CBTC ve Fallback sistemlerinin kullanılabilirlik değerlendirmesi
Evaluation of availability of railway signalling systems
Yazar: VEDAT SÖYLEMEZ
Danışman: PROF. DR. MEHMET TURAN SÖYLEMEZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Raylı Sistemler Ana Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Kentleşme oranının çok yüksek değerlere ulaştığı şehir merkezlerinde ve özellikle metropol alanlarında ulaşım ihtiyaçlarına çözüm olarak raylı sistemler düşünülmektedir. Yine yapılaşmadan kaynaklı fiziksel alanların yetersizliği de raylı sistemlerin tercihinde önemli etkenlerden birisidir. Raylı sistemler enerji verimliliği ve çevre duyarlılığı açısından bakıldığında da diğer ulaşım sistemlerine üstünlük sağlamaktadır. Trenlerle yapılan yolculuk uçaktan sonra ikinci güvenli taşıma olarakta bilinir. Tüm bunlara rağmen demiryollarında da bazı istenmeyen özellikler bulunmaktadır. Bunlardan birisi kötü frenleme koşullarından dolayı trenlerin durma mesafeleri fazladır. Bu sebeple hem emniyeti sağlamak hemde işletmede esnekliği artırmak için sinyalizasyon sistemleri kullanılır. Farklı tasarımlar yapılsada temelde iki tren arasında güvenli bir mesafede tutulması gerekmektedir. Geleneksel sinyalizasyon sistemlerinde bu ayrım fiziksel bloklarla sağlanmaktaydı. Her bloğa tek bir trenin girişine izin verilebildiği için sefer sıklığı yüksek olan bu sistemler kapasiteyi de sınırlamaktaydı. Bu sınırlamanın çözümü olarak hareketli blok sinyalizasyon sistemleri karşımıza çıkmaktadır. Her bir trenin kendi başına blok kabul edilmesi sebebiyle artık sabit fiziksel bloklar yoktur.  
Kent içi demiryollarında kullanılan CBTC sistemleri de hareketli blok yapısına sahiptir. Çift yönlü ve yüksek kapasiteli very haberleşme sistemine sahip olduğu için de CBTC sistemlerinin peroformans değerleri çok üst düzeye dayanmaktadır. CBTC sistemlerini oluşturan donanımların yüksek güvenilirliğe ve düşük arıza oranına sahip olması gerekmektedir. Sistem tasarımında performans gerekliliklerini sağlamak için RAMS süreci yürütülmektedir. Günümüzde CBTC haberleşmesi daha çok radyo tabanlıdır ve kullanılabilirliğin en fazla olduğu alt sistemlerdendir (5-9s). CBTC sistemlerinin ray devrelerinden bağımsız çalışıyor olması hatta ki donanım sayısını da azaltmaktadır. Bazı durumlarda CBTC sistemine ek olarak ikincil bir sinyal sistemi (Fallback) veya koruma sistemi kurulmak istenir. Olası arızalar, karma işletme, hattın bir üst sinyalizasyon seviyesine çıkarılması, bakım araçlarının kontrolü gibi biröok sebep için tasarlanabilir. Çalışmalar göstermektedir ki CBTC sistemleri yüksek güvenilirliğe sahip donanımları barındırması ve modüler tasarımından dolayı (MTTR değerinin düşük olması) yüksek bir kullanılabilirliğe sahiptir. Eğer zorunlu bir durum yoksa kesinlikle bu gibi sistemlerden kaçınılması gerekmektedir. 30 yıl üzerinden bakım ve lojistik maliyetleride dahil edildiğinde bakım araçlarının kontrolü içinde hiç uygun olmayacaktır. CBTC sistemlerine ikincil bir sistem daha kurma ancak hattın yenileme çalışmalarında işletmenin aksamaması veya daha az etkilenmesi adına bir çözüm olabilir. Karma işletme artık çok nadir düşünülen çalışmalar olsa gerek ki kent içi raylı sistemlerde tamamen sürücüsüz çalışan sistemler yapılmaktadır.

Abstract:
Rail transport is universally accepted as one of the safest public transport. Over the last decade, there has been a huge focus on railway systems. The implementation of railway systems depend on infrastructure cost. Nowadays there are many projects that planned to expand the rail network in Turkey, especially metro systems. Also European rail market is expected to grow to approximately 176 billion euro by 2017.
A feature of railways is the low friction between steel rails and steel wheels; this has its advantages but means long distances to stop are required due to the severely limited adhesion. Trains can also be extremely heavy and travel at considerable speeds; a moving train therefore has significant momentum and there is a lot of kinetic energy to convert to other forms of energy during the process of stopping a train. Hence braking assumes a much bigger significance within the design of a railway system than it is within the design of a road network for example. The most fundamental role of a railway signalling system is to communicate to the driver information needed for the train to be driven safely and efficiently. Since braking distances are generally far greater than the distance over which direct observation is possible, the warning to brake for an unseen hazard is the most important piece of information to transfer to the driver.
The Signalling system is an essential part of the railway. Its principal task to ensure that trains run safely. The overall purpose of signalling is to facilitate the safe and efficient movement of trains on the railway. Trains can be run without signals but at the risk of human error; it is the signalling that allows this risk to be managed to very low levels. 
Operation of the conventional signalling system is based on the fixed physical blocks. In this approach, railway line is divided into blocks. Block lengths have effect on the capacity of the line. The safe train separation is performed by the allowing one train in each block. Signalling system is one of the most costly systems in the railway field. It compromise so many subsystems and components.
The fixed block concept employs fixed sections each made up of track circuits to detect the presence of trains. The track is divided into block sections with fixed boundary locations. The minimum length of these block sections are chosen to ensure protection under all operating conditions; the maximum lengths are determined by desired headway and operating speeds. In CBTC systems generally, the moving block concept can be defined as an automation system allowing each train to receive a "movement authority info" from the zone controller. The zone controller has to have a continuous communicate with all the trains and to know continuously their speed and position. On this basis, the zone controller can calculate and dispatch to each train the current "movement authority info". Together with the "movement authority info" safe data,  the control centre can also dispatch to the train some non-safe data, related to traffic regulation purposes. The fixed block technology leads to lower dynamic performances with respect to the moving block. However, to obtain a full exploitation of the performances of the moving block, the control centre has to perform a continuous traffic regulation, suggesting the best drive parameters to the trains
Fallback mode is an auxiliary method of detecting trains using conventional means, it is not the primary method. Secondary train control system is defined, "… a secondary train control system is defined as signaling equipment that, when integrated with the primary CBTC system, provides a level of automatic train protection functionality for trains either:
Dependability is measure of the capability of a system to perform its intended function.  Dependability affects financial justification of the project. Dependability consist of the four elements. These elements have been extensively applied to the signalling field. Moreover, this process called the RAMS that are known as   Reliability, Availability, Maintainability and Safety techniques. RAMS techniques allow reliability engineers to forecast failures from the observation of operational field data. RAMS parameters that may be forecast are reliability, availability, failure rate, Mean Time Between Failures (MTBF), Mean Down Time (MDT), among others. In this thesis, one of the RAMS parameters, availability is examined to evaluate that luxury or neccesity the case of fallback system focusing on the rail component.
From the railway standards, there are two type of failure: random and sistematic. This analysis is based on random failures. When a failure occurs in the railway signalling system, other operational scenarios are used to continue railway service, e.g. degraded mode, reduced speed. Signalling system failures have great effect on availability and safety. In railway environment there is an increasing demand for greater reliability and availability of systems and for closer integration of systems. To succeed these aims, it is required to improve overall system reliability and safety and maintainability. In early devolopment process, in terms of achieving reliability goals, safety and availability are critical parameters. Railway systems provide a service to customer by performing more functions. There are a certain number of key system attributes which must be considered during the design and devolopment of a system required to be reliable. These are logistic reliability, operational reliability and operational avalability.
Depends upon the sum of the reliability of each individual component, system reliability are influenced by some technical and operational conditions. Such as level of fault tolerance level, early warning fault, latent faults, immunity, degradation mode, duplication of subsystem or equipment etc. The greater the availability of system the less chance of needing to operate in degraded conditions. Availability is extremely affected by reliability of components that comprise the system. In terms of failure rate, low number of failure minimise the probability of degraded mode working. In addition to this, system configuration have important effect on system availability. In some cases, with redundant equipments, a subsytem failure does not cause the system to be functionless and this permits it to be repaired/replaced when reasonably convenient rather than as a matter of urgency to restore service. Conversely if the means of achieving the availability target is arranging the system architecture to have much diversity, this inevitably has an adverse effect on reliability because the increased quantity of equipment subject to suffering failures. It also increases system complexity and as a consequence the possibility of human error is increased; unexpected failure modes may ocur. 
Maintainability means ability of a system to be retained in, or restored to, a state in which it can perform a required function. The better the maintainability of system the less time which is spent operating in degraded mode. Maintainability considerations include: accessiability physical and from overall system viewpoint, protection against environment (corrosion, vibration, temperature, humidity), preventing of wear or lubrication, regular replacement of components with relatively short lifespan, training of personnel, availability of spares and system documentation and proccedures etc.