In order to analyze the advantages of the complementary measurement with HF and UHF sensors applying suitable processing and analysis tools, four types of insulation defects were generated and measured simultaneously in a laboratory experimental setup. The PD sources implemented were: two internal defects in XLPE, corona in air and corona in SF6 inside the metal cladding of a GIS compartment.
4.1. Generation of PD Sources
Internal discharges (voids inside solid dielectrics) are considered the most harmful for dielectric elements and lead to insulation failures. Corona discharges pose no risk if they are generated in air, however, if they are produced close to silicone insulators or in SF6, the consequences also lead to insulation faults.
Internal defect type 1. A cavity type defect was caused in a cable termination at the end of the semiconducting external layer. The cavity was done by a transversal cut of 1.5 mm depth in the main insulation (XLPE), as shown in
Figure 16a. This type of defect can be caused due to an incorrect handling of the splicing tools at the edges of semiconducting shield cut-backs, in accessories assembling processes [
20].
Internal defect type 2. Another cavity type defect was caused in a cable termination making a void in the main insulation (XLPE). A hole of 1.5 mm depth was drilled using a 1 mm drill bit removing previously a part of the cable semiconducting external layer, as shown in
Figure 16b. The semiconducting layer was fixed again to the main insulator and the cuts were reconstructed using a semiconducting varnish (ref. Raychem EPPA 220). This type of defect occurs due to failures in the cable extrusion process that causes a void in the main insulation, or when a void remains inside the insulation elements after the assembly process of the cable accessories.
Corona in air (tip as HV electrode). Corona discharges were caused by means of a point-plane configuration assembled in a holding device that allows the adjustment of the gap between the point and the plane electrodes (see
Figure 16c).
Corona in a GIS compartment (tip as ground electrode). To simulate a corona defect in SF
6, a 1 mm radius tip was fixed in the enclosure of a GIS compartment that was manufactured with a stainless steel metal cladding of 3 m length. This chamber has a thickness of 3 mm and an external diameter of 500 mm. A conductor of 54 mm diameter was assembled inside the compartment, see
Figure 17b. The internal conductor was supported by two epoxy resin cone spacers. The metal cladding is provided with two inspection windows available for the assembling of invasive PD sensors as shown in
Figure 17a.
The mechanical and electrical connection of the tip with the metal cladding was performed using neodymium magnets, which provides a reliable joining and prevents the machining of the structure, see
Figure 16d. The gap between the tip and the internal conductor can be adjusted with a screw. For a gas pressure inside the compartment of 4 bars and applying a test voltage of 20 kV, the distance between the tip and the internal conductor was adjusted in 3 mm.
Figure 16d shows the tip assembled inside the GIS chamber. In practice, corona discharges in SF
6 can be caused by protrusions due to errors in the installation process, to the friction between moving parts, to burrs or to material degradations caused by switching operations during the service life of the GIS [
40].
Figure 16.
PD sources. (a) Transversal cut in the main insulation of a cable termination. (b) Internal void inside the main insulation of a cable; (c) Point-plane corona in air; (d) Corona in a GIS chamber.
Figure 16.
PD sources. (a) Transversal cut in the main insulation of a cable termination. (b) Internal void inside the main insulation of a cable; (c) Point-plane corona in air; (d) Corona in a GIS chamber.
Figure 17.
(a) Outside view of the GIS compartment; (b) Interior view.
Figure 17.
(a) Outside view of the GIS compartment; (b) Interior view.
4.4. Experimental Results and Analysis
The performance of the described sensors and processing tools is validated by the measurement of the PD sources in the presence of noise interferences. A significant amount of pulses were processed in order to obtain representative patterns for the individual sources. In practice, a thorough PD analysis is required, since up to four defects are present in the HV installation, with the added difficulty of the emplacement of two of them in the same site (position C).
Firstly the measurements performed with the HFCT sensors are analyzed. The PRPD patterns for the complete acquisition obtained with both sensors after the application of the wavelet noise filter are shown in
Figure 19. As the individual patterns generated in each source are mixed, the insulation condition of the HV system can not be determined in a first assessment.
Figure 19.
PRPD patterns obtained with the HFCT sensors. (a) HFCT 1 in position C; (b) HFCT 2 in position E.
Figure 19.
PRPD patterns obtained with the HFCT sensors. (a) HFCT 1 in position C; (b) HFCT 2 in position E.
A further analysis is required for an accurate identification of the defects. In a first step the pulse-type noise sources and interferences measured in the HV installation are removed. Pulse-shaped electric noise can be differentiated from PD pulses with the PD classification tool by pulse waveform. The results of this processing tool are shown in
Figure 20. For the measurement performed with the sensors HFCT 1 and HFCT 2, 6 and 5 clusters have been differentiated respectively. Analyzing the PRPD patterns for each group of pulses it has been possible to correlate all the clusters obtained with each sensor except the cluster 6 corresponding to the sensor HFCT 1.
Figure 20.
Result of the PD classification tool for the acquisitions with HFCT sensors 1 (a) and 2 (b).
Figure 20.
Result of the PD classification tool for the acquisitions with HFCT sensors 1 (a) and 2 (b).
By selecting the pulses corresponding to cluster 1, the electronic noise generated by the power electronic (IGBTs) of the resonant system used to apply high voltage is identified. These interferences are synchronized with the test voltage reference signal. Furthermore, a significant amount of random pulse-shaped signals conducted by the earth network of the installation were also identified by selecting the pulses of cluster 2. The PRPD patterns corresponding to clusters 1 and 2 are shown in
Figure 21a,b. Once these noise signals are removed, the remaining pulses considered PD, are shown in
Figure 21c,d.
Figure 21.
PRPD patterns of the noise pulses measured with the HFCT sensor 1 (a) and HFCT sensor 2 (b) corresponding to clusters 1 and 2. PRPD patterns of the PD pulses measured with the HFCT sensor 1 (c) and 2 (d).
Figure 21.
PRPD patterns of the noise pulses measured with the HFCT sensor 1 (a) and HFCT sensor 2 (b) corresponding to clusters 1 and 2. PRPD patterns of the PD pulses measured with the HFCT sensor 1 (c) and 2 (d).
In a second step, the classification by location tool is applied to the PD pulses and the PD mapping shown in
Figure 22 is obtained. Analyzing the time delay Δ
ti for correlated pulses, two different emplacements of PD sources were detected (positions C and D in
Figure 18 and
Figure 22). These sites correspond with the emplacement of the sensor HFCT 1 and with the joint in the cable system respectively.
Figure 22.
PD classification by location. PD sources located in positions C and D of the cable system.
Figure 22.
PD classification by location. PD sources located in positions C and D of the cable system.
In a third step, the classification tool by pulse waveform is applied to those pulses corresponding with a specific location. In position C two clusters have been identified for the measurements performed with sensor 1, see
Figure 23. These two groups of pulses correspond with clusters 3 and 4 in
Figure 20a. The individual PRPD patterns are obtained by selecting the formed clusters, see
Figure 23b,c.
Figure 23.
(a) Classification of PD pulses positioned in C, (b) and (c) PRPD patterns for the sources positioned in C.
Figure 23.
(a) Classification of PD pulses positioned in C, (b) and (c) PRPD patterns for the sources positioned in C.
The pattern displayed for cluster 3 is characteristic of a corona defect (tip as HV electrode). PD pulses appear on the crests of the reference voltage signal. There are stable amplitude values within a certain range in both half-periods and there are less pulses and with higher amplitude in the positive half-cycle.
The pattern corresponding to cluster 4 is characteristic of an internal defect in a solid dielectric. PD pulses occur slightly before the zero crossings and in the increasing intervals of the voltage signal. There is certain symmetry and a similar repetition rate when comparing the patterns of both half-periods.
In position D only the group of pulses corresponding to cluster 5 was identified (see
Figure 20a and
Figure 24a). The pattern obtained is also characteristic of an internal defect, see
Figure 24b. The patterns shown for positions C and D were obtained for the signals acquired by the sensor HFCT 1.
Figure 24.
(a) Classification of PD pulses positioned in D; (b) PRPD pattern for the source positioned in D.
Figure 24.
(a) Classification of PD pulses positioned in D; (b) PRPD pattern for the source positioned in D.
In order to corroborate the location of the defects in the HV installation, it must be considered that:
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When the pulses are positioned by the location tool in the same site where a HFCT sensor is coupled the location of the defect can not be totally assured,
i.e., the source can be in that position or in a previous one. This is because in both cases the delay in the arrival time of the pulses to the sensors is the same, see
Figure 25a,b, so the pulses are always positioned in the emplacement where the HFCT sensor is placed.
- -
Only when the defect is in an intermediate point between the sensors it can be assured that the positioning of the focus is correct; a certain time delay corresponds only to one emplacement of the PD source, see
Figure 25c.
According with this and considering the internal defects indicated in the PD mapping it can be assured only the correct emplacement for the defect of position D (positioned between the sensors).
The emplacement of the corona defect in position C can also be verified, as it is only in this point where the high voltage conductor of the cable system is exposed to air.
Figure 25.
(a) Defect before the section of cable between sensors; (b) Defect located in the same emplacement as one of the sensors; (c) Defect inside the section of cable between sensors.
Figure 25.
(a) Defect before the section of cable between sensors; (b) Defect located in the same emplacement as one of the sensors; (c) Defect inside the section of cable between sensors.
All clusters obtained with the classification tool by pulse waveform have been analyzed except cluster 6, see
Figure 20a and
Figure 26a. The PD source corresponding with this cluster was detected with the sensor HFCT 1 but not with the HFCT 2 due to the attenuation effect of the pulses, so the location of this source was not possible. Analyzing the characteristics of the pattern it seems that it is related with a corona defect (tip as ground electrode), probably located in the GIS compartment.
Figure 26.
(a) Cluster 6 detected with the sensor HFCT 1; (b) PRPD pattern corresponding to cluster 6.
Figure 26.
(a) Cluster 6 detected with the sensor HFCT 1; (b) PRPD pattern corresponding to cluster 6.
Once analyzed the results obtained with the measurements performed applying the HFCT sensors the following statements can be made.
- -
In position C two types of sources were identified: corona in air and an internal defect.
- -
The location of the corona defect in position C was ratified. However, the emplacement of the internal defect could not be confirmed.
- -
In position D an internal defect in a joint was detected and located.
- -
A corona type PD source related with a tip referenced to ground (cluster 6) was detected with the sensor HFCT 1. The location of this source could not be determined.
In order to complement the previous diagnosis, the measurements performed with the invasive UHF sensors were analyzed. Furthermore, to confirm the location of the internal defect in position C, the mobile non-invasive sensor UHF 3 was coupled in each cable termination of this emplacement.
The PRPD patterns for the acquisition with the invasive couplers located in positions A and B of the GIS compartment are shown in
Figure 27a,b. Due to the selective approach of the measurements in UHF only a single pattern has been obtained for each sensor. Moreover, it can be noticed the immunity that this technique presents to background noise interferences.
Figure 27.
PRPD patterns obtained with the UHF sensors. (a) Sensor UHF 1 in position A; (b) Sensor UHF 2 in position B.
Figure 27.
PRPD patterns obtained with the UHF sensors. (a) Sensor UHF 1 in position A; (b) Sensor UHF 2 in position B.
The PRPD pattern detected with the invasive UHF sensors in the GIS compartment is characteristic of a corona defect in SF6 with a tip referenced to ground. PD pulses appear on the crests of the reference voltage signal and there are more pulses in the positive half-cycle.
As the captures were performed with invasive UHF sensors, it can be assured that the pulses were generated inside the compartment; UHF sensors are insensitive to the low frequency spectrum of the pulses generated in the cable system that propagate to the GIS. Furthermore, this PRPD pattern has the same shape as the one measured with the sensor HFCT 1 corresponding with cluster 6, so it can be concluded that the non-localized corona defect measured with this sensor is the one generated in the GIS chamber.
Due to the high propagation speed of PD signals inside the GIS compartment (0.28 m/ns) and the short distance between sensors (2 m), in this case the localization tool can not be applied, as the sampling period of the measuring instrument is 1 sample every 10ns. An approximate location of the defect is possible comparing the amplitude of the integrated pulse of a PD measured with both sensors; the coupler with the higher signal level is closer to the PD source. The higher amplitude obtained for the pulse measured with the sensor UHF 1 evidences that the defect is closer to the position A, see
Figure 28. An accurate location can be obtained by analyzing the travelling time of the UHF signals measured with a dual-channel oscilloscope of at least 1 GHz of bandwidth and 5 GS/s of sampling rate.
Finally, the PRPD pattern shown in
Figure 29 was obtained with the mobile sensor UHF 3 coupled in the right cable termination of position C. No PD signals were detected in the left termination. This pattern is characteristic of an internal defect and is very similar to the one obtained with the sensor HFCT 1, see
Figure 23c. The detection with the sensor UHF 3 of this internal pattern confirms that the PD source is in this cable termination.
Figure 28.
(a) PD pulse measured with the sensor UHF 1 and converted into a HF pulse; (b) PD pulse measured with the sensor UHF 2 and converted into a HF pulse.
Figure 28.
(a) PD pulse measured with the sensor UHF 1 and converted into a HF pulse; (b) PD pulse measured with the sensor UHF 2 and converted into a HF pulse.
Figure 29.
PRPD pattern obtained with the mobile non-invasive UHF sensor coupled in the right termination of position C.
Figure 29.
PRPD pattern obtained with the mobile non-invasive UHF sensor coupled in the right termination of position C.
The measurements performed with the UHF sensors made it possible to identify and locate correctly the corona defect of the GIS compartment and also to confirm the emplacement of the internal defect in the right cable termination of position C. The complementary measurements with both techniques enabled the identification and location of all the insulation defects generated in the HV installation.