Title of Invention

A DETECTION OF CORONA USING SCINTILLATING FIBER OPTICS THROUGH AN ELECTRONIC DETECTOR .

Abstract

The present invention relates to detection of corona using scintillating fibre optics through an electronic detector comprising a scintillating fibre (1) provided with a thin cladding core (3) and a mirror (2) at the first end and a coupling (5) at the second end, laid around a high voltage equipment (7) or insulator for sensing ultraviolet emission of corona and emanating visible light of the end of said scintillating fibre (1) a first of a light transmitting fibre (4) connected to one end of said scintillating fibre by a coupling for carrying said visible light transmitting fibre (4), an electronic detector (6) coupled to said second end of said transmitting fibre (4) for receiving said visible light for detecting the ultraviolet radiation and setting in an alarm (14) on detection of corona characterized in that the said corona detector continuously monitor incipient failures in insulation in high voltage systems and prevents large scale brake down by setting up an alarm in the system device in case of emergency.

Full Text

The present invention relates to a corona detector. More specifically, the invention relates to a corona detector using fibre optics. Corona is observed in high voltage systems and is the result of field intensified ionization. An ultra violet purple and blue radiation is emitted during this phenomena. Detection of corona may lead to detection of incipient failures in high voltage systems. Corona or field intensified ionization eventually causes breakdown of insulation resulting in failure of electrical equipment. Corona is a low power arc while true breakdown is a high power arc. It is primarily an alternating current phenomena. Corona discharges that recur at regular intervals are referred to as continuous corona. Examples of this are, discharges that occur every cycle of an applied AC voltage or once in every 30 sec. of an applied direct voltage. The effect of corona on insulation is degradation, which will eventually lead to failure of components. Corona sets up white noise, which can have an adverse effect on communications and sensitive electronic equipments. In addition, corona deteriorates the insulation causing electrical equipment to fail prematurely. This degradation occurs through three mechanisms. Firstly, heat is produced and this has direct and deleterious effect on insulation especially in the case of long term exposure. Secondly, the ozone formed is a powerful oxidizing agent which acts to degrade both natural and synthetic polymer insulation, making it brittle and producing cracks. Finally, the free ions produced by the corona currents have a cutting effect on the insulating material and thus leads to the formation of carbonization tracks and eventual insulation failure. The corona problem grows more severe as voltage and frequency of the electrical system increases. Traditionally for detection, corona can be observed using ultraviolet or infrared cameras. However, the use of such devices in an enclosed systems and panels is rather inconvenient. Such devices are normally used as inspection tools. These are also very expensive devices. One object of the present invention therefore is to provide a low cost corona detector using fibre optics. Another object of the present invention is to provide a corona detector which can be safely arranged inside the high voltage bus ducts or switchgear. An optical device using fibre optics can easily be accommodated inside high voltage bus ducts or switchgear. A further object of the present invention is to provide immunity to electric noise and insulation. The scintillating fibre used is made of flexible plastic material which is immune to electric noise and provides insulation. The present invention utilizes the property of scintillating fibres which can be used as sensor for detecting ultraviolet emission of the corona and for emitting visible light at the two ends of the fibre. Scintillating fibres are plastic fibres containing dyes for absorbing ultraviolet emission and for emanating light in visible band of the light spectrum. The corona detector using fibre optics can thus be used to detect corona and arcing for switchgear and bus ducts, transformers and other cubicle mounted electrical equipment. The scintillating fibres made from flexible plastic material can be conveniently laid within a cubicle. The optical fibre being immune to electrical noise provides insulation, and therefore it is a safe device. The ultraviolet radiation from a corona is sensed through the sides of the scintillating fibre, which has a scintillating efficienty of 3.4 to 7%. A light emanating end of the scintillating fibre is coupled to a light transmitting glass/plastic fibre. The other end of the transmitting fibre carries the light radiation to an electronic detection circuitry for corona detection and setting in an alarm. Thus the present invention provides a corona detector using fibre optics, comprising a scintillating fibre laid around a high voltage equipment or insulator for sensing ultraviolet emission of corona and emanating visible light to the ends of said scintillating fibre; a first end of a light transmitting fibre connected to one end of said scintillating fibre by a coupling for carrying said visible light to a second end of said light transmitting fibre; and an electronic detection circuit coupled to said second end of said transmitting fibre for receiving said visible light for detecting the ultraviolet radiation and setting in an alarm on detection of corona. The invention will now be described in detail with the help of the accompanying drawings, where Figure 1 shows the scintillating fibre used in the present invention; Figure 2 shows the arrangement of a mirror at one end of the scintillating fibre; Figure 3 shows the arrangement of the corona detection device of the present invention; Figure 4 shows a typical installation of a corona detection device of the present invention. Figure 5 shows in block diagram form the electronic detection circuitry of the corona detection device of Figure 3. A free running scintillating fibre 1 is laid around high voltage equipment like potential transformers and current transformers, or a high voltage insulator. The scintillating fibre 1 contains dyes to absorb ultraviolet radiation from the corona and for emanating light in visible band of light spectrum. The scintillating fibre 1 can thus be used as a sensor to detect the ultraviolet radiation of the corona and emit visible light at the two ends of the scintillating fibre 1. A thin cladding 3 can be used for the core 8 of the scintillating fibre 1. This cladding 3 is with relatively lower refractive index plastic material which makes the light to be conduited efficiently to either end of the scintillating fibre 1. The cladding thickness can be of the order of microns, hence it does not effect the sensitivity of detection. The ultra violet radiation sensed through the sides of the fibre is then converted into visible range of light and transmitted through the ends of the fibre. The ultraviolet radiation threshold for the detection can be varied by the electronic detection circuit. One of the light emanating ends of the scintillating fibre 1 is connected toa first end of a light transmitting fibre 4 of glass or plastic by coupling 5. On the other end of the scintillating fibre 1, a mirror 2 can be provided which is in contact with the ight emitting phase. The light emanating from that end of the scintillating fibre 1 is reflected back by the mirror 2 into the fibre so that the entire light flux can pass through one end of the scintillating fibre only. This arrangement as shown in Fig.2 enhances the sensitivity of the corona detector. This coupling 5 can be with the help of a lens 5' as shown in Fig. 3. The coupling lens 5' has the additional advantage of matching the numerical apertures of the scintillating fibre 1 with those of the light transmitting fibre 4. The coupling lens 5' as shown in Fig. 3 can transfer almost complete light from the scintillating fibre into the transmitting fibre. The numerical aperture of the scintillating fibre is 0.58 while the numerical aperture of the transmitting fibre is 0.51. Therefor, a small double convex lens can be used as coupling. The second end of this light transmitting fibre 4 carries the light radiation to an electronic detection circuit 6 for detection of corona and for setting in an alarm. In order to avoid the influence of high voltage noise, electromagnetic interference, etc, the electronic detection circuit 6 is mounted suitably away from the high voltage system. Thus, the light emitting from the scintillating fibre is transmitted to a distant location through a light piping fibre. The transmitting fibre 4 can be of plastic or glass fibre suitable for the visible operation of the light spectrum. The electronic detection circuit 6 comprises a photo detector 9 followed by amplifiers 10, a comparator 11 and a circuit for setting an alarm on detection of corona as shown in Fig. 5. The amplifier section 10 comprises three amplification stages. The comparator 11 is used for elimination of noise. Base noise level can be set in the comparator so that only signals stronger than this base noise level can be amplified. The signal is then given to the counter 13 through a buffer 12 with a provision for counting the number of pulses. The number of pulses can be adjusted by the user. Such a provision allows elimination of spurious signals. The counter 13 is reset by an auto reset timer 15 at an adjustable time interval. The output of the counter 13 is given to a flip-flop circuitry for generating the alarm signal. After an alarm signal has been generated, it remains locked, till it is reset manually by manual reset 14, after the emergency has been satisactorily addressed. When the corona is sensed by the scintillating fibre,the electronic circuitry 6 detects the presence of corona and sets in an alarm. To avoid detection of spurious signals (noise), an automatic reset circuit is provided which prevents the sensor from sounding an alarm, if the minimum number of pulses (set by the user) does not occur during a certain period of time (say 2 minutes). The alarm is in the form of a relay contact and causes an LED display to light up. The relay contact additionally can actuate the tripping of the high tension system or any interlocking systems that may be provided for such emergencies. In one preferred embodiment of the present invention, the fibre optic corona detector has been made with components having system specifications as shown in Table I below : SYSTEM SPECIFICATION Scintillating Fibre: Emission Response Time 10 to 20 nsec Core refractive index 1.60 Cladding refractive index 1.49 Numerical Aperture 0.58 Scintillating Efficiency 3.4 to 7% Temperature Range -20 to 50°C Core Material Florescent polystyrene Cladding Material Acrylic(PMMA) Cladding Thickness 80u Coupling Lens: Focal length 4.60mm Numerical Aperture 0.53 AR coating MgF2 for 95% transmission Transmitting Fibre: Core Refractive index 1.492 Clad Refractive index 1.402 Numerical Aperture 0.52 Attenuation db/km 190 Electronics Response time of the 10 nsec photo-detector Bandwidth of the system DC to 400 KHz The fibre optic corona detector of present invention can be used in enclosed high voltage systems for continuous monitoring. The fibre optic detector can be used to cover a larger area by coupling the scintillating fibres serially from panel to panel. This helps in detection of insulation degradation at a very early stage, preventing large scale breakdowns. Having disclosed the embodiments, modifications and variations may be made to the embodiments while remaining within the scope of the present invention as defined by the appended claims. WE CLAIM 1. A corona detector comprising scintillating fibre optics through an electronic detector comprising: - scintillating fibre provided with a thin cladding core and a mirror at the first end and a coupling at the second end, laid around a high voltage equipment or insulator for sensing ultraviolet emission of corona and emanating visible light to the end of said scintillating fibre; - a first end of a light transmitting fibre connected to one end of said scintillating fibre by a coupling for carrying said visible light to a second end of said light transmitting fibre and - an electronic detector coupled to said second end of said transmitting fibre for receiving said visible light for detecting the ultraviolet radiation and setting in an alarm of detection of corona characterized in that the said scintillating fibre acts as a sensor to detect ultraviolet radiation of corona and emit visible light which is fed to an electronic detection circuit which generates an alarm. 2. The corona detector as claimed in claim 1, wherein the other free end of said scintillating fibre is provided with a mirror for reflecting back the light emanating from that end into the fibres. 3. The corona detector as claimed in claims 1 or 2, wherein said coupling between the scintillating fibre and said first end of said transmitting fibre is made with the help of a lens. 4. The corona detector as claimed in claims 1 or 3, wherein said lens for coupling between scintillating fibre and transmitting fibre is a double convex lens. 5. The corona detector as claimed in the preceding claims., wherein the core of said scintillating fibre is provided with a thin cladding with relatively lower refractive index plastic material. 6. The corona detector as claimed in the preceding claims, wherein said electronic detection circuit comprises a photo detector, amplifiers, a comparator and an alarm circuit. 7. The corona detector as claimed in the preceding claims, wherein said alarm is in the form of a relay contact with a LED display. 8. The corona detector as claimed in the preceding claims, wherein said alarm circuit is provided with a resetable counter with an automatic reset timer for avoiding detection of spurious signals. 9. A corona detector using fibre optics substantially as herein described and illustrated in the accompanying drawings. The present invention relates to detection of corona using scintillating fibre optics through an electronic detector comprising a scintillating fibre (1) provided with a thin cladding core (3) and a mirror (2) at the first end and a coupling (5) at the second end, laid around a high voltage equipment (7) or insulator for sensing ultraviolet emission of corona and emanating visible light of the end of said scintillating fibre (1) a first of a light transmitting fibre (4) connected to one end of said scintillating fibre by a coupling for carrying said visible light transmitting fibre (4), an electronic detector (6) coupled to said second end of said transmitting fibre (4) for receiving said visible light for detecting the ultraviolet radiation and setting in an alarm (14) on detection of corona characterized in that the said corona detector continuously monitor incipient failures in insulation in high voltage systems and prevents large scale brake down by setting up an alarm in the system device in case of emergency.

Documents:

522-KOL-2003-CORRESPONDENCE.pdf

522-KOL-2003-FORM 15.pdf

522-kol-2003-granted-abstract.pdf

522-kol-2003-granted-claims.pdf

522-kol-2003-granted-correspondence.pdf

522-kol-2003-granted-description (complete).pdf

522-kol-2003-granted-drawings.pdf

522-kol-2003-granted-form 1.pdf

522-kol-2003-granted-form 13.pdf

522-kol-2003-granted-form 18.pdf

522-kol-2003-granted-form 2.pdf

522-kol-2003-granted-form 26.pdf

522-kol-2003-granted-form 3.pdf

522-kol-2003-granted-form 5.pdf

522-kol-2003-granted-reply to examination report.pdf

522-kol-2003-granted-specification.pdf

522-KOL-2003-PA.pdf