Claims:WE CLAIM:

1. A method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS, comprising:
Injecting external Partial Discharge (PD) pulse through INJ1 [09];
Measuring PD magnitude at sensors S4 [08D] and S5 [08E];
Evaluating the sensitivity of sensors S4 [08D] and S5 [08E] based on received PD signals in-comparison with the injected PD signal;
Injecting external PD pulse through INJ2 [10];
Measuring PD magnitude at sensors S3 [08C] and S4 [08D];
Evaluating the sensitivity of sensors S3 [08C] and S4 [08D] based on received PD signals in-comparison with the injected PD signal;
Injecting external PD pulse through INJ3 [11];
Measuring PD magnitude at sensors S1 [08A] and S2 [08B]; and
Evaluating the sensitivity of sensors S1 [08A] and S2 [08B] based on received PD signals in-comparison with the injected PD signal.

2. The method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS as claimed in claim 1, wherein the electrical distance of sensors S1 & S2 from PD source (MES1) is measured;
PD pulse is injected using MES1;
Time of occurrence of PD at sensors S1 & S2 is measured;
The distance of PD source (MES1) from S1 & S2 is calculated based on the time difference; and
The location error is calculated based on the actual and calculated distance of PD source (MES1) from sensors S1 & S2.
3. The method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS as claimed in claim 1, wherein the external PD is injected using insulated opening of earthing switch in GIS which has sufficient large insulated surface to inject the external PD signals wherein the insulated portion is totally uncovered, with no metallic portion thereby providing an ideal port for injecting the external PD signals for sensitivity verification.
4. The method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS as claimed in claim 6, wherein the insulation used for insulating earthing switch is Polyoxymethylene (POM)/ Acetal or Polytetrafluoroethylene (PTFE).
5. The method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS as claimed in claim 1, wherein the sensors (S1 – S5) are UHF sensors at predefined locations depending on GIS configuration.
6. The method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS as claimed in claim 1, wherein the PD pulses are injected using earthing switch modules FES, MES2 and MES1.
7. The method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS as claimed in claim 1, wherein the PD pulses are injected through PD pulse injection points INJ1 (09), INJ2 (10) and INJ3 (11).
, Description:FIELD OF THE INVENTION
[001] The present invention in general relates to the field of Gas Insulated Substation (GIS). The present invention in particular relates a method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS.
BACKGROUND OF THE INVENTION

[002] Gas insulated substations (GIS) require very less space in comparison to the same rating air insulated substations. Thus GIS substations have become ideal solution where there is a space/land constraint. In GIS, two major insulations used are SF6 gas (under pressurized condition) and the epoxy material (for insulators which support the High Voltage conductor). Any degradation in the GIS insulation may lead to generation of partial discharges (PD). Partial discharges are the localized discharge point under energized condition, of the insulation. In GIS, PD can originate due to voids in the solid insulation (epoxy insulator) or due to presence of foreign particles/components (metallic or non-metallic and floating or fixed) inside the GIS enclosure.
[003] Partial Discharge (PD) or localized insulation defects, in Gas Insulated Substation (GIS) can be measured using conventional electrical method or non-conventional Ultra High Frequency (UHF) method. Sometimes acoustic method is also used, which is also a non-conventional method. The conventional method of PD measurement is used at laboratory or GIS manufacturing plants while the non-conventional methods are mostly employed for on-site online monitoring of PD at site installed GIS. The UHF method of on-line PD monitoring uses special sensors or couplers which detect the PD generated UHF signals in the GIS enclosure. Using online UHF based PD monitoring, any potential problem in the GIS insulation could be known in advance and can be addressed accordingly. But for effective monitoring, the PD sensing capability/sensitivity of the installed sensors needs to be correctly ascertained at site. The PD sensor captured data can only be reliable, if one has precisely verified the sensitivity of the installed sensors as well as the complete monitoring system under site conditions.
[004] Measurement of partial discharge provides an indication of the condition of the GIS insulation. Partial discharge phenomena creates measurable effects like current pulse, UHF emissions, sound, light and chemical byproducts. The conventional method (or electrical method) involves measurement of the PD generated current pulse using appropriate test setups. Alternatively, unconventional methods are being employed for continuous online PD monitoring of GIS insulation at site installations. Out of these methods, the UHF method has been the most acceptable and accurate method for partial discharge measurements at GIS installations.

[005] The generation of UHF signals inside GIS insulation is due to PD generated current pulse which has a very short rise time of few picoseconds. The generated UHF signal is captured by installing UHF sensors/couplers at strategic locations across the GIS bay. The captured UHF signals represent direct indication of the partial discharges (PD) occurring inside the GIS modules. But to have effective and reliable PD monitoring/measurement using the UHF sensors,the sensitivity of these sensors needs to checked, verified and quantified during installation of the Online UHF PD monitoring system. This is a mandatory requirement as per relevant IEC standards. The invention presented is related to this onsite sensitivity verification of UHF sensors, particularly the externally mounted type of sensors.

[006] Prior art:
[007] In general, to monitor healthiness of GIS insulation, partial discharge (PD) is measured. The conventional or electrical method which is used for routine PD measurement in GIS cannot be used for PD measurement at site installations. This is because of significant noise signals present at site installation which affect the measurement of PD generated electrical pulses. To overcome this problem UHF based PD measurement and monitoring is being employed for GIS site installations. Over the years this method has gained prominence and is being widely accepted by power utilities as an effective tool to know the insulation health of the operating GIS.

[008] In case of conventional PD measurement it is clearly indicated in relevant IEC standards that any PD levels of more than 5 pico coulomb (pC) in GIS is not acceptable. The same cut-off acceptable PD levels cannot be specified for the UHF method as in this method, apparent charge (i.e. pico coulomb-pC) based output from the monitoring system is not available.

[009] In UHF method, the intensity of generated UHF waves (as a consequence of PD source) is measured in terms of dBm. There cannot be any direct correlation between the pC levels (of conventional method) and the dBm levels (of the UHF method). Thus in absence of any correlation between the two, when the UHF method is used for PD monitoring at site, the sensitivity of this method (which depends on the sensors design, the PD processing hardware as well as the UHF signal loss inside GIS due to its configuration/design) is ascertained by the utilities. For undertaking this sensitivity verification, a known PD pulse is injected inside the GIS and the same is measured using the installed UHF sensors. This verification also confirms that the installed sensors cover the full GIS bay and PD from any defect inside the GIS would be picked up by one or multiple sensors.

[0010] For injecting the PD inside the GIS, disc insulator ring opening or open insulator (ref: Patent CN103278787A)is used. These ring openings are provided in the GIS for external PD sensor mounting or sometimes they are the port for epoxy filling during manufacturing of the GIS insulator.

[0011] Sometimes for measurement and monitoring of the UHF PD, suitable UHF couplers are fitted to the GIS pressure vessels. Generated UHF signals are taken from the UHF couplers mounted on inside of the hatch cover formed on the bus chamber (ref: Patent US 5396180A). For sensitivity verification purpose, these internally fixed sensors can also be used. They form an ideal point to inject artificial known magnitude PD pulse inside the GIS.

[0012] All the major inventions for on-site sensitivity verification of the UHF PD monitoring method uses the following as a point for injecting the external PD pulse: a) An internal PD sensor, b) An external sensor mounted on open or bare insulator and c) An external sensor mounted on small opening or aperture of GIS insulator. In first two cases, sufficient amount of PD can be injected for sensitivity verification. It may be noted that the use of open or bare spacers in ultra high voltage class GIS may not be reliable due to electromagnetic interference related problems in control equipment of GIS. Thus, when we use external type of PD sensors, they mostly are mounted on an opening/slot/aperture of the GIS insulator (point no. c). In case where one uses this arrangement i.e. external sensor mounted on small aperture of the GIS spacer, it is practically impossible to inject sufficient amount of external PD pulse for sensitivity verification purposes.

[0013] The present patent offers a novel and simple method to undertake this onsite PD sensitivity verification. Also the required provisions in the GIS modules for enabling this on-site verification form a part of this invention.

[0014] The present invention is related to a process which overcomes this problem of sensitivity verification in case of external type of UHF PD sensors mounted on an aperture of GIS spacer. This invention can also be used for PD sensitivity verification of internal sensor based PD monitoring system in case one does not want to open the UHF cable connections of the internal sensors (for PD injection)or there is a difficulty of accessibility of the internal sensors, for injecting the artificial/pre-defined PD pulses.
OBJECTS OF THE INVENTION
[0015] It is therefore an object of the invention to propose a method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS.
[0016] Another object of the invention to propose a method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS, particularly the external type of sensors at site installations.
[0017] Yet another object of the invention to propose a method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS, using a method for PD pulse/signal injection inside GIS enclosure.
[0018] Yet another object of the invention to propose a method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS, using a technique for quantification of the PD source/location accuracy.

SUMMARY OF THE INVENTION
[0019] A method for on-site sensitivity verification for ultra high frequency (UHF) sensors in GIS comprising the steps of injecting of external PD pulse using INJ1 [09] and measuring the PD magnitude at the nearby sensorsS4 [08D] and S5 [08E]. The sensitivity of these two sensors is evaluated based on received PD signals in-comparison with the injected PD signal. Another external PD pulse is injected using INJ2 [10] and the PD magnitude at the nearby sensorsS3 [08C] and S4 [08D] is measured and the sensitivity of these two sensors based on received PD signals in-comparison with the injected PD signal is evaluated. Lastly, another external PD pulse is injected using INJ3 [11] and PD magnitude at the nearby sensors S1 [08A] and S2 [08B] is measured for evaluating the sensitivity of these two sensors based on received PD signals in-comparison with the injected PD signal.

BRIEF DESCRIPTION OF DRAWNGS
The proposed invention will be better understood by the following description with reference to the accompanying drawings:

[0020] Figure 1: Conventional Measurement of PD signal through Support insulator without Metallic Cover
[0021] Figure 2: Insulated aperture on LT of support insulator for external UHF sensors
[0022] Figure 3:Insulated viewing port in GIS for mounting of external UHF sensors
[0023] Figure 4:Insulated terminal in Earth Switch (ES) for PD injection
[0024] Figure 5: PD injection for on-site sensitivity verification
[0025] Figure 6: Schematic for PD monitoring sensitivity verification
[0026] Figure 7: Schematic for PD location accuracy verification

DETAIL DESCRIPTION OF THE INVENTION

The present invention is described in detail below.

[0027] The UHF PD sensors which are used for onsite PD monitoring can be of two types: 1) Internal Type of sensors and 2) External Type of sensors. The usage of a particular type of sensor depends on the provision available in the GIS for installation of these sensors. Internal sensors are mounted on ports specially designed for this purpose on the GIS enclosure. External sensors are mounted either on small apertures made on the LT of the support insulators or on the viewing window opening in the GIS. The couplers/sensors which are traditionally used for capturing of PD are mounted inside the GIS and are directly exposed to PD induced UHF signals. High Voltage design of GIS modules also takes into account these sensors and their positions in the GIS enclosure as these sensors may affect the dielectric field strength and electrical field distribution inside the GIS. Since these couplers are mounted inside the GIS they are also required to withstand the require SF6 gas pressure continuously. Thus the mechanical design of the GIS enclosure with respect to pressure withstanding ability also caters to the position and location of the UHF couplers/sensors. Further, once mounted, the positions of the sensors cannot be charged online as they are embedded inside the GIS enclosure. Also in case the sensors become defective, the replacement of the defective sensors would require a complete shutdown of the concerned GIS bay. But the advantage of using these internally mounted couplers is that they have extremely good sensitivity which is comparable to the PD measurement sensitivity of the conventional electrical method. The described process of invention caters to achieve comparable PD monitoring sensitivity of the internally mounted UHF couplers/sensors using a novel non-invasive approach.

[0028] The UHF couplers used for this process are non-invasive passive type which can be mounted externally without the need to open the GIS chamber or to take a shut-down of the GIS installations. Special provision is made on the GIS for fixing the couplers to them from outside. The installed couplers from outside capture the leaking UHF signal which is generated inside the GIS due to a PD source. The typical output of the coupler is a voltage signal. The coupler output depends on the UHF field strength inside the GIS enclosure, coupler design and the external port provision on the GIS spacer.

[0029] The discharge signal emanates from insulated flange of support insulator. Figure 1 shows the conventional support insulator [01] without metallic flange/cover. High frequency discharge signal emanate from insulated spacer/support insulator [01] is sensed by sensor fixed to it. Different novel locations that can be used for sensing can be predefined. In insulating flange type insulator, multiple numbers of copper strips [02] are connected for ground current transfer. The sensor [03] can be located between copper strips [02]. It is important to note that, in all the installations support insulators may not be purely insulated type. Further, in extra high voltage and ultra high voltage class GIS, most of the support insulators are integrated with metallic cover [04]. These insulators shall have circular holes for processing of insulators but these holes may not be good enough to measure PD signals. Hence opening shall be provided with higher surface area and preferably rectangular / elliptical [05] shape. This is mainly due to limited width (thickness) of support insulator.
[0030] Figure 2 shows the profile of this rectangular slot and its cutting view on support insulator. The special provision invented for implementing this non-invasive PD monitoring in the GIS is as shown in Figure 2. In recent years, for long bus ducts, it is understood that support insulators can be mounted inside the enclosure so that provision for collecting signal with non-invasive type is not possible. In such situations, it is proposed to use existing viewing ports [06] in the shape of rectangular or elliptical or circular depending on availability of sensor base. These viewing ports are generally available / shall be made on switching elements enclosures like disconnector switch, earthing switch to check the status of contacts. The response of these insulated openings varies depending on orientation, medium through which PD signals emanate out of GIS. Precisely, in viewing ports, through the insulated glass PD signals will be received by sensor [03].
[0031] Figure 3 shows the Insulated viewing port in GIS for mounting of external UHF sensors.
[0032] On-site sensitivity verification of the installed UHF sensors is a must before the sensors are employed for actual monitoring of PD. This verification mandatorily needs to be carried out at site after the UHF sensors are installed across the GIS bay. The verification helps to check the proper functionality as well as PD signals sensing accuracy of the installed UHF sensors. The verification process aims to ascertain the fact that any PD phenomenon occurring in the GIS bay can be detected successfully by the installed on-line PD monitoring system.
[0033] The on-site sensitivity verification procedure is defined in IEC. As per this, to verify the sensitivity of the sensors, a PD pulse (short duration voltage signal) which is generated by a special electronic module, is injected at all the installed UHF sensors one-by-one and the corresponding generated UHF signal inside GIS is measured on the adjacent sensors. This procedure helps to verify the sensitivity of UHF PD measurement across the GIS bay. Even though this looks to be simple procedure but there is practical problem in using this technique particularly when one is using external type of UHF PD sensors. As already mentioned, the external sensors are mounted on the exposed epoxy aperture of the GIS spacers/cone insulators. Traditionally the apertures which are available on the GIS insulators are of small dimensions. Sometimes the epoxy filling small ports are used as an aperture for PD sensors. Larger dimension apertures are not preferred (although for PD monitoring purpose they are beneficial) as they may reduce the mechanical strength of the insulator during its manufacturing process. Further this type of design may result in higher electromagnetic interference levels that may affect the performance of control equipment which in turn may create mal-operations in the GIS main circuit.
[0034] During sensitivity verification at site it is observed that, it is practically impossible to inject sufficient amount of PD pulse inside the GIS when external type of sensors is being used. This is because of the following reasons: a) The dimension of the aperture (exposed epoxy surface) is normally small thus reducing the magnitude of external PD pulse to propagate inside the GIS, b) Since the magnitude of the injectable PD pulse (generated using an electronic PD pulse generator) is also small, it is very difficult for it to propagate effectively through the dense solid epoxy insulation of spacer and enter the GIS enclosure, and c) The external sensors are fixed/screwed on the LT metallic portion and covers the aperture or exposed epoxy surface on the spacer. This mounting arrangement may create air gap between the external sensor and the epoxy surface. This air gap leads to considerable attenuation of the injected PD pulse, thereby further reducing the chances of injecting sufficient amount of PD signal.
[0035] The present invention caters to address the above issue of launching sufficient amount of PD signals inside GIS for sensitivity verification purpose particularly in case of external sensors being used for on-line PD monitoring. A special provision of Delrin/ PTFE insulation [07] with limited diameter has been used in the earth switch of GIS modules, so that injection of good quality PD signals can be achieved. The design of these insulators `meets two requirements. First one is to meet the requirement of insulated flange type of gas insulated earthing switch. Second one is to feed PD calibrating signal through UHF sensors. A novel double insulator concept has been adopted to isolate the grounded high voltage circuit from grounded enclosure. Most importantly these two insulators are of same profile and dimensions. One of these insulators shall be used for feeding the calibrating PD signal. One more insulator [07A] can be used to verify the feeding signal intensity as there is negligible attenuation of signal between the two insulators. Figure 4 shows the designed insulated opening of earthing switch in GIS. The designed insulated portion in the ES has sufficient large insulated surface to inject the external PD signals. This insulated portion is totally uncovered, with no metallic portion and thus provides an ideal port for injecting the external PD signals for sensitivity verification. Most importantly this particular insulator is near the grounding side of the enclosure and electromagnetic transients appear at this portion are limited and only two to three sets depending on the substation configuration are available in the entire GIS bay. Further, the dimensions of this insulator are much smaller as compared to insulated support insulators.
[0036] Figure 5 shows the injection of PD signals inside GIS using this insulated port on earthing switch. As shown in the figure, PD pulse signal is generated using an external electronic circuit/module. The output of this pulse generator is connected to an external UHF sensor. When a pulsed voltage (5V, 10V or 20V) is applied to the UHF sensor, its sensing portion emits UHF PD pulses. The sensor’s sensing portion is placed firmly on the insulated portion of the earth switch module. Since this insulated portion on ES has sufficiently large surface, the UHF PD pulses can easily be injected into GIS. There is a direct contact of the sensor with the insulated ES portion and thus there is very less attenuation of the PD pulse at the interface. Also the profile/design of this insulating port offers easy propagation of the PD pulse in comparison to the epoxy material of the spacers/ complex support insulator. Thus through this arrangement, sufficient amount of PD pulse can be launched inside the GIS for sensitivity verification. The launched PD pulse can be rechecked by keeping one more PD sensor on other Delrin insulator [07A] of the earthing switch. Most importantly, these insulators of insulated type earthing switch can also be used for measuring PD signals. This type of provision is available for both maintenance earthing switch and fast acting earthing switch. Figure 5 shows insulated opening of earthing switch in GIS.
[0037] Figure 6 shows the schematic and implementation of the developed sensitivity verification scheme on actual GIS bay. The schematic shows a GIS bay layout which consists of circuit breaker, disconnector switches (DS), and current transformer (CT), maintenance earthing switch (MES), fast acting earthing switch (FES) etc.MES1, MES2 and FES are the ES modules of the bay. S1 to S5 [08] are the external type of UHF PD sensors installed across the GIS bay. The location of these PD sensors is determined based on the calculated UHF signal loss in this GIS configuration. As explained earlier, the three ES modules (FES, MES2 and MES1) are used for injecting the PD pulses for sensitivity verification. Thus INJ1 [09], INJ2 [10] and INJ3 [11] are the three PD pulse injection points in the GIS bay. The sensitivity of each of the sensors can be verified using this scheme. If INJ1 [09] is used as PD injection point, the nearby sensors S4 [08D] and S5 [08E] receives the PD signals and their sensitivity can be ascertained. If INJ2 [10] is used, sensitivity of sensors S3 [08C] and S4 [08D] can be established. Similarly, with INJ3 [11] the sensitivity of sensors S1 [08A] and S2 [08B] can be checked. Thus the sensitivity of all the 5 sensors spread across the GIS bay can be verified using this technique. Ideally, it is also possible to find the response of all sensors for each injection INJ (INJ1, INJ2, and INJ3). This data will be useful for accurate estimation of attenuation rate of GIS configuration between each sensor.
[0038] Apart from PD measurement/monitoring sensitivity verification of individual sensors, the accuracy of PD location assessment (by the installed PD monitoring system) can also be quantified using this technique. Figure 7 shows the typical schematic for “PD location accuracy” verification method. External PD pulses are injected inside the GIS using MES1 earth switch (ES). In other words, MES1is the module, which is a source of the partial discharges in the GIS bay. The two adjacent sensors to this PD source are marked as sensor 1 and 2 (S1 & S2). The electrical distance of these sensors from PD source (i.e. MES1) is A meters and B meters respectively. On injection of PD signal in the ES, the sensor 1 will sense the PD signal first in comparison to sensor 2 as distance A is less than B. Based on this time difference we can calculate the distance of the PD source from the sensor 1 as well as from sensor 2. For this the speed of UHF signal propagation in GIS with SF6 gas and solid insulation can be taken as approx. 0.97c (where c is the speed of light in vacuum). The difference between the calculated value and the actual values (A & B) of the PD source location from sensors 1 &2 will provide us an estimation of the error in locating the PD source. This error quantification should also be carried out using other earthing switches of the GIS bay as PD injection point. The average of all the measurements can provide us the estimated “PD location measurement accuracy” of the implemented online monitoring technique. Once this process is carried out during installation stage, it would be very helpful during in-service PD location assessment of GIS.

[0039] The invention has been described in an illustrative manner, and it is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the specification, the reference numerals are merely for convenience, and are not to be in any way limiting, the invention may be practiced otherwise than is specifically described.