Introduction to Aircraft Design and Aviation Systems (ENG3005)

Transcription

1 Chapter 3 Emergency power generation 1. Power distribution 1.1 The power source for many civil aircraft: a.main aircraft generator : through a Generator Control Breaker (GCB). Controlled by Generator Control Unit (GCU) b.auxiliary Power Unit (APU): Through APU-GCB. Controlled by BPCU. c. Ground Power: Through External Power Contractor (EPC). Controlled by Ground Power Monitor. d. Backup Converter: Through Converter Control Breaker (CCB). Controlled by supplied by a VSCF. e. Ram Air Turbine (RAT): An air turbine use air stream to deliver emergency hydraulic and electric power. f. Battery: To assist in DC system power transient. Provide power in startup mode when no other power sources. Common types are Nickel-cadmium (Ni-Cd) and Liion batteries 1.2 Power system Fig 1: Electrical Power System 1

2 2. Power usage Motor and actuation: Engine control actuation, fuel valve control, start motors, fuel pump, fans and ventilation. Lighting : Navigation, landing, taxi, Emergency evacuation, cockpit, passengers cabin, Bay, etc Heating : Anti/deicing, windscreen, cabin air-conditioning Subsystem controller: Supplied by 15V, 12V, 5V and has its power supplier. Each unit is integrated with a power converter to provide the voltage needed from 28V or 270V DC. Avionics system: Most of the units are using building block or Line replaceable units (LRUs) that facilities the replacement. Ground power : It is 115V AC 400Hz. Protected by EPC. Ensure essential function 3. Backup power generation a) Ram Air turbine (RAT): Usually 5kVA to 15kVA depends on the aircraft. Provide power to go the nearest air field. Released into air-stream through a hatch in the aircraft fuselage Only operated in emergency and costly maintenance b. Backup converters: Using AC-DC-AC principle. The same principle is the VSCF. Obtain power from backup VF generator and to provide electrical variable frequency to VSCF. Give constant frequency and voltage output c. Permanent magnet generators Provide critical power such as Full Authority Digital Engine Control (FADAC) Small power such as several hundreds of Watts. 2

3 3.1 APU Fig 2: Backup generators A compact, self-contained unit to provide electrical power and compressed air (for main engine starting) during periods of aircraft ground activity or in-flight if needed Consists of a small gas turbine engine with engine controls, mountings and enclosures Installed in the tail cone of most aircrafts and isolated from flight critical structure A battery, which is charged by the aircraft electrical system, is provided for APU starting Depending upon the aircraft, the APU could drive one or two generators which could supply all or most of the electrical load needed by the aircraft Before the power is connected to the electrical systems, auxiliary generator control unit (AGCU) monitors the power to ensure that it is at the proper voltage and frequency. No Thrust Gas Turbine Engine Constant Speed Generator Three Phase 400Hz 115V L-N No Electrical Conversion Needed 3

4 3.2 RAT Fig 1. Example of APU Fig 5 RAT (United Technologies) Quite an old technology, but still in use as a stand-by generator for emergence The drive consists of a two-bladed fan or air turbine which connects to a generator. Constant Speed Generator Three Phase 400 Hz 115 L-N No Electrical Conversion Needed The RAT is at the heart of an aircraft s emergency power system. In extremely rare instances when airplanes lose power, the RAT deploys from the airplane's wing or fuselage and rotates to extract sufficient power from the airstream to control and land the aircraft. 4

5 4. Rectifier Using Transformer Rectifier Unit (TRU), the 115VAC is rectified into DC voltage. It can be used to provide 270VDC or 28VDC. The characteristic is: Any power level available: typical 1-200kW Efficiency 95-98% 12 pulse < 13% THD Conversion : 115Vac to 270VDC, 115VAC to 28VDC, 230VAC to 540VDC, 230VAC to 270VDC. The typical voltage conversion for 6-pulse TRU is: 3 6N V 2 o = V in cosα πn1 where N 2 : N 1 is the transformer turns ratio (Secondary to Primary). V in is the input phase voltage, α is the thyristor firing angle. Fig 7: 6-pulse TRU 5

6 5. Fire Detection System 5.1 Detection Fire and ignition detection is automatic Fig 6: 12-pulse TRU using series connection Thermal sensors include overheat and rate-of-temperature rise are used in cargo holds, engine APU, toilet, toilet waste bins, high temperature bleed air leaks and landing gear bays. 6

7 Fume detector: Accurate communication from cabin crew to flight crew about detected fumes in the passenger cabin The origin of fumes may be local to the source Faulty electrical equipment, the overheated contents of a galley oven, or a failed cabin florescent light fitting; or it may be spread away from the source through the air conditioning system whether the fumes could be in any way related to a fire hazard aircrew must also decide if they may be hazardous to crew or passengers Last type is just the Observation by crewmembers 5.2 Type of Fire: Engine Fire. Cabin Fire. Hidden Fire An engine fire is normally detected and contained satisfactorily by the aircraft fire detection and suppression systems. Fire is such that onboard systems may not be able to contain the fire and it may spread to the wing and/or fuselage. there is still a risk that the fire may reignite The crew to land the aircraft as soon as possible carry out a visual examination of the engine. A fire within the cabin will usually be detected early and be contained by the crew using onboard fire fighting equipment. Advisable to land the aircraft as soon as possible and carry out a detailed examination of the cause of the fire and any damage. A hidden fire may be detected by onboard fire detection systems or by the crew or passengers noticing smoke or fumes, a hot spot on a wall or floor, or by unusual electrical malfunctions particularly when the systems are unrelated. Dangerous type of fire for 2 reasons: Hidden fires are difficult to locate and confirm Difficult to access in order to fight them. Delay will do considerable damage to the aircraft. 7

8 A hidden fire is difficult to confirm and the crew may be slow to initiate an emergency landing. The consequence of such a delay may delay opportunity to land. 5.3 Aircraft Fire Extinguishing Systems Four types of fire extinguishing installations are found on commercial transport aircraft. Portable Extinguishers installed at specified locations in both the main cabin and the flight deck Hold fire extinguishing systems (with automatic detection) Engine fire bottle extinguishing systems (with automatic detection) Toilet waste bin bottle extinguishing systems Because fire is one of the most dangerous threats to an aircraft, the potential fire zones of modern multiengine aircraft are protected by a fixed fire protection system. The term fixed describes a permanently installed system in contrast to any type of portable fire extinguishing equipment, such as a hand-held fire extinguisher. Typical zones on aircraft that have a fixed fire detection and/or fire extinguisher system are: 1. Engines and auxiliary power unit (APU) 2. Cargo and baggage compartments 3. Lavatories on transport aircraft 4. Electronic bays 5. Wheel wells 6. Bleed air ducts 5.4 Examples of sensor Fig 7: The thermal coupler circuit 8