Aircraft Supply System

Question: Design an aircraft power distribution system and explain the features of your system that help to ensure the continuing integrity and safety. Answer: The description of single line diagram: As shown in diagram, a design of twin engine busbar system for a aircraft which required less than 12KW power in each channel. The system is of two 28VDC. As shown in fig the two main DC busbars namely No.1 and No.2 are operating in parallel and getting power from two 28VDC generators. Non essential DC services are thereby directly connected to busbar No.1. For feeding the Non-essential AC consumers, the two DC busbars are individually connected with a inverters. The output derived from the inverters is 115VDC at 400Hz. There is one centre busbar which is connected directly to the two main DC busbars No.1 and No.2, which is further providing DC power for the essential consumers of aircraft. This busbar also provides a load of 115VAC, if inverter is powered off. When aircraft is on ground without the engine running, we can connect the centre busbar with the external power source by regulating the switch. At the middle of the diagram, there is battery bank of a aircraft which is connec ted to the battery busbar which in turn feeds the vital consumer services. The essential consumers can also be feed by this battery busbar by connecting it through the DC essential busbar, if required. Protection system To make system run efficiently and affordable, several protections are required. Thus for proper protection at several levels are provided by proper switching and protection devices. Thus over voltage protection relay is used in order to avoid over voltage condition and is part of the primary power generation protection system. This also consists of reverse current circuit breaker (RCCB) for controlling the reverse current. These together come under voltage regulator. Thus it regulates the output of the generator to the No.1/No.2 DC busbars by controlling the generator feed contactors. As we have discussed the centre busbar and essential consumers are connected to the main busbars, thus a circuit breaker or smart contactor can be used to provide protection. The use of circuit breakers can be done to protect individual loads or groups of loads fed from the supply or feeder busbars. The main principle for fault protection should be that, minimum of disruption occur on a electrical system. If in case faults occur, it should trip circuit breaker of that particular load but should not hamper the next level protection, in order to avoid cascade failure and ensure that this does not occur again. Components: Inverter- The main function of inverter is to convert single phase 28VDC into 115VDC.The inverter can also be used as alternative source in case of power failure. The inverter comprises of SCRs which through rapid switching synthesize the AC waveform from the DC input. Circuit Breakers- The purpose of circuit breaker is same as the current limiter or fuse. They break the contacts of circuit in case of overload and protect the circuit. They have a mechanical trip mechanism which is activated by a bi- metallic strip. The current carrying capacity of circuit breakers differ. Ram Air Turbine(RAT)- The use of RAT is required when most of the power generation system has failed or are unavailable. The RAT is an air driven turbine, normally stowed in the aircraft nose. The size of RAT generator varies from 5 to 15 KVA. Auxiliary power unit- as the name suggests this is used as a backup or auxiliary unit, in case of system failure or extra requirement. Parallel Operation- The main requirement in case of muti-engined aircraft that uninterrupted power supply is required, thus in parallel operation each engine dries its own generator. Due to this a equal proportion of electrical load is shared between them. This is also known as equalizing circuit, but for parallel operation the voltage regulators of two generators must be interlinked. The parallel operation is not possible in frequency-wild (FWG) or variable frequency (VF) generators. Variable frequency generation- This is the most reliable form of power generation. They dont have any effect on frequency sensitive aircraft loads. VF is being widely adopted in the business jet community as their power requirements are above 28VDC/12KW VSCF( Variable speed constant frequency)- in this the generated power of variable frequency is electronically converted by solid state power switching device into constant frequency 400Hz,115VAC power.