Elektrik dağıtım sistemlerinde kayıp azaltımı için fider düzenlemesi
Distribution feeder reconfiguration for loss reduction
- Tez No: 66722
- Danışmanlar: DOÇ. DR. ADNAN KAYPMAZ
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Elektrik Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 99
Özet
ÖZET Günümüzde artan elektrik enerjisi ihtiyacını karşılamak üzere, yeni enerji üretim merkezleri kurulması ve daha çok üretim yapılması çalışmaları yanında, mevcut sistemi en küçük kayıplarla işleterek enerji tasarrufu yolu ile yeni kaynaklar oluşturulması yönündeki çalışmalar hız kazanmış olup, iletim kayıpları yanında dağıtım kayıpları da gündeme gelmiştir. En gözde enerjilerden biri olan elektrik enerjisinin üretildiği yerden, en az kayıpla tüketicilere ulaştırılması, üzerinde önemle durulması gereken hususlardan biridir. Kayıpları azaltmanın pek çok yolu vardır. Bunlardan“Besleme Otomasyonu”, dağıtım sistemlerinde kayıpları azaltmak için yapılan çalışmalardan sadece biri olup, tezin konusunu teşkil etmektedir. Bu konu kapsamında yapılan çalışmalarda, normal işletme koşullarında, sistemin yapısını yeniden düzenleyerek güç kayıplarının minimize edilmesi amaçlanmaktadır. İşletme sırasında dağıtım sisteminin dallı yapışırım korunmak istenmesi, sistemde bir anahtar kapandığı zaman diğerinin açılması ile mümkün olmaktadır. Anahtarlama işlemleri denilen bu işlemlerin sayısı sistemin boyutları ile değişmektedir. Hesap yoğunluğu ve işlem süresini azaltmak için çeşitli kriterler uygulanarak bu sayı azaltılmaya çalışılmakta ve bununla ilgili algoritmalar sunulmaktadır. Bu amaçla tezde şimdiye kadar yapılan çalışmalar incelenmiş, kayıp azaltımı için ele alınan yaklaşımlar karşılaştırılmıştır. Bu çalışmalardan biri olan, Civanlar ve arkadaşlarının ele aldığı yöntem ve kriterler üzerinde daha ayrıntılı olarak durularak örneklerle açıklamalar verilmiştir. Daha sonra dağıtım sisteminde meydana gelen kayıpların hesaplanmasına çok-uçlu eleman kavramıyla yaklaşılmış, elde edilen sonuçlar ile öncekiler karşılaştırılmıştır. vıı
Özet (Çeviri)
SUMMARY DISTRIBUTION FEEDER RECONFIGURA TION FOR LOSS REDUCTION Network reconfiguration is the process of altering the topological structures of distribution feeders by changing the open/closed status of the sectionalizing and tie switches. Sectionalizing switches are normally closed and tie switches are normally open. An electric power distribution network is a dynamic system in which various switches are continously opened and closed for various reasons. In respons to a fault, some of the normally closed switches would be opened in order to isolate the faulted network branches. At the same time, a number of normally open switches would be closed in order to transfer part or all of the isolated branches to another feeder or to another branch of the same feeder. All switches would be restored to their normal positions after removal of the fault. Under normal operating conditions, distribution engineers periodically reconfigure distribution feeders by opening and closing of switches in order to increase network reliability and reduce line losses. Most electric distribution feeders are configured radially for effective coordination of their protection systems. Because the resulting feeders must remain radial and meet all load requirements. Figure. 1 shows a shematic diagram of a simplified primary circuit of a distribution system together with sectionalizing switches. SSI CB5 SS2 Figurel. Schematic diagram of a primary circuit of a distribution system VUlIn the figure, load points, where the distribution transformers are tapped off from the primary circuit, is marked by dots,“. ”. SSI and SS2 are substation nodes. As also shown in the figure, there are two types of switches in the system: Sectionalizing switches connecting the line sections (CB1-CB6), and tie switches connecting either two primary feeders (CB7) or two substations (CB8) or loop type laterals (CB9). Switches CB7 and CB8 can be closed and CB3 and CB6 can be opened to transfer load from one feeder to another. The loads of a distribution system are diverse and time varying. This results in the peak load for different areas in the distribution system occuring at different times. One circuit or section of a circuit may be heavily loaded while at the same time another may be lighdy loaded. At off-peak periods, the normal configuration of a distribution system may be very inefficient. This lowers the overall efficiency of the distribution system since periods of system peak demand are relatively short. By reconfiguring the distribution system the network can be made more efficient for every-loading conditions. In recent years, the benefits of distribution automation have been widely acknowledged. One of the important functions of the distribution automation is the configuration management. In modern power networks, switching operations are fully automated and instructions are given at control centers without any need for maintenance crew to be on site to execute the switching. Thus the dispatcher would be able to transfer loads from one feeder to another while minimizing the system losses. When a significant change of load is detected, a loss-minimization algorithm can help the dispatcher to generate a minimum -loss configuration. It is not uncommon to employ manually operated switches to provide seasonal reconfigurations for loss reduction; even monthly reconfigurations are seldom practised in typical utilities. Owing to the relatively recent introduction of this type of procedure and the conservative nature of the power industry, the implementation of distribution reconfiguration has been limited to pilot projects and schemes involving seasonal and infrequent reconfiguration. The reconfiguration algorithms can be classified by the solution methods that they employ: those based upon a blend of heuristic and optimization methods, those making use of heuristics alone, and those using some form of artificial intelligence (AT). The switching operation problem in distribution system is to find the optimal switching candidates so that the objective function, which consists of system loss, is minimized and the operation constraints are satisfied at the same time. Such a problem can be formulated as a combinatorial optimization problem with both equality and inequality constraints. Since the combination of all possible switching operations is very complex and the number of switching cadidates is numerous, a lot of computation time will be required during the optimization process. Such a heavy computing loading will degrade the efficiency of problem solution. The combinatorial nature of the distribution reconfiguration problem has prompted investigation into the use of techniques based purely on heuristic solution methods. The heuristic based methods are characterized by the development of empirical formulas to assess the loss reduction associated with a switching operation and the introduction of rules to reduce the number of candidate switching operations. While the heuristic methods offer solutions in minimum times, the resulting configurations are dependent upon the initial system configuration and offer only loss reductions and do not offer detailed models of the non-linear loads typical at the IXdistribution level. The methods based on a blend of heuristics and optimization techniques present more accurate system models, and result in optimal solutions independent of the initial configuration. The main drawback of employing a blend of heuristics and optimization is that this type of solution methodology requires longer solution time. Artificial neural networks (ANNs), genetic algorithms (GAs), and expert systems (ESs) are techniques based upon artificial intelligence (AT). They have been used to implement the distribution reconfiguration procedure. Although the use of AI based methods has proven them to be valuable in a wide variety of applications, the caution is that they do not represent the best solution in every implementation. The purpose of this thesis is to identify tie and sectionalizing switches that should be closed and opened, respectively, to achieve a maximum reduction in losses. The change in losses can easily be computed from the results of two load flow studies simulating the system configurations before and after the feeder reconfiguration. The time-consuming nature of the rigorous problem formulation prompted Civanlar et al. to propose a formulation based solely upon heuristics. Civanlar et al. justified the use of a purely heuristic algorithm by the two distinct advantages that it offered: 1) the definition of two heuristic rules to reduce the number of candidate switching operations, 2) the formulation of an equation to determine the relative reduction in losses due to a particular switching operation. The two heuristic rules of Civanlar et al. were based upon typical system operation requirements; by analyzing the voltage drop relationships at system buses, Civanlar et al. were able to determine if a particular switching operation would increase or reduce system losses. The use of these heuristic rules was particularly useful as certain switching options could be ruled out without performing any calculations. The rules of Civanlar et al. are defined as follows: Rule 1. Open tie switches. Loss reduction can only be attained if there is a significant voltage difference across an open tie switch. Rule 2. Open switches. Loss reduction will be achieved if loads on the higher voltage drop side of the tie switch are transferred to the other side. Once the two heuristic rules were applied, the relative loss reduction afforded by the remainder of the switching options could be determined using an empirical formula: AP = Re[2.(Xt)(Em-En)*] + Rloop.| £L|2 (1) ieD ieD In Equation (1), D is a set of buses which are disconnected from second feeder and connected to first feeder, m and n are tie buses, Ii complex bus current at bus i. E", and En are components of Ebus = R^ Ibus corresponding to bus m and n, respectively. Rioop is series resistance of the path connecting the two substations. For an ideal distribution system with perfect VAR compensation, all the variables in Eq.(l) assume real values, since there is negligible reactive current flow. In spite of the advanteges of the method, it must be remember that it has some disadvantages. The disadvanteges of this method can be stated as: Only one pair ofswitching operations could be considered at a time. The method assured only a reduction in losses, and not minimization of losses. The proposed network configuration would be dependent on the initial switch status and the presence of line equipment could not be accounted for by the empirical formula. In network theory, multi-terminal components constitues an important concept. It has been thought that if this concept could be used to solve the problem of reducing power losses in distribution systems. The bus impedance matrix Zbus of a power network is a part of the mathematical model of a multi-terminal component which all of its nodes are the terminals. In power networks because of the necessary alterations and new additions done on the network, some changes occure in time. Sometimes the changes are made according to a definite program such as for fault, sometimes for reducing losses in period of times. It is important to know as to how these changes are reflected into bus impedance and power losses. It is assumed that the bus impedance matrix is symetric and there are no couplings between the elements. The changes that occur in electrical power networks usully be considered under two categories: (1) Addition of a new element between any two of the buses of the power network (which also covers the case where alteration of the value of an element already exists between these buses is required), (2) Addition of a new element only one of its terminal is connected to any one of the buses of the power network so that a new bus is introduced into the power system. The changes which occure in power network also affect the power losses in distribution networks. These changes in power systems are desirable to reduce the losses. The power losses are shown as follow before and after changing: P.J^I^.R^.rT (2) Pwm = i,w.RtaF,.l,w (3) The difference occuring between losses is stated as follow: AP=PiJ2)-P,J1) (4) If Pioss(2)
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