B3-101 STRATEGIES FOR OPTIMIZING THE USE OF SUBSTATION ASSETS. W. DEGEN K. LASKOWSKI Siemens AG (Germany)

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1 21, rue d'artois, F Paris B3-101 Session 2004 CIGRÉ STRATEGIES FOR OPTIMIZING THE USE OF SUBSTATION ASSETS G. BALZER * Darmstadt University of Technology W. DEGEN K. LASKOWSKI Siemens AG M. HALFMANN ABB Utilities GmbH T. HARTKOPF EnBW AG C. NEUMANN RWE Transportnetz Strom GmbH SUMMARY The reliability calculations point out the weak points or equipment of the system, so that maintenance measures should be efficient under the consideration of the restricted financial resources of the utilities. An overall procedure for a maintenance strategy is presented in this paper: a life cycle cost based approach to optimize the service strategy of the substation and a succeeding renovation strategy based on Fuzzylogic. Life cycle cost calculations assist in finding the optimal technology, configuration or operating strategy. Applying life cycle cost calculation on different service strategies helps to decrease the maintenance costs considering all relevant parameters of the regarded substation. After the life cycle cost investigation the maintenance strategy for a complete substation has to be provided. The principle of the system approach to apply the RCM strategy leads to the result, which asset should be maintained or replaced. If the RCM assessment of a substation leads to the conclusion, that a renovation of a substation should be advisable, the question is, if the entire substation should be replaced or single items. The usage of Artificial Intelligence (Fuzzy-Logic) gives an answer for the decision making process. Keywords: Life-cycle costs, renovation strategy, GIS, Fuzzy-Logic 1. INTRODUCTION Electric utilities are placing increasing emphasis on cost-effectively extending the life of existing substations while maintaining adequate levels of reliability and availability. Power quality and re-liability requires appropriate switchgear performance. However the long-term switchgear integrity compromised nowadays with continual budget costs in investment spending and operations as well as maintenance. Utilities are not willing to invest in switchgear improvements unless they are deemed critical or there is an immediate return of revenues or at least in the near future. Deregulation processes are even supporting these attitudes of asset management. Due to the extended time of use of substation assets it is necessary to find the right strategy for the life time of the equipment. Nowadays the average age of the switchgear used in substations varies between 15 to 25 years, the life expectancy is between 35 and 50 years. The variety of life time expectancy comes along with many different technologies and concepts of high voltage substations. Therefore strategies are needed to optimize the initial investment in switchgear technology, the economic operation of the * Landgraf-Georg-Str. 4, D Darmstadt; gerd.balzer@eev.tu-darmstadt.de

2 installation and the renovation strategy of the equipment. This paper focuses on the tools to assist optimizing the service and renovation strategy for high voltage substations. 2. EVALUATING SERVICE STRATEGIES WITH LIFE-CYCLE COST 2.1. General Life cycle considerations cover the overall cost, giving advice for the decision of the best solution for the individual requirements, respecting investment cost comprising system, engineering and installation cost as well as operating cost evaluating the different aspects of Life Cycle Cost. One of the dominating parts of the operating costs are the maintenance and all relating costs. In order to optimize the service strategy all relevant cost portion have to be considered. Different maintenance strategies such as time based, corrective, condition based or reliability centered maintenance have different impact on life cycle cost. The decisive parameters have to identify and their influence on the overall cost has to determine. Furthermore different switchgear technologies and different arrangement influence strongly the results of these calculations. Individual calculations are necessary for the practical application of these principles. In this contribution the results of a typical sample arrangement are presented. The duly investigation on the life cycle cost and a tight asset management reduces the overall cost of a high voltage substation and contributes to the economical benefit of the operating utility Life Cycle Cost structure The basis of any life cycle cost evaluation is to select an appropriate cost break down structure. LCC + Cost + Cost = Costacquis. ownership disposal The cost breakdown structure is based on the recommendations of IEC [1] which is proposing a generic cost breakdown structure for high voltage applications. In this contribution the analysis of the ownership cost is essential so that these costs will be introduced more in detail. The following cost portions will be considered: Operating cost - maintenance of GIS building (for GIS) and control building (for HIS & AIS) - switchgear yard maintenance, incl. maintenance of cable ducts, gantries etc. (HIS & AIS) Scheduled maintenance - labor, material and travel expenses - cost of scheduled outage (no interruption of energy transmission considered) Unscheduled maintenance - labor, material and travel expenses according Outage cost - financial loss in case of interruption - penalties due to the interruption of energy transmission The introduced parameters base on many sub parameters which can not be introduced due to the high amount. All life cycle cost calculations are dynamic calculations according to the method of discounted cash flow taking into account the changes of the value of the money. An individual calculation of an individual configuration is indispensable for correct conclusions because multivariate dependencies. All calculations will be done with the help of one sample arrangement according to Figure 1. The reliability of the analyzed equipment is one of the core parameters influencing the impact of various service strategies on life cycle cost. Different reliability values are not only to observe between different technologies (such as GIS vs. AIS) but also in different properties in the equipment of different suppliers. An comparison of the mean time between failures ( MTBF ) as one of the key dependability figures between the results published by Cigré [2,3] and the MTBF of GIS equipment of one representative of the major suppliers shows big differences in the reliability of the equipment as shown in Figure 2. The reliability of the

3 equipment of the major supplier is significantly higher than the appropriate Cigré data. Therefore the need of scheduled and unscheduled maintenance and the referred outage costs are appreciably lower. The data collected by Cigré are averaged data from the world high voltage supplier s market. MTBF [years] MTBF of GIS of typical major supplier SC13-C.B. (1st inq.) SC13-C.B. (2nd inq.) SC23-GIS Figure 1: Analyzed sample arrangement: 145kV H-Scheme with 3 circuit breakers (AIS and HIS) years Figure 2: Comparison of reliability data of Cigré and comparable data of major supplier [2,3] (SC13 data is only relevant for C.B., all other data include whole switchgear) Separate calculations are useful to distinguish the effects due to the different level of reliability of Cigré data and the major supplier s data and will lead to different results applying various service strategies. In the case of the more reliable major supplier s equipment the operating cost are significantly lower in respect to the total life cycle cost (Figures 3,4). The investment cost for the system and the balance of plant portion of the sample substation was set to equal values in both calculations not to differ more parameters than necessary for the analysis of the service strategies. 125% 125% 100% 75% 50% 25% 0% GIS HIS AIS Operating cost Investment cost Replacemt. of Equipm. Disposal Cost Outage Cost Unscheduled Maint. Scheduled Maint. Operating cost Balance of Plant System Cost 100% 75% 50% 25% Land acquisition cost not considered 0% GIS HIS AIS Operating cost Investment cost Replacemt. of Equipm. Disposal Cost Outage Cost Unscheduled Maint. Scheduled Maint. Operating cost Balance of Plant System Cost Land acquisition cost not considered Figure 3: Life Cycle cost structure for 145kV sample arrangement based on Cigré data (left) and major supplier (right), HIS data is derived from GIS data 2.3. Evaluation on service strategies The cost of ownership is determined over the life time up to 50 years mainly by the appropriate service strategy. It shall be shown how the different service strategies influence the life cycle cost of different high voltage technologies. These following four service strategies are considered for the evaluation: - Time Based Maintenance (TBM) - Corrective Maintenance (CM) - Condition Based Maintenance (CBM) - Reliability Centered Maintenance (RCM) The maintenance strategies are described briefly and the results of the life cycle cost analysis discussed. The examples show the values for GIS and AIS as far as the operating costs of HIS equipment are very close to the comparable GIS costs. The different scenarios were explained by Figure 4, this figure present the changes of Life Cycle Cost depending on failure cost. All the scenarios are related to the Cigre basic data, which are indicated with 100 % failure cost (x-axis).

4 Time Based Maintenance (TBM) - The basis data of the reliability examinations worked out by Cigré can be assumed related on time based maintenance strategy mainly. Preventive maintenance will be carried out in predefined intervals based on experience of usage. Increasing the maintenance interval from the average interval between two routine inspections, which are seven years according Cigré (non-metal enclosed) respectively eight years (encapsulated), will have the sketched effect. The life cycle cost decrease by about 20 k but the cost due to a higher failure rate will overcompensate this benefit from an increase of failures by about 10 % (Figure 4, left). The gradient of this straight line is related to reliability of the equipment: a high reliability leads to a small gradient. The analysis of the life cycle cost based optimization of the time based maintenance strategy shows that a reduction of maintenance intensity only recommended for reliable equipment or substations with weak consequences of an outage (no big financial losses or no penalties). An individual analysis of the individual circumstances is necessary to derive the optimal configuration. LCC 300 TEUR 200 higher LCC AIS Cigré GIS 300 TEUR 200 LCC higher LCC 100 lower failure cost higher failure cost 0 0% 50% 100% 150% 200% 250% Major-Supplier lower failure cost higher failure cost Major-Supplier 0 0% 50% 100% 150% 200% -100 GIS lower LCC Cigré AIS lower LCC Figure 4: Impact on life cycle cost while applying service strategies based on TBM (left) and CBM (right) Corrective Maintenance (CM) - In case of corrective maintenance no preventive maintenance at all is carried out, so that the cost of ownership are reduced by these maintenance cost. Maintenance is only performed when a failure occurs. Reducing the scheduled maintenance will result in higher failure rate (similar to consequences of variation the maintenance interval in case of TBM). Depending on the reliability of the equipment and the financial losses in case of outage the quantity of failures can increase up to 3 times related to the original failure rate until an increase of the overall life cycle cost can be detected. The curves are similar to the curves of the TBM strategy, they are just shifted down by the constant amount of saved maintenance expenses, Figure 4 (left). Condition Based Maintenance (CBM) - The maintenance is driven by the technical condition of the equipment so that investment in additional monitoring devices is necessary in order to reduce failure rate and maintenance effort. The additional investment reduces in an increase of life cycle cost in comparison to the non monitored base case. Especially for low reliable equipment the additional investment has a positive effect on the life cycle cost, Figure 4, right. Reliability Centered Maintenance (RCM) - Beside the technical condition the importance of the equipment in the network determines the preventive maintenance. The life cycle cost based evaluation of this strategy has to be derived from the CBM-analysis.

5 3. RENOVATION STRATEGY WITH FUZZY-LOGIC 3.1. General Based on the result of these assessment procedures, engineers determine which installations and/or components are to be subjected to maintenance. Especially in the case of refurbishments, it may be necessary to decide whether exchanging single items of equipment is the most cost-effective solution or whether replacing the entire installation would be the better choice. It is not easy to find an answer to this question that is both clear and derived methodically. In addition it takes budgetary pressures as well as technical quality requirements into account. The following section present a decision-making process based on Fuzzy Logic that has been designed to help answer the question of whether an entire system is to be replaced or whether exchanging some of the components is what is technically necessary and economically reasonable. The overall condition of an installation is determined on the basis of the condition of the individual equipment. The result virtually represents the average condition of all items involved [4]. In order to identify which maintenance measure is best suited in a particular case, the analysis will not only be founded on objective criteria, such as condition and importance of the installation and of components, but also - and not least - on empirical operating data that has been collected over many years. A renewal of an aged installation, for instance, may also require that the surrounding building be replaced or modernized. Conversely, there may be good technical and/or economical reasons to continue to operate an aged electrical installation, even if extensive building rehabilitation measures appear to be necessary. The decision for or against partial or complete rehabilitation often has to be taken in a grey area where the options can hardly be systemized. This is also true for installations (e.g. outdoor installations) where only electrical equipment needs to be assessed. The question of whether to decide in favor of a partial rehabilitation or a complete renewal may be difficult to answer at times. The Fuzzy Logic is an option that presents the complex correlations of this issue and provides a methodical decision-making process Main features of the Fuzzy Logic In a solution concept using Fuzzy Logic, the response of the solution function is first described by verbal and indistinct "If-then" rules, i.e.: - If item 1 is in condition c 1 and item 2 in condition c2, then the result is E. - In terms of substation maintenance, the rule might look like this: If the building is in poor condition and the primary equipment is in poor condition, then a total rehabilitation of the substation is advisable. The basic principle of a decision-taking process using Fuzzy Logic is described in detail in [5] - [6]. The individual functional blocks of the Fuzzy process can be defined as follows: Fuzzification The purpose of fuzzification is to translate "distinct" input variables into "indistinct" Fuzzy variables with the help of what is referred to as the membership functions. This means, a current measured value is assigned a certain qualitative variable. The "condition of the substation, is the distinct variable (a specific numerical value between zero = very good, and one hundred = very poor). This "distinct" input variable is represented by three "indistinct" membership functions (good, medium, poor). Rules As explained above, the rules established for the processing of input variables describe the relationship between the input variables and the output variables. For instance, the rules may be: - If item 1 is in "good" condition and item 2 is in "medium" condition, the output is "good"; - If item 1 is in "medium" condition and item 2 is in "medium" condition, the output is "medium";

6 - etc. The "output or result may be the overall condition of an installation that consists of items 1 and 2. The rules reflect the decision-making process for replacing either individual parts or an entire system. They are based on relevant practical experience. Knowledge processing and defuzzification In knowledge processing, the indistinct Fuzzy variables are logically combined (mainly AND, OR, NOT operations). The operations used here are exclusively AND operations (conjunctions). The resulting degree of membership is determined by the minimum of the degrees of membership of the various input variables. With all non-distinct input membership functions used in practice, the Fuzzy output computed with the help of center-of-gravity defuzzification is never exactly zero or exactly hundred. This is why the result needs to be normalized, e.g. to hundred. The technical rules defined for the decision-taking process now take over the indistinct Fuzzy variables for further condition assessment in order to arrive at the decision on whether to replace the installation or replace individual components. With the rules chosen here, if-then conclusions and conjunctions will always be sufficient. For defining the maintenance approach for the entire substation, area G is assigned the activities replace individual items or no measure. Area P represents the activity replace substation up to replace individual items. The "medium" area M overlaps over both of the aforementioned Fuzzy areas. Table 1 gives some exemplary rules for condition assessment for the substation. For n = 3 components and x = 3 types of conditions, the result is n x = 27 individual rules. The complete pattern of relations between the resulting output function and the condition values of the two given items, based on the three membership functions, is illustrated in Figure 6. The condition influence of the circuit-breaker/current transformer against the other primary equipment regarding the renovation is illustrated. Table 1: Rules for determination the maintenance measure for h.v. Gas-Insulated Substation no. circuitbreaker/current -transformer disconnector other primary equipment renovation 1 P P P P 2 P P M P 3 P P G M P G P P G G P M 26 G G M G 27 G G G G P poor M medium G good probability renovation other primary equipment circuit-breaker/current transformer Figure 6: Membership functions of items 1 and 2 in relation to the output 3.3. Condition assessment procedure The procedure presented here is used for defining the best suited maintenance activity of h.v. substations. The condition of a network substation is assessed by applying a number of criteria and by ranking the condition of the following components: - circuit-breakers - transformers - disconnectors/earthing switches - instrument transformers (current, voltage) - secondary equipment and so on. Following the separate assessments, different equipment are combined in groups which can reasonably be used for formulating the renovation strategy for the substation. In this example, current transformers and circuit-breakers are combined in a group, disconnectors are considered separately, and the other items are combined in another group. The reason is, that the equipment of the last group can be replaced inde-

7 pendently of the condition of other items. For the purpose of simplification, it is assumed that the power transformer is exchanged regardless of the condition of the other GIS-equipment. The technical condition of a power transformer therefore does not have any impact on the decision for or against a total renovation of the substation. The Fuzzy Logic is therefore applied to the following decisions: - circuit-breakers/current transformers - disconnectors - other primary equipment. Figure 7 shows the decision-taking tree for implementing the maintenance strategy. Starting with the above mentioned equipment, Fuzzy Logic is used for identifying whether a total renovation of the substation would be advisable. The next step is to decide - on the basis of condition c of the single equipment - how each is to be dealt with. As can be seen from Figure 7 it is assumed, that a condition index c higher than 60 represents a condition of the single equipment, which leads to the conclusion, that the equipment should be replaced. Before a final decision on how to proceed with a piece of equipment is taken, it is necessary to find answers to important system issues. The examination is built on five specific questions: - Are there intentions, to change the feeding nodes of the system? - Are there any plans for making changes in the rated voltage? - Are there any probabilities or plans to change the power flow? - Have there been any changes regarding the technical requirements (capacity, reliability, customer requirements, etc.)? - Is it possible, that the company will merge or is any change of the shareholder? If the answer to one of the above questions is "yes" for the coming examination period (e.g. the next three years), there will neither be an exchange of the entire substation nor a replacement of individual equipment, since it will be necessary to draw up a new concept that will be suited for the changed outline conditions. Figure 7: Implementation of a maintenance strategy for substations Figure 8: Replacement of GIS circuit-breaker 3.5. Example To check the complete process a special Gas-Insulated Substation (GIS, Figure 8), 123 kv, was assessed to consider the technical condition. Outgoing from this the technical condition of further 20 other substations are theoretically changed to test the Fuzzy procedure. The result is the probability that a com-

8 plete renovation of the substations are recommended. In this context several probability classes can be defined and the number of substations, which should be replaced: - probability > 70 %: 3 -probability %: 4 - probability %: 4. Thus it is possible, to perform a ranking of the substation, which should be replaced first, second, and so on. In the other cases the replacement of the single component is advisable. Figure 12 shows the example, that a circuit-breaker of an substation, which was commissioned in 1975, was replaced by a new circuitbreaker. 4. CONCLUSION Life cycle cost consideration can assist in optimizing the maintenance strategy of a high voltage substation which is determining the most important part of operating cost of a HV substation. The life cycle cost based analysis of maintenance strategies leads to the following results: - The more reliable the equipment the more suitable is the reduction of maintenance. - The less reliable the equipment or the higher potential consequences of an outage are, the more suitable is the investment cost in monitoring devices for condition based maintenance. However conclusions on the impact of modifications of maintenance strategies on life cycle costs can only derived when calculations with the consideration of an individual set of parameters are applied. The RCM strategy to maintain the asset of a system leads to the result, which equipment should be maintained first, second, and so on. This description clearly demonstrates how a maintenance strategy can be derived from the results of a substation condition assessment. Based on the importance of the stations within the network, it is then possible to define the order in which capital needs to be invested in this installation. The maintenance strategy is derived with the help of Fuzzy Logic, as it allows for consideration of the logic correlations between individual items of equipment that may have to be replaced. The RCM procedure was implemented in a software tool and the most important advantage is, that a documentation of the total process is provided. 6. REFERENCES [1] IEC : Dependability management, Part 3 Application guide Section 3: Life cycle costing. First edition [2] Jansen, A. et al.: Final report on the Second International Inquiry on High Voltage Circuit Breaker Failures and Defects in Service, CIGRE WG 13.06, 1994 [3] Kopejtkova, D., et al.: Report on the Second International Survey on High Voltage Gas Insulated Substations (GIS) Service Experience, CIGRE WG 23.02, 2000 [4] Balzer, G; et. al.: Life Cycle Assessment of Substations: A Procedure for an Optimized Asset Management. Cigre 2002, Paris, report [5] Lunze, J.: Künstliche Intelligenz für Ingenieure, Vol. 1 and 2, Verlag Oldenbourg 1995 [6] Bothe, H.-H.: Neuro-Fuzzy-Methoden, Springer Verlag 1997