Electrical power generating system producing alternating and direct current

The present invention is an electrical power generating system and method of operation. An inverter (18) is responsive to a DC potential for producing a fundamental frequency alternating current output such as 400 Hz. three-phase AC including harmonics. A first harmonic suppression filter (22), coupled to the alternating current output, produces an output having reduced harmonics compared to the harmonics in the input of the inverter. An isolation transformer (61) has a primary coupled to an output (24) of the first harmonic filter, a first secondary for outputting the alternating current at the fundamental frequency, and a second secondary for also outputting the alternating current at the fundamental frequency. A first rectifier (65) is coupled to the output (63) of the first secondary and produces direct current including harmonics produced by the rectification. A second harmonic suppression filter (69), coupled to the output (67) of the rectifier, outputs direct current having reduced harmonics compared to the harmonics in the output of the fullwave rectifier.

TECHNICAL FIELD 
The present invention relates to electrical power generating systems which 
produce alternating current and direct current utilizing an inverter and 
more particularly, to systems of the aforementioned type which minimize 
the number of harmonic suppression filters and transformers utilized 
therein. 
BACKGROUND ART 
FIG. 1 illustrates a block diagram of an electrical power generating system 
of the type used on airframes for producing 400 Hz. three-phase 
alternating current with an inverter for supplying electrical power to 
various alternating current loads and further 28 volt DC used for 
maintaining potential on batteries therein. In an airframe using 
electrical power generating systems of the aforementioned type, which are 
known as variable speed constant frequency (VSCF), each of the propulsion 
engines is provided with a so-called wild frequency generator which 
generates three-phase alternating current having a frequency proportional 
to the shaft speed of the propulsion engine. A three-phase variable 
frequency alternator 10, which is powered by a propulsion engine, produces 
variable frequency electrical current on an output 12. The variable 
frequency three-phase alternating current on output 12 is applied to a 
rectifier 14 which produces direct current on an output 16. The direct 
current on the output 16 applied to a DC to a three-phase 400 Hz. inverter 
and control 18 of conventional construction which includes a plurality of 
power semiconductor switches for producing the respective 400 Hz. phases. 
The three-phase 400 Hz. output 20 is applied to a harmonic suppression 
filter 22 of conventional design for the purpose of reducing odd voltage 
harmonics which are present in the output 20 from the three phase 400 Hz. 
inverter and control 18. The output 24 of the harmonic suppression filter, 
which contains reduced harmonics in comparison to the input 20 from the 
three-phase 400 Hz. inverter and control, is applied to an isolation 
transformer 26. The magnetic flux coupling between the primary and 
secondary of the isolation transformer 26 provides DC electrical isolation 
on the output 30 which is connected to the secondary thereof. A contactor 
32 of conventional construction is controlled by the control portion of 
the three-phase 400 Hz. inverter and control 18 for the purpose of 
disconnecting the transformer 26 output 30 from the 400 Hz. three-phase 
bus 34 which is representative of the alternating current power conducting 
structure present on an airframe. The 400 Hz. three-phase bus 34 has 
several outputs, one of which is an output 36 which is connected to a 
circuit breaker 38 which functions to disconnect the 400 Hz. three-phase 
bus 34 from connection to a transformer rectifier unit (TRU) 40 as 
described below in conjunction with FIG. 2 which produces an output 
voltage of 28 volts DC which provide electrical power to various 28 volt 
DC loads. 
FIG. 2 illustrates a block diagram of the TRU 40 of FIG. 1. An alternating 
voltage input 50 is applied to an input filter 52 which removes 
undesirable harmonics present in the alternating input current. The output 
54 of the input filter 52 is applied to voltage step down and isolation 
transformer 56. The alternating current present on the output 58 of the 
isolation transformer is applied to a rectifier 60 which produces direct 
current on output 62 which contains undesirable voltage harmonics. A 
harmonic suppression filter 64 reduces the undesirable harmonics present 
in the output of the fullwave rectifier 62 and produces a DC output 66 
which is at 28 volts. 
The addition of a TRU of FIG. 2, which is a stand alone system, to an 
airframe electrical power generating system by connection to the 400 Hz. 
three-phase bus has disadvantages. In the first place, the TRU requires an 
input filter 52 and isolation transformer 56 which add substantially to 
TRU weight and additional cost to the electrical power generating system 
of an airframe which requires both 400 Hz. three-phase AC and 28 volts DC 
for providing the DC potential of the batteries in the airframe. Reduction 
of weight, complexity and cost are of crucial importance to improving 
electrical power generating systems on airframes. 
See U.S. Pat. Nos. 4,103,325, 4,587,436, 5,181,169, 5,291,119, 5,404,093 
and 5,418,708 for descriptions of direct current electrical power 
generation. U.S. Pat. No. 4,103,325 discloses an aircraft power and phase 
converter including 28 volt DC. U.S. Pat. No. 4,587,436 discloses 28 and 
270 volt DC electrical power generation. U.S. Pat. No. 5,181,169 discloses 
conversion of 270 volt DC to 28 volt DC and conversion of 28 volt DC to 
270 volt DC. U.S. Pat. Nos. 5,291,119 and 5,404,093 disclose auxiliary DC 
outputs. U.S. Pat. No. 5,418,708 discloses conversion of 270 volt DC to 28 
volts DC. 
DISCLOSURE OF THE INVENTION 
The present invention is an improved electrical power generating system and 
method of electrical power generation which eliminates the input filter 52 
of the TRU 40 of FIG. 2 and the isolation transformer 56 of the TRU of 
FIG. 2. As a result, when it is desired to generate 28 volts DC or another 
DC potential from 400 Hz. three-phase AC or another alternating current 
potential which is generated with an inverter, it is unnecessary to 
utilize a separate input filter and additional isolation transformer which 
have been used in TRU described above in conjunction with FIG. 2. 
Furthermore, the present invention may be used to generate other DC 
potentials, such as 270 volts DC used in airframes, from another output of 
the rectifier used for producing DC which is applied to the three-phase 
400 Hz. inverter and control of the VSCF electrical power generating 
system in an airframe. As a result of eliminating the aforementioned 
filter 52 and a transformer 56 of the TRU 40 for generating 28 volt DC in 
an airframe, the number of components, cost and complexity of a hybrid 
electrical power generating system for generating both alternating current 
and direct current on an airframe is reduced from that using a TRU which 
is connected to the regulated three-phase 400 Hz. bus of VSCF electrical 
power generating system.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 3 illustrates a block diagram of an electrical power generating system 
in accordance with the present invention. Parts identical to the prior art 
system of FIG. 1 will not be discussed in detail. The embodiment of FIG. 3 
differs from the embodiment of FIG. 1 in that the transformer rectifier 
unit 40 of FIG. 1 has been replaced with a simplified system which 
eliminates the input filter 52 of the TRU and further combines the 
isolation transformer 56 of the TRU with the isolation transformer 61 
which performs the same function as in the prior art isolation transformer 
26 of FIG. 1 to produce a first output 30 from a first secondary. In 
addition, the isolation transformer 61 contains a second secondary for 
outputting three-phase 400 Hz. alternating current on an output 63 through 
contactor 72 to a rectifier 65 for producing DC on an output 67 which 
includes harmonics produced by the rectification. The output 67 is applied 
to a harmonic suppression filter 69 of conventional construction which 
outputs on output 70 direct current having reduced harmonics compared to 
the harmonics in the output 67 of the rectifier 65. The contactor 72 is 
controlled by the control of the three-phase 400 Hz. inverter and control 
18 to disconnect the output 63 from the 28 volt DC bus 74 when a fault 
occurs which could cause damage or failure of the solid state switches in 
the three-phase 400 Hz. inverter and control 18. 
A comparison of the prior art for generating both 400 Hz. three-phase AC 
and 28 volt DC with the present invention reveals that the present 
invention eliminates the necessity for an input filter 52 which functions 
in the same manner as the harmonic suppression filter 22 and further, the 
isolation transformer 56 the function of which is now performed by adding 
a second secondary to the isolation transformer 61. As a result, the 
number of components required to generate three-phase 400 Hz. AC and 28 
volt DC in comparison to the prior art has been reduced, the weight of the 
electrical power generating system has been reduced and further, the 
complexity and cost are reduced. The present invention has a preferred 
application in VSCF electrical power generating systems in an airframe but 
it should be understood that the present invention is not limited thereto 
and has applications where ever simplified hybrid generation of both 
alternating current and direct current is required utilizing an inverter 
as the source of the fundamental frequency of the alternating current and 
further the alternating current is rectified to produce the direct 
current. 
As illustrated in FIG. 3, the second output 76 from the rectifier 14 may 
output 270 volt DC which is filtered by a harmonic suppression filter 78 
to reduce the presence of harmonics present on the output 80 which is a 
270 volt DC power bus for loads in airframes or other applications. 
While the invention has been described in terms of its preferred 
embodiment, it should be understood that numerous modifications may be 
made thereto without departing from the spirit and scope of the invention. 
It is intended that all such modifications fall within the scope of the 
appended claims.