SMART GRID REFERENCE ARCHITECTURE
Railway DC Micro-grid is a small scale smart grid. Thus, it should respect the Smart Grid Reference Architecture (SGAM) specified by “CEN-CENELEC-ETSI Smart Grid Coordination Group”. The use case’s components, functions, data flow and communication should be classified into layers, zones and domains to organize the system and ensure interoperability.
Interoperability is the main key of a smart micro-grid. According to IEC 61850-2010: “Interoperability refers to the ability of two or more devices from the same vendor, or different vendors, to exchange information and use that information for correct co-operation.” Therefore, the GridWise Architecture Council (GWAC, 2008) defined interoperability categories describing requirement and methodology in order to achieve this mutual understanding within a micro-grid. For each interoperable function, all categories have to be covered, by means of standards or specifications.
To simplify the task, the interoperability categories were aggregated into five abstract layers: Business, Function, Information, Communication and Component [CEN00]. They are presented in Figure 37.
According to the Smart DC micro-grid architecture, the PMS should be able to communicate with the distributed control units. Therefore, it should contain more or less the five layers listed in Figure 37.
The business layer defines the objectives of the system based on a specific business model. It’s where policies are decided. It includes the business processes, services and organizations that are related to the defined use case.
In this use case, the railway DC Micro-grid is mainly intended to:
Recuperate trains’ braking energy.
Charge electrical hybrid buses.
The Function layer describes functions needed to fulfil objectives and roles defined by the business layer. Functions and their relationships are independent of the physical implementation of the system (applied technology or assigned actor). They represent the use case functionality. In the DC Micro-grid, the PMS main functions are to:
recuperate the trains braking energy while respecting power exchange between trains.
store the maximum of trains braking power in the hybrid storage system.
regulate the DC Busbar voltage to avoid over or under voltages.
This layer defines communication protocols within the smart micro-grid. In order to choose the type of communication, it is first required to define what type of networks will be used in this application. Figure 38 shows an example of different communication networks in communication layer.
The micro-grid mainly covers from the Process domain up to the Operation domain. Therefore, the selected networks should also cover these domains and be able to communicate (same communication technologies). For example, based on Table 2, the “Low-end intra substation” and the “field area” networks could be chosen.
It is recommended to use the IP for the communication because it is a well-known open standard that fits smart grid’s new requirements. It also offers migration path for some non-IP protocols and implementations like DNP, Modbus and KNX.
It consists of the physical components of the smart micro-grid. This includes measurement devices, power system equipment (converters, storage…), protection and remote-control devices (programmable logic controllers, network infrastructure (wired / wireless communication connections, routers, switches, servers) and any kind of computers (in case there is an interface). For IP communication technologies, Ethernet or Wi-Fi could be used because the micro-grid is considered as a small area network.
STORAGE SYSTEM TECHNOLOGIES
As Katie Fehrenbacher said, “a next-generation smart grid without energy storage is like a computer without a hard drive: severely limited” [ENS02]. Therefore, the DC Micro-grid should contain a storage system to ensure flexibility and better energy management. A hybrid storage system stores the braking energy coming from the railway network. Primary energy sources, such as batteries, are usually used as a continuous source of low power. However, they cannot efficiently handle peak power demands because of their charging/discharging low power density. Power sources are needed to complement these energy sources in order to meet today’s applications. They consist together a hybrid storage system providing both high power and energy density (Figure 39).Pumped hydroelectric storage facilities (PHS) store energy by pumping water from a lower reservoir to an upper reservoir. Power is then restored by releasing stored water through turbines the same as a conventional hydropower plant. The energy’s round-trip efficiency (pumping/generating) is greater than 80%. However, this technology requires a large volume which is not compatible with urban applications. Compressed air energy storage (CAES) is an alternative to PHS but instead of pumping water, ambient air is compressed and stored under pressure in an underground cavern. When electricity is required, the pressurized air is heated and expanded in an expansion turbine driving a generator for power production [ENS00]. It can be employed in small-scale on-site energy storage solutions as well as in a very large storage with big energy reserve. CAES power-to-power efficiency varies between 40% (Diabatic Method) and 70% (Adiabatic Method). Thermal storage presents three main categories: Pumped heat electrical storage (PHES), hydrogen energy storage (HES) and liquid air energy storage (LAES). PHES stores energy by pumping heat form the “cold store” to the “hot store” like in a refrigerator. To restore this energy, the engine takes heat from the hot store, delivers waste heat to the cold store and produces mechanical work which will drive an electric generator. The expected AC to AC around trip efficiency is 75-80% [ENS01]. HES is where electricity is converted to hydrogen by electrolysis. The produced hydrogen can be stored and burned later to produce electricity. Unfortunately, HES’s efficiency today is low (30-40%) but the interest in this technology is growing due to its storage capacity that is much higher than batteries (small scale) or PHS and CAES (large scale). LAES uses electricity to cool air until it liquefies. The liquid is then stored in a tank. Energy production is done by bringing back the air to its gaseous state using ambient air or waste heat from industrial process. The gas turns the turbines driving electric generators. This technology is usually used for large-scale and long duration energy storage.
Concerning batteries, they can be classified in two categories: solid state batteries (conventional batteries) and flow batteries. Conventional batteries consist of electrochemical cells that convert stored chemical energy into electrical energy. Each cell contains a positive terminal (cathode) and a negative terminal (anode). Ions move between terminals through electrolytes. New technologies such as electrochemical supercapacitors can be charged and discharged instantly and provide almost “unlimited” life cycles (commercially up to 1 Million cycles). The most common rechargeable batteries are: lead-acid battery the oldest, Nickel Cadium (Ni-Cd) battery and lithium-ion (Li-ion) battery. The latter is the most promising battery chemistry. It is replacing the others in many applications. A flow battery is an electrical storage device that is a cross between a conventional battery and a fuel cell. Mechanically activated by pumps, flow batteries perform best at a size above 20kWh which make them suitable for bulk energy storage. For example, a flow battery for frequency regulation is being installed in Japan that will deliver a whooping 60MWh. The figure below compares the energy density of main battery technologies:
Table of contents :
SECTION A: OVERVIEW ON RAILWAY SYSTEMS
I – Historical railway evolution
II – Railway electric traction systems
III – Environmental impact and existing solutions
III.a. Environmental impact
III.b. Train braking system
III.c. Solutions for reducing energy consumption
IV – Conclusion
SECTION B: SMART DC STATION FOR URBAN SYSTEMS
V – DC Micro-grid integration in railway system
V.b. Context and Concept
V.c. Smart grid reference architecture
V.d. Storage system technologies
V.e. Power converters
V.f. Power management system
V.g. Use case: Application On Paris metro line
VI – Stability of DC Micro-grid
VI.a. Instability of low damped systems
VI.b. Small signal stability of the DC Micro-grid
VI.c. Backstepping approach
VII – Technical recommendations
VII.a. Inverter’s specifications
VII.b. Storage converter’s specifications
VII.c. Railway converter specification
SECTION C: ENERGY OPTIMIZATION FOR SUBURBAN AND HIGH SPEED LINES
VIII – Introduction
IX – Speed profile and energy optimization
IX.b. Methodology general specifications
IX.c. Definition of the algorithm
X – Trains synchronization
X.a. Dwell times modification
X.b. Delay compensation
XI – Simulations: Application on Paris-Lyon high speed line
XI.b. Results of speed profile optmization
XI.c. Results of substations power optimization
CONCLUSION AND PERSPECTIVES
A- OSIRIS PROJECT
B- MERLIN PROJECT
C- ELBAS SOFTWARE USED FOR ENERGY SIMULATIONS
D- TIMETABLE OF PARIS-LYON HIGH SPEED LINE