Dağıtım Şebeke Yatırım Planlamasında Çok Kriterli Karar Verme Yönteminin Kullanımı

Abstract

Günümüzde elektrik enerjisinin kullanımı oldukça yaygın bir hale gelmiş olup, hem günlük hayatta hem de iş hayatında bir zorunluluk haline gelmiştir. Oluşan kısa süreli elektrik kesintileri, hayatımızı durma noktasına getirmektedir. Ekonomik büyüme, bilimsel-teknolojik gelişme ve nüfus yoğunluğundaki artış elektrik enerji talebini her geçen gün arttırmaktadır. Bu nedenle üretim tesislerinden iletim, dağıtım şebekeleri ve son tüketiciye kadar elektrik enerjisinin sağlanmasında görev alan tüm birimler bu talebe hazırlıklı olmalıdır. Bu hazırlık, gelecek dönemlerin önceden talep tahminin yapıldığı planlama çalışması sonucunda gerçekleştirilebilir. Elektrik enerjisinin üretiminden tüketicilere kadar ulaşmasında görev alan tüm şebekeler incelendiğinde en karmaşık yapıya sahip şebekenin dağıtım şebekeleri olduğu açık bir gerçektir. Dolayısıyla dağıtım şebekelerinin planlanmasında farklı kriterlerin göz önünde bulundurulması gerekmektedir. Karmaşık şebeke yapısının doğal sonucu olarak belirlenen kriterlerin karşılandığı birden çok çözüm seçeneği ortaya çıkabilmektedir. Bu nedenle, yapılan planlama çalışmalarının belirlenen kriterleri karşılamakla birlikte, kriterleri karşılayabilen olası alternatif çözümlerden en uygun olanının seçimini de içermesi gerekmektedir. Farklı değerlendirme kriterlerinin bulunduğu durumlarda karar verme, sadece dağıtım yatırımları için değil tüm kararlar için oldukça zordur. Bu zorluğun aşılması amacıyla çok kriterli karar verme yöntemleri geliştirilmiştir. Çok kriterli karar verme yöntemlerinin amacı; birden fazla kriterin olduğu durumlarda karar verme aşamasındaki belirsizlikleri ortadan kaldırmak ve karar verme sürecini belirli yapıya sokarak formülize etmek ve dökümantasyona hazırlamaktır. Yapılan bu çalışmada, ele alınan örnek bir dağıtım şebekesi için geleceğe ait tahmini yük artışı belirlenmiştir. Hem mevcut hem de gelecek dönem yük talebi dikkate alınarak zayıf nokta analizleri yapılmış ve problemli noktalar tespit edilmiştir. Bu problemlerin giderilmesi için; mevcut şebekenin güçlendirilmesi, gerilim dönüşümü ve dağınık üretim birimlerinin bağlanması şeklinde üç farklı alternatif çözüm sunulmuştur. Alternatiflerin değerlendirilmesi için genel anlamda ekonomik, teknik ve çevresel etkiler bazında sekiz farklı hedef ve kriterler belirlenmiştir. Alternatiflerin değerlendirilmesinde çok kriterli karar verme yöntemlerinden TOPSIS (For the technique for order preference by similarity to ideal solution) yöntemi kullanılmıştır. Her bir kriterin önemi, farklı ağırlık katsayıları verilerek belirlenmiştir. Ağırlık katsayıları değerlendirilerek belirtilen çözümler karşılaştırılmış ve uygun çözüm/çözümler belirlenmiştir. Her üç duruma ait karşılaştırma sonuçlarından çok kriterli karar vermede ağırlık katsayılarının önemi görülmüştür. Bu nedenle yatırımlara karar vericilerin hedeflerin, kriterlerin belirlenmesine ve ağırlıklandırılmasına ne kadar önem vermeleri açıkça ortaya çıkmaktadır.

Nowadays, the usage of electrical energy has become a quite common. It has become a necessity in both daily life and business life. The short term power interruptions, bring the human life to a standstill. The demand to the electrical energy has increased day by day regarding to economic growth, scientific-technologic improvement and population growth. For this reason, all units which take a role from generation units to transmission, distribution system and also to the last consumer should be ready and well prepared for the increased demand. When the systems which are the parts from generation to the end customer are examined, there is no doubt that the distribution systems are the most complex one and they are in touch with the end customer. Therefore, so many criterias are needed to be taken into account in distribution system planning. In the past distribution planning studies, the planners were just taking into accounts the technical criterias like maximum voltage drop, overloading issues, technical losses etc. However, distribution utilities have realised that criterias just only for the technical issues is not enough in the competitive energy market. If the distribution utilites would like to survive in this market, they need to take into account the economical criterias besides the technical criterias. Otherwise, they can not withstand. Recent researches show that the economics of the distribution systems can be divided in three categories as capital cost, operation-maintenance cost and cost of losses. Some of these criterias is directly related to the technical issues, but some of them is related to financial problems in the utility. And also, some researches are done for combining these two things like selection of the optimal conductor for the system. Furthermore, technical and economical criterias are not enough to be sure about the investments. Some new criterias are involved to this process regarding to the changes in regulations, developments of economonics, technology. These criterias can be the supply quality, average interruption duration, average interruption frequency, customer satisfaction, environmental impacts (Green gas effect, CO2, NOx emission etc.), visual esthetics, safety etc. As a result, the number of criteria are increasing with the development of the life. There can be so many alternative solutions as a natural result of the complex distribution systems. Implementing the first solution into the distribution system can cause a mistake. Thus, the distribution system planning study needs to include selection of the best alternative solution from alternative solutions which provides the defined criterias and goals. Deciding in a case with too many evaluation criterias and goals is very difficult for the decision makers in distribution utilities. However, this problem is not only for the distribution utilities. Hence, multiple criteria decision methods are improved to handle this problem. Recent researches show that evaluation of the real estate value,financial activities of the businesses and banks are analyzed with multiple criteria decision methods. The main aim of multi criteria decision methods is eliminating the uncertainties in decision making process with multiple criteria, creating a structure to be formulized and documentated. There are too many multi criteria decision methods regarding to the uncertainty and alternative number constraints. One of the multi criteria decision method TOPSIS (For the technique for order preference by similarity to ideal solution) can be used when no uncertainty appears in the problem variables and the comparability among the alternatives is not limited. In this study, multi criteria decision method is used for selection of the best alternative in distribution planning. A sample distribution system is modeled as geographically in PSS®Sincal v9.0 which is a network analysis software. The geographical network model let users see the routes of feeders, connections between the feeders and problematic areas better than schematic model. This sample distribution system is supplied from 31,5/15,8kV MV/MV substation via 10(12,5)MVA power transformer. There are 5 outgoings from 15,8kV busbar, 142 distribution transformers (15,8/0,4kV) and 141km medium voltage lines (overhead lines and cables) in the sample distribution system. Existing active power demand of the system is 10,11 MW. Load growth factor is defined as 5,8% for the sample distribution system for future power demand. The future active power demand of the systems is 13,392 MW. The weak point analyzes are done for both existing and future term load conditions, and the problems in the system are detected for each term. The main reason of analyzing future term is understanding the future requirements and be ready for future with the new investments. In weak point analyzes, Newton-Raphson load flow method is used. As a result of weak point analyzes, the technical results and especially the voltage drop problems are shown with colour filter regarding to the limits in the graphics. For solving these problems, three different alternative solutions are proposed. First solution alternative is keeping the operating voltage as it is (15,8kV) and reinforcing of the existing system. In this solution alternative, the mainlines are reinforced, the overloaded distribution transformers are replaced with suitable higher capacity and adding a new power transformer 10(12,5)MVA to the MV/MV substation. Second alternative solution proposes a voltage conversion of the whole system. The operation voltage of the system is converted 15,8kV to 31,5kV. All feeder outgoings are disconnected from 15,8kV busbar and reconnected to the 31,5kV busbar. This MV/MV substation is converted as a MV distribution substation. According to the 31,5kV operating voltage, all the distribution transformers are replaced with the new ones. Third solution alternative proposes to add a new distributed generation units by using the hydro potentials in the area of the distribution system. These units are connected to the end of feeders where the voltage drop problems are seen. To evaluate the solution alternatives for selecting the best one, the goals and criterias are defined and can be classified as economical, technical and environmental impacts like in recent studies. The cost of capital, operating, maintenance cost and cost of losses are the parts of economical criterias. Maximum voltage drop, loss ratio in medium voltage level, system constraints (maximum power capability), supply quality (power can be supplied in an interruption case) are the technical criterias. The only criteria for environmental impact is CO2 emission because of the fossil fuel of the generation units. In the evaluation part, TOPSIS (For the technique for order preference by similarity to ideal solution) method which is the common method in multi criteria decision methods is used. The criterias are weighted with different coefficients according to the importance in evaluation process. Regarding to the weighted coefficients in the first evaluation case, second solution alternative is selected as the best solution. The reason of the selection, second alternative takes the first position in the criterias which have 60% weight, and in rest of the criterias it takes the second position with a tiny differences from the first position. These reasons make the second alternative as the best. To emphasize the importance of weighted coefficients for the criterias, the evaluation studies are done with two other weighted coefficient cases. In the second evaluation case of the coefficients, the first alternative selected as the best alternative. The reason of this selection is that capital cost criteria has 40% weight where the first alternative is the best solution. And supply quality criteria which has 25% weight where the first alternative has no solution and shares second position with its biggest competitive second alternative solution. In other criterias second alternative solution has advance rather than first, but this advantage is not enough to be selected the second solution as best one. In the third evaluation case of the coefficients, the third alternative solution selected as the best alternative. The reason of this selection depends on the 40% weight in the supply quality. In this criteria, just only the third alternative provides a solution. The other two alternatives can not provide any solution, and this makes the third alternative solution as the positive ideal solution in evaluation process. Although, the third alternative has not taken the first position in any other criterias, it takes the whole points from supply quality criteria and win the competition against to the other alternatives. In conclusion, all three alternative solutions are compared with each other with taking into account the weighted coefficients. As a result of this comparison, the crucial contribution of the weighted coefficients in multi criteria decision making is shown up. Accordingly, it is absolutely clear that the decision makers in distribution utilites have to put emphasis on defining the goals, criterias and weighted coefficients to select the best alternative. Therefore, the distribution utilities should create their own decision team which includes employees from different responsibilities and experiences. However, these decision teams may not be fair. This situation can cause an unfair competition between the distribution companies. To prevent this problem, energy market regulatory authorities have to define the limits for the criterias, audit each distribution companies’ decision teams and check the decisions to balance the rights with the other distribution companies. Also, the investments which are the results of this decision process can be monitored according to the criterias. These results can be used as a feedback for the importance of the weighted coefficients.