On Line Diagnostics for Condition Based and Reliability Centered Maintenance

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1 1 On Line Diagnostics for Condition Based and Reliability Centered Maintenance Carl Öhlen* STRI Abstract-- Maintenance is required for almost all equipment in order to guarantee its performance, prevent failures and to extend the life expectancy. Modern high voltage equipment as well as protection & control equipment requires less maintenance compared to older technology. Unnecessary maintenance can also be a cause of failures by itself. Both failures and traditional maintenance will normally result in a disconnection of a part of the power system and by this reduce the availability. This paper describes on-line monitoring and on-site measurement for interruption free diagnostics of the power system. It also describes possible web-based On Line Analyser with database and trends. This creates information for Condition Based Maintenance (CBM) and Reliability Centred Maintenance (RCM). The goal is to achieve the perfect balance between maintenance and operation to optimize both availability and cost using on-line monitoring and web based diagnostics together with state-of-the art protection & supervisory control. I. INTRODUCTION The unavailability of the power system can be simplified as MTTR/MTBF + MTTM/MTBM: - MTBF = Mean Time Between Failure - MTTR = Mean Time To Repair - MTBM = Mean Time Between Maintenance - MTTM = Mean Time To Maintain The ideal situation with zero unavailability and 100% availability can theoretically be obtained if MTBF and MTBM is extremely high (infinity) which means that there are no faults and no need for maintenance. This is naturally impossible to obtain. The other alternative is that MTTR and MTTM is zero. This is however possible to obtain with a combination of redundancy and on-line diagnostics. Redundancy according to the n-1 criteria means for example that one component can be lost with maintained availability of power between two points. Still full redundancy for all components in a power system (lines, transformers, generators) is costly and un-realistic. If however the interruption can be limited to spontaneous faults (normally lightning) followed by a rapid auto re-closing the unavailability can still me minimised. By using on-line monitoring to discover stresses and ageing before a failure occurs and plan suitable maintenance (or replacement) the interruption of that particular part of the system can be done when other redundant part can maintain the operation. This is called CBM, Condition Based Maintenance and replaces the earlier TBM, Time Based Maintenance or in worst case NMA = No Maintenance at All. CBM can be further refined to RCM, Reliability Centered Maintenance which is using RBA, Risk Based Analysis to give the priority to maintenance and replacement activities. * carl.ohlen@stri.se

2 2 II. AN HOLISTIC APPROACH TO ASSET MONITORING A. Closing the Information Gap This vision of interruption free diagnostics of a complete network with substations, lines and cables requires a holistic approach identifying the critical components and possible supervision methods including both existing and new ways of monitoring. Today the data about the condition of different equipment is spread among different persons, organisations and measurement equipment. This includes everything from paper reports from manual inspection to digitally stored disturbance recordings. The main challenge is to convert raw data islands to useful information for Asset Management and Operation. This includes an efficient communication infrastructure and data management where for example high speed LAN/WAN according to IEC can be utilized. The major task is however still to analyse the data and present useful information for different users. This can be done manually or automatically. Since different organisations or even companies owns the assets, operate the assets and maintenance the assets another challenge is to improve the information exchange between these organisations. Some utilities has therefore created a special Asset monitoring function to handle both short term, mid term and long term analysis and planning. [1] In order to efficiently managing the data and then analyze the data to give directly applicable information to different users this paper will present a web based On Line Analyzer. This can directly communicate with on-line monitoring devices but also be the portal to enter inspection data. This means that all information about a certain asset is integrated in one portal = Asset Monitoring Site. For manual on site inspection of for example insulators and conductor joints it is however essential to have a common guide and template how to inspect and rate each object. These guides are available today together with the On Line Analyzer. How to get from raw data to information!asset Monitoring/Management!Systems integration!automatic data analysis!manual data analysis!data management!communication infrastructure!data capture Fig. 1. Data to information framework [1].

3 3 B. The right action in the right time The power system and each equipment part is designed for a certain voltage level and a certain current level. If these levels are exceeded the equipment will either fail or degenerate faster towards the technical and economical life length. There are certain defined limits, which should be verified in order to guarantee the performance of the system. You may in a very simplified manner divide this in three main areas. TABLE I EVENT/CAUSE/TIME/FUNCTION Event Cause Time to action Function Electrical short-circuit fault Lightening etc. ms Relay protection Abnormal operation Emergency load etc. seconds -hours Supervisory control Stress and ageing Insulation/contact wear days months -years On-line monitoring Lightning and other weather effects are most often the cause of an electrical short circuit and the location is most often on a transmission line. (75% - 95% of all faults) This means that it is not possible to completely eliminate. Instead a reliable and fast protection is required together with an auto re-closing scheme. Differential protection is the most accurate, rapid and selective method and can today with better communication also be used for lines even though distance protection is still in majority. The auto reclosing can be done three pole, multi pole or single pole. More advanced schemes can provide multi pole auto re-closing of two or several parallel lines in order to guarantee that a minimum of three different phases between two points are in operation. This type of logic is now relatively simple to introduce with the IEC high speed inter bay communication in a substation. Both stress, ageing and abnormal operation may lead to an electrical fault if not taken care of. But by right dimensioning of the towers, insulators, distances in combination with surge arresters and point-on-wave switching can reduce the probability for electrical failures due to lightning and switching. The software Line Performance Estimator available from STRI is such a design tool. Line Performance Estimator Fig. 2. Fault frequency and successful auto re-closing rate as a function of insulation and environment

4 C. Monitoring all links in the chain The overall availability of the chain depends on each individual link. It is therefore important to identify those critical links and assure that the condition is monitored either with continuous monitoring or manual inspection. Transformers and other equipment are monitored for temperature, breakers and GIS for SF6 density etc. But there are still no overall diagnostics, which can accurately predict if and when a more detailed test and maintenance should be done. And although a lot of data exist today there are still major parts of a substation with connected lines and cables, which are not being monitored or diagnosed. This includes insulation, operating mechanism, conductor joints, disconnectors as well as influence of pollution and weather. In the following some possible methods are presented which can be used for on-line monitoring and on-line diagnostics. Condition Monitoring 4 Generation Transmission kv Distribution kv Industry To be monitored and/or tested: " Line ending and joints (Resistance/Temp/IR) " Conductors (Temperature) " Towers (Corrosion) " Cable ending and joints (PD, Temp/IR) " Arresters (Leakage current) " Earthing switch (Resistance to ground) " Voltage transformer (Oil analysis) " Current transformer (Oil analysis) " Disconnector (Resistance/Temp) " Circuit Breaker (SF6, time, resistance, vibration) " Insulators (Temp/IR) " Power Transformer (Oil and gas analysis) " Generator (PD, temperature, vibration) Fig. 3. Different methods for condition monitoring Partial Discharge or PD monitoring is one of the methods, which now is available to monitor the insulation of generators, transformers, cables and GIS without taking the equipment out of service. The advantage is that a degeneration of the insulation can be detected well in advance and save the equipment from a complete and costly failure. The main challenge also here is to have an efficient on-line monitoring and diagnostics, which can establish when the measured PD is dangerous and requires an action. Circuit Breakers are essential for the system availability. The reliability of circuit breakers has improved during the years. International statistics shows that modern SF6 breakers with spring operating mechanisms have a low failure rate and a low maintenance requirement. It is also possible to monitor the breaker contacts by monitoring the number of operations and calculating I 2 t as a measure for contact wear. Besides for reactor breakers the wear is normally not a problem. According to statistics the operating mechanism is the major source of a failure. Traditionally time, travel or dynamic resistance has been used for breaker testing. It is however difficult to use these methods for on-line monitoring. Vibration testing is a new method, which can be used for monitoring (continuous or intervals) with a normal open-close operation.

5 Both PD monitoring and vibration monitoring gives high frequency data which has to be compared with an original signature to indicate any trends. This requires a database with old data as well as a user-friendly tool to analyse the result. Insulators are the most common part of the power system either as line insulators or as part of equipment and bushings. This means that the insulators have the highest risk for flashovers. Still there is normally no monitoring of insulators. Ageing or vandalism resulting in cracks in porcelain is an example of a mechanical defect. This can be detected by inspection. Snow and ice can also be detected visually. Pollution is more difficult. Pollution monitors available to utilities have been evaluated by CIGRÉ Taskforce first report, containing a review of monitors for characterizing pollution severity, as many as 33 different monitors were identified. The work was followed by a comparative field study where in total 11 monitors and site severity measurements were evaluated at 13 sites for a period of 24 months. [3,4] The protection and control system of modern design is to great extent self monitored. This includes both the electronics, communication but also connection to current and voltage transformers and trip circuits. These so called IEDs (Intelligent Electronic Devices) also includes disturbance, event and fault recorders, which can be used to analyse faults but also detect incorrect functions before they result in a fault. Modern IEDs designed for IEC can provide new flexibility, redundancy, functionality and communication. The standard separate hardware from functions and allows new architecture with duplication of all functions, interchange of data for more intelligent automation etc. More and more will also install high speed Internet or WAN in the station which allow rapid access of all data. 5 III. EXAMPLES OF ON-LINE MONITORING AND DIAGNOSTICS A. On-line monitoring of a generating station STRI has together with a LDIC, a German manufacturer of monitoring equipment and VB Kraft, a Swedish Utility installed a system for on-line monitoring and diagnostics of a generating station using SOLA, STRI On Line Analyser to monitor partial discharge, load and temperature. The data is collected in the station and then routed via the utility LAN and the Internet to the Asset Monitoring Site. All users can then access this site via Internet and get all historical and actual data as well as trend analysis. Fig. 4. STRI On Line Analyzer for a generating station

6 6 B. On-line monitoring of a substation STRI has together with Statnett, the Norwegian TSO installed an advance on-line monitoring and on-line diagnostics system for a 400 kv substation in Norway. Besides monitoring of electrical and environmental data, three cameras are monitoring pollution and swing angles on insulators. This is done by a picture analysing software. The data is collected in the station and then routed via the utility WAN and the Internet to the Asset Monitoring Site. All users can then access this site via Internet and get all historical and actual data as well as trend analysis. Fig. 5. STRI On Line Analyzer for a substation IV. REFERENCES [1] On-Line Condition Monitoring of High Voltage Equipment in Two Transmission Networks in Australia, CIGRE 2006 A3-201, S. Jones, A. Kingsmill, M. Blundell, J. Gabb, G. Slade. [2] Vibration Analysis for Diagnostic Testing of Circuit Breakers, IEEE Transactions, Vol 11, No. 4 Oct 1996, M Runde, G.E. Ottesen, B. Skyberg, M. Öhlén. [3] CIGRÉ Taskforce , Round Robin Pollution Monitor Study, 11 August 2000 [4] CIGRÉ Taskforce , Insulator pollution monitoring, Electra, No. 152, pp , Feb [5] IEEE DEIS Laboratory Tests and Web Based Surveillance to determine the Ice- and Snow Performance of Insulators. To be published S.M. Berlijn, I. Gutman, K.Å. Halsan, M. Eilertsten, I.Y.H. Gu. V. BIOGRAPHIES Carl Öhlen was born in Sundsvall, Sweden, on July 4, He graduated as M Sc in Electric Power Engineering at the Royal Institute of Technology in Stockholm and he has worked within the international power industry located in Brazil, Switzerland, Sweden and USA for more than 30 years especially with protection, automation and diagnostics. He is now employed by STRI, an independent technology consulting company and accredited high voltage laboratory, located in Ludvika, Sweden.