The aircraft power transmission and distribution system realizes the transmission, distribution, control and protection of electrical energy to electrical equipment. The power transmission and distribution system is referred to as the power distribution system, also known as the aircraft power grid, which is composed of wires or cables, power distribution devices, protection devices and testing instruments.
1. The composition of the aircraft power distribution system
The aircraft power distribution system can be divided into two parts: the power supply network and the distribution network. The power supply network is the part from the aircraft power supply, the power bus bar to the electrical equipment bus bar. The distribution network is the part from the electrical equipment bus bar to the electrical equipment. The electrical equipment bus bar is also called the load bus bar.
Aircraft conductors consist of a core and an outer insulating layer. The core is twisted by multiple strands of fine copper wires. In order to improve the strength, the thinner wires are also twisted with multiple strands of fine copper alloy wires. The copper wire is coated with a protective layer of tin, silver or nickel. Outer insulation material determines the type of wire. For example, the operating temperature of FVN type PVC insulated nylon sheathed wire is -60~80℃, and the operating temperature of AF-250 fluoroplastic insulated wire and TFBL-2 polytetrafluoroethylene insulated wire is -60~250℃. On some aircrafts with large power consumption, aluminum platoons are used as the main line of the power supply network to reduce weight. A single wire on an aircraft must have a protective sleeve, and multiple wires are often wrapped into wire bundles to improve electrical and mechanical strength and to be easy to install.
There are three types of power distribution devices according to their functions. The first type is used to directly make, break or switch circuits, such as buttons, switches, transfer switches, etc. The second category is used for making, breaking and switching circuits over long distances, such as relays and contactors. The third type is the end-point or cam switch, which is widely used in aircraft control mechanisms.
Terminal blocks, plug sockets, etc. are connecting elements between wires or cables to facilitate installation, removal and inspection of circuits.
Power grid protection devices include fuses and automatic switches (circuit breakers), etc., which are used to protect the power grid, prevent faults from expanding and eliminate faults.
There are also filtering and shielding, bonding circuits, etc. in the aircraft power distribution system.
2. Wire system of aircraft power distribution system
In the case of using metal materials as the structure of the aircraft body, the DC power grid of the aircraft can be used in a single-wire system, using the body as a negative loop. The advantages of single-wire system are: only the positive wire is used, and the weight of the power grid is light; the negative wire is a metal body, and the voltage loss is small; the number of connections, the size of the wire and the switch are reduced, the installation, use and maintenance are convenient, and the electrostatic induction between the wire and the metal body is eliminated. Its disadvantage is that it is prone to short-circuit to ground.
For some airframes using composite materials, only two-wire system can be used because the airframe is not conductive. The short circuit to ground (body) of the conductors in the two-wire grid will not cause excessive short-circuit current and grid short-circuit faults. Electromagnetic interference can be reduced if the positive and negative wires are wired together, or if twisted pairs are used. On some unmanned aircraft, in order to improve the reliability and vitality of the electrical system, a two-wire grid is used.
The three-phase four-wire structure of the AC power grid generally takes the body as the neutral line.
Use a two-wire system in a High voltage DC power supply system.
3. Requirements for aircraft power distribution systems
The distribution of the aircraft power distribution system depends on the distribution position of the aircraft electrical equipment, which is almost distributed in the whole body of the aircraft. As a result, aircraft power distribution systems are complex and prone to failure. In order to ensure safe flight and complete flight tasks, the following requirements are put forward:
(1) The aircraft power distribution system must have high reliability and strong vitality. It is required to ensure uninterrupted power supply of electrical equipment under normal and various fault conditions, especially to ensure continuous power supply of equipment for safe return to flight. When a short-circuit fault occurs, there should be the ability to eliminate the fault, or limit the scope of the fault, avoid the expansion of the fault, and prevent accidents such as fires.
(2) Ensure the quality of electric energy at the end of the electrical equipment. The power quality directly affects the performance of the electrical equipment and the accuracy of the instrument. It is not only necessary to ensure the quality of power supply during normal power supply, but also to ensure that there is still a high quality of power supply under various fault conditions.
(3) The weight of the power grid should be light. For low-voltage DC power grids, the voltage is low, the current is large, and the wires are thick, so it is necessary to reduce the weight of the power grid.
(4) Easy installation, inspection, maintenance and modification.
(5) Reduce electromagnetic interference to electronic and communication equipment. The metal body should have low resistance, and there should be filtering and shielding facilities in the power grid.