variable frequency powered more electric future aircraft

VARIABLE FREQUENCY POWERED MORE

ELECTRIC FUTURE AIRCRAFT Seminar 2011

Dept Of EEE VAST 1

Chapter 1

INTRODUCTION

Future aircraft electrical power systems will likely move from constant-

frequency (CF) AC to variable-frequency (VF) AC power systems or a hybrid

configuration. The existing primary electric power on commercial aircraft is provided

by CF 3-phase AC power at 400Hz and 115V rms. These power systems employ a

constant-speed-regulated generator (CSRG). This is a hybrid system that consists of

integrated mechanical and hydraulic mechanisms to convert the mechanical power, at

variable-speed rotation, of each aircraft engine into a CF AC power at 400 Hz. In such

a CSRG unit, the variable-speed input power is first converted to a regulated constant-

speed power by a built-in mechanical/hydraulic mechanism. An alternator coupled on

the shaft of the CSRG at the constant rotating speed then generates the CF AC power at

400 Hz. A block diagram of the electrical power architecture of a popular commercial

aircraft is shown in Fig. 1. AC loads are run directly from the 400 Hz AC power bus,

while simple transformer-rectifier units (TRUs) transform the AC power into DC for

loads that require DC input.

Approximately 95% of all in-service aircraft employ the mechanical-regulated CF

power systems. However, the mechanical-regulated generation system is complex and

has relatively low efficiency of power conversion. On the other hand, the CF power

system is not optimized for many AC loads such as AC motors that need adjustable-

frequency control to obtain the desirable operating speed or torque. For those reasons,

many aircraft manufacturers are seriously considering VF power systems as an

alternative or have begun their in-house design.



Dept Of EEE VAST 2

Chapter 2

ANALYSIS AND EVALUATION OF VARIOUS AC LOADS IN VF

POWER SYSTEM

In an aircraft power system, there are many different types of loads that require

power supplies at specific voltages and frequencies that are different from each other

and different from those produced by the main alternators. While many existing on-

board AC loads, such as galley and turbo-fan loads, can operate satisfactorily with a VF

power source, many electrical motors that drive various on-board systems or

equipments cannot be driven directly by the VF power bus. This is because of the

mismatch between the required motor excitation frequency and VF-bus frequency that

is variable in a wide range during a particular time frame. The mismatch may cause the

motor output torque and speed to significantly deviate from the desired operating

characteristics. The motors torque would also naturally decrease with increasing input

power frequency above its base frequency (400 Hz in this case). The operating point

can possibly shift to an unstable operating range. Possible considerations can be given

to use larger motors or use power electronic converters (motor drives) to control the

motor speed. However, the use of larger motors will result in unacceptably increased

weight, size, and cost.

On the other hand, existing conventional power converters and motor drives are

not optimized for aircraft applications. They carry size and weight penalties. Perhaps

more importantly, they generate harmonic pollution on the power bus, which can

potentially cause malfunction to other avionic equipment connected to the power bus.



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Therefore, it requires power converters to be specially developed that meet the

following requirements:

1) Highly compact

2) Low weight

3) Can directly interface the VF-input power, and

4) Low harmonics and meet electromagnetic interference (EMI) requirements.

The new converter becomes an essential technology element supporting a VF aircraft

power system.

There are also differences in the system requirements between small and large

aircraft. Small aircraft (e.g., business or regional jets), which contain few motor loads

that require high constant torque (e.g., one or two air conditioning fans), can

accommodate the size, weight, and harmonic noise associated with adding a couple of

motor drives. However, medium and large aircraft, which contain large numbers of

motor loads, require constant high torque. For example, a medium-size passenger

aircraft, i.e., 150300 passengers, may employ as many as 10 air conditioning fan

motors; a large aircraft may even contain 20 such motors. The overall impact of add-on

size, weight, and harmonic noise from all the converters and motor drives will become

significant to aircraft design.

In addition, a small aircraft has low wiring impedance due to its short power

distribution lines, while a large aircraft has high wiring impedance due to much longer

power cables. This means the same current harmonics generated by a motor drive will

result in small line voltage distortion on a small aircraft, but large line voltage distortion

on a large aircraft. For these reasons, existing deployment of VF power systems are

limited so far to small aircraft (e.g., Global Express, SAAB 2000), while the

deployment on medium and large aircraft still await for additional technological

improvement. To facilitate understanding the system, background information of an

aircraft electrical aircraft power system is also given at the end.



Dept Of EEE VAST 4

In the following sections, we present a new configuration of a potential VF-

power system architecture and discuss the key supporting technologies, such as VF-

input pulse width-modulated (PWM) power converters, for optimal distributed system

and more efficient utilization of electrical power.

Fig1. Typical electrical power system architecture for commercial aircraft



Dept Of EEE VAST 5

Chapter 3

NOVEL VF-POWER SYSTEM ARCHITECTURE FOR

AIRCRAFT

Based on the above analysis and a new concept of distributed VF power system,

new aircraft power system architecture is conceived and proposed in Fig. 2. Although,

in practice, it is possible to make many variations about the system configuration to

best fit the specific requirements for individual aircraft platform, we use Fig. 2 as

concrete VF-power system architecture to facilitate our discussion. The proposed power

system has a distributed structure with two parallel and coordinated power conversion

subsystems, namely subsystem 1 (or right, facing the front direction of aircraft) and

subsystem 2 (or left). Each subsystem consists of stages of:

1) VF power generation without using CSRG (or IDG, Integrated Drive

Generator)

2) VF AC Bus

3) CF AC Bus at 115/200 V, 400 Hz

4) 270 V DC bus

5) 28 V DC bus for low-voltage point-of-load regulators (POLRs), and

6) Emergency supply and tie-in mechanisms for external/ground power supplies.

The 270 V and 28 V DC sources can be also obtained directly from the VF AC

bus using solid-state AC-DC converters, achieving more-distributed power system

architecture. Although the architecture in Fig. 2 is at a conceptual and evaluation stage

and proposed for medium to large aircraft, the overall concept of AC load partitioning

and grouping, and the distributed architecture can be applicable for other aircraft power

systems.



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This VF power architecture is developed based on the minimization of a cost

index defined as the throughput of total kilowatts times the number of stages of power

conversions from the alternators. For example, each parallel subsystem structure has

two groups of AC loads that take the AC power directly from the VF power bus. The

first group is labelled as Turbo-fan, Galley and other VF AC Loads in Fig. 2. Those

are low-performance AC loads that can operate satisfactorily with a VF power source.

The second box below group 1 in Fig. 2 is designated for emerging AC loads including

electromechanical actuators (EMAs) and electro-hydraulic actuators (EHAs). The air-

conditioning fan motors can be classified into this group. Those electrical subsystems

are expected to operate at a medium power level, i.e., at tens of kilowatts, and require

VF-input power converters to control the speed (positioning) or torque of the AC

machines that drive the actuators. Taking the AC power directly from the VF power bus

without going through the traditional stages of CSRG and AC-DC conversion stage will

result in significant reduction in the power losses associated with each of the

conversion stages, thus improving the overall efficiency and fuel economy.

A comparison of estimated power conversion losses of the VF approach (for VF

load) in Fig. 2 and the conventional mechanical CSRG system is given in Table I.

In Table I, the average efficiency of the power conversion stage for the two groups of

AC loads fed directly from the VF power bus is estimated = 9697% depending on

the type of power devices and the control strategy employed for the solid-state power

converters. By employing advanced wideband-gap power devices and integrated

modules, it is reported that the converters switching losses can be reduced by up to

50%. In this case, the average efficiency of the power conversion can possibly achieve

97.5% in the future. On the other hand, the efficiency of a typical CSRG unit at

medium power is estimated between 75.280%, although higher efficiency might be

possible by recent technology advancements. If the total power throughput in this group

is 150 kW, for instance, the power-loss reduction by the new approach is estimated in

double digits of kWs for the power conversion, with a consideration of additional

factors that are not listed in Table I.



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Replacing the CF AC power system with VF AC system will increase overall

energy efficiency, and improve fuel economy. However, as shown in Fig. 2, new power

conversion topologies and new converters are needed for the various power and

frequency transformation between the VF-AC, CF-AC, and DC power. The key power

conversion and regulation functions required are listed below.

1) VF-VF: to control electrical AC motors fed from the VF power bus. The converters

have VF power input and produce variable-voltage and variable-frequency (VVVF)

output [12].

Also to facilitate the applications such as engine starter/generator, bi-directional power

converters are necessary.

2) VF-CF: to convert the unregulated VF power to a well-regulated CF power at 400

Hz and 110 V rms.

3) AC-DC and DC-AC. On the other hand, the impact of power converters (such as

input harmonic distortions and EMI) on the VF distribution systems must be carefully

considered.



Dept Of EEE VAST 8

Fig2. Potential VF Power system architecture for large aircraft using bi-directional power

converter technology



Dept Of EEE VAST 9

Chapter 4

VF-Input Power Converters

The VF power system, such as Fig. 2 or its variations, calls for a number of new

VF-input power converters. Todays conventional power converters employ two-stage

power conversion process, AC-DC-AC, including transformer-rectifier units (TRUs)

(AC-DC) plus a VF inverter (DC-AC). The TRU converters or its variations employ a

front-end multiple-phase transformer to convert three-phase voltages, for example, to

six-phases of voltage supplies feeding a twelve-pulse rectifier, achieving reduced input

current harmonics. Due to its simplicity in the solid-state rectifier, the TRU converters

are considered reliable. However, the front-end transformer is heavy and the TRU

products are often designed for specific applications and are not capable of bi-

directional power control. It is desirable to employ more-silicon power converter

technology in the future.

A simplified block diagram of a VF-input PWM converter that needs no input

multiple-phase transformer is illustrated in Fig. 3. The circuit topologies of the solid-

state PWM converter that can be selected include, but are not limited to, the following

types:

1) Dual-PWM bridge converter that is a three-phase PWM rectifier plus a PWM

inverter,

2) AC-AC one-stage converter based on nine bi-directional integrated power-switching

devices,

3) Cycloconverters based naturally-commutated power devices, such as 18 or 36

thyristors.



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Fig 3. General block diagram of VF input PWM Converter



Dept Of EEE VAST 11

The cycloconverters have been successfully used for variable-speed CF

conversion for small to medium size of aircraft, including MD 90 and Boeing 737, as

well as military applications. However, the cycloconverter circuit is also known for its

limited range of frequency conversion ratio requiring the input frequency to be greater

than three times of the output frequency. This, together with the rich harmonic contents

in comparison with the PWM converters, would limit the cycloconverter in the

application of VF-VF conversion systems.

Fig4. Typical 6-pulse Front End Converter for AC drives



Dept Of EEE VAST 12

Chapter 5

OPTIMAL PARTITION OF ELECTRICAL LOADS

Supported by the new technology development, the onboard electrical AC loads

and power control blocks, as shown in the system diagram of Fig. 2, can be further

classified into three major groups that need especially designed power converters.

1) Application Group 1, Localized AC Motor Control:

As mentioned previously, implementing a VF power system necessitates the

introduction of VF-input converters or AC motor drives for certain motor loads that

require controlled speed/torque. Additional motor loads on a large aircraft are identified

and listed in Table II. AC motor drives are required for air-conditioning fan/blower

motors and for flap/slat motors because their output torque must remain high regardless

of the AC bus frequency.

Motor drives are also required for fuel jettison pumps and override fuel pumps

because these pumps must operate at high speed regardless of the AC bus frequency

(e.g., must dump fuel quickly before emergency landing). In Table II, fuel boost pumps

and hydraulic pumps are preferred to operate with controlled speed by adjustable motor

drives. But it is also possible to obtain adequate performance in a VF system without

using motor drives. For reference, Table II also shows the number of motors and the

power requirement associated with each type of application for an existing popular

aircraft (which uses a standard CF AC bus). Future VF-powered medium aircraft will

have similar loads, while a large aircraft may have twice as many loads, as well as

higher power requirements for each load. For these applications, a converter such as

VF-input one-stage converter can be used as a direct AC-to-AC converter with VF

input and VVVF output to serve as an adjustable-speed drive. This not only allows AC

loads to operate properly in a VF system, but also improves energy efficiency.



Dept Of EEE VAST 13

2) Application Group 2, VF-to-CF Power Bus Converter:

A VF-input one-stage converter can also be used to generate a CF supply from the

VF input for a CF power bus. One example is the in-seat power for passenger portable

electronics, such as laptop computers, on future commercial aircraft. A possible power

distribution architecture for this application interested by the in-flight entertainment

industry is to convert the on-board AC power (VF or 400 Hz CF) to 60 Hz AC and

distribute it to the passenger seats where the passengers can plug in a standard adapter

for their portable devices.

In present CF power systems, AC-to-DC conversion is performed by using TRUs;

that is a mature technology. However, the TRU has a couple of shortcomings. In

addition to bulky size and heavy weight due to the use of transformers, the output

response is poor due to lack of voltage regulation. For these reasons, the commercial

aviation industry has shown increasing interest in new AC-to-DC conversion

technology which can overcome the problems of the traditional TRU. The modular VF-

input one-stage converter is a potential candidate for this application, with its ability to

control both input current and output voltage. Although, other converter topologies

requiring fewer semiconductor devices (such as the standard 6-switch boost topology)

can be used for AC-to-DC conversion .

3) Application Group 3, PWM-Controlled Electronic IDG and Engine

Starter/Generator:

In the high power range, a possible future application of the modular VF-input

one-stage converter is to provide bi-directional power control to support the engine

starter and generator subsystems. On the other hand, it is desirable to use a bi-

directional power converter as a PWM-controlled electronic IDG that converts the

output of the VF alternator into 400 Hz CF power to service all CF loads. This would

require over 100 kVA range. However, confidence in the reliability of power converters

in lower power applications must first be established.



Dept Of EEE VAST 14

Chapter 6

VF-INPUT ONE-STAGE POWER

CONVERTERS FOR MULTIPLE FUNCTIONS

Based on the analysis and understanding above, we include in this section an

improved approach of VF-input one-stage power converter for multiple functions. A

VF-input multi-functional power converter (MFPC) has been developed and reported

by our recent works. This is a one-stage VF-VF (or VF-CF) solid-state power converter

that is advanced from matrix converters. The power train uses advanced integrated self-

turn-off IGBT AC switches. In this converter, we design the control and modulation at

both input and load sides as voltage sources (or modes), instead of current sources (or

current modes). To further improve the systems operational tolerance of the line

imbalance and large input distortions, we design our control architecture in a complete

open-loop way on both input side and output side, without using any conventional

feedback control loops. `

The converter system is controlled by PWM approaches at both input and output

sides using a DSP controller. In Fig. 4(a), the converter power circuit consists of three

identical three-phase to one-phase (3-to-1) conversion blocks, named U, V, and W at

their output side. At the input side, three-phase voltage sources produced by VF

alternator are named Va, Vb, and Vc. Upon an optimal partitioning of possible basic

building-block circuits, the AC power switches, Sjk, are grouped and designed to form

integrated bi-directional power modules (IBPM), as shown in the dash-line blocks in

Fig. 4(a). Each of the phase circuits consists of only one IBPM and forms a 3-to-1

phase power conversion bank that can be used as a common building block for multiple

phase converters with multiple functions. More detailed circuit diagrams and a picture

of our fabricated IBPM module are included.



Dept Of EEE VAST 15

In the upstream of power flow, as shown in Fig. 4(a), this converter accepts a

wide variation of input power frequency that is in proportion to the shaft rotating speed

of the alternator driven by the power engine. The capability of accepting widely VF

power input is achieved by a precise identification of the input frequency and phase

angle in real time. The IBPMs on the input side are switched synchronously with the

frequency change to achieve an adaptive VF control, as shown in Fig. 4. On the output

side, the converter produces VVVF output or CF AC power with regulated voltage

control. This, therefore, becomes a new breed of VF-VF or VF-VVVF converter or a

variable-speed VF one-stage power conversion if we include the stage of power

generation in our consideration. The block of SMP represents the switching-mode

power supplies that are designed as the control power supplies for the electronics

controller and the integrated bipolar transistor (IGBT) gate circuits of the integrated

semiconductor power models. The converter power train uses advanced integrated self-

turn-off AC switches and is controlled by PWM approaches at both input and output

sides. This differentiates it from the cycloconverters that are based on the principle of

natural commutation and phase angle control technique. The output frequency of a

conventional cycloconverter is limited to less than 1/3 of the input frequency as

aforementioned. This is due to its excessive harmonics in both voltage and current that

adversely affects the system performance.



Dept Of EEE VAST 16

Fig5. Configuration of 3 phase VF VVVF Converter

Fig6. VF DC Converter by Modular Configuration



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Table1. Airborne Equipment Input Harmonic Current Limits

Limits for input harmonic currents defined in DO-160 and ISO-1540 are

compared below in Table II. Note that ISO-1540 has a special allowance permitting

higher harmonic distortion produced by 12-pulse rectifiers, which is not included in

Table II. ISO-1540 also has limits for total harmonic current distortion (THD) in

addition to the limits for individual harmonics. The THD limit is 12% for the 12-pulse

rectifiers and 8% for all other loads. The highest order of harmonic limited by DO-160

is the 40th for both CF and VF systems, while ISO-1540 limits apply to harmonics

under 25 kHz in CF systems and 50 kHz in VF systems. In general, the allowable

harmonic current amplitude decreases as the harmonic frequency increases. This makes

sense considering the higher impedance of the distribution lines at higher frequencies

(i.e., the same amount of current at higher frequency will produce higher voltage

distortion). However, that is not the case in the October 2000 draft of MIL-STD-704F

where higher order harmonics are allowed to have higher magnitudes.



Dept Of EEE VAST 18

Table2. Potential Motor Drive Applications for Aircraft with VF Power System

Fig7. Difference between Linear & Non-Linear loads



Dept Of EEE VAST 19

Chapter 7

CHALLENGES AND RECOMMENDATIONS FOR

CF AC POWER SOURCE

When used to generate CF AC power to supply multiple loads, the power

converter may have to meet additional regulatory requirements for power quality at its

output. This would be the case when the converter serves as the AC-DC unit in Fig. 2to

generate the CF AC bus from VF power. Since multiple user equipment will be

connected to the CF AC bus, the converter must be controlled as a voltage source so

that its output has similar characteristics of conventional AC power sources. ISO-1540

and DO-160 defined all performance requirements including voltage modulation, phase

imbalance, frequency variation voltage distortions, and transient characteristics. Both

DO-160 and ISO-1540 require that the maximum harmonic voltage distortion of the

AC bus be less than 8% of the fundamental voltage, with each individual harmonic

component not exceed 6% of the fundamental. These limits apply to both CF and VF

systems and are independent of the bus voltage (115 V/230 V). Since the output voltage

of the converter contains high-frequency switching harmonic, output filtering therefore

becomes necessary, which can be accomplished by using advanced harmonic filters.



Dept Of EEE VAST 20

Chapter 8

BACKGROUND INFORMATION OF ELECTRICAL POWER SYSTEM

Electrical Power System Components: An example of an electrical power

system supplies 115 V CF AC and 28 V DC electrical power to the airplane. The

related key power sources of a popular large commercial aircraft are list below.

1)Two integrated CSRGs.

These type of units are also called IDGs.

2) APU generator.

3) Two backup generators.

4) Ram air turbine (RAT) generator.

5) Main and APU batteries.

6) External power.

There is one CSRG (IDG) on each engine. They are the primary source of AC

power in light. An additional source of AC power is the APU generator. Each generator

supplies up to 120 kVA. There is one backup generator on each engine. They are

variable-speed, VF generators. Each supplies up to 20 kVA of AC power. A backup

converter changes the VF power to CF power. Each backup generator also contains two

permanent magnetic generators (PMGs) that supply power to three flight control DC

(FCDC) power supply assemblies. A RAT generator is another source of backup AC

power. For ground operations, there are two external power connectors. These are on

the forward, right side of the fuselage. Each external power connector is rated for about

100 kVA of AC power. Electrical Power System: The electrical power system normally

operates as two independent left and right power channels. Each channel has a main AC

bus. The left main AC bus receives power from the left IDG and the right bus receives

power from the right IDG.



Dept Of EEE VAST 21

The APU generator and external power connections are also sources of AC power

for either main bus. The right main AC bus supplies power to the ground service bus.

When the right bus does not have power, the APU generator or primary external

connector can supply power to the ground service bus. On the ground, the APU

generator or primary external power source supplies power to the ground handling bus.



Dept Of EEE VAST 22

Fig8.Harmonic Trap filter



Dept Of EEE VAST 23

POWER SYSTEM DISTRIBUTION

Fig9. Power System Distribution in an Aircraft



Dept Of EEE VAST 24

Chapter 9

CONCLUSIONS

A novel VF-system architecture employing modular VF-input converter

technology has been developed. Optimal partitioning and grouping of onboard AC

loads has been discussed with specific system data. The system-level optimization is

achieved based on the cost index minimization of the power throughput times the

number of stages of power conversion (from the VF power bus). Three major

applications calling for advanced converter technology for future aircraft have been

identified and their key requirements have been discussed. The input current harmonic

distortion has been identified as an important and challenging issue, and the applicable

regulatory requirements have been reviewed. This presents a unique opportunity of

increased demands for new power converters, including VF-input MFPC technology

and its variations.



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REFERENCES

[1]. Jie Chang, Anhua Wang.: Variable Frequency power System Evaluation & Analysis

for Future Aircraft, IEEE Transactions On Aerospace And Electronic Systems, Vol

42, No:2, April 2006.

[2]. Weimer J A.: Electrical Power Technology For The More Electric Aircraft

Proceedings of the IEEE 12th Digital avionics Systems Conference, Oct 1993.

[3]. Murphy, F G TurnBull. : Power Electronic control Of AC Motors,1988 , Pergammon

Press, Elmsford , New York, 1988.

[4]. Praneet Atalya, dragan Maskimovic : IEEE Power Electronic Letters, Vol 2, No: 4,

Dec 2004

variable frequency powered more electric future aircraft

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