Adapting Smart Grid, RES Penetration, Electromagnetic Compatibility and Energy Efficiency Concepts to Electric Ship Power Systems Marios Moschakis1, a, Fotis Kanellos 2, b and John Prousalidis3, c 1
Technological Educational Institute of Larissa, Department of Electrical Engineering, 41110, Larissa, Greece
2
Technical University of Crete, Department of Production Engineering and Management, 73100 Chania, Greece 3
National Technical University of Athens, School of Naval Architecture & Marine Engineering, Zografou 15780, Athens, Greece a
mmoschakis@teilar.gr, bfkanellos@dpem.tuc.gr, cjprousal@naval.ntua.gr
Keywords: Maritime electric power systems, electromagnetic compatibility, power quality, smart grids, renewable energy sources, energy efficiency.
Abstract. This paper deals with the adaption of Smart Grid, RES penetration, Electromagnetic Compatibility and Energy Efficiency concepts from land to ship power systems. It presents a review on the state-of-art on those areas as regards the land power systems and attempts to propose some of those advances and how they can be adapted to ship power systems. Ship power systems have many resemblances mainly with autonomous land power systems. Thus, autonomous power systems will be used as a model for an accurate and successive adaption of those concepts. Introduction Smart grids refer to the use of computer, communication, sensing and control technology which operates in parallel with an electric power grid in order to enhance the reliability of electric power delivery, to minimize the cost of electric energy to consumers and facilitate the interconnection of renewable generating sources to the grid. Microgrids are a concept of smart grids referring to the ability in operating either in interconnected or islanded (autonomous) mode of operation [1]-[3]. The formation of smart grids is crucial in order to achieve better operation and control, improved Electromagnetic Compatibility (EMC) or Power Quality (PQ), increased energy efficiency and penetration of Renewable Energy Sources (RES), reduced cost and lower emissions for the production of electric energy. There are many opportunities and research challenges in the modern land and ship power systems. Those power systems may benefit from the advances in a particular area and concepts may be adapted from land to ship power systems and vice versa. The basic aim of this paper is to present the state-of-the art in land and maritime electric power systems and to list the future challenges. Moreover, recommendations on the adaption of concepts and techniques developed from one to the other type of power systems will be given. Smart Grids and Ship Power Systems Maritime electrical systems are autonomous power systems and include power generation, distribution and control. Ship power systems face similar challenges with land power systems. Smart Grid and Microgrid concepts can be adapted by those systems for similar benefits with the land power systems. The concept of “Smart Grids” refers to the designation of an electrical power system that is inherently flexible, accommodates a variety of energy production sources and adapts to and incorporates new technologies as they are developed. Specifically, the “Smart Grid” concept refers to research and development in the following area [4]-[5]:
Power management systems. A fast, efficient and smart power management system is the key component of Smart Grids. It consists typically by a central (master) power management system, a number of smart meters and sensors, a smart communication network, control and automation systems and a supervision system. Such a configuration is shown in Fig. 1. Active participation by consumers. Electrical appliances are no longer merely passive devices but active participants in the system’s infrastructure. This means that the electrical loads communicate with the control system, they are variable in a smart way and ideal complements to the renewable sources engaged, which are variable in energy production. Smart appliances can also contribute to peak power haircuts. “Self-healing” from power disturbance events. The rapid detection, analysis, response and restoration of the grid’s operation in cases of any power system disturbances including physical destructions and cyber attacks are critical functions of a smart grid. PQ/EMC and Reliability. A certain level of power quality and reliability is desirable in both land and naval power systems. A smart grid should assure this level of electromagnetic compatibility between the grid and the electrical equipment according to strict and applicable standards and requirements. Power converters. They are used as an interface between the electric power generation units and the grid. The most common types are the DC/AC, AC/DC/AC and AC/AC. Accommodation of renewable energy generation and storage units. Modern power systems have substantially larger contribution from low-carbon and renewable sources, which are characterized by intermittency of operation and a lack of ability to dispatch. The high penetration of such sources in an automation and control system is therefore a challenging issue as regards the maintenance of a balance between instantaneous supply and demand. In such cases, storage technologies and their control are crucial [5]. A typical configuration of a power network incorporating renewable and storage units for the electricity supply of a ship’s electrical loads is shown in Fig. 2.
Fig. 1 Typical configuration of the control, communication and power network of a smart grid
Obviously, the aforementioned are the most important common areas of research and development for land and ship power systems. There are also some research domains related with ship power systems and smart grids. These include “cold ironing”, shaft generator systems and electric energy saving devices (EESD’s). “Cold ironing” refers to a shore-side power supply connection of a ship with the ship's machinery shut-down and with the requirement that the port has the infrastructure and the green– energy (e.g. renewables) to support the effort [6]. Cold ironing and smart-grids can be combined successfully, if any deficiency problems in infrastructure aboard and ashore have been resolved. Specifically, instead of directly connecting the ships to the grid mains, efficient energy storage units
are involved, which provide the ships with the energy needed. Additionally, the storage interfaces are charged when energy demand is low e.g. at night. Thus, ports with cold-ironed ships are turned into “smart-loads”. Consequently, the environmental pollution provoked by ships in the port ambiences is almost completely eliminated and the power demand of inland grid is smoothened taking into account that covering the power demands of a ship is on the order of few MW’s. Moreover, energy storage units of large power capacity can be based upon novel battery technologies developed recently, which combine high storage capacity in smaller weight and volume than in the past. Any voltage and/or frequency matching can be realized via modern transformer-less power electronic converters, already exploited in inland electric network for “smart-grid” applications.
Fig. 2 Typical configuration of a ship power system engaging RES
Improved configurations of shaft generator systems can be used for enhanced ship operation efficiency [6]. Specifically, the installation of a sophisticated static four-quadrant power converter (which is actually an EESD) in the shaft generator could eliminate the rotary synchronous capacitor and all its operating and maintenance costs. Moreover, such a four-quadrant power converter would facilitate the reverse operation of the shaft generator as a shaft motor, which either assists the propulsion engine as a boosting device or acts as an emergency propulsion engine. This latter scheme can be a readily available solution to be adopted, when the concept of “safe sail” in case of severe damage of the main engine eventually becomes prerequisite for all ships. Renewable Energy Sources and Ship Power Systems The trend towards using renewable and alternative energy sources on land has gathered momentum over the last decade or so as the general public and policy makers tackle the issues of pollution, energy security and climate change. However at sea, the shift towards the widespread adoption of alternative energy is only now beginning to take shape. Over recent years the shipping industry has begun to seriously look at ways to reduce fossil fuel consumption and operate in a more environmentally friendly way. The concept of Green Shipping or Sustainable Shipping is now becoming an important issue for ship owners, shipping lines and ship builders globally In [7], considerations are held about the specifications in which Photovoltaic (PV) plants have to fulfil so that they can be installed on marine vessels. It is concluded that PV system must be tolerant to the special marine environmental conditions and especially the wind, humidity, shading, corrosion problems, and limited installation areas. The resulting restrictions are the parameters that define not only the type of the solar cells but also the applied PV system technology which refers to the interconnections types between panels and converters. A better approach would be to design a system that could use wind and sun together as a source of energy and propulsion (along with the ship's main engines) for reduced harmful emissions and
lower fuel consumption. This will make the ships better for business and better for the environment. Of special interest are also the fuel cells and hydrogen-based systems. Thus, cost efficient solutions with low fuel consumption and low exhaust emissions but also sufficient high level of system safety and availability, is a dominating activity. The research challenge is to develop analytical means that allow combined system performance evaluation at steady state and at transient operation. EMC-Power Quality and Ship Power Systems Power Quality (PQ) or Electromagnetic Compatibility (EMC) is a term referring to a wide variety of disturbances in electric networks either ship or continental ones. Thus, PQ/EMC problems refer to deviations of electric quantities from nominal values and wave-shapes, which can cause several malfunctions, interruption or even damages of equipment and systems. Considering the advent of All-Electric Ship (AES), where all equipment, including main propulsion systems, will be completely electrified, PQ/EMC problems will be of primary importance for the safe and normal operation of any waterborne vessel. PQ/EMC phenomena in land and ship power systems mainly include harmonics, short duration voltage events or momentary voltage deviations (spikes, overvoltages or swells, sags or dips), voltage and steady-state or momentary frequency deviations. A coherent description and classification of the PQ/EMC phenomena (causes, consequences, characteristic parameters etc.) occurred onboard highlighting their significance in the successful accomplishment of vessels’ missions is presented in [8], [9]. “Poor” power quality is mainly influenced by the propeller, the thrusters and the converters. Additionally, voltage quality problems are due to pulsed loads, which impose significant stress on the power system, and leakage capacitive currents, which is a distinctive characteristic of ships with particular effect on personnel safety, short-circuit faults, harmonic distortion, transients and EMC [8], [9]. As PQ/EMC phenomena in ship power systems present many similarities with those on land power systems, solutions will also have similarities and adapt to the nature of PQ/EMC phenomena in ships. Thus, Uninterruptible Power Supply (UPS), active and passive filters and other power conditioning devices may be used also in ship power systems [10], [11]. There are many national and international requirements and standards on the field of PQ/EMC for land and ship power systems. IEC 61000, EN50160 and IEEE standards are the main international standards and requirements on this field. On the other hand, the main requirements on PQ/EMC ship power systems include STANAG 1008 [12], IEC 60533 [13] and IEC 60092-101 [14]. In Table 1, the convergence between Standards IEC 60092-101, STANAG 1008, EN50160 and IEC 61000-2-4 is investigated. Table 1 PQ/EMC requirements applied for ship and land power systems
Ships IEC 60092-101 Steady-state voltage deviations Momentary voltage deviation Voltage transients recovery time Voltage unbalance Harmonics THD Steady-state frequency deviations Momentary frequency deviation Frequency transients recovery time
Land power systems EN 50160 IEC 61000-2-4
STANAG 1008
+6%, -10%
±5%
±10%
±10%
±20%
±16%
-
-
1,5 s
2s
-
-
3%
2%
3%
3%
5%
5%
±5%
±3%
±10%
±4%
5s
2s
2% 2% Even harmonics up to 1% Odd harmonics up to 6% 8% 8% Autonomous systems: ±4% ±2% for 95% of a week ±15% for 100% of a week -
-
Energy Efficiency and Ship Power Systems Energy efficiency is a crucial issue of the ship operation. According to the policy of the International Maritime Organization-IMO, the ship efficiency is to be quantitatively evaluated via two indices Energy Efficiency Design Index (EEDI) and Energy Efficiency Design Index (EEOI). Both of them express the produced CO2 per ship capacity and/or transport work, with EEDI-index addressing new buildings, while EEOI-index existing ones. The production of CO2 is mainly due to the operation of the main propulsion engine(s) and the auxiliary engines used as prime movers to the electric generators. Several measures are under study in order the efficiency indices EEDI and EEOI are improved. Some of them are [6]. Cold ironing. It is often used to describe a new generation of high voltage shore connections with fast plugs and seamless load transfer without blackouts, which allow the full range of inport activities to continue. It is most effective when applied to ships making frequent calls to the same port. Several studies have shown that, irrespective of the kind of electricity production used, there is an overall benefit in using shore-side electricity Shaft generator systems. In those systems, electricity is produced using fuels less expensive than diesel fuel, i.e. heavy fuel oil or natural gas, as the shaft generator system can be coupled to the main engine. The noise level of shaft generators is proven to be fairly lower than those of conventional gen-sets. In voyages of long distance and significant cargo, the EEOI values corresponding to operation with shaft generators can be more appealing than the ones corresponding to auxiliary generators. Currently used shaft generator schemes have certain disadvantages, as they comprise complicated configurations in order to achieve an identical performance to that of auxiliary generators. Thus, in most cases, a power electronic converter is connected downstream the generator, in order to match the output frequency of the generator with that of the rest of the electric grid. A companion synchronous motor is also connected, in order to provide the reactive power of the shaft generator system. Electric Energy Saving Devices (EESD’s). They refer to either Electric Energy Storage Units or Power Quality Enhancement devices. Their difference consists in the amount of energy they can provide (i.e. their capacity in power and time interval they can support). They can be used to alleviate peak energy demands causing voltage and frequency fluctuations. However, until very recently they have not been extensively applied aboard with the exception of the batteries as emergency supply units. Advanced Power Management Systems (PMS). They have been installed in ships for a long time, offering to the ship operator numerous advantages. Existing ships, especially those built in the last decade, have sufficiently good PMS’s. However, certain aspects dealing with energy savings need further investigation in order to be possibly incorporated in the Ship Energy Efficiency Management Plans suggested by International Maritime Organization. Summary The current document attempts to adapt concepts and advances from modern land power systems to ship power systems. The proper way for this adaption is examined and recommendations on the applicable concepts and techniques from one to the other power system are given. Benefits, standards and indices applicable to ship power systems as regards power quality and energy efficiency are provided. Experience gained in continental systems regarding smart grids, renewable energy technologies, power quality and energy efficiency issues, is valuable for naval systems and it can be exploited by the corresponding standards as long as a common framework is established. By taking advantage of the developments and the research in those areas, naval standards can provide guidelines for design and operation that will ensure higher levels of energy efficiency and power quality in ships, which become more important due to the eventual complete electrification of all systems onboard.
Acknowledgment This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: THALES: Reinforcement of the interdisciplinary and/or inter-institutional research and innovation References [1]
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