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