Patent Description:
Vehicles, such as aircraft, often include electrical power systems with an on-board generator that converts rotational movement from an engine to electrical power. The generated electrical power is used to power on-board electrical components such as flight controls (e.g., ailerons and rudders), sensors, and/or other on-board electrical devices. These items collectively act as a baseline electrical load that requires a baseline amount of electrical power.

The vehicle may also include pulse loads that require supplemental power, causing a spike in the overall load of the electrical system. Some example pulse loads could include a radar or a directed energy weapon. On-board energy storage devices that charge from the generator can be used to provide supplemental power for pulse loads. <NPL>, describes a coordinated control strategy for a hybrid energy storage system (HESS) to stabilize DC bus voltages in a bipolar type DC grid.

An example electrical power system includes a direct current (DC) bus including a positive rail configured to provide a positive DC voltage, a negative rail configured to provide a negative DC voltage, and a ground rail. A first energy storage module (ESM) and a second ESM are connected to the DC bus. Each ESM includes an energy storage device. The first and second ESM are connected to and configured to provide an output voltage to a respective one of the positive and negative rail. A node connects the first and second ESMs to each other and to the ground rail. A controller is configured to determine values for the output voltages for use during at least one of a discharging mode in which the energy storage devices discharge onto the DC bus and a charging mode in which the energy storage devices charge from the DC bus based on a difference between a state of charge value of each of the first and second energy storage devices.

An example method of operating an electrical power system includes measuring a state of charge of each of a first energy storage device of a first ESM and a second energy storage device of a second ESM. The first ESM is connected to and configured to provide an output voltage to a positive rail of a DC bus that is configured to provide a positive DC voltage. The second ESM is connected to and configured to provide an output voltage to a negative rail of the DC bus that is configured to provide a negative DC voltage. A node connects the first and second ESMs to each other and to a ground rail of the DC bus. Values are determined for the output voltages for use during at least one of a discharging mode in which the energy storage devices discharge onto the DC bus and a charging mode in which the energy storage devices charge from the DC bus based on a difference between a state of charge value of each of the first and second energy storage devices.

<FIG> is a schematic view of an example aircraft <NUM> that includes an electrical power system <NUM>. The electrical power system <NUM> includes a generator <NUM>, AC/DC converter <NUM>, and DC bus <NUM>. The generator <NUM> converts rotational movement (e.g., of a rotor of a gas turbine engine) to electrical power. The AC/DC converter <NUM> converts an alternating current (AC) voltage from the generator <NUM> to a direct current (DC) voltage, and provides the DC voltage to the DC bus <NUM>.

A baseline load <NUM> and one or more pulse loads <NUM> are powered over the DC bus <NUM>. The baseline load <NUM> could include flight controls, sensors, and/or other electrical devices of the aircraft <NUM>, for example. The pulse load <NUM> could include one or more of a radar device and a directed energy weapon, for example.

The electrical power system <NUM> includes a plurality of energy storage modules (ESMs) 24A-B that are configured to charge from the DC bus <NUM> in a charging mode, and to act as supplemental power stages by providing supplemental power to the DC bus <NUM> in a discharging mode when the one or more pulse loads <NUM> are utilized. Each pulse load <NUM> needs a relatively large amount of power over a relatively short time duration, and the ESM <NUM> is able to meet that power demand. Although only two ESMs 24A-B are shown in <FIG>, it is understood that additional ESMs could <NUM> be used if desired.

<FIG> is a schematic view of an example of the power control system <NUM>. In the example of <FIG>, the DC bus <NUM> is a split DC bus that includes a positive rail 30A that provides a positive voltage, a negative rail 30B that provides a negative voltage, and a ground rail 30C. A node N1 connects the ESMs 24A-B to each other and to the ground rail 30C. Thus, the ESMs 24A-B can be described as being stacked in series. Also, a resistor R1 is connected between the node N1 and the ground rail 30C. The resistor R1 limits a flow of current from the node N1 to the ground rail 30C.

In one example, the magnitude of positive voltage on positive rail 30A and the magnitude of the negative voltage on the negative rail 30B are approximately the same (e.g., on the order of <NUM> volts). In such an example, the DC bus <NUM> can be said to provide a voltage of ± <NUM> volts, or a "total bus voltage" of <NUM> volts.

In the case of a vehicle such as an aircraft, the ground rail 30C may be a neutral rail that acts as a ground. In such an example, the ground rail 30C could connect to a chassis of the vehicle or a neutral terminal of the generator <NUM>, for example.

Each ESM <NUM> includes at least one energy storage device <NUM>, a DC/DC converter <NUM>, and a power filter <NUM>. The energy storage devices <NUM> can include a variety of devices, such as batteries, fuel cells, and ultracapacitors, for example. Ultracapacitors are high-capacity capacitors that can typically store <NUM> - <NUM> times more energy per unit volume or mass than electrolytic capacitors. In one example, each ESM 24A-B includes the same type of energy storage device <NUM>.

Certain types of energy storage devices <NUM> are well-suited for certain types of pulse loads. For example, ultracapacitors are well-suited for the power requirements of radar pulse loads, whereas batteries are well-suited for the energy requirements of laser pulse loads.

The DC/DC converters <NUM> provide an output voltage from their respective energy storage devices 40A-B to the DC bus. In particular, DC/DC converter 42A provides an output voltage to positive rail 30A, and DC/DC converter 42B provides an output voltage to negative rail 30B. As used herein, the "output voltage" of an ESM <NUM> refers to the output voltage of its DC/DC converter <NUM> to the DC bus <NUM>.

The DC/DC converters <NUM> are operable as buck/boost converters to adjust the voltage on the DC bus <NUM> with respect to the voltage of their respective energy storage device <NUM>.

In one example, the voltages of the energy storage devices <NUM> have a lower magnitude than that of the DC bus. As an example, the DC bus <NUM> may have a voltage of ±<NUM> volts (i.e., approximately +<NUM> volts on positive rail 30A and -<NUM> volts on negative rail 30B), and each energy storage device <NUM> may have a voltage on the order of <NUM> volts. In such an example, the DC/DC converters <NUM> operate in a buck mode when charging the energy storage devices <NUM> and operate in a boost mode when discharging the energy storage devices <NUM>.

In the buck mode, the DC/DC converters <NUM> convert the larger magnitude DC voltage from the respective rail <NUM> to which the ESM <NUM> is connected to a lower magnitude DC voltage for charging the energy storage device <NUM>. This is also known as "sinking" current from the DC bus <NUM>.

In the boost mode, the DC/DC converters <NUM> convert the lower magnitude voltages of the energy storage devices <NUM> to the higher magnitude voltages of the respective rail <NUM> to which the ESM <NUM> is connected for discharging the energy storage devices <NUM> onto the DC bus. This is also known as "sourcing current" to the DC bus <NUM>.

Power filters 44A-B provide for electromagnetic interference (EMI) filtering when charging or discharging the energy storage devices <NUM>.

A controller <NUM> controls an output that each DC/DC converter <NUM> provides to the DC bus <NUM> to control whether the ESMs <NUM> are in the discharging mode or the charging mode. If the sum of magnitude of the output voltages (e.g., <NUM> volts from ESM 24A and -<NUM> volts from ESM 24B, for a "total ESM voltage" of <NUM> volts) is the same as the total DC bus voltage (e.g., <NUM> volts), the ESMs <NUM> will be "off" (i.e., connected to and providing a voltage to the DC bus <NUM>, but neither charging nor discharging). If the sum of the magnitude of the output voltages is greater than the total DC bus <NUM> voltage, the ESMs <NUM> enter the discharging mode and will provide power to the DC bus <NUM> for discharging. Conversely, if the sum of the magnitude of the discharge voltages is less than the total DC bus voltage, the ESMs <NUM> will enter the charging mode and will draw power from the DC bus <NUM> for charging.

Because the voltage on the DC bus <NUM> may vary in actual operational conditions (e.g., it may actually be <NUM> volts or <NUM> volts instead of <NUM> volts), the controller <NUM> monitors the DC bus <NUM> voltage and adjusts the ESM <NUM> discharge voltages to be either above or below the DC bus <NUM> voltage based on whether the controller <NUM> wants the ESMs <NUM> to charge, discharge, or be connected in the off state in which the ESMs <NUM> are neither discharging nor charging. A "midpoint" refers to DC/DC converter <NUM> output voltage values that match that of the DC bus <NUM>, such that the ESMs <NUM> are in the off state.

The controller <NUM> could include a microprocessor, application-specific integrated circuit (ASIC), or the like, for example.

Each energy storage device <NUM> has a "state of charge" (SoC) which refers to a charge level of the energy storage device <NUM>, and is akin to a fuel gauge for an energy storage device <NUM>. A SoC is generally measured as a percentage, where a SoC of <NUM> means that an energy storage device <NUM> has no charge and is "empty" and a SoC of <NUM> means that an energy storage device <NUM> is fully charged and is "full.

During operation of the electrical power system <NUM>, it is possible that the SoC values of the energy storage devices <NUM> may diverge. This can occur due to variations in efficiency between the ESMs <NUM>, for example. If left uncorrected, this divergence can lead to an overvoltage fault condition within one of the ESMs <NUM> due to overcharging, and could also lead to undercharging of another of the ESMs <NUM>. The controller <NUM> is operable to determine a difference between a SoC of each of the energy storage devices <NUM> using respective sensors 52A-B, and to determine values for the DC/DC converter <NUM> output voltages for either of the discharging and charging modes based on a difference between the SoC values of the energy storage devices 40A-B. The values are determined such that when they are implemented, the difference between the SoC values will decrease.

<FIG> summarizes an example method of operating the electrical power system <NUM> in a flowchart diagram <NUM>. The controller determines a difference between the SoC of ESM 24A and the SoC of ESM 24B at <NUM>, and determines whether a difference between the SoC values is greater than a predefined threshold at <NUM>. In one example, the predefined threshold is a difference of <NUM>% between the SoC values.

If the difference is less than the threshold (a "no" at <NUM>), the controller <NUM> uses default values for the output voltages at <NUM>. As used herein, "default values" means that the magnitude of the output voltage for each of the ESMs <NUM> is approximately the same as one another, whether in the charging mode, discharging mode, or off state.

If the SoC difference is greater than the predefined threshold (a "yes" at <NUM>), the controller <NUM> determines whether the ESMs 24A-B are in the charging or discharging mode. The controller <NUM> then adjusts the respective DC/DC converter <NUM> output voltages for the ESMs <NUM>.

In particular, if the ESMs <NUM> are discharging, the controller <NUM> increases the output voltage for the ESM <NUM> having the higher SoC value, and decreases the output voltage for the ESM <NUM> having the lower SoC voltage at <NUM>. This forces the ESM <NUM> having the higher SoC value to provide more power to the DC bus <NUM> than the ESM <NUM> having the lower SoC value.

Conversely, if the ESMs <NUM> are charging, the controller <NUM> increases the output voltage for the ESM <NUM> having the lower SoC value, and decreases the output voltage for the ESM <NUM> having the higher SoC voltage at <NUM>. This forces the ESM <NUM> having the lower SoC value to draw more power from the DC bus <NUM> than the ESM <NUM> having the higher SoC value.

Either of the adjustments <NUM>, <NUM> causes the difference between the respective SoC values of the ESMs <NUM> to decrease.

In one example, the increase and decrease implemented in either of the adjustments <NUM>, <NUM> have a same magnitude. Thus, if X volts is added to the discharge voltage of one of the ESMs <NUM> at <NUM>, X volts is subtracted from the discharge voltage of the other of the ESMs <NUM>. In a similar example, if Y volts is added to the charging voltage of one of the ESMs <NUM> at <NUM>, Y volts is subtracted from the charging voltage of the other of the ESMs <NUM>. In a non-limiting example, X and Y could be approximately <NUM> volts.

In one example, the output voltage adjustments of <NUM>, <NUM> are gradually implemented over an implementation period (e.g., on the order of several minutes). In particular, to increase the output voltage for one of the ESMs <NUM> (whether charging or discharging), the controller <NUM> is configured to gradually implement a voltage increase for the one of the ESMs <NUM> from a current value to an increased target value; and to decrease the output voltage for the other of the ESMs <NUM>, the controller <NUM> is configured to gradually implement a voltage decrease for said one of the ESMs from a current value to a decreased target value. If during the gradual adjustments, the SoC difference between the ESMs <NUM> falls below the predefined threshold, then the gradual adjustments can be ceased and use of the default output voltages can be resumed. The gradual adjustments avoids large instantaneous changes to the output voltages.

<FIG> illustrates an example of how controller <NUM> could implement the method of <FIG>. In a voltage adjustment determination section <NUM>, the controller <NUM> determines a voltage adjustment to be implemented in either <NUM> or <NUM>. In particular, the controller <NUM> receives a SoC value for ESM 24A (shown as 62A with label "ESM1_SoC_Fdbk") and a SoC value for ESM 24B (shown as 62B with label "ESM2_SoC_Fdbk"). A summer <NUM> determines a difference between the SoC values 62A-B, and a conversion block <NUM> converts that SoC difference from a percentage value to a current value. If the current value from conversion block <NUM> is non-zero, a limiter <NUM> provides a predefined adjustment amount to be used for each of the DC/DC converter <NUM> output voltages (i.e., to be subtracted from one of the output voltages and added to another of the output voltages). In one example, the predefined adjustment amount is approximately <NUM> volts.

A current command for ESM 24A (shown as 70A with label "ESM1_Cmd") and a current command for ESM 24B (shown as 70B with label "ESM2_Cmd") are input into a sign selector <NUM> that determines whether the ESMs 24A-B are charging, discharging, or are in the off state during which they are neither charging nor discharging. In one example, if the ESMs 24A-B are discharging / sourcing current to the DC bus <NUM>, the current commands 70A-B have positive values, and the sign selector <NUM> outputs a value of <NUM>. If the ESMs 24A-B are charging from / sinking current from the DC bus, the commands 70A-B have negative values, and the sign selector <NUM> outputs a value of -<NUM>. If the ESMs 24A-B are in the off state, the current commands 70A-B have a value of <NUM>, and the sign selector outputs a value of <NUM>.

A multiplier <NUM> multiples the output of the sign selector <NUM> by the output of the limiter <NUM> and provides a target voltage adjustment <NUM> as an output. If the output of sign selector <NUM> is zero (indicating that the ESMs <NUM> are in the off state) or the output of limiter <NUM> is zero (indicating that the ESMs <NUM> have the same SoC), then the target voltage adjustment <NUM> is zero.

However, if the target voltage adjustment <NUM> is non-zero, a filter <NUM> (e.g., a low pass filter) implements the gradual adjustment feature above, so that instead of immediately implementing the predefined adjustment amount (e.g., ±<NUM> volts), the controller <NUM> incrementally adjusts the DC/DC converter <NUM> output voltages to implement the voltage adjustment <NUM> over a time period (e.g., on the order of several minutes).

The controller <NUM> also includes a trim section <NUM>. In a first portion of the trim section <NUM> (prior to element <NUM>), the controller <NUM> determines whether a difference between the respective SoC values 62A-B of each ESM 24A-B exceeds the predefined difference threshold, and in a second portion of the trim section <NUM> (element <NUM> and after) the controller <NUM> calibrates a voltage of the ESMs 24A-B to that of the DC bus <NUM>.

In the discussion below, a "total ESM voltage" of the ESMs <NUM> refers to a sum of the magnitude of the output voltage of each DC/DC converter <NUM>. Thus, if one provides +<NUM> volts and one provides -<NUM> volts, the total ESM voltage would be <NUM> volts. If the total voltage of the ESMs <NUM> matches the total bus voltage on the DC bus <NUM>, then the ESMs <NUM> will neither charge nor discharge. If the total ESM voltage of the ESMs <NUM> exceeds that of the DC bus <NUM> the ESMs <NUM> will discharge, and if the total ESM voltage of the ESMs <NUM> is less than that of the DC bus <NUM> the ESMs will charge. Because the actual voltage on the DC bus <NUM> as provided by the generator <NUM> and AC/DC converter <NUM> may vary based on operational conditions, it is useful to calibrate the ESMs <NUM> to that actual voltage to avoid inadvertent charging or discharging. The trim section <NUM> provides that calibration.

A desired SoC <NUM> is provided corresponding to a desired SoC for each of the ESMs (e.g., a <NUM>% SoC). Block <NUM> determines a minimum between the ESM SoC values 62A-B, and a summer <NUM> determines a difference between the desired SoC <NUM> and that minimum value. If both ESMs <NUM> have a SoC that is equal to the desired SoC, then they are deemed fully charged at the desired SoC level. If the output of summer <NUM> is non-zero, indicating a differential between the SoC of ESM <NUM> and ESM 24B, a relay <NUM> is used to determine whether the difference meets the predefined SoC difference threshold discussed above (e.g., of <NUM>%). If that threshold is met, the relay <NUM> turns on an adjustment function to decrease the SoC difference. A converter <NUM> converts the percentage to a desired current for each ESM <NUM>.

The remainder of the trim algorithm section <NUM> calibrates the voltage of the ESMs <NUM> to an actual voltage of the DC bus <NUM>. A summer <NUM> determines a difference between the desired current command <NUM> and the presently-used current commands 70A-B, and provides that difference to a proportional integral (PI) regulator <NUM> as a current error.

A PI regulator <NUM> provides an anti-windup feature for quickly switching between the charging and discharging modes of the ESMs <NUM>. As the current error calculated in summer <NUM> enters PI regulator <NUM>, it is initially limited by saturation block <NUM>. The output of saturation block <NUM> is multiplied by the proportional gain <NUM> Kp to obtain the proportional response of the PI regulator <NUM>. The proportional response is added to the integral I response by summer <NUM>. The output of the PI regulator <NUM> is limited by the saturation block <NUM>.

The integral response is calculated by taking the output of the PI regulator <NUM> and subtracting the integral I term by sum block <NUM>. The output of summation block <NUM> is multiplied by KiKp <NUM>. This value is then integrated by the integrator block <NUM>.

The PI regulator <NUM> provides a voltage trim command <NUM> that indicates an amount that the each ESM output voltage should be adjusted to be calibrated with the DC bus <NUM> (i.e., a midpoint adjustment) to avoid inadvertent charging or discharging. The voltage trim command <NUM> is summed with the magnitude of the bus voltage <NUM> on either of the rails 30A, 30B by summer <NUM> to obtain an adjusted bus voltage <NUM>.

This adjusted bus voltage <NUM> is summed with the filtered voltage adjustment <NUM> by summers 97A and 97B to determine a voltage command for ESM 24A (shown as 98A with label "ESM1_V_Cmd") and voltage command for ESM 24B (shown as 98B with label "ESM1_V_Cmd"). Which one of the voltage commands <NUM> is an increase and which is a decrease depends on the output of the sign selector <NUM>.

The embodiments discussed herein provide for improved efficiency in the electrical power system <NUM> by avoiding the overvoltage conditions possible when utilizing ESMs <NUM> having differing SoC values. Weight reduction may also occur due to a reduction in a number of battery cells required to reach a desired voltage level (e.g., <NUM> volts). If one were to build a <NUM> volt battery, it would typically require many heavy cells. By using the ESMs 24A-B, however, one can build a system that is lighter and uses fewer cells while still providing the desired voltage (e.g., with one ESM providing +<NUM> volts, and the other providing -<NUM> volts).

Claim 1:
An electrical power system (<NUM>), characterized by comprising:
a direct current (DC) bus (<NUM>) comprising a positive rail configured to provide a positive DC voltage, a negative rail configured to provide a negative DC voltage, and a ground rail (30C);
a first energy storage module (ESM) (24A) and a second ESM (24B) connected to the DC bus (<NUM>), each ESM comprising an energy storage device and each ESM connected to and configured to provide an output voltage to a respective one of the positive and negative rails, a node connecting the first and second ESMs (24A,B) to each other and to the ground rail (30C); and
a controller (<NUM>) configured to determine values for the output voltages for use during a discharging mode in which the energy storage devices discharge onto the DC bus (<NUM>) and a charging mode in which the energy storage devices charge from the DC bus (<NUM>) based on a difference between a state of charge value of each of the first and second energy storage devices; and wherein each ESM (24A,B) includes a DC/DC converter that connects its respective energy storage device to the DC bus (<NUM>), and the output voltages are DC/DC converter output voltages.