Vessels Electrical Distribution System - Term Paper Example

A Vessel’s Electrical Distribution System Introduction A vessel carries with it people, machines, equipments, and many other technology-related and tools for industrial, commercial and individual uses. Since it carries people and resources and conducts business in many places, it can be considered a virtual city. Problems and challenges when it comes to resources, in particular energy consumption, are almost similar with problems and challenges encountered by a city. A ship can be as large as a city in terms of population, resources, and needs. Ships need power or electricity to make them operational. There are ships which transport specifically oil or gas, and bring them to the different parts of the world. These are vessels that apply complicated operational systems as extra precaution against accidents and that also needs effective management system, even employing well-tested computer software for electricity distribution and communication. Great example of modern large vessels are cruise ships, commercial and industrial ships and tankers, military ships, pipe layers, and drill, which have complicated electrical distribution systems with multiple generators, distribution boards, and significant essential services located throughout the vessel. The type of distribution system needs a power management system which is responsible for the automatic start/stop control of the generators, loads, and all interconnections inside the distribution ring. A ring-main configuration provides at least two power sources for each essential component. Electrical distribution and control systems become problematic sometimes due to the growing number of consumers and distribution units. In this scenario, we must be able to provide detailed attention on the complex distribution and control systems and a focus on the special operation and reliability. Working on these systems requires a collaborative effort as this is a real challenge for engineers and technicians. The crew who will operate need proper training. The worldwide shortage of qualified technical personnel affects the marine industry and superimposes the human factor on the operation issues. Engineering activities during the design of large distribution systems include the use of new modern tools, software packages, and often dynamic simulation analysis, all of which can lead to an increasing number of errors. One particular problematic area is the application of converters, which are related with so-called harmonic distortion. Converters need filtering equipment, which are quite expensive. Multiple installation or converters with active front ends are customarily used to solve the problem of harmonic distortion. This project can be applied to large ships, but more specifically for the liquid natural gas carrier, ‘Al Khaznah.’ The details of this carrier are specified here. Al Khaznah was built sometime in 1993, with steam turbine as engine type; the electrical supply has a 440 volt 60 Hz capacity, and the total power generation is 28,700 kW or 28.7 MW. The generators it carries are: 2 turbo generators with output power of 2,700 kW, 440 V, 3, 60 Hz., and 0.8 PF; 1 stand-by diesel generator, with output power of 2,700 kW, 440 V, 3, 60 Hz., and 0.8 PF; and 1 emergency diesel generator, with output power of 560 kW, 440 V, 3, 60 Hz., and 0.8 PF. We have provided the electrical equipment, and what we do need to describe now are the switchboards. At least five switchboards are needed to make connections for the ship. The ship has two main switchboards, an additional two for cargo and another one to act as emergency switchboard. This emergency switchboard needs an emergency generator. Each switchboard is strategically situated in the ship, with each own switchboard room. In other words, they must be separated for security and to make them operational even in case of emergency. Fresh Perspective for Marine Electrical Distribution Direct current (DC) is an alternative to alternating current (AC). Electronic circuits are mostly operated and run by low-voltage DC, and this is applicable and ideal for equipments and machines in large ships. A direct current is also produced by an electronic circuit, with capacitors, resistors and transformers, to convert the alternating current. DC has many advantages compared with AC. One of the most significant advantages is the absence of inductance losses and harmonic problems. Certainly, DC systems would be preferred over AC systems if the advantages of AC devices such as transformers, three-phase induction, and synchronous machines were not present. AC electromagnetic machines do no face any real competition because of the poorer performance and higher maintenance of any such alternatives. AC electromagnetic machines are unlikely to be replaced with anything having compatible performance characteristics in the near future. The combination of electric generator with a diesel engine or a gas turbine is perhaps the most effective way to produce electrical power for the foreseeable future. DC generators no longer have a real place in large marine electrical systems, as they are heavy, expensive, and can provide only limited voltage, imposed by the limitations of commutators. The concept of using limited DC distribution on modern vessels has been around for several years. DC can be produced by using an AC generator with an uncontrolled rectifier to convert the AC to DC. A suggested configuration involves a standard AC generator with an external rectifier, and a better arrangement may be a two-windings machine where the windings are displaced 30 electrical degrees and connected to two uncontrolled rectifiers in parallel (or in series) to reduce the harmonic load on the generator and improve the output waveform. These can be called “DC generators” which should be connected to main DC distribution boards located throughout the vessel, which, in turn, would be interconnected with a two-wire distribution in a ring-main arrangement. A vessel can be divided into “distribution zones.” Each zone would comprise an inverter module/s, transformer, and conventional AC distribution units such as switchboards and distribution panels. The propulsion converters could be of a standard pulse width modulated (PWM) type and could connect to the DC distribution directly without front-end rectifiers. Connected to the DC distribution at several points are capacitor banks to provide reactive power for converters and inverters. An alternative may be a fixed capacitor bank with a current-limiting reactor for each inverter. This is shown in the diagram in figure 1. Figure 1. Generator, local distribution, and propulsion motor Generators Most of the large electrical distribution systems require a medium-voltage installation. The choice of medium voltage is determined by the level of fault current in the system. Having a low-voltage system of more than 10 MW of installed generation usually results in a system with a high fault level requiring special and expensive circuit breakers. We need to switch to a medium-voltage installation. But we can also provide a design in which the system operates with closed tie breakers when the power demand is low and automatically opens tie breakers when power demand is high to avoid operation when the short circuit current level would be highest. However, this type of design takes away many advantages of the single united system, such as reduced fuel consumption and less running equipment. Different medium-voltage standards exist, but current guidelines suggest large ships would use 6,600 volts for commercial vessels and 4,160 volts for military ships. (Rozine and Adams 90). A medium-voltage uncontrolled rectifier bridge is standard “off-the-shelf” equipment. If two bridges are connected in series, the use of standard single medium-voltage diodes without any sharing resistors is possible. The bridges are readily available for a 4 MW medium-voltage machine that could be used in a large electrical distribution system. Rectifiers have the characteristic to withstand a fault current and this should be taken into consideration during the design process. The rectifier is an inexpensive and simple component with the main disadvantage being that it will require some space onboard. The rectifier could be incorporated into the generator housing, using the existing cooling systems. Main distribution boards Main distribution boards are extremely simple because of two-wire distribution systems. It is believed that this type of switchboard will require less copper bus-work than the conventional type. The control system is uncomplicated as there is no need for any AC synchronizing equipment. This allows the system to have an unfussy manual control that is readily usable in emergency situations. Main distribution is straightforward and is the most advantageous component of the whole system. The electromagnetic emission problems associated with long AC power cables are eliminated automatically. The DC distribution can be implemented by either cables or bus bars. The need to support a specific ship’s magnetic field signature shall be taken into account with this type of distribution system. At the same time, the DC current does not produce any eddy currents in surrounding ferromagnetic materials, or penetrations, so single conductors can be used without the normal AC limitations. A grounded DC system would be suitable for this installation. One idea is to use sections of inexpensive and readily replaced fusible links, distributed throughout the system for connection of external cables. The links may be designed to act as fuses at the local point of a ground fault or for section isolation in the event of a fault. Grounded systems have advantages, but their main disadvantage is their low reliability. Any ground fault may lead to a power interruption, and additionally the ground fault close to a source of power may produce a high fault current in large power systems. Isolated large medium-voltage systems that do not have this disadvantage, on the other hand, are vulnerable because they tend to have severe over voltages during an intermittent ground fault as a result of distributed capacitance. The only real alternative to the grounded system is a high-resistance grounded system. As there are only two wires in a DC system, we would need two resistors to provide an effective grounding system. The DC system can use the grounded system due to specifics of the distribution system, and it is more reliable than the standard AC medium-voltage grounded system. This conclusion is based on the idea that there is less equipment directly connected to a DC medium-voltage distribution, and it is easier to protect only the one energized wire in the system. The Distribution Zones Each distribution zone should have an inverter module, transformer, and conventional AC distribution units for local services. The distribution zones are allocated throughout the vessel and can be interconnected using DC or AC links to provide a more reliable service. In case of blackout, there should be emergency generator, or several emergency generator sets that can easily supply all the essential loads in local distribution zones. Taking into account the local character of this distribution, the option of using a higher-frequency system such as 400 Hz is possible, allowing the use of lighter induction motors, smaller transformers, and longer life and better and lighter fluorescent lighting. Additional windings on the power transformers could provide a power supply for lighting and small consumers. The use of AC power in local distribution zones permits standard equipment and simplifies local operations. Separate and independent inverter modules for local distribution zones provide a high-quality clean power supply for all consumers with superior frequency stability. Capacitors Capacitors are devices which store electrical current. These are installed in different locations throughout the vessel and are connected to the main DC distribution to provide reactive power for converters and inverters. As these sources are distributed, it provides a reduction in current through the main DC distribution and generators’ windings. It also increases overall system efficiency as the reactive power is supplied from static sources as well as smoothing the output voltage from the generators. A capacitor bank is a standard item of electrical equipment and can be easily purchased and installed in a protective metal enclosure. A fixed capacitor bank with a current-limiting reactor could be a part of each inverter. This type of distribution system for an electrical supply in a ship has a significant advantage in terms of survivability. This is because of the simple DC medium-voltage, possibly grounded distribution system. The main distribution, as mentioned earlier, might be implemented in several ways, and one of them could be the very thin sheet of copper located, for instance, outboard. This location is often ideal as it does not take useful place inside the ship. However, if it is a traditional three-phase heavy cableway, it could be easily destroyed if battle damage occurs. It is hard to assess the suitability of thin sheets without detailed tests but it seems that it could be done, and if not then special DC fuses might be used. The thin sheet conductor can act as a fuse if damage occurs; the metal area around the damage point (ground fault) would vaporize leaving the surrounding areas and the whole system intact without any power supply interruption. Low- or High-frequency Trip The General Automatic Control System (GAC-21) is the appropriate system installed in the ship’s control system. It has the capability of “bus frequency monitoring,” which checks or sets the alarm once the frequency changes to lower or higher than the prescribed frequency. The system automatically alarms the Integrated Monitoring System (IMS). GACS has the following parameters. If trouble is high frequency, the rated value is 60 to 66 Hz, and the system rings an alarm; if trouble is low frequency, the rated value is 55 Hz or more, and the system sets the alarm with the automatic standby diesel to start; and when the trouble is “bus low-low frequency,” the rated value is 55 Hz or less, the automatic standby diesel generator starts, along with an alarm with preferential trip. Air Circuit Breaker or Gas-filled Circuit Breaker Air circuit breaker or SF6 gas circuit breakers interrupt current with a chamber that extinguishes an arc created during an over voltage event. SF6 circuit breakers can be enclosed in a pressure vessel, referred to as a dead-tank breaker, or open to the atmosphere, also called live-tank circuit breaker. Gas is used to insulate the surrounding system when the breaker opens to allow for separation of the contacts. When an arc is exhausted by SF6 gas pressure, the interruption is referred to as a puffer system. Circuit breakers have gone through an evolution from the advent of the first oil circuit breaker to today’s SF6 puffer style breaker. Just as new product technologies have evolved, so have maintenance techniques. Maintenance activities have gone from time based external and internal invasive inspections to full scale noninvasive maintenance including procedures such as SF6 leak detection, thermal imaging, radiography (x-ray), corona recording, etc. ACBs that are required in the switchboards are the “under voltage trip device” (UVT) and the “shunt trip device” (SHT), which provide grips on the interlocking function. Figure 2. Schematic of under voltage trip device, a function of ACB The UVT is DC-fed and this current should be supplied after ACB has tripped in order for the latter to reclose. The circuit demonstrates how the two ACBs in parallel are related. First, the coil of A has to be “excited” in order for the ACB to close. Figure 3. Photograph of Air Circuit Breaker ACB photograph above is from Siemens. ACB’s current capability, or amperage, ranges from 630A to 6,300A, and can be used for voltage capacity of up to 1,150V. These can provide for demands from large operation and monitoring with the use of electronic control systems and management. Fuses The fuse acts as circuit protection when there is overload or overcurrent and when short circuit occurs in the system. A fuse is not like a circuit breaker which has a mechanism that acts and reacts to what happens in the system or circuit. However, troubleshooting is no problem as the technician or engineer only has to change the fuse in case of malfunction. But the troubleshooter should be able to detect other malfunctions or trouble in the system, as the fuse would not be busted if there is no other trouble occurring in the system, which means that a busted fuse is only a symptom of another defect or fault. A fuse can be either a small glass cartridge for smaller electronic equipment or circuit, or a cartridge type, which is filled with sand for motors and lighting circuits. Overload Relay Figure 4. Photograph of overload relay A relay is an electronically controlled switch which operates a circuit through a low-power signal “with complete electrical isolation between control and controlled circuits” (Rajagopalan, Hayagrivan and Praveenkumar 62). Thermal overload relays were engineered to stop the power by opening the circuit when the motor gets too much current from the supply for an extended time causing increase of temperature. Under-voltage Trip System The UVT is another device for monitoring large equipments inside the vessel. The device is DC-fed and needs control from external source. Figure 5. Photography of an under-voltage trip device (UVT) As shown in figure 5, the under-voltage trip device is an electro-mechanical device, which has 12 essential parts: 1.) Undervoltage trip coil; 2.) Reset lever assembly; 3.) Trip plunger; 4.) Breaker trip coil; 5.) Trip shaft; 6.) Gag bolt; 7.) Reset mechanism; 8.) Trip hammer assembly; 9.) Trip plunger spring; 10.) Trip bar spring; 11.) Trip bar; 12.) Breaker hex shaft. Preferential Trip System A function of the Power Management System (PMS), the preferential trip is the process of protecting the generators by disconnecting the other loads in the system. The main point here is to prevent total power loss. The current must not exceed the preferred value. The Preferential Trip System protects several important equipments in the ship, like cargo pumps, ballast pumps, main air conditioning units, and the galley and laundry equipment. The tripping system disconnects all the important loads or connected circuits and equipments from the generator when an overload occurs. All circuit breakers used in ships are fitted on board to protect and connect the generators and the mains, and their function is surely to protect generators. How the preferential trip system works Once there is an overload in the system, the current in the coil increases, thereby producing a magnetic field and a temporary magnet. This attracts the frame, and the overload current pulls the armature and a trip occurs. A diagram is shown in figure 6. Figure 6. Schematic of preferential trip system Reverse-Power Relay Another device that monitors the generator is the “Reverse Power Relay,” which, as the name suggests, acts when there is reversal power in the electrical circuit. It reacts as soon as the generator takes the role of motor for 10 seconds caused by power abnormality. The relay functions and separates the generator from the “Bus Bar,” ensuring that the generator remains in a normal state before it has to be reconnected by another system, or manually. Generator Automatic Control System It functions as automatic control and for power management. GACS helps act on generator voltage, “Bus-Bar” voltage and frequency, standby diesel generator, automatic control of standby generator, and automatic control load sharing among the various generators in the system. GACS provides constant monitoring on all functions of the system, particularly the Bus-Bar. This guards the following settings and ensures there are no changes in fixed current and voltages: Trouble Rated value Operation caused Voltage Bus High 450 V up to 560 V Alarm only Voltage Bus Low 450 V to 330 V Automatic standby diesel generator start and alarm The generators can be controlled in three separate locations, such as: 1.) the engine side, where the operation might be manual start/stop or remote, and use for diesel generator and turbo generator; 2.) the main switchboards, where the diesel generator can be given manual start or stop; and 3.) GACP in engine control room, where the operation can be auto start/stop and manual start/stop; emergency trip operation for turbo generator, and power management system for both diesel and turbo generators. The General Auto Control Panel, which is situated in the engine control room, provides an array of several controls for the different generators and equipments in the ship’s system. This can be demonstrated in the following manner. For manual position, operations can be manual start/stop control on GACP, the Air Circuit Breaker, and manual synchroscope and load share/shift control. For auto position, the control system provides for stand-by diesel generator auto start control, auto sychroscope and load/share/shift control, and large motor start blocking. The Power Management System (PMS) takes control of all generators on line, large motors, preferential trip system, and fault recovery. Fault recovery and control Figure 7. Diagram for fault recovery and control When there is fault or malfunctioning on any part in the system that creates current abnormality, the engineer expects to have several other abnormal conditions in the turbine generator, voltages and frequencies, the generator load, and possible blackout within the vessel. The Switchboards The generators supply power on the two main switchboards (MSB), which are strategically placed at the second deck of the engine room. Heavy equips are connected directly to the MSBs, and these are the forced draft fans, ballast pumps, and others. Group start panels are connected and specifically placed at the back of each board. Start panel B1 is linked with MSB1, while group start panel B2 is directly linked with MSB2. The emergency switchboard is also directly linked with either MSB1 or MSB2. Figure 8. The vessel’s main switchboard Switchboard’s Description The vessel’s main switchboard has a deck head which is used for exhaust of the heat in the switchboard. It has a removable door that provides easy maintenance and inspection. The cabinets are properly ventilated comfortably built for the technicians and operators to work on. The “Bus Duct,” with a capacity of 6,000 amperes, connects the switchboards. This is doubled because there are two bus ducts connecting MSB1 AND MSB2. Fire is preventable with the application of sheet steel partitions between the adjacent frameworks. The switchboards are also fitted with connecting switches. This is to provide a double isolation in case emergency requires to disconnect a generator or feeder panel from the main bus bar. Safety Measures The vessel and the various systems involved have to be fitted with all necessary safety measures to avoid accidents and a subsequent loss of lives and property. A safety measure known as matting is provided on the five main switchboards for safety and so that the switchboards cannot come in contact with the earth. High-voltage rubber gloves are provided for ready use in emergency situations. The system also has emergency internal communication coming from the Telephone Exchange, which can be used in case of blackout. Safety directions are also provided in the different parts of the vessel to ensure that personnel and workers adhere to safety precautions. The safety signs include “CO2,” “live equipment,” “ear protection,” and other similar fire-warning notices that should enable workers to be extra careful. The ship is also provided with an emergency switchboard in case of fire, which should be located outside the engine space, per guidelines from international bodies. In this particular vessel, the emergency switchboard has been strategically located at the “A” deck starboard. Only the necessary emergency equipments can be connected temporarily with the emergency switchboard. Some equipments are prohibited from being connected to the emergency switchboard. The volts relay can act on the emergency generator to start and re-connect to the board automatically. Moreover, the emergency switchboard can supply power to the radio equipment, instrument and alarm system service transformer, UPS, general service battery switchboard, instrument and alarm system service battery switchboard, nos. 1 and 2 steering gears, radars, electro-hydraulic pumps, breathing apparatus for air compressor, elevator for personnel and goods, main pump for valve control, and the number one engine room supply fan. The ship also has a cargo shipboard, which takes its power form the main switchboard and is strategically situated at the Upper Deck. It supplies the cargo and spray pumps, low duty compressors, pipe-passage fans, and so forth. Uninterruptible Power Supply Another important equipment necessary for the vessel is the “uninterruptible power supply” (UPS). This is very important because it provides emergency power in case of sudden blackout. Computers and on-line equipments need UPS to prevent essential information from being lost and/or protect the equipment from sudden low current in the system. The UPS supplies power to the fire detection system, integrated navigation system, shipboard management system, emergency shutdown system, ballast console, loading computer, main turbine manoevering control box, boilers, gas detecting systems, fast alarm printers, fire alarms, and many other equipments in the ship. Figure 9. Picture of a large Uninterruptible Power Supply that powers several equipments simultaneously Voltage Requirements There are some equipments that need 24 volts. These are supplied from the system batteries known as the General Services Battery. The batteries are also constantly charged by the charging and regulating unit, which is situated in a special location. Radio Service Battery Radio equipments that need battery supply are coursed through the GMDSS equipments. Radios are classified as MF, HF, VHF radio systems, and dual Sat-C systems. Batteries for radio are specifically placed in the battery room on F-deck port. This also provides emergency supply for equipments in case of blackout. Lighting Needs All lighting needs of the vessel are supplied by the main or emergency switchboards, as required by SOLAS rules and regulations about lighting. Emergency lighting must also comply with the same rules. For example, lights for cabins, alleyways and the machinery space must have light to come from the emergency supply, or the emergency switchboard, and these are marked the letter “E”. Conclusion The vessel’s power needs are provided by diesel generators, which have to be in constant shape at all times. When there are malfunctions and faults, they are easily addressed to by the various functions in the system, either automatically or manually. A computer software and effective management system are significant in the attainment of the vessels objectives. All power supplies are connected and controlled in the distribution system. A fault or accident can cause a symptom in any part of the vessel, and these faults are detectable which makes the system effective. There are automatic checks and automatic start-ups/stoppages in the different generators, but each malfunction and fault is linked with an alarm system so that technicians and engineers can provide emergency measures. A diesel generator can automatically shut down once power consumption is down to 75 percent for ten minutes of the capacity of the control system. There are specific motors which need large current to start. In this situation, a control known as “large motor start blocking control” is provided to prevent a sudden drop the ship’s power demand. Other equipments cannot be driven to start with normal load conditions, like water spray pump, ballast pumps, cargo pumps, and high duty compressors. The large motor start blocking control will be responsible for their re-functioning into the system. Works Cited Rajagopalan, Badrinarayanan, Mugundhan Hayagrivan and Mahesh Praveenkumar. “Ferrofluid Actuated Thermal Overload Relay.” Smart Grid and Renewable Energy. 3 (2012): 62-66. ProQuest. Web. 17 Apr. 2015. Rozine, Vassili and Max Adams. “New Conceptual Approach to Large Marine Electrical Distributions Systems.” Marine technology and SNAME News. 45.2 (2008): 89-93. Web. ABI/INFORM Complete. 17 Apr. 2015. Siemens: Air Circuit Breakers 2015. Web. 18 Apr. 2015. . Read More