Research on Reliability and Economy of Maintenance Plan for Generator Sets

1.1 Three levels of the unit maintenance plan The generator unit maintenance plan can solve power plant power companies and interconnected power systems from three levels. As far as the power plant level is concerned, the goal of the generator maintenance plan is to give each unit the most adequate maintenance interval so as to obtain a satisfactory level of equipment availability. This level of constraint is related to the availability of the operator's repair shop. Too few overhauls will result in huge economic losses in the future, and over frequency will cost more to repair. For balance, a probabilistic model based on reliability-centered overhaul is proposed from the power company level. Its objective should be to minimize the maximum economic cost of power generation availability. Constraints In addition to the power plant floor, constraints related to load supply in the area should also be added, and power contracts with regions outside the region should be exchanged. Many current generator maintenance plans consider this level of questioning. In the interconnected system layer, in addition to the constraints of the power plant and single area, tie-line switching capacity should also be added to limit the requirements of the interconnected system reliability that should be met, mutual support policies, and business unit load-bearing order and seasonal factors.

1.2 Development and Application of Maintenance Plan for Generator Sets The development of the power system has greatly promoted the research and development of the maintenance plan of the generator set, and put forward higher requirements on reliability and economy. This is mainly reflected in the fact that more and more power plants have begun to adopt pilot inspections to reduce maintenance costs and to extend the inspection cycle. 2 The nuclear power industry has achieved rapid development. The special features of the nuclear power plant are the nuclear energy transportation fund project supported by the National Natural Science Foundation of China No. 59977004. No energy can be refueled during the hardships. At least 4 weeks are needed to refuel the nuclear reactor. 3. The unit is overhauled and restored during actual production. Fueling is carried out at the same time, which requires a very rigorous plan to optimize the unit maintenance. 3 The development of the interconnected system makes the overhaul of the interconnected system a feasibility lesson that needs urgent solution. The interconnected system can achieve higher reliability and lower production costs for the entire power system. It is imperative that multi-area joint repairs are performed under the interconnection conditions. 4 In the process of transitioning to the electricity market, the power plants do it all, and the power grid management is the consensus of the world's power industry restructuring. 4. Under the separate power grid market, there is an overall coordination of economy and reliability. From a system point of view, the plan to maximize the reliability of the system is not to make the owner of the generating unit consume the maintenance plan with the lowest economic cost, so that there is a decentralized coordination problem. Independent system operators Independent power companies Transmission suppliers all need to develop a comprehensive long-term plan, and the unit maintenance plan is an important and fundamental part of this.

1.3 Maintenance Plans Covered by the Maintenance Mode The power plant-level unit maintenance plan involves six types of maintenance methods. 1 Reliability-centered maintenance. 1 Sets special procedures to determine effective and economical preventive maintenance tasks for equipment and components, and specifies maintenance. Or system methods for monitoring intervals. (2) Condition maintenance The diagnosis of equipment is performed based on the analysis of measurement data such as temperature and vibration, and appropriate maintenance is performed in a timely manner. 8 Corrective Maintenance The only way to repair a device if it fails. 4 Preventive Maintenance Periodically overhaul equipment to increase the usefulness of power generation equipment. 5 Active overhauling and monitoring If any abnormality occurs in various parameters, the equipment has not yet undergone material deterioration or degradation of performance. Find the root cause of the failure and design or modify the equipment to eliminate the possibility of failure. 6 Periodic maintenance consists of a complete set of maintenance plans for different types of maintenance and their maintenance intervals, and regular maintenance.

2.1 Reliability Model The user's power loss has a close relationship with the reliability of the system. The generator maintenance plan must consider the system reliability. Although the overhaul plan is actually a multi-objective and multi-constrained optimization problem, due to the complexity, only one optimization indicator is selected as the objective function in practical application, and other specific requirements are considered in the constraint. Regardless of whether it is an objective function or a constraint, reliability must be taken into consideration. And economics will be uncertain in mathematics models, such as equal risk. The reliability model takes reliability as the objective function and can be divided into two categories: deterministic and random. The objective function of the reliability determination is to consider the maximum system net reserve after the unit is overhauled and outage, and the standby method is such. Because this concept is simple and easy to implement, it is often used in practice. However, the determined reliability objective function ignores the availability of the generator set and the uncertainty of the load. Therefore, having different periods of the same net standby does not mean that the reliability is the same. This is its main drawback. The stochastic reliability objective function can compensate for the above-mentioned deficiencies by taking into account the effects of random forced outage of the generator unit. One such risk method belongs to this category. The random reliability objective function generally uses a 101 Yang reliability index as a target value.

2.2 Economic Model The economic cost objective function mainly consists of maintenance costs and production costs. The total maintenance cost of a generator set consists of two parts, which is the maintenance investment loss due to the wear of Zhu, which is caused by the overhaul of the maintenance unit. The other part is the actual maintenance cost. This part is a function of the previous maintenance time. The total repair cost is basically a convex function. The production cost is the fuel cost required to produce the fixed electrical energy. The objective function that is commonly used when optimizing a maintenance plan only contains the production expense item. The optimization of the minimum production cost plan is quite difficult because it is a non-linear function of the operating unit. 5.9 It is worth emphasizing that economic research shows that the most reliable planning method is not necessarily the lowest production cost. Optimizing maintenance plans that consider production costs is better than simply considering reliability. In the electricity market conditions, economic costs should also include reliability costs and the purchase of electricity from outside the region.

There are two major types of heuristic optimization methods and mathematical optimization methods for solving generator set plans.

3.1 Heuristic optimization methods A heuristic algorithm is a technique that allows finding the best solution within the acceptable computational cost, but it can indefinitely guarantee the feasibility and optimality of the solution obtained, even in most cases, It is impossible to explain the degree of approximation of the obtained solution to the optimal solution. The generator maintenance plan is a combinatorial optimization problem. Its characteristic is that the dimension is discretized and nonlinear. Accompanied by the current tabu search and other solutions. 1 The question of the heuristic algorithm of global optimization, heuristic algorithm is widely used in maintenance planning.

3.1.1 The alternate method, such as the standby method, is to make the net standby of the system as equal as possible during the whole year in order to formulate the maintenance plan after taking into account the maintenance and outage of the generator set. The basic principle is very intuitive and is a traditional method of scheduling repairs, but it does not reflect the availability of units and the uncertainty of the load.

3.1.2 Equal risk method 8 This is a kind of method with a random reliability objective function. Through the optimization of the overhaul plan, the system risk is nearly equal in each period of the research cycle. Its implementation steps are similar to other alternative methods, and it is also an embarrassing process. It overcomes the alternative standby method 3.1.3 Simulated Annealing. This method is a random algorithm that converges to the global optimal solution with a probability of one. The advantage is that it can be more effectively prevented from falling into a local optimum, but it takes a long time and is a single point optimization. With this method, the system's deterministic optimization time for a 60-cylinder unit has reached a time-to-value ratio of about 21%, but its effectiveness for large-scale systems is not ideal.

3.1.4 Evolutionary Algorithms The typical evolutionary algorithm models include genetic algorithms evolutionary strategies and evolutionary programming. The advantage is that it is suitable for numerically solving the 3 optimization problem with multi-variable multi-parameters and multi-objectives and in multiple regions but with poor connectivity. It is quite suitable for generator maintenance plan, and it is multi-point optimized and compatible with other heuristic algorithms. It has disadvantages such as slow calculation speed and sometimes converging control parameters that converge to the local optimum and are not easy to grasp. The literature uses evolutionary strategies and fuzzy decision-making to solve maintenance plans that include generator sets and transmission equipment. It is a man-machine interactive optimization program similar to that of expert systems, in which fuzzy decisions are used to evaluate the resulting maintenance plan like an engineer. In particular, it also considers the real-time maintenance demand status of generator sets and transmission equipment, so as to flexibly handle maintenance according to actual conditions.

3.1.5 Taboo search algorithm The tabu search algorithm is the successful application of artificial intelligence in combinatorial optimization algorithms. The advantage is that the convergence speed is fast, and the disadvantage is that the convergence is affected by the initial solution and the skill is very strong. The literature combines the simulated annealing taboo search of genetic algorithms to optimize the maintenance plan of thermal power units, and uses their respective advantages to achieve a definite result.

3.1.6 Lagrangian relaxation method Simulated annealing taboo search genetic algorithm The heuristic algorithm gives the upper bound of the optimal value, while the Lagrangian relaxation method gives the lower bound of the optimal value. The basic principle is to absorb the constraints that cause difficulties to the objective function and maintain the linearization of the objective function, making it easy to solve. The Lagrangian relaxation heuristic algorithm mainly contains two parts of the feasibility of the gradient optimization calculation and solution. The literature uses the Lagrangian relaxation method to develop a maintenance plan for a nuclear power unit and gives the order of overhauls for 58 nuclear power units in France.

3.1.7 Expert System The expert system stores the knowledge and experience of human experts in a computer in the form of a knowledge base, and imitates the reasoning methods and thinking processes of human experts to solve questions, and makes judgments and decisions about the questions in reality. In a large-scale power system, if the factors such as maintenance load change of transmission equipment are taken into account, the maintenance plan becomes an extremely large-scale optimization problem. Simple mathematical optimization techniques are difficult to solve, and expert systems can do it well.

3.2 Mathematical Optimization Methods 3.2.1 The linear programming of power is achieved so that the power reserve is maximized. This is a deterministic method that only evaluates the genset maintenance schedule. Document 3 uses such micro-increase risk method in the multi-regional generator set maintenance plan, mainly as a deterministic method. Document 4 combines linear programming with heuristic algorithms, first using linear programming, and then using heuristic algorithms. The advantage is that the maintenance period changes from the original weekly unit to the daily unit. However, the reliability of the latter heuristic algorithm is worthy of progress.

3.2.2 Dynamic programming Dynamic programming is a mathematical method to solve the optimization of the decision-making process. The main idea is to convert the multi-stage process into a series of single-stage question-making decisions. It is discrete and complex. It is more appropriate to use dynamic programming to optimize. of. However, the biggest drawback of dynamic programming is the dimensioning of disasters, especially the high dimensionality of generator maintenance schedules. Document 5 uses a successive approximation method and groups all the units for solving. Document 6 integrates the semi-invariant method on the basis of the former, and solves the objective function as the weighted sum of production costs and reliability costs.

3.2.3 Goal planning Goal planning is a kind of multi-objective planning. Its main idea is to optimize the weighting and gradual optimization of each target. It is worth mentioning that it uses a computer language designed specifically for optimization to quickly and easily solve the optimization goals. Therefore, the research on the optimization of maintenance plan for generator sets should start from many aspects.

3.2.4 Integer Programming Integer programming is a mathematical plan that requires variables to take integer values. This is consistent with the integer nature of the maintenance schedule of the generator set. However, it builds a random model that requires a large amount of storage space and a long calculation time. The literature 6 uses mixed integer programming to consider the production of transmission pits for interconnected systems consisting of two regions. Since the integer programming takes a long time, although the integer programming model is established in the literature 7, the solution is still implemented using a heuristic algorithm.

3.2.5 Penders Decomposition Method 819 The Benders Decomposition Method decomposes the maintenance plan of the generator set into an integer plan and a stochastic plan for decentralized coordination, which can simultaneously include the characteristics of integerity, randomness, non-linearity, etc., without processing, and the results are quite high. Take into account network maintenance line maintenance plans, etc.

With the promotion of the electricity market, the power system is developing toward the direction of large-scale systems and multiple regions, and the issue of transmission and maintenance overhauls is gradually exposed.

Should focus on the actual question, the focus should be on the two aspects of computing speed and convergence to find a better way. In addition, we should also consider the overall maintenance and safety of the transmission and distribution network.

赍 Editor Li Bo, page 21, 1. Evaluation of the corrosion resistance of carbon steel No.20 steel surface by three methods including alternating current impedance and constant potential step phenanthroline spectrophotometry. Yes.

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8 weeks country setting. Electrochemical Method Applied to Corrosion and Protection of Power Equipment Corrosion Research Editor

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