1. analysis of new step-up and step-down direct sysmmetric 18-pulse topologies for aircraft atru

8/8/2019 1. Analysis of New Step-up and Step-down Direct Sysmmetric 18-Pulse Topologies for Aircraft ATRU

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This work made use of Engineering Research Center Shared Facilities supported by the National Science Foundation under NSF Award Number EEC-9731677 and the CPES Industry Partnership Program. Any opinions, findings and conclusions or recommendations expressed in this material are those of the

author(s) and do not necessarily reflect those of the National Science Foundation. This work was conducted with the use of Saber Sketch, donated in kind by

Synopsis Inc of the CPES Industry Partnership Program.

Analysis of New Step-up and Step-down Direct

Symmetric 18-pulse Topologies for Aircraft

Autotransformer-Rectifier Units

Alexander Uan-Zo-li, Rolando P. Burgos, Frederic Lacaux, Arman Roshan, Fred Wang and Dushan Boroyevich

Center for Power Electronics Systems (CPES)

Department of Electrical and Computer Engineering

Virginia Polytechnic Institute and State UniversityBlacksburg, VA 24061-0111 USA

AbstractAmong possible 18-pulse Autotransformer-

Rectifier-Unit (ATRU) topologies, Direct Symmetric circuit (DS-

ATRU) demonstrates relatively low complexity, it is insensitive to

impedance mismatch and distortions of input voltage and has low

common mode voltage. This paper proposes new step-up and step-

down Direct Symmetric 18-pulse ATRU topologies for aircraftapplication. Serious study of proposed topologies was performed.

The authors analyzed the kVA ratings, effects of leakage

inductance on compliance with input current harmonic

requirements, power sharing between diode bridges, common

mode voltage and output ripple. The comparison is carried out for

the full range of the input line frequency from 400 to 800Hz. Key

analyses and results obtained with Saber simulations are

presented for validation of the presented work. Some

experimental results are shown.

I. INTRODUCTION

Today, the More Electric Aircraft (MEA) initiative aims for

the replacement of Constant Frequency Generators (CFG) by

variable frequency generators (VFG), which eliminates all thecomplexities associated with the integrated mechanicalfrequency controller of CFG [1], [2]. This approach was

successfully proven on business jets (Bombardier Global

Express), airliners (McDonnell Douglas MD-90), and is the

selected technology for the Airbus A-380 [2], [3]. This in turnhas drastically changed aircraft electric power systems, which

may now employ either variable frequency AC, DC, or a

hybrid AC and DC system for power distribution [4], [5] and[6]. Aircraft power electronics have also been affected by this

trend, as the need for conversion and driving equipment has

become manifest. A number of static power converters havealready been studied and tested [3], [7], [8], [9], [10] and [11].

The work on this area however is far from being mature; andnew technologies, structures, and topologies have yet to beestablished.

Recently, the usage of AC-DC conversion has become a

common feature upon the aircraft electric power distribution.For constant frequency systems for instance, Transformer

Rectifier Units (TRUs) are employed to generate low voltage

DC. For variable frequency systems, the AC-DC conversion isused to generate the DC bus, which supplies power to both

DC-DC and DC/AC secondary converters. There exist two

alternative types of AC/DC conversion techniques, the PWMactive front-ends and passive multi-pulse converters.

Seemingly, the former approach still requires considerable

development for aircraft applications given the stringent

operational and reliability goals that must be met. The latter onthe contrary may be readily employed given its sheer

simplicity and the vast existent knowledge in industry. Also,given that there is no need for isolation, the autotransformers

have obvious advantages over transformers due to reduced

kVA ratings, size and weight. Given the stringent requirements

for the input harmonics and output voltage ripple of theAC/DC equipment, 18-pulse topologies are a natural choice for

the aircraft application.A number of possible topologies and winding configurations

were recently proposed [12], [13], [14], [15] and [16]. Among

possible 18-pulse Autotransformer-Rectifier-Unit (ATRU)

topologies, Direct Symmetric circuit (DS-ATRU) demonstrates

relatively low complexity, is insensitive to impedancemismatch and distortions of the input voltage and has lowcommon mode voltage [17]. Due to these reasons, 18-pulse

DS-ATRU is a logical choice for passive rectification for the

aircraft application.

This paper presents new step-up and step-down variations ofDS-ATRU. The proposed circuits are analyzed in order to

assess the feasibility of employing them in electrical powersystems of aircrafts. Particular attention is given to their correct

operation at relatively high and variable line frequency. The

following items are chosen to be analyzed: KVA ratings, input

harmonic distortion (harmonics and THD), DC voltageregulation, common-mode voltage, impact of impedance

mismatch between converter paths, impact of harmonic voltagedistortion, and impact of leakage inductance on the operation

of ATRUs at high frequency.

II. DIRECT SYMMETRIC TOPOLOGY

A. Operation of DS-ATRUThe block diagram of DS-ATRU is shown in Fig. 1 and the

vector diagrams of three consecutive conduction intervals are

demonstrated in Fig. 2. As can be observed, the

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autotransformer is used to create two additional 3-phasesystems, one leading the input AC supply voltage by 40

degrees, the other lagging by 40 degrees, with the amplitudes

of the created phase voltages equal to the amplitude of the

supply voltages. The three resultant three-phase systems arethen connected to the diode bridges (it is possible to use

compensating inductors to counteract the impedance mismatch between conduction paths). The outputs of the rectifiers are

directly fed to the load. At each moment, there are two bridgesconducting load current, with one rectifier producing positive

current, and the other negative current. The angle between the

utilized phase vectors is 160 degrees, so the line-to-line voltage

used by rectifiers is 14% higher than the line-to-line voltage ofthe input line. In average, each rectifier bridge conducts one

third of the total output power. Each bridge diode conducts fullload DC current for the duration of 40 degrees.

AC Supply

CompensatingInductors

Autotransformer

ForwardBridge

LaggingBridge

ThroughBridge

Fig. 1 Block Diagram of DS-ATRU

Vb

Va

Vap

Vapp

Vcpp

Vbp

Vc

Vbpp

Vb

Va

Vap

Vapp

Vcpp

Vcp

Vbp

Vc

Vbpp

Vb

Va

Vap

Vapp

Vcpp

Vcp

Vbp

Vc

Fig. 2 Vector diagram of three consecutive conduction intervals

B. Description of possible winding configurations andtheir comparison

Fig. 3 shows three winding configurations of the DS-ATRU, accordingly T-Delta [15], Delta [15] and Polygon

[18]. Fig. 4 demonstrates how each of the winding

configurations creates three 3-phase systems shifted by 40

degrees. Table 1 shows the comparison of the windingconfigurations based on autotransformer manufacturability

(number of unconnected windings per 3-phase

autotransformer leg) and power rating. It is clear that Deltaand T-Delta configurations offer the lowest possible power

rating, which translates in smaller physical size of the

autotransformer. It can be seen that Polygon has the

smallest number of unconnected windings, whichtranslates in higher manufacturability and potentially the

lowest leakage.

Va

Vap Vapp

VbpVcpp

Vb

Vbpp

Vc

Vcp

Va

Vap Vapp

VbpVcpp

Vb

Vbpp

Vc

Vcp

VappVap

VbppVcp

VbpVcpp

Va

VbVc

Fig. 3 T-Delta, Delta and Polygon winding configurations of DS-ATRU

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k1

k2

VaVb

Vc

Vbpp Vap

Vapp

Vcpp

Vbp

Vcp

k1

k2

VaVb

Vc

VbppVap

Vapp

Vcpp

Vbp

Vcp

Vc

Vbpp

Vb

Va

Vap

Vapp

Vcpp

Vcp

Vbp

k1

k2

Fig. 4 Vector Diagrams of T-ATRU, D-ATRU and P-ATRU windings

Table 1 Comparison of DS winding configurations

Parameter of

Comparison\Winding

T-

Delta

Delta Polygon

Number of unconnected

windings per transformerleg [#]

5 3 2

kVA Rating [%] 55.4 51.4 83

C. Variations of Direct Symmetric Topology forincreased and decreased voltage

In order to satisfy the requirements of the aircraft powersystem, it may become necessary to modify the existing

topology of DS-ATRU in order to provide lower or highervalue of the output voltage [19]. Fig. 5 shows vector diagrams

of some of the proposed step-down and step-up variations of

the DS-ATRU, which are based on the Delta winding

configuration. It is also possible to show the same variations

for TS-ATRU. All proposed topologies feature very small

increase in number of windings, while preserving thesymmetry between the created phases and providing required

voltage. The main problem with the shown topologies is thatall created phases are at different angle to the input phase. This

would create additional harmonics due to imbalance in

impedances. The authors also looked at these topologies with

more symmetric output vectors. Fig. 6 and Fig. 7 show thesimulated the input currents and the harmonic contents of the

input current for the step-down Delta DS-ATRU with

symmetric distribution of the AC vectors and input voltage of231V, 400Hz and 800Hz input line. SaberR Synopsis Simulator

was used to obtain the simulation results. It can be easily

noticed that the circuit exhibits the 18-pulse operation withrelatively small harmonic distortion of the input voltage. The

leakage and parasitic parameters used for the simulation were

obtained from the analysis of the existing ATRU technologies.

Vb

VaD

Vap

Vapp

Vcpp

Vcp Vbpp

VbD

Vbp

40 40

Va

Vc

VcD

VaD

Vapp

Vapp

Vcpp

Vcp

Vbpp

VbD Vbp

40

40Va

Vb

Vc

VcD

Fig. 5 Vector Diagrams for step-down and step-up variations of Delta DS-ATRU

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THD=5 .3%

Fig. 6 ATRU Input current and its harmonic content at 400Hz, 231V input line

T H D = 5 . 3 %T H D = 3 . 7 4 %

Fig. 7 ATRU Input current and its harmonic content at 800Hz, 231V input line

III. PERFORMANCE EVALUATION

A serious study of DS-ATRU topologies was performed.

The authors analyzed the kVA ratings, effects of leakage

inductance on compliance with input current harmonic

requirements, power sharing between diode bridges, commonmode voltage and output ripple using Synopsis Saber

simulations on the switching models of the equipment.

A. Effects of leakage coupling on the input current distortion

Because the decreases in voltage with additional winding is

normally relatively low, the authors studied the influence of

coupling on the standard ATRU topologies, without the step

down or step up capability. Fig. 8 shows detailed schematic ofthe winding configuration of the T-Delta DS-ATRU. Thistopology was evaluated by [20], but still some questions

remained unanswered. In order to carefully analyze the effects

of coupling, the authors propose to use a fully coupled model

of the leakage, which takes into account the coupling betweenall leakages of all windings of the same autotransformer core

leg. Such approach is rather complex, but it allows to observethe influence of all circuit leakage parameters, and provides

invaluable aid to designers of the autotransformer [21]. Fig. 9

shows the approach to modeling, for example, of the five

windings of one autotransformer core leg where each leakageinductance (defined as inductance, which does not create flux

coupled with windings positioned on the other legs) is coupled

with all windings on the same leg [22]. Figs. 10-15 show somepreliminary data, demonstrating the effects of coupling of the

winding of the same leg on the THD of the input current. As

can be observed, such dependence is far from simple and someeffects were not expected.

Based on the analysis of the relationship of THD of theATRU input currents and winding leakage coupling, the

authors came up with the following preliminary conclusions:

Only couplings of the windings on the same leg areimportant;

Couplings between Nk1 windings are not important and

can be ignored;

Couplings between adjacent Nk1 and Nk2 are notimportant and can be ignored;

Couplings between opposite windings Nk1 and Nk2 are

important and must be modeled.

B. Simulation and Assessment of normal ATRU OperatingConditions

Figs. 16 and 17 show the harmonic content of the inputcurrent of the step-down D-ATRU supplied by balanced 3-

phase voltage, 230V, 400Hz and 800Hz accordingly as afunction of output load. As one can see, the harmonics are

relatively small. Fig. 18 shows the common mode voltage of

the ATRU as a function of output load for balanced 3-phase

voltage. Again, the proposed ATRU topology demonstrateslow common mode voltage, being less than 12V at maximum

output power, while supplied by 230V, 800Hz supply.

C. Simulation and Assessment of Abnormal ATRUOperating Conditions

Figs. 19-24 show simulated input current, input voltage,output common mode voltage and output ripple for the cases of

abnormal operating conditions. The results can be summarizedas follows:

The step-down D-ATRU presents a minimum common-modevoltage with balanced power supply of 9V at 360 Hz and 3V

at 800 Hz.

With 6Vac unbalance, the common mode voltage is 62 V at

360 Hz and 24 V at 800 Hz

With 12 Vac unbalance the common mode voltage of the

step-down D-ATRU is 118V at 360 Hz and 47V at 800 Hz

The input voltage unbalance together with phase

displacement introduces the 2nd order voltage and current

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harmonics and affects the common-mode voltage, whichsurpassed 62V. Voltage modulation introduces current sub-

harmonics, i.e., harmonics below fundamental frequency (360

Hz in this case). That leads to low frequency harmonics on the

DC voltage, i.e., between DC and 2nd order harmonics.Voltage harmonic distortion (5th harmonic injection) increases

the ATRU input current distortion. Presence of the voltage

unbalance and voltage modulation generates 3rd and evenorder harmonics of relatively high level. DC offset in the AC

input voltage supply generates further power unbalance

between phases. The combined effect of the dc offset,

harmonic distortion, and phase unbalance and voltagemodulation can saturate the autotransformer core, even though

a small gap was used to prevent saturation.

Va

Vap Vapp

VbpVcpp

Vb

Vbpp

Vc

Vcp

Nk1a

Nk2a

Nk1b

Nk2b

Np

Fig. 8 Autotransformer winding configuration

Rk1

L NK1B

L NK1A

L NK2B

L N K 2 A

LNP

Rk1

Rk2

Rk2

RP

Fig. 9 Modeling of the autotransformer core winding

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.5 0.8 1

THD

Fig. 10 THD as a function of the Nk1a-Nk1b

coupling

-0.005

0.005

0.015

0.025

0.035

0.045

0.055

0.065

0 0.2 0.4 0.5 0.8 1

THD

Fig. 11 THD as a function of the Nk2aNk2b

coupling

0.03

0.035

0.04

0.045

0.05

0 0.2 0.4 0.5 0.8 1

THD

Fig. 12 THD as a function of the Nk2Np

coupling

-0.01

0.01

0.03

0.05

0.07

0 0.2 0.4 0.5 0.8 1

THD

Fig. 13 THD as a function of the Nk1Np

coupling

0.03

0.035

0.04

0.045

0.05

0.055

0 0.2 0.4 0.5 0.8 1

THD

Fig. 14 THD as a function of the Nk1bNk2a

coupling

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.2 0.4 0.5 0.8 1

THD

Fig. 15 THD as a function of the Nk1aNk2

coupling

I-5I-7

I-11I-13

I-17I-19

I-35I-37

0%

25%

50%

100%

200%

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

[%]

HarmonicOrder

Load Current [%

Fig. 16 Harmonic of Input Current as a function of

output load, balanced supply400Hz, 230V

I-5I-7

I-11I-13

I-17I-19

I-35I-37

0%

25%

50%

100%

200%

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

[%]

HarmonicOrder

LoadCurrent [%

Fig. 17 Harmonic of Input Current as a function of

output load, balanced supply400Hz, 230V

0

2

4

6

8

10

12

14

0 100 200 300 400 500

Volts

Fig. 18 Output common mode voltage of th

ATRU, balanced supply 400Hz and 800Hz,230Vrms

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Fig. 19 Input Currents and voltage for 6Vac

input unbalance

Fig. 20 Common mode voltage for 6Vac

input unbalance

Fig. 21 Output voltage ripple for 6Vac input

unbalance

Fig. 22 Input Currents and voltage for 12Vac

input unbalanceFig. 23 Common mode voltage for 12Vac

input unbalanceFig. 24 Output voltage ripple for 12Vac

input unbalance

IV. EXPERIMENTALRESULTS

In order to validate the proposed circuits, the authors built ascaled-down prototype of the step-down DS-ATRU with

symmetric outputs. Figs. 25-27 show the input phase voltages

Va, Vb and Vc and the created step-down voltages VaD, VbD,

VcD, as well as the leading the input supply by 40 degreesvoltages Vap, Vbp and Vcp, and lagging by 40 degrees

voltages Vapp, Vbpp and Vcpp under no load conditions.

Figs. 28-29 show the voltages Va, VaD, Vap and Vapp under

load conditions, supplied by 400Hz and 800Hz input line. Figs.30-31 show the currents flowing into the rectifier from the

voltages VaD, Vap and Vapp, under load conditions when

ATRU is supplied by 400Hz and 800Hz input line. Lastly,Figs. 32 and 33 show the input current of the ATRU under load

at 400Hz and 800Hz supply. It can be seen that the ATRU

demonstrates 18-pulse rectification of the input voltage, whileproviding necessary step-down of the input voltage.

Fig. 25 Voltages Va, Vad, Vap, Vapp under noload conditions

Fig. 26 Voltages Vb, VbD Vbp, Vbpp under no

load conditions

Fig. 27 Voltages Vb, VbD Vbp, Vbpp under no-

load conditions

Fig. 28 Voltages Va, Vad, Vap, Vapp, 400Hz,

under load

Fig. 29 Voltages Va, Vad, Vap, Vapp 800Hz,

under load

Fig. 30 Currents Iad, Iap, Iapp 400Hz, Voltage

Va, under load

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Fig. 31 Currents Iad, Iap, Iapp, Voltage Va,800Hz, under load

Fig. 32 Currents Ia, Ib, Ic, Voltage Va, 400Hz,under load

Fig. 33 Currents Ia, Ib, Ic, Voltage Va, 800Hz,under load

CONCLUSION

This paper proposes new step-up and step-down Direct

Symmetric 18-pulse ATRU topologies for aircraft application.

Also, it includes comparison of different types of the DS-Topologies and detailed analysis of new step-up and step-down

versions of DS-ATRU. Some key figures and results were

shown in this paper. The authors analyzed the kVA ratings,effects of leakage inductance on compliance with input current

harmonic requirements, power sharing between diode bridges,

common mode voltage and output ripple. The comparison is

carried out for the full range of the input line frequency from400 to 800Hz. Key analyses and results obtained with Saber

simulations are presented for validation of the presented work.

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1. analysis of new step-up and step-down direct sysmmetric 18-pulse topologies for aircraft atru

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