Asset management for power cable systems - Total cost optimization based on the Application of diagnostics -

Transcription

1 21, rue d Artois, F PARIS B1_212_2012 CIGRE 2012 http : // Asset management for power cable systems - Total cost optimization based on the Application of diagnostics - T. NISHIKAWA Y. SUDOH S. TSUCHIYA Tokyo Electric Power Co. Tokyo Electric Power Co. Tokyo Electric Power Co. D. SHIMOHIRO T.IIDA K.WATANABE Chubu Electric Power Co. Kansai Electric Power Co. Kyusyu Electric Power Co. Japan SUMMARY In Japan, electric power companies used to upgrade and expand electricity facilities in accordance with the high growth of electric power demand. However, demand has become saturated in recent years, and electric power companies strengthen the more efficient operations and maintenance of existing facilities. At the same time, the amount of aged facilities has been increasing. d power cable facilities over 30-years old have also been increasing, and the amount will continue to grow rapidly over the next 10 or 20 years. In this situation, electric power companies face challenges to reduce future risks and retain their present electric supply reliability while maintaining supply costs at a reasonable level. Therefore, the diagnostic technology and asset management for the effective maintenance of power cable systems are becoming very important. In this paper, we describe the process of asset management for power cables. We chose XLPE cables without water barriers and Self-Contained Oil Filled cables as the scope of the asset management model because these are especially aged cable systems in Japan. The purpose of the asset management modeling is to optimize total costs. We conducted the sensitivity analysis of the asset management model, and compared the total costs of the Corrective Maintenance, a Time Based Replacement and a Condition Based Replacement. As a result, we indicated the optimization method of the total cost per the Time Based Replacement or the Condition Based Replacement, especially for XLPE cables without a radial water barrier which causes a concern of the growing risk of water-tree deterioration. KEYWORDS Asset management - XLPE cables without radial water barrier - SCOF cables Replacement - Diagnostics [email protected]

2 1. Introduction In Japan, electric power companies used to upgrade and expand power facilities in concert with the strong growth of electric demand more than 10 years ago. However, demand has become saturated during the past 10 years and it is not expected to grow largely in the future. Therefore, electric power companies value the efficient operation and maintenance of existing facilities rather than upgrading and expanding. The age distribution of power cable systems is relatively younger than substation & overhead transmission facilities. However, the aged power cable facilities over 30 years old have also been increasing, and their amount will grow rapidly over the next 10 or 20 years. In this situation, we face challenges to reduce future risks and retain our present reliability with reasonable supply costs. Therefore, the diagnostic technology and the asset management for the effective maintenance of power cable systems are becoming very important. In this paper, we describe the process of asset management for power cables by using the model based on the type and age distribution of Japanese power cable facilities. 2. Background Fig.1 shows the age distribution of the power cables in Japan. This shows that more aged type of power cables are Self Contained Oil Filled cables (SCOF cables). The deterioration of the main insulator has not been found, however, the number of oil leakages has been increasing. We think it is possible that the planned replacements are effective for maintenance. XLPE cables without a radial water barrier (XLPE cables w/o water barrier) are also aged. The number of breakdowns due to the water-tree deterioration has increased, and the diagnostics of the deterioration have begun to be adopted by some electric power companies in recent years. For example, the diagnostics by harmonics in the AC loss current has been adopted on the water-tree deteriorated XLPE cables for the judgement of the replacement.[1] We think it is possible Circuit length of cables [km cct] Others SCOF (Alumi Sheath) SCOF (Lead Sheath) XLPE (with water barrier) XLPE (w/o water barrier) Fig.1 Distribution of the power cable in Japan that diagnostics is effective for efficient maintenance. The newest type of cable is the XLPE cable with a radial water barrier, and no deterioration has been found in this type so far. 3. The idea of the asset management model The target facilities need to be clarified in the asset management study. In this study, we defined the power systems in terms of 4 types of clusters; the first to the fourth. Fig.2 shows the schematic illustration of the 4 types of clusters. (1) The first cluster The whole power system (Generation to Distribution) (2) The second cluster Units of transmission systems (Bulk Power System, local power system, etc.) (3) The third cluster Families of equipment (XLPE cables without radial water barrier, SCOF cables, etc.) (4) The fourth cluster Individual equipment (A line, a substation facility, etc.) The higher cluster we select, the more overall optimization we can realize. On the other hand, asset management modeling is more difficult. 1

3 In Japan, one characteristic is that the amount of aged XLPE cables without a radial water barrier and the aged SCOF cables are large. Besides the application of a planned maintenance over same type of cables is a good and important method because the each types of cable will have a similar deterioration characteristic. Therefore, we discussed asset management modeling on the third cluster, XLPE cables without a radial water barrier and SCOF cables. Fig.2 Schematic illustration of modeling clusters. 4. Characteristics of the asset management model Characteristics of the asset management models are as follows; (1) The asset management model of the XLPE cable without a radial water barrier (XLPE cable model); The risk of the breakdown increases with age The possibility of a breakdown due to the water-tree deterioration increases with age. These cables will be replaced by XLPE cables with a radial water barrier, and after that breakdowns due to aging will cease according to our experience. It is difficult to maintain insulation properties via partial repairs, therefore, the primary maintenance method is the replacement. (2) The asset management model of SCOF cable (SCOF cable model); increase with age The possibility of an oil leakage from a lead sleeve increases with age. There will be no rising of a breakdown possibility with age. It is possible to maintain insulation properties via partial repairs. 5. The idea of maintenance adopted in the asset management model 5.1 XLPE cable model It is difficult to repair XLPE cables w/o water barrier partially which have characteristics of water-tree deterioration. Therefore it is very important to determine the time of replacement for the optimization of total costs by using the cables for as long as possible. In this study, we categorized the determination method of the replacement time into 3 patterns as follows; (1) Corrective Maintenance (CM) To replace XLPE cables w/o water barrier after the breakdown (Without planned replacements) (2) The Preventive Maintenance (PM) Time Based Replacement (TBR) To replace XLPE cables w/o water barrier after a specified age during a given period of time Condition Based Replacement (CBR) To replace XLPE cables w/o water barrier based on the result of the diagnostic technologies 5.2 SCOF cable model The checks and repair costs of the SCOF cables rise with age due to increasing oil leakage from the lead sleeve of a joint and so on. On the other hand, the check costs of the XLPE cables with radial water barrier are constant with age, and the cables are highly reliable. Therefore, it is very important to 2

4 determine the time of replacement from aged SCOF cables to XLPE cables with the radial water barrier for the optimization of total costs. (1) CM To replace SCOF cables after the breakdown. (Without the planned replacement) (2) PM TBM To replace SCOF cables after a specified age during a given period of time. CBR We didn t consider CBR for SCOF cables because it is difficult to apply diagnostics for determining the time of their replacement. 6. The idea of cost parameters in the asset management model In this study, we classified the costs for families of equipment into 4 categories, and calculate the cumulative maintenance cost by the total of the annual maintenance cost. We considered optimizing the cumulative maintenance costs. In addition, we applied the present value method based on the interest rate over these costs. 6.1 are defined as the inclusive term of costs for checks and repairs for families of equipment. costs in the XLPE cable model are constant, and that in the SCOF cable model rises with age. It is possible that the costs for SCOF cables will be increasing for avoiding the loss risk of human resources and supply chains. Eq.1 shows the annual maintenance costs, which are expressed as the product of the unit price of maintenance and the amount of equipment. Cumulative maintenance costs XLPE cable SCOF cable Fig.3 The idea of the total cumulative maintenance cost of XLPE cables w/o water barrier and SCOF cables [] = [A unit price of maintenance] [The total amount of equipment] (Eq.1) Fig.3 shows the concept of cumulative maintenance costs for a lifetime of each cable type. The cumulative maintenance costs are defined as the total amount of annual maintenance costs by the age of the cables. 6.2 Trouble costs Trouble costs are defined as the inclusive term of emergency response costs, inspection costs, and blackout losses and so on. In this study, we divided the replacement cost during the breakdown from trouble costs, and classified it as replacement costs to be hereinafter described. Annual Trouble Costs w/o planned replacement with planned replacement Replacement period of CBR Cumulative trouble costs w/o planned replacement with planned replacement Start time of planned replacement Fig.4 Annual trouble costs of a power cable family Fig.5 Cumulative trouble costs of a power cable family Eq.2 shows the trouble costs which are defined as the product of a unit price of the trouble cost and the annual amount of the family of the breakdown equipment. [Trouble costs] = [Unit price of the trouble cost] [A breakdown possibility] [The total amount of the family of existing equipment] (Eq.2) 3

5 Fig.4 and Fig.5 show the annual trouble costs and cumulative trouble costs for a lifetime of a cable cluster. We calculated the possibility of a breakdown based on the idea of the bath-tub curve as Fig.6 shows. In this study, we didn t consider the early failure period, because the period of these cables has already passed. Through our research and analysis on the past record of breakdowns, we found that the breakdown possibility of XLPE cables w/o water barrier changes between the chance failure period and the wear-out failure period, and the major part of XLPE cables w/o water barrier seem to be in the transition period Fig.6 The idea of the breakdown possibility in the asset management model of XLPE cables and SCOF cables The breakdown possibility could increase in the future. Therefore, we evaluated the possibility for 2 cases in the asset management model; The possibility is kept at the present level (chance failure period) The possibility rises with age (wear-out failure period) The breakdown possibility of the SCOF cable could be evaluated as in the chance failure period because the past record of breakdowns is fewer than that of XLPE cables w/o water barrier. It is difficult to calculate the possibility of breakdowns. Therefore, we defined it by referring to the former possibility of the XLPE cable model (chance failure period). 6.3 Replacement costs Replacement costs are defined as the inclusive term of the construction costs for the replacements of breakdown cables or planned replacements. The annual replacement costs are the product of the unit price of the replacement and the amount of the family of equipment, as Eq.3 shows below; Annual replacement costs w/o planned replacement with planned replacement Increase of replacement cost due to breakdown caused by aging Increase of replacement cost in TBR Cumulative replacement costs w/o planned replacement with planned replacement Starting of TBR Fig.7 Annual replacement costs of a power cable family Fig.8 Cumulative replacement costs of a power cable family [Replacement costs] = [A unit price of replacement] [(A breakdown possibility) (Existing amount of the family of equipment) + (Amount of the family of equipment to be replaced according to a plan or the result of diagnostics)] (Eq.3) Fig.7 and Fig.8 show annual replacement costs and cumulative replacement costs. 6.4 Diagnostics costs Diagnostic costs are defined as the costs for assessing the deteriorating conditions of the equipment. In this study, we evaluated the availability of the diagnostics of the XLPE cables w/o the water barrier in the asset management model. Eq.4 shows the annual diagnostic costs, which are expressed as the product of a unit price of the diagnostics and the existing amount of a family of equipment. 4

6 [Diagnostic costs] = [Unit price of diagnostic costs] [The total amount of the family of existing equipment] (Eq.4) The item to be considered is as follows; We can reduce trouble costs by adopting the diagnostics and replacement of deteriorating equipment which has a breakdown risk. However, it is possible that we would replace the healthy equipment due to the false result of the diagnostics, and it could increase the replacement costs via the advanced change of equipment. 6.5 Total costs We defined the total costs as the summation of maintenance costs, trouble costs, replacement costs, and diagnostic costs. We optimized the total costs by selecting an effective start time of replacement and method. Table.1 and Fig.9 show the values of the parameters in the asset management models. In Fig.9, the unit price is normalized with a construction cost for the one span cable derived from past results. Table.1 Values of parameters in the XLPE cable model and the SCOF cable models 0.05 Unit price parameters Trouble costs Replacement costs Diagnostics costs* Possibility of the diagnosis XLPE model p ** = 0.95 p **= 0.01 SCOF model SCOF cable; shown in Fig.9 XLPE cable ; *for CBR ** The details of these parameters are to be described in Sec.8 Fig.9 The unit price of maintenance costs for SCOF cables. 7. Results and discussions 7.1 The XLPE cable model Fig.10 Fig.12 show the result of the asset management model of XLPE cables w/o water barrier with CM, TBR and CBR, under the condition that the breakdown possibility rises with age (wear-out period). The cost is normalized with a construction cost for the one span cable derived from past results. Replacement costs Trouble costs Fig.10 of CM in the XLPE cable model 0.0 リプレイス Replacement 費 用 costs 事 Trouble 故 費 用 costs 維 Maintenance 持 費 用 costs Fig.11 of TBR in the XLPE cable model 60 In this condition, the cumulative total costs are, in ascending order, CBR < TBR <CM. On the other hand, if the breakdown possibility is kept at the present level, it will be understood that the cumulative total costs of CM are the lowest in these methods. This result suggests that we can save total costs by examining the wear-out period, using the diagnostic technology, and adopting the planned replacement. Diagnostics 診 断 費 用 costs Replacement リプレイス 費 用 costs Trouble 事 故 費 用 costs Maintenance 維 持 費 用 costs Fig.12 of CBR in the XLPE cable model 5

7 7.2. SCOF cable model Fig.13 and Fig.14 show the result of the asset management model with PM and TBR. 0.0 Replacement costs Trouble costs Fig.13 The cumulative total cost of PM model for SCOF cables 60 This result suggests the possibility that total costs can be optimized by adopting the planned replacement to the SCOF cables whose maintenance costs rise with age. Because the maintenance costs will be increasing as we mentioned in 6.2, and the replacement cost will be decreasing owing to future development of replacement technology. On the other hand, we are required to examine the start time and the period of the replacement in view of the various cost trends such as the increase of maintenance costs. 8. Sensitivity analysis We conducted the sensitivity analysis on these asset management models under the 2 points below. 8.1 The precision of the diagnostics in the XLPE cable model We examined the influence of the 2 index parameters of the diagnostics of total cumulative costs. p means that the possibility; a rate to pick out bad cables before the breakdown. p means the diagnostic error rate; a rate to diagnose good cables as bad. Fig.15 shows the examination result of these parameters. We found that p has a large influence on costs. If p is high, we are forced to replace cables before their lifetime. It could cause the loss of the remaining value. For instance, p and p can be evaluated in the reference of [2]. at the 60 ages 5 5 p'=0.01 p'=0.03 p'=0.05 p'=0 0.0 Replacement costs Trouble costs Fig.14 The cumulative total cost of TBR model for SCOF cables (To replace the cable during 20 years since 31 years old) at the 60 ages CM CBR Precision factor "p" Fig.15 Two parameter study of the diagnostics technology s precision Timing of CBR Fig.16 The total cumulative cost in the CBR model for the SCOF cable at the 60 ages (wear-out failure pattern) If p is low enough, we can optimize the total costs by avoiding replacing before its lifetime. Therefore, it is important to raise the precision of the diagnostics. 8.2 The time of the planned replacement We examined the below 2 cases for the case studies of the time of the planned replacement. (1) The start time study of the diagnostics on the XLPE cables w/o the water barrier in the CBR 6

8 Fig.16 shows the distribution of the start time of the diagnostics and the total cumulative costs for 60 years. In this model case, the CBR was more effective than the CM, and it shows that the total costs were optimized the most if we had started the diagnostics in the mid-30s. The result is derived from the trade-off between the diagnostic costs and the trouble costs (In other words, the earlier we start the diagnostics, the higher the diagnostic costs are, and the lower the trouble costs will be). Moreover, the results will vary widely according to the period of cumulative costs to be optimized, whether the most optimized start time is practical for real equipment or not, and so on. (2) The study of the replacement start time and the replacement period for the SCOF cables in the TBR. Fig. 17 shows the distribution of the total costs according to the start time of the TBR and the replacement period. This shows that the total costs can be optimized if we started the TBR from 30 to 40 years old, and replaced the cable in 10 or 15 years. Duration of replacement [year] to start TBR Comulative cost at the 60 years Fig.17 Total cumulative costs of SCOF cables adopted the TBR at the 60 years 9. Conclusion The result of the study shows that the replacement time of the aged cables should be examined because the unit price of replacement are quite larger than the maintenance costs, and the former widely impacts the total costs. In the case of the XLPE cable model, the results indicate that the CBR is the best to control total cumulative costs because it can reduce trouble costs which are influenced by aging deterioration. are sensitive to the precision of the diagnostics. As a result, the increase of the advantage of the CBR is to be expected by improving its accuracy. In the case of the SCOF cable model, the increase of maintenance costs with age is considered. In such a model, it is necessary to carefully study the replacement time because the time will vary widely according to the idea of the parameters. We described the XLPE cable model (mainly CBR) and the SCOF cable model (mainly TBR) for the asset management of the underground transmission facilities in this paper. The results indicated that these models are expected to contribute to the suitable maintenance of power cables, and we should improve the various parameters including the breakdown possibility. BIBLIOGRAPHY [1] Tanaka.A et al ; Actual application of on-site diagnostic method for water treed XLPE cable by harmonics in AC loss Current, (JICABLE 03, C.8.3.2) [2] A. Toya et.al ; Trends in Degradation Diagnostic Technique for XLPE Cables in Japan, (CIGRE B1-110, 2004) 7