C1-108 ASSESSING THE IMPACT OF MAINTENANCE STRATEGIES ON SUPPLY RELIABILITY IN ASSET MANAGEMENT METHODS

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1 21, rue d'artois, F Paris C1-108 Session 2004 CIGRÉ ASSESSING THE IMPACT OF MAINTENANCE STRATEGIES ON SUPPLY RELIABILITY IN ASSET MANAGEMENT METHODS M. SCHWAN * K.-H. WECK FGH e.v. Mannheim M. ROTH GEW RheinEnergie AG Cologne GERMANY 1 INTRODUCTION Liberalisation puts the utility companies under a steadily increasing cost pressure. The experiences in several markets have shown that typical measures taken by the affected companies to reduce their cost with respect to uncertain earnings in the future include the implementation of leaner and simplified network structures with components operating at higher loads, increasing the operation period of components due to reduced reinvestments, reduced maintenance efforts, and staff reductions. While especially changing the network structures surely will only have an effect in the long term, staff reductions and reduced maintenance efforts are expected to affect network operation also in the short term view. Also, it is expected that in general all these cost saving measures will have a negative impact on the quality of supply [1]. As customers and regulating authorities not only focus on the price of electrical energy, but also on its quality, the question to find the optimal balance between cost effectiveness and appropriate quality of supply is a key task for network operators in liberalised markets. Network planning including both technical and also economic aspects thus requires detailed and of course quantitative information in this framework. While this is less a problem for the financial part, the assessment of supply quality is more challenging. An appropriate tool which gains ever more importance in the field of supply reliability is probabilistic reliability analysis. The usage of modern probabilistic reliability calculation tools can be considered to be standard in today's network planning. The indices determined by these calculations considering e.g. the economic effects of the system's reliability performance under the given regulatory framework are also used in asset management methods. * schwan@fgh-ma.de

2 As the network operators are pushed to introduce cost saving measures as described above, the concern for the quality of supply requires that the effect of these measures on e.g. supply reliability should be assessed in appropriate planning studies. In principle, these effects are very clear: the individual measures will either have an impact on the component reliability, described by failure rates and outage durations, directly on the system reliability, e.g. by prolonging switching durations due to staff reductions, or on both aspects. However, quantifying the influence of such cost saving measures on the reliability indices of network components and on operational indices, such as fault clearing durations, is a very difficult task. In a recent analysis [2], the effect of increased component age and loading, of decreased maintenance activities and of staff reductions on the system supply reliability was estimated by applying appropriate factors on the input data for the reliability calculation. The reliability indices for these scenarios are compared to the indices calculated for the base variant using standard input data for the reliability calculation. The results showed that considering single cost saving measures one by one only has a limited effect on the system reliability performance. But in practice, typically all these measures are introduced simultaneously. Analysing this combined scenario, the effect shows to be very significant, as Figure 1 presents: Unavailability in min/a ' Base variant Scenario A B C D E F G H I J Figure 1 Unavailability for substations A to J of a 110 kv network in the base variant and in a scenario considering the effect of increased component age and loading, of decreased maintenance and of staff reductions on a 15 year horizon In this example, which was calculated for the different substations of a 110 kv network, the unavailability in the system is rising very sharply considering the effect of the described cost saving methods. Although the absolute numbers are based on rough estimates, it is obvious that the effect on supply reliability will be significant. Therefore, the effect of changes in the network structure and in the network operation should be precisely evaluated and monitored in order to be able to maintain the required level of quality of supply. In this paper, first some basic aspects about maintenance and asset management are presented, as far as they are relevant for the described method. Then, the general idea of a comprehensive asset management method is described, and special attention is paid to the interrelation between maintenance strategies and system supply reliability. 2

3 2 BASIC ASPECTS In the field of maintenance, there are several different measures which are often characterized as follows: Inspection: This planned measure serves to consider the actual condition of network components. Servicing: This planned measure is a preventive action taken to preserve the desired component condition. Measures include e.g. scheduled services or revisions of network components. Reinvestment: Although reinvestment is not part of maintenance in the strict theoretical sense, for practical reasons it is included in this list as a planned measure to restore the desired component function in a system. If a network component reaches a condition or age, respectively, where it is not (economically) possible to service or repair it, the replacement is the only solution to provide the required function in the system. Repair/replacement: This term includes all actions taken to change the component condition back to the desired state, or at least as close to the desired state as possible. Repair of course is necessary after a network component was affected by an active failure, but it might also be necessary because of the effects of operational wear or mere ageing, which might be detected e.g. by an inspection. If a repair is not (economically) possible, the component has to be replaced. Fault elimination: This measure clearly is not part of maintenance. However, it is often considered in close combination with maintenance measures, as the fault elimination is often performed by the same organisational units and staff as maintenance. Whereas inspections, servicing and also the reinvestment of network components can be planned deterministically in advance, fault elimination and repair/replacement depend on the occurrence of failures in the network and thus are highly stochastic. The distinction between deterministic and stochastic aspects is also of importance for the modelling of different cost segments. For network operators, typically the most significant cost segments are capital costs, representing the installed components in the network, and the maintenance costs as the largest part of the operational costs. For the considerations presented in this paper, capital costs and certain parts of the maintenance costs costs for inspections, servicing, and reinvestment can be considered to be deterministic. Other parts of the maintenance costs costs for repair/replacement and the costs for fault elimination are depending on the stochastic occurrence of failures. Also, the ongoing trend to impose penalties on the violation of power quality targets set by regulating authorities (e.g. in the UK [3] or in Norway [4]) will put more importance on stochastic cost components. Of course, also the costs designated as deterministic are subject to stochastic aspects the reinvestment costs e.g. are influenced by time or budget overspending which might be caused by a multitude of reasons. However, the probability distributions describing those stochastic aspects are typically much narrower than the distributions of the very rarely occurring failure events. 3

4 In general, the high relevance of stochastic processes requires the application of suitable risk management methods [5, 6]. Risk management methods can be used both in the assessment of the system reliability performance with its respective cost segments [7], and in the assessment of the scheduled costs for servicing and reinvestment [8]. 3 COMPREHENSIVE ASSET MANAGEMENT APPROACH With the background described above, trying to solve the key question of how to find the optimal balance between cost effectiveness and appropriate quality of supply has to be supported by comprehensive asset management methods. These methods have to consider both economical and technical aspects, and have to represent the stochastic nature of certain cost components. The principle of a comprehensive asset management approach is described by the schematic diagram shown in Figure 2: Prognosis Component - reliability data Network and component data e.g. component age, maintenace history Strategies Inspections Servicing Reinvestment Fault elimination Reliability calculation. Risk analysis Calculation Stochastic costs Fault elimination Repair / replacement Penalties / compensations Deterministic costs Inspections Servicing Reinvestment Other fixed costs Analysis and optimisation Figure 2 Principle of a comprehensive asset management approach The bases for this method are of course network and component data, like e.g. the number, age structure and maintenance history of network components, together with data on the current component reliability, eventually considering the current component condition. Together with a model of the chosen strategies for inspections, servicing, reinvestment and fault elimination e.g. describing the time interval for inspections, or the number of personnel available for fault elimination and repair, which determines the duration of fault elimination and of component outages the different cost components for the actual situation can be calculated for a given time period. For the deterministic costs this calculation is comparatively easy. For the stochastic costs depending on the occurrence of failure events, a probabilistic reliability calculation of the actual situation, together with appropriate risk management methods, is required to derive costs for fault elimination, repair and for penalties or compensations according to the given regulation. 4

5 In the final analysis of the actual situation, the separate components are evaluated and the overall effectiveness of the scenario under consideration is assessed. Changes in the chosen strategies may be derived in order to investigate the effect of different strategies and in order to achieve preset target values. With the adapted modelling of the maintenance strategies, a fictive situation is calculated in the next step. The modules required for this comprehensive asset management method also considering the system's reliability performance depending on the strategies for inspections, servicing, reinvestment and fault elimination are all available today, except for one key point. It is quite obvious that the chosen strategies do have an effect on the reliability of the network components. While the qualitative interrelations are described easily the less maintenance activities are performed and the older the component, the worse is the component reliability it is typically not at all possible to describe these interrelations in a detailed and quantitative way. Especially the ability to quantify the effects is of course a prerequisite for any calculation. So, the prognosis of the component reliability indices depending on the age of the component and on the chosen strategies is of crucial importance for asset management. But for most components appropriate models are not available. Even if rough models are known, gathering the required input data for these models is critical in their practical application. 4 MAINTENANCE STRATEGY AND SYSTEM SUPPLY RELIABILITY PERFORMANCE 4.1 Modelling As described above, the effect of the chosen strategies for servicing, reinvestment and fault elimination on the component reliability has to be modelled in a suitable way. Basically, this means to quantitatively describe the effect of ageing as the age of the network components is determined by the chosen reinvestment strategy and the effect of maintenance activities on the course of the component failure rate λ over time (Figure 3). λ [1/a] Without maintenance With maintenance t [a] Figure 3 Component failure rate λ over time, with or without maintenance activities For the modelling, two different approaches are possible. With the first approach, the component reliability is just considered as a property depending on the described aspects. By analysing suitable statistics, a quantitative interrelation is described from the documented experience. The physical interrelations themselves are kept as a black box. 5

6 With the second approach, the physical interrelations themselves are modelled explicitly. This requires first to identify a physical property of the component being relevant for the functioning of the component in the system and being influenced by the ageing process under consideration. Second, from historical experience or laboratory tests, the effect of ageing and of maintenance activities on this physical property has to be determined. Also, the operational stress relating to this physical property that the component is exposed to in the system has to be determined. Combining these quantitative descriptions, the resulting failure rate of the component with respect to the ageing process under consideration can be calculated. 4.2 Example for VPE cables The modelling of individual failure processes is demonstrated for the example of VPE cables affected by water trees [9, 10]. As said before, usually detailed models and statistical data are not available in the required level of detail in order to derive such models. However, some of the first VPE cables manufactured in the 1970ies proved to suffer from water-trees, rapidly deteriorating the insulation capability of the cables. As this caused massive problems in network operation, very detailed statistics and analyses were performed on this special case. The physical property describing the insulation capability is the disruptive discharge voltage of the VPE insulation, and the corresponding operational stress is the voltage stress. Experience from the field and from laboratory experiments showed that both the disruptive discharge voltage and the voltage stress depend on the operating conditions, i.e. normal operation, operation under earth fault conditions and very rarely the occurrence of transient overvoltages. The course of the disruptive discharge voltage over the operation duration and the distribution of voltage stresses are shown in Figure 4. Disruptive discharge voltage in kv Operation duration in years Frequency per year ,1 0,01 0, Voltage stress in kv Power frequency voltages Transient overvoltages Earth fault overvoltages Switching overvoltages Lightning overvoltages Continuous voltage Transient overvoltage Earth fault overvoltage a) b) Figure 4 a) Disruptive discharge voltage of certain VPE cables b) Typical voltage stresses in a 20 kv distribution network Combining the description of the disruptive discharge voltage and the corresponding operational voltage stress, the resulting failure rate of the VPE cable can be derived. The result is shown for certain VPE cables manufactured in 1975 in Figure 5 a. This figure both presents the results of the theoretical modelling as described above, and of the operational experience documented in a specific cable statistic which was introduced to cover this topic. It can be seen that the results fit very well. Also, the effect of the increasing failure rates is 6

7 shown in Figure 5 b. Here, reliability indices for a distribution network were calculated, assuming a failure rate for cables in this network according to Figure 5 a. It can be clearly seen that for a certain period, the system s reliability performance is not affected. However, as soon as the effect of cable ageing becomes visible, the decrease of reliability is very sharp. Failure rate in 1/cable system/year 0,6 0,5 0,4 0,3 0,2 0,1 0 Production year 1975, earth fault conditions Production year 1975, normal operation Cable statistic Cable age in years Interruption frequency in 1/year Interruption frequency Energy not supplied Time in years Energy not supplied in MVAh/year a) b) Figure 5 a) Failure rate of certain VPE cables depending on the cable age b) Corresponding reliability indices for a distribution network 4.4 Research project As mentioned before, it is an exception that quantitative descriptions of the ageing processes of network components are available. Because of the high relevance of such models for asset management methods, a research project was initiated to cover this topic. In this research project, 12 network operators, 2 service companies and 4 academic institutions take part. The aim is to derive models describing the component reliability in dependency of the operation conditions, the component age and the actual maintenance strategy. The project consists of three main tasks: Task 1: Data collection on component failures, including cost aspects Task 2: Determination of the failure rates of network components using the statistical data Task 3: Implementing the derived models in a comprehensive asset management method Currently, the recording scheme for the statistic on component failures is defined and the data collection has started. The final results of this project are expected to be available in CONCLUSION Asset management methods are essential tools for network operators acting in the conflict area between cost saving programs on one side and power quality requirements on the other side. The principle of a comprehensive asset management method, considering both deterministic cost components, like costs for preventive maintenance, and stochastic cost components, like costs for fault elimination or penalties depending on the reliability performance of the system, is described. The modules required for this method are all 7

8 available today and are widely used by network operators. The only exception is the prognosis of the expected component reliability. This prognosis is a key point of such methods, as the strategies for preventive maintenance, reinvestment and network operation clearly have an impact on component reliability and on the reliability performance of the system. Today, these effects cannot be described systematically and quantitatively for most network components, as suitable models and corresponding parameters are not available. The theoretical approach for deriving such models is described in this paper, and an example for VPE cables of certain production years is presented. Finally, a research project is presented which is covering this topic. It is expected that the results of this project will greatly increase the insight and the modelling capabilities for ageing processes of network components, which is of high relevance for asset management methods. 6 REFERENCES [1] E. van Geert: Towards a customer oriented approach of quality of supply in an open market. (38 th CIGRE Session, 2000, paper ) [2] M. Schwan, W.H. Wellßow, A. Schnettler: Einflussfaktoren auf die Zuverlässigkeit. (ETG-Fachtagung Zuverlässigkeit in der Stromversorgung (ETG-Fachbericht 92), 2003, p. 7-13) [3] OFGEM: Guaranteed and Overall Standards of Performance. (OFGEM, 2000) [4] T. Langset, F. Trengereid, K. Samdal, J. Heggset: Quality dependent revenue caps a model for quality of supply regulation. (CIRED, 2001, paper 6-4) [5] J. Backes, O. Bertoldi, H.-J. Haubrich, A. Montebaur, H.-J. Koglin, M. Schwan, A. Sorg, C. Tagliabue, W.H. Wellßow, M. Zdrallek, U. Zimmermann: Service reliability in a competitive market. Tools, criteria and new approaches for risk management and monetary evaluation. (38 th CIGRE Session, 2000, paper 37/38/39-106) [6] B. Buchholz, H.-J. Koglin, J. Nilges, O. Schmitt, A. Schneider, M. Schwan, A. Sorg, Th. Weber, W.H. Wellßow: Insurance models as an appropriate measure for risk based asset management in the competitive European electricity market. (39 th CIGRE Session, 2002, paper ) [7] M. Schwan, W.H. Wellßow, H.-J. Koglin: Identification of probability distributions of probabilistic reliability indices for monetary evaluation and risk assessment. (14 th PSCC, 2002, paper S04P01) [8] J. Teupen, S. Meinig: Moderne Verfahren zur Unterstützung eines Asset-Managers bei der Entscheidungsfindung. (Int. ETG-Kongress 2003 (ETG-Fachbericht 94), 2003) [9] M. Böhme, I. Krage, K.-H. Weck: Instandhaltung von Mittelspannungskabelanlagen. (ETG-Fachtagung Verteilungsnetze im liberalisierten Markt (ETG-Fachbericht 82), 2000, p ) [10] C. Neumann, K.-H. Weck: Störungs- und Schadensanalyse Feedback für Diagnostik von Betriebsmitteln aus dem Netzbetrieb. (ETG-Fachtagung Diagnostik elektrischer Betriebsmittel (ETG-Fachbericht 87), 2002) 8