Optimum utilization of power transformers based on reliability concepts

Dissertation

Master's degree

Field: Electrical Engineering - Power

Abstract

The transformer is one of the most important components of the power system and due to its high price and the importance of the transformer's performance in the power system and The time-consuming nature of its replacement, the process of examining and choosing the optimal and optimal operation method for transformers, taking into account economic and reliability issues, has always been one of the most important and necessary issues of power systems. Therefore, in this thesis, a possible new method for determining the range of economic operation of the transformer is presented. The main idea of ??this thesis is based on the intentional withdrawal of one of the parallel transformers of the distribution or transmission substations and placing it in a standby state and includes five main steps: 1) calculating the availability and unavailability based on the Markov model and converting it into an economic index using the concept of unsupplied expected energy; 2) Calculating the total no-load and copper losses of the transformer based on the loading information and converting it into an economic index; 3) Determining a critical load point for the substation and determining the economic range of operation based on steps 1 and 2. 4) Investigating the effect of the new operation method on the average life of the transformer. 5) Calculation of system reliability in the erosion period using the thermal model and the relationship of acceleration of wear and using the normal distribution function. For the purpose of a case study, the necessary simulations have been carried out based on the real data of one of the 230/63 kV substations of the Iranian electricity network. The results of the case study indicate the efficiency of the new proposed method. As it will cause economic savings, significantly increase the average life of transformers and increase the opportunity for preventive repairs. Also, the simulations performed in this thesis can be used to monitor the reliability and remaining life of the transformer on-line. Transformer power, intentional discharge, transformer erosion

Foreword

Transformers are one of the main equipment of the power network to provide energy to subscribers. Due to the high cost of repairs and maintenance of power transformers and since their repair, supply and installation in case of failure requires spending a long time, the occurrence of errors and breakdowns in transformers cause long-term power outages and subsequently reduce energy sales and income of power companies. Therefore, the optimal use of transformers in order to reduce their cost and failure rate is one of the issues that are currently facing power grid management and operation companies in many countries of the world. So far, many studies have been conducted in the field of economic and optimal use of power transformers based on different perspectives. Among them, we can use economic power transformers based on total power and reactive power, characteristics and technical parameters of transformers, improvement of load factor, barrow displacement. pointed out [7]. This is despite the fact that not many studies and researches have been done in the field of economic exploitation of power transformers considering reliability indicators. On the other hand, in the few researches that have been conducted, the mutual effect of the influencing factors on the reliability indicators and finally their cumulative effect on the optimal use of the transformer has not been seen.

The challenging point in this regard is that paying too much attention to reliability will cause a waste of budget and capital and increase costs, on the other hand, unreasonable and illogical attention to economic issues and efforts to reduce the cost of operation without considering Reliability indicators will place the system in the area of ??risk and danger, for this reason, creating a compromise and balance between operating costs and reliability indicators has always been one of the most important and complex issues in power system studies.Therefore, the most important principle for policy in the optimal use of transformers of distribution and transmission substations based on the concepts of reliability is to find a suitable balance between costs and reliability. The main concerns of most of the owners of these equipments are, especially when the transformer is under continuous and periodic overload.

One ??of the important factors in determining the average life of transformers in the range of power, super distribution and distribution transformers is to pay attention to the insulation issues of these equipments. There are various factors in this field, each of which can affect the insulation life of the transformer.

Insulation decay is a function of working temperature, humidity and oxygen in the insulation. Nowadays, with modern systems for processing and maintaining oil and other insulations of transformers, oxygen and moisture of insulations have been minimized, since the temperature distribution in the transformer is not uniform, research has focused on determining the hottest point of the transformer (the so-called hot spot) and it is referred to as the main factor in determining insulation erosion. C57.91-1981, C57.92-1982 and C57.115-1991 are provided. But in the C57.91-1995 standard related to the loading of oil transformers, these tables have been removed and instead of them, a thermal transfer model has been provided, which the user can determine the hot spot temperature based on its transient state relationships and using the thermal characteristics of the desired transformer obtained from the experiment. To determine the hot spot temperature in this thermal model, loading is entered into relations. Therefore, loading is one of the factors affecting transformer temperatures, especially the hot spot temperature.

On the other hand, until the transformer enters the medium life range, only the accidental failures of the components during the useful life period play a role in determining reliability [8]. But when the transformer enters the medium life range, erosion will also affect the reliability of the transformer. The most important factor in determining the reliability in the erosion stage is the average life of the transformer [8,9]. As mentioned, the average life of the transformer is affected by the temperature of the hot spot and as a result of the loading.

Research objective

Evaluation of the availability and reliability of the substation with two parallel transformers in a long-term and specific operating period by applying the proposed method assuming the intentional disconnection of one of its transformers and placing It is in standby mode.

Calculation of total losses of transformers and unsupplied expected energy[1] and converting them into economic indicators and comparing two common operation methods and the proposed new method with the approach of minimizing the total cost of operation of the transformer in a certain time horizon. them and also the reliability of the system during the erosion period.

In this dissertation, with a case study on one of the country's transmission substations, it will be shown that the operation of the parallel transformers of a substation based on the new method proposed in this research will have an effect on the cost of operation, reliability, lifetime, and the time of the start of the erosion of the transformers, while this operation method has not been discussed in any research so far. is