Orta gerilim şebekelerinde kullanılan gerilim kademelerinin optimizasyonu
Medium voltage network optimization
Yazar: ÖNER ÇELEBİ
Danışman: PROF.DR. NESRİN TARKAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Yüksek Lisans, 1994
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering
Dizin: Gerilim = Voltage ; Optimizasyon = Optimization ; Orta gerilim = Medium voltage ; Şebeke = Network

Özet:
BZET Bir arta gerilim şebekesi tasarlanırken, şebekenin toplam güç ihtiyacı, üretim tesislerine uzaklığı, iletim ve dağıtım gerilimi, enerji kaybı, cihazların uygunluğu, kablo hattı tesis edilecekse ideal güzergahın secimi gibi hususların gözönüne alınması gerekir. Dağıtım şebekeleri, salt ve indirici transformatör merkezleri, orta gerilim hatları, transformatör postaları ve nihayet alçak gerilim besleme kablolarından meydana gelir. Orta gerilim şebekelerinde kullanılan kablo ve cihazların maliyetleri ve imalat şekilleri gerilim kademelerine göre bir çok farklılıklar gösterir. Bu çalışmada, önce seçilen pilot bölgedeki mevcut orta gerilim şebekesinin toplam maliyeti bulunmuştur. Sonra sırasıyla 6,3 kV, 15 kV ve 34,5 kV gerilimlere göre tekrar tasarlanıp bu gerilim kademelerindeki şebeke maliyetleri hesap edilmiştir. Bu maliyetler kıyaslanarak bir gerilim maliyet grafiği çizilmiştir. Elde edilen maliyet değerlerinden gerilim yükseldikçe cihaz maliyetlerinin arttığı, bunun yanında kablo kesitlerinin dolayısıyla maliyetinin azaldığı görülmüştür. Cihaz maliyetlerindeki artmalar, kablo maliyetlerindeki azalmalardan daha az olduğundan seçilen pilot bölge için 34,5 kV gerilim kademesi optimum gerilim olarak hesap edilmiştir. -vı-

Abstract:
SUMMARY MEDIUM VOLTAGE NETWORK OPTIMIZATION In design, building and maintanence of any physical system engineers have to take under consideration many technological and managerial factors and have to make decision at several stages. The main objective of considering of all these factors and making decisions is either to minimize the effort or cost required or to maximize the profit in constructing a system which is physical. Since the effort and cost required or the benefit desired in constructing a system or in implementing an objective can be expressed as a function of certain variables, the optimization teen i que can be defined as the process of finding the parameters or control signals which minimize or maximize a function. In a more general sense the optimization is the process of obtaining the most appropriate solution in building a physical system or imlementing a control operation. These are a number of methods available for solving optimization problems. The optimization techniques can be used to salve any engineering problems. In its broadest sense, the optimization can be applied to any physical problem. Optimization problems can be classified as follows: 1. Optimization with or without constraints. Depending upon whether the constraints exist or not in the problem, the optimization can be classified as; a) Optimization with equality and inequality contraints, b) Optimization without constraints. -vi i-2. Optimization based on the nature of design variables, a) Parameter Optimization: For this category of optimization problem is to find the values of the system parameters which make some performance measure of these parameters minimum subject to certain constraints, b) Another type of optimization problem is to find the variations of a set of the system parameters which are the continuous function of other parameters, that minimize a performance measure subject to the prescribed constraints. In an optimal control problem there are two types variables. These are the control and the state variables. The state variables describe the behaviour of the system when the control variables are given. Thus the control variables bring the system from a given operating point into another one. When we use this vocabulary, then for the defination of the optimization problem the following statement can easily be given. The optimization problem is to find a set of control variables such that the objective function computed for all control stages is minimized subject to prescribed constraints put on the state and control variables. The most important factors for medium voltage networks optimization are network cost, network power, network voltage and cross section area of line. There are many voltage steps in medium voltage networks. In cases of selection of insufficient voltage step during design stage, network power losses increase. Therefore, while voltage steps in medium voltage networks are being selected zoning and construction plan should be feasibilitied, more effective prediction for energy requirement in accordance with improving in future, should be done and medium voltage networks should be installed to prevent long duration energy cut- off. - vxn-Losses in networks, which do not ensure energy requirement will increase and energy cut- off will be in long duration. Constraints should be specified during optimization in medium voltage network. These can be voltage step, power cross section area of the lines, installed in system. Furthermore, constant outgoings for networks can come to the fore and be considerable as a constraints especially in developed cities in medium voltage network optimization, most important constraint is the power. Since power requirement of network and increasing in future can be guessed with little approximation, power is usually selected as a constant and voltage steps and cross section area of lines, used in medium voltage networks are optimizied. Obtained results in this study to design medium voltage networks are as follows; 1. In regions, where power density is more. Power distribution should be done with upper medium voltage step. Being more of count of lower transformer station is the most optimal solution. 2. According to result of network design, doing for various voltage steps, voltage of 34,5 kV is determined as optimal. Voltage steps in value of 6,3 kV, 10 kV and 15 kV, used in our country, are not economic in regions where power density and esspecially transmission distance are more. Even per unit prices of cables and transformer station devices are cheaper than be in upper voltage step, the transmitted power is less. 3. When voltage value is rised, constant costs also increase. At that case, outgoings are to be more since transformer station is found on big area. -IX-Since medium voltage steps, which are higher than 34,5 kV, are not used, the devices for these voltage steps are not produced. Price of devices for these voltages steps is not included in Turkish Electricity Authority's price list. If schalt devices are produced for voltage steps, being higher than now, in our country, medium voltage networks can be installed that is more optimal. The economics of electric power transmission revolve around three topics: 1) The actual cost of transmitting unit energy over distance. 2) The relative transmission cost as it varies from one type of line to another and particularly of underground lines with respect to overhead lines. 3) The energy lass per unit energy conveyed over unit distance. These there aspects will be discussed for all types of cable and the treatment of the oil- paper insulated self- contained cable serves as the model for this discussion. To arrive at the actual cost of power transmission, it is necessary to have up to date information on three major cost components, which are: a) The installed cost of transmission lines per unit length, b) The cost of the fixed energy losses (dielectric, pumping, etc.), which accrue all the time regardless of -x-the amount of energy that the line is carrying. c) The cost of the joule losses, which are proportional to the square of current or power flow. Another important economic parameter is the "annual capital charge." This refers to depreciation, interest that would have been earned or saved had the capital not been built, and the annual maintenance cost, which for transmission lines is quite low. The annual capital charge as a fraction of the installed cost of transmission lines used throughout reference is 15 percent. Depreciation rates do not vary with inflation and interest charges have.remained relatively constant. Relatively few self-contained oil- paper insulated cables have been installed in the United States. Their costs were not compiled in the ADL- report. Yet these cables are widely used in Europe and paticularly in Great Britain. A graph comparing the installed cost of self-contained with pipetype oil- paper insulated cables was published in 1967. The British cost actually applies to 1965 and it is seen to be only about half that of U.S. pipe- type cable installations at corresponding voltage and power levels. In the discussion of it was argued that part of the differential might be explained by the higher wages that prevailed in the United States. Most expert seem to agree that self- contained cables are more economical than pipe-type cables. But since no hard figures are available for U.S. installations it will be assumed that the installed cost of the self- contained cable in the United States is the same as that of the pipe- type cable for equal voltages and conductors. -xi-