Patent Application
20180106852
The present invention relates to a high-voltage cable accessory and, more particularly, to a field control element for a high-voltage cable accessory.
High-voltage cable accessories are used to connect high-voltage power cables to electrical equipment such as overhead lines, transformers, search arrestors, switchgears, etc. The accessories either insulate or control the electrical field of power cables at different ambient conditions. High-voltage cable accessories are expected to have a lifetime of more than 40 years without any failure. In order to achieve this requirement, the insulation system of the cable accessory and its performance have to be checked regularly. Moreover, due to the fact that a manual installation is always necessary for erecting a termination or installing a cable joint, faults during the installation must also be considered; such mistakes can lead to an electrical breakdown with a subsequent arc inside the termination. Consequently, there is a need for a diagnostic method that provides information about the current status of the cable accessories including their remaining lifetime.
The most commonly used diagnostic method for providing real time information about the status and the remaining lifetime of cable accessories is the electrical partial discharge (PD) measurement. Electric discharges that do not completely bridge two electrodes or conductors in close proximity to each other are called partial discharges. Originating with defects such as cavities or inclusions and interfaces to other materials, in particular to conductors carrying high-voltage, partial discharges lead to the formation of “trees” that grow over time and eventually cause an electric break down. Such PD trees typically have a length of more than 1 μm. The magnitude of such partial discharges is usually small and the amount of charge transferred is in the range of 10 to a few hundred Pico-Coulombs (pC). For a partial discharge test, a suitably high AC or DC voltage is applied to the conductors surrounding the insulation material under test. Alternatively, the discharge can be detected under normal operating conditions depending on the defect and energized system characteristics.
The most commonly used diagnostic method for obtaining the status of high-voltage cable accessories is the electrical partial discharge measurement according to the IEC standard 60270 (“High-voltage test techniques—Partial discharge measurement”, IEC 60270:2000). This setup consists basically of a high-voltage source (which could be based on different techniques such as standard 50 Hz or resonant system), a coupling capacitance for signal extraction, and a quadrupole for the adaption of the PD signals for direct measurement. Each partial discharge event causes a short current signal which can be detected with the connected measuring device.
PD diagnostics is therefore based on the measurement of electrical signals with very small amplitude. A disadvantage of this technique is that due to the small amplitude it is very sensitive to electrical noise caused by external electrical fields such as from transformers, overhead lines, etc. Consequently, the electrical partial discharge measurement in a noisy environment (e. g. during on-site tests) does not always allow a proper interpretation of the partial discharge measurement results and thus an understanding of the condition of high-voltage equipment is not possible.
Partial discharges also produce light and therefore it is possible to detect partial discharges by measuring the light generated thereby. Such an optical PD measurement advantageously is not affected by electrical noise from the surrounding high-voltage equipment. For instance by using fiber optic sensors that are integrated into high-voltage accessories, a real time damage monitoring based on an optical PD measurement could be demonstrated. Such a fiber optic sensor arrangement using fluorescent optical fibers is for instance described in the article W. R. Habel et al.: “Fibre-optic sensors for early damage detection in plastic installations of high-voltage facilities”, XVII International Symposium on High-voltage Engineering, Hannover, Germany, Aug. 22-26, 2011. A detailed description of this system used with stress cones is also given in the PhD Thesis “Dielectric strength behavior and mechanical properties of transparent insulation materials suitable to optical monitoring of partial discharges” by Chaiyaporn Lothongkam, Fakultät für Elektrotechnik and Informatik der Gottfried Wilhelm Leibniz Universitat Hannover, 25 Jul. 2014.
This conventional system, however, suffers from non-satisfactory signal strength in case the partial discharge occurs at a site distanced away from the fluorescent optical fiber. Moreover, only specific fluorescent fiber optic sensors can be used, which have to be mounted as close as possible to the location where the partial discharges have to be detected. In order to achieve a sufficiently high signal yield, this known system needs a large surface of the fluorescent optical fiber to be in contact with the stress cone. Therefore, a long fiber is embedded in an outer region of the stress cone to be arranged helically wound around a longitudinal axis of the stress cone. For mounting the stress cone, however, the cone normally has to be expanded, sometimes by 400% of the initial diameter. The optical fiber suffers from the problem that it cannot be expanded in its length to a comparable extent, so that it can be damaged or break during the mounting process.
A field control element for a high-voltage cable accessory according to the invention comprises an electrically insulating material. The electrically insulating material includes a fluorescent dye adapted to convert a first radiation having a first wavelength and generated by an electrical discharge into a second radiation having a second wavelength longer than the first wavelength.
The invention will now be described by way of example with reference to the accompanying Figures, of which:
Exemplary embodiments of the present invention will be described hereinafter in detail with reference to the attached drawings, wherein like reference numerals refer to like elements. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure will be thorough and complete and will fully convey the concept of the disclosure to those skilled in the art.
The term “high-voltage” as used in the following is intended to relate to voltages above approximately 1 kV. In particular, the term high-voltage is intended to comprise the usual nominal voltage ranges of power transmission, namely medium voltage (about 3 kV to about 50 kV), high-voltage (about 50 kV to about 110 kV), and also extra high-voltage (up to presently about 500 kV). Of course also higher voltages may be considered in the future. These voltages may be direct current (DC) or alternating current (AC) voltages. In the following, the term “high-voltage cable” is intended to signify a cable that is suitable for carrying electric current of more than about 1 A at a voltage above approximately 1 kV. Accordingly, the term “high-voltage accessory” is intended to signify a device that is suitable for interconnecting high-voltage facilities and/or high-voltage cables. In particular, a high-voltage accessory may either be an end termination or a cable joint.
The term “partial discharge” as used in the following therefore is intended to describe short term, low-energy and locally limited discharges within an insulation, which do not lead to a complete electric break down but irreversibly damage the insulation. The term “partial discharge” in particular is intended to comprise so-called “inner partial discharges” which occur within non-gaseous insulating materials.
The wavelength range 101 for discharges in air, as shown in
Within solid materials most of the discharges are related to air discharges as the void contains air or the failure is often caused by air entrapment. Based on this, the characteristic wavelength of partial discharges in plastic insulations, such as field control elements for high-voltage cable accessories, can be assumed to correspond to the wavelength range 101 of discharges in air. However, a direct real time monitoring of the radiation which is generated by discharges within air is currently difficult or impossible due to the very short wavelength; the wavelength of discharges in air has to be transferred into a range at which such detection can be properly conducted.
The fluorescent dye may comprise any suitable fluorophore that does not impair the insulation or the mechanical performance of the insulation material. In an embodiment, the fluorescent dye has an absorption fluorescence between 350 nm and 500 nm, and an emission fluorescence between 500 nm and 800 nm.
A fluorophore (or fluorochrome) is a fluorescent chemical compound that can re-emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or plane or cyclic molecules with several 7C bonds. Fluorophores can be covalently bonded to a macromolecule, for instance polysiloxane. However, fluorophores may also be used alone, for instance be dispersed in a liquid insulator such as silicone oil. The fluorophore absorbs light energy of a specific wavelength and re-emits light at a longer wavelength. The absorbed wavelengths, energy transfer efficiency, and time before emission depend on both the fluorophore structure and its chemical environment, as the molecule in its excited state interacts with surrounding molecules. Wavelengths of maximum absorption (approximately corresponding to the excitation wavelength) and emission (for example, Absorption/Emission=485 nm/517 nm) are the typical terms used to refer to a given fluorophore, but the whole spectrum may be important to consider. The excitation wavelength spectrum may be a very narrow or broader band, or it may be all beyond a cutoff level. The emission spectrum is usually sharper than the excitation spectrum, and it is of a longer wavelength and correspondingly lower energy. Excitation energies range from ultraviolet through the visible spectrum, and emission energies may continue from visible light into the near infrared region. Main characteristics of fluorophores are:
Depending on the chosen fluorescence dye, the re-emission of radiation from the excited molecule is of a lower wavelength 105. This effect is the Stokes shift exhibited by most fluorescent materials. When comparing the re-emitted wavelength range 105 with the wavelength range 106 where the photodiode usually is sensitive, it becomes clear that the photodiode is able to detect almost all of the re-emitted radiation in the range 105, but would be able to detect very little of the primarily emitted radiation in the range 101.
The wavelength conversion takes place within the bulk material of the insulator, especially within a field control element, by providing a fluorescent dye directly within the material of the insulator. Any fluorescent particles which do not have an influence on the installation performance of the insulation material and have a suitable absorption range 104 and re-emission range 105 can be used according to the present invention. In particular, when providing a field control element with the fluorescent dye, the emission by the partial discharges can be transformed into radiation with a higher wavelength than directly at the site of the formation of the partial discharge. This lower energy radiation can pass through the insulator material to be detected by an optical detector. The re-emitted radiation with its lower energy suffers less damping when travelling through the insulator material than the high-energy primary radiation. This enhances the sensitivity of the monitoring system compared to conventional systems that detect the primary radiation by means of fluorescent optical fibers.
The re-emitted radiation may be absorbed by other fluorescence dye molecules because the absorption and re-emission spectra 104, 105 overlap. Consequently, the radiation is amplified on its path to the detector and therefore the light yield can be enhanced compared to known systems using fluorescence dye clad optical fibers. As fluorescence re-emission takes place very shortly after the absorption event, no substantial time delay is caused by this additional step (for commonly used fluorescent compounds, typical excited state decay times for photon emissions with energies from the UV to near infrared are within the range of 0.5 to 20 nanoseconds).
An embodiment of a high-voltage outdoor termination 200, commonly used for overhead lines, is shown in
The high-voltage cable 202 is inserted into the cable end termination 200 and is deprived of its outer layers, such as the cable jacket, cushioning layers and the metallic shield. A normally semi-conductive outer cable shield 220 is inserted into the area of a field control element 212 within the housing 208. Inside the insulating area 218 with the insulating filling, only a cable conductor 210 of the cable 202 surrounded by a cable insulation is present.
The field control element 212 shown in
As shown in
Alternatively or additionally, only limited areas or layers of the field control element 212 may contain a fluorescent dye. The dye may also be embedded in electrically conductive or semi-conductive plastic parts, provided these parts are sufficiently transparent for the absorbed and re-emitted radiation. Moreover, the insulating filling of the insulating area 218 may also contain a fluorescent dye; in this case the filling comprises electrically insulating oil, such as silicone oil with fluorescent particles dispersed therein. Providing the insulating filling with a fluorescent dye can be done in addition or alternatively to providing the field control element 212 with a fluorescent dye. Of course, the same or different fluorophores can be used in the solid and the liquid matrix, respectively.
A detection of the occurrence of partial discharges can be achieved by optically detecting the re-emitted radiation by an optical detector such as at least one photodetector, a photodiode, a CCD, or the like. The optical detector may be located in the vicinity of the field control element 212, but can also be arranged distanced therefrom, for instance within the insulating area 218. Most of the partial discharges are caused in the area of the field control element 212, and in particular close to the deflector 216, because of an improper installation of the field control element 212 or a defective cable treatment which causes air entrapment. Discharges in other areas can be detected for instance electrical discharges in oil due to moisture ingress into the insulating area 218.
An embodiment of the high-voltage outdoor termination 200 with an integrated optical detector for detecting the re-emitted radiation is shown in
As shown in
A coupling end of the optical fiber 222, as shown in
The optical fiber 222 is a polymeric optical fiber (POF) that this inserted into the insulating field control body 214 in the embodiment shown in
In another embodiment shown in
The switchgear termination 300 shown in
The field control element 304, as shown in
A switchgear termination 300 having an optical fiber 222 and an optical transducer 224, analogously to the arrangement of
When a partial discharge occurs, light is emitted and this primary radiation is converted by the fluorescent dye in the polysiloxane material into a secondary re-emitted radiation having lower energy and a longer wavelength. The radiation re-emitted by the fluorescence dye partly enters the optical fiber 222 and is transmitted to the optical transducer 224. The transducer 224 converts the optical signal into an electric signal that may be further processed by additional external electronic devices.
In another embodiment shown in
The cable joint 400, as shown in
The field control elements 406, 408 as well as the main body 414 may be provided with a fluorescent dye. Partial discharges that occur inside the cable joint 414 are absorbed by the fluorescent dye. A re-emitted secondary radiation is generated from the primary radiation caused by the PD and can be detected by any suitable optical detector.
An optical fiber 222, as shown in
The optical transducer 224 can also be connected to a superordinate electronic unit for further processing of the electric signal.
The integration of optical partial discharge detection on the basis of fluorescent silicone rubber in high voltage cable accessories is possible; during different tests it was verified that the modified silicone rubber has no negative impact to the fluorescent behavior and can be used for optical partial discharge detection. The on-site-PD-measurement of the cable accessories disclosed herein have a sensitivity comparable to an electrical partial discharge measurement but avoids complications of noise. Partial discharges can be detected which occur within the fluorescence tagged insulator(s) but also discharges that occur in the optically coupled environment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15173098.3 | Jun 2015 | EP | regional |
This application is a continuation of PCT International Application No. PCT/EP2016/061242, filed on May 19, 2016, which claims priority under 35 U.S.C. § 119 to European Patent Application No. 15173098.3, filed on Jun. 22, 2015.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2016/061242 | May 2016 | US |
| Child | 15847200 | US |
