Chapter 11 AC Power Generation Systems

航空发表于 2011-9-20 08:25:22

**** Hidden Message *****

航空发表于 2011-9-20 08:26:07

1 Chapter 11 AC Power Generation Systems 2 Chapter 11 AC Power Generation Systems • AC supply systems vary in complexity depending on aircraft type and electrical requirements. There are two categories of AC systems commonly used dependent on whether the output frequency of the generator is controlled or not. They are known as frequency wild and constant frequency systems and are fully described below. 3 11.1 Frequency-Wild AC System • 11.1.1 A Typical Frequency-Wild AC System Architecture • In this system, the AC generators are fitted directly to each engine, and unless the engines run at a constant speed, the output frequency varies (frequency-wild). 4 11.1 Frequency-Wild AC System (continue) • The output from each generator is normally 200 V three-phase and varies in frequency between 280 and 540 Hz, which corresponds respectively to tow and high engine rpm. 5 11.1 Frequency-Wild AC System (continue) • The generators in this system should not be run in parallel under any circumstance, so their AC output is normally used to feed heating elements only. This is because the elements are purely resistive and are unaffected by changes in frequency. 6 11.1 Frequency-Wild AC System (continue) • In some systems, part of the frequency-wild output is rectified in a transformer rectifier unit (TRU) and provides an alternative DC supply. The DC supplies may also be paralleled provided that the voltages are matched. 7 Figure 11-1 A Typical Frequency-Wild AC System 8 11.1.3 Fault Protection in a Typical Frequency-Wild AC System • The following fault protections exist in a twinengine turbo-propeller frequency-wild AC system: •  Overheat • If the generator overheats due to inadequate cooling or overload, a warning light illuminates on the flight deck, and the generator should be manually switched off. 9 •  Earth-Leakage • If there is low insulation in the alternator system or loads, a warning light illuminates. If this occurs, switch off the generator. 10 •  Under-Voltage • This fault normally uses the same warning light as that used to indicate an earth leakage fault. The system voltmeter is used to discriminate between an earth leakage fault and an under-voltage fault. 11 •  Over-Voltage • If an over voltage occurs, a sensing circuit automatically de-excites the generator and removes it from the busbar. One attempt is usually allowed to reset the system by cycling the control switch between RESET and RUN. 12 •  Differential Protection • This system is used to: • ● Monitor line-to-line faults • ● Monitor line-to-earth faults • ● Ensure that the output current flowing from the generator is the same as that flowing to the loads and returning to the generator. 13 Figure 11-2 Fault Protection in a Typical Frequency-Wild AC System 14 • If one of the above faults exists, the generator is automatically de-excited and is removed from the busbar. One reset may be attempted, but even if the system resets satisfactorily for the rest of the flight, the fault must still be reported on landing. 15 11.2 Constant Frequency Split Busbar AC System • The following electrical system is typically used on a twin-jet engine aircraft whose AC power supply is 200 V 400 Hz threephase. 16 11.2 Constant Frequency Split Busbar AC System (continue) 17 11.3 Constant Frequency Parallel AC System • Advantages: • ● Provides a continuity of electrical supply • ● Prolongs the generator life expectancy, since each generator is normally run on part load • ● Readily absorbs large transient loads 18 11.3 Constant Frequency Parallel AC System (continue) • Disadvantages: • ● Expensive protection circuitry is required since any single fault may propagate through the complete system. • ● Parallel operation does not meet the requirements for totally independent supplies. 19 11.3 Constant Frequency Parallel AC System (continue) • The following conditions must exist before paralleling can take place between two generators: • 1. Voltages must be within tolerance. • 2. Frequencies must be within tolerance. • 3. Phase displacement must be within tolerance. • 4. Phase rotation must be correct. 20 11.3 Constant Frequency Parallel AC System (continue) generator circuit breakers (GCB) split system breaker (SSB) 21 11.3.2 Reactive Load Shearing • Reactive load sharing is achieved by a load-sharing loop which automatically adjusts the excitation of the paralleled generator fields simultaneously via their individual voltage regulators. 22 11.3.2 Reactive Load Shearing (continue) 23 11.3.3 Real Load Shearing • Real load sharing is achieved by a loadsharing loop, which adjusts the magnetic trim in the mechanical governor of the CSDUs simultaneously via their load controllers. 24 11.3.3 Real Load Shearing (continue) 25 11.3.4 Paralleling • Manual Paralleling is an old method of paralleling generators. To facilitate this method, a lamp is fitted across the main contacts of the GCB. When both generators' outputs are the same, the lamp will darken and go out. When this occurs, the engineer closes the oncoming generator's control switch. This is known as the lamps dark method of paralleling. 26 11.3.4 Paralleling (continue) • Automatic Paralleling. When using the automatic paralleling method, the generator switch is selected to on at any time, and once the auto paralleling circuits sense that both generators are ready for paralleling, the GCB automatically closes. 27 • Over-Excitation (Parallel Fault) protection devices operate whenever the excitation to the field of one of the generator increases. This is sensed when the over-excited generator takes more than its share of reactive load. The fault signal has an inverse time function that trips the BTB of the overexcited generator. The voltage regulator or reactive load-sharing circuit could cause this fault. 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 28 • Over-Voltage protection devices operate whenever the system voltage exceeds 225 V. They protect the components in the system from damage due to excessive voltages. This protection device operates on an inverse time function, which means that the magnitude of voltage determines the time in which the offending generator is de-energised by tripping the GCR and GCB. The GCR de-energises the field, and the GCB trips the generator off the busbar. 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 29 • Under-Excitation (Parallel Fault) protection devices operate whenever the excitation of one of the generator fields is reduced. This is sensed when the under-excited generator takes less than its share of reactive load, and a fault signal causes the BTB to trip in a fixed time (3-5 sec). This type of fault could be caused by a fault in the: •  Reactive load sharing circuit •  Generator •  Voltage regulator 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 30 • Under-Voltage protection devices operate to prevent damage to equipment from high currents and losses in motor loads, which may cause over-heating and burn out. When this device operates, it trips the GCR and GCB in a fixed time (3-5 sec), resulting in the shut-down of that generator. 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 31 • Differential Protection devices operate in the same way as stated in the split-busbar generator system. They operate if any of the following faults exist: •  A line-to-line or line to-earth fault •  If the current flowing to the busbar is different from the current flowing from the generator 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 32 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) • Instability Protection (Parallel Fault) devices are incorporated in the system to guard against oscillating outputs from the generators, which may cause sensitive equipment to malfunction or trip Off. 33 • Negative Sequence Voltage Protection devices detect any line-to-line or line-toearth faults after the differentially protected zone and cause all the BTBs to trip. 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 34 • Overheat warning lights illuminate if a temperature sensor fitted in the generator senses an overheat condition. If this warning occurs, the pilot should operate the GCR switch, which will Cause the GCR and GCB to trip. 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 35 • Over-speed (Over Frequency) devices operate if a fault occurs in the CSDU, which may cause the generator to exceed its specified frequency limits. If an overspeed condition occurs, it causes the GCB to trip and puts the CSDU into under-drive. 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 36 • Under-speed (Under-Frequency) of the CSDU is sensed by an oil pressure switch in the CSDU. This causes the GCB to trip, removing the generator from the busbar, and protecting the loads from an underfrequency. 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 37 • Time delays are fitted in the generator protection system to give the normal circuit protection devices (i.e. circuit breakers and fuses) time to operate, rather than removing a generator from the system. 11.3.5 Fault Protections in A Constant Frequency AC Parallel System (continue) 38 11.4 DC Power Supplies • Primary aircraft DC power supplies are derived from transformer rectifier units, which are supplied from the 200 V AC busbars. The TRUs are normally run in parallel, although some systems have isolation relays installed, which are designed to separate the DC busbars during fault conditions. 39 11.4 DC Power Supplies (continue) 40 11.5 Emergency Supplies • In the unlikely event that both IDGs and the APU generator fail, AC can still be obtained from: •  The aircraft battery which automatically feeds the AC essential busbar via a static inverter. •  A Ram Air Turbine (RAT) can be automatically or manually dropped into the airstream to drive an AC generator, which produces a constant frequency output for the AC essential busbar. 41 11.5 Emergency Supplies (continue) • If the emergency power supplies are selected, it is normal to shed any nonessential loads (e.g. galleys) in order to prevent overloading the remaining generators, which is known as Load Shedding. 42 11.6 Battery Charger • Modern aircraft are fitted with battery chargers that are supplied from AC power supplies. These provide a DC supply to charge a battery in the shortest possible time, within certain voltage constraints, and without causing excessive gassing. 43 11.6 Battery Charger (continue) • The charger provides a DC current of 45- 50 Amps until the charge reaches completion. It then reverts to the pulse mode to prevent the battery voltage from becoming excessive. 44 11.6 Battery Charger (continue) • Comprehensive protection circuitry is provided in the battery charger to give protection against: •  Over voltage •  Overheating •  Battery disconnection 45 11.6 Battery Charger (continue) • If the battery over-volts, the battery charger is automatically switched off and can only be reset by a push-switch situated on the front of the battery charger. • If the charger overheats, it is automatically shut down but resets itself when cooled. • If the battery is disconnected, the charger cannot be switched on. 46 11.7 Battery Power • The batteries supply secondary DC power on most aircraft, they also feed essential DC and, through a static inverter, essential AC for a period of 30 minutes or more. • Some batteries are additionally fitted in non-pressurised areas in the fuselage and are provided with electrically heated blankets to prevent freezing. 47 11.8 Ground Handling Bus • The ground handling busbar is powered from either an APU generator or an external power unit. The busbar is powered automatically whenever external or APU power is available. This busbar is used mainly on the ground to power lights and the refuelling system. 48 END OF CHAPTER 11

linairm 发表于 2011-10-22 20:27:26

这个东东真好

页: [1]

航空论坛_航空翻译_民航英语翻译_飞行翻译's Archiver

Chapter 11 AC Power Generation Systems