The propeller is commonly known as the vehicle blade, and its appearance is shown in Figure 2-1. The propeller is usually installed at the stern of the ship (but there are also some special ships equipped with propellers at the fore and aft, such as …
Current status of electric propulsionAt the end of the 20th century, countries all over the world are keen on researching ship electric propulsion technology. More than 80% of newly built ships use electric propulsion, and the proportion is getting higher and higher. In addition, shipbuilding …
Ship electric propulsion is not a new term. This term appeared in 1838 and has a history of more than 160 years. Ship electric propulsion develops with the continuous progress of the ship and shipping industry and the electrical and power electronic technology. In the early 20th century, AC and DC electric propulsion systems have been used in ships. Looking back at the development of electric propulsion, there are roughly the following stages:
- Test period
At the end of the 19th century, Germany and Russia first began to test the application of electric propulsion using batteries as energy. After that, the first generation of electric propulsion was put into use in 1920, and the effect was obvious on small passenger ships across the Atlantic. This period lasted from the birth of electric boats to the beginning of the 20th century. During this period, most of the electric propulsion was powered by batteries, and DC motors were used as propulsion motors, and the power was below 75kW.
- Wide application period
In the 1920s and 1930s, although the technology of high-power steam turbines as the prime mover of ships was mature, due to the lack of mechanical processing level and capability, high-power ships from civil freighters, passenger ships, oil tankers to aircraft carriers mostly used electricity. Advance. Electric propulsion has been widely used in the popular period. In addition to special engineering ships such as submarines and icebreakers, the United States alone has 226 frigates and 488 civilian ships that use electric propulsion. The “New Mexico” electric propulsion battleship built by the United States uses steam turbines to generate electricity, and the total shaft power propelled by asynchronous motors has reached 4000-22000k W.
- Make full use of electric propulsion for a long period of time
In the late 1940s, due to the advancement of machining technology, especially the improvement of the processing capacity of gear transmissions, steam turbines and diesel engines became large-scaled, and the mass production capacity was also improved. However, the electric propulsion at that time was due to technical conditions. Restrictions, the device is large, heavy, low efficiency, and high cost, which severely limits its wide application. Therefore, most surface ships are propelled by steam turbines, diesel engines, gas turbines and various combined power plants.
In the 1950s, electric propulsion was mainly an adjustable-speed “generator-motor” DC system, and the speed regulation was realized by the variable resistance of the motor’s excitation circuit.
In the 1960s, semiconductor technology can ensure that the excitation is controlled by the thyristor system, which promotes the development of electric propulsion systems. In the mid-1960s, AC electric propulsion with variable pitch appeared.
In the 1970s, electric propulsion was characterized by the use of high-current semiconductor components to transfer the current of the three-phase AC generator used for the work of the ship’s general power grid to the electric propulsion device. However, the ship’s DC propulsion motor has a commutator and an electric Brushes have many shortcomings in use, such as sparks during heavy load and reverse rotation, commutator wear, brush burnout, electromagnetic interference, and maintenance difficulties. As the frequency conversion technology is still new under the conditions at the time, the available AC propulsion devices cannot provide the necessary capacity. The AC commutator motor has the same shortcomings as the DC variable speed system.
- A period of vigorous development
After the 1980s, the AC propulsion system that adjusts the speed of the propulsion motor by changing the current frequency and voltage supplied to the motor replaced the DC propulsion system, and various propulsion schemes realized by inverters and frequency converters have been widely used. The use of more compact and lighter AC propulsion motors-synchronous motors and asynchronous motors can enable the system to obtain higher frequencies and greatly simplify equipment maintenance. The following two systems using modern AC converter technology have been widely used:
(1) A system with a direct frequency converter and a quiet synchronous propulsion motor, suitable for 1~40MW power use;
(2) A system with a frequency converter with an intermediate DC link and an asynchronous propulsion motor. The motor speed range is 800 ~1500r/min, and it has a deceleration transmission device connected to the propulsion shaft. This type of propulsion device is suitable for 7~8MW Power usage.
In the late 20th century, the continuous improvement of power electronic device manufacturing technology and continuous improvement of control technology have greatly promoted the application level of commercial ship electric propulsion technology, and greatly improved the effective power level of the electric propulsion system. Electric propulsion has appeared in the application field of civil ships. With the unprecedented development and prosperity, the application scope of electric power merchant ships is expanding day by day.
In addition, the ship’s propulsion structure ranges from a single engine such as gas turbine, diesel engine or nuclear power to a variety of prime movers; the power level ranges from 100 kilowatts to tens of megawatts; the propulsion mode is more diversified, such as uses Extensive pod-style propulsion. Due to the use of control technologies such as pulse width modulation and cyclic frequency conversion, the control of the propulsion motor in electric propulsion is more reliable, and the power quality of various equipment on the ship is guaranteed. The above-mentioned series of changes make electric propulsion the development trend of ship propulsion technology.
Electric propulsion is mostly used on ships with the following characteristics:(1) Ships requiring high maneuverability;(2) Ships with special nature of work are required;(3) Ships with large-capacity auxiliary machinery;(4) Military ships.The use of electric propulsion on some of the following ships has particularly outstanding advantages, and …
Compared with direct or geared propulsion, the main advantages of electric propulsion are: (1) Flexible layout and installation. The prime mover of large ships is almost without exception installed in the lower space of the stern, and at the same time, a longer drive shaft …
The ship’s electric propulsion system is generally composed of propellers, electric motors, generators, prime movers and control equipment. Its structure is shown in Figure 1.
Among them, the mechanical energy of the prime mover Y is transformed into electrical energy by the generator G, which is transmitted to the propulsion motor M, and the electrical energy is transformed into mechanical energy by the motor, which is transferred to the spiral diesel J to promote the movement of the ship. Due to the large power required by the propeller, the propulsion motor Y-primary mover, G-generator, M-propulsion motor, K-digging equipment, and J-propeller cannot be powered by the general daily power grid, and a separate generator must be installed. (Group) or more powerful power source, so the electric propulsion ship can set up two independent power stations, or a comprehensive power station.
The prime mover for electric propulsion can be diesel engine, steam turbine or gas turbine. At present, high-speed or medium-high-speed diesel engines are generally used, and steam turbines or gas turbines are used for high power.
The generator can be a DC separately excited generator, an AC synchronous generator or an AC rectifier generator.
The motor can be a DC separately excited motor, AC synchronous motor, asynchronous motor or permanent magnet motor. Ship propellers generally use fixed pitch propellers because of their high efficiency and small size.
The electric propulsion device uses a rotating electric motor to transfer power from the prime mover to the propeller. The generator driven by the prime mover supplies power to the propulsion motor directly or through a solid-state rectifier or frequency converter, and the propulsion motor is coupled to the propeller directly or through a reduction gear device. Due to the use of solid-state converters, the electric energy generated by the generator for the propulsion motor does not need to be of the same type, and can be alternating current or direct current. Therefore, the marine electric propulsion system can be classified according to the type of prime mover used, the type of main circuit current and the function of the device.
- Classified by prime mover type
(1) Diesel engine electric propulsion
Diesel engines are currently the most widely used prime mover in ship electric propulsion, especially for small and medium-sized ships. Diesel engines are more economical than steam turbines. In order to reduce weight and volume, electric propulsion generally uses high and medium speed diesel generator sets.
(2) Steam turbine electric propulsion
Generally suitable for high-power electric propulsion and ships that require a large amount of steam consumption. Steam turbines can use low-grade and cheap fuels to reduce the cost of ship operation. The disadvantage of steam turbines is that they require a steam boiler, which makes the power plant bulky and heavy.
(3) Gas turbine electric propulsion
The gas turbine has high power, small size, light weight, simple structure and fast starting, and is the prime mover that can be considered for electric propulsion.
(4) Electric propulsion of nuclear reactor installation
The thermal energy generated in the nuclear reactor is passed through the heat exchanger to heat the steam or inert gas, and then the steam turbine is used to generate electricity. As a ship’s electric propulsion, it can sail for a long time without fuel storage, so it is especially suitable for large and medium-sized ships such as icebreakers, submarines, and ocean-going ships.
(5) Fuel cell electric propulsion
A fuel cell is a chemical cell that directly or indirectly uses the free energy of fuel oxidation. It is different from a normal battery in that it can continuously generate electricity as long as it is continuously supplied with fuel. In addition, it is reliable, noise-free, and can be connected in series or parallel according to needs. These advantages make fuel cells have broad development prospects in the application of electric propulsion.
- Classified by current type
According to the type of main circuit current, it can be divided into DC, AC, AC and DC and DC system electric propulsion.
(1) DC electric propulsion
According to the principle of system regulation, it can be divided into constant voltage electric propulsion, simple G-M electric propulsion, constant power electric propulsion and constant current electric propulsion.
(2) AC electric propulsion
The propulsion motor adopts AC motors, including asynchronous motors, synchronous motors, and permanent magnet motors. The speed regulation methods mainly adopt AC-AC variable-frequency speed regulation, AC-DC-AC variable speed regulation, etc. At present, most ships use this type of propulsion.
(3) AC and DC electric propulsion
AC and DC systems use power electronics technology to combine AC power and DC motors into one system.
(4) Direct AC electric propulsion
The direct-ac system uses power electronic technology to combine the direct-current power supply and the alternating-current motor into one system.
- Classified by propulsion function
(1) Independent electric propulsion
The propeller is driven exclusively by the propulsion motor, the main engine
In addition to supplying power to the propulsion motor, the electric motor can supply part of the electric power to the ship’s power grid.
(2) Joint electric propulsion
Joint electric propulsion, this kind of propulsion is shown in the figure2 shown,
It can have four working conditions:
1) The propeller is driven by the propulsion motor (the main engine is separated from the propeller at this time) for low-speed sailing;
2) The propeller is directly driven by the main engine (primary mover) (at this time the propulsion motor is disconnected from the propeller);
3 The propeller is driven by the main engine (primary mover) and the propulsion motor for high-speed sailing;
4) When sailing, the propulsion motor is driven by the main shaft to generate electricity and feed the electric energy back to the grid (equivalent to a shaft generator).
(3) Special electric propulsion
Special electric propulsion includes side thrust electric propulsion, azimuth pod electric propulsion, superconducting electric propulsion, magnetic fluid electric propulsion and pump-jet electric propulsion.
(4) Integrated electric propulsion
Power system integration technology is used to realize the generation, transmission, transformation, distribution and utilization of ship electric energy to meet the needs of ship propulsion, daily loads, high-power pulse loads, etc. It integrates ship power generation and propulsion power, and shipboard equipment power use in a unified system, so as to achieve unified dispatch and centralized control of power generation, power distribution, power propulsion power and other equipment power use.