A three phase, high voltage, high frequency distributed bus system for advanced aircraft is discussed. A system model is given. Available components are described. Recommendations are given

Finke, R. C.

English

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. N84 I0o0:" "
THREE-PHASE, HIGH-VOLTAGE, HIGH-FREQUENCYDISTRIBLITED
BUS SYSTEM FOR ADVANCED AIRCRAFT
Robert C. Finke
National Aeronautics and Space Administration a
Lewis Research Center
Cleveland, Ohio 44135
The NASA Lewis Research Center is developing higher voltage, higher fre-
quency components for spacecraft. The technology is directly applicable to
aircraft power-generatingsystems except that aircraft systems are about an
order of magnitude 1_rg_r than spacecraft systems.
For th_s paper the followin_ _ umptions were used: The system is gen-
eric. Everything is powered from the generators, if all the switches were
turned on at once, the load would be 500 kVA. The distance from genu,:_or to
load tan be 250 ft. All generators and motors are i_hement!y alternating-
current machines. There is no such thing as a brushless direct-currentmotor.
It's an alternating-currentmotor with electronic commutation. There is also
no such thing as a direct-currentgenerator. Alternating-curr_ntgenerators
with rectifiers are used. All of the systems are inherentlyalternating cur-
rent, and anything else requires hardware to be added to the system.
Figure 1 shows the system model. The model is simple: It ha: a generator
and some level of current and voltage. The current and voltage multiply to-
gether to give the total power. All of the diodes, transistors, etc., are
represented in the circuit by a forward volta§e drop across the diode. There
is also resistance of the line, connectors, etc. And finally, the load.
Forward voltage drop in semiconductorsis relatively constant. It varies
from about i to 2 V depending on whether the semiconductor is a si,icon-
controlled rectifier or a transistor and on what kincs of voltages and currents
are going through it. But if _here are many diodes in _he line, as the system
voltage goes down, the wattage lost through forward voltage drop goes up (fig.
: 2). In a system with a large number of semiconductorsthe loss curve begins
to flatten out around 300 V. That is, the system losses due to all the semi-
conductors start getting appreciably lower.
For cable with a 2-percent loss, system weight increases significantly
(fig. 3). At about 50 V it becomes excessive. For this reason, and because
the semiconductor losses are lower there, high voltages would be preferred.
To be able to use higher voltages in power systems, semiconductordevices with
adequate ratings must be available.
The following components are available for use in aircraft p_wer-
generating systems:
(I) High-current power-switchingtransistor DYST (fig. 4). The DYST is
the successor to the D6OT, which Westinghouse is now producing under a NASA
contract. The D7ST features are a voltage of 400 to 500 V at a current up to
150 A, with 400-A peak, and power of 50 kW. Its rise and fall times are 0.75
; usec with a 4-psec storage time. It can therefore be used in a relatively
• | high-frequencyconverter to switch high power levels. Although it is a fairly i_
i,_ impressivedevice, it can be improved.!
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(2) Augnmnt_d pnwer transistor. This transistor is in the development
stage. The contract _pecifications are a voltage of 800 to 1000 V, current of
70 to 112 A (gain of I0) with a 400-A peak, power handling of 75 kW, power
dissipation of 1.25 kW at 75" C, rise and fall times of 0.5 usec, and storage
time of 2.5 _sec. This device can also be used in power inverters or switches
at high vo]tage and high power.(3) Fast-recovery, high-voltage power diode. A 1200-V diode with a 50-A
forward current (fig. 5) has been developed by Power Transistor Corp. under
contract to NASALewis it is a fast diode, with reverse recovery time of 200
nsec. The applicat_uns in a system are myriad and include rectification, pro-
tection, filters, and clippers.
For higher-voltage systems, up to i000 V, corona breakdown could be a
problem. In a Boeing report done for Wright-Patterson AFB by William Dunbar,
corona initiation voltages ranged from 1200 V at 40 000 ft to 400 V at 80 000
: ft (fig. 6). With commercial aircraft flyin9 around 40 000 ft, the higher
voltages look pretty reasonable for power distribution.
Figure 7, produced by James Triner of NASA Lewis, shows efficiency and
weight versus frequency for a 50-kVA transformer with advanced materials. As
with voltage, as frequency increases Lhere is a significant drop in weight and
arouhd I0 kHz the curve starts to flatten out. However, at that frequency the
_- efficiency has alredy begun to decrease. A little under I0 kHz is a good fre-
quency at which to do inversion and perhaps distribution because the magnetics
are maintaining their efficiency but th_'_r weight is substantially reduced.
Figure 8 is a comparison of transformers for space and commercial uses.
Both are 25-kCA, single-phase transformers. The co_,mrcial pole transformer
weighs 400 lb, is 97.9 percent efficient, has a voltage of 120/240 V, and has
a high voltage of 3850 V. The space tra_scor_,_r has the same power-handling
capacity but at a frequency of 20 kHz instead of 60 Hz. That makes a sizeable
difference. And it has also been designed extremely carefully from a thermal
standpoint. The pie windings are shaped and fastened to aluminum plates for
heat removal. Another paper ir this conference, by Gene Schwarze of Lewis,
describes this transformer technology and its characteristics. The weight
difference is also significant - 400 lb at 60 Hz as compared with 7 lb atm
, 20 kHz. The space transformer is also more efficient (99.2 versus g7.g per- :
cent) and has a lower temperaturerise.
The type of distributionsystem selected for an aircraft power system can
have a substantial effect on the conductor weight as shown in table I. A two-
wire, direct-currentsystem has been selected as the base and allotted a weight
• of 100 percent. A very substantialweight reduction can be had by going to
the three-wire Edison system. The two three-phase, alternating-currentsystems
both exhibit a weight savings over the base. However, the three-phase, four-
wire wye shows the ]argest gain even with a full-size neutral.
NASA Lewis has done research on remote-power-controllertechnology. The
driver for this technology, however, is the semiconductordevice, so we first
need big enough, fast enough, semiconductorsto make and break the circuit.
i Figure 10 shows early solid-state, remote power controllers (RPC's) developed
._ for use in the space shuttle. These are 28-V RPC's at various sizes, 3 to 5 A
• : up to 15 to 20 A. The next generation of remote power controllers (fig. Ii)
_.: have voltages up to 120 V dc and currents to 30 A. They were developed by _
,: Westinghouse.
i
1984001988-029
.4
Figure 12 shows a Lewis-developedhigh-voltage, high-power circuit .
breaker. The figure shows the breadboarl and a schematic of the system. The
load-switchingtransistor has been the tecnnology driver. Only recently have
1-kV transistors been developed to go into this circuit. But this circuit
breaker has been built and tested. It has been used primarily with the elec-
tric propulsion systems. Ion thrusters use a 1-kV screen drive at several
amperes. And this circuit breaker has been used as the main bus-interruptfor
electrical propulsion systems. So it has been tested in that sort of appli-
cation and now the technology is ready for other applications.
The following characteristicsare recommendedfor aircraft power-
generating systems:
(I) High voltage distribution. High voltage of about 200 to 300 V re-
duces conductor weight and reduces the effect of semiconductordrops. This
: provides a lighter and more efficient power system. Corona does not appear to
be a problem at these voltages for any aircraft.
(2) Alternating-currentdistribution. Since all rotating machinery
generates or consumes ac power, it appears to be most efficient and simple to
distribute the power in that form. In addition, it is much easier to interrupt
alternatingcurrent than direct current.
(3) High frequency distribution. Frequencies in the range of 10 kHz
cause a substantialreduction in the weight of magnetics while still providing
a relatively high efficiency.
(4) Multiphase distributien. A three-phase distribution system provides
a substantialreduction in conductor weight. It can also be very low in
radiation.
i
TABLE I.- EFFECTOF SYSTEMARCHITECTURE
ON CONDUCTORWEIGHT
System Weight,
percent
of basellne
t . Two-wiredc (baselinesystem) 100
Three-wlreEdison 37.5L
: I Three-phasedelta 75
;_I Three-phase,four-wirewye 33.3
(neutral, full size)
1
1984001988-030
,_ ORIGINAL PAGE ISOF POOR QUALITY
,500kVA
Generatedvoltage
T Current
I Forwzrdvoltaicdrop
_Oft
I I Lineresistance
FigureI. - System odel.
I h....T ....... j1
FigureZ - Lossduetoformnl_tageclrgtasafuncllonof
systemvoltup.
15,1o3
b"_I_ OF pOOR QuALrT'Y •
_.: Oi_IC-IINALpAr_,_Ig
OF POOR QU[
": Ih
i
t
32 ;"
1984001988-033
qk
=b ORIGINAL pAGE |g
OF pOOR QUALITY
=o/ t, I I
40 (_ 80 100zl03
Altitude.M
Figure6. - C_onaInlt_Ionv_tageasafunctl_nof
altitude.
33
1984001988-034
ORI_NAL P::,,'_-_i",
OF POOR QUALFa'Y
)
,i
-'4 .... &__. ---.,--- _ lu
t
" 25 XVA TRANSFORMFR
_..__. , , SINGLE PI_ASE
TTI NASA teRC COMMFHf,IAL
W[ICJHT _l BS_ 7 100
EFFICIENCY I' ,) 99 2 97 9
I FEMPERA TUBE RISE IC} 50 85
'_ FREQUENCY_HZ_ 20 000 b0
LOw vOLTAGF _VOL TSp ?OO 120 240
NIGHVOL'AGFFV()LTS_ 1500 ,,_850 415(3 14713
Figure 8. - Con.Rarison of space and commerical transformers,
IF. ORI_,_L ;_c_7 i,j
OF POOR (_UAL[I_I' ..
II1
D " L
Figure 10. - Next generation of remote power controllers.
t _KV.2sA D.C.
SOLIG _TATE CIRCUIT
BREAKER BREAG:OARD
DNbVP L;_L)IT CAPACITOCq
y
/. ";-'m, * OPTICALLY IBOLATED
_-' , i ,{"_;IPcoNr_o_=_,c .,oucTo_ * PROGRAMFD TRAN81ENT
PROTECllON
•¢-_L=o,wIT.STAN=._ i
c,.cuIT.EQu,._.FO S.Om
MULTI HUNDRED KILOWATT
POWER MANAGEMENT
F|gure ]]. - H|gh-vo]tage, fltgh-power C|,cutt. ! t
t
35 '
1984001988-036


Three-phase, high-voltage, high-frequency distributed bus system for advanced aircraft

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