Views:58 Author:Lionel Publish Time: 2018-12-28 Origin:Site
1. Ship Electric power system composition
The ship electric power system is mainly composed of power supply, power distribution device, power grid and load.
Power supply: A device that converts mechanical energy, chemical energy, and other energy into electrical energy. The power supply units commonly used on ships are diesel generator sets and batteries.
Power distribution device: A device that distributes, monitors, measures, protects, converts, and controls power and load. The power distribution device can be divided into main switchboard, emergency switchboard, distribution board, charge and discharge board
Grid: It is the general term for cable and wire for all ships. It is the intermediate link between the generator, the main switchboard, the distribution board and the load. It is the medium that transports the power of the power supply to the load end. According to the nature of the connected load, the power grid can be divided into: power grid, lighting grid, emergency grid, low-voltage grid, and weak grid.
Load: Ship load can be roughly divided into cabin machinery, deck machinery, ship lighting, navigation equipment and other point facilities.
2. Differences between ship electric power systems and land electric power systems
1) The capacity of the ship power station is small. Due to the small capacity of the ship power station, some large load capacity can be compared with the capacity of a single generator. When such a load is started, it will cause a great impact on the power grid, thus powering the ship. The stability of the system puts forward higher requirements.
2) Ship power grid transmission line is short Because of the limitation of ship volume, electrical equipment is relatively concentrated, the length of the power grid is not long and cables are used, so the protection of generators and power grids is simpler than that of onshore systems.
3) The working environment of marine electrical equipment is harsh. High ambient temperature, high vibration, high relative humidity, etc. can cause damage to electrical equipment, poor contact or malfunction.
It can be seen that marine electrical equipment must meet marine conditions.
3. Basic parameters of ship power system
Mainly the type of current, rated voltage, rated frequency
The current is divided into DC and AC; the electrical equipment of the AC ship has much less work in maintenance and maintenance than the DC ship, and the AC motor has a simple structure, small size, light weight, reliable operation, and the corresponding equipment is simple. The AC ship is divided into single-phase AC, three-phase three-wire insulation system, and three-phase four-wire system.
Three-phase three-wire insulation systems are most commonly used. This method is safe and reliable. There is no direct connection between the lighting grid and the power grid, and the mutual influence is small. When the grid is well insulated to the ground, the crew accidentally hits any line of the power grid and does not cause electric shock and casualties; When single-phase grounding occurs, a short circuit is not formed and the normal operation of the electrical equipment can be maintained.
Three-phase four-wire system, because it is not an insulation system, when the crew encounters any one of the power grid lines, it is prone to electric shock and casualties; when a single-phase ground fault occurs, a short circuit is formed, and a power jump attempt may occur, so the ship is more likely to Less adopted.
The rated voltage of the ship's power system directly affects the technical and economic indicators and personal safety issues such as the weight, size and price of all electrical equipment in the power system. The rated voltage of the power grid of the main power station of the ocean or sea vessel currently under construction or under construction is not 380V or 440V. The rated voltage of the lighting grid is not 220 or 110. The temporary emergency lighting grid and the weak grid generally use 24V.
The frequency is selected on the standard level of the land, 50HZ 60HZ. Increasing the frequency within a certain range can increase the speed of the automation system, reducing the weight and size of the motor, transformer, transducer, and automation components, but has the opposite effect on the cable and other components of the power system. The increase in frequency also requires matching high-speed mechanical devices and high-speed bearings, which also leads to an increase in AC impedance and an increase in losses. At the same time, the mechanical noise at high speeds is also large.
4. Ship network connection method
Generally, the voltage level of civil ships is below 500V, and the line connection methods mainly include feeder type (radial type) and trunk line type.
Each cable of the feeder-type junction cable is directly led out from the main distribution board, and is independently supplied to only one electrical device or one distribution panel. The advantage of this method is that it is convenient for centralized control. The failure of one branch feeder line only affects the power supply of one power supply device or the distribution power box powered by this branch, and other branches can still work normally. The disadvantage is that the main switchboard has a large size, the feed cable has a large demand, and the cost is high.
The trunk line has a main distribution board that leads to several trunk cables. All the power equipment is powered by a junction box connected in series on the trunk. The advantage is that the main switchboard is small in size, uses fewer cables, and has lower shipbuilding costs. The disadvantage is that when the mains feeder cable fails, all the electrical equipment powered by the mains has to be powered off, so the power supply reliability is poor.
The primary distribution network of maritime transport ships usually uses feeder-type power supply. The power grid in the secondary distribution network also uses feeder-type power supply. Most of the lighting grids use trunk-line power supply.
5. Grid protection
1) Overload protection
Since the civil ship power system is in the form of a single main power station, the ship power grid is mostly a radial feeder-type distribution network, and the feeder cross-section is integrated with the capacity of the generator and the electric equipment, so the overload protection of the power grid is generally not required. Special considerations and installation of special protective devices.
The power supply network between the generator and the main switchboard. Because this section of cable is selected according to the rated capacity of the generator, the cable overload is the generator overload, so this section of cable overload protection is responsible for the generator overload protection device.
The cables between the switchboards at all levels are less likely to be overloaded because their cross-sections are calculated by all the load currents on the distribution boards and considering the simultaneous operating factors, so the overload of the individual loads does not cause the cable to be caused. The overload, so this cable does not have an overload protection device.
The electrical equipment is connected to the cable between the main (sub) switchboards. The cross section of the cable is selected according to the rated capacity of the electrical equipment. The electrical equipment has overload protection devices. This device also protects the cable from overload. effect.
2) Short circuit protection
Since the short-circuit protection device of the generator and the electric equipment in the ship power system is as close as possible to the outlet end of the power supply side, the short-circuit protection of the power grid does not require the installation of a special protection device.
The most important issue of ship network short-circuit protection refers to the selectivity of the protection device, that is, when the fault occurs, the protection device only removes the faulty part of the circuit, and the front-level protection device should not operate, thus ensuring that other non-faulty equipment can continue. normal work.
6. Power distribution unit
Mainly divided into main switchboard, emergency switchboard, charge and discharge board, shore power board, distribution electric box.
The main switchboard is composed of a generator control panel, a load screen, a parallel screen, and a busbar (busbar).
7. Shore power supply
When the ship enters the factory and is in port inspection, or when some ships are docked or parked, they can use the power supply of the land to supply power, which is called “shore power”.
Considerations for shore power
1) When the shore power is connected, the shore current and the ship's current type are the same.
2) When the shore power is used, the rated frequency and rated voltage of the shore power should be consistent with the ship's power.
3) When the shore power is three-phase four-wire, the shore power needs to be The neutral line is connected to the terminal block of the hull on the shore power box. Only after the hull and the shore power neutral line are connected can the shore power be connected.
4) Close the switch on the shore power box, and only when the shore power phase sequence is consistent with the ship power phase sequence, the power can be forwarded to the front of the main switchboard.
5) When the ship is connected to the shore power, the ship generator is prohibited from being powered on. Only after the shore power is cut off can the generator be closed for power supply, and the same applies to the shore power switch when the ship is powered. It is worth noting here that for ships with three-phase three-wire insulation systems, once the three-phase four-wire shore power is switched, the ship power grid is not an insulation system at this time, so the grid insulation monitor should be turned off.
8. Monitoring and alarm functions
Fault alarm: Whether the various devices are normal or not, whether there are some related parameters are within the allowable upper and lower limits. In the event of a failure of most devices, the relevant parameters will not be automatically restored after the relevant parameters are exceeded. Only after the device is repaired can the parameters be restored. This type of fault is called a normal fault or a long-term fault. Some important equipments are often equipped with 2 sets, one set, one set, and automatic switching. When a fault occurs and an alarm is issued, the standby system is automatically switched, the fault disappears, and the running parameters return to normal in a short time. This type of fault is called a short-term fault.
Parameter display and alarm indication: Commonly used display instruments include pointer display instruments, digital display instruments and CRT displays.
Print records: Timed tabulation and summoning records
Blocking alarm: If the host stops, the related data is abnormal.
Extended alarm: unmanned cabin, transmitting alarms to public places
Dereliction of duty alert:
Duty Alarm: Room Transition Alarm for Handover
Functional experiment: test light button; function experiment button