltage. The Mth level of voltage may be higher than the M-1th level of voltage. Each range of voltages may be the same size. Each range of voltages may be proportional in size to the number of windings in a respective Mth set of windings.

[0052] The tap control unit may be operable to open and / or close the one or more taps simultaneously.

[0053] The autonomous multi-ratio transformer system may comprise a reserve battery. The autonomous power harvesting system may comprise a reserve battery. The reserve battery may be operable to power the tap control unit when the power harvesting unit cannot power the tap control unit alone.

[0054] The tap control unit may be connected to the plurality of sets of secondary windings to draw power. The tap control unit may be connected to the first set of secondary windings to draw power. The tap control unit may be connected between a point, between the first set of secondary windings and the first tap, and the common reference. The autonomous multi-ratio transformer system may comprise a diode bridge via which the tap control unit draws power. The autonomous power harvesting system may comprise a diode bridge via which the tap control unit draws power. The diode bridge may be connected in parallel with the first set of secondary windings. One input of the diode bridge may be connected to the point, between the first set of secondary windings and the first tap. The input of the diode bridge may be directly connected to the point, between the first set of secondary windings and the first tap. The other input of the diode bridge may be connected to the common reference. The other input of the diode bridge may be directly connected to the common reference. The outputs of the diode bridge may be connected to the tap control unit. The outputs of the diode bridge may be directly connected to the tap control unit. The negative output of the diode bridge may be connected to the common reference. The negative output of the diode bridge may be directly connected to the common reference.

[0055] The first set of secondary windings of the plurality of sets of secondary windings is directly connected between a common reference and a first tap of the plurality of taps. Each Nth set of secondary windings (where N is an integer and N>1) of the plurality of secondary is directly connected between an Nth tap and an N-1 th tap of the plurality of taps. The autonomous power harvesting unit may be only connected to the plurality of sets of secondary windings via the plurality of taps and the common reference.

[0056] The sets of secondary windings may be directly connected in series.

[0057] Each set of secondary windings may be wound around a core. Each set of secondary windings may be wound around a respective core. Each set of secondary windings may be wound around the same core. The core or each core may be iron. The core or each core may be laminated iron.

[0058] Each multi-ratio transformer may have a respective common reference.

[0059] Each set of secondary windings may have the same number of windings. The term winding is equivalent to turn. Each set of secondary windings may have a different number of windings. The number of windings may be 120 for each set of secondary windings. The number of windings of the first set of secondary windings may be 100 . The number of windings of the second set of secondary windings may vary from set to set.

[0060] The autonomous multi-ratio transformer system may comprise a DC level quantisation unit operable to quantise the output of the voltmeter. The power harvesting system may comprise a DC level quantisation unit operable to quantise the output of the voltmeter. The DC level quantisation unit may be connected between the voltmeter and the zero-crossing detection unit.

[0061] The shunt may be operable to totally block the multi-ratio transformer from a load of the power harvesting unit when closed and connect the two when open. The shunt may be connected between the plurality of taps and the load of the power harvesting unit. The shunt may be a MOSFET shunt.

[0062] The autonomous power harvesting unit may comprise a shunt control unit operable to open and close the shunt. The shunt control unit may be operable to detect the voltage across the power harvesting unit. The shunt control unit may be operable to open and close the shunt dependent upon the determined current and / or voltage. The shunt control unit may be operable to open the shunt when below a danger current level and / or danger voltage level, and close the shunt when at or above the danger current level and / or danger voltage level.

[0063] The shunt may be directly connected to the plurality of taps. The shunt may be directly connected to the ammeter.

[0064] The rectifier may be operable to rectify the output of the plurality of taps. The rectifier may be a MOSFET bridge. The rectifier may be connected between the plurality of taps and the load of the power harvesting unit. The rectifier may be connected between the shunt and the load of the power harvesting unit. The rectifier may be directly connected to the shunt. The autonomous power harvesting unit may comprise a capacitor to filter the output of the rectifier. The capacitor may be connected across the outputs of the rectifier. The capacitor may be connected between the positive output of the rectifier and the DC ground (GND).

[0065] The shunt may be located on a first side of the rectifier. The load may be located on the second side of the rectifier, the second side being opposite to the first side. The multi-ratio transformer may be on the first side of the rectifier.

[0066] The autonomous power harvesting unit may comprise a power harvesting voltmeter. The power harvesting voltmeter may be connected between the positive output of the rectifier and the DC ground (GND). The power harvesting voltmeter may be connected in parallel with the capacitor.

[0067] The autonomous power harvesting system may comprise a plurality of multi-ratio transformers and power harvesting units, wherein:

[0068] (a) each multi-ratio transformer comprises a plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings being connected in series, and a plurality of taps connecting the sets of secondary windings to the power harvesting unit, wherein the plurality of sets of secondary windings are arranged around a single core, and wherein the core is configured to be arranged around the primary wire;

[0069] (b) each first set of secondary windings of the plurality of sets of secondary windings is connected between a common reference and a first tap of the plurality of taps, and each Nth set of secondary windings (where N is an integer and N>1) of the plurality of sets of secondary windings are connected between an Nth tap and an N-1 th tap of the plurality of taps;

[0070] (c) each power harvesting unit is connected between each plurality of taps and the common reference of a respective multi-ratio transformer, the power harvesting unit being operable in use to draw power by induction from the primary wire via the one or more of the sets of secondary windings when one of the taps is closed;

[0071] (d) the tap control unit being operable to detect the current and / or voltage of the active set of secondary windings and close and / or open one or more of the taps based on the detected values; and

[0072] (e) each plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and M+1th and higher taps are open, the sets of secondary windings upto and including the Mth set of secondary windings form one closed circuit with the power harvesting unit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

[0073] Each power harvesting unit may comprise a respective rectifier. Each power harvesting unit may comprise a respective shunt. The shunt control unit may be operable to open and close one or more of the shunts. The shunt control unit may be operable to open and close each shunt dependent upon the determined current and / or voltage. The shunt control unit may be operable to open and close each shunt dependent upon the determined voltage across the power harvesting unit. The shunt control unit may be operable to open and close each shunt dependent upon the reading of the power harvesting voltmeter.

[0074] The shunt control unit may be operable to associate each multi-ratio transformer with a set voltage level for voltage across the power harvesting unit. A first multi-ratio transformer may have a first voltage level. A Lth multi-ratio transformer (L being an integer and L>1) may have an Lth set voltage level, wherein the Lth set voltage level is lower than the L-1 th set voltage level. The shunt control unit may be operable to close the Lth shunt, and any higher shunts, when above the Lth set voltage level for the respective multi-ratio transformer, and open any L-1 th shunt and lower.

[0075] Each multi-ratio transformer may have a respective common reference.

[0076] The autonomous power harvesting system may comprise one ammeter. The power harvesting system may comprise one transformer voltmeter. The power harvesting system may comprise one power harvesting voltmeter.

[0077] The tap control unit may be operable to detect the current through the plurality of sets of secondary windings in the first multi-ratio transformer to detect the current through the primary wire. The ammeter may be positioned to measure the current through the common reference and the closed tap or each closed tap of the first multi-ratio transformer. The ammeter may be connected in series with the sets of secondary windings of the first multi-ratio transformer. The ammeter may be connected between the first tap and the common reference of the first multi-ratio transformer. The ammeter may be connected between the first set of secondary windings and the second set of secondary windings of the first multi-ratio transformer. The ammeter may be connected between the first set of secondary windings and the common reference of the first multi-ratio transformer.

[0078] The transformer voltmeter may be connected between the plurality of taps and the common reference of the first multi-ratio transformer. The transformer voltmeter may be directly connected to each of the plurality of taps of the first multi-ratio transformer. The transformer voltmeter may be directly connected to the common reference of the first multi-ratio transformer.

[0079] The autonomous power harvesting voltmeter may be connected between the positive output of the rectifier of the first power harvesting unit and the DC ground (GND), the first power harvesting unit being associated with the first multi-ratio transformer.

[0080] The power harvesting units may share DC ground (GND).

[0081] The tap control unit may comprise a plurality of multi-ratio controllers. Each multi-ratio controller may be operable to open and / or close the taps of a respective multi-ratio transformer.

[0082] According to a third aspect of the present invention there is provided an autonomous current measurement system for measuring the alternating current in a primary wire, the autonomous current measurement system comprising a multi-ratio transformer, a metering unit, and a tap control unit, wherein:

[0083] (a) the multi-ratio transformer comprises a plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings being connected in series, and a plurality of taps connecting the sets of secondary windings to the metering unit;

[0084] (b) a first set of secondary windings of the plurality of sets of secondary windings is connected between a common reference and a first tap of the plurality of taps, and each Nth set of secondary windings (where N>1) of the plurality of sets of secondary windings is connected between an Nth tap and an N-1 th tap of the plurality of taps;

[0085] (c) the metering unit is connected between the plurality of taps and the common reference, the metering unit being operable in use to measure the alternative current in the primary wire via the one or more of the sets of secondary windings when one of the taps is closed;

[0086] (d) the tap control unit being operable to detect the current through the primary wire and close and / or open one or more of the taps based on the detected current; and

[0087] (e) the plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and M+1th and higher taps are open, the sets of secondary windings up to and including the Mth set of secondary windings form a closed circuit with the metering unit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

[0088] The position of the taps means each section of windings of the multi-tap transformer does not form part of a complete circuit unless the associated tap or higher is closed. As such, the windings which are not required can be completely isolated from the circuit and so not draw power.

[0089] The metering unit may be only connected to the plurality of sets of secondary windings via the plurality of taps and the common reference.

[0090] The metering unit may be operable to receive the current value from the ammeter.

[0091] The autonomous current measurement system may comprise a power harvesting unit operable to power the metering unit and / or the tap control unit. The current measurement system may comprise a separate auxiliary current transformer for providing power to the power harvesting unit for powering the current measurement system. The power harvesting unit may be connected to the auxiliary current transformer. The power harvesting unit may be directly connected to the auxiliary current transformer. The auxiliary current transformer may be a set of windings. The power harvesting unit may be operable to draw power from the primary wire via the auxiliary current transformer. The autonomous current measurement system may comprise a shunt operable to block the auxiliary current transformer from the power harvesting unit when closed and connect the two when open.

[0092] The metering unit may be a microcontroller.

[0093] The autonomous current measurement system may comprise an ammeter to which the tap control unit is connected to measure the current.

[0094] The autonomous current measurement system may comprise a zero-crossing detection unit operable to determine the zero crossing detection points of the current through the sets of secondary windings.

[0095] The autonomous current measurement system may comprise a transformer voltmeter operable to detect the voltage output to the power harvesting unit. The transformer voltmeter may be connected across the power harvesting unit.

[0096] The auxiliary current transformer may be on the first side of the rectifier.

[0097] The autonomous current measurement system may comprise a reserve battery.

[0098] The autonomous current measurement system may comprise a DC level quantisation unit operable to quantise the output of the voltmeter.

[0099] The rectifier may be connected between the auxiliary current transformer and the load of the current measurement system. The shunt may be connected between the auxiliary current transformer and the load of the current measurement system. The shunt may be connected between the auxiliary current transformer and the rectifier.

[0100] The autonomous current measurement system may comprise a plurality of multi-ratio transformers, wherein:

[0101] (a) each multi-ratio transformer comprises a plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings being connected in series, and a plurality of taps connecting the sets of secondary windings to the metering unit;

[0102] (b) each first set of secondary windings of the plurality of sets of secondary windings is connected between a common reference and a first tap of the plurality of taps, and each Nth set of secondary windings (where N is an integer and N>1) of the plurality of sets of secondary windings is connected between an Nth tap and an N-1th tap of the plurality of taps;

[0103] (c) the metering unit is connected between the plurality of taps and the common reference, the metering unit being operable in use to measure the alternative current in the primary wire via the one or more of the sets of secondary windings when one of the taps is closed; (d) the tap control unit being operable to detect the current through the primary wire and close and / or open one or more of the taps of each plurality of taps based on the detected current; and

[0104] (e) each plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and M+1th and higher taps are open, the sets of secondary windings upto and including the Mth set of secondary windings form one closed circuit with the metering unit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

[0105] The autonomous current measurement system may comprise a plurality of power harvesting units and a plurality of auxiliary current transformers, each power harvesting unit and auxiliary current transformer associated with a respective multi-ratio transformer, the power harvesting unit being connected between a positive output of the respective auxiliary current transformer and a common reference of the auxiliary current transformer , the power harvesting unit being operable in use to draw power by induction from the primary wire via the auxiliary current transformer.

[0106] The third aspect of the present invention may have any or all of the optional features of the second aspect, as desired or required.

[0107] Detailed Description of the Invention

[0108] In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

[0109] Figure 1 is the prior art apparatus of WO2017138026 A1 and WO2019030781 A1 ;

[0110] Figure 2 is one set of windings, its respective circuitry, and the burden, of the prior art apparatus of WO2017138026 A1 and WO2019030781 A1 ;

[0111] Figure 3 is the prior art apparatus of US2021 / 0257158 A1 ;

[0112] Figure 4 is an autonomous power harvesting system;

[0113] Figure 5 is an autonomous current measurement system;

[0114] Figure 6 is the autonomous power harvesting system of figure 4 in more detail;

[0115] Figure 7 is the autonomous current measurement system of figure 5 in more detail;

[0116] Figure 8 is a graph of the AC voltage and zero cross detection over time;

[0117] Figure 9 is a graph of the state of each tap in figures 6 and 7;

[0118] Figure 10 is the autonomous current measurement system of figure 5, showing the multiratio transformer in more detail;

[0119] Figure 11 is the multi-ratio controller in figure 10; Figure 12 is the autonomous power harvesting system of figure 4;

[0120] Figure 13 is an autonomous power harvesting system;

[0121] Figure 14 is an autonomous power harvesting system; and

[0122] Figure 15 is the power supply for an autonomous power harvesting system or an autonomous current measurement system.

[0123] As show in figures 4 and 5, the autonomous power harvesting system and the autonomous current measurement system 100, 112 comprise a number of sets of secondary windings 101 , 102, 103, 104 (in these embodiments, four sets of secondary windings) which, in use, are applied to a phase wire 105 (or primary wire - in some embodiments, an overhead power line). In this embodiment each set of secondary windings contains 120 windings. In other embodiments the number of windings can be different, and can vary from set to set in the same system. The sets of secondary windings 101 , 102, 103, 104 are connected in series, the first set of secondary windings 101 to the second set of secondary windings 102, the second set of secondary windings 102 to the third set of secondary windings 103, and so on. The first set of secondary windings is connected between the common reference 110 and the second set of secondary windings 102.

[0124] The secondary windings are wound around a core of iron.

[0125] The power harvesting circuit also comprises a respective tap 106, 107, 108. 109 for each set of secondary windings 101 , 102, 103, 104. The first tap 106 is directly connected to a point between the first set of secondary windings 101 and the second set of secondary windings 102. The second tap 107 is directly connected to a point between the second set of secondary windings 102 and the third set of secondary windings 103. This continues such that each Nth tap is directly connected to a point between the Nth set of secondary windings and the N+1 th set of secondary windings, until the last tap which is directly connected to the end of the series of sets of secondary windings, to the last set of secondary windings (in this case, the fourth set of secondary windings 104).

[0126] In the autonomous power harvesting system of figure 4, each tap 106, 107, 108, 109 then connects to the power harvesting circuit 11 1 (or power harvesting unit), which is connected between them and the common reference 110. In the autonomous current measurement system of figure 5, each tap 106, 107, 108, 109 then connects to the metering circuit 113 (or current measurement unit), which is connected between them and the common reference 110.

[0127] The sets of secondary windings and taps together form a multi-ratio transformer 126.

[0128] As shown in figures 6, the autonomous power harvesting system 100 also comprises a multi-ratio controller 114 (which forms part of a tap control unit), a zero-crossing detection unit 115 (another part of the tap control unit), an ammeter 116, a multi-ratio transformer voltmeter 117, and a diode bridge 118. The ammeter 116 is connected between the common reference 110 and the first set of secondary windings 101 , while the voltmeter is connected in parallel with the power harvesting unit 11 1 , between the common reference 110 and a point between the plurality of taps 106, 107, 108, 109 and the power harvesting unit 11 1 . In use, the ammeter 116 measures the current through the phase wire indirectly, by measuring the current through the one or more sets of secondary windings 101 , 102, 103, 104. In use, the multi-ratio transformer voltmeter 1 17 measures the voltage outputted from the plurality of taps 106, 107, 108, 109 to the power harvesting unit 1 1 1.

[0129] The zero-crossing detection unit 115 is connected to the ammeter 116 and multi-ratio transformer voltmeter 117 so as to obtain the readings from each in use. The multi-ratio transformer voltmeter 117, as well as giving a voltage reading, analyses the AC waveform of the voltage for zero-crossing detection and provides this analysis as a reading as well. In use, from the voltmeter readings and the ammeter readings the zero-crossing detection unit 115 determines when the zero-crossing occurs. From the ammeter readings the zero-crossing detection unit 115 determines a current value, in some embodiments this being the RMS current value. In alternative embodiments, this may be the peak current value. The zero-crossing detection unit 1 15 then sends a pulse showing the zero-crossing detection and the current value to the multi-ratio transformer 114. The pulses sent are shown in figure 8.

[0130] Based on the current reading, the multi-ratio controller 114 changes the tap 106, 107, 108, 109 which is closed. When the autonomous power harvesting system 100 is first powered up, the first tap 106 is closed and the rest remain open. This means that the first set of secondary windings 101 form part of a closed circuit and draw power from the phase wire 105 and supply it to the power harvesting unit 11 1. The remaining sets of secondary windings 102, 103, 104 form part of an open circuit, and so do not draw any power.

[0131] Each set of secondary windings 101 , 102, 103, 104 has an associated primary current value, with the third set of secondary windings 103 having a higher primary current value than the second set of secondary windings, and the fourth set of secondary windings 104 having a higher primary current value than the third set of secondary windings 103. This relationship carries on, such that the Nth set of secondary windings has a higher primary current value than the N-1 th set of secondary windings.

[0132] When the current value associated with the second set of secondary windings 102 is reached, the multi-ratio transformer 114 will open the first tap 106 and, at the same time, close the second tap 107. The multi-ratio transformer 114 will open and close taps at the leading edge of the next zero-crossing detection pulse. This results in the first and second sets of secondary windings 101 , 102 together forming one combined set of secondary windings, through which power is drawn from the phase wire 105. The remaining sets of secondary windings continue to be part of open circuits, and so draw no power.

[0133] The increased number of windings in the closed circuit means that the autonomous power harvesting system can cope with the increased current. In some embodiments the zero-crossing detection unit 115 also sends the entire voltmeter reading to the multi-ratio controller 114. From this reading the multi-ratio controller 114 can determine if a knee-point distortion and / or dangerously high voltage levels are reached. If either are reached, the multi-ratio controller 114 will open and close taps to increase the number of windings in use, regardless of the current level, and so stabilise and / or reduce the voltage.

[0134] As shown in figure 9, as the current increases further higher taps are opened, and the others are closed. When the Nth tap is closed, the sets of secondary windings up to the Nth set form a combined set of secondary windings and draw power together from the phase wire 105. The N+1 th set of secondary windings and higher form part of a open circuit and so draw no power.

[0135] In use, as the current value falls below the set current value the multi-ratio controller 114 will open the Nth tap and close the N-1 th tap. Accordingly, the number of secondary windings in use is reduced as the current reduces.

[0136] The autonomous power harvesting system 100 can thereby adjust for the present current level, which allows it to operate well at lower current levels while also avoiding damage at higher current levels.

[0137] The diode bridge 118 is connected between the common reference 110 and a point between the first set of secondary windings 101 and the first tap 106. The diode bridge 118 is also connected to the multi-ratio controller 114, so as to power it.

[0138] As shown in figure 7, the current measurement system 112 lacks the diode bridge 118 of the autonomous power harvesting system 100, but has the ammeter 1 16, the multi-ratio controller 114, the zero-crossing detection unit 118, the multi-ratio transformer voltmeter 117 and their respective connections. It has a metering circuit 120 in the position of the power harvesting circuit 11 1 in the autonomous power harvesting system 100. It does have a power harvesting circuit 11 1 , but this is connected to its own, separate set of secondary windings 121 (the auxiliary transformer) to draw power from the phase wire 105 and thereby power the metering circuit 120 and the multi-ratio controller 114.

[0139] The metering circuit 113 takes readings from the ammeter 116, for use in determining the metering reading.

[0140] The first tap 106 is a normally closed setup while the remaining taps are in a normally open setup. As such, the default (when the system is off or just starting up) is the first tap 106 closed and the remaining taps open. The system will return to this default when switched off.

[0141] The multi-ratio controller 114 takes the form of a microcontroller, as shown in figure 11 . It is powered from the VDD line, and receives inputs of the voltage and current from the zerodetection crossing unit 115. An output runs to each tap, to adjust whether it is open or closed. Figures 10 and 12 shows the circuitry involved in the power harvesting circuits 111. Between a load of the power harvesting circuit 11 1 (which, for the current measurement, is the multi-ratio controller 114) and the plurality of taps (or, forthe current measurement system 112, the auxiliary transformer 121), and connected between the output of the plurality of taps (orthe auxiliary transformer 121) and the common reference 110 (or DC ground 119, beyond the rectifier), is, in order, a MOSFET shunt 122, a MOSFET bridge 123 (which is the rectifier), and a power harvesting voltmeter 125.

[0142] The MOSFET shunt 122, when open, connects the multi-ratio transformer 126 (or, for the current measurement system 112, the auxiliary transformer 121) to the load of the power harvesting circuit 11 1. When closed, this connection is blocked and the two are disconnected. The MOSFET shunt 122 is controlled by a state latch and shunting control unit 127 (or shunt control unit). The voltage of the power harvesting voltmeter 125 is taken and quantised by a DC level quantisation unit 128. This quantised reading is then passed to the zero-crossing detection unit 1 15, and then passed to the state latch and shunting control unit 127.

[0143] When a system comprises a plurality of multi-ratio transformers (each with a respective MOSFET shunt) there is a respective voltage level associated with each multi-ratio transformer. The first multi-ratio transformer is associated with the highest voltage level - this is the voltage level above which there is danger of damage. The second multi-ratio transformer has a lower voltage level, and so on such that the Lth multi-ratio transformer has a voltage level which is lowerthan the voltage level of the L-1 th multi-ratio transformer but higherthan the voltage level of the L+1 th multi-ratio transformer. The state latch and shunting control unit 127 acts to close the MOSFET shunts of the multi-ratio transformers with a voltage level less than the current measured voltage. As such, as the voltage rises fewer multi-ratio transformers are used, and as the voltage lowers more multi-ratio transformers are used. This acts to counter-act the change in the voltage value. If the voltage rises above the maximum limit, all the shunts are closed, resulting in no power being drawn and the voltage lowering back to safer levels.

[0144] The MOSFET bridge 123 rectifies the output of the multi-ratio controller 126, before it goes to the power harvesting circuit 11 1.

[0145] Figure 13 shows an embodiment with two multi-ratio transformers 126. Each multi-ratio transformer has its own respective power harvesting unit, in the form of a MOSFET shunt 122 and MOSFET bridge 123. They share the multi-ratio controller 114 and state latch and shunting control unit 127, which respectively control the taps and MOSFET shunts 122 of both multiratio transformers 126. The multi-ratio controller 114 opens and closes the same taps of each multi-ratio transformers 126 (e.g. so the second tap is closed and the rest open for both multiratio transformers 126 at the same time).

[0146] There is only one each of the ammeter 116, multi-ratio transformer voltmeter 117, power harvesting voltmeter 125, zero-crossing detection unit 115 and DC level quantisation unit 128, each of which form part of or are connected to only one of the multi-ratio transformers 126. This multi-ratio transformer 126 is the first multi-ratio transformer, such that it is the last multi-ratio transformer 126 to be disconnected as the voltage rises.

[0147] Figure 14 shows the layout of microcontrollers for the autonomous power harvesting system 100. The zero-crossing detection unit 115, multi-ratio controller 114, DC level quantisation 128, and state latch and shunting control unit 127 are all part of the microcontrollers shown. A similar layout will work for the circuit measurement system 112.

[0148] As shown in figure 15, the multi-ratio controller 114 of the autonomous power harvesting system 100 can have three different power supply sources: a temporary power supply 129 (which is the diode bridge 118), the power harvesting circuit 1 1 1 , and the reserve battery 130. These three connect to a voltage regulator 131 , which determines which to use, and so supply to the multi-ratio controller 114. When the system is fully operational, the power harvesting circuit 111 supplies power. When the circuit is off or in other circumstances when only low power is required, the battery 130 is used to power the multi-ratio controller 114. When the circuit is first turned on, when the power harvesting circuit 11 1 is not yet generating power but the power from the battery 130 is insufficient, the multi-ratio controller 114 is supplied by power generated in the multi-ratio transformer (i.e. the temporary power supply 129).

[0149] The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

CLAIMS1 . An autonomous multi-ratio transformer system for drawing current from an alternating current in a primary wire, the autonomous multi-ratio transformer system comprising a multi-ratio transformer, an application circuit, and a tap control unit, wherein: a. the multi-ratio transformer comprises a plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings being connected in series, and a plurality of taps connecting the sets of secondary windings to the application circuit, wherein the plurality of sets of secondary windings are arranged around a single core, and wherein the core is configured to be arranged around the primary wire; b. a first set of secondary windings of the plurality of sets of secondary windings is connected between a common reference and a first tap of the plurality of taps, and each Nth set of secondary windings (where N is an integer and N>1) ofthe plurality of sets of secondary windings is connected between an Nth tap and an N-1 th tap of the plurality of taps; c. the application circuit is connected between the plurality of taps and the common reference, the application circuit being operable in use to obtain the current by induction from the primary wire via the one or more of the sets of secondary windings when one of the taps is closed; d. the tap control unit being operable to detect the current and / or voltage ofthe active set of secondary windings and close and / or open one or more of the taps based on the detected values; and e. the plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and M+1th and higher taps are open, the sets of secondary windings up to and including the Mth set of secondary windings form one closed circuit with the application circuit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

2. An autonomous multi-ratio transformer system according to claim 1 , wherein the tap control unit is operable to close the Mth tap and open the remaining taps of the plurality of taps.

3. An autonomous multi-ratio transformer system according to any preceding claim wherein the tap control unit is operable to have a respective range of primary current associated with each tap of the plurality of taps.

4. An autonomous multi-ratio transformer system according to claim 3, wherein the first range of currents, associated with the first tap, runs from zero to a first level of current.

5. An autonomous multi-ratio transformer system according to claim 4, wherein the Mth range of currents, associated with the Mth tap, runs from an M-1th level of current to an Mth level of current. The Mth level of current may be higher than the M-1th level of current.

6. An autonomous multi-ratio transformer system according to any preceding claim wherein the tap control unit is operable to detect the voltage across the plurality of sets of secondary windings and close and / or open one or more of the taps based on the detected voltage.

7. An autonomous multi-ratio transformer system according to any preceding claim, wherein the tap control unit is operable to open and / or close the one or more taps simultaneously and / or instantaneously.

8. An autonomous multi-ratio transformer system according to any preceding claim which is a power harvesting system for harvesting power from the alternating current in the primary wire, the power harvesting system comprising the multi-ratio transformer, the application circuit which is a power harvesting unit, and the tap control unit, wherein: a. the multi-ratio transformer comprises the plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings being connected in series, and the plurality of taps connecting the sets of secondary windings to the power harvesting unit, wherein the plurality of sets of secondary windings are arranged around a single core, and wherein the core is configured to be arranged around the primary wire; b. a first set of secondary windings of the plurality of sets of secondary windings is connected between the common reference and the first tap of the plurality of taps, and each Nth set of secondary windings (where N is an integer and N>1) of the plurality of sets of secondary windings is connected between the Nth tap and the N- 1th tap of the plurality of taps; c. the power harvesting unit is connected between the plurality of taps and the common reference, the power harvesting unit being operable in use to draw power by induction from the primary wire via the one or more of the sets of the secondary windings when one of the taps is closed; d. the tap control unit being operable to detect the current and / or voltage of the active set of secondary windings and close and / or open one or more of the taps based on the detected values; and e. the plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and M+1th and higher taps are open, the sets of secondary windings up to and including the Mth set of secondary windings form one closed circuit with the power harvesting unit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

9. An autonomous multi-ratio transformer system according to claim 8, comprising a plurality of multi-ratio transformers and power harvesting units, wherein: a. each multi-ratio transformer comprises a plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings being connected in series, and a plurality of taps connecting the sets of secondary windings to the power harvesting unit, wherein the plurality of sets of secondary windings are arranged around a single core, and wherein the core is configured to be arranged around the primary wire; b. each first set of secondary windings of the plurality of sets of secondary windings is connected between a common reference and a first tap of the plurality of taps, and each Nth set of secondary windings (where N is an integer and N>1) of the plurality of secondary is connected between an Nth tap and an N-1th tap of the plurality of taps; c. each power harvesting unit is connected between each plurality of taps and the common reference or a respective multi-ratio transformer, the power harvesting unit being operable in use to draw power by induction from the primary wire via the one or more of the sets of windings when one of the taps is closed; d. the tap control unit being operable to detect the current and / or voltage of the active set of secondary windings and close and / or open one or more of the taps based on the detected values; and e. each plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and M+1th and higher taps are open, the sets of secondary windings up to and including the Mth set of secondary windings form one closed circuit with the power harvesting unit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

10. A power harvesting system according to claim 9, when dependent upon claim 17 and 18, wherein each power harvesting unit comprises a respective shunt and the shunt control unit is operable to open and close one or more of the shunts and wherein the shunt control unit is operable to open and close each shunt dependent upon the determined voltage across the power harvesting unit.

11. An autonomous multi-ratio transformer system according to any of claims 1 to 7 which is an autonomous current measurement system for measuring the alternating current in the primary wire, the current measurement system comprising the multi-ratio transformer, a application circuit which is a metering unit, and the tap control unit, wherein: a. the multi-ratio transformer comprises the plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings beingconnected in series, and the plurality of taps connecting the sets of secondary windings to the metering unit; b. a first set of secondary windings of the plurality of sets of secondary windings is connected between a common reference and a first tap of the plurality of taps, and each Nth set of secondary windings (where N>1) of the plurality of sets of secondary windings is connected between the Nth tap and the N-1th tap of the plurality of taps; c. the metering unit is connected between the plurality of taps and the common reference, the metering unit being operable in use to measure the alternative current in the primary wire via the one or more of the sets of secondary windings when one of the taps is closed; d. the tap control unit being operable to detect the current and / or voltage of the active set of secondary windings and close and / or open one or more of the taps based on the detected values; and e. the plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and M+1th and higher taps are open, the sets of secondary windings up to and including the Mth set of secondary windings form a closed circuit with the metering unit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

12. The current measurement system according to claim 11 , comprising a plurality of multiratio transformers, wherein: a. each multi-ratio transformer comprises a plurality of sets of secondary windings configured to be applied to the primary wire, the sets of secondary windings being connected in series, and a plurality of taps connecting the sets of secondary windings to the metering unit, the plurality of taps being connected in parallel; b. each first set of secondary windings of the plurality of sets of secondary windings is connected between a common reference and a first tap of the plurality of taps, and each Nth set of secondary windings (where N is an integer and N>1) of the plurality of sets of secondary windings is connected between an Nth tap and an N-1 th tap of the plurality of taps; c. the metering unit is connected between the parallel plurality of taps and the common reference, the metering unit being operable in use to measure the alternative current in the primary wire via the one or more of the sets of secondary windings when one of the taps is closed; d. the tap control unit being operable to detect the current and / or voltage of the active set of secondary windings and close and / or open one or more of the taps based on the detected values; ande. each plurality of taps are arranged so that when the Mth tap (where M is an integer and M31) is closed and M+1th and higher taps are open, the sets of secondary windings up to and including the Mth set of secondary windings form one closed circuit with the metering unit and the remaining sets of secondary windings of the plurality of sets of secondary windings form an open circuit.

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