The capacity of the aircraft power supply system refers to its main power supply capacity, which is equal to the product of the number of main generators on the aircraft (the number of power generation channels for variable-speed constant-frequency power supplies) and the rated capacity of the motor (or power supply). The unit of DC power capacity is kilowatts (kW), and the unit of AC power capacity is kilovolt-ampere (kV·A). The rated capacity of the power generation system refers to the maximum output capacity when the power quality meets the technical requirements and long-term continuous operation under certain environmental conditions.
The actual capacity of a generator or power generation channel is not necessarily equal to its rated capacity. For example, when a generator cooled by external air flow is on the ground, it can only be cooled by its own fan, and the amount of cooling air is much smaller than the required value, and the capacity of the generator is correspondingly reduced. Since the effect of ventilation cooling is related to the intake air temperature and mass flow, it is related to the altitude and speed of the aircraft. As the flight height increases, the intake air temperature decreases, but the atmospheric density also decreases, and the heat dissipation effect of the motor becomes worse. During supersonic flight, the temperature of the cooling air entering the motor increases rapidly, which also reduces the generator capacity. The capacity of the aircraft DC generator in variable speed operation is also limited by the speed, the low speed operation is limited by the overheating of the motor excitation winding, and the high speed operation is limited by the motor commutation. The AC and DC power supply capacity correction curves stipulated by the national military standard GJB-860A-2006 indicate the relationship between the actual capacity of the motor, the cooling air temperature and the flight altitude.
When multiple generators operate in parallel, the capacity reduction caused by the unbalanced load distribution in parallel also needs to be involved. The capacity of the power system is not only related to the generator capacity, but also to the feeder capacity between the motor and the power bus. Usually the feeder should be able to withstand the full capacity output of the power supply.
Aviation power supplies have short-term overload requirements. For example, an alternator should be able to work at 150% of rated load for 2min and at 200% of rated load for 5s. The DC generator is generally required to work under 125%~150% rated load for 2min and under 150%~200% rated load for 30s. The overload requirement is given after the generator reaches the rated temperature under full load, so the motor temperature will exceed the safe limit when overloaded, but the motor will not be damaged due to the short duration of the overload. For DC generators that work in a large speed range, it is not required to be able to operate with overload in the entire operating speed range.
The overload requirement of 5s is to meet the requirements of the large starting current of the motor and eliminate the short-circuit fault of the wires in the distribution network. Modern aircraft power grid protectors are mostly thermal protectors, which remove short-circuit faults by means of overcurrent thermal tripping. Therefore, the generator of the constant speed and constant frequency power supply system also has the requirement that the three-phase steady-state short-circuit current is not less than three times the rated current, and the short-circuit current of the variable frequency alternator should not be less than 250% of the rated value. At the same time, it is stipulated that the constant speed and constant frequency power supply should reach the steady-state value of the short-circuit current within 70ms, so that the thermal protector can act quickly. The DC generator usually does not specify the short-circuit current value, because the DC generator is often connected in parallel with the battery, and the battery can provide a large current in the event of a short circuit.
The solid-state power controller is a new type of switching protection device composed of semiconductor power devices and detection control elements. It performs overcurrent and short-circuit protection by directly detecting current, so it has fast response characteristics and can achieve short-circuit protection within μs level. The application of solid-state power controllers has changed the short-circuit requirements for power systems.