The aircraft power supply system has three working states: normal working state, abnormal working state and emergency working state. During the flight of the aircraft or other tasks, the state in which the power supply system performs the predetermined task without failure is the normal working state. At this time, the speed of the engine that drives the main generator may change, the generators may be connected in parallel or the bus bars may be switched, and the loading/unloading of loads may also occur. The abnormal working state of the power supply system is a short-term out-of-control state, which causes a large change in the voltage or frequency of the bus bar. For example, the distribution line or feeder is short-circuited to the body, resulting in a short-circuit protection state, and the system returns to normal operation after the protector trips. The emergency working state is a state in which the main power supply fails and cannot supply power and must be powered by the emergency power supply. In this state, the aircraft must return and land as soon as possible. The power quality of the normal power supply is high, and the power quality of the abnormal or emergency power supply is low.
In addition to the difference in steady-state voltage and frequency range between the abnormal working state and the emergency working state, the indicators such as transient voltage peak value and recovery time are also quite different. For example, the US military standard MIL-STD-704 stipulates that the steady-state voltage range of the AC power supply is 108~118V, the minimum and maximum transient voltages are 80V and 180V, and the voltage duration is less than 0.01s; when the power supply is abnormal, the steady-state voltage range is 100~125V, the maximum transient voltage is 180V and the duration is 50ms, and the minimum transient voltage is 0V and the duration is 7s. The standard RTCADO-160 of the American Radio Technical Committee on civil aircraft stipulates that the steady-state voltage range of the AC power supply in the normal state is 104~122V, the interruption time of the power supply is less than 200ms, the voltage surge is 160V (maximum) and 60V (minimum), and the duration is 30ms; the abnormal power supply voltage is 97~134V, the undervoltage is 60V (sustainable time is 7s), the overvoltage surge is 180V (sustainable time is 100ms), and 148V (sustainable time is 1s). In the abnormal working state, not only the power quality is significantly reduced, but also the phenomenon of interruption of power supply will occur.
On a single-engine aircraft with low-voltage DC power supply, there is often only one generator driven by the engine to form the main power supply. At this time, if the main power supply fails, it can only enter the emergency state powered by the emergency power supply (aviation battery). On a multi-engine aircraft, the number of primary power sources is generally the same as the number of engines. In this case, once a generator fails, it will not immediately switch to the emergency power supply state, but will switch to the main power system emergency procedure, the faulty power supply will be withdrawn from the grid, and the load will be powered by the remaining non-faulty power supply. If the normal working power supply capacity is sufficient, the aircraft can continue to perform the flight mission. If the remaining capacity is insufficient after one or two generators are withdrawn, some minor loads should be removed by the pilot or automatically, which is the monitoring of the load. The main reason for using the main power emergency procedures is that the main power source of modern aircraft is very reliable and safe. The emergency power supply is only supplied when all main and auxiliary power sources (many auxiliary power sources are also allowed to operate while the aircraft is in flight) fail. It must be pointed out that there is generally a period of power supply interruption when the main power supply running in non-parallel is converted, and the system enters an abnormal working state at this time.
The failure of the flight control system means that the aircraft is out of control, and redundancy technology must be used to ensure high reliability. Redundant technology requires redundant power. Usually, the flight control system adopts four redundant power sources, namely two sets of independent main power sources, one set of backup power sources, and one set of emergency power sources. If the emergency power source is a battery, it must be ensured that the battery is in a fully charged state under normal circumstances, so that it has sufficient capacity in an emergency.
Important electrical equipment usually adopts three redundant power supply mode, powered by two sets of main power supply and one set of backup power supply, and the electrical equipment can work selectively.
General-purpose electrical equipment is usually powered by mains, and when one mains fails, it can be switched to another.
During the normal operation of the power supply system, the general electrical equipment should have complete technical performance and ensure safety. During the interruption of power supply during bus bar switching, there is no requirement for the performance of the electrical equipment, but after the power supply system returns to normal, the characteristics of the electrical equipment should be fully restored.
During the abnormal operation of the power supply system, the performance of the electrical equipment is generally not required, and even loss of function is allowed, but the safety must be guaranteed, and all the characteristics of the electrical equipment will be restored after the power supply characteristics return to normal.
During emergency power supply, equipment that needs to perform tasks should be able to provide specified technical performance and ensure safety and reliability. After the power supply characteristics are restored, all characteristics shall be restored.
Some electrical equipment requires uninterrupted power supply. Usually, the aircraft DC power supply system is easy to achieve uninterrupted power supply, because multiple DC power supplies are easy to work in parallel, and when the power supply adopts reverse protection diodes to connect to the power bus bar, the failure of the power supply itself will not cause the power supply of the electrical equipment to be interrupted. The AC power uninterrupted power supply technology is more complicated.