GB2510121A

GB2510121A - An aircraft electrical generator supplemented by an energy store until the generator is ramped up to meet the load requirement.

Info

Publication number: GB2510121A
Application number: GB1301223.2A
Authority: GB
Inventors: Huw Llewelyn Edwards; Arthur Laurence Rowe; Robert Slater
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2013-01-24
Filing date: 2013-01-24
Publication date: 2014-07-30
Also published as: GB201301223D0

Abstract

An aircraft electrical system 210 comprises a first electrical generator 212 driven by an engine and an energy store 226. A control system monitors the power output of the generator and the power requirement of the loads. If the load demand exceeds the generator 212 output, the energy store 226 provides the required power until the generator 212 output is increased to meet the load demand. The engine speed or compressor pressure ratio may be monitored to determine power output and demand. The engine speed may be increased to charge the energy store 226 as well as meet the load demand. An AC bus 220 and a DC bus 222 may be provided, with a bi-directional convertor 224 between the two. The engine may be started by a motor powered by the energy store 226. The energy store 226 may be a battery or a flywheel. The flywheel may be charged by an auxiliary power unit or by the first electrical generator 212 and may drive an inductive generator.

Description

An Electrical System for an Aircraft and a Method of Control

Technical Field of Invention

This invention relates to an electrical system for an aircraft and the control of that system. In particular, the invention relates to an electrical system which utilises energy storage for supplying power to the electrical loads of the aircraft.

Background of Invention

Figure 1 shows a conventional ducted fan gas turbine engine 10 comprising, in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. A nacelle 32 generally surrounds the engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle 36.

Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by interconnecting shafts 38, 40, 42.

In current gas turbine engines, electrical power is typically generated by a wound field synchronous generator 44, although it will be appreciated that other electrical machines could be used subject to reliability and electrical performance requirements. The generator 44 is driven via a mechanical drive train 46 which includes an angle drive shaft 48, a step aside gearbox 50 and a radial drive 52 which is coupled to the intermediate pressure spool shaft 40 via a geared arrangement. Thus, due to the choice of generator 44 and transmission 46, the rotational speed of the generator's rotor and the electrical frequency which is outputted into the electrical system of the aircraft is proportional to the speed of the engine. It will be appreciated that other arrangements are possible, including the use of constant speed drives which help provide a fixed frequency electrical supply or through core mounting the generators.

The electrical power provided by the engines supplies the various loads of the airframe and gas turbine engines themselves. These loads typically increase in size with each new generation of civil aircraft to the point where the electrical loading on current aircraft and aircraft in development accounts for a significant portion of the fuel consumption. For example, some state of the art aircraft have the capacity to generate a continuous 500kVA per engine under normal operating conditions.

The generators are capable of being safely overloaded for short periods to accommodate changes in the load. Consequently, conventional aircraft are configured to operate the gas turbine engines at a higher idle speed during a steady state flight conditions. This allows any step change in electrical power demand to be readily accommodated without having to wait for the engine speed and power output from the engine increased.

This approach, although adequate, results in increased fuel burn to keep the engine idle speed higher than it need otherwise be. For example) the idle of a gas turbine engine can be as high as 5% above the level required for propulsive and electrical requirements during steady state flight conditions, simply to meet any unexpected electrical load increases. It will be appreciated that the idle may also be increased to meet other needs of the aircraft such as to protect surge margins and the like.

Other methods of managing the electrical power system are known in the art. In US8237308, a method is used to provide dynamic electrical power management which may minimize the potential for overload conditions and may ensure that system performance limits are maintained. The method dynamically limits the primary load system power draw in response to the net power draw of all other electrical power users on the aircraft which may ensure that the total power levels remain below critical limits. The described method also provides predictive controls to handle rapid load transients. Additionally, if vital functions are not being met, the method may shed other selected aircraft electrical loads which may ensure that adequate power is provided to the primary load system.

The present invention seeks to provide an improved method of operating the gas turbine engine to help improve overall efficiency of the aircraft.

Statements of Invention

In a first aspect the present invention provides a method of providing electrical energy to an electrical system of an aircraft, wherein the aircraft includes: a first electrical power source for providing electrical power to one or more electrical loads on the aircraft, the first electrical power source being driven by a propulsive engine in use; an energy store which is arranged to provide power to one or more loads; and, a control system configured to monitor a predetermined engine condition which is indicative of amount of electrical power which can be provided by the first electrical power source and electrical power required by the one or more loads; the method including the steps of: a) monitoring a condition of the engine which can be used to determine the available electrical power from the first electrical power source; b) determining the electrical power required by the one or more loads; c) determining when the available electrical power from the primary power source is insufficient to meet the required power; d) providing electrical power to the electrical loads from the energy store; and, e) increasing the power output of the engine until the required electrical power can be provided by the primary electrical power source.

The use of an energy store allows the idle of the engine to be lowered during part of a flight cycle.

The use of the energy store allows the idle of the aircraft engine to be reduced thereby reducing fuel burn. The use of energy stores in aircraft is known. For example, EP2447726 describes a method of controlling torque oscillations in a mechanical drive train of an electrical generation. However, there is no suggestion that the energy system could be used for an alternative purpose or for reducing fuel burn.

The monitored condition may be one which gives an indication of the compressor condition. For example, the monitored condition may be the pressure ratio. Alternatively or additionally, the monitored condition may be the the speed of the engine.

The rotational speed of the engine may be increased to increase the power output of the engine.

The power output of the engine may be increased sufficiently to provide all of the required electrical power for the one or more loads and to recharge the energy store. The power output may be increased by increasing the shaft speed. The controller may be configured to assess the required power demand and available power and determine when to charge the energy store. The decision as to when to charge the energy store may be determined by considering one or more of: the allowable charging limits of the energy store; the efficiency of the gas turbine; the predicted load and the associated requirement for having the energy store charged; and the part of the flight cycle.

In a second aspect) the present invention provides an electrical system for an aircraft) comprising: a first electrical power source for providing electrical power to one or more electrical loads on the aircraft, the first electrical power source being driven by a propulsive engine in use; an energy store which is arranged to provide power to the one or more loads; and, a control system configured to: a) monitor a condition of the engine which can be used to determine the available electrical power from the first electrical power source; b) determine the electrical power required by the one or more loads; c) determine when the available electrical power from the primary power source is insufficient to meet the required power; d) re-configure the electrical system to provide electrical power to the electrical loads from the energy store; and, e) increase the power output of the engine until the required electrical power can be provided by the primary electrical power source.

The electrical system may further comprise an AC bus and a DC bus and a bidirectional power conditioning unit for providing power from the AC bus to the DC bus and vice versa.

The electrical system may include a motor for starting the gas turbine engine which is electrical drivable with power provided by the energy store.

The energy store may be an electrical energy store.

The energy store may be a DC electrical energy store.

The energy store may be a mechanical energy store. The mechanical energy store may be a flywheel.

The energy store may be arranged to be charged by an auxiliary power unit.

The mechanical energy store may be arranged to drive a doubly fed induction machine.

The mechanical energy store may be driveably connected to an electrical machine which receives electrical power from the primary electrical power source such that driving the electrical machine charges the mechanical energy store.

Description of Drawings

Embodiments of the invention will now be described with the aid of the following drawings of which: Figure 1 shows a conventional gas turbine engine.

Figure 2 shows an electrical system according to the invention.

Figure 3 shows an operational mode diagram for the electrical system shown in Figure 2.

Figure 4 shows an alternative electrical system in which an energy store is connected directly to a DC sub-system.

FigureS shows a yet further alternative electrical system in which the energy store is mechanical.

Detailed Description of Invention

Figure 2 shows an electrical system 210 for an aircraft and includes a main generator 212 which is powered by a power take-off taken 214 from a spool of the gas turbine engine, similar to that described in relation to Figure 1. The main electrical generator 212 provides power to various AC 216 and DC 218 loads via an electrical bus system. The electrical bus system includes an AC bus 220 and a DC bus 222. The AC bus 220 provides a direct electrical connection to the AC loads 216 on the system. The DC bus 222 is provided with electrical power from the AC bus 220 via a convertor 224 which converts the AC power taken from the AC bus 220. The DC bus 222 is connected to the DC loads 216 on the system.

Also included in the arrangement is an energy store, or more particularly in the described embodiment, a DC energy store 226. The DC energy store 226 can be of any form suita ble for providing the energy storage for the required purpose which is elaborated on below. For example, the energy store 226 may be a bank of batteries or supercapacitors which are of a suitable capacity to provide the required power for a short duration. The required power and duration will be dependent on the electrical system and how reactive the engines can be but a typical duration would be between 3 to 5 seconds. The energy store 226 is connected to the AC bus via a convertor 228 which converts the DC power produced by the energy store 226 into an AC power having the necessary electrical parameters for safe synchronisation with the AC system. Generally, the electrical parameters in any one instance are determined by the output of the main generator 212 and the connected loads. The DC energy store 226 can also be directly connected to the DC bus 222.

There are a number of switches 230 shown in the electrical system 210. These provide the necessary connectivity between the various electrical power sources 212, 226 and loads 216, 218 in the system 210 and are controlled by a control system which monitors and configures the electrical system to meet a required power demand and also control and reconfigure the system should a fault occur.

In an exemplary embodiment, the control system includes an Electronic Engine Controller 232, EEC, which is part of the aircraft Full Authority Digital Engine Control (FADEC) system (not shown) and which communicates with the Engine Power Management System 234 or EPMS, which is communicably connectable to the main generator 212 and the energy store 226. EEC 232, FADEC, EPMS 234 and the associated implementation of the necessary control strategies are well known in the art. Generally, such a system includes a communication system and electronic hardware and algorithms which are configured to sense electrical power demands, receive command signals from central aircraft systems and coordinate power take-off between the shafts of the main engines) as well as operating engine functions such as bleed-air and variable guide vanes, for example.

In this described embodiment, the control system is additionally adapted to monitor the charging, discharging and level of charge in the energy storage device 226. This is achieved by monitoring various parameters which are representative of the electrical condition of the energy storage device 226. In one embodiment, the voltage state of the energy storage device 226 and the current drawn from it are measured using any one of a number of known techniques. These may include the use of current transformers for monitoring the current and or op amps combined with potential divider circuits for measuring the voltage.

Using the stated parameters) the instantaneous condition of the energy store 226 can be ascertained and an appropriate level of charge or discharge determined. The factors which decide the desired level of charge or discharge may be governed by a load requirement and generation capacity at any one time but may also include an assessment of the capability of the chosen energy store 226. For example, some types of energy store may have safety limits on acceptable levels of charge and discharge or require certain conditions such as constant voltage. Further, the rate of charging may affect the longevity of a type of energy store which will need to be factored into any charging schedule.

Figure 3 shows an operational flow diagram 310 for the main steps of the electrical system 210.

Once the system is activated 312, the control system is employed to monitor a condition of the engine in the form of the engine speed 314, using known techniques. The speed is then used by the control system to determine the available electrical power which can be provided from the main generator as driven by the gas turbine engine. It will be appreciated that other suitable conditions which indicate the amount of electrical power which may be available can be used in place of speed if appropriate.

The current electrical demand is also monitored at step 314 and compared with the required power to determine whether the demand can be met using the present engine operating conditions. The monitoring of the required power and supplied power can be done actively by sensing the condition of the electrical network and parameters which are indicative of whether there has been an increase (or reduction) in load on the network. Thus, the monitoring can be achieved from the generator control unit, based on the voltage and current output of the generator as is known in the art or any other suitable way. Alternatively a signal may be received from the aircraft giving a real-time update on the electrical demand as is known in the art.

When the voltage and current at the main generator terminals reveals that the power demand is greater than the power which can be supplied, power sharing can be introduced so that the electrical system is reconfigured to provide the electrical loads with energy which comes at least in part from the energy store.

In the described embodiment, this is achieved by configuring the electrical system to connect the DC energy store to the AC bus using the aforementioned switching arrangements 230. However, it will be appreciated that the way in which the stored energy is provided to the loads 216, 218 will depend on the type of the energy store 226 and available electrical architecture. Hence, for example, for the embodiment of FigureS described below which utilises a mechanical energy store 526, the stored energy is converted into electrical energy using a co-located generator 528 which is then provided on to the network as required.

In order to successfully connect the energy store 226 to the electrical system 210 for power sharing it is necessary to ensure that the electrical condition of each is controlled so as to be synchronised.

In one embodiment, this is achieved using a frequency monitoring technique in the form of frequency droop control. Assuming the main generator is AC, the frequency from the two supplies are matched which is achieved using the convertor 228 which is used to control the power delivered from the DC energy store 226. In such a case, the main generator 212 is designated as a master with the energy store 226 a slave. The main generator 212 is controlled to maintain a predetermined level of voltage, with the energy store 226 acting as a current supply. More specifically, the frequency is monitored on the network, for example, at the generator terminals. The electronic generator control unit (eGCU) determines if the frequency droop is outside of a predetermined threshold and controls the output of the generator 212 by adjusting the output terminal voltage within the available constraints of the generator capacity at the current engine speed. If the frequency continues to droop then current can be injected by the convertor 228 by appropriate switching schedules.

If connecting two power sources to a DC bus, voltage droop control can be used with a similar master and slave system.

Although of limited application, another option for power sharing is to use the available power sources independently such that the main generator 212 and energy store 226 are controlled in a make before break scheme. Here, the main generator 212 is switched out of service when it can no longer meet the electrical demand with energy store 226 being switched in to deliver the necessary power. It will be appreciated that this will only be possible where the energy store 226 is of sufficient size to fully meet the required demand and that a minor disruption in the electrical supply can be tolerated.

Once the energy store 226 is providing the additional power required for the increase in load, or whilst the system is being reconfigured, the control system can instruct an increase in the engine power output 316 so as to increase the amount of electrical energy which can be provided by the main generator 212. when the engine speed has increased, the power taken from the energy storage will reduce to the point where the main generator is providing all of the required power.

At step 318, a decision is taken to determine whether the energy store 226 should be recharged once the engine speed has been increased, or delayed in preference of a future window in expected power demand. This decision can be made on predicted load changes and or the impact on the efficiency of the aircraft for a given rate of charging at the present thrust and power output requirements. Alternatively, the recharge decision can be made on a consideration of the energy requirements in terms of longevity or charging rate safety limits of the energy store. Any of these deciding factors may be used to provide a default for the recharge so that an active decision is not taken but occurs automatically when certain criteria have been met.

In one exemplary embodiment) it may be determined that the present electrical power demand may not feasibly increase in the short to medium term based upon knowledge of the flight cycle or current loads being used. In this case, the charging of the energy store 226 may be delayed until such a time when the power demand has reduced and before the engine speed is reduced.

Alternatively, it may be, depending on the particular part of the flight cycle, that it is more efficient to increase the engine speed further to provide the recharge capacity, or that the energy store needs to be recharged to assist with potential power transients.

Another point of consideration is that the charging rate can be controlled so that it is much reduced compared to the discharge rate. This may be up to a factor of ten or hundred times. In this case, the burden of charging the energy store 226 on the performance of the gas turbine engine is much reduced and can be incorporated more readily which lends itself to a default condition of recharging the energy store 226 when the engine speed has been sufficiently increased and all of the power is being provided by the main generator. In this way, the energy store can be returned to full capacity and readied for use as soon as possible.

In the described embodiment depicted in Figure 3, the default position is that the recharge phase begins as soon as the generator can do so. Hence, the recharge simply becomes an additional load on the network. With this in mind, the desirable charging rate and overall power requirement can be ascertained and the engine speed set to allow enough power to be produced by the main generator 212. Further, depending on the predicted requirement of the energy store 212, the engine may be idled to account for additional transients loads or have no excess in the idle speed beyond what is ordinarily required for non-electrical related safety margins and the like. The increased idle may be as high as 5% above the required level.

In another embodiment, the energy store 212 could be used to accept excess power from the gas turbine, for example, when there is an unexpected drop in load. This would allow the control of the engine to be better adjusted to match the new load requirements.

Once the energy store 226 is recharged, the idle can be reduced to the applicable level such that the system is in state las indicated at 314.

Figure 4 shows a modified embodiment of the electrical system 410 shown in Figure 2. Here, the electrical system 410 includes a direct connection 412 between the energy store 427 and the DC bus 422 which is used to provide power to the DC bus 422 and AC bus 420. In this case, the convertor 424 described in relation to Figure 2 for providing power to the DC bus is a bidirectional unit capable of rectifying the electrical power from the AC bus 420 and also inverting power from the DC bus 424 for supply to the AC bus 420 and AC loads 416. In some embodiments, this allows the DC energy store 426 to be used to motor the main generator for starting purposes. It will be appreciated that in some embodiments, the convertor may not be bidirectional and load swapping to the DC bus may be sufficient to allow the necessary power share without conversion.

This arrangement is particularly advantageous for use on so-called more electric aircraft in which a large proportion of the electrical loads are supplied via a DC distribution system, typically greater than 50%. Hence, a direct connection allows the energy store to remove a large amount of load from the engine with relative ease. This is in part due to the reduced requirements for frequency matching and synchronisation of the loads and energy store. It is therefore lighter and simpler to implement. Ideally, the energy store would be placed local to the DC buses) which are conventionally located on the aircraft. However, an acceptable alternative would be to have a DC connection running from a remote energy storage system to the DC bus.

S FigureS shows an alternative embodiment in which the energy store 526 is a mechanical device in the form of a flywheel. In the described embodiment, the mechanical energy store 526 is used to drive an auxiliary generator 527 to generate electrical power when required. Thus, there is shown a main generator 512 which is connected to AC 516 and DC 518 loads via respective buses 520, 522 and the energy storage system 526 which is connected to the AC bus via a convertor 528 to condition the output electricity to match that of the AC bus 520.

The auxiliary generator 527 of the embodiment is in the form of a wound field generator using converter 528 to syncronise the power with bus 520. An alternative would be to include machine that has a controllable AC field on the rotor, such as a Doubly Fed Induction Generator (DFIG), which is particularly advantageous as the electrical output frequency of the machine can be adjusted with relative ease and with relatively low power electronic requirements by altering the relative speed of the rotating fields in the rotor and stator as is known in the art. This means converter 520 would no longer be needed, with the power electronics needed for frequency control being part of the DFIG's winding power control system. With either system, the rotational frequency of the flywheel 526 need not be matched mechanically to the electrical frequency on the AC electrical bus 520. Rather, it can be used to drive the auxiliary generator 527 whose output can be tailored to the required frequency. In other embodiments, the generator could a permanent magnet or switched reluctance machine.

To store energy in the flywheel 526 it is rotated at high speeds when there is surplus power available from the gas turbine engine. In one embodiment, the rotation is carried out by the auxiliary generator 527 which receives power from the primary electrical power source 512, or sources, through the convertor 528. In other embodiments, the mechanical energy store 526 can be charged using an auxiliary power source such as the auxiliary power unit (APU). The APU may be any known in the art and will typically be a relatively small gas turbine or rotary engine which is used to provide electrical power to the aircraft in addition to the main electrical generators.

It will be appreciated that the mechanical energy store may need to be coupled to the generator and APU via one or more clutch arrangements.

It will also be apparent that the electrical system described in the embodiment may be larger than is shown. For example, the electrical system may include numerous generators with each one being driven by a different engine or engine spool. Alternatively, each spool may be configured to drive numerous generators.

In the event of a failure of the energy store or electrical system, the controller can be adapted to revert to a safe operating mode in which the engine has an increased idle during steady state flying conditions as is the current practice in conventional electrical systems. This is also the case where the capacity of the energy store is insufficient.

In some arrangements, the energy store may be placed local to the engine along with the associated power electronics needed for conversion.

Although the described embodiments use the speed of the engine to determine the power output of the engine, it is possible to use any monitored condition which is indicative of the condition of the compressor) such as the pressure ratio.

Claims

Claims: 1. A method of providing electrical energy to an electrical system of an aircraft, wherein the aircraft includes: a first electrical power source for providing electrical power to one or more electrical loads on the aircraft) the first electrical power source being driven by a propulsive engine in use; an energy store which is arranged to provide power to one or more loads; and, a control system configured to monitor a predetermined engine condition which is indicative of amount of electrical power which can be provided by the first electrical power source and electrical power required by the one or more loads; the method including the steps of: a) monitoring a condition of the engine which can be used to determine the available electrical power from the first electrical power source; b) determining the electrical power required by the one or more loads; c) determining when the available electrical power from the primary power source is insufficient to meet the required power; d) providing electrical power to the electrical loads from the energy store; and, e) increasing the power output of the engine until the required electrical power can be provided by the primary electrical power source.
2. A method as claimed in claim 1, wherein the monitored condition is one or more of the speed of the engine and the pressure ratio of the compressor.
3. A method as claimed in either of claims 1 or 2, wherein the rotational speed of the engine is increased to increase the power output of the engine.
4. A method as claimed in claim 3, wherein power output of the engine is increased sufficiently to provide all of the required electrical power for the one or more loads and to recharge the energy store.
5. An electrical system for an aircraft, comprising: a first electrical power source for providing electrical power to one or more electrical loads on the aircraft) the first electrical power source being driven by a propulsive engine in use; an energy store which is arranged to provide power to the one or more loads; and, a control system configured to: a) monitor a condition of the engine which can be used to determine the available electrical power from the first electrical power source; b) determine the electrical power required by the one or more loads; c) determine when the available electrical power from the primary power source is insufficient to meet the required power; d) re-configure the electrical system to provide electrical power to the electrical loads from the energy store; and, e) increase the power output of the engine until the required electrical power can be provided by the primary electrical power source.
6. An electrical system as claimed in claim 5, further comprising an AC bus and a DC bus and a bidirectional power conditioning unit for providing power from the AC bus to the DC bus and vice versa.
7. An electrical system as claimed in either of claims S or 6, wherein the electrical system includes a motor for starting the gas turbine engine which is electrical drivable with power provided by the energy store.
8. An electrical system as claimed in any of claims S to 7, wherein the energy store is an electrical energy store.
9. An electrical system as claimed in claim 8, wherein the energy store is DC electrical energy store.
10. An electrical system as claimed in any of claims 5 to 7, wherein the energy store is a mechanical energy store.
11. An electrical system as claimed in claim 10, wherein the mechanical energy store is a flywheel.
12. An electrical system as claimed in either of claims 10 or 11, wherein the energy store is arranged to be charged by an auxiliary power unit.
13. An electrical system as claimed in any of claims 10 to 12, wherein the mechanical energy store is arranged to drive one or more of a wound field synchronous drive, a permanent magnet machine) a switched reluctance machine or a doubly fed induction machine.
14. An electrical system as claimed in any of claims 10 to 11, wherein the mechanical energy store is driveably connected to an electrical machine which receives electrical power from the primary electrical power source such that driving the electrical machine charges the mechanical energy store.