Experience on Deterioration Survey of Aged 550 kv GIS and Optimized Evaluation Model in Thailand

Transcription

1 PS1 Experience on Deterioration Survey of Aged 550 kv GIS and Optimized Evaluation Model in Thailand S. Singharerg T. Watanabe Electricity Generating Authority of Thailand Chubu Electric Power Co., Inc. Thailand Japan T. Abe A. Okada Hitachi T&D Systems Asia Pte. Ltd. Hitachi, Ltd. Singapore Japan SUMMARY The first Gas Insulated Switchgear (GIS) was installed in Electricity Generating Authority of Thailand (EGAT) in While recently, GIS gained in the system has been newly constructed rapidly and renovated from existing Air Insulated Substation (AIS). On the other hand, existing GIS should have optimistic maintenance management to prevent major failures with high maintenance cost, long recovery time, especially expanded shutdown in some parts making them difficult to maintain the high reliability of the transmission networks. So EGAT started to perform GIS refurbishment in 2013 according to manufacturer s recommendation. However, the portion maintenance and spare parts which were recommended by manufactures did not match up to the proper conditions of GIS. These became countermeasures to be taken for the aged GIS which were used for long terms together with how to co-operate and actually investigate their conditions among manufacturers, maintenance specialists and local maintenance engineers. The main objective of this study is optimized maintenance such as refurbishment, replacement, and improvement of the design and quality, especially aging or wearing components of GIS in The study results of this paper are the deterioration analysis of age and maximum of short circuit interruption of 550 kv GIS in Thailand. The survey project including the investigation details and analysis of the GIS (indoor) manufactured and installed in 1997 were done on site by GIS manufacturers and the Japanese utility acting as a third party so that we can design the optimized maintenance lists and required countermeasures for further years of usage. Besides, the deterioration survey was implemented in various items such as O-ring analysis in gas sealing portions, internal and external grease chemical analysis, internal inspection of accumulator in hydraulic system, auxiliary devices, flange condition check of outdoor except routine and detail maintenance items. Gas Insulated Bus (GIB) is required to maintain GIS as good performance. Keyword GIS, GCB, GIB, Deterioration, Lifetime, Optimized maintenance, O-ring, Grease suthep.si@egat.co.th 1

2 1. INTRODUCTION In Electricity Generating Authority of Thailand (EGAT) system, the first gas insulated switchgear (GIS) was put in service in 1988, and then it was gained more due to demand increase and high reliability. GIS have been installed by not only new construction but also renovation of aged switchgears in air insulated substation (AIS) with limited space. Although service experience of GIS in the form of failure statistics was very low when compared to AIS, scheduled maintenance and refurbishment for different manufacture, model, location and service conditions are very important for preventive malfunction and failure reasonable cost. Therefore, EGAT started to perform GIS refurbishment in The GIS was audited by the manufacturer. For example, collection operating and failures data, maintenance history, visual inspection, recommendation maintenance items, procedure and spare parts for the whole GIS were inspected. As a result that some maintenance items and spare parts did not match up to the proper conditions of GIS, these are caused from over conservative maintenance and then became costly. On the other hand, there are lack of spare parts in some portions, such as auxiliary relays and control equipment. From the background, EGAT has created basic GIS maintenance policy firstly. And next, the pilot cooperation project of GIS maintenance and investigation among manufacturers, users and third party has produced an effect of early evaluation of the conditions before planning to refurbish for substations. 2. TARGET GIS SELECTION By considering and evaluating various factors such as age, importance in power system, operation history, possibility of shutdown, experience of the parties, the target GIS and one sampling bay of the pilot maintenance project was selected. Figure 1 and Table 1 show the appearance and the specification of the target GIS, respectively. Figure 1. Target GIS Table 1. Specifications of Target GIS Items Spectification Rated voltage 550 kv Rated current 4000 A Rated breaking current of GCB 50 ka Manufacturing year 1997 Age 19 Operation drive of GCB Hydraulic Use Transmission Line Installation Indoor Number of operation of GCB About 500 (A : 496, B : 499, C : 492) Number of fault interruption Total 19 times in 3 phases, Maximum current: ka Failure records 2 minor failures Maintenance history No internal maintenance 3. INVESTIGATION FOR CONDITION ASSESSMENT EVALUATION The detail maintenance and investigation work were carried out on the target sampling bay of GIS. The condition of the GIS were evaluated by the analysis of the results of the work. 3.1 Investigation of O-ring Even though the low gas alarm within GIS has never occurred for 19 years of service. However, the present condition of O-rings on flanges of GIS and driving mechanism should be checked to estimate their lifetime. 2

3 The number of 27 O-rings were taken from 11 portions including static and dynamic function. The deterioration of O-ring is estimated by permanent compression set factor. The O-ring material is Nitrile Butadiene Rubber (NBR). The estimation result is shown in Figure 2. The life time of O-ring which the permanent compression set factor is 80 % is estimated by using the degradation curve of NBR. As the result, the estimated lifetime of static O-rings (on flange surface of GIS tank) is around 90 years, and that of dynamic O-rings is around 35 years. This result can be decided that it is not necessary to replace the static O-ring for preventive maintenance during their lifetime such as 30 or 40 years in this GIS condition, and then corrective maintenance may be used if the alarm of gas pressure low is found case by case. For the condition of dynamic O-rings, the data is not enough for evaluation, therefore, it is important to collect the field data. 3.2 Grease Analysis Figure 2. Life Evaluation by O-ring Analysis (Static O-ring of GIS Tank) The internal grease applied to the contact surface in interrupter of GCB is hardened or dried up due to deterioration. The deterioration level of grease can be checked by chemical analysis that is measuring rate of base oil of the grease. The rate of base oil is measured by using thermogravimetric analysis (TG). [3] Figure 3 shows grease analysis results. It shows that the grease lifetime can be estimated around 38 years from the average of main contacts and around 24 years from the worst data. The internal grease condition is different of the operation condition. This target GCB (bay) is the one with the highest current interruption in the GIS in the substation. The age of years is the good timing for checking internal condition of GCB for this GIS. Figure 3. Life Evaluation by Grease Analysis 3

4 3.3 Investigation of Control Devices, Auxiliary Relays and switches Sampling investigation of control devices, auxiliary relays and switches on GIS was carried out. It was checked by two methods, such as contact resistance measurement and chemical analysis of the material on relay/switch contacts and relay case. Figure 4 shows the results of the contact resistance measurement. The measured contact resistance is lower than 150 µω. It is good condition level according to the Japanese field experience. As a result of the chemical analysis, it was no problem. Some materials except original ones are found on contacts and relay case. They are same as the ones of dust. Sulphur, which causes Ag 2S and contact resistance increase, is detected, however, the value is very small. As a result of this investigation, it is not necessary to replace auxiliary relays in the GIS in this substation every 12 or 18 years of the period that the manufacturer recommendation. However, it is necessary to have some spare parts for the emergency cases Maximum Resistance (m-ohm) Minimum Average CB Line-DS Line-DS LCC LCC Auxiliary switch of CB Auxiliary switch of L-DS Magnet contactor (89X, 89Y) for L-DS 12.9 Auxiliary relay for control panel (OMRON RY) Control SW, LR SW (Age: 19 years) (Age: 19 years) (Age: 19 years) (Age: 19 years) (Age: 19 years) Figure 4. Auxiliary Relays & Swiches and Control Switches Evaluation by Contact Resistance Measurement 3.4 Visual Inspection of GCB Interrupter As a result of internal inspection of GCB, arcing contacts and nozzles were slightly wearing and there is no grease on the arcing contacts due to number of breaking fault current with 19 times in three phases. Insulator, grading capacitor, corona shield, all connections was checked by visual inspection, and then there are not any signs of track mark or cracks. Decomposed materials are found in the bottom of tank inside the compartment. We estimated that the condition of arcing contacts, main contacts and nozzles were still acceptable. Sampling internal inspection of GCB is important to check the condition of GCB in GIS, and it is necessary to carry out internal inspection with the specified operation number or fault current breaking number of GCB. 3.5 Inspection of Hydraulic Operating Mechanism The GCB is drove with hydraulic operating mechanism. For the sampling phase, this mechanism was approximately used 500 times or 25 % of design criteria, and the running time of hydraulic pump reached 400 hours. The parts of the mechanism such as pilot valves, pressure switches, hydraulic oil and filters were checked and replaced. One of accumulators was visually investigated. As a result, T- ring on piston looks deteriorated though there is no N 2 gas leakage. We will consider and make decision of the maintenance method of hydraulic mechanism with manufacturer cooperation for long term use of GIS. 4

5 3.6 Visual Inspection of Flange Surface Condition of Outdoor GIB Some flanges of outdoor GIB such as hand holes, bottom tank cover below bushing, were checked visually to evaluate their conditions. As a result, the flange condition of hand holes was good conditions. On the other hand, the flange surface and bolts of bottom tank cover below bushing became rusted that the rust reached O-ring but not though the groove yet as shown in Figure 5. The liquid gasket and sealing washers were designed and used for the hand holes of GIB for preventive corrosion, however, they are not used for the tank cover below bushing. Therefore, bolts with sealing washer and caulking flanges and bolts should be applied at those portions as rain water proof for the long term use of the outdoor GIB. Figure 5. Rusted Flange and Bolts of GIB 4. CONCLUSION As a result of this survey, there were some remarkable points revealed. For example, lifetime of GIS targeted can be extended by optimized maintenance and improvement or countermeasures for further years of usage than the general design lifetime. This phenomenon will be repeated for the other GCB and GIS. The results of the joint survey also showed us as the followings; 1. General deterioration factors such as plating of contacts, O-rings and grease might be several deterioration potentials which still remained in itself as per each manufacturer s design, even if that aged GIS had good performance without specific trouble with no open maintenance. 2. The optimized maintenance for the long term use of GIS shall be studied with considering the pilot project results. In addition, maintenance data management is important, and further investigation for data collection is desirable to estimate GIS life time. 3. Various types of aged GIS have to be continuously investigated and the effective maintenance and countermeasures should be also scheduled that based on the urgency and importance of the network operation. Furthermore, the method of optimized evaluation for several aged GIS with the experiences of refurbishment works are introduced as well. 4. The period of years is the important timing to perform the joint investigations and analysis of aged GIS s life extension by the utility and manufacturer. This model is effective for evaluation condition of GIS before planning for refurbishment. The pilot bay with most severity condition in the GIS can use to forecast remaining bays refurbishment program. Using data by user, knowledge and skill by manufacture and experience by consultant integrate the real condition to optimize maintenance program and ensure that GIS having performance under manufacture standard and reliable according to user need. Furthermore, by sharing knowledge and experience between utility and manufacturers, and the other parties with such experience if possible, will surely improve user s maintenance skill and become beneficial in the future. 5

6 BIBLIOGRAPHY [1] A. Okada, A. Cheang, K. Kawakita, Maintenance planning aiming at long term use of indoor GIS in Southeast Asia, CIGRE SC B3 & D1 Colloquium Brisbane 2013, No. 190, 2013 [2] CIGRE SC B3 WG B3.17, Residual Life Concepts Applied to HV GIS TB No. 499, 2012 [3] Y. Nakada, T. Watanabe, K. Takahashi, K. Iwabuchi, S. Ishizeki, K. Obayashi, Survey and Research on Deterioration of 300 and 550 kv GCB, 2013 CIGRE Auckland Symposium, No. 252,