PS2: Lifetime management, deterioration and ageing of substation equipment

Transcription

1 PS2: Lifetime management, deterioration and ageing of substation equipment Ageing and deterioration of composite post insulators exposed to high electric field in 220 kv and 400 kv switchyards in Swedish network M. RADOSAVLJEVIC 1, I. GUTMAN 2, C. AHLHOLM 2, P. SIDENVALL 2 1 SVENSKA KRAFTNÄT, 2 STRI SWEDEN SUMMARY The majority of station post insulators in the world are porcelain insulators. However, based on in general positive service experience of line composite insulators, and especially composite apparatus insulators, composite station post insulators are also used on lower scale. However, it is important that the requirements for such insulators to ensure their long-term performance shall be the same as for line/apparatus composite insulators. It is well-known for line composite insulators that it is very important to limit the levels of E-field to avoid permanent corona from the metal parts and water drop corona located on insulator surface. Based on historical use of porcelain post insulators for substations, the need for corona/grading rings is underestimated for composite post insulators. Comprehensive 3D electric field calculations on different composite post insulators (few designs/manufacturers with/without corona/grading rings) at Swedish TSO revealed that in many cases the levels of E-field are well above the internationally accepted criteria. Deviation from these criteria varied depending on the specific design. Due to concern for life time of the exposed to high electric field composite post insulators a comprehensive test program was launched, which included: field inspections using IR/UV cameras and visual inspections and laboratory investigation of some removed from service insulators 220 kv and 400 kv. The laboratory investigations included switching and steep-front electrical tests, evaluation of adhesion, dissection of insulators, etc. The results of inspections/investigations revealed for some insulators weak adhesion between the silicone rubber and fibreglass core, which in combination with too high electric field in some cases led to the punctures of silicone rubber housing in service. Another new type of internal deterioration as a result of combination of weak adhesion and high electric field was revealed. The paper describes in details results of investigations and recommendations for further improvement of representative tests and calculations to be considered by CIGRÉ/IEC. This is to avoid low quality insulators to reach the service. Keyword Composite post insulator, simulation, high voltage, life expectancy, ageing Milan Radosavljevic, Milan.radosavljevic@svk.se 1

2 INTRODUCTION AND GOAL General service experiences of both line and apparatus composite insulators are rather positive at present. This is summarized in few CIGRÉ Technical Brochures [1], [2], [3]. According to [2] based on preliminary and sparse information an exponential increase of installations of line composite insulators can be seen (Figure 1). It is discussed in [1] that if not considering the failures of the first generation of composite insulators their reliability at present can be comparable with the strings of glass insulators. Figure 1. Estimated growth in composite insulator use in AC overhead transmission lines (adopted from [2]). Swedish TSO Svenska kraftnät (Svk) uses apparatus composite insulators with success for a number of years. Field inspection of about 30 silicone rubber circuit breakers performed after years in service did not reveal any significant deterioration and the hydrophobicity of this insulators was still very good (Hydrophobicity Class, HC 1-2, i.e. hydrophobic). This paper concentrates on specific experience with composite station post insulators, which rather recently came into the Swedish network (a few years in service). Even having positive service experience in general, composite insulators in the market can be of rather different quality and the issue of quality is not fully covered by the existing IEC standards relating to such insulators. The basic IEC standard [4] was issued in 2012 and a product standard for station post composite insulators IEC [5] mostly follows this standard. It is well known that it takes about 3-4 years to issue a new standard and it is based on the results of earlier performed research, thus it can be considered that the present standards were developed based on a decade old data. The goal of this paper is to summarise shortly the results of the comprehensive investigation of composite station post insulators including E-field calculations, service inspections and after-service laboratory tests. This is to evaluate the quality of insulators and to identify and highlight the needs for further improvement of the specifications (via CIGRE/IEC work) to prevent insulators with lower quality to reach the network. The comprehensive test program included: E-field calculations performed for 11 types of insulators with/without corona/grading rings and intended for kv Service inspections of insulators under voltage (IR and UV observation techniques) Service inspections of insulators without voltage (close visual observations and hydrophobicity measurements) 2

3 After-service laboratory investigations using IEC standard and specially developed new tests E-FIELD CALCULATIONS It can be concluded that at present there are three main criteria for a composite insulator for the maximum electric field (to prevent corona from the metal parts and water drop corona on surface). These are based on comprehensive research performed by STRI and EPRI independently of each other about 10 years ago [6]. The industry practice is that most of manufacturers and utilities are using similar criteria when accepting a new design. The criteria are listed in Table 1 and have been verified by STRI numerously for more than 30 different insulator designs [7]. Further in this paper the criteria are presented using kv/mm units. Table 1. Criteria for electric field on composite insulators [6]. Part of insulator Criterion [kv/cm] End fitting, grading ring and arcing horn 18 End fitting seal (the so called triple point) 3.5 Sheath of insulator housing 4.2 (max average along 10 mm) The first criterion is for corona from metal, thus applicable on end fittings and hardware (such as yoke, grading ring, arcing horn, etc.) both at HV side and at grounded side. This criterion is slightly more conservative than the CIGRÉ recommendation 2.1 kv/mm [8] because it considers minor manufacturing flaws and ageing of the hardware in service. Such reduction was proposed by some power utilities during discussions at the working groups based on practical service experience. The use of this criterion is for dry conditions. The second and the third criteria are for the maximum electric field along the housing material, i.e. in the direction of the surface. The calculations are made in dry conditions as a dry background electric field; however, they are applied for wet conditions (water drop corona). The reason for this is that corona from water droplets may deteriorate the housing. The second criterion is for the triple point, i.e. where the metal end fitting, housing material and air joins. This is a vital part of the insulator which makes it crucial not to have any corona at this position. The third criterion is the maximum average electric field along 10 mm of the surface of the insulator. It is an average due to the fact that minor calculation flaws (such as sharp angles) can be neglected. All criteria are applicable regardless if it is a line insulator, station post or apparatus insulator. Deterioration from corona has not been considered in the past and therefore only audible noise and RIV were the driving forces to test insulators from corona point of view. The first criterion can be thus verified with a standard RIV and corona test according to the IEC [9]. There was no test available to verify the second and the third criteria. However, recently this test method called Water Drop Corona Induced (WDCI) has been developed [10]. The WDCI test method has been evaluated thorough a Round Robin Test during 2016 in five laboratories (four in Europe and one in Asia). The preliminary results from the electric field calculations for two set-ups are presented in Figure 2, showing that the first test set-up should not suffer from any corona, but the second test set-up should lead to corona. The results from the Round Robin test, shown in Table 2 and example images in Figure 3, solidly verified the results predicted by the calculations in all participating laboratories. 3

4 Figure 2. Table 2. Left : Test set-up 1 which should pass the test based on electric field calculations. Right : Test set-up 2 which should not pass the test based on electric field calculations. Summary of results from Round Robin test of water induced corona test method (green = accepted, red = not accepted) [10]. Laboratory Water drop corona on housing (Yes/No) Correct grading ring Incorrect grading ring position position 1 No Yes 2 No Yes 3 No Yes 4 No Yes 5 No Yes Figure 3. Left : example of accepted test (no corona on insulator housing). Right : example of failed test (corona on insulator housing). The developed criteria for water drop corona have also been verified by service observations. For example the electric filed calculations for one of the insulators of interest ended with maximum electric field along 10 mm of 1.3 kv/mm and the criterion is 0.42 kv/mm, which is shown in Figure 4 (left). This should lead to corona on the insulator in service. This prediction was confirmed by service inspection in wet conditions (Figure 4, middle) and the close up inspection revealed even deterioration from corona activity on the housing on the first shed to the HV end. The corona activity was so intensive that a crack in the sealing could be found, shown to the right in Figure 4. 4

5 Figure 4. Left: electric field along insulator (dark red = above 0,5 kv/mm). Middle : daylight UV camera showing corona activity on insulator in wet service conditions. Right: deterioration of the first shed of the insulator. The example shown in Figure 4 started the process of checking of all installed composite station post insulators in service at Svk from the electric field point of view. Calculations of these revealed that only three insulators out of eleven were properly graded (two 220 kv and one 400 kv). A summary of the results of the electric field calculations is shown in Table 3. The codes in Table 3 mean that e.g. A- 220 is insulator type A of 220 kv without grading ring, while B-400C kv is insulator type B of 400 kv rating with grading/corona ring. All products having at least one grading/corona ring are designated with suffice C. To summarize, eight out of eleven insulators were found overstressed which led to further investigations. These were conducted via service inspections and laboratory investigations. Table 3. Summary of calculations of composite station post insulators (green = accepted by the criterion; yellow = close to the criterion; red = not accepted by the criterion). Maximum average electric field along 10 mm of insulator Maximum electric field at triple point (end fitting/housing/air) Insulator (kv/mm) (kv/mm) NN Criteria (kv/mm) A B C-400C D E F G G-400C H-220C I J-220C SERVICE INSPECTIONS OF INSULATORS WITH VOLTAGE Service inspections of the insulators overstressed electrically revealed that the intensive corona can be observed on the insulators in service close to the HV end. It is important to note that the corona was not observed in the same conditions on apparatus composite insulators installed at the same substations, thus it can be concluded that the apparatus insulators were designed properly from this point of view. In the worst cases the corona was also observed along the insulator away from the HV end. Inspection of the same insulators by IR revealed hot spots in the same places along the insulator where corona was detected, see Figure 5. 5

6 Figure 5. Left: example of corona along the insulators detected by UV; right: example of corona along the insulators detected by IR. SERVICE INSPECTIONS OF INSULATORS WITHOUT VOLTAGE The close inspection of the de-energized insulators revealed that in the locations where UV/IR detection showed corona and hot spots, punctures or cracks of the housing were observed (see example in Figure 6, left). Depending on the level of electrical overstress at the HV end, deterioration of the housing (close to the HV end) due to intensive corona was also observed; see example Figure 6, right. The hydrophobicity of insulators was good in general, except the parts affected by corona, which were hydrophilic. Figure 6. Left: example of puncture; right: example of deterioration due to intensive corona. AFTER-SERVICE LABORATORY INVESTIGATIONS Test program The main objective of the after-service laboratory testing and analysis was to determine the severity of the deterioration of the punctured insulators. The results of the tests together with service observations were intended to quantify the risk for reduced life expectancy or even failure. Thus, the test program contained the following tests: Standard switching impulse test. Non-standard adhesion test. Standard steep-front impulse test. Dissection and visual observation. Standard dye penetration test. 6

7 Test results The punctured insulators passed the switching impulse test with no indication that the impulse voltage level was reduced. Thus it was considered that the probability to fail in short term is low. However, it was also considered that the test with the simulation of humid service conditions by short-term water immersion might not properly simulate the actual service environment. In the frame of this project there were developed and applied two very practically methods for the evaluation of adhesion, i.e. the so-called stripe-test and square-test. Stripe test: Two parallel cuts with a parallel distance of 10 mm are made. Then a single cut perpendicular with the two earlier cut is made. All cuts need to be fully conducted through the silicon rubber. The 10 mm stripe is then pulled out with pliers from the edge of the perpendicular cut. Square test: Squares of roughly 100 mm 2 were made. As for the stripe test it is very important that all cuts are fully conducted through the silicon rubber. The squares are then pulled with pliers. This simple non-standard adhesion test clearly revealed areas of low level on tested insulators, see example in Figure 7, left. The low level of adhesion is considered as the primarily cause of the damage of the punctured insulators. The overstress by the E-field just accelerated the process. The experience with standard IEC steep-front test showed that this test was not able to reveal insulators with low level of adhesion. The dissection followed by visual inspection and dye penetration test confirmed that the damage under the silicon rubber is extensive, see Figure 7, right. The tracking (conductive path) in the glass fibre rod makes the epoxy resin to erode, leaving only glass fibres. The damaged part of the glass fibre rod is also conductive. This means that the punctured insulator can fail in long-term either mechanically or electrically. Figure 7. Left: example of weak adhesion revealed by simple home-made test; right: example of deterioration in the interface rod/housing penetrating the rod. DISCUSSION AND RECOMMENDATIONS In contradiction to in general positive experience with composite insulators world-wide, the comprehensive test program applied for some composite station post insulators revealed that some of them have very weak adhesion between the rod and the hosing. This is considered as the main reason for punctures and cracks observed in service. These insulators were also electrically overstressed, which made the process of the degradation even faster. This leads ultimately to shorter live expectancy of the insulators. The standard steep-front test is intended for revealing of internal defects in ceramic and composite insulators often in the form of different voids and intrusions. It was believed by international community that this test would also be able to reveal low level of adhesion. Unfortunately, this was not a case and the insulators with clearly low level of adhesion detected by adhesion test passed the 7

8 standard steep-front test and even one trial at higher steepness that recommended by the IEC. The results show that a modification of the test is needed to reveal low adhesion. The simple non-standard adhesion tests presented in this paper clearly revealed the weakness of some insulators. Such tests do not exist in the present IEC standards, however are applied by many manufacturers in-house. This issue should be a topic of interest for CIGRE/IEC in the future, i.e. to develop new representative test methods to evaluate the level of adhesion (might be both sample and design tests) and to revise relevant IEC. Similar proposals can be found in recent publications [11]- [13]. The criteria for the limitation of electric field proposed in IEEE paper [6] and backed by service experience reported in this paper should also be of interest for CIGRE/IEC (recently they were presented to CIGRÉ WG B2.57). BIBLIOGRAPHY [1] CIGRÉ WG B2.21, Guide for the Assessment of Composite Insulators in the Laboratory after their Removal from Service, CIGRÉ TB 481, December 2011 [2] CIGRÉ WG B2.21, Assessment of in-service Composite Insulators by using Diagnostic Tools, CIGRÉ TB 545, August 2013 [3] CIGRÉ WG A3.21, Aspects for the Application of Composite Insulators to High Voltage ( 72kv) Apparatus, CIGRÉ TB 455, April 2011 [4] IEC 62217, Ed. 2.0, Polymeric HV insulators for indoor and outdoor use General definitions, test methods and acceptance criteria, 2012 [5] IEC , Ed. 1.0, Composite station post insulators for substations with AC voltages greater than V up to 245 kv Part 1: Dimensional, mechanical and electrical characteristics, 2015 [6] A.J. Philips, A.J. Maxwell, C.S. Engelbrecht, I. Gutman: Electric Field Limits for the Design of Grading Rings for Composite Line Insulators, IEEE Transactions on Power Delivery, Vol. 30, No. 3, June 2015, p.p [7] P. Sidenvall, I. Gutman, L. Carlshem, J. Bartsch, R. Kleveborn: Development of the Water Drop Induced Corona WDIC Test Method for Composite Insulators, IEEE Electrical Insulation Magazine, November/December 2015, Vol. 31, No. 6, p.p [8] CIGRÉ WG B2.03, Use of corona rings to control the electrical field along transmission line composite insulators, CIGRÉ TB 284, December 2005 [9] IEC Standard, Overhead lines Requirements and tests for fittings, IEC Standard 61284, Second edition, [10] P. Sidenvall, I. Gutman, L. Carlshem, J. Bartsch: A Round Robin Test of the Water Induced Corona Test, ICOLIM-2017, Strasbourg, France, April 2017, paper 0017 [11] C. Ahlrot, J. Lundengård, I. Gutman, Need of standardized adhesion test for composite insulators: lessons learned from service experience, 20 th ISH-2017, Buenos Aires, Argentina, August 28 September 01, 2017, paper 145 (to be published) [12] I. Gutman, P. Sidenvall, T. Condon: Evaluation of composite insulators with internal deterioration: lessons learned from service and after-service testing, CIGRÉ Winnipeg 2017 International Colloquium & Exhibition, Winnipeg, Canada, September 30 October 6, 2017, paper 142 (to be published) [13] F. Zhang, Z. He, Y. Liao, G. Wang, B. Luo, Research on the adhesiveness between core rod and sheath for composite insulators on the transmission lines, INSUKON-2017, Birmingham, UK, May 2017, p.p