Abstract:
An electrical accumulator unit wherein an energy storage device is utilized in conjunction with an actively controlled bidirectional power converter to provide auxiliary power to an electrical network is disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/198,452 filed Nov. 6, 2008 entitled “Electrical Accumulator Unit for Providing Emergency Power to an Electrical Network” which is hereby incorporated by reference in its entirety. 
     
    
     GOVERNMENT RIGHTS CLAUSE 
       [0002]    This invention was made with Government support under Contract Number FA8650-04-D-2409 awarded by the U.S. Air Force. The United States Air Force has certain rights in the invention. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0003]    The present disclosure relates generally to electrical energy storage devices and, more specifically, to an electrical accumulator unit for providing auxiliary power to an electrical network. 
       BACKGROUND OF THE INVENTION 
       [0004]    The movement to more-electric aircraft (MEA) architectures during the past decade in military and commercial aircraft systems continues to increase the complexity of designing and specifying the electric power system (EPS). The addition of numerous high-power electric loads has drastically altered the dynamics of power flow on the electrical bus. Such loads include electro-hydrostatic actuators (EHAs), electromechanical actuators (EMAs), advanced radar, and directed energy weapons (DEW). Although these loads represent a relatively small portion of the average power draw from the EPS, the short-term transient power may exceed twice the average power capabilities of the generator, with peak-to-average power ratios in excess of 5-to-1 for brief periods of time (50-5000 ms). In addition to this high peak-power, some of the loads can produce regenerative power flow during deceleration of motors and drive trains which is equal to peak power draw for brief periods of time (typically 20-200 ms). 
         [0005]    There exists a wide variety of architectures which are capable of addressing the challenges of this dynamic power profile. For example, one architecture is to force regenerative power to be handled locally with diodes and/or power resistors and to size the generator (including the gearbox, shafts, etc.) to be capable of peak power generation. Such architecture can be challenging to design and may lead to an unnecessarily large increase in system weight due to increased demands on the thermal systems and derating of key mechanical components in the generator drive-train. 
         [0006]    Another viable approach is to allow the electrical bus to support bidirectional power flow all the way back to the engine. Aircraft generators often already have the requisite power electronics to support bi-directional power flow due to their dual role of providing main-engine start capability. While this approach reduces the thermal concerns associated with burning regenerative power locally, it actually increases the derating factors required in the mechanical drive-train of the generator which again could result in increased system weight. In addition, such architecture requires all sources (i.e. emergency power units, auxiliary power units, battery, and ground power carts) to support bi-directional power flow. The resulting increase in size, weight and cost associated with these sources may be unacceptable in relation to the system design and cost constraints. A need exists for an improved design which increases the load-handling capabilities of the aircraft electric power system while minimizing the weight and size requirements of the associated components. 
       SUMMARY OF THE INVENTION 
       [0007]    According to one aspect, a device for providing auxiliary power to an electrical network is disclosed, comprising an energy storage device, and an actively controlled power converter operatively coupled to the energy storage device, wherein said actively controlled power converter is configured to provide automatic bidirectional power flow into and out of the electrical network from and to, respectively, the energy storage device in response to at least one measured electrical property within the electrical network. The device may further comprise a plurality of electronic switches and a plurality of diodes, with each one of the diodes connected across the collector and emitter of one of the electronic switches such that the forward bias current of each one of the diodes is from the emitter to the collector of the corresponding electronic switch. 
         [0008]    In another aspect, the plurality of electronic switches comprises two low side electronic switches and two high side electronic switches, wherein the emitter of each low side electronic switch is connected to a neutral bus and the collector of each low side electronic switch is connected to at least one inductor in the actively controlled power converter. The collector of each high side electronic switch is connected to the positive bus in the electrical network and the emitter of each high side electronic switch is connected to the at least one inductor in the actively controller power converter. 
         [0009]    According to another aspect, a method for controlling power flow into an electrical network from an EAU and out of the electrical network to the EAU is disclosed, comprising the acts of (a) sensing at least one electrical property of the electrical network; and (b) providing automatic bidirectional power flow into and out of the electrical network from and to, respectively, the EAU in response at least in part to the at least one sensed electrical property. The method may further comprise the acts of (a) sensing a required load power; (b) determining an upper EAU power limit which will prevent a primary power source from exceeding a primary power source upper limit; (c) determining a lower EAU power limit which will prevent the primary power source from exceeding a primary power source lower limit; (d) determining a commanded power value based on a voltage differential between the energy storage device and a nominal voltage; (e) ensuring that the commanded power value does not exceed the upper EAU power limit or the lower EAU power limit; (f) determining a first duty ratio of the actively controlled power converter based on the commanded power value; and (g) applying the first duty ratio to the actively controlled power converter to sink or source power to the electrical network. 
         [0010]    In yet another aspect, the first duty ratio D is used to control at least one low side electronic switch within the power converter and a value of (1−D) is used as a second duty ratio to control at least one high side electronic switch within the power converter. The same relative switching order may be used to control the low side and high side electronic switches in the power converter regardless of whether the EAU is sourcing or sinking power to the electrical network. A proportional integral controller may also be used to determine an idealized output voltage of the power converter based on a differential between a first commanded power value and a measured EAU power value. The duty ratio may further be determined based on the idealized power converter output voltage, a measured voltage across the energy storage device, and a measured voltage across a bus of the electrical network. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is graph of a transient power profile in a representative modern MEA. 
           [0012]      FIG. 2  is block diagram of an electrical accumulator unit (EAU) according to one embodiment of the present disclosure. 
           [0013]      FIG. 3  is a schematic diagram of an EAU according to one embodiment of the present disclosure. 
           [0014]      FIG. 4  is a switch timing diagram for power electronic switches within the EAU of  FIG. 3  during a time period when the energy storage device within the EAU is being discharged. 
           [0015]      FIG. 5  is a switch timing diagram for power electronic switches within the EAU of  FIG. 3  during a time period when the energy storage device within the EAU is being charged. 
           [0016]      FIG. 6  is a schematic control diagram of the controller logic used to determine the duty cycle for the switching signals of  FIGS. 4 and 5 . 
           [0017]      FIGS. 7 a -7 d    depict typical EAU transient performance results against a 1 Hz step load with a duty ratio of 0.25. 
           [0018]      FIGS. 8 a -8 d    depict typical EAU transient performance results against a 20 Hz, 100 KW (p-p) sinusoidal load. 
           [0019]      FIGS. 9 a -9 d    depict typical EAU transient performance results against a representative aircraft load. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and alterations and modifications in the illustrated device, and further applications of the principles of the invention as illustrated therein are herein contemplated as would normally occur to one skilled in the art to which the invention relates. 
         [0021]      FIG. 1  shows a representative power profile  102  which might be seen on a typical modern electric aircraft during a significant maneuver. As illustrated, large swings in required dynamic power are common, however these transients typically occur for brief periods of time between periods with significantly reduced dynamics (i.e., when the aircraft is cruising at a steady altitude). It shall be understood that the exact distribution of power delivery requirements is highly dependent upon the species of a given aircraft application and the power profile  102  shown in  FIG. 1  is only one potential distribution profile. It shall be further understood that the embodiments described herein may be implemented in other types of electrical networks, including those found in terrestrial and marine vehicles and those utilized in non-moving environments. 
         [0022]    In the example of  FIG. 1 , the maximum generator power capability is approximately 1.6 per unit (pu), which is approximately 1 pu below the peak power required during the maneuver. The generator is also not generally capable of handling regenerative loads, however, the peak regenerative power required during the maneuver is approximately −1 pu. The EPS generator could be sized to accommodate the peak and regenerative power transients such as those shown in  FIG. 1 , although this would likely result in a significantly oversized architecture and impose an unnecessary weight penalty on the aircraft. 
         [0023]      FIG. 2  shows a block diagram of an electrical power system  200  according to one embodiment of the present disclosure. An electrical accumulator unit (EAU)  205  is placed across the electrical power bus lines  210 , 215  between the main power source (shown here as generator  220 ) and the load  225 . The EAU is sized to handle a portion of the peak power transients which can occur for short times during a significant aircraft maneuver, but are beyond the capabilities of the generator  220 . It shall be understood that the ratio of power sharing between the EAU  205  and the generator  220  may be adjusted depending on the requirements of the particular application. It shall be further understood that while the illustrated electrical power bus line  210  is shown as a +270 Vdc bus, other voltage bus levels are contemplated to be compatible with use of the system  200  of the present disclosure. The EAU  205  comprises an energy storage device (ESD)  230  which is operatively coupled with a power converter  235  as shown. In addition, the EAU  205  is optionally configured to sense the bus voltage and source current to generate a reference duty ratio which controls the power-electronic device switching within the power converter  235 . 
         [0024]      FIG. 3  shows a more detailed schematic diagram of the EAU  205  according to one embodiment of the present disclosure. As shown, the energy storage device  230  optionally comprises a capacitor  302 , a safety bleed resistor  304 , a disconnect switch  306 , and a balancing unit  308 . The capacitor  302  may comprise an ultra-capacitor, super-capacitor, conventional capacitor, or any combination thereof in order to achieve the energy reserve capacity required by the specific application. In the preferred embodiment, the capacitor  302  is chosen to have a nominal voltage of approximately 250 Vdc at full charge. It shall be understood that while the illustrated embodiment utilizes the capacitor  302  as the main energy storage component, other types of electrical components may be used to store the reserve energy such as inductors, electro-chemical batteries, mechanical springs, fuel cells, rotating masses (i.e., a fly-wheel), or pressurized fluids or gases. In addition, still further types of main energy storage components may be used which employ a temperature gradient, a chemical storage, or harness potential energy or mechanical inertia of a mass (e.g., a hydroelectric energy storage device). Certain types of rotating inertia masses may also be used which can absorb energy through a heat pump cycle (e.g., when connected to an engine). 
         [0025]    The bleed resistor  304  is connected in parallel with the capacitor  302  to provide safe power drainage from the capacitor  302  when the electrical power system  200  is not in use. Disconnect switch  306  is optionally provided to allow for isolation of the ESD  230  during testing or maintenance procedures. Balancing unit  308  utilizes a standard resistive loss based method and is optionally provided to prevent the voltage across any given capacitive element within the capacitor  302  from exceeding a desired maximum value or from becoming negative. 
         [0026]    The power converter  235  is operatively coupled to the energy storage device  230  and preferably comprises a primary inductor  310 , leakage inductors  312  and  314 , four insulated gate bipolar transistor (IGBT) switches  316 , 318 , 320 , 322 , and a snubber capacitor  324 . It shall be understood that while IGBT type switches are shown in the illustrated embodiment, other types of electronic switching devices may be used to open or close the electrical paths between the various circuit components including bipolar transistors, field effect transistors, such as junction field effect transistors (JFETs), metal oxide semiconductor field effect transistors (MOSFETs), relays, and the like. 
         [0027]    A control unit  325  actively controls the timing of the IGBT switches  316 , 318 , 320 , 322  as discussed hereinbelow. The control unit  325  may comprise any suitable digital signal processing (DSP) unit known in the art, such as the TMS320F2812 DSP manufactured by Texas Instruments, 12500 TI Boulevard, Dallas, Tex. Control unit  325  may further comprise additional electronic components and integrated circuits (not shown) to enable the control unit  325  to interface with the various circuit elements shown in  FIG. 3 . The control unit  325  is therefore able to sense and filter multiple circuit variables including, but not limited to, main bus voltage (across bus lines  210 , 215 ), EAU  205  voltage, temperature, EAU  205  current, generator  220  current, load  225  current, ESD  230  current, inductor  310  current, and link voltage. In addition, the control unit  325  may be configured to sense the rate of change of any of the above variables for use in a control algorithm. For the sake of clarity, many of the connections between control unit  325  and the other portions of the EAU  205  are omitted from the drawings. 
         [0028]    In the preferred embodiment, the power converter  235  is configured in a modified bidirectional interleaving boost arrangement as shown in  FIG. 3 . This allows the power converter  235  to achieve the reduced switching losses characteristic of discontinuous mode converters while maintaining the large-scale system dynamics of a continuous mode converter. It shall be understood, however, that other converter configurations known in the art may be employed within power converter  235  and are considered to be within the scope of the present disclosure. For example, power converter  235  may comprise a three-phase electromechanical machine drive with an optional electromechanical machine. 
         [0029]    The output of the primary inductor  310  is connected to the input of both leakage inductors  312 ,  314 . The component values of the primary inductor  310  and leakage inductors  312 , 314  are chosen to limit the injected current ripple to 20 amps at the worst case operating point and 5 amps during fully charged operation. Applying these criteria in a preferred embodiment, the primary inductor  310  is approximately 100 micro Henries (μH) and the leakage inductors  312  and  314  are approximately 1.5 μH each. The output of leakage inductor  312  is connected to the collector of IGBT switch  316  and the emitter of IGBT switch  320  as shown. The output of leakage inductor  314  is connected to the collector of IGBT switch  318  and the emitter of IGBT switch  322  as shown. The collectors of IGBT switches  320  and  322  are connected to the positive output  326  of the power converter  235  as shown. The emitters of IGBT switches  316  and  318  are connected to the neutral output  328  of the power converter  235  as shown. Diodes  317 , 319 , 321 , 323  are connected across the collector and emitter of IGBT switches  316 , 318 , 320 , 322  respectively as shown. The configuration of the IGBT switches  316 , 318 , 320 , 322  and diodes  317 , 319 , 321 , 323  allows the power converter  235  to automatically operate in a bidirectional fashion. The snubber capacitor  324  is connected across the outputs  326 , 328  of the power converter  235 . 
         [0030]    The outputs  326 , 328  of the power converter  235  are fed through an optional electro-magnetic interference (EMI) filter  330  to attenuate the current ripple injected into the aircraft electrical power system to levels which are compliant with the appropriate specifications which govern such injection. In a preferred embodiment, EMI filter  330  is configured in a double-L arrangement and includes capacitors  332 , 334 , 336 , inductors  338 , 340 , and resistor  342 . The output  326  of the power converter  235  is connected to the input of the inductor  338  and capacitor  332 . The output of inductor  338  is connected to the input of the capacitor  334 , resistor  342 , and inductor  340 . The output of resistor  342  is connected to the input of capacitor  336 . The output of inductor  340  is connected to the +270V bus  210  via switch  344 . The outputs of capacitors  332 ,  334 , and  336  are connected to the neutral bus  215  via switch  346 . In the preferred embodiment, the component values in the EMI filter  330  are chosen to limit the injected current ripple to military standard MIL-STD-461 levels (much less than 1 amp at most frequencies) under all operating conditions. Using this criteria, capacitors  332  and  334  are chosen to be 160 μF, capacitor  336  is chosen to be approximately 300 μF, inductors  338  and  340  are chosen to be approximately 25 μH, and resistor  342  is chosen to be approximately 0.4 ohms. It shall be understood that the component values within EMT filter  330  may be adjusted based on the needs of the particular application. 
         [0031]      FIG. 4  shows a detailed description of the timing for the four actively controlled IGBT switches  316 , 318 , 320 , 322  when the energy storage device  230  is being discharged (supplying power to the EPS). Likewise,  FIG. 5  shows a similar description of the switch timing when the energy storage device  230  is being charged (absorbing power from the EPS). In the illustrated embodiment, the duty ratio of the switching signals is generated such that if the load power exceeds the transient limits of the generator  220  as specified in the control unit  325 , the EAU  205  will attempt to source or sink power as necessary to hold the actual source power to within the specified transient limits. After the load has returned to within the transient limits, the EAU  205  will charge or discharge as necessary to maintain the nominal voltage on the capacitor  302  of 250 Vdc. 
         [0032]      FIG. 6  illustrates a schematic block diagram of the logic used by the control unit  325  to determine the duty ratio of the IGBT switches  316 , 318 , 320 , 322 . The symbols used within  FIG. 6  are generally based on standard calculation symbols used within the MATLAB/Simulink programming platform (published by The MathWorks, Inc., of Natick, Mass., USA). In the illustrated embodiment, the main objectives of the controller are: (1) limiting the total power delivered by the generator  220 , and (2) maintaining the state of charge of the energy storage device  230  at a nominal value. The first objective is preferably considered to take priority over the second objective. That is, the EAU  205  will not attempt to charge or discharge the energy storage device  230  if such action will cause the load on generator  220  to exceed the specified limits. 
         [0033]    Referring again to  FIG. 6 , the control unit  325  senses the required load current  602  and the main bus voltage  604 . These values are multiplied at block  606  to determine the required load power. The required load power is then compared to the source (generator  220 ) upper power limit  610  at block  608 , to determine the maximum allowable EAU power that would not cause a limit violation on the generator  220 . The required load power is also compared to the source lower power limit  614  at block  612  to determine the minimum allowable EAU power that would not cause a limit violation on the generator  220 . In short, the upper and lower limits of saturation block  616  ensure that the EAU  205  does not supply too much or too little power and cause the generator  220  to run above or below its allowable loading limits. In further embodiments, the upper and lower limits of saturation block  616  may be further adjusted to ensure that the power supplied or absorbed by the EAU  205  does not cause the generator  220  to exceed any upper or lower load rate of change limits. 
         [0034]    At block  622 , the energy storage device voltage  618  is compared with the specified nominal voltage (250 volts in this case) and fed to block  624 . Block  624  multiplies the measured voltage difference by a specified charge rate per volt to determine the charge (or discharge) rate as a unit of power. The output of block  624  is then fed through saturation block  626  in order to limit the charge rate to a specified level, typically based on the capabilities of the chosen energy storage device  230 . 
         [0035]    Dynamic saturation block  616  receives the output power value of saturation block  626  and compares it with the maximum and minimum allowable EAU power values which will maintain the generator  220  loading limits. If the input value is within the limits, the value is passed unchanged. If the input value is outside either the maximum or minimum allowable EAU value, the value of the respective limit is output. 
         [0036]    Saturation block  628  receives the output of dynamic saturation block  616  and further compares the value to the upper and lower power limits of the EAU  205 . In other words, saturation block  628  ensures that the commanded power  630  does not cause the EAU  205  to exceed its own charging or discharging limits. 
         [0037]    Once the commanded power  636  is determined, it is repeated at block  638  and compared to the measured EAU power at block  646 . The measured EAU power is determined by multiplying the measured EAU current  640  by the bus voltage  642  at block  644 . The resulting EAU power error signal is output from block  646  and fed to proportional integral controller  648  which determines a desired voltage drop through the EMI filter  330 . 
         [0038]    The desired EMI filter voltage from block  648  is then fed to input block  654 , along with the measured energy storage device voltage  650  and bus voltage  652 . These values are then fed to block  656  and used to calculate a duty ratio based on the input/output relationship of an ideal boost converter as follows: The duty ratio of an ideal boost converter is 
         [0000]        V   out   /V   in =1/(1 −D ) 
         [0039]    where D is the duty ratio. In the circuit of  FIG. 3 , V in  is equal to the voltage across ESD  230 , V out  is equal to the idealized voltage across EMI filter  330  (V EMI ) plus the main bus voltage (V bus ). Substituting V EMI +V bus  for V out  in the equation above, we arrive at the following equation for the duty ratio D: 
         [0000]        D= 1−( V   ESD /( V   bus   +V   EMI )
 
         [0040]    This equation is applied at block  656  to determine the duty ratio for each of the low side IGBT switches  316  and  318 . As calculated above, the duty ratio D corresponds to the period of a half cycle in the overall switching diagrams, that is, from time T 1  to T 4  or time T 4  to T 7 . Each of the high side IGBT switches  320  and  322  will have a corresponding duty ratio of (1−D), where D is the duty ratio of the corresponding low side IGBT switch. The output of block  656  is then fed through saturation block  658  to limit the duty ratio to a specified upper and lower bound. The upper and lower bounds of saturation block  658  are typically a function of the non-idealities of the circuit and may vary depending on the chosen implementation. 
         [0041]    In certain embodiments, the control methodology used above may be modified such that the EAU  205  will emulate the unidirectional or bidirectional power draw characteristics of another load on the power system  200 . In still further embodiments, a mechanism of average power generation and/or absorption may be added which interfaces with a source and/or sink which is external to the power system  200 . 
         [0042]    A more detailed description of the switching signals will now be presented. Again,  FIG. 4  illustrates the situation where the energy storage device  230  is discharging and supplying power to the main bus. Starting at time T 1 , IGBT switch  316  is turned on, inducing an increasing current flow through inductor  312  until time T 3 . The turning on of switch  316  causes the bulk of the current flowing through inductors  310  and  312  to be directed through switch  316  instead of through diode  321 , since switch  316  provides a less resistive path to the neutral bus  328 . The rate of increase changes slightly at time T 2  due to the different inductance values of inductors  310  and  312 . At time T 3 , switch  316  turns off, allowing current to flow from inductor  312  through diode  321  to the output  326 . Switch  320  also turns on at time T 3 , however the main current will be flowing through diode  321  at this time. Inductor  312  continues to discharge, supplying power to the main bus from time T 3  to time T 4 . At time T 4 , switch  320  turns off and switch  318  turns on, inducing an increasing current to flow through inductor  314  and switch  318  until time T 6 . From time T 4  to time T 6 , inductor  314  is therefore charging. At time T 6 , switch  318  turns off, allowing current to flow from inductor  314  through diode  323  to the output  326 . Switch  322  also turns on at time T 6 , however the main current will be flowing through diode  323  at this time. Inductor  314  continues to discharge, supplying power to the main bus from time T 6  to time T 7 . At time T 7  the process begins again when switch  322  turns off and switch  316  turns on. 
         [0043]    Referring again to  FIG. 5 , the situation will be described where the energy storage device  230  is being charged by the main generator  220  using the power converter  235 . Starting at time T 1 , switch  316  turns on and switch  322  turns off. Because current was flowing through inductor  314  prior to switch  316  turning on, current will now flow from the neutral bus  328 , through the diode  319 , through inductors  314  and  310  and into energy storage device  230 . At time T 2 , switch  316  turns off and switch  320  turns on. This causes current to begin flowing from the main generator  220  to node  326 , through switch  320 , and through the inductors  312  and  310 . The current through inductors  312  and  310  continues to increase negatively until time T 4 . At time T 4 , switch  320  turns off and switch  318  turns on. Again, because currently was previously flowing through inductor  312 , current will now flow from the neutral bus  328 , through diode  317 , through inductors  312  and  310 , and into the energy storage device  230 . At time T 5 , switch  318  turns off and switch  322  turns on, allowing current to begin flowing from the main generator  220 , through switch  322 , and through the inductors  314  and  310 . The current through inductors  314  and  310  continues to increase negatively until time T 7 . At time T 7 , switch  322  turns off and switch  318  turns on, starting the process over again. 
         [0044]    As can be seen from  FIGS. 4 and 5 , the modified bidirectional interleaving boost arrangement illustrated in  FIG. 3  allows the EAU  205  to both source and sink power from the electrical network  200  as needed while maintaining the same relative switching order of switches  316 , 318 , 320 , 322 , with only the duty ratio being altered. It shall be understood that additional dead time appropriate for IGBT switches may be inserted between the high side (switches  320 ,  322 ) and low side (switches  316 ,  318 ) switching signals shown in  FIG. 4  before being fed through appropriate isolation and gate-drive circuitry (not shown) to the gates of IGBT switches  316 , 317 , 318 , 320 . 
         [0045]      FIGS. 7 a -7 d    provide detailed waveforms illustrating the performance of the specified embodiment of the EAU  200  when connected to a representative power network including a generator with transient limits at 160/0 kW (max/min) and a 1 Hz step load with a 25% duty ratio. As shown in  FIG. 7 a   , once the load power jumps to 260 kW at approximately time 0.05 seconds, the EAU  205  begins supplying the 100 kW needed to keep the generator  220  operating at 160 kW. At approximately time 0.3 seconds, when the load drops to approximately 140 kW, the EAU  205  begins sinking 20 kW in order to recharge the energy storage device  230 , while allowing the generator  220  to remain operating safely at its 160 kW limit. This continues until time 1.05 seconds when the load power again jumps to 260 kW. 
         [0046]      FIG. 7 b    illustrates the charging and discharging cycles of the energy storage device  230  in response to the 1 Hz step load. From time 0.05 seconds to time 0.3 seconds, the voltage across the capacitor  302  is decreasing as the EAU  205  supplies power to the system bus  210 ,  215 . From time 0.3 seconds to time 1.05 seconds, the voltage across capacitor  302  is increasing as the EAU sinks power to charge the capacitor  302 . 
         [0047]      FIG. 7 c    shows the resulting current through inductor  310  as the 1 Hz step load is applied. At time 0.05 seconds, the current through inductor  310  jumps in response to the step load and continues to rise until time 0.3 seconds, at which point the current reverses as the capacitor  302  is recharged.  FIG. 7 d    shows the resulting voltage across the system bus  210 ,  215 , which is held close to the target value of 270 volts throughout the sample time with the exception of minor spikes at the step transitions. 
         [0048]      FIGS. 8 a -8 d    provide detailed waveforms illustrating the intended performance of the specified embodiment of the EAU  205  when connected to a representative power network including a generator  220  with transient limits at 160/0 kW (max/min) and a 20 Hz sinusoidal load with a 100 kW peak-to-peak loading. Again, as soon as the sinusoidal load exceeds the generator  220  capabilities at time 0.05 seconds (indicated by arrow  802 ), the EAU  205  power flow is adjusted to hold the generator loading to within the specified transient limits. After the load falls within the transient limits at approximately time 0.075 seconds (indicated by arrow  804 ), the EAU  205  attempts to recharge the energy storage device  230  without exceeding the limits of the generator  220 . 
         [0049]      FIG. 8 b    illustrates the charging and discharging cycles of the energy storage device  230  in response to the sinusoidal load. From time 0.05 seconds to time 0.075 seconds, the voltage across the capacitor  302  is decreasing as the EAU  205  supplies power to the system bus  210 ,  215 . From time 0.075 seconds to time 0.1 seconds, the voltage across capacitor  302  is increasing as the EAU sinks power to charge the capacitor  302 . The process repeats when the load power begins to increase again at time 0.1 seconds. 
         [0050]      FIG. 8 c    shows the resulting current through inductor  310  as the sinusoidal load is applied. From time 0.05 seconds to time 0.075 seconds, the current through inductor  310  increases and decreases in response the portion of the sinusoidal load which is beyond the limits of the generator  220 . From time 0.075 seconds to time 0.1 seconds, the current through inductor  310  reverses as some portion of the power supplied by the generator  220  is used to charge the capacitor  302 .  FIG. 8 d    shows the resulting voltage across the system bus  210 ,  215  in response to the sinusoidal load. 
         [0051]      FIGS. 9 a -9 d    provide detailed waveforms illustrating the intended performance of the specified embodiment of the EAU  205  when connected to a representative power network  200  including a generator  220  and an aggregate load representative of a full aircraft loading profile. As shown in  FIG. 9 a   , from time 0 seconds to time 0.2 seconds, the generator  220  is safely supplying all of the power to the network  200  with the EAU  205  holding steady at 0 kW. Once the load jumps above the 160 kW generator limit at time 0.2 (indicated by arrow  902 ), the EAU  205  power flow is adjusted to hold the generator  220  loading to within the 160 kW limit. At approximately time 0.45 seconds (indicated by arrow  904 ), the load falls below the 160 kW, at which point the EAU  205  begins sinking power to recharge the energy storage device  230  while holding the generator at its 160 kW limit. In the case where the load actually drops below zero (between times indicated by arrows  706  and  708 ), the EAU  205  begins sinking a greater amount of power in order to prevent the generator  220  from going into regenerative mode. 
         [0052]      FIG. 9 b    illustrates the charging and discharging cycles of the energy storage device  230  in response to the aircraft loading profile. From time 0 seconds to time 0.2 seconds, the voltage across the capacitor  302  remains steady at 250 volts. From time 0.2 seconds to time 0.45 seconds, the voltage across the capacitor  302  is decreasing as the EAU  205  supplies power to the system bus  210 ,  215 . From time 0.45 seconds to time 0.7 seconds, the voltage across capacitor  302  is increasing as the EAU sinks power to charge the capacitor  302 . 
         [0053]      FIG. 9 c    shows the resulting current through inductor  310  as the aircraft load is applied. From time 0 seconds to time 0.2 seconds, the inductor  310  current remains at zero. From time 0.2 seconds to time 0.45 seconds, the current through inductor  310  increases and decreases in response the portion of the load which is beyond the limits of the generator  220 . From time 0.45 seconds to time 0.7 seconds, the current through inductor  310  reverses as some portion of the power supplied by the generator  220  is used to charge the capacitor  302 .  FIG. 9 d    shows the resulting voltage across the system bus  210 ,  215  in response to the aircraft load. 
         [0054]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.