Planning the maintenance of production units based on the purchase proposal in order to maintain the adequacy of the system

Dissertation for Master's Degree

In Electrical Engineering - Power Orientation

Abstract

In this thesis, a new method for planning maintenance and repair of production units in restructured environments is presented. In a traditional environment with a vertical structure, the system operator determines the maintenance schedule to maintain reliability and also tries to reduce costs. But such actions are not acceptable in a competitive environment. In the restructured environment, the operator is still in charge of maintaining the reliability at the desired level. While the goal of each manufacturer is to increase its own profit, which may conflict with the goal of maintaining reliability. In this thesis, a multi-stage method is proposed for maintenance planning. In the first stage, according to the cost/benefit evaluation, the producers prepare a proposal for the purchase of maintenance for the production unit they are considering and present it to the independent operator of the system. In the second stage, the independent operator of the system, according to the suggestions sent by the producers and also the system adequacy, determines the output timing of the production units with the aim of maximizing the satisfaction of the producers and maintaining the system adequacy. In the third stage, due to the non-profit nature of the independent operator of the system, an impartial solution for market settlement is proposed.

Key words: proposal-oriented maintenance planning, power systems planning, reliability, restructuring. 

Definition of maintenance[1]

Repair and maintenance is an operation in which a device is repaired at certain intervals. Maintenance is to prevent, reduce or eliminate the deterioration of the device.

The purpose of maintenance is to extend the life of the equipment or increase the average time until the next failure. Apart from this, it is expected that effective maintenance policies can reduce the frequency of service interruptions and many of the undesirable outcomes of those interruptions. Maintenance clearly affects the reliability [3] of components and the whole system. Lack of attention to maintenance leads to a large number of costly breakdowns and poor system performance, thus reducing reliability. Paying too much attention to maintenance, although it increases reliability, but also greatly increases its cost. In an effective maintenance planning, a balance should be made between cost and reliability [2].

In the existing standards for the power system, the maintenance of electrical equipment is defined as follows:

"Maintenance is the preservation and maintenance of the conditions necessary for the operation of electrical equipment in the direction in which it is used" [3]. Therefore, maintenance ensures that the system in the fields it is designed to work in is electrically in the condition work safely.

In general, maintenance is divided into two main groups: preventive maintenance[4] and corrective maintenance[5]. Preventive maintenance is done in order to maintain the accuracy of the system and provide the required reliability. For this purpose, regular operations such as system performance inspection, cleaning, adjustment, lubrication, etc. are carried out, and the parts that are in the beginning of the erosion stage or have failed as parallel surplus parts are replaced by new and healthy parts. The purpose of this operation is to protect the system in avoiding the occurrence of downtime beyond the desired limit. Corrective maintenance is performed in the event of any malfunction in the system's functions, and the purpose of its implementation is to quickly restore the system to optimal conditions for full operation. This work is done by replacing, repairing or adjusting the parts that have stopped the system [4].

1-2- Maintenance solutions

Power companies rely on maintenance to keep their equipment in good working condition as much as possible. In the past, the main maintenance routine mostly consisted of pre-defined activities that were performed at regular intervals (scheduled maintenance [6]). But, such a maintenance method may be somewhat inefficient, because it may be very costly in the long run, and it may not be able to extend the life of the equipment as much as it could.. Therefore, many companies have replaced the maintenance planning with fixed intervals with flexible programs based on required checks and priority, or studying the information obtained through condition monitoring (predictive maintenance [7]).

Predictive maintenance methods include a group of programs that are called reliability-oriented maintenance [8]. In a reliability-oriented maintenance solution, it is possible to compare different alternative maintenance methods and choose which one is more economical to maintain the reliability of the equipment. Reliability-oriented maintenance programs have been used by some electric companies as a useful tool. Many articles are only related to replacement [9] in post-fault situations and during repairs, and ignore the possibility of other types of maintenance that lead to less improvement at a lower cost. The oldest replacement methods are replacement methods based on equipment life [10] and mass replacement [11]. In replacement based on age, an equipment is replaced at a certain age or when it breaks down. In bulk replacement, all equipment in a class is replaced within a predetermined interval or when they fail. The solution of replacing parts is very easy to implement, especially if the age of the equipment is unknown, it can also be more economical than the solutions based on the replacement of a single piece of equipment. Newer replacement solutions are sometimes based on probabilistic models, so they can be much more complex. In any case, in many electrical applications, maintenance that results in limited improvement is used as the primary solution, and replacement models play only a secondary role.

Maintenance programs cover a wide range and can be very simple or very complex. Perhaps the simplest program is to adopt an inflexible maintenance schedule [12], where predetermined activities are performed at fixed time intervals. When an equipment fails, it is repaired or replaced. Repair and replacement are both much more expensive than a single maintenance operation. Maintenance intervals are usually chosen based on long-term experience. Reliability-oriented maintenance is strongly based on regular evaluation of equipment conditions and therefore does not employ inflexible maintenance scheduling. It has been observed that reliability-oriented maintenance is a somewhat fluid concept and is defined in different ways in different sources. Reliability-oriented maintenance is not always based on condition monitoring [13], but also depends on other characteristics such as failure states [14], investigation of effects [15], review of operational needs and priority rights [16]. Optimizing preventive maintenance is more based on mission analysis [18] than system analysis and has the ability to effectively reduce the number of missions required for repairs.  Programs such as reliability-oriented maintenance and optimization of preventive maintenance are very useful to ensure the economic operation of power plants.

For a complete evaluation of the effectiveness of maintenance methods, first, it is necessary to know how much the use of those methods will extend the life span or the average time to failure [19] of an equipment. To realize this, a mathematical model of the equipment's deterioration process [20] is needed, which is then combined with a model that describes the effects of maintenance. Several mathematical models have been proposed for this purpose, which establish a link between maintenance and reliability. Once a mathematical model is built, the process can be optimized by changing one or more variables.

Simpler mathematical models are based on fixed maintenance intervals and the result of optimization is finding the least expensive maintenance interval. More complex models use the concept of condition monitoring, where the decision about the time and length of maintenance intervals depends on the actual condition (stage of deterioration) of the equipment.  Therefore, some type of monitoring (for example, inspection) should be part of the model.