# Patent

## Title
Electrical power distribution system and method with active load control

## Abstract

An electrical power distribution system and a method of implementation are provided. The electrical power distribution system includes: a first AC power source; a second AC power source; a first DC bus; a first AC to DC rectifier; a second AC to DC rectifier; and an electrical load management control unit. The first AC power source is connected to the first DC bus via the first AC to DC rectifier. The second AC power source is connected to the first DC bus via the second AC to DC rectifier. The electrical load management control unit shifts power extraction between the first and second AC power sources by changing an AC voltage output of one of the first and second AC power sources relative to an AC voltage output of the other of the first and second AC power sources, thereby changing an DC voltage output to the first DC bus from one of the first and second AC to DC rectifiers.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a continuation of application Ser. No. 11/199,151, filed Aug. 9, 2005, now U.S. Pat. No. 7,400,065 which claims the benefit of provisional patent application No. 60/603,568 filed Aug. 24, 2004 under 35 U.S.C. § 119(e), which is hereby incorporated by reference.


FIELD OF THE INVENTION
The present invention relates to an electrical power distribution system and method, and more particularly, to an electrical power distribution system and method which implement an electrical load sharing technique among a plurality of power sources.
BACKGROUND OF THE INVENTION
At present, multiple generator and prime mover (e.g., aircraft engine or auxiliary power unit (APU)) sizing methods typically require that the worst case electrical load power extraction requirements be satisfied by both the generators and the prime mover design. In the case of the generators, the total electrical load of the vehicle is summed up over all operational scenarios so as to represent the maximum rated capacity of the generators. For “More Electric” vehicle applications, which have been proposed to shift the primary power sources used for systems and services from pneumatic (engine bleed) and hydraulic sources to electric sources, this generator capacity can be so large that it poses a hardship for the prime mover to provide both the generator input horsepower as well as any other horsepower extraction (e.g., propulsion) tasks that it is required to perform over its operational envelope.
When sizing such prime movers, additional power output downsizing can be achieved in an effort to optimize program objectives (such as weight reduction), but usually at the expense of derating the output of the prime mover to various services under certain operational conditions (e.g., periods of multitasking, high temperature, low ambient air pressure). As a result, some or all of the prime movers typically must be increased in power extraction capacity or otherwise oversized to carry the load of the generators along with other accessories and provide the necessary propulsion. As a result, either or both of the prime movers and the generators must be oversized to ensure that electrical capacity is maintained and that the prime movers operate properly through their mission profile. This oversizing of the prime movers and the electrical system equipment drives up cost and weight for the prime mover and the electrical system and may hence present impediments to the vehicle program goals.
SUMMARY OF THE INVENTION
The present invention overcomes the aforementioned drawbacks by providing an electrical power distribution system which implements an electrical load sharing technique among a plurality of power sources.
According to one aspect of the present invention, an electrical power distribution system comprises: a first AC power source; a second AC power source; a first DC bus; a first AC to DC rectifier; a second AC to DC rectifier; and an electrical load management control unit. The first and second AC power sources are connected to the first DC bus via the first AC to DC rectifier and second AC to DC rectifier, respectively. The electrical load management control unit shifts an power extraction between the first and second AC power sources by changing an AC voltage output of one of the first and second AC power sources relative to an AC voltage output of the other of the first and second AC power sources, thereby changing an DC voltage output to the first DC bus from one of the first and second AC to DC rectifiers.
According to another aspect of the present invention, an electrical power distribution system comprises: a first AC power source; a second AC power source; a first DC bus; a first AC to DC rectifier; a second AC to DC rectifier; and an electrical load management control unit. The first and second AC power sources are connected to the first DC bus via the first AC to DC rectifier and second AC to DC rectifier, respectively. The electrical load management control unit shifts power extraction between the first and second AC power sources by adjusting either or both of a current flowing through the first AC to DC rectifier and a current flowing through the second AC to DC rectifier.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein:
FIG. 1 is a schematic block diagram of an electrical power distribution system in accordance with an embodiment of the present invention;
FIG. 2 is a schematic block diagram of an electrical power distribution system in accordance with another embodiment of the present invention;
FIG. 3 is a schematic block diagram of an electrical power distribution system in accordance with another embodiment of the present invention;
FIG. 4 is a schematic block diagram of an electrical power distribution system in accordance with another embodiment of the present invention; and
FIG. 5 is a schematic block diagram of an electrical power distribution system in accordance with another embodiment of the present invention.



DETAILED DESCRIPTION
On a “More Electric” vehicle, the electrical system output can include a large portion of rectified DC power that is obtained through AC to DC power conversion units from power originally produced and supplied by the engine driven primary AC generators. When the proportion of rectified DC power is significant compared to the total generated AC power, sufficient leverage exists to use electric power flow blended from each generator as a means to share the total electrical load in different proportions from each generator. In this way, the horsepower extraction from each prime mover can be increased or decreased in proportion to the other prime movers. By ensuring that each installed generator capacity can carry the diverted loads and that the dissimilar engine performance specifications are mutually exclusive, shifting the horsepower extraction between prime movers can be accomplished without a reduction in electrical power output.
FIG. 1 is a schematic block diagram of the electrical power distribution system in accordance with an embodiment of the present invention. The electrical power distribution system 100 includes a first AC power source 1a; a second AC power source 1b; and a third AC power source 1c. 
In the illustrated embodiment, the first and third AC power sources 1a and 1c are a first AC generator 10a and a second AC generator 10c, respectively, and the first and second AC generators 10a and 10c are driven by a first aircraft engine 11a and a second aircraft engine 11c, respectively. The aircraft engine 11a or 11c can be, but not limited to, an aircraft propulsion engine, an aircraft turbine engine, an internal combustion engine (diesel, gasoline, propane, natural gas, etc.), or an steam powered engine, etc. In addition, the illustrated embodiment can also apply to any “more electric” land or sea vehicles which may use propulsion engines that are of limited size for the total electrical power extraction necessary for a more electric vehicle.
In the illustrated embodiment, the second AC power source 1b is a third AC generator 10b, and the third AC generator 10b is driven by an engine 11b, such as a third aircraft engine (e.g., an aircraft propulsion engine, an aircraft turbine engine, an internal combustion engine (diesel, gasoline, propane, natural gas, etc.), or an steam powered engine, etc.), an auxiliary power unit, a spool associated with one of the first and second aircraft engines 11a and 11c, etc. In this embodiment, the prime mover associated with the second AC power source 1b has performance characteristics which allow it to carry the electrical load when the first and second aircraft engines 10a and 10c associated with the first and third AC power sources 1a and 1c are unable to do so. It is also possible to allow some relief for power extraction of the second AC power source 1b. 
The electrical power distribution system 100 further includes: a first DC bus 6a; a second DC bus 6b; a first AC to DC rectifier 5a; a second AC to DC rectifier 5b; a third AC to DC rectifier 5c; and a fourth AC to DC rectifier 5d. The first and second AC power sources 1a and 1b are connected to the first DC bus 6a via the first and second AC to DC rectifiers 5a and 5b, respectively. The second and third AC power sources 1b and 1c are connected to the second DC bus 6b via the third and fourth AC to DC rectifiers 5c and 5d, respectively.
The electrical power distribution system 100 further comprises an electrical load management control unit 8. In the illustrated embodiment, upon receiving the vehicle power load change commands from an engine control system (e.g., a computer) 9, the electrical load management control unit 8 shifts the power extraction between the first and second AC power sources 1a and 1b by changing an AC voltage output of one of the first and second AC power sources 1a and 1b relative to an AC voltage output of the other of the first and second AC power sources 1a and 1b, thereby changing an DC voltage output to the first DC bus 6a from one of the first and second AC to DC rectifiers 5a and 5b. The engine control system 9 is responsible for the engine output horsepower regulation and manages the vehicle status including the engine power setting, and the human operator controls (switches, rheostats, position transducers etc) that indicate changes to the electrical power loading which may be independent of computer monitored or generated signals. The vehicle power load change commands are any signals that may increase or decrease engine output power, increase or decrease electrical power demand, turn off or turn on electrical power sources or any relevant vehicle or environmental data that can have an influence on the complete engine power extraction control algorithm.
Similarly, upon receiving the vehicle power load change commands from the engine control system 9, the electrical load management control unit 8 shifts an power extraction between the second and third AC power sources 1b and 1c by changing an AC voltage output of one of the second and third AC power sources 1b and 1c relative to an AC voltage output of the other of the second and third AC power sources 1b and 1c, thereby changing an DC voltage output to the first DC bus 6a from one of the third and fourth AC to DC rectifiers 5c and 5d. 
In the illustrated embodiment, the electrical power distribution system 100 further includes: a first regulation control unit 2a; a second regulation control unit 2b; and a third regulation control unit 2c. The first, second and third regulation control units 2a, 2b, and 2c are connected to the electrical load management control unit 8. The electrical load management control unit 8 regulates the AC voltage output of the first, second and third AC power sources 1a, 1b, and 1c via the first, second and third regulation control units 2a, 2b, and 2c, respectively.
In the illustrated embodiment, the electrical power distribution system 100 further includes: a first AC bus 3a; a second AC bus 3b; and a third AC bus 3C. The first, second and third AC buses 3a, 3b, and 3c are connected to the first, second and third AC power sources 1a, 1b, and 1c, respectively. The first, second and third AC buses 3a and 3b supply the AC voltage output from the first, second and third AC power sources 1a, 1b, and 1c to vehicle AC loads 4.
In addition, the first AC bus 3a supplies the AC voltage output from the first AC power source 1a to the first AC to DC rectifier 5a. The second AC bus supplies the AC voltage output from the second AC power source 1b to the second and third AC to DC rectifiers 5b and 5c. The third AC bus supplies the AC voltage output from the third AC power source 1c to the fourth AC to DC rectifier 5d. 
In the illustrated embodiment, the first DC bus 6a supplies the DC voltage output from the first and second AC to DC rectifiers 5a and 5b to vehicle DC loads 7, and the second DC bus 6b supplies the DC voltage output from the third and fourth AC to DC rectifiers 5c and 5d to vehicle DC loads 7.
In the illustrated embodiment, the electrical load management control unit 8 acquires the DC voltage output of the first and second DC buses 6a and 6b to regulate the AC voltage output of the first, second and third AC power sources 1a, 1b and 1c. 
In order to provide a closed loop control, the electrical load management control unit 8 monitors the output of the DC buses to ensure that the voltage on a specific DC bus is within acceptable limits for the DC equipment attached. In the case that the rectifiers are passive, for a constant input AC voltage, the DC output voltage will reduce as a function of output current. If the electrical load management control unit 8 was operating in an open loop and was changing the regulation control of the AC power sources, it could be done through a succession of regulation adjustments and DC load variations to lower each AC source alternately such that the rectifier DC output fell above or below the acceptable DC limits. In a closed loop, the electrical load management control unit 8 would know whether to trim up or down each AC Power Sources, thereby ensuring that the DC output is within acceptable limits.
In another embodiment shown in FIG. 2, each of the first and second AC to DC rectifiers 5a and 5b is a passive AC to DC rectifier. In this embodiment, each includes of the first and second AC to DC rectifiers 5a and 5b a diode 51a or 51b connected to the first DC bus 6a, as shown in FIG. 2. By changing the AC voltage output of the one of the first and second AC power sources 1a and 1b relative to the AC voltage output of the other of the first and second AC power sources 1a and 1b, a reverse bias on the diode of one of the first and second AC to DC rectifiers 5a and 5b, i.e., diode 51a or 51b, will restrict the DC voltage output to the first DC bus 6a from the one of the first and second AC to DC rectifiers 5a and 5b. 
Similarly, in the embodiment shown in FIG. 2, each of the third and fourth AC to DC rectifiers 5c and 5d is a passive rectifier. In this embodiment, each of the third and fourth AC to DC rectifiers 5c and 5d includes a diode 51c or 51d connected to the second DC bus 6b, as shown in FIG. 2. By changing the AC voltage output of the one of the second and third AC power sources 1b and 1c relative to the AC voltage output of the other of the second and third AC power sources 1b and 1c, a reverse bias on the diode of one of the third and fourth AC to DC rectifiers 5c and 5d, i.e., diode 51c or 51d, will restrict the DC voltage output to the second DC bus 6b from the one of the third and fourth AC to DC rectifiers 5c and 5d. 
For example, assuming that upon receiving the vehicle power load change commands from the engine control system 9, the electrical load management control unit 8 decides to offload the first AC Power Source, the electrical load management control unit 8 instructs the first regulation control unit 2a to reduce its regulated AC voltage output relative to the regulated AC voltage output of the second regulation control unit 2b. In this embodiment, since the first AC to DC rectifier 5a is a passive device, the output DC voltage of the first AC to DC rectifier 5a will decrease. On the other hand, since the second AC power source 1b still keeps outputting the same AC voltage, the DC voltage outputted by the second AC to DC rectifier 5b would be relatively higher than the DC voltage outputted by the first AC to DC rectifier 5a. This will causes a reverse bias on the diode 51a of the first AC to DC rectifier 5a, which restricts the DC voltage outputted from the first AC to DC rectifier 5a to the first DC Bus 6a. 
Therefore, by controlling the AC power sources to increase or decrease the AC voltage output, the voltage change at the DC output of the corresponding AC to DC rectifier will generate a reverse bias on a specific diode of an AC to DC rectifier. Accordingly, electrical power flow and horsepower extraction from the corresponding AC power source can be reduced or discontinued.
FIG. 3 is a schematic block diagram of the electrical power distribution system in accordance with another embodiment of the present invention.
The embodiment shown in FIG. 3 presents a variation of the embodiment shown in FIG. 1. In this embodiment shown in FIG. 3, to reduce the parts count, the two AC to DC rectifiers of the embodiment shown in FIG. 1 connected to the second AC power source 1b are replaced by a dual output AC to DC rectifier 5e in the electrical power distribution system 200. In another embodiment as shown in FIG. 4, the dual output AC to DC rectifier 5e is a passive rectifier and includes a first diode 51e connected to the first DC bus 6a and a second diode 52e connected to the second DC bus 6b. By changing the AC voltage output of the one of the first and second AC power sources 1a and 1b relative to the AC voltage output of the other of the first and second AC power sources 1a and 1b, a reverse bias on one of the first diode 51e of the second AC to DC rectifier 5e and the diode 51a of the first AC to DC rectifier 5a will restrict the DC voltage output to the first DC bus 6a from the one of the first and second AC to DC rectifiers 5a and 5e. Similarly, by changing the AC voltage output of the one of the second and third AC power sources 1b and 1c relative to the AC voltage output of the other of the second and third AC power sources 1b and 1c, a reverse bias on one of the second diode 52e of the second AC to DC rectifier 5e and the diode 51d of the third AC to DC rectifier 5d will restrict the DC voltage output to the second DC bus 6b from the one of the second and third AC to DC rectifiers 5e and 5d. 
Therefore, by controlling the AC power sources to increase or decrease the AC voltage output, the voltage change at the DC output of the corresponding AC to DC rectifier will generate a reverse bias on a specific diode of an AC to DC rectifier. Accordingly, electrical power flow and horsepower extraction from the corresponding AC power source can be reduced or discontinued.
Another application of the concepts described above is to use an active rectifier in place of the passive AC to DC rectifier. In this embodiment, the active rectifier can directly affect the degree of power flow through itself and minimize the amount of AC Power Source voltage change necessary or eliminate it entirely.
The embodiment in FIG. 5 provides a more sophisticated method of achieving the same end with more dedicated and accurate power blending control. The active rectifiers 5f, 5g, 5h and 5i are shown in one possible configuration in FIG. 5 that combines a power section that provides the power switching circuits and devices (5f1, 5g1, 5h1, 5i1) and switching device driver control electronics, regulation and protection logic (5f2, 5g2, 5h2, 5i2). In other possible configurations, the power section and the control section may be realized as separate units communicating through data buses or discrete connections. In another more integrated solution, the electronic controls of the active rectifier may be incorporated in the electrical load management control unit with the switching device driver outputs connected only to the power section. Either architecture described or other physical arrangements of controls that can be envisioned do not change the fundamental controls or the intention of power blending scheme described. In such a scheme the electrical load management control unit 8 does not modify the regulation control unit for each AC generation source as in FIGS. 1-4. Previously this was required since the AC to DC rectifier was passive and the only opportunity to vary the current flow was to effect the regulation of the upstream generator regulation control. However, with the active rectifier, control electronics already exist as part of the active rectifier concept such that the electrical load management unit 8 can maintain a direct connection to the current and voltage regulation controls of each active rectifier. In monitoring the voltage and current from each active rectifier as well as the electrical power system and engine status of the aircraft, the electrical load management control unit 8 can adjust the flow of current through each active rectifier individually or in combination to obtain the desired engine power extraction and electrical power supply. Functionally, an active rectifier is more efficient because instead of reverse biasing the passive rectifiers by varying the AC source output voltage to switch off a particular input AC channel, and then performing two power conversion stages, the active rectifier uses switching semiconductors to synthesize (chop) and filter the input AC power signal to provide a constant DC output voltage. This results in a higher efficiency (less weight, volume and heat generation) since the power required at any time only passes through one conversion operation instead of being subjected to individual transformer and rectification stages. Simplicity is also introduced by allowing the generator regulation control units to maintain a constant voltage output for the general aircraft buses. Speed of response is also improved since the generators and transformers of the AC to DC rectifier naturally exhibit electromagnetic inertia in response to a regulation control input change and therefore, by affecting the active rectifier directly, this inertia can be minimized. The electrical load management control unit 8 therefore would directly vary the degree of DC power synthesis within the active rectifier to regulate the amount of power that would flow through any particular rectifier. Hence, this would result in the restricting or passing of electrical power in the appropriate proportion to the other rectifiers with respect to the permissible engine power extraction and the electrical power demand as determined by the electrical load management control unit. Therefore the desired blending effect as principally described by this patent earlier using passive rectifiers is similarly achieved using an active rectifier approach.
Although FIGS. 1-5 illustrate a plurality of discrete elements for the electrical load management control unit and the multiple regulation control units, it should be recognized that this illustration is for ease of explanation and the associated control functions can be combined in one or more elements, including by implementing software, hardware, ASIC, etc., based techniques.

## Claims

1. A method of sharing an electrical load in an electrical power distribution system, the method comprising:
providing a first AC power source, a second AC power source, a first DC bus, and a second DC bus;
connecting the first AC power source to the first DC bus via a first AC to DC rectifier;
connecting the second AC power source to the first DC bus via a second AC to DC rectifier;
connecting the second AC power source to the second DC bus via the second AC to DC rectifier;
extracting DC power simultaneously from the first and the second DC buses to satisfy a total DC load requirement;
monitoring an amount of DC power extracted from first and the second DC buses;
determining a desired proportion of the total DC load requirement that is to be provided by each of the DC buses;
changing a proportion of the total DC load provided from the first DC bus by changing an AC voltage output of the first AC power source relative to an AC voltage output of the second AC power source, to correspondingly change DC voltage on the first DC bus relative to DC voltage on the second DC bus and thereby partially shift power extraction between the first and second AC power sources.

2. The method of claim 1, further comprising the step of:
applying a reverse bias on a diode of the second AC to DC rectifiers, thereby restricting DC voltage output to the second DC bus from the first AC to DC rectifier.

3. The method of claim 1, further comprising the steps of:
acquiring the DC voltage output of the first DC bus; and
regulating the AC voltage output of the first AC power unit relative to the AC voltage output of the second AC power sources via a first regulation control unit and a second regulation control.

4. The method of claim 1, wherein the step of providing the first power source and the second power source comprises:
driving a first AC generator with a first spool of a first aircraft engine; and
driving a second AC generator with one of;
a second aircraft engine,
an auxiliary power unit, and
a second spool of the first aircraft engine.

5. An electrical power distribution system, comprising:
a first AC power source;
a first regulation control unit for controlling AC voltage output of the first AC power source;
a second AC power source;
a second regulation control unit for controlling AC voltage output of the first AC power source;
a first DC bus;
a second DC bus;
a first AC to DC rectifier, the first AC power source being connected to the first DC bus via the first AC to DC rectifier;
a second AC to DC rectifier, the second AC power source being connected to the first DC bus via the second AC to DC rectifier;
the second AC power source being connected to the second DC bus via the second AC to DC rectifier; and
an electrical load management control unit interconnected to provide desired output-voltage commands to the first and the second regulation control units;
the electrical load management control unit interconnected to the first and second DC buses to determine DC voltages on the first and second DC buses;
the electrical load management control unit varying DC voltage on the first and second DC buses by varying AC output voltage of the first and second AC power sources with commands to the first and second regulation control units;
the electrical load management control unit providing a desired allocation of a total DC load to the first and second DC buses by varying the DC voltage on the first and second DC buses.