Abstract:
A DC-AC converter includes a DC-DC converter providing bi-directional conversion between a first DC power signal and a second DC power signal, the first DC power signal being on a first DC bus and the second DC power signal being on a second DC bus. The DC-AC converter also includes an inverter providing bi-directional DC-AC conversion between a third DC power signal and a first AC power signals the third DC power signal being on the second DC bus and the first AC power signal being on a first AC bus.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to an electric power processing device, and more particularly, to a low-mass bi-directional DC-AC power converter. The low-mass, bidirectional DC-AC power converter can be incorporated into, for example, an aircraft power conditioning unit that interfaces generation equipment with various load equipment utilizing independent voltages levels and frequencies. 
         [0002]    Many industries can benefit from lightweight power conditioning systems that are also flexible in providing a variety of voltages of different magnitudes and frequencies. One such industry is the aviation industry where advances in aircraft design (both manned and unmanned) are necessitating new electric power system architectures. For example, emerging aircraft have 270 VDC electrical power equipment while still maintaining legacy 115 VAC/400 Hz or variable frequency equipment. The 115 VAC is generated by a power converter that uses the 270 VDC as its input. 
         [0003]      FIG. 1  illustrates a related art power conditioning system for an aircraft. The power conditioning system includes generator  10 , Generator Control Unit (GCU)  15 , External Power Connection (EPC) DC ground cart interface  40 , high-voltage battery  20 , low-voltage battery  30 , DC-DC converter  50  and inverter/transformer unit  60 . A typical aircraft may have two power conditioning systems similar to that illustrated in  FIG. 1 . 
         [0004]    Generator  10  typically includes a wound field synchronous motor (WFSM)  12  that is configured to be used as a generator. The output of generator  10  forms high-voltage DC bus  25  by rectifying the output of WFSM  12  using rectifier  11 . GCU  15  controls the excitation voltage of WFSM  12  to maintain a desired DC voltage at the output of generator  10 . High-voltage DC bus  25  supplies most of the electrical power for the aircraft, and high-voltage DC bus  25  may, for example, have a magnitude of 270 VDC. 
         [0005]    Connected to high-voltage DC bus  25  is on-board high-voltage battery  20 . During normal operation, the charge on high-voltage battery  20  is maintained by generator  10  via the high-voltage DC bus. A battery charger and disconnect switches (both features not shown) may be connected between high-voltage DC bus  25  and high-voltage battery  20 . When generator  10  is not available or if the power from generator  10  is insufficient, the system may be configured such that high-voltage battery  20  provides power to high-voltage DC bus  25  to operate the equipment. In some modern, more electric aircraft, the high-voltage battery is not connected to the bus, but is separated by a contactor, which is closed only when the main generator fails. 
         [0006]    The input power to DC-DC converter  50  is provided by high-voltage DC bus  25 , and the output of DC-DC converter  50  forms low-voltage DC bus  35  that supplies control power to the system avionics. During normal operation, the charge on low-voltage battery  30  is maintained by DC-DC converter  50  via low-voltage DC bus  35 . A battery charger and disconnect switches (both features not shown) may be connected between low-voltage DC bus  35  and low-voltage battery  30 . If DC-DC converter  50  is not operational or if the power from DC-DC converter  50  is insufficient, the system may be configured such that low-voltage battery  30  will provide power to low-voltage DC bus  35 . The magnitude of low-voltage DC bus  35  may be, for example, 28 VDC. 
         [0007]    Inverter/transformer unit  60  is a DC-AC converter that provides power to legacy equipment that run on AC power. Inverter/transformer unit  60  gets its supply from high-voltage DC bus  25  and converts it to AC power at, for example, 115 volts, 400 Hz. 
         [0008]    EPC DC ground cart  41  can be connected to the aircraft&#39;s high-voltage DC bus  25  through DC ground interface  40  when the aircraft is on the ground. EPC DC ground cart  41  powers the high-voltage DC equipment and also provides power to the 115 volt, 400 Hz equipment via inverter/transformer unit  60 . Other systems may have an EPC AC ground card interface that connects directly to the legacy AC bus. 
         [0009]    Inverter/transformer unit  60  represents a related art solution employing a DC bus inverter with an output isolation transformer. Inverter/transformer unit  60  may also be configured as a DC-DC converter with an isolation transformer or a DC-DC converter with bi-polar voltage and a direct DC-AC stage. 
         [0010]      FIG. 2  illustrates an exemplary topology for inverter/transformer unit  60  shown in  FIG. 1 . The supply power from high-voltage DC bus  25  is fed to DC filter  61 , which filters out any noise on the high-voltage DC supply power. In this example, the input DC supply is uni-polar, i.e., one leg of the DC supply is grounded to the chassis. Multi-phase inverter  62  receives the filtered DC voltage and produces a biased, poly-phase AC output that is sent to transformer  63 . Transformer  63  provides isolation and converts the AC signal from multi-phase inverter  62  to a desired AC voltage. AC filter  64  receives the AC signal from transformer  63  and provides a filtered AC output. An AC signal at the output of transformer  63  may be sent to controller  65  as a feedback signal to adjust multi-phase inverter  62  if the output voltage from transformer  63  deviates from a desired voltage. 
         [0011]    In the related art topology of  FIG. 2 , the output of AC filter  64  is a non-biased, isolated AC voltage supply. In addition, the topology is such that the power can flow in either direction (bi-directional). However, use of transformer  63  in an aircraft is not desirable because the transformer is heavy and awkward (approximately 70 lbs for a 6-10 kVA unit operating at 400 Hz). 
         [0012]      FIG. 3A  depicts a transformer-less topology, inverter  70 , that can be substituted for the inverter/transformer unit  60  of  FIG. 1 . The negative DC rail of the DC voltage supply is grounded, i.e., at chassis potential. Therefore, although inverter  70  does not have a transformer, it produces an AC signal that is biased. That is, the output AC signal from AC filter  71  will not be centered at zero volts (see  FIG. 3B ). Multi-phase inverter  72  can be configured such that the “average” value of the output sine waves and the peak-to-peak values of the output sine waves can vary as shown by the solid curve and the dotted-line curve in  FIG. 3B . However, because the DC bus negative is grounded, inverter  70  will produce output sine waves whose values are always positive. 
         [0013]    An output sine wave that is always positive is a problem if the AC system is “expecting” a neutral referenced AC sine wave, i.e. a sine wave whose values are positive and negative (for example, the legacy AC system is typically 115 VAC/400 Hz). Therefore, in order to use the topology of  FIG. 3A  to provide a neutral referenced AC source, an isolation stage will be required between the uni-polar DC bus and the input side of inverter  70 . This will increase the complexity, cost and weight of the system. 
         [0014]      FIG. 4  depicts another related art topology, inverter  80 , that can produce a non-biased, isolated AC output. In this topology, an isolated converter, DC-DC converter  86 , is located between DC filter  81  and multi-phase inverter  82 . DC-DC converter  86  has a high frequency transformer that provides the isolation for the system. The high frequency transformer is lower in weight (less than 10 lbs) than the transformer in  FIG. 2 . However, this topology also has drawbacks. 
         [0015]    For example, DC-DC converter  86  must process the total power and derive a “new” isolated DC voltage that can be center tapped grounded to the chassis. In addition, DC-DC converter  86  does not provide bidirectional power flow. Therefore, an AC source, such as an EPC AC ground cart, cannot be used to generate the DC bus when the aircraft is grounded. To add bi-directional capability to the topology shown in  FIG. 4 , additional components and controls must be added to both sides of DC-DC converter  86 , which increases cost, complexity and weight. 
         [0016]    Accordingly, the related art power conditioning units are awkward and heavy (transformers), do not easily provide bi-directional power flow capability (i.e., without requiring additional components), and/or produce an output voltage supply that is not optimal for an aircraft. Therefore, it is desirable to have a transformer-less power conversion unit that produces a non-biased, balanced isolated AC voltage supply. Preferably, the power conversion unit can also produce positively biased AC outputs, negatively biased AC outputs and non-biased AC outputs. 
         [0017]    Main engine start has traditionally been done using an air-turbine starter (ATS). The ATS uses compressed air generated from a compressor powered by the on-board auxiliary power unit (APU) (e.g., a small or dedicated device used to compress air such as an electric generator), an external ground cart or the aircraft&#39;s other engine (if there is more than one). In order to be more autonomous, however, the aviation industry is requiring that emerging aircraft start their main engines with less ground support. Accordingly, aircraft are being designed to start their engines electrically (i.e., without an ATS), which requires a strong DC source. 
         [0018]    One option for providing the electrical start is to provide a ground cart that not only powers DC equipment as described earlier but is also sized to perform a main engine start by providing sufficient power to inverter  120  to run the WFSM  12  in engine-start mode or to assist the on-board battery in performing the main engine start. Another option is to use an EPC AC ground cart (not shown in  FIG. 1 ) to provide supplemental power via an AC to DC inverter to help the on-board battery in performing the main engine start. A third option is to size the on-board battery to perform the main engine start without requiring supplemental power from an external ground cart. The third option allows the aircraft to be more autonomous than either of the other two options. 
         [0019]    Unfortunately, all these options have their drawbacks. DC ground carts typically have a power limit of approximately 90 kVA or less, based on historical equipment and EPC connectors used on aircraft. High-power ground carts that are rated to perform main engine starts are not very common and are typically only found in the largest airports or military bases. Future aircraft will have to be flexible and have the ability to start their engines anywhere in the world. For the AC ground cart option, the additional circuitry needed to allow the EPC AC ground cart to assist in the main engine start will add additional weight and complexity to the related art power conditioning units. Although the third option of providing an engine start capable on-board battery will provide this flexibility, the weight and cost of the battery makes this option prohibitive. 
         [0020]    In addition, related art power conditioning units are not designed to regulate the power going to the legacy AC bus during main engine start. Without regulation, main engine starts may create power fluctuations that cause power blackouts in the legacy AC system, which would necessitate longer aircraft commissioning times. 
         [0021]    Therefore, along with having a transformer-less power conditioning unit that produces a non-biased, isolated AC voltage supply from the high-voltage DC bus, it is also desirable to have a power conditioning unit that will help enable electric main engine start by using commonly available EPC AC ground carts and low-power EPC DC ground carts to supplement the power from the on-board battery. Preferably, power to the legacy AC bus is regulated to minimize blackouts during main engine starts. 
       SUMMARY OF THE INVENTION 
       [0022]    In an embodiment of the invention, a DC-AC converter includes a DC-DC converter providing bi-directional conversion between a first DC power signal and a second DC power signal, the first DC power signal being on a first DC bus and the second DC power signal being on a second DC bus. The DC-AC converter also includes an inverter providing bi-directional DC-AC conversion between a third DC power signal and a first AC power signal, the third DC power signal being on the second DC bus and the first AC power signal being on a first AC bus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiment of the invention which is schematically set forth in the figures, in which: 
           [0024]      FIG. 1  is a block diagram of a related art power conditioning system in an aircraft. 
           [0025]      FIG. 2  is an example of a related art topology for the inverter/transformer of  FIG. 1 . 
           [0026]      FIG. 3A  is an example of a related art topology for a power converter that can be used in the system of  FIG. 1 . 
           [0027]      FIG. 3B  illustrates examples of the output AC waveforms from the related art power converter of  FIG. 3A . 
           [0028]      FIG. 4  is an example of a related art topology for a power converter that can be used in the system of  FIG. 1 . 
           [0029]      FIG. 5  is a block diagram of a power conditioning unit with a bi-directional DC-AC converter that is consistent with the present invention. 
           [0030]      FIG. 6A  illustrates a more detailed block diagram of the bi-directional DC-AC converter shown in  FIG. 5 . 
           [0031]      FIG. 6B  illustrates examples of the output AC waveforms form the bi-directional DC-AC converter of  FIG. 6A . 
           [0032]      FIG. 7A  is a circuit diagram of a bi-directional Auk converter that can be used in the bi-directional DC-DC circuit of  FIG. 6A . 
           [0033]      FIG. 7B  is a circuit diagram of a bi-directional Buck-Boost converter that can be used in the bi-directional DC-DC circuit of  FIG. 6A . 
           [0034]      FIG. 8  is a block diagram of another power conditioning unit with another bi-directional DC-AC converter that is consistent with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    The present invention will be explained in further detail by making reference to the accompanying drawings, which do not limit the scope of the invention in any way. 
         [0036]      FIG. 5  illustrates a block diagram of a power conditioning unit comprising a low-mass, bi-directional DC-AC converter that is consistent with the present invention. Specifically, power conditioning unit  100  comprises DC bus  101 , DC-DC converter  110 , inverter circuit  135 , rectifier  140 , switch  145 , DC-DC converter  150  and low-mass, bi-directional DC-AC converter  180 . DC bus  101  is connected to high voltage DC bus  25  by terminal  102 . 
         [0037]    Inverter circuit  135  is a bi-directional AC-DC converter that converts the power flowing between WFSM  220  and DC bus  101  into the appropriate voltage form (i.e., AC and DC). Inverter circuit  135  is connected to WFSM  220  at terminal  103  and comprises inverter  120  and GCU  130 . GCU  130  regulates the output voltage of inverter circuit  135 . 
         [0038]    Engine gear box  200  has a shaft for accepting WFSM  220 . The shaft is engaged to engine gear box  200  during main engine startup and normal flight operation. During normal flight operation, WFSM  220  is a generator that supplies inverter circuit  135  with AC power. Inverter  120  of inverter circuit  135  converts the AC power from WFSM  220  to DC power. The DC power is then supplied to DC bus  101 , which is connected to the high voltage bus  25  at terminal  102 . 
         [0039]    PMG  230 , which is mounted on the same shaft as WESM  220 , supplies power to rectifier  140 , which then feeds DC-DC converter  150 . DC-DC converter  150  feeds low voltage DC bus  35 , which supplies control power to the system avionics. Although PMG  230  and DC-DC converter  150  are shown directly supplying low voltage DC bus  35  in  FIG. 5 , other configurations may be employed to interface PMG  230  to a DC bus (low or high) during normal or emergency conditions. For example, switch  145  enables PMG  230  to supply high voltage DC bus  25 , if needed. 
         [0040]    Along with feeding high-voltage DC bus  25  via DC Bus  101  during normal flight operation, inverter circuit  135  also feeds bi-directional DC-AC converter  180 . Bi-directional DC-AC converter  180  converts the power flowing between the legacy AC bus and DC bus  101  into the appropriate voltage form (i.e., AC and DC). Bi-directional DC-AC converter  180  is connected to the legacy AC bus at terminal  104  and comprises bi-directional DC-DC converter  160 , smoothing capacitors  170 A and  170 B, multi-phase inverter  190  and filter  195 . 
         [0041]    Multi-phase inverter  190  provides bi-directional AC-DC conversion between the legacy AC bus and the connection to bi-directional DC-DC converter  160 . Bi-directional DC-DC converter  160 , as the name implies, provides bi-directional DC-DC conversion between DC bus  101  and the connection to multi-phase inverter  190 . Bi-directional DC-DC converter  160  may be configured as shown in  FIG. 6A . In  FIG. 6A , bi-directional DC-DC converter  160  comprises DC filter  161 , which is optional, and non-isolated, bidirectional DC-DC circuit  162 . 
         [0042]    Consistent with the present invention, bi-directional DC-DC circuit  162  may incorporate an inverting DC-DC topology. For example, bi-directional DC-DC circuit  162  may include a bi-directional Ćuk converter ( FIG. 7A ) or a bi-directional buck-boost converter ( FIG. 7B ). These converters invert the incoming voltage, i.e., they accept a voltage V dc  and output, for example, a voltage −V dc . 
         [0043]    The switches K 1  and K 2  in  FIG. 7A  for the Ćuk converter and switches K 1  and K 2  in  FIG. 7B  for the buck-boost converter can be any type of switching device that blocks forward voltage and has controlled on-off gating such as, for example, transistors, MOSFETS, IGBTs, etc. One skilled in the art is familiar with the operation of a Ćuk converter and a buck-boost converter. Therefore, the operation of these converters will not be further described here. 
         [0044]    An inverting DC-DC topology, such as that provided by the Ćuk converter and the buck-boost converter, is desirable because the bi-directional DC-DC converter  160  can then be configured to provide a bi-polar DC voltage to multi-phase inverter  190 . 
         [0045]    As shown in  FIG. 6A , bi-directional DC-DC converter  160  receives uni-polar DC voltage from DC bus  101 . An optional DC filter  161  may be included in bi-directional DC-DC converter  160  (or externally) to filter the DC bus signal. Bi-directional DC-DC converter  160  then outputs a bi-polar DC voltage to multi-phase inverter  190 . Multi-phase inverter  190  can then develop balanced neutral AC waveforms that are bi-polar in nature, if needed. One skilled in the art is familiar with the operation of multi-phase inverter  190  and it will not be further described here. 
         [0046]    The uni-polar to bi-polar conversion by bi-directional DC-DC converter  160  is accomplished by connecting one input terminal of bi-directional DC-DC circuit  162  to the positive output, Vdc, of DC filter  161 , and the other input terminal to chassis ground via DC filter  161 . Therefore, bidirectional DC-DC circuit  162  will have an inverted output, with one rail grounded to the chassis and the other rail forming the −Vneg bus. Although the output is inverted, the magnitude of Vneg does not necessarily have to equal the magnitude of Vdc. Any asymmetry can be accounted for by appropriately controlling the modulation of the multi-phase inverter  190 . 
         [0047]    Typically, the voltage Vdc is 270 VDC and, in related art systems, this value is just adequate for multi-phase inverter  190  to generate 115 VAC. However, with the embodiment shown in  FIG. 6A , the voltage across multi-phase inverter  190  can be greater than 270 VDC, which ensures adequate voltage supply to produce a 115 VAC output. That is, when the power flows from the high-voltage DC bus to multi-phase inverter  190 , bi-directional DC-DC converter  160  can adjust the output voltage from 0 to −270 VDC. This will produce a differential of 270 VDC to 540 VDC across multi-phase inverter  190 . 
         [0048]    In addition, by using the topology of  FIG. 6A , bi-directional DC-DC converter  160  only processes ½ the total power used by the load connected to multi-phase inverter  190 . This is because the positive bus of the DC bus, e.g. the 270 VDC bus, is sent directly to multi-phase inverter  190  and only the power in the negative bus is processed by bidirectional DC-DC circuit  162 . 
         [0049]      FIG. 6B  illustrates examples of AC output waveforms that can be produced using the embodiment shown in  FIG. 6A . As seen in  FIG. 6B , in a bi-directional DC-AC converter consistent with the present invention, the AC output waveforms can be positively biased, negatively biased or non-biased (neutral). 
         [0050]    When not in flight, an aircraft using a power conversion unit consistent with the present invention can receive electrical power from AC or DC ground carts. For example, when power is received from EPC AC ground cart  46  ( FIG. 5 ) through AC ground interface  45 , the AC power supplied by EPC AC ground cart  46  is converted to DC by bi-directional DC-AC converter  180 . Specifically, multi-phase inverter  190  receives the AC power signal and feeds a DC power signal to bi-directional DC-DC converter  160 , which supplies high-voltage bus  25  via DC bus  101 . This configuration can be used to charge the high-voltage battery  20  via battery charger  32  and provide power to any DC equipment that is operating. Battery charger  22  will be described in more detail below. 
         [0051]    If EPC AC ground cart  46  is used to assist in main engine start, power from bi-directional DC-AC converter  180  is used to supplement the power from on-board high-voltage battery  20  feeding inverter circuit  135 . Inverter circuit  135  then converts the combined DC power to AC to start WFSM  220 . 
         [0052]    When power is received from DC ground cart  41  through DC ground interface  40 , the DC power is fed directly to high-voltage DC bus  25 . DC ground cart  41  can then be used to charge high-voltage battery  20  and provide power to any DC equipment that is operating. 
         [0053]    If EPC DC ground cart  41  is used to assist in main engine start, the DC power supplied by DC ground cart  41  supplements the power from on-board high-voltage battery  20  feeding inverter circuit  135 . As before, inverter circuit  135  converts the combined DC ower to AC to start WFSM  220 . If the rating of DC ground cart  41  is high enough, DC ground cart  41  can provide sufficient power to perform the main engine start without any need for supplemental power from on-board high-voltage battery  20 . 
         [0054]    Conversely, if on-board high-voltage battery  20  is big enough, then on-board high-voltage battery  20  may be used to perform the main engine start without the use of any ground carts. Although this configuration would allow the aircraft to be most autonomous, such a big on-board battery is typically not practical. 
         [0055]    In the embodiment shown above, DC bus  101  is uni-polar and inverter circuit  135  and bi-directional DC-AC converter  180  are configured to accept a uni-polar bus. However, power conditioning units with other topologies are also within the scope of the present invention. Another exemplary embodiment is shown in  FIG. 8 . 
         [0056]    In  FIG. 8 , bi-polar DC bus  101 A connects inverter  120 A of inverter circuit  135 A to multi-phase inverter  190  of bi-directional DC-AC converter  180 A. In this topology, bidirectional DC-AC converter  180 A is configured such that when connected to inverter circuit  135 , bi-directional DC-DC converter  160 , multi-phase inverter  190  and inverter circuit  135  are connected in “parallel.” Bi-directional DC converter  160  is connected to high-voltage DC bus  25  via uni-polar DC bus  101 B. 
         [0057]    In this embodiment, because inverter circuit  135 A provides a bi-polar DC bus, bi-directional DC-DC converter  160  is not needed to convert a uni-polar bus to a bi-polar DC bus for multi-phase inverter  190 . However, bi-directional DC-DC converter  160  is configured to perform DC-DC conversion between uni-polar high-voltage DC bus  25  and bi-polar DC bus  101 A. Bi-directional DC-DC converter  160  in this embodiment may also be configured as shown in  FIG. 6A . 
         [0058]    As described above, power conditioning units having topologies that incorporate bi-directional DC-AC converters consistent with the present invention can produce a non-biased, isolated AC voltage supply to the legacy equipment. Because these bi-directional DC-AC converters are transformer-less, the size and weight of the power conditioning unit will be optimal for an aircraft. 
         [0059]    In addition, because these bi-directional DC-AC converters include a bi-directional DC-DC converter that supplies regulated power to the multi-phase inverter feeding the legacy AC system, the power blackouts on the legacy loads are minimized during main engine starts. 
         [0060]    However, some DC equipment on the high-voltage DC bus  25  that require reboot sequences if power is interrupted can still be adversely affected during main engine start (avionics are typically on the low-voltage, e.g. 28 volt, bus and, thus, generally not affected by voltage fluctuations on the high-voltage bus). The problems associated with the DC equipment requiring reboot sequences will only increase as more equipment is transferred from the legacy AC bus to the high-voltage DC bus. 
         [0061]    Accordingly, an embodiment of the present invention includes a high-voltage battery charger that comprises a bi-directional DC-DC converter. As shown in  FIGS. 5 and 8 , bi-directional DC-DC battery charger  22  can be placed between high-voltage battery  20  and high-voltage battery bus  25 . Disconnect switches S 1  and S 2  allow for various modes of operation described in more detail below. 
         [0062]    Typically, an unregulated battery system can vary from −40% to +15% of the rated DC bus voltage due to processes ranging from heavy load engine start to “charging” the battery. This variation in voltage may create problems with DC equipment, on high-voltage bus  25 , that require reboot sequences if power is interrupted. By placing bi-directional DC-DC battery charger  22  between high-voltage DC bus  25  and high-voltage battery  20 , regulated DC bus  26  can be created. The DC equipment requiring reboot sequences can then be reconfigured to receive power from regulated DC bus  26 . Using this arrangement, regulated DC bus  26  will have approximately a ±5% window of regulation, which will ensure that the DC equipment remain on-line. If the voltage on high-voltage battery  25  varies due to the main engine being started up via WFSM  220 , bi-directional DC-DC battery charger  22  will regulate the DC voltage going to regulated DC bus  26  at a desired voltage level (e.g., 270 volts). 
         [0063]    When the main engine is being started by WFSM  220  using EPC DC ground cart  40  and/or high-voltage battery  20 , switch S 1  is open and switch S 2  is closed. With S 1  open, the DC equipment requiring reboot sequences will be isolated from high-voltage DC bus  25  during engine start. With S 2  closed, power from high-voltage DC battery  20  can supplement power from EPC DC ground cart  40  to start WFSM  220  via power conditioning unit  100  or  100 A. Power also flows to bi-directional DC-DC battery charger  22  to supply regulated DC bus  26 . During main engine start-up, the voltage at the output of high-voltage DC battery  20  and, hence, high-voltage bus  25  may vary significantly. However, high-voltage battery charger  22  will ensure that the voltage on regulated DC bus  26  remains relatively constant. Similarly, bi-directional DC-AC converters  185  and  185 A of power conditioning units  100  and  100 A, respectively, ensure that the AC voltage on the legacy AC system remains at a desired level, e.g., 115 volts. 
         [0064]    If EPC AC ground cart  46  is used to provide supplemental power to help on-board high-voltage battery  20  in starting the main engine, then, along with powering legacy AC equipment, the power from EPC AC ground cart  46  will flow to WFSM  220  via power conditioning unit  100  or  100 A. Similar to the scenario given above, the voltage on high-voltage DC bus  25  and, hence, high-voltage battery  20  could vary. As before, bi-directional DC-DC battery charger  22  will ensure that the voltage on regulated DC bus  26  remains relatively constant. 
         [0065]    When the main generator, WFSM  220 , is on-line, switch SI may be closed to provide regulated DC voltage to high-voltage bus  26  from the output of power conditioning unit  100  or  100 A. If WFSM  220  is on-line and switch S 1  is closed, high-voltage battery  20  may be recharged and can be put on “float charge” by bi-directional DC-DC battery charger  22 . “Float charge” is the condition where the high-voltage battery  20  is maintained in the fully charged state during normal operation. During normal operation, high-voltage battery  20  will remain on “float charge” with switch S 1  closed and switch S 2  open. 
         [0066]    If both switches S 1  and S 2  are closed, high-voltage battery  20  will recharge at its maximum rate. In this mode, bi-directional DC-DC battery charger  22  is inactive. 
         [0067]    By using switches S 1  and S 2 , high-voltage battery  20  can be maintained in-flight, including specially recharging high-voltage battery  20  if required due to battery chemistry. Traditionally, to “equalize charge” the battery, it was necessary to either remove the battery from the aircraft or provide special ground cart equipment to service the battery. “Equalize charge” is the condition where maintenance is performed on a battery by applying a voltage that is higher (e.g., by 15%) than the normal rated voltage. 
         [0068]    However, with the embodiments shown in  FIGS. 5 and 8 , the high-voltage battery  22  can be put on “equalize charge” whenever WFSM  220  on-line (in-flight or on the ground) by closing switch S 1 , opening switch S 2  and increasing the output voltage setting of bi-directional DC-DC battery charger  22 . 
         [0069]    When the aircraft is not in flight and the main generator is off-line, EPC ground cart power (AC or DC) can be connected to the respective ground interfaces  40  and  45  to provide external power to the aircraft. If switch S 1  is closed and switch S 2  is open, the external power can charge (float or equalize) high-voltage batter  20 . 
         [0070]    Additionally, if the aircraft is on the ground and WFSM  220  is on-line, then aircraft power may be used to supply power externally. If switch S 1  is open and switch S 2  is closed, power from WFSM  220  can be sent externally via EPC terminals  40  or  45  while still maintaining voltage on regulated DC bus  26  using bi-directional DC-DC battery charger  22 . 
         [0071]    In the above embodiments, power conditioning units  100  and  100 A are uni-polar bus units that generally use the aircraft metal chassis as a conductor path similar to a 12 Volt negative ground system on an automobile. However, the present invention can also be applied to a bi-polar bus arrangement. 
         [0072]    Although the present invention has been taught with 270 VDC and 115 VAC systems, the present invention is also applicable to emerging “double voltage” 540 VDC and 230 VAC systems. 
         [0073]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.