WO2018174711A1

WO2018174711A1 - Measurement system for monitoring gas insulated system

Info

Publication number: WO2018174711A1
Application number: PCT/NL2018/050174
Authority: WO
Inventors: Armando Rodrigo Mor; Luis Carlos CASTRO HEREDIA
Original assignee: Technische Universiteit Delft
Priority date: 2017-03-20
Filing date: 2018-03-20
Publication date: 2018-09-27
Also published as: NL2018552B1

Abstract

Measurement system for monitoring a gas insulated system, the gas insulated system comprising at least two GIS segments (2). Each of the at least two GIS segments (2) has a central conductor (3) and an outer conductor (4) coaxially positioned around the central conductor (3). An insulating spacer (5) is positioned between two adjacent outer conductors (4) of the at least two GIS segments (2), and an electrical conducting element (6) is in electrical connection with the two adjacent outer cylindrical conductors (4). The measurement system (1) further has a current sensor (7) in measuring relationship with the at least one electrical conducting element (6), which is e.g. formed by one of a plurality of fixing rods (8) extending through the spacer (5).

Description

Measurement system for monitoring gas insulated system

Field of the invention

The present invention relates to a measurement system for monitoring a gas insulated system (GIS), the gas insulated system comprising at least two GIS segments, each of the at least two GIS segments comprising a central conductor and an outer conductor coaxially positioned around the central conductor, and an insulating spacer positioned between two adjacent outer conductors of the at least two GIS segments.

Background art

The international patent publication WO2007/097491 discloses a system for partial discharge (PD) detection of gas insulated switchgear, using an ultra-high frequency (UHF) antenna installed in a recess formed in an insulating spacer of the switchgear. In general, using a UHF antenna for PD detection requires the presence of a window in an outer side of the switchgear, which allows transmission of radiofrequency (RF) radiation to detect RF radiation inside the gas insulated switchgear originating from a PD event.

US patent publication US2016/349303 and the equivalent British patent publication GB-A- 2538199 disclose PD detection using a slot antenna, parts of which are formed by bolts connecting end flanges of GIS compartments with an insulating spacer in between. Although the insulating spacer is here used to accommodate the antenna, this system still only allows to detect the radiation caused by a partial discharge event.

British patent publication GB-A-2444613 describes a partial discharge detection device for gas-insulated equipment in which a high voltage conductor is supported by an insulator inside hermitically sealed and gas-filled metal containers.

US patent publication US2007/1 15008 describes a pulse current sensor (not a GIS system) comprising an entrance coaxial conductor and an exit coaxial conductor that are interconnected by a continuous inner conductor. The outer conductors are interrupted and are interconnected by a sensing resistor with a substantially constant resistance.

US patent publication US-B-7,741 ,853 describes a ground-fault sensor that has a plurality of conductors each disposed one inside of another except for an outer conductor and a field sensor configured to sense an electric field, a magnetic field, or both. In some embodiments, the field sensor is disposed adjacent the outer conductor.

US patent publication US2015/204936 describes a partial discharge sensor system similar to the one disclosed in GB-A-2444613, with a plurality of bolts connecting flanges formed on the outer tubes. A bowtie antenna is positioned inside a metal casing in a direction orthogonal to a circumferential direction of the flanges, to allow reception of a signal caused by a partial discharge.

British patent publication GB-A-2474125 describes a partial discharge detector system similar to the one disclosed in GB-A-2444613, with a plurality of bolts connecting flanges formed on the outer tubes. A slot antenna is positioned in contact with or near to an insulating spacer between two flanges.

Summary of the invention

The present invention seeks to provide an improved measurement system for detecting partial discharges in a gas insulated system.

According to the present invention, a measurement system as defined above is provided, the gas insulated system further comprising an electrical conducting element in electrical connection with the two adjacent outer cylindrical conductors, and the measurement system comprising a current sensor in measuring relationship with the at least one electrical conducting element. This measurement system allows to measure a current caused by a partial discharge event in one of the at least two GIS segments. This would furthermore allow to evaluate the pulse charge by means of partial discharge charge evaluation techniques.

Short description of drawings

The present invention will be discussed in more detail below, with reference to the attached drawings, in which

Fig. 1 shows a schematic view of a gas insulated system having multiple segments;

Fig. 2 shows a partial cross sectional view of a first embodiment of the present invention measurement system;

Fig. 3 shows a partial cross sectional view of a second embodiment of the present invention measurement system;

Fig. 4 shows a partial cross sectional view of a third embodiment of the present invention measurement system;

Fig. 5 shows a schematic view of a measurement system according to an embodiment of the present invention; and

Fig. 6 shows a partial cross sectional view of an even further embodiment of the present invention measurement system.

Description of embodiments

The present invention relates to a measurement system 1 for gas insulated systems (GIS) or Gas insulated lines (GIL) as employed in distribution of electrical energy, e.g. as an underground cable. The measurement system 1 is particularly suited for detecting a partial discharge event in a GIS or GIL (which in the description hereinafter will be indicated in general by the term 'GIS').

Fig. 1 shows a schematic view of a gas insulated system having multiple GIS segments 2. Each GIS segment 2 comprises an inner or central conductor 3 and an outer conductor 4. Two adjacent GIS segments 2 are connected to each other by means of an insulating spacer 5 and a plurality of fixing rods (or bolts) 8. The space between the central conductor 3 and outer conductor 4 are filled with an insulating gas in operation, allowing transport of high voltage energy through the GIS segments 2. The arrangement of central conductor 3 and outer conductor 4 can be fully symmetrical, and in case of a cylindrical outer conductor 4 this results in a coaxial arrangement. Alternatively, several central conductors 3 may be present in an arrangement allowing maximum spacing from the outer conductor 4.

A complete GIS installation is a combination of various GIS segments 2, in the form of GIS sections, such as non-switching compartments, gas insulated lines, and elements like bushings, spacers, circuit breakers, disconnector switches, earthing switches, voltage and current measuring transformers. In a GIS segment 2, an insulation defect may occur during operation, which usually results in a partial discharge (PD), indicated by reference numeral 10 in Fig. 1 . When a PD occurs at a certain location of the insulation system, resulting PD current pulses 1 1 a, 1 1 b propagate along the GIS as the GIS behaves as a transmission line for the (high frequency content) PD pulses.

In existing GIS installations, UHF antennas are used to detect electromagnetic waves created by the PD pulse and propagating along the GIS segments 2. This necessitates placement of openings or dielectric windows in a GIS segment 2 in which antennas can be placed to pick up the radiated signal produced by a PD event.

According to the present invention embodiments, PD induced current pulses are measured in the outer conductors 4 of a GIS installation. This is possible as the arrangement of a GIS segment 2 having a central (or inner) conductor 3 and an outer conductor 4, i.e. a coaxial structure, acts as receiving electrodes of current pulses 1 1 a, 1 1 b induced by a PD event 10 within the GIS segment 2.

As shown in the general structure of the GIS installation of Fig. 1 , the insulating spacers 5 which are used to connect two GIS segments 2 together, would interrupt the coaxial structure, and the PD current pulses would flow through the fixing rods 8 connecting two end parts of two adjacent outer conductors 4. The current which would be homogeneously distributed over the outer conductor 4 is split into several current paths, i.e. over the plurality of (metallic) fixing rods 8 extending through the insulating spacer 5. By measuring the current through one of the fixing rods 8, or in more general terms (see also the embodiment described with reference to Fig. 4 below) through an electrical conducting element 6 connecting two adjacent outer conductors 4, it is then possible to detect the PD current pulses 1 1 a, 1 1 b.

In a generic sense, the present invention embodiments relate to a measurement system 1 for monitoring a gas insulated system (e.g. for detecting partial discharges in the gas insulated system), the gas insulated system comprising at least two GIS segments 2, each of the at least two GIS segments 2 comprising a central conductor 3 and an outer conductor 4 coaxially positioned around the central conductor 3, an insulating spacer 5 positioned between two adjacent outer conductors 4 of the at least two GIS segments 2, and an electrical conducting element 6 in electrical connection with the two adjacent outer cylindrical conductors 4. The measurement system 1 comprises a current sensor 7 in measuring relationship with the at least one electrical conducting element 6. The measuring relationship of the current sensor 7 with the at least one electrical conducting element 6 may be implemented using different techniques and structures, as will be explained with reference to the embodiments as shown in Fig. 2-4.

Fig. 2 shows a partial cross sectional view of a first embodiment of the present invention measurement system, wherein the current sensor 7 is integrated into the insulating spacer 5. The current sensor 7 e.g. comprises a casing 20 with an insulating inner sleeve 21 , in combination encasing a ferrite core and sensor coil.

In this embodiment, the at least one electrical conducting element 6 is one of the plurality of fixing rods 8, mechanically connecting adjacent end parts 9 of the two adjacent outer conductors 4 and extending through the insulating spacer 5. As shown in this exemplary embodiment, the end part 9 comprises a flange 9 extending radially from the respective outer conductor 4. This allows to easily and effectively connect the two outer conductors 4 in a mechanically strong manner, e.g. by providing holes in the flanges 9 and spacer 5, and fastening the fixing rods 8 using a nut 15 and ring 16. As indicated in Fig. 2, an insulating sleeve 17 may be provided in the hole in flange 9 serving dielectric purposes as well as mechanical purposes, e.g. to allow easy insertion of the fixing rods 8. If the plurality of fixing rods 8 is provided evenly distributed over the circumference of the end part 9, a proper sealing and structural integrity of two adjacent GIS segments 2 is ensured.

Fig. 3 shows a partial cross sectional view of a second embodiment of the present invention measurement system, wherein the insulating spacer 5 is not modified to include the current sensor 7. In this embodiment (and in the embodiment shown in Fig. 2), the at least one electrical conducting element 6 is one of the plurality of fixing rods 8. In other words, the current sensor 7 is provided as a coaxial arrangement around one of the fixing rods 8.

In the embodiment shown in Fig. 3, the flow of the current originating from the partial discharge event will follow the entire length of the (conducting) fixing rod 8 because of the insulating sleeve 17, in order to have the total current flow through the core of the current sensor 7. In order to minimize disadvantageous effects (because of the added path length for the current) the width of the current sensor 7 is chosen to be as low as possible. With reference to Fig. 3, the two outer conductors 4 of adjacent GIS segments 2 are attached to each other with an insulating spacer 5 in between by sixteen fixing rods 8. Without the nuts 15 and rings 16 there would be no electrical connection between the fixing rods 8 and the outer conductors 4, because of the insulating sleeve 17. In the embodiment shown in Fig. 3 (with the installed nuts 15 and rings 16) the current coming from one outer conductor 4 flows through the (metallic) fixing rod 8 and the current sensor 7, and passes to the other outer conductor 4 via the housing 20 of the current sensor 7. As at the location of the current sensor 7 the partial discharge induced current flows through an inner conductor (fixing rod 8) and returns through an outer conductor (housing 20 of current sensor 7), this configuration is also dubbed a coaxial arrangement.

In a specific embodiment, the current sensor 7 comprises a magnetic core surrounding the at least one electrical conducting element 6, a coil being wound around the magnetic (e.g. ferrite) core. The current sensor 7 may be a high frequency current transformer (HFCT), wherein the at least one conducting element 6 can be seen as the primary winding, and the coil around the magnetic core as the secondary winding. In alternative arrangements, the current sensor 7 comprises a magnetic field sensor positioned in vicinity of the at least one electrical conducting element 6, e.g. attached to an outer surface of the insulating spacer 5. In high voltage systems, it is possible to implement this embodiment specifically using a Rogowsky coil. In general, the current sensor 7 is in a measuring relationship with the at least one electrical conducting element 6, and transforms a current flowing in the at least one electrical conducting element into a measurement voltage across terminals of the current sensor 7.

Fig. 4 shows a partial cross sectional view of an example of a third embodiment of the present invention measurement system, wherein the at least one electrical conducting element 6 is an external conductor 6 connecting adjacent end parts 9 of the two adjacent outer conductors 4, and is positioned external to the insulating spacer 5. Compared to the embodiment shown in Fig. 2, the external conductor 6 may be connected in electrical connection to the end parts 9 using the nut 15 to secure the external conductor 6 to the surface of the end part 9. If the fixing rod 8 is a metallic material bolt, the external conductor 6 shunts the fixing rod 8, and the current induced by a PD pulse normally running through that fixing rod 8, will spread in two separate current paths, still allowing to detect the PD current pulse in the path of the external conductor 6.

In a further embodiment, one of the plurality of fixing rods 8 is an insulating rod, and the external conductor 6 is mechanically connected to the insulating rod (and electrically to the end part 9). In this case, depending on the (HF) impedance of the external conductor 6, a larger current part will flow through the external conductor 6 allowing easier detection of a PD pulse.

In both embodiments, the external conductor 6 may be selected from the group comprising a flexible cable (e.g. a braided wire, a Litze wire, etc.), a metal strip (in the form of a jumper, clip, etc.), or a stack of metal strips.

Furthermore, in all embodiments described above, the part of the current flowing through the at least one electrical conducting element 6 may be enhanced by properly selecting the material of the at least one electrical conducting element 6, as well as the material of the remaining ones of the plurality of fixing rods 8. E.g. in the exemplary embodiments shown in Fig 2 and 3, the fixing rod 8 acting as the at least one electrical conducting element 6 may be made of a better conducting material, such as copper.

Using the present invention embodiments, it is possible to have the current sensor 7 to be associated with e.g. only one of the fixing rods 8, even though in that case only a part of the PD induced current can be measured. To improve the accuracy, sensitivity and/or interference resistance of the measurement set-up, in further embodiments the PD induced current is channelled to only one or less than all fixing rods 8. In a specific embodiment, this is implemented by use of non-conductive nuts 15 and rings 16 for one or more of the fixing rods 8, thus increasing current through specific ones of fixing rods 8 acting as the at least one electrical conducting element 6 operatively coupled to current sensor 7. It will be apparent that also further isolating elements may be used to channel a possible PD induced current away from one or more of the fixing rods 8. In an even further group of alternative embodiment, accuracy, sensitivity and/or interference resistance of the measurement set-up is improved using multiple current sensors 7 associated with a similar number of fixing rods 8 in electrical connection with adjacent outer conductors 4 (or the end parts 9 thereof).

Fig. 6 shows a partial cross sectional view of yet a further embodiment of the present invention, wherein the outer conductors 4 are connected at their respective end parts 9 using an alternative embodiment of the spacer 5 (and fixing rods 8). In this embodiments the spacer 5 comprises a main spacer body 5a of isolating material, fixing the central conductor 3 in position with respect to the outer conductors 4. The main spacer body 5a in this embodiment does not extend radially all the way up to the end of flanges 9 of the outer conductors, but to a more limited radius, e.g. about equal to the outer radius of the outer conductors 4. In addition a secondary spacer body 5b is used, which radially extends from the spacer main body 5a, having through holes for installing the fixing bolts 8. This allows for an easier manufacture of the secondary spacer body 5b with the through holes (in the form of a single material ring shaped body). The material of the secondary spacer body 5b may be the same as the spacer main body 5a, but alternatively may be made of a conducting material.

If the secondary spacer body 5b is of a conducting material, the present invention embodiments can still be implemented if an isolating layer 5c is provided, which as indicated in the embodiment of Fig. 6 isolates the conductive secondary spacer body 5b from the flange 9 of one (or both) of the outer conductors 4. Especially the embodiment described above with reference to Fig. 3 and 4 can still be easily applied.

The isolating layer 5c can be implemented in various embodiments, e.g. as a disc shaped ring of insulating material (plastic, paper, etc.). The radial dimensions can be similar to the radial dimension of the secondary spacer body 5b, or alternatively extend further inward to (partly) overlap the spacer main body 5a. It is noted that the insulation properties of the isolating layer 5c only need to be considered in relation to the small currents induced by the PD's and the relatively low resistance of the at least one electrical conducting element 6, and as a result the isolating layer 5c only needs to be relatively thin. The isolating layer 5c may even be implemented as a non-conductive (or isolating) coating of the surface of the flange 9 of the outer conductor 4 and/or a non-conductive (or isolating) coating of the surface of the secondary spacer body 5b.

In all variants, it is possible to add the current sensor 7 and associated arrangement to a GIS in a very easy and cost-effective manner, without needing to modify any (standard) structural parts of the GIS, such as the GIS segments 2.

Fig. 5 shows a schematic view of a measurement system according to an embodiment of the present invention, wherein one of the above described arrangements of current sensor 7 is applied. The measurement system 1 further comprises an evaluation unit 12 connected to the current sensor 7, the evaluation unit 12 being arranged to execute a charge evaluation of detected current pulses. A charge evaluation of the detected current pulse can to a predetermined level of uncertainty result in a reliable and robust detection of a partial discharge (or in a further alternative of an insulation defect) in one of the GIS segments 2. It is noted that normally, charge evaluation is not used for detection, but to assess the severity of the defect causing the partial discharge. In the standards, the maximum amount of charge is specified (normally in picoCoulomb (pC)) that is allowed in factory tests. The present invention measurement system has the advantage that it is possible to calculate a charge, as a result of which it is possible to compare on-line field measurements with laboratory tests. To ensure proper detection the current sensor 7 and evaluation unit 12 circuitry have appropriate characteristics, such as a lower cut-off frequency of30kHz and an upper cut-off frequency above 60 MHz. In short a compromise will be selected with a trade-off between sensitivity and bandwidth.

In a further embodiment, the evaluation unit 12 is further arranged to determine the ratio of measurement current (or charge) through the at least one electrical conducting element 6 and a current (or charge) through the outer conductor 4 caused by a pulse event (such as a partial discharge pulse or an insulation defect). From this ratio, which can be determined by (electromagnetic) simulation, or by calibration, the original pulse shape can be reconstructed and hence a proper charge estimation can be accomplished. Calibration can be implemented using a controlled current injection and measurement. E.g. a PD calibrator may be placed at a location in a GIS segment 2, and connected between the central conductor 3 and outer conductor 4. Note that for such calibration standards may apply, such as IEC60270. A predetermined current of a known charge value is then injected, and the current sensor 7 is used to measure the resulting current at the measurement location, allowing the evaluation unit 12 to calculate the associated charge. The ratio of the measured charge and the injected charge is then the ratio of the charge measured by the current sensor 7 to the total charge flowing in the outer conductor 4. Knowing this ratio of charges, the total charge flowing through the fixing rods 8 and at least one electrical conducting element 6 of the insulating spacer 5, and thus through the outer conductor 4 can be estimated.

As an example, the measurement system 1 may be applied to measure at the location of an insulating spacer 5 between two GIS segments 2 which are mechanically connected to each other using sixteen rods 8 at a circumference of the associated outer conductors 4. It is noted that the measurement system 1 , and especially the form and type of combination of current sensor 7 and at least one conducting element 6, may be of an influence on how the current pulse 1 1 a, 1 1 b originating from the partial discharge 10 spreads over the outer surface of outer conductors 4 and through each of the rods 8. In this respect, especially the resistance and inductance from the current sensor 7 play a role. The ratio in an exemplary embodiment using sixteen rods 8, and a current sensor 7 in a coaxial relation to one of the rods 8, has been determined using the injection calibration method to be 3%. In a further embodiments, where the at least one conducting element 6 was embodied as a stack of ten copper mutually isolated strips and electrically connected to two adjacent outer conductors 4 at the position of an isolating rod 8, in combination with an HFCT sensor 7, the ratio was determined to be 1 .5% (again using the injection calibration method).

In an even further embodiment (as shown in the exemplary schematic view of Fig. 5), the measurement system further comprises at least one additional current sensor 7a connected to the evaluation unit 12, the at least one additional current sensor 7a being associated with an additional insulating spacer 5' between two further GIS segments 2. If current pulses are detected at several spacers 5, 5'it is possible to determine the location of a partial discharge, e.g. by evaluating the polarity of the current pulses. As shown in the embodiment of Fig. 5, a PD will cause a left travelling half pulse 1 1 a and a right travelling half pulse 1 1 b. If all current sensors 7, 7a are installed with the same polarity, a PD pulse from a GIS segment 2 outside of all current sensors 7, 7a will have the same polarity. A PD pulse from a GIS segment 2 within all current sensors 7, 7a will cause a different polarity in some of the current sensors 7, 7a, allowing determination of the PD event location.

Also further complex determinations can be made by proper testing protocols being executed by the evaluation unit 12. E.g. the evaluation unit 12 may be further arranged to detect a position of a partial discharge in one of the GIS segments 2 by measuring timing differences between congruent current pulses as detected by the current sensor 7 and one or more additional sensors 7a.

Furthermore, these embodiment would allow even better detection of a partial discharge event in one of the associated GIS segments 2, as the measurements by the current sensor 7 and at least one additional current sensor 7a may be used in a redundant manner.

Alternatively or additionally, the evaluation unit may be further arranged to detect a faulty GIS segment 2 by matching current pulse measurements from the current sensor 7 and one or more additional sensors 7a. A faulty GIS segment 2 may e.g. be caused by the presence of a leakage current, or a short circuit. In general such events cause currents which are several orders of magnitude larger than a current pulse caused by a PD event. This may be detected and evaluated by the evaluation unit 12. The measured current magnitude in several measurement locations (current sensors 7, 7a) will indicated the location of the faulty GIS segment 2.

These more complex measurement methods (location determination of a PD and fault detection) may be advantageously used when gas insulated lines (GIL) are employed, which typically have a larger dimension in length, and which (in operation) may be much more difficult to access physically.

The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.

Claims

1 . Measurement system for monitoring a gas insulated system, the gas insulated system comprising

at least two GIS segments (2), each of the at least two GIS segments (2) comprising a central conductor (3) and an outer conductor (4) coaxially positioned around the central conductor (3), an insulating spacer (5) positioned between two adjacent outer conductors (4) of the at least two GIS segments (2), and

an electrical conducting element (6) in electrical connection with the two adjacent outer conductors (4),

the measurement system (1) comprising a current sensor (7) in measuring relationship with the at least one electrical conducting element (6).

2. Measurement system according to claim 1 , wherein the at least one electrical conducting element (6) is one of a plurality of fixing rods (8) mechanically connecting the two adjacent outer conductors (4) and the insulating spacer (5).

3. Measurement system according to claim 1 or 2, wherein the current sensor (7) comprises a magnetic core surrounding the at least one electrical conducting element (6).

4. Measurement system according to claim 3, wherein the current sensor (7) comprises a high frequency current transformer (HFCT).

5. Measurement system according to claim 1 or 2, wherein the current sensor (7) comprises a magnetic field sensor positioned in vicinity of the at least one electrical conducting element (6).

6. Measurement system according to claim 3 or 5, wherein the current sensor (7) comprises a Rogowsky coil.

7. Measurement system according to any one of claims 1 -6, wherein the current sensor (7) is integrated into the insulating spacer (5).

8. Measurement system according to claim 1 or 2, wherein the at least one electrical conducting element (6) is an external conductor (6) connecting adjacent end parts (9) of the two adjacent outer conductors (4), and is positioned external to the insulating spacer (5).

9. Measurement system according to claim 8, further comprising

a plurality of fixing rods (8) mechanically connecting the two adjacent outer conductors (4) and the insulating spacer (5), wherein one of the plurality of fixing rods (8) is an insulating rod, and the external conductor (6) is mechanically connected to the insulating rod.

10. Measurement system according to claim 8 or 9, wherein the external conductor (6) is selected from the group comprising:

a flexible cable, a metal strip, a stack of metal strips.

1 1 . Measurement system according to any one of claims 1 -10, wherein the measurement system (1) further comprises an evaluation unit (12) connected to the current sensor (7), the evaluation unit (12) being arranged to execute a charge evaluation of detected current pulses.

12. Measurement system according to claim 1 1 , wherein the evaluation unit (12) is further arranged to determine a ratio of measurement current through the at least one electrical conducting element (6) and a current through the outer conductor (4) caused by a pulse event.

13. Measurement system according to claim 12, wherein the evaluation unit (12) is further arranged to determine the ratio by simulation.

14. Measurement system according to claim 12, wherein the evaluation unit (12) is further arranged to determine the ratio by calibration.

15. Measurement system according to any one of claims 1 1 -14, further comprising at least one additional current sensor (7a) connected to the evaluation unit (12), the at least one additional current sensor (7a) being associated with an additional insulating spacer (5') between two further GIS segments (2).