Patent Description:
Overvoltage events caused by lightning strikes, switching operations or temporary surges, can result in power outages in power stations, substations, and electric power transmission and distribution lines. Surge arresters, which are configured to have a high impedance at normal operating voltage and a low impedance during a surge event, are usually installed between phase and ground to provide protection from excessive overvoltages caused by such overvoltage events. When the voltage across the surge arrester terminals exceeds a predetermined threshold voltage, the surge arrester conducts the current caused by the overvoltage to the ground.

The condition of a surge arrester can be monitored by measuring the leakage current through the ground conductor at the grounded side of a surge arrester. The total leakage current of a surge arrester is composed of a capacitive component and a resistive component, whereby the capacitive component is much larger than the resistive component. An increase in the resistive current, and consequently the total leakage current, indicates that the condition of the surge arrester has degraded.

A number of measurement methods have been utilized to reveal signs of deterioration and provide clues to impending surge arrester failure. Many of these measurements utilize sensitive leakage current measurement circuitry that can measure small changes in the total leakage current. When making such measurements, the leakage current at normal operating voltage is measured, but the sensitive measurement circuitry must at the same time be protected from overcurrents to which a surge arrester may be exposed during surge events.

One known method to measure the leakage current is a standard pass-through solution in which a solid metallic conductor carrying a primary current is passed through a toroidal core conversion circuit to generate a secondary current that is picked up by a leakage current measurement circuit. Conventional diodes are then used to rectify the secondary current signal. A problem with this approach is that very small secondary currents are generated in the toroidal core's secondary winding so passive electronics and cost-effective analog meters cannot be used to measure those small currents. Instead, active electronics with digital displays must be used to measure the leakage current through the conductor. A device with active electronics is likely to require external power sources such as solar cells, auxiliary power or powered by a battery that needs to be replaced periodically. Therefore, such an implementation increases the cost and complexity of the leakage current measurement device.

Another known method is to use a circuit comprising a metal oxide (MO) block to divert the leakage current to a measurement circuit. A drawback of this method is that the MO block must be selected to match the characteristics of the surge arrester whose leakage current is being measured (and which also contains MO blocks) and the process of assembling such a measurement circuit is complex.

It is also known to use a spark gap to divert the leakage current to a measurement circuit. Drawbacks with this method are the difficulties involved in providing a specific gap distance between electrodes and ensuring a safe failure mode. A very rigid housing is needed so that the spark gap distance is always accurately controlled, and there may be problems concerning the aging of the spark gap and fluctuations of the spark gap characteristics due to climate and environmental conditions, such as humidity, temperature or altitude. Prior art can be found in document <CIT>.

An object of the invention is to provide an improved device for measuring an alternating (AC) leakage current though a conductor, i.e. the total leakage current through a conductor.

The term "measuring an AC leakage current" is intended to mean ascertaining the amount of leakage current flowing through a conductor and/or identifying the presence of a leakage current in a conductor. The word "measure" is therefore intended to mean "measure" and/or "detect".

This object is achieved by a device comprising the features recited in claim <NUM>. The device comprises a conversion circuit comprising a magnetic core, and a leakage current measurement circuit. The device also comprises a synchronous rectification circuit that comprises a plurality of metal oxide semiconductor field effect transistors (MOSFETs).

The conversion circuit is configured so that the conductor, such as a ground conductor, is arranged to pass through, or wind around the magnetic core in one or more turns, and the conversion circuit comprises multiple secondary windings, i.e. a plurality of pairs of secondary windings, or a single secondary winding with a plurality of taps, such as a plurality of pairs of taps, whereby the plurality of pairs of secondary windings or the single secondary winding is wound around the magnetic core. Each pair of secondary windings or each pair of taps handles a positive or negative part of the AC current.

The conversion circuit is configured to convert a primary AC current in the conductor to a secondary AC current in the plurality of pairs of secondary windings or in the single secondary winding. At least one first pair of secondary windings or at least one first pair of taps is configured to apply a voltage to the plurality of MOSFETs. At least one second pair of secondary windings or at least one second pair of taps is connected to the synchronous rectification circuit, whereby the synchronous rectification circuit is configured to rectify the secondary AC current in the at least one second pair of secondary windings or at least one second pair of taps to a direct current (DC) and supply the DC current to the leakage current measurement circuit.

The device is configured to transfer electrical energy from the conductor carrying the AC leakage current, which constitutes a primary winding of the conversion circuit, to the leakage current measurement circuit via the synchronous rectification circuit. The leakage current measurement circuit is consequently powered by the leakage current that it is arranged to measure and thereby lacks an external power supply and active electronic components. Since electric energy can be transferred to the measurement circuit, active electronics with digital displays do not namely have to be used to enable measurement of the leakage current through the conductor.

Such a device enables a low loss conversion of the leakage current from the conductor to the multiple secondary windings or the single secondary winding of the conversion circuit. Additionally, since rectification is provided using a synchronous rectification circuit, rather than conventional diodes, high losses are not introduced during conversion or rectification. The device according to the present invention thereby enables a very low leakage current to be measured, such as a leakage current of <NUM> to <NUM> mA, or less than <NUM> mA, or less than 1mA, or less than <NUM> mA. Furthermore, the conductor carrying the AC leakage current to be measured forms a continuous current path through the device.

The synchronous rectification circuit of the device according to the present invention improves the efficiency of rectification by replacing conventional semiconductor diodes with MOSFETs. Conventional semiconductor diodes have a fixed voltage drop of about <NUM> to <NUM> volts, whereas MOSFETs behave as resistances and can have a very low voltage drop. Even though a MOSFET is an active electronic component, it requires almost no input current to control the load current, compared to a bipolar junction transistor (BJT).

According to an embodiment of the invention, the gate threshold voltage of a MOSFET, or the minimum voltage that is needed to create a conducting path between its source and drain terminals, is selected to be extremely low (<NUM> Volts) so as to minimize the rectification voltage drop once the at least one second pair of secondary windings or the at least one second pair of taps opens the MOSFETS of the synchronous rectification circuit.

According to an embodiment of the invention, the leakage current measurement circuit comprises an analog meter (a passive electronic component) configured to display the leakage current. Such a device enables a low leakage current to be measured whilst part of the leakage current is used to power the analog meter, which results in a cost-effective solution. No active electronic component with a digital display is required to measure or detect a leakage current through a conductor.

According to an embodiment of the invention, the device comprises a plurality of overvoltage protection devices, such as transient-voltage-suppression (TVS) diodes, connected across the at least two pairs of secondary windings or the at least two pair of taps. The overvoltage protection devices are configured to protect the measurement circuit and the plurality of MOSFETS during surge events by limiting the maximum voltage across the measurement circuit and the MOSFET gates at high primary current.

The device according to the present invention is configured to isolate the leakage current measurement circuit from the conductor carrying the leakage current during a surge event. During normal operation, i.e. when there is no surge event and the leakage current in the conductor is to be measured, leakage current will be transferred to the leakage current measurement circuit via the synchronous rectification circuit. When a surge event occurs, the voltage being applied to the overvoltage protection devices will change and this will change the electrical conductivity of the overvoltage protection devices ensuring that overcurrents do not flow through the leakage current measurement circuit.

According to an embodiment of the invention, the conversion circuit is configured so that the conductor winds around the magnetic core in a single turn, which constitutes a primary winding having a single turn. Alternatively, the conversion circuit is configured so that the conductor winds around the magnetic core in a plurality of turns, which constitutes a primary winding having multiple turns. The conversion circuit may be configured so that the conductor winds around the magnetic core in any number of turns, whose number may be selected depending on the magnitude of the leakage current being measured and the desired measurement sensitivity level.

It should be noted that the number of turns in the conductor (which constitutes the primary winding of the conversion circuit) and the number of turns of the secondary windings or in the single secondary winding may be selected depending on the current range to be measured, magnetic core characteristics, MOSFET characteristics and the type of analog meter used. The ratio between the number of turns in the conductor and in each secondary winding or in the single secondary winding is selected so as to achieve the desired output voltage.

According to an embodiment of the invention, the at least one first pair of secondary windings or at least one first pair of taps comprises a higher number of turns or a higher number of turns therebetween than the at least one second pair of secondary windings or than between the at least one second pair of taps. For example, the ratio of turns of the at least one first pair of secondary windings or between the at least one first pair of taps to the at least one second pair of secondary windings or between the at least one second pair of taps may be at least <NUM>:<NUM>, at least <NUM>:<NUM>, at least <NUM>:<NUM>, at least <NUM>:<NUM>, at least <NUM>:<NUM>, at least <NUM>:<NUM>, at least <NUM>:<NUM>, at least <NUM>:<NUM>, at least <NUM>:<NUM>, or at least <NUM>:<NUM>.

According to an embodiment of the invention, the conversion circuit comprises four, six or eight or ten secondary windings in total.

According to an embodiment of the invention, the device comprises a surge counting circuit.

According to an embodiment of the invention, the magnetic core has a high permeability to enable a high magnetic flux to be generated in the magnetic core, which enables a very low primary current to be converted and measured using the device according to the present invention. For example, the magnetic core may comprise at least one of the following materials: nickel, carbon steel, martensitic stainless steel, ferritic stainless steel, iron having a purity of at least <NUM>%, cobalt iron, Mu-metal, permalloy, metglas. The magnetic core is preferably a construction comprising a plurality of laminated sheets or layers of one or more materials having a high permeability.

According to an embodiment of the invention the magnetic core may be a toroidal magnetic core. The magnetic core may however be of any suitable shape such as a core having a circular, oval, square, rectangular or polygonal cross section.

According to an embodiment of the invention the device comprises the step of providing the device with a surge counting circuit, such as a surge counting circuit that includes a magnetic core and that is configured so that said conductor is arranged to pass through, or wind around said magnetic core. The conductor will thereby form a continuous current path through both the conversion circuit and the surge counting circuit.

The present invention also concerns a method for measuring an alternating (AC) leakage current though a conductor, such as a ground conductor. The method comprises the step of providing a device, such as a device according to any of the embodiments of the invention, whereby the device comprises a conversion circuit comprising a magnetic core, a synchronous rectification circuit that comprises a plurality of metal oxide semiconductor field effect transistors (MOSFETs), and a leakage current measurement circuit.

The method comprises the steps of passing the conductor through the magnetic core or winding the conductor around the magnetic core, whereby the conductor constitutes a primary winding of the conversion circuit and providing the conversion circuit with a plurality of pairs of secondary windings or a single secondary winding with a plurality of taps, such as a plurality of pairs of taps, whereby the plurality of pairs of secondary windings or the single secondary winding is wound around the magnetic core, whereby the conversion circuit is configured to convert a primary AC current in the at least one primary winding to a secondary AC current in the plurality of pairs of secondary windings or in the single secondary winding.

The method also comprises the steps of configuring at least one first pair of secondary windings or at least one first pair of taps to apply a voltage to the plurality of MOSFETs, and connecting at least one second pair of secondary windings or at least one second pair of taps to the synchronous rectification circuit, and configuring the synchronous rectification circuit to rectify the secondary AC current in the at least one second pair of secondary windings or the at least second first pair of taps to a direct current (DC) and supplying the DC current to the leakage current measurement circuit. The steps of the method according to the present invention may be carried out in any suitable order.

According to an embodiment of the invention, the method comprises the step of providing the leakage current measurement circuit with an analog meter configured to display the leakage current.

According to an embodiment of the invention, the method comprises the step of connecting a plurality of overvoltage protection devices, such as transient-voltage-suppression (TVS) diodes, across the plurality of pairs of secondary windings or across the plurality of taps to protect the measurement circuit and the plurality of MOSFETS from surge events.

According to an embodiment of the invention, the method comprises at least one of the following steps: arranging the conductor to wind around the magnetic core one or more turns, providing the at least one first pair of secondary windings or the at least one first pair of taps with a higher number of turns or a higher number of turns therebetween than the at least one second pair of secondary windings or than between the at least one first pair of taps, and/or providing four, six or eight or ten secondary windings or pairs of taps in total. The device may however comprise any number of pairs of secondary windings or taps.

According to an embodiment of the invention the method comprises the step of providing the device with a surge counting circuit, such as a surge counting circuit that includes a magnetic core and that is configured so that said conductor is arranged to pass through, or wind around said magnetic core.

According to an embodiment of the invention the method comprises the step of providing a magnetic core comprising at least one of the following materials: nickel, carbon steel, martensitic stainless steel, ferritic stainless steel, iron having a purity of at least <NUM>%, cobalt iron, Mu-metal, permalloy, metglas.

According to an embodiment of the invention the method comprises the step of connecting the device to the grounded side of a surge arrester and to ground, for safety reasons.

The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended schematic figures where;.

It should be noted that the drawings have not necessarily been drawn to scale and that the dimensions of certain features may have been exaggerated for the sake of clarity.

Any feature of any embodiment of the device or method depicted or described in any part of this document may be combined with any one or more features of any other embodiment, unless such a combination is explicitly excluded herein.

<FIG> schematically shows a surge arrester <NUM> that is arranged to protect at least one electric device, connected to a power transmission or distribution line for example, from surge events. A device <NUM> according to an embodiment of the present invention <NUM> is arranged so that a grounded conductor <NUM> of a surge arrester <NUM> passes through or is wound around the magnetic core of a conversion circuit <NUM> of the device <NUM>. The grounded conductor <NUM> thereby forms a continuous current path through the device <NUM>.

The magnetic core <NUM> and has a high permeability and may be a toroidal magnetic core comprising high purity ferromagnetic material, such as laminated iron, iron powder, or ferrite.

The device <NUM> also comprises a synchronous rectification circuit <NUM> that comprises a plurality of MOSFETs <NUM>, and a leakage current measurement circuit <NUM>.

The ground conductor <NUM> constitutes a primary winding of the conversion circuit <NUM> and may have be wound around the magnetic core <NUM> in one or more turns. The ground conductor <NUM> carries the AC leakage current that is to be measured, such as a <NUM> AC leakage current. The conversion circuit <NUM> also comprises four secondary windings, i.e. two pairs of secondary windings <NUM>, <NUM>, that are wound around the magnetic core <NUM>.

The conversion circuit <NUM> is configured to convert the primary current in the ground conductor <NUM> to a secondary current in the secondary windings <NUM>, <NUM>, whereby the conductor <NUM> draws electrical energy from the grounded side of the surge arrester <NUM> and the secondary windings <NUM>, <NUM> deliver the electrical energy at the transformed voltage to other parts of the device <NUM>.

A first pair of secondary windings <NUM>, which may each comprise <NUM> turns for example, is configured to apply a voltage to the gates of the MOSFETs <NUM>, and a second pair of secondary windings <NUM>, which may each comprise <NUM> turns for example, is configured to rectify AC current in the second pair of secondary windings <NUM> to a direct current (DC) and supply the DC current to the leakage current measurement circuit <NUM>.

The lower turn (<NUM> turns) second pair of secondary windings <NUM> provides electrical energy for the leakage current measurement circuit <NUM>. The higher turn (<NUM> turns) first pair of secondary windings <NUM> generate a voltage that is high enough to operate the gates of the MOSFETs <NUM>. MOSFETs <NUM> with an extremely low gate threshold are selected so as to minimize the rectification voltage drop once the higher turn first pair of secondary windings <NUM> opens the MOSFET <NUM>.

The leakage current measurement circuit <NUM> also comprises an analog meter <NUM> configured to display the leakage current being measure which does not require a battery or separate power supply.

The illustrated device <NUM> comprises a plurality of overvoltage protection devices <NUM>, namely a plurality of TVS diodes, connected across the two pairs of secondary windings <NUM>, <NUM> which is configured to protect the leakage current measurement circuit <NUM> and the plurality of MOSFETS <NUM> of the synchronous rectification circuit. A TVS diode operates by shunting excess current when the induced voltage exceeds the avalanche breakdown potential. A TVS diode suppresses all overvoltages above its breakdown voltage. It automatically resets when the overvoltage goes away.

Any suitable overvoltage protection devices <NUM> may be used to protect the leakage current measurement circuit <NUM>. All of the overvoltage protection devices <NUM> in a plurality of overvoltage protection devices <NUM> need not necessarily be of the same type. For example, a plurality of overvoltage protection devices <NUM> may comprise at least one of the following: a TVS diode, an avalanche diode, a metal oxide varistor, a crowbar circuit, a discharge tube, a Zener diode and/or an arcing horn.

The illustrated device <NUM> also comprises a surge counting circuit <NUM> comprising a magnetic core <NUM>. The surge counting circuit <NUM> is configured to register, and optionally display, surges. A conductor <NUM> may be arranged to pass through or wind around the magnetic core <NUM> of a device <NUM> and then to pass through or wind around the magnetic core <NUM> of a surge counter <NUM> as shown in <FIG>. Alternatively, a conductor <NUM> may be arranged to pass through or wind around the magnetic core <NUM> of a surge counter <NUM> and then to pass through or wind around the magnetic core <NUM> of a device <NUM>.

A device <NUM> according to the present invention may comprise one or more additional condition monitoring devices to facilitate predictive maintenance for a surge arrester <NUM>.

Rather than providing a device with a plurality of secondary windings <NUM> as illustrated in <FIG>, a device <NUM> according to the present invention may comprise a single secondary winding with multiple taps.

<FIG> shows the essential steps of a method for measuring a leakage current though a ground conductor. The method comprises step of providing a device having a conversion circuit comprising a magnetic core and at least two pairs of secondary windings or a single secondary winding with a plurality of taps, a synchronous rectification circuit comprising a plurality of MOSFETs, and a leakage current measurement circuit. The method also comprises the step of passing a conductor that is to carry the AC leakage current that is to be measured through the magnetic core of the conversion circuit or wind the conductor around the magnetic core in one or more turns. The method further comprises the step of configuring at least one first pair of secondary windings or at least one first pair of taps to apply a voltage to the gates of a plurality of MOSFETs. The method further comprises the steps of connecting at least one second pair of secondary windings or at least one second pair of taps to the synchronous rectification circuit to rectify alternating current (AC) to direct current (DC) and supply the direct current (DC) to the leakage current measurement circuit. The steps do not have to be carried out in the order presented in <FIG> but may be carried out in any suitable order.

Such a method may be used to identify in good time whether a surge arrester is capable of reliably arresting surges or whether it has been damaged or is at risk of failure and may thereby reduce downtime, increase electric grid reliance, and reduce maintenance costs.

Preferably, the method may comprise at least one of the steps of providing the leakage current measurement circuit with an analog meter configured to display the leakage current, connecting a plurality of overvoltage protection devices, such as TVS diodes, across the at least two pairs of secondary windings or across the at least two pairs of taps that are configured to protect the measurement circuit and the plurality of MOSFETS, providing the device with a surge counting circuit and/or connecting the device to the grounded side of a surge arrester and to ground.

Claim 1:
Device (<NUM>) for measuring an alternating (AC) leakage current though a conductor (<NUM>), whereby said device (<NUM>) comprises: a conversion circuit (<NUM>) comprising a magnetic core (<NUM>) and a leakage current measurement circuit (<NUM>),
characterized in that
- said device (<NUM>) comprises a synchronous rectification circuit (<NUM>) that comprises a plurality of metal oxide semiconductor field effect transistors (MOSFETs) (<NUM>), and
- said conversion circuit (<NUM>) is configured so that said conductor (<NUM>) is arranged to pass through, or wind around said magnetic core (<NUM>), and said conversion circuit (<NUM>) comprises a plurality of pairs of secondary windings (<NUM>, <NUM>) or a single secondary winding with a plurality of taps, whereby said plurality of pairs of secondary windings (<NUM>,<NUM>) or said single secondary winding is wound around said magnetic core (<NUM>), whereby said conversion circuit (<NUM>) is configured to convert a primary AC current in said conductor (<NUM>) to a secondary AC current in said plurality of pairs of secondary windings (<NUM>, <NUM>) or said single secondary winding, whereby
- at least one first pair of secondary windings (<NUM>) is configured to apply a voltage to said plurality of MOSFETs, and
- at least one second pair of secondary windings (<NUM>) or at least one first pair of taps is connected to said synchronous rectification circuit (<NUM>), whereby said synchronous rectification circuit (<NUM>) is configured to rectify said secondary AC current in said at least one second pair of secondary windings (<NUM>) or in said at least one second pair of taps to a direct current (DC) and supply said DC current to said leakage current measurement circuit (<NUM>).