DEVICE AND METHOD FOR TESTING PROTECTION CIRCUITS IN HIGH-VOLTAGE/SWITCHGEAR

A test device to test a Rogowski coil and an associated protection device connected to the coil via conductors. The test device includes means to generate at least one test voltage, transformation means to transform the test voltage into a voltage representative of a secondary signal generated by the Rogowski coil, the transformation means including output terminals for connection to conductors, and means to process an output voltage of the coil.

TECHNICAL FIELD AND PRIOR ART

Rogowski sensor are known to measure currents in Gas Insulated Switchgears (GIS).

They can be implemented on a printed circuit board, as disclosed in U.S. Pat. No. 5,414,400.

FIG.1Ashows a Rogowski coil4according to U.S. Pat. No. 5,414,400, comprising a plate11of a printed circuit, for example made of epoxy resin; the coil comprises deposits of copper3on each face of the plate, the deposits of one face being connected to the deposits of the other face via plated through holes5passing through the plate. The coil has terminals7,9.

As illustrated onFIG.1B, a Rogowski coil4is usually connected to a protection circuit8which can open switches or circuit breakers15in case of any default, for example a surge of current in the GIS or a short circuit17. Such default can generate currents as high as several tens of A or event higher than 100 000 A, which are very dangerous both for operators and for the whole system and can result in interruptions of several months. It is necessary to test whether the protection circuit and the switches or circuit breakers15operate properly. But it is of course impossible to simulate a real default17because of the very high currents (up to 100 000 A) which are required: a portable generator can generate at most several tens or hundreds of A.

For these reasons test circuits have been developed to test the protection circuits8,15. A conventional test of the protection circuit is schematically illustrated onFIG.2A, on which reference2designates a conductor around which a Rogowski coil4is arranged in order to measure a current circulating therein, the coil4generating a secondary voltage signal which is supplied to a system8comprising an integrator6, a compensation stage61and a digitalization stage63in order to test the protection devices15against, for example, overcurrents.

More precisely, the magnetic field created by the primary current which usually circulates in conductor2generates a voltage at the terminals7,9of the coil. This voltage is for example approximately of the order of 50 mV/KA at 50 Hz. As shown onFIG.2B, this voltage vrogo is integrated by integrator6; the resulting voltage Vout is representative of the primary current circulating in conductor2and can be digitized and processed. OnFIG.2B, the coil is represented by an equivalent circuit comprising an inductance41and a resistance42in series.

Usually, as shown onFIG.2A, a test current is directly injected into the protection system8, in particular directly to the digitalization stage63, in order to test the protection functions and devices15. However, the components located upstream of the digitalization stage, in particular the wires13and the integrator6and the coil4itself are not tested. Furthermore, the current which is injected as a test current is not representative of the secondary voltage signal output by the coil4; in other words, the test is performed under conditions which are not representing the actual signal generated by the coil4when it is measuring a current in the conductor2.

There is thus a problem of finding a new test method and a new test device of a Rogowski coil and its associated circuit in order to overcome one or more of the above problems.

In particular, there is a need for a new test method and a new device adapted to test the protection circuit of a Rogowski coil and the conductors between said circuit and said coil.

There is also a need for a new test method and a new device adapted to generate test signals which are representative of the actual signals generated by the coil4when it is measuring a current in the conductor2and when a default (like default17onFIG.1B) appears.

SUMMARY

The present invention first concerns a test device, to test for example a Rogowski coil and an associated protection device connected to said coil via conductors, comprising:means to generate at least one test voltage, comprising for example an overvoltage, and to apply said test voltage to the terminals of a Rogowski coil;means to process an output voltage of said coil.

For example, according to one embodiment of the invention, a test device to test a Rogowski coil and an associated protection device connected to said coil via conductors, comprises:means to generate at least one test voltage;transformation means to transform said test voltage into a voltage comprised between 0.5 V and 10V, representative of a secondary signal generated by said Rogowski coil, said transformation means comprising output terminals for connection to conductors;means to process an output voltage of said coil.

The voltage which is output by device according to the invention is representative of the output signal of a Rogowski coil.

Said transformation means can comprise a capacitive circuit, comprising for example at least 1 capacitor, for example 2 capacitors. A resistor can be arranged in parallel to said at least one capacitor.

Preferably, said at least one resistor has an impedance much smaller than the impedance of said coil and said at least one capacitor.

In a test device according to the invention, said means to generate a test voltage can comprise an alternative current generator. For example, said alternative current generator comprises a controller and preferably at least one memory storing or to store data of at least one, preferably several, waveform(s) which can be used for testing.

In a test device according to the invention, said transformation means can comprise one or more electrical components, for example at least one resistor and/or at least one capacitor, and can further comprise means to vary at least one electrical characteristic at least one of said electronic components.

Said at least one electrical characteristic of at least one of said electronic components can be varied on the basis of one or more tests performed on a Rogowski coil tested with a device according to the invention.

In an embodiment, said transformation means can comprise terminals to provide said means to process with a signal from a Rogowski coil under test.

The invention also concerns a Rogowski coil and an associated protection system connected to said coil via conductors, said conductors comprising connectors to connect a test device according to the invention.

Said Rogowski coil can further comprise a memory storing one more characteristic, for example the sensitivity and/or the resistance and/or the inductance, of said coil. One or more conductors can connect said memory to at least one part, for example a compensation stage, of said protection system.

Said protection system can comprise one or more switch(es) and/or one or more circuit breaker(s), the test device comprising means to detect whether at least one switch or at least one circuit breaker opens when a default is detected.

The invention applies to a single-phase conductor or to one or more phase(s) of a system having several, for example 3, phases. Thus, the invention also concerns a single phase or a 3 phase GIS, comprising at least one conductor, at least one Rogowski coil around said at least one conductor and at least one test device according to the invention.

The invention also concerns a method to test a Rogowski coil according to the invention, said coil being arranged around a main conductor, said method comprising:interrupting any primary current circulating in said main conductor;connecting a test device according to the invention to said conductors;injecting in said conductors one or more test signal(s) generated by said test device;processing a signal from said coil to decide whether the coil and its protection system is defective or not, for example by detecting whether said protection system is triggered.

In a device or a method according to the invention, a protection system connected to a Rogowski coil can comprise one or more of:integration means, or an integration stage or device,compensation means, or a compensation stage or device;digitization means or a digitalization stage or device;signal filtering means, or a filtering stage or device;protection means, for example one or more switch(es) and/or one or more circuit breaker(s).

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

An embodiment of the present invention is shown onFIG.3.

It is applied to a Rogowski coil4, for example as disclosed above in connection withFIG.1A, said coil4being arranged around a conductor2in order to measure a current circulating in said conductor. The coil4generates a secondary voltage signal which is supplied to an integrator6forming part of protection system8which controls one or more protection device(s), for example one or more switch(es) and/or circuit breaker(s)15, in case, for example, of overcurrents.

A test device20according to one embodiment of the invention is connected to the conductors13, between the coil4and the integrator6, in order to inject in said conductors a current representative of a signal at the terminals of coil4when it is measuring a current circulating in conductor2. In other words, a test current of some A or some tens of A, for example between 1 A and 50 A or 100 A, results in a current at the outlet of the test device20very comparable to the current generated by the coil when a primary current of several tens of kA circulates in conductor2.

Test device20comprises for example a current generator or source21able to generate a current of for example between 0 and 100 A and an interface tool or device22. Alternatively, a voltage source could be used instead of the current source and of the impedance220(see below) but it is preferable to use a current source in order to simulate a primary current.

FIG.4shows a more detailed representation of an embodiment of test device20.

The generator21comprises an electronic circuit210which controls an AC source213. to generate a 1stalternating voltage between its outlet terminals211,212. Said terminals can be connected to inlet terminals221,222of the interface device22. Thus generator21is able to supply a voltage (1stalternating voltage) to the interface device22. This 1stalternating voltage applied to a resistor220and capacitors223,224of the interface device is able to generate a test voltage Vtest representative of the voltage at the terminals of the coil4.

The generator21can further comprise one or more memory/ies214to store waveform data in order to simulate one or more default(s), for example over-currents or over intensities or over-voltage on one or more phases, to test the elements or components downstream of coil4, including the protection device8. An over-current, respectively an over-voltage, is a current, respectively a voltage, that the machine cannot accept except during a very short time: the machine has a rated current, of for example 3000 A eff, which it can permanently accept, but it can accept an overcurrent higher than the rated current, for example at least 2 times the rated current, only for a short period of time, for example for some 10 ms and some s, or between 50 ms and 5 s; beyond the end of that period of time, the machine must be stopped or the current interrupted.

An operator and/or a computer or a controller can select one or more of these data files and the electronic circuit210can provide the system (interface device22) with the corresponding signal comprising or simulating said one or more simulated default(s).

Interface device22comprises a resistor220and inlet terminals221,222to supply said resistor220with the outlet voltage provided by generator21. Resistor220has a value such that the alternating current i1circulating therein is representative of the primary current usually circulating in conductor2, for example between 50 and 100 kA.

As explained below, the value of resistor220can be adapted depending on the test performed.

The terminals of the resistor can be connected, for example through one or more capacitor(s)223,224, to outlet terminals231,232of interface device22.

Said one or more capacitor(s)223,224are test capacitors. In particular, this/these capacitor(s) will block any direct current or direct voltage component so that it is not supplied to the system comprising the conductors13and the protection device8. As explained below, the value of these capacitor(s) can be adapted depending on the test performed.

The voltage between outlet terminals231,232is representative of, or very comparable to, the secondary voltage at the outlet of coil4; for example it is comprised between 0.5 V and 10V.

Interface device22can furthermore comprise a microcontroller226and a display device228as explained below.

Outlet terminals231,232of interface device22can be connected to the conductors13(upstream of the protection system8) via test plugs or connectors131,132of said conductors. Thus, the system20can provide the conductors13with a test signal (a voltage) which is very similar to the secondary voltage generated by the coil4detecting a primary current in conductor2. The test signals allow testing the conductors13and all other components between (and including) the coil4and the system8, which comprises the integrator6but also a compensation stage61(this stage being fed with information from a memory, for example an EEPROM80, about the sensitivity and/or the resistance and/or the inductance of the coil4) and/or one or more filter(s) and/or one or more protection device(s)15(for example one or more switch(es) and/or circuit breaker(s)) to protect against defaults such as over currents or over intensities, and/or faults to ground etc.

In particular, it is thus possible to test the one or more protection device(s) of the system8,15: if the test device20injects into the conductors13a signal comprising one or more default(s), but the protection device does not trigger the switches or circuit-breakers15and/or does not interrupt the current circulating in conductor2, then it is decided that the protection system does not work properly and should be checked and possibly repaired. In other words, the simulated default is seen by the system and it is possible to check whether a signal is generated to open the switch(es) and/or the circuit breaker(s)15. If the protection device triggers the switches or circuit-breakers15or interrupts the current circulating in conductor2, then it is decided that the protection system works properly.

A signal representing the current inside the system8can be derived through conductors93which can for example be connected to test plugs831,832of conductors83connecting the memory80to the compensation stage61. This signal can be supplied to inlet terminals241,243of the interface device22and then, for example, to microcontroller226and to display device228.

FIG.8is an example of a test voltage (curve I) generated by a test device according to the invention, curve II (90° out of phase with respect to curve I) being the voltage measured or detected between the plugs831,832.

The microcontroller226can be programmed or adapted to regulate the parameters of the electrical components of the interface device22, for example resistor220and/or capacitor(s)223,225, depending on the signal received from the system8. One or more data of memory80, in particular one or more of the above-mentioned data about the coil4, can also be provided to microcontroller226. Rogowski coils can have different properties from each other, in particular different sensitivities; the information about these properties, for example the sensitivity, can therefore be read from said memory80and can be used when regulating the electrical components of the interface device22.

FIG.5shows an equivalent diagram of the circuit disclosed above in connection withFIG.4together with the integration stage6(which forms part of the protection system8). The references designate the same elements or components as above. Reference213′ is a current source connected to the terminals of the resistor220. The Rogowski coil4is represented by resistor42and capacitor41. The secondary signal generated by the coil4is integrated by the integrator6.

When a test according to the invention must be performed, any current circulating in the primary circuit (in the conductor2) is interrupted. The Rogowski coil therefore does not generate any secondary voltage and is ready for a test.

For a test, the system8and the conductors13remain connected to coil4. The interface device22is connected to generator21and to conductors13as shown onFIG.4. The test circuit is therefore connected in parallel to the Rogowski coil4.

The current source213′ generates a test current IT which is representative of the default current, for example of a short-circuit current.

The test current overwhelmingly circulates through resistor220(which has an impedance R220which is small with respect to the impedance of the other branch of the circuit (capacitors222,223and coil4); for example: 10×R220<ZCT1+Z4+ZCT2.

The voltage at the terminals of220(for example 1V for 5 A and R220=0.2□) is very comparable to the voltage at the terminals of coil4when a primary current is circulating in conductor. The voltage URS (in phase with current IT) between the terminals of resistor220is equal to the sum of the voltages between the terminals of the capacitors223,224and of the coil4: URS=VC1+VC2+Vtest. Vtest simulates the behaviour of Rogowski coil4in reaction to a primary current and depends on the respective values of the capacitors223,224, of resistor42and of inductance41. Preferably, the values of capacitors223,224and of resistor42are selected so that Vtest is comparable to the voltage at the terminals of coil4when it is subject to primary current IT. The values of capacitors223,224are also preferably selected so that Vtest is phase shifted by 90° with respect to the current generated by generator213′ as shown onFIG.8(curve I being the test voltage and curve II being the current, for a circuit illustrated on this figure and having the following characteristics: R220=200 m□, C224=C223=6 μF): for this reason, their impedance is preferably much higher than the impedance Lcoil of the coil41(10×Z41<ZCT1+ZCT2).

The one or more capacitors CT1, CT2behave like coupling capacitors allowing a derived or a test alternative current to circulate in the Rogowski coil which is in an idle state (no primary current is circulating in conductor2). The voltage generated at the terminals of the Rogowski coil mirrors the behaviour of the coil as if it was detecting a current in conductor2.

When a current (primary signal) circulates in conductor2, the Rogowski coil generates a secondary signal, wherein the resistance of the coil does not play any significant role.

A test performed according to the present invention makes use of the resistance of the Rogowski coil (by circulating a current therein) to generate the secondary voltage, independently from the Rogowski effect.

Example values which can be used for a test device according to the invention are as follows:an effective current (rms) IT of some A;a resistor220between 0.1 Q and 1 Q;an amplitude of voltage URS of about 1 V;capacitors223and224between 1 iiF and 10 iiF each;Rcoil (resistance of resistor42) between 10 Q and 100 Q;Lcoil (inductance41) between 10 iiH and 200 iiH;an effective voltage (rms) of Vtest between 10 mV and 100 mV (for example with peak values of Vtest up to 10V);a current Itest between 1 iiA and 10 iiA.

More precise values will be selected based on the actual characteristics of each system.

A test device according to the invention offers the following advantages.

The main circuit (including coil4, conductors13and system8) is protected from common mode signals through the capacitors223,224.

A default of the Rogowski coil (for example a cut or a short-circuit of wire3) can be detected since it will affect Vtest. Actually, the whole chain from and including the Rogowski coil4to the complete protection system8, including any filtering and/or integrating stage, can be tested, which is not the case with usual digital signals directly sent to the protection system8(as onFIG.2A).

Complex waveforms can be injected through the test circuit according to the invention, just like with conventional circuits.

A test circuit according to the invention allows injecting currents of the order of some Amp into the protection circuit8and the coil4, which are representative of a primary current (in conductor2) of several tens or hundreds of kA.

When a test device according to the invention is implemented, the primary circuit circulating in the conductor2is interrupted and the Rogowski coil4therefore does not generate any voltage. A test implementing the invention can be performed.

The invention was disclosed in connection with one conductor2or one phase.

FIG.6represents an enclosure60containing 3 conductors2,102,202, for example conductors of a 3-phases system, a Rogowski coil4,104,204being arranged around each of said conductors. Each of these coils can be tested with a test device or a test method according to the invention. For example, the same test device20can successively be connected to the conductors13associated with each of the 3 Rogowski coils.

In any embodiment according to the invention, the signal from each test can be provided to a control unit, for example a computer or a controller or a microprocessor (not illustrated on the figures), to sample and/or process said signal; it can generate a test signal as shown onFIG.9A, including some default(s).FIG.9Ashows for each of 3 phases a test voltage including a simulated default (a sudden voltage drop on 2 phases) at about t=0.1 which generates a surge in current i of these phases (seeFIG.9B). Device21or210(FIGS.3,4) can be programmed so that an alarm is triggered when the simulated current increases above a threshold300, here at about 5 A.

FIG.7represents a Rogowski coil4implemented around a conductor2. The signals from the coil can be sent to a converter32to convert said signal into optical signals. Other signals from one or more other sensor(s), for example a capacitive sensor34, can also be converted by a converter36into optical signals. The optical signals can be merged in a merging unit28. The merging unit can comprise a microprocessor to process data and can have calculation capabilities. Rogowski coil4can be tested according to the teaching of the present invention. The same system may be implemented for several coils, like for example onFIG.6.

A test device20according to the invention can be easily disconnected from and connected to test plugs131,132of the conductors13connected to the terminals of a Rogowski coil4. A test according to the invention therefore does not need to implement or to activate any switching element which could create disturbances. Indeed, currents circulating on the secondary side of a Rogowski coil are rather weak: if the conductors had to be interrupted to make a test, other test, namely continuity tests, should be performed to reconnect the conductors (the contact impedance of the connections could result in a fainting of the signal).

A test device20according to the invention offers flexibility because the interface device22can be:disconnected from the conductors13and connected to the conductors13of another Rogowski coil;and/or disconnected from generator21and connected to another test signals generator.