Research On Optimal Preventive Maintenance Based On Reliability Cost-benefit Analysis

Generators, transformers, and transmission lines are the key components ofcomposed power system, the reliability level of these components is essential to thesafety and reliability of power system operation. Electrical equipments suffer fromaging, wearing and hidden failures in operation, thus the component failure rate isincreasing over time. In order to avoid the power system risk rising caused by theincreasing failure probability of components, maintenance for components is needed.The preventive maintenance is the most commonly used and cost-effective techniquesto weaken failure rate growing of components and improve the reliability of composedpower system.The preventive maintenance is divided into four levels A, B, C and D, according tothe power electrical equipment maintenance standards. The A-level maintenanceconducts a disintegration inspection and repair thoroughly. The B-level maintenanceconducts disintegration inspection and repair partly according to some problems. TheC-level maintenance conducts the maintenance and replacement of small parts,according to wearing, aging of equipments. The D-level maintenance is conducted onsub-systems when the equipment is in good operation overall. The preventivemaintenance cycles in each level are relatively fixed, such as the A-level maintenancecycle is generally4-6years, the B-level maintenance cycle is2-3years, and the C-levelmaintenance period is1year. Since the maintenance cycles of each level are determinedreferring to the maintenance standards, without considering the operating conditions andenvironment, the location in the network topology, the wearing, and aging of theequipments, it is essential to study the optimal preventive maintenance cyclesconsidering the main outage factors of various types of electrical equipments, andprovide a useful reference for power system planning and operation.The main contents of this thesis are as follows:①Power system components suffer from aging, wearing and hidden failures inoperation, thus the component failure rate is increasing over time. Considering the mainoutage types of generators, transformers and transmission lines, the correspondingincreasing failure rate model is established. Then the average unavailability formulaincorprationg the impact of C-level preventive maintenance is derived, assuming thatequipments can be repaired "as good as new" under the C-level maintenance. Based on reliability cost-benefit analysis, the LOLP, EENS and the total cost of the system can bededuced as a function of the preventive maintenance cycles of components respectively.②The sensitivity of the total system cost respect to each component preventivemaintenance rate are derived according to the analytical expressions of the total systemcost taking the preventive maintenance into account. Then, this thesis proposed aheuristic itereative algorithm in which the sensitivity is ranked, to find the optimalpreventive maintenance. According to the studies on RBTS and RTS79, the heuristicitereative algorithm is verified that the component optimal maintenance cycle varieswith type, capacity, reliability and the position of that component. Finally, the effects ofthe aging factor and unit power shortage cost on maintenance optimization are studiedbased on the RBTS system.③For the gradual deterioration electrical equipment, the recovery effect cannotreach the perfect state "as good as new", which is defined as the imperfect preventivemaintenance. Based on the effective age and the failure rate function, the thesis selectsthe important A-level and C-level maintenance to establish the growing failure ratemodel incorporating the imperfect preventive maintenance, in which A-level andC-level preventive maintenance are treated as perfect and imperfect maintenancerespectively. From the model, the average unavailability formula incorporating A-leveland C-level preventive maintenance is derived. Based on average unavailability, thesystem reliability indices, the A-level and C-level preventive maintenance costs, thecorrective maintenance costs and the total cost of the system can be deduced as afunction of the C-level preventive maintenance cycles and A/C preventive maintenancecycle ratios respectively.④To minimize the sum of outage costs and the maintenance costs, which is thelarge-scale mixed-integer nonlinear programming problem, the sensitivity of the totalsystem cost respect to each component C-level preventive maintenance cycle, and thedifferential of he total system cost respect to each component A/C preventivemaintenance cycle ratios are derived. Based on the thought of sensitivity and thedifferential，this thesis proposed a heuristic iterative algorithm to optimize the A/C levelpreventive maintenance coordinated. According to the studies on RBTS andIEEE-RTS79, it’s analyzed and compared between the optimal preventive maintenanceand the traditional preventive maintenance mode. Finally, this thesis discusses the trendsof the system reliability and economy indices with the A/C level preventivemaintenance cycle ratios increasing, in the situation of remaining A-level or C-leverl preventive maintenance cycles unchanged, and the impact of recovery factors onoptimal preventive maintenance are researched as well.