Current Status and Development Trend of Electric Propulsion

Current Status and Development Trend of Electric Propulsion
  1. Current status of electric propulsion
    At 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 powers have also put forward research plans for electric propulsion technology. Electric Propulsion) research plan, etc.

The electric propulsion ships designed and built in China in the 20th century mainly used traditional DC propulsion technology. By the end of the 20th century, China also began to study AC electric propulsion ships with modern technology based on the integrated power system, and it was the first ship built in China. The AC electric propulsion ship is the “Palaus Pella” chemical tanker manufactured by Shanghai Edward Shipbuilding Co., Ltd. for the Swedish DONSOTANK company. It was put into operation in 2000; in December 2002, Guangzhou Shipyard International built a semi-submersible ship for COSCO Guangzhou Company. “Taiankou” was officially delivered. The ship uses an advanced pod-type AC electric propulsion system, and its sister ship “Kang Shengkou” has also been put into operation. The above-mentioned ships are designed by foreign companies and only built by domestic manufacturers. The first domestically designed AC electric propulsion ship with modern technology is a scientific research ship designed by Shanghai Ship Research and Design Institute and built by Jiangnan Heavy Industry. It was put into use in the South China Sea in 2005. The Shanghai Ship Research and Design Institute also designed for the Ministry of Railways. Two AC electric propelled ro-ro passenger ships “Yanda” ferry were built by Tianjin Xingang Shipyard, with a length of 182.6m, a width of 24.8m, a full-load displacement of 16299t, a service speed of 18kn, and an 8 class anti-wind and wave capability.

The integrated ship power system mainly includes power generation, power distribution, electric energy conversion, electric propulsion, monitoring and control, etc. Its structure is shown in Figure 1.

Current Status and Development Trend of Electric Propulsion
Figure 1 The composition diagram of the ship’s integrated power system

The electric propulsion system is mainly composed of propulsion motors, propulsion frequency converters, propulsion loads, monitoring and control systems, etc. The application status and development trends of these devices are as follows:

(1) Propulsion motor
According to the type of motor, propulsion motors are divided into DC propulsion motors, AC propulsion motors, permanent magnet propulsion motors and superconducting propulsion motors. In the electric propulsion of surface ships, AC propulsion motors and permanent magnet propulsion motors coexist. DC propulsion motors currently dominate in the electric propulsion of underwater vessels (including submarines).
1) DC propulsion motor
Because of its good speed, overload, starting and running performance, DC propulsion motors still dominate the electric propulsion systems of underwater ships (including submarines). In order to meet the needs of the development of the situation, under the background of adopting new technologies, new structures, new processes and new materials, the design of DC propulsion motors has been continuously improved, and its propulsion performance has been continuously improved.
2) AC propulsion motor
Before the frequency conversion technology was mature, due to the limitation of AC motor speed regulation performance, AC propulsion motors were mainly used for ships that did not require high mobility, or as an auxiliary propulsion for certain ships. For example, active energy motors used as auxiliary propulsion and AC propulsion motors used to drive pitch propellers.
With the development of power electronics technology, digital control technology, modern control theory, especially vector control technology and direct torque control technology, the speed regulation performance of AC propulsion motors can be comparable to DC propulsion motors. The application of AC propulsion motors in foreign commercial special surface ships has been very extensive, and the application of AC propulsion motors in domestic electric propulsion ships is also taking off.
As a power type AC motor, in principle, it can be used as an AC propulsion motor. However, there are mainly the following types in practical applications: wound rotor asynchronous motors, cage asynchronous motors, advanced induction motors and synchronous motors.
3) Permanent magnet propulsion motor
With the development of permanent magnet materials and AC speed regulation, permanent magnet propulsion motors are shown on the world ocean with their obvious advantages. The successful application of the permanent magnet propulsion motor on the German submarine 212 and the Russian submarine “Amor”, as well as the fruitful results of practical research in the United States, Britain, France and other countries, strongly prove that it is the preferred power unit in the near and mid-term. , Is the updated product of the ship’s DC propulsion motor. Permanent magnet propulsion motors can be divided into radial flux permanent magnet motors, axial flux permanent magnet motors and transverse flux permanent magnet motors according to the direction of the air gap flux. According to the waveform of the armature winding back EMF, it can be divided into a sine wave permanent magnet motor and a square wave (trapezoidal wave) permanent magnet motor.
The permanent magnet motor uses permanent magnet materials for excitation, and there is no excitation winding and excitation loss. Compared with general excitation motors, permanent magnet motors have the advantages of high efficiency and small size, and are especially suitable for ship propulsion motors.
The permanent magnet materials used in ship propulsion motors are mainly neodymium iron boron and banknote cobalt two rare earth permanent magnet materials. NdFeB has a high magnetic energy product and is cheap, but its temperature coefficient is relatively large and the Curie temperature is low. Banknote cobalt has a higher Curie temperature and a small temperature coefficient, but it is expensive and has a low magnetic energy product.
4) Superconducting propulsion motor
With the development of high-temperature superconducting materials, superconducting propulsion motors have gradually attracted the attention of countries all over the world, and they have become propulsion motors that can provide high-efficiency and high-power power in the medium and long term. Superconducting propulsion motors include superconducting unipolar motors, superconducting synchronous motors, superconducting heteropolar motors (ie, superconducting commutator-type DC motors), special superconducting motors, and so on. Among them, superconducting unipolar motors and superconducting synchronous motors have a certain research foundation and are relatively mature.
The DC superconducting power propulsion system uses superconducting unipolar motors. So far, more than 20 low-temperature superconducting unipolar motors of different powers and different types have been successfully developed in the world, 2200kW and 1000kW superconducting unipolar power generation The machine and the electric motor have already started the actual ship test of the propulsion system.

(2) Frequency conversion speed regulation device
Divided by the main circuit structure, the most common types of marine electric propulsion frequency conversion speed control devices are mainly four types: rectifiers or choppers, AC-AC cyclic inverters, AC-DC-AC current source inverters (also It is called synchronous frequency converter) and AC-DC-AC voltage source frequency converter (also called PWM frequency converter), which can drive different types of propulsion motors.
1) Rectifier and chopper
In the application of marine electric propulsion, the rectifier can directly drive the DC propulsion motor to realize the speed regulation function.
In ship electric propulsion, 12 or 24 pulse uncontrollable rectifiers are often used to reduce harmonics and provide high-quality DC power for AC propulsion inverter circuits. The output adopts 12-pulse uncontrollable rectifier with series and parallel structure as shown in Figure 2 and Figure 3.

Current Status and Development Trend of Electric Propulsion
Figure 2 Series output 12-pulse rectifier
Current Status and Development Trend of Electric Propulsion
Figure 3 Parallel output 12-pulse rectifier

The chopper is a direct current-direct current (DC/DC) conversion method, which includes three types of chopper-buck converter circuit, boost converter circuit and buck-boost converter circuit. Among them, the chopper step-down circuit speed regulation is the most common. This method can be used in battery-powered submarine or deep-submersible equipment, and the DC propulsion speed can be adjusted by changing the voltage by chopping.
2) AC-AC cyclic inverter
AC-AC cyclic frequency converter (also known as cycle frequency converter) is a kind of frequency converter that directly converts alternating current of one frequency into alternating current of another frequency without passing through an intermediate direct current link. The AC-AC converter is composed of thyristor rectifier circuits connected in a certain way. When the output of each rectifier circuit is controlled by a certain rule, the output of the inverter can be composed of a multi-phase rectified voltage envelope that meets the requirements. Frequency of alternating current.

Current Status and Development Trend of Electric Propulsion
Figure 4 Single-phase AC-AC inverter circuit and voltage and current waveforms

The single-phase output AC-AC inverter circuit and voltage and current waveforms are shown in Figure 4. It is essentially a set of three-phase bridge type non-circulating current anti-parallel reversible rectifier device, the thyristor working in the device is turned off by the natural commutation of the AC voltage of the power supply.
The advantage of the AC-AC converter is that it uses the power supply voltage to commutate and does not require a special converter circuit. It can use a large-capacity, relatively reliable and low-cost thyristor as a power device; there is no DC link, and only one conversion process. High energy efficiency and strong overload capacity; the current flowing through the motor is similar to a three-phase sine, with low additional loss and small pulsating torque. However, the shortcomings of the AC-AC variable-frequency speed control system are also obvious, mainly because its frequency conversion range is small, the maximum output frequency is only 1/3 of the input frequency, and the power factor is low, and the harmonic pollution is serious. In addition, due to the low power density of the AC-AC converter, the output harmonics are large, which will generate greater vibration and noise.
AC-AC circulating frequency converter is the first choice for low-speed operation occasions, especially icebreaking ships, but it is also used for low-speed and dynamic positioning ships and passenger ships that require high maneuverability.
3) AC-DC-AC current source inverter
AC-DC-AC current source inverters are also called synchronous inverters, sometimes also called load rectification and commutated inverters (LCI). The distinguishing feature of this type of inverter is that a large inductance is connected in series with the DC power supply to achieve the filtering effect. . Due to the current limiting effect of the large inductance, the DC current provided by the inverter has a flat waveform and small ripple, which has the characteristics of a current source. In addition, this makes the AC current output by the inverter a rectangular wave, regardless of the nature of the load, and the output AC voltage waveform and phase change with the load. For the electric propulsion speed control system, this large inductor is also an energy storage element that buffers the reactive power energy of the load.
The current source inverter needs to obtain a specific reverse induced voltage (EMF-electromotive force) from the motor to complete the commutation. At low speed, typically when it is lower than 5% to 10% of the rated speed, the EMF is too low to complete normal commutation, which will cause excessive motor torque pulsation. Therefore, the design of the propulsion system must be carefully considered. Moment pulsation and shaft vibration to reduce vibration and noise.
Synchronous frequency converters usually drive synchronous propulsion motors. Compared with three-phase synchronous motors, six-phase synchronous motors can reduce torque ripple. Figure 5 is the main circuit topology diagram of a current-type frequency converter driving a six-phase synchronous motor.

Current Status and Development Trend of Electric Propulsion
Figure 5 Topological diagram of the main circuit of a six-phase synchronous motor driven by a current-type frequency converter

4) AC-DC-AC voltage source inverter
AC-DC-AC voltage inverter is also called PWM inverter. Because the large-capacity filter capacitor C is connected in parallel on the DC side, the internal resistance of the DC side is small, and the output voltage
With voltage source characteristics. The large capacitor in parallel also clamps the AC output voltage of the inverter to a rectangular wave, which has nothing to do with the nature of the load. The waveform and phase of the AC output current are determined by the load power factor. In the variable frequency speed regulation system of the propulsion motor, the large capacitor is also an energy storage element to buffer the reactive power of the load.
The inductance of the DC loop acts as a current limiter, and the inductance is very small.
AC-DC-AC voltage inverter is the most commonly used variant in ship propulsion applications. It provides the most flexible, precise and high-performance drive, and it can drive asynchronous motors, synchronous motors and permanent magnets. Synchronous motor. In the high-power range, it can compete with other types of inverters. Now it can be used in high-power drive applications over 30MW.
The structure of the inverter is flexible and diverse, such as the traditional single-arm type, H-bridge type, multi-level type, etc. These inverters have their own characteristics and have been widely used in the field of ship propulsion.
5) Selection of variable speed regulating device
Taking into account factors such as propulsion power, maintainability and economy, the current mainstream of ship electric propulsion is to use alternating current

Current Status and Development Trend of Electric Propulsion
Figure 6 AC-DC-AC voltage prototype inverter topology diagram:(a) Three-phase single-arm voltage inverter, (b) H-bridge voltage inverter, (c) three-phase three-level voltage inverter

The motive is propelled, so the corresponding frequency converter is AC-AC cyclic frequency converter or AC-DC-AC frequency converter. Cyclic frequency converters are mainly used in occasions with extremely low speed and extremely high torque, such as icebreakers. In addition, the power density of this type of frequency converter is low, and the output harmonics are large, resulting in greater vibration and noise. Synchronous frequency converters and PWM frequency converters are the current mainstream frequency conversion speed regulation devices. Synchronous frequency converter is mainly used for frequency conversion speed control of synchronous motors. Its technological development has been very mature. Because it mainly uses thyristors as the main switching device, it has advantages in voltage, power level and economy, so many large-scale electric propulsion Civilian ships, such as oil tankers and passenger ships, all use frequency converters of this structure. Its main disadvantage is that the power density is not high, and there are also problems with noise and vibration.
Therefore, for ships with higher requirements for stealth and maneuverability, synchronous inverters may not be able to meet the needs. The main choice is the PWM inverter with better performance. It has more advantages than other types of inverters in terms of power density, output harmonics, and control performance. In addition, its control objects are also more flexible and can be asynchronous. Electric motors, synchronous motors or permanent magnet motors provide more options for the development of electric propulsion.

(3) Propeller
The types of thrusters include conventional shafting thrusters, Z-type thrusters and pod-type thrusters.
1) Conventional shafting electric propulsion
In conventional shafting electric propulsion, the propeller is directly driven by the propulsion motor to drive the ship to move through the shafting, which is basically the same as the transmission equipment of conventional mechanical propulsion.
2) Z-type electric propulsion device
The Z-type electric propulsion device is a full-slewing device driven by two pairs of bevel gears to drive the propeller. The propulsion motor is in the hull and is mainly used in tugboats and offshore work ships with speeds below 15kn. Due to the mechanical strength and the difficulty of bevel gear processing, when the device is used as the main propulsion, the maximum power of a single machine is generally less than 4000kW.
3) Pod type propulsion system
The pod propulsion system is a very advanced new electric propulsion system developed in recent years. Its main working principle is: install the high-power motor used to directly drive the propeller in a streamlined pod under the hull. The pod can rotate 360° horizontally with the vertical rotation axis to achieve the best flexibility.
The propulsion motor in the pod adopts an air-cooled low-speed synchronous motor. The low-speed motor is directly connected with the fixed-pitch propeller, eliminating the traditional transmission gear. The pod, the propulsion motor and the propeller are connected as a whole and placed in the water at the bottom of the ship. The principle structure is shown in Figure 7.

Current Status and Development Trend of Electric Propulsion
Figure 7 Pod propulsion system

Synchronous motors use synchronous inverters for speed control. For low power range or special requirements, PWM converter can be used for speed regulation. The propeller’s azimuth angle is controlled by an electric-hydraulic device, and 2~4 electro-hydraulic driving devices drive the steel gear to control the rotation of the propeller. The number of hydraulic motors depends on the requirements for maneuverability and redundancy. Adjusting the speed of the propulsion motor and the azimuth angle of the propeller can realize various sailing conditions such as forward, reverse, steering, speed regulation, and braking of the ship, so that the flexibility and maneuverability of the ship are in the best condition.
In addition to the characteristics of general electric propulsion, the pod-type propulsion system also has more significant advantages: ①High efficiency; ②Flexible control, good maneuverability; ③Quick reversing, short emergency braking distance, fast dynamic response; ④More noise Low, less vibration.

(4) Monitoring and control system
Electric propulsion monitoring and control systems mainly involve electric propulsion systems, actuator systems, sensor systems and control computer systems. The task of design and manufacturing is to integrate these subsystems purposefully in accordance with engineering standards and specifications to complete the monitoring and control functions that meet the requirements.
The control computer system is composed of hardware and software platforms, at least one controller or processor containing CPU and I/O, equipped with basic software and application software, the cabinet has one or several operating stations, and a series of interfaces as sensors Input, each controller I/O unit and network switch. The control computer system is designed in accordance with the principle of electric propulsion monitoring, and performs the calculations required by the monitoring and command equipment to realize a set of planned or system-specific functions. Other functions involve related security, system integrity, correct failure protection and operation, and safety strategies (such as active reconstruction) for system failures and limited adaptation to the external environment.

  1. The development trend of electric propulsion
    The flexibility, high reliability, high efficiency, and high power density of equipment must be the goals pursued by electric propulsion. With the continuous advancement of science and technology, marine electric propulsion technology will also continue to move forward.
    The United States, Britain, France and other countries are conducting research on key technologies for a new generation of integrated power systems. Among them, some new technologies such as the use of high-temperature superconducting motors for propulsion motors, the use of silicon carbide-based power devices for variable frequency speed regulation devices, and the use of DC power grids for transmission grids, etc., will greatly reduce the volume and weight of the new generation of integrated power systems and increase their application range. And application flexibility.