Variable frequency AC power supplies and hybrid power supplies

Inverter AC power supply

The aircraft variable frequency AC power system is another method that does not use a constant speed transmission device, that is, the engine directly drives the generator to generate variable frequency AC power for the load.

At present, the maximum installed capacity of the constant speed and constant frequency AC power supply system is 150kV·A. As a power supply to replace the constant-speed and constant-frequency AC power supply system, the variable-frequency AC power supply system has obvious application advantages in the high-power AC power supply system. The main advantages of the variable frequency power supply system compared with the constant speed and constant frequency power supply system are: the variable frequency generator is small in size, light in weight, high in reliability, high in efficiency, and easy to achieve AC starting/generating.

The goal of the variable frequency power supply system is to provide the same power quality as the current constant frequency system in the entire frequency conversion range and meet the requirements of standards such as MIL-STD-704E/F and DO-160C/DE, including all technical parameters (voltage transients, etc.) except the frequency range. China has also formulated the corresponding navigation mark “Aircraft Electrical System Characteristics” (HB7745-2004) according to ISO1540. The power quality of the variable frequency power supply should also meet the relevant standards of my country, and the reliability and airworthiness should also meet the relevant standards.

Therefore, the difference between the variable frequency power supply system and the constant frequency power supply system will only be that the operating frequency range of the variable frequency power supply system is wider. The purpose of this is to minimize the impact of the system on electrical equipment.

The maximum and minimum frequency limits for variable frequency power supplies are as follows:

(1) The minimum frequency is determined to be 360Hz, because this frequency includes the minimum frequency of the external power supply, and is also the minimum abnormal operating frequency specified in the current DO-160D electrical equipment requirements document. Therefore, the electrical equipment will not need to add too much electromagnet core material, and the maximum frequency is limited to 800Hz.

(2) There are two types of variable frequency power supply: ① narrow variable frequency power supply, the frequency range is 320~640Hz; ② wide variable frequency power supply, the frequency range is 360~800Hz, suitable for all current and foreseeable future use of turbofan engine aircraft.

Hybrid power

A power supply with two or more main power sources installed on an aircraft is called a hybrid power source. Many aircraft in the Soviet Union used low-voltage DC power supplies, but due to insufficient power supply, AC power supplies were installed to form a hybrid power supply. Low-voltage DC power supplies mostly use starter generators. There are two kinds of AC power supply, one is the variable frequency AC power supply, and the alternator is directly driven by the engine. Due to the large frequency variation range of the AC power, it is only suitable for loads that do not require frequency, such as lighting and heating. Some aircraft engines (such as turboprop engines) have a narrow operating speed range, generally only changing by about 5%, and the generator produces alternating current with a near constant frequency. Some aircraft engines are also equipped with generators driven by constant-speed transmissions to output constant-frequency alternating current. This type of hybrid power supply is relatively complex, with large volume and weight, and poor power quality. It is a transitional aircraft power supply.

Each aircraft engine of the new generation hybrid power supply drives a variable frequency brushless alternator, and then obtains 270V high-voltage direct current, 115/200V400Hz constant frequency alternating current, and even 28.5V low-voltage direct current through the power electronic converter. The unconverted part of the variable frequency alternating current can be used for lighting, heating and other equipment that does not require frequency. The Gulfstream business jet (G-3, G-4) put into operation in the 1980s adopts this scheme, its generator capacity is about 30kV·A, and it obtains 22.5kV·A 400Hz constant frequency alternating current and 28.5V low-voltage direct current (7.5kW) through the converter. The characteristics of the new generation of hybrid power supply are: (1) There are few moving parts of the power generation device, and the required installation space on the engine is small; (2) The power quality is high; (3) The efficiency is high and the loss is small; (4) The use and maintenance are simple.