GIS Diagnostics - Thermal Imaging Systems used for Poor Contact Detection

D. Avital, V. Brandenbursky, A. Farber,Central Electrical Lab., Israel Electric Corporation.

1 Shemen St. Haifa, Israel 31000 Tel: 972-4-8688819, Fax: 972-4-8646867, Cell: 972-52-7467445 e-mail: doron_a@iec.co.il

ABSTRACT

The reliability of GIS is very high but any failure that occurs can cause extensive damage result and the repair times are considerably long. The consequential losses to system security and economically can be high, especially if the nominal GIS voltage is 420 kV and above. In view of these circumstances, increasing attention is being given to diagnostic techniques for in-service maintenance undertaken to improve the reliability and availability of GIS. Recently considerable progress has been made in diagnostic techniques and they are now used successfully during the service life of the equipment [1-3]. These diagnostic techniques in general focus on the GIS insulation system and are based on partial discharge (PD) measurements in GIS. There are three main methods for in-service PD detection in GIS: -

the chemical method that rely on the detection of cracked gas caused by PD, the acoustic method designed to detect the acoustic emission excited by PD, and, the electrical method which is based on detection of electrical resonance at ultra high frequencies (UHF) up to 1.5 GHz caused by PD excitation in GIS chambers (UHF method).

These three dielectric diagnostic methods cannot be used for the detection of poor current carrying contacts in GIS. This problem does not always produce partial discharges and at early stages it does not cause gas cracking. An interesting solution to use two techniques - the current unbalance alarm scheme and partial discharge monitoring was advised by A. Salinas from South California Edison Co. [4]. Unfortunately this way is complicated and very expensive.The investigations performed in Japan on standing alone SF6 breaker showed that joule heating of the contact accompanied by released power of 1600 Watt produce temperature difference on the enclosure up to 7 degrees centigrade that could be detected by infra-red Thermal Imaging System [5].According to CIGRE Joint Working Group 33/23.12 Report [6], 11% of all GIS failures are due to poor current carrying contacts in GIS.The Israel Electric Company (IEC) in seeking a solution to this problem have undertaken experimental work to examine the possibility of in-service diagnostic of poor contact problem in GIS via direct local heating detection, using a Thermal Imaging System. The experiments were carried out on the part of the GIS with nominal SF6 pressure. The following aspects of the problem were examined:

- the range of power released in the defective contact that could give the practical temperature rise on the surface of enclosure; - temperature distribution on the surface of enclosure; - the influence of spacer type (with holes or without) on the heat transfer process; - the influence of the length of SF6 tubes and there position (horizontal or vertical); - the temperature difference between upper and lower parts of the tubes in horizontal position; - practical use of the Thermal Imaging System for detecting poor contact problem in GIS.

1. IEC’s Experimental set-up and test procedure

Tests were carried out on one phase of a GIS module that was a part of the busbar having the rated voltage of 170 kV. This module with aluminium-welded enclosure, included in different tests from 4 to 6 SF6 chambers separated by spacers (accordingly the length of the modules differs from 3.8 m to 6.8 m). The GIS module was filled with SF6 gas at nominal pressure (3.5 bar). The experiments were carried out on a poor contacting of 0.5 milliOhm at different power ranges: 100, 200, 300 and 400 Watt. All the measurements were made for a steady state temperature, the final temperature rise was reached after about 20 hours. The inner temperature on the defective contact and on the central conductor away from the contact was measured by Precision Centigrade Temperature Sensor type LM35D from National Semiconductor whose output voltage is linearly proportional to the Celsius temperature. The sensor provide accuracy of 0.75 0C over a full - 400 to +110 0C temperature range and has very low self-heating, less than 0.1 0C. Temperature measurements on the surface of the enclosure were made directly by the contact method using thermocouple and at the same time, the temperature distribution along the surface of enclosure was obtained by Thermal Imaging System type SC3000. This Thermal Imaging System is based on QWIP focal plane technology and gives high precision temperature measurements ( +/- 1 0C) with thermal sensitivity of less than 0.02 0C.Two modules with different lengths 3.8 m and 6.8 m were examined. The drawings of the modules and the pictures are shown on Fig. 1. The short model included four SF6 compartments: three tubes with a defective contact situated between two of them (point 9) and a corner chamber. For the long module two tubes of 2 and 1 metre length were added to the corner chamber. For this module the tests were carried for two types of the spacer situated in the corner chamber - the whole spacer and the spacer with holes that permits gas flow. For the long module the tests were also performed in a vertical tube position.

2

Fig.1 Experimental GIS modules showing the temperature measuring positions:

1, 2, 3, 4, 5, 6, 7, 8 - on the surface of the enclosure; 9 - on the defective contact; 10 - on the central conductor 1 metre away from the defective contact

3

2.Experimental Results and Discussion

Fig. 2 shows the comparison results for the models with different lengths for the same power released in the contact.

Fig. 2 Surface temperatures obtained for different module length LP = 400 Watt, T9 = 75 o C, T 10 = 58 o C, T room = 23 o C

Fig. 3 Thermal pictures for the module 3.8 m length, P = 400 Watt

It could be seen that the length grows does not change the temperature distribution along the tubes. The absolute values of temperature changed a little bit but the temperature difference between the point on the surface above the bad contact and the point at 1 metre away from it remains the same and equal to about 4 0C. At the end of the long model (point 8) the temperature of the tube is equal to the room temperature that allows to conclude that additional growth of the tube length will not change the temperature distribution received in the region of the bad contact.

4

34

36

Line Min Max Cursorli01 32.8°C 37.6°C -

°C IR01

21.7°C

37.7°C

25

30

35LI01

Fig. 4 Thermal pictures for the module 6.8 m length, P = 400 Watt

The temperature distribution obtained by Thermal Imaging System for short and long models is shown on fig. 3 and 4. It could be seen that thermal pictures exactly reproduce the detected temperature distribution (fig. 2) for these two models for specified conditions when the power released in the bad contact was 400 Watt.

Fig. 5 Surface temperatures obtained for different power ranges:P = 400 Watt, T 9 = 75 o C, T 10 = 58 o C; P = 300 Watt, T 9 = 61.8 o C, T 10 = 51 o C;

P = 200 Watt, T 9 = 50.6 o C, T 10 = 43.5 o C; P = 100 Watt, T 9 = 38 o C, T 10 = 34.7 o C

The small disturbances on the thermal graphs obtained by Thermal Imaging System are corresponding to the flange connection of the SF6 tubes.Fig. 5 shows the temperature distribution on the enclosure for the model 6.8 m length for different ranges of power released in the bad contact.With decrease of joule heating from 400 Watt to 100 Watt the temperature of the contact decreases from 75 0C to 38 0C when it overcome the room temperature only by 16 0C. In accordance the temperature distribution on the enclosure surface becomes more flat, at 1 metre away from the bad contact position the temperature difference is only about 1 0C, but maximum temperature on the enclosure above the bad contact is overcoming the temperature of it distant parts by 4.8 0C. This could be compared with the results obtained for the power range of 400 Watt where the temperature

5

25

30

35

Line Min Max Cursorli01 24.3°C 37.8°C -

°C IR01

22.0°C

39.8°C

25

30

35

LI01

difference at 1 metre away from the bad contact area is 4.6 0C and for distant parts this difference is equal to 14.4 0C. The obtained thermal picture for power range 100 Watt is shown on fig. 6. It could be seen that for the power of 100 Watt the area of overheating on the surface is relatively small but at the same time the temperature difference between the point above the bad contact and the distant points of the enclosure is still could be detected by Thermal Imaging System.

Fig.6 Thermal picture for the module L = 6.8 m at power P = 100 Watt

The difference of heating between upper and lower parts of the tubes is illustrated on fig. 7. In the area of the bad contact the temperature of the upper point of the tube is 3 0C higher than for the lower one, but this difference is already negligible at a distance of 1 metre away.

Fig. 7 Surface temperatures for upper and lower parts of the tubesL = 6.8 m length, P = 400 Watt

On the thermal pictures the temperature difference between upper and lower tube parts could be clearly seen in the area of bad contact (fig. 4). According to this result the practical detection in GIS has to be provided on the upper parts of the tubes. The obtained temperature difference between upper and lower part of the tube shows that the convection has a vital importance in the heat transfer process.

6

22

24

26

Line Min Max Cursorli01 26.4°C 27.6°C -

°C IR01

21.8°C

29.5°C

22

24

26

28

LI01

22

24

26

28

30

32

Line Min Max Cursorli01 26.0°C 32.4°C -

°C IR01

Fig. 8 Temperatures obtained for the

module L = 6.8 m, P = 300 Watt with different spacer types

The influence of spacer type on the heat transfer is illustrated on fig. 8.

Fig. 9 Thermal picture for the module with the spacer with holes

The existence of holes in the spacer slightly changed the absolute temperatures on the enclosure and the temperature distribution along the tubes. As it could be expected the gas flow between the chambers cooled a little bit the bad contact and led to higher temperature at the distant parts of the

7

21.9°C

32.4°C

22

24

26

28

30

32

LI01

enclosure. These changes do not have the remarkable influence on the temperature difference between the point above the bad contact and the point at 1 metre away from it.This temperature difference for specified test conditions remains almost the same (about 3 0C, for P = 300 Watt) for both types of the spacer. Corresponding thermal pictures for the spacer with holes is shown on fig 9.Fig. 10 illustrates the results obtained for the module with vertical arrangement of the tubes. As a result of convection a sharp decrease of temperature was received for the points lower the bad contact. At the same time the maximum temperature appears at the point situated 30 cm higher than the bad contact position.

Fig. 10 Temperatures obtained for horizontal and vertical positions of the tubes

The corresponding thermal pictures are shown on fig. 11

Fig 11 Thermal picture for vertical position of the tubes

8

25

30

35

40

Line Min Max Cursorli01 25.0°C 38.9°C -

°C IR01LI01

The above mention changes in temperature distribution are clearly detected by Thermal Imaging System. The thermal picture reproduce the different temperature distribution on the tubes at both sides if the bad contact position. It could be concluded that detected sharp temperature decrease in the area of bad contact for vertical arrangement of SF6 tubes could only simplify the in-service control for GIS. The use of the Thermal Imaging System for in-service inspection of GIS was examined on 420 kV GIS in service. It was found that for GIS without defects the nominal current does not heat the GIS enclosure. For the bays under voltage with nominal current and for the bays out of voltage the enclosure temperature was equal to room temperature. The only places in GIS that were found with the temperature higher than the room one were the voltage transformers (VT). The corresponding thermal pictures are shown on fig. 12.

25.0°C

32.0°C

26

28

30

Fig. 12 - Thermal picture of the VT taken for the 420 kV GIS in-service

For vertical arrangement of VT an uniform heating of the upper part of the transformer takes place. For horizontal position of the VT it could be definitely seen that the heat spread upper from the central part of the transformer where the core and the windings are situated. The drawing of the VT is shown on fig. 13.

Fig. 13 The drawing of 420 kV voltage transformer

9

25.0°C

31.8°C

26

28

30

The obtained result confirms the practical possibility of overheat detection for GIS in-service situated inside the building using Thermal Imaging System.The observations that can be determined from the results are as follows: Relatively small overheating of the busbar contact in GIS (up to 75 0C) causes significant

temperature differences on the enclosure surface, (local temperature rise up to 4.6 0C);According to IEC 60694 [7] contacts in GIS are designed to operate with a maximum temperature of 105°C under normal conditions, (maximum current load and maximum room temperature of 40°C). The obtained results show that using the Thermal Imaging System it is possible to detect and locate the contact overheating at temperatures of contact even lower than permissible one. With defective contacts that could have higher values of temperature under these circumstances the detection and location of the defect could be easily performed with a Thermal Imaging System.

For GIS inside the building the temperature distribution obtained with Thermal Imaging System reproduce with very high accuracy the temperature on the GIS surface.

The convection plays an important role in heat transfer process and as it was obtained the enclosure overheating displays more vividly for the upper parts of the tubes and for the tubes in vertical position. The practical inspection for GIS in-service it is better to perform scanning the GIS from the upper point of view.

The type of spacers - with holes or without - did not have a remarkable influence on the temperature distribution on the enclosure.

The power of about 100 Watt released in the defective contact is a minimum range of power for GIS with rated voltage 170 kV that could be detected using Thermal Imaging System.

The use of Thermal Imaging System for in-service inspection for GIS inside the building is practically proved during inspection of 420 kV GIS.

3. ConclusionsThe experimental results reported by IEC verify that the infra-red Thermal Imaging System technique is suitable for identifying and locating poor current carrying contacts in GIS. It has been proved that even minor anomalies such as contact local heating up to a temperature below the permissible value can be easily detected by infra-red Thermal Imaging System technique.

4. CaptionsFig. 1 - Experimental GIS modules showing the temperature measuring positionsFig. 2 - Surface temperatures obtained for different module length L.Fig. 3 - Thermal pictures for the module 3.8 m length, P = 400 Watt.Fig. 4 - Thermal pictures for the module 6.8 m length, P = 400 Watt.Fig. 5 - Surface temperatures obtained for different power rangersFig. 6 - Thermal picture for the module L = 6.8 m at power P = 100 Watt.Fig. 7 - Surface temperatures for upper and lower parts of the tubes for the module L = 6.8 m length, P = 400 Watt.Fig. 8 - Temperatures obtained for the module L = 6.8 m, P = 300 Watt with different spacer typesFig. 9 - Thermal picture for the module with the spacer with holesFig. 10 - Temperatures obtained for horizontal and vertical positions of the tubes

10

Fig. 11 - Thermal picture for vertical position of the tubesFig. 12 - Thermal picture of the VT taken for the 420 kV GIS in-serviceFig. 13 - The drawing of 420 kV voltage transformer

11

5. References[1] TF 15/33.03.05 "Partial discharge detection system for GIS: sensitivity verification for the UHF method and the acoustic method" ELECTRA, No 183 April, 1999.[2] J.S. Pearson, O. Farish, B.F. Hampton, M.D. Judd, D. Templeton, B.M. Pryor, I.M. Welch "Partial discharge diagnostics for GIS" IEEE Trans. On Dielectrics and Electrical Insulation", Vol. 2, No 5, October 1995.[3] R. Kurrer, K Feser: "The Application of Ultra-High-Frequency Partial Discharge Measurements to Gas Insulated Substations", IEEE Trans. on Power Delivery, Vol. 13, No 3, July 1998.[4] A. R. Salinas "Detecting Trouble Through Diagnostic Techniques", T&D World, May 2001. [5] H. Kawada, K. Ando, T. Maeda, M. Tamura, Y. Murakami, T. Nitta"Application of diagnostic techniques to gas insulated switchgears" CIGRE, Int. Conf. On Large Electric Systems, 1988[6] CIGRE JWG 33/23.12 "Insulation coordination of GIS: return of experience, on site tests and diagnostic techniques", ELECTRA No 176, February 1998.[7] IEC - Publication. 60694: "Common specifications for high-voltage switchgear and controlgear standards", 2001-05.

12