Uninterrupted energy exchange

Abstract

Device comprising: a DC-DC converter (112) comprising a filter circuit (124) and a DC-DC converter output (115), wherein the filter circuit (124) comprises a filter circuit input (127) and a filter circuit output (131) comprising the DC-DC converter output (115); an AC-DC converter (114) comprising an AC-DC converter output (132); and a connector (141) connecting the AC-DC converter output (132) to the filter circuit input (127) in the DC-DC converter (112), wherein the AC-DC converter (114) has a transformer rectifier unit (137) which has a transformer (138) connected to a rectifier (140), wherein the transformer (138) has: a number of transformer inputs (305);a number of winding sets (311, 312, 313), wherein each winding set in the number of winding sets (311, 312, 313) has a primary winding (314, 340, 365) and secondary windings (315, 341, 366), wherein the primary winding (314, 340, 365) in each winding set (311, 312 or 313) is connected between two of the number of transformer inputs (317, 318, 343) and the secondary windings (315, 341, 366) in each winding set (311, 312, 313), which consists of six secondary windings (319, 320, 321, 322, 323, 324); and a number of transformer outputs (306) connected to the rectifier (304), the four transformer outputs (306) being for each winding set in the plurality of winding sets, each of the four transformer outputs (306) for a winding set in the number of winding sets (311, 312, 313) being connected to a number of the secondary windings (315, 341, 366) in the winding set (311, 312, 313).

Description

BACKGROUND INFORMATION1. Area:The present disclosure relates generally to electrical power systems, and more particularly to aircraft electrical power systems. More specifically, the present disclosure relates to a method and an apparatus for switching electric power provided for a load between different electric power sources and a transformer rectifier unit for converting AC power to DC power.2. Background:In aircraft, various systems and devices may use electrical energy. By way of example and not limitation, electrical energy may be used by a flight control system, in-flight entertainment systems, computers, actuators, lights, fans, pumps, or other systems and devices onboard an aircraft. Some systems and devices on board an aircraft may use DC power. Other systems and devices on board an aircraft may use AC power.Electrical power for systems and devices onboard an aircraft may be provided by an electrical power system. An electrical power system on board an aircraft may include various sources of electrical power and various structures for supplying electrical power from the electrical power sources to various systems and devices on board the aircraft. An electrical power system on board an aircraft may include a number of AC power sources, a number of DC power sources, or both. Without limitation, electrical power may be provided, for example, from different power sources to different systems and devices that use electrical power on board the aircraft via a number of buses. For example, AC power may be provided from a number of AC power sources to aircraft onboard systems and devices that use AC power via a number of AC buses onboard the aircraft. DC power may be provided from a number of DC power sources to aircraft systems and devices that use DC power via a number of DC buses on board the aircraft.Alternating current power on board an aircraft may be provided by a plurality of generators, by a plurality of other suitable alternating current power sources, or by various combinations of alternating current power sources. Generators for providing AC power on board an aircraft may be operated by the aircraft engines, in any other suitable manner, or in a combination of suitable ways.An AC-DC converter may represent an example of a DC power source on board an aircraft. An AC power may be converted to DC power by a plurality of AC-DC converters on board an aircraft. Without limitation, an AC-DC converter may be configured to receive AC power provided by an AC power source via an AC bus onboard the aircraft and provide DC power via a DC bus onboard the aircraft, for example. An AC-DC converter on board an aircraft may be configured to provide DC power at a desired voltage for use by various systems and devices that use DC power on board the aircraft from the AC power provided by the AC power source over the AC bus lines.A battery could represent another example of a DC power source on board an aircraft. An aircraft-onboard battery may provide DC power at a certain voltage. A DC-DC converter on board the aircraft may be configured to convert a DC energy provided by the battery at a first voltage to a DC energy at a second voltage desirable for use by various systems and devices that use a DC energy on board the aircraft. For example, a DC-DC converter on board an aircraft may be connected to provide DC power at a desired voltage via a DC bus on board the aircraft to distribute the DC power to various systems and devices on board the aircraft via the DC bus.DC power to various systems and devices on board an aircraft may be provided via a DC bus on board the aircraft by various DC power sources at different times. Without limitation, DC power may be provided, for example, via a DC bus onboard an aircraft, by an AC-to-DC converter at times when the aircraft engines are operating and the AC-to-DC converter receives AC power from a generator powered by an aircraft engine. At other times, DC power may be provided via the DC bus on board the aircraft by a DC-DC converter that receives DC power from a battery or other backup power source. Therefore, at various times, it may be desirable to switch from providing power via a DC bus onboard an aircraft by an AC-DC converter to providing power via the DC bus of the aircraft by a DC-DC converter, and vice versa.It may be desirable that switching the provision of power over a DC bus on board an aircraft between different DC power sources does not affect the DC power provided to the aircraft systems and devices over the DC bus in an undesired manner. Currently, switching the provision of power via a DC bus on board an aircraft between different DC power sources may result in an undesired interruption of the power provided via the DC bus. A method and apparatus for switching the provision of power over a DC bus between different DC power sources without power interruption over the DC bus may be desirable.An AC-DC converter on board an aircraft may be implemented as a transformer rectifier unit. A transformer rectifier unit includes a transformer and a rectifier. Without limitation, a transformer in a transformer rectifier unit on board an aircraft may be configured to transform a polyphase AC input signal provided by an AC power source into a polyphase AC output signal. The transformer may be configured to transform the voltage, phase, other characteristics, or various combinations of the characteristics of the polyphase AC input signal received from the transformer to provide the polyphase AC output signal. The rectifier in the transformer rectifier unit may be configured to convert this polyphase AC output signal provided by the transformer in the transformer rectifier unit into a DC power signal.It may be desirable that the quality of the DC power provided by the transformer rectifier unit on board the aircraft be adequate. Without limitation, for example, it may be desirable that voltage ripple in the DC power signal provided by a transformer rectifier unit be reduced. It could also be desirable to simplify the construction of a transformer rectifier unit on board an aircraft.Document US 6 414 403 B2 relates according to its abstract to a power supply unit with a built-in battery for supplying voltage from the battery over a long period of time by fully exploiting the capabilities of the built-in battery. The power unit supplies electric power to a load even when a battery having a higher battery voltage than the voltage of the external power source is used by charging the battery from an external power source even when the voltage of the external power source is lower than the battery voltage. By charging the battery when electric power is supplied from an external power source to a load, the power unit supplies electric power from the battery to the load when supply of electricity from an external power source is cut off. The power unit includes an inverter to boost the output voltage from the battery and supply the voltage to the load when the battery is discharged and the voltage of the external power source is higher than the battery voltage. Alternatively, the power unit includes a converter for boosting the voltage of the external power source and charging the battery when the battery is used at a voltage lower than the voltage of the external power source.Document US 7 928 701 B2 relates, in summary, to an AC-DC converter comprising a rectifier for rectifying an alternating current supplied from an alternating current source, a power factor controller connected to an output side of the rectifier for improving a power factor and also outputting either a power or a current limited to a predetermined value, a DC-DC converter converting a voltage output from the power factor controller into another voltage, a capacitor for storing energy, and a two-way converter to which an input / output terminal is connected, the output side of the power factor controller, and the other input / output terminals connected to the capacitor for performing two-way power conversion.According to its summary, the publication US 2006 / 0 227 582 A1 relates to a rectifier comprising a transformer, a rectifier bridge, a connection capacitor and at least one first resonant loop. The transformer is operable to receive three-phase input signals at a source frequency and generate a plurality of transformer phase signals. The rectifying bridge has a number of stages corresponding to the number of transformer phase signals and is operable to generate a rectified output signal based on the transformer phase signals. The link capacitor is coupled via the rectifier bridge. The first resonant loop is coupled between the rectifier bridge and the link capacitor. The first resonant loop has a first resonant frequency corresponding to a harmonic frequency of the source frequency.Document US 6 577 513 B1, in summary, relates to an on-line seamless power supply system (UPS) with built-in lighting equipment, capable of supplying sinusoidal alternating current from a single direct current source by increasing the direct current voltage of its battery unit and converting it via a direct current into a sinusoidal alternating current output, to an alternating current inverter circuit according to pulse width modulation (PWM) signals supplied in succession by a control circuit during the power failure, and at the same time starts the built-in lighting equipment to illuminate the environment. The AC output power is intensified and stabilized by series and parallel connections of the internal circuit components of the UPS.US 4 340 823 A, in summary, relates to an uninterrupted power supply comprising a rectifier for receiving a first alternating current signal and for converting the first alternating current signal into a first direct current signal for generating a direct current power output of the uninterrupted power supply, a battery for supplying a second direct current signal, a controlled rectifier for receiving a second alternating current signal and converting the second alternating current signal into a controlled output power, and a switch connected to receive the second direct current signal from the battery for generating the second direct current signal as direct current output power of the first alternating current signal is interrupted. Output terminals of the rectifier are connected in series with output terminals of the controlled rectifier and the battery, whereby the first DC power signal and the controlled output power are added and applied to the battery to charge the same.Therefore, it would be desirable to have a method and apparatus that would take into account at least some of the items discussed above, as well as other possible items.SUMMARYAn illustrative embodiment provides an apparatus comprising a DC-DC converter, an AC-DC converter, and a connector. The DC-DC converter includes a filter circuit and a DC-DC converter output. The filter circuit has a filter circuit input and a filter circuit output having a DC-DC converter output. The AC-DC converter has an AC-DC converter output. The connector connects the AC-DC converter output to the filter circuit input in the DC-DC converter. The AC-DC converter includes a transformer rectifier unit including a transformer connected to a rectifier, the transformer including: a number of transformer inputs; a number of winding sets, each winding set in the number of winding sets including a primary winding and secondary windings, the primary winding in each winding set being connected between two of the number of transformer inputs and the secondary windings in each winding set consisting of six secondary windings; and a number of transformer outputs connected to the rectifier and including four transformer outputs for each winding set in the plurality of winding sets, each of the four transformer outputs for one winding set in the number of winding sets being connected to a number of the secondary windings in the winding set.An illustrative embodiment also provides a method for transferring energy to a load. An AC-to-DC converter output for an AC-to-DC converter is connected to a filter circuit input in a DC-to-DC converter via a connector, such that an uninterrupted power change occurs when switching between providing power to the load by the AC-to-DC converter and providing power to the load by the DC-to-DC converter. The AC-DC converter includes a transformer rectifier unit including a transformer connected to a rectifier, the transformer including: a number of transformer inputs; a number of winding sets, each winding set in the number of winding sets including a primary winding and secondary windings, the primary winding in each winding set being connected between two of the number of transformer inputs and the secondary windings in each winding set consisting of six secondary windings; and a plurality of transformer outputs connected to the rectifier and including four transformer outputs for each winding set in the number of winding sets, each of the four transformer outputs for a winding set in the number of winding sets connecting a number of the secondary windings in the winding set to the rectifier.The features and functions may be achieved independently in various embodiments of the present disclosure or may be combined in still further embodiments in which further details may be seen with reference to the following description and drawings.BRIEF DESCRIPTION OF THE DRAWINGSThe novel features characteristic of the illustrative embodiments are set forth in the appended claims. The illustrative embodiments, as well as a preferred mode of use, other objects and features thereof, will, however, be best understood by reference to the following detailed description of an illustrative embodiment of the present disclosure when read in conjunction with the accompanying drawings, wherein: FIG. 1 illustrates an illustration of a block diagram of an electrical power system in accordance with an illustrative embodiment; FIG. 2 illustrates an illustration of a circuit diagram of an electrical power system according to an illustrative embodiment; FIG. 3 illustrates an illustration of a block diagram of a transformer rectifier unit according to an illustrative embodiment; FIG. 4 illustrates an illustration of a circuit diagram of a transformer rectifier unit according to an illustrative embodiment; FIG. 5 illustrates an illustration of a pointer diagram of a transformer according to an illustrative embodiment; FIG. 6 illustrates an illustration of a flow chart of a process for changing power according to an illustrative embodiment; FIG. 7 illustrates an illustration of a voltage graph during a conventional power change; FIG. 8 illustrates an illustration of a voltage graph during a power change according to an illustrative embodiment; FIG. 9 illustrates an illustration of a graph of an output voltage provided by a conventional transformer rectifier unit; and FIG. 10 illustrates an illustration of a graph of an output voltage provided by a transformer rectifier unit according to an illustrative embodiment.DETAILED DESCRIPTIONIllustrative embodiments recognize and consider a number of considerations. "A number" as used herein with reference to items means one or more items. "A number of different considerations" represents, for example, one or more different considerations.Illustrative embodiments recognize and take into account that, at present, switching between different electrical energy sources to provide electrical energy to a load may result in interruption of the electrical energy provided to the load by the electrical energy sources. Currently, for example, an AC-DC converter and a DC-DC converter may be connected in parallel to a DC bus on board an aircraft to provide DC power to a load on board the aircraft via the DC bus alternately through the AC-DC converter and the DC-DC converter. In this case, for example, changing from providing power to the DC bus by the AC-DC converter to providing power to the DC bus by the DC-DC converter may result in a relatively large voltage drop that may exceed power quality limits. An example of such a relatively large voltage drop resulting from such a change of supplying power by the AC-DC converter to supplying power by a DC-DC converter is illustrated in FIG. 7 and will be described below with reference to FIG. 7.Illustrative embodiments recognize and consider that interruption of the electrical energy provided to a load that results from switching between different electrical energy sources to provide electrical energy to the load may affect the load in an undesired manner. Currently, aircraft systems and devices that receive electrical energy via a DC bus on board the aircraft may be designed to sustain a break and operate normally in response to a break in the electrical energy provided via the DC bus, the break resulting from a transition between providing energy by an AC-to-DC converter via the DC bus and providing energy by a DC-to-DC converter via the DC bus. Illustrative embodiments recognize and consider that aircraft-onboard systems and devices configured to sustain such electrical energy interruptions may be more expensive, complex, and heavier, result in higher costs for operating an aircraft, or result in various other undesired constraints, or various combinations of undesired constraints.Illustrative embodiments recognize and contemplate that seamless power changes between different sources of electrical power may avoid some or all of the limitations of current systems and methods for changing between different electrical power sources. Therefore, illustrative embodiments provide a method and apparatus for seamless power changes between different electrical power sources. Illustrative embodiments may maintain seamless performance, for example, when switching between providing power to a load by an AC-DC converter and providing power to the load by a DC-DC converter. Without limitation, illustrative embodiments may provide seamless power changes, for example, by connecting an AC-DC converter output to a filter circuit input in a DC-DC converter to integrate the AC-DC converter and the DC-DC converter into a single unit.Illustrative embodiments recognize and contemplate that an AC-DC converter used as a DC power source may include a transformer rectifier unit. Currently, a transformer rectifier unit used as a DC power source in an aircraft or other application may provide an output voltage including a relatively large ripple, which may be greater than desired. An example of such a relatively large voltage ripple in the output of a transformer rectifier unit is illustrated in FIG. 9 and will be described with reference to FIG. 9. Illustrative embodiments recognize and consider that aircraft systems and devices using the power provided by a DC power source may be affected by a voltage ripple greater than desired in the power provided by the DC power source in an undesirable manner. Illustrative embodiments recognize and take into account that, therefore, reducing DC voltage ripple in an aircraft power system is one of the goals in designing modern aircraft.Illustrative embodiments recognize and consider that currently additional phase-rotation transformers or inductors and capacitors may be used in a transformer rectifier unit on board an aircraft to reduce output voltage ripple of the transformer rectifier unit to meet design requirements. Illustrative embodiments recognize and contemplate that voltage ripple at the output of a transformer rectifier unit may be reduced by using a transformer in the transformer rectifier unit configured to provide a relatively large number of output pulses per cycle and a corresponding rectifier to rectify the relatively large number of output pulses to provide the output to the transformer rectifier unit. Without limitation, a transformer that provides 24 output pulses per cycle and a 24-pulse rectifier may be used in the transformer rectifier unit, for example, to reduce voltage ripple at the output of the transformer rectifier unit. Currently, such a 24-pulse transformer may require a relatively large number of windings. For example, current 24-pulse transformers may require 24 or 33 transformer windings. Illustrative embodiments recognize and consider that current solutions for reducing DC voltage ripple in an aircraft power system may require the use of transformer rectifier units that include additional components. Such current solutions may be more expensive, more complex, heavier, result in higher cost for operating an aircraft, or result in various other undesired constraints or various combinations of the undesired constraints.Illustrative embodiments provide a transformer rectifier unit that provides a relatively low output voltage ripple while avoiding some or all limitations of current solutions to reduce voltage ripple at the output of a transformer rectifier unit. Illustrative embodiments provide a transformer for a transformer rectifier unit that reduces voltage ripple at the output of the transformer rectifier unit using fewer transformer windings. Without limitation, illustrative embodiments provide, for example, a 24-pulse transformer with only 21 windings. A transformer according to an illustrative embodiment may be less complex, simpler and less expensive to manufacture and maintain and may have higher reliability than transformers currently used to reduce output voltage ripple in current transformer rectifier units.Illustrative embodiments also recognize and consider that the voltage on a DC bus on board an aircraft may need to be within an allowable voltage range. The allowable voltage range may represent, for example, a DC bus voltage range in which aircraft onboard systems and devices receiving power from the DC bus may properly operate. Currently, the voltage ripple at the output of the transformer rectifier unit that supplies power to a DC bus on board an aircraft may be so large that the voltage provided across the DC bus by the transformer rectifier unit may vary over the entire allowable voltage range for the DC bus or large portions thereof when the nominal DC output voltage for the transformer rectifier unit is set to the middle of the allowable voltage range.The voltage ripple at the output of a transformer rectifier unit according to an illustrative embodiment may be sufficiently small that the transformer rectifier unit may be configured to provide power over the DC bus that is included within a smaller first voltage range in the allowed voltage range. A DC-DC converter may be configured to provide power over the DC bus that is included within a separate second voltage range in the allowed voltage range. In this case, the source of DC power provided via the DC bus at a particular time can be determined relatively easily by determining the voltage on the DC bus at that time and by determining whether the identified voltage on the DC bus is in the first voltage range or in the second voltage range.Turning now to FIG. 1, an illustration of a block diagram of an electrical power system according to an illustrative embodiment is illustrated. For example, without limitation, an electrical energy system 100 may provide electrical energy to an aircraft 102.The aircraft 102 may be a commercial passenger aircraft, a cargo aircraft, a private or personal aircraft, a military aircraft, or any other suitable type of aircraft used for any suitable purpose. The aircraft 102 may be a fixed wing aircraft, a rotorcraft aircraft, or a lighter-than-air aircraft. The aircraft 102 may be a manned aircraft or an unmanned aircraft.The aircraft 102 represents an example of a vehicle 104.Illustrative embodiments may include the electrical energy system 100 for providing electrical energy on board the vehicle 104 that is different from the aircraft 102. The vehicle 104 may be any vehicle configured for operation in the air, room, land, water, underwater, or any other medium or combinations of media.The vehicle 104 represents an example of a platform 106. Illustrative embodiments may include the electrical power system 100 for providing electrical power on the platform 106 that is different from the vehicle 104. Without limitation, platform 106 may be, for example, a building, an oil platform, or any other suitable mobile or fixed platform.The electrical power system 100 provides DC power to a load 108. The load 108 may include a number of loads on the aircraft 102, the vehicle 104, or the platform 106. The load 108 may include any system or device configured to use DC power. Without limitation, the load 108 on board the aircraft 102 may include, for example, instruments, controllers, motors, other systems or devices, or various combinations of systems and devices on board the aircraft 102 that use DC power.The electrical power system 100 may be configured to provide DC power to the load 108 via a DC bus 110. The DC bus 110 may have any suitable structure for providing DC power from a DC power source to the load 108. Without limitation, the DC bus 110 may include, for example, suitable wires that connect a DC power source to the load 108 for carrying a DC electrical power signal from the DC power source to the load 108. The DC bus 110 may be considered a part of the load 108 to which DC power is provided by a DC power source.The electrical power system 100 is configured to provide DC power to the load 108 through a DC-DC converter 112 and an AC-DC converter 114. The DC-DC converter 112 and the AC-DC converter 114 are examples of DC power sources for the load 108.The DC-DC converter 112 is configured to provide a DC power to the load 108 via a DC-DC converter output 115 from a DC power provided by the DC power source 116. The DC-DC converter output 115 may be connected to the DC bus 110 in any suitable manner to provide DC power from the DC-DC converter 112 to the load 108.The DC power source 116 may include any suitable DC power source. Without limitation, the DC power source 116 may include, for example, a battery 118.Alternatively, or in addition, the DC power source 116 may include a DC bus different from the DC bus 110, a DC-DC converter different from the DC-DC converter 112, an AC-DC converter different from the AC-DC converter 114, any other suitable DC power source, or various suitable combinations of DC power sources.The DC-DC converter 112 may be configured to convert the DC power provided by the DC power source 116 at one voltage to a DC power at another voltage suitable for the load 108. Without limitation, the DC-DC converter 112 may be configured to convert the DC power provided by the DC power source 116 having a relatively high voltage into DC power for the load 108 having a relatively low voltage, for example. Alternatively, or additionally, the DC-DC converter 112 may be configured to convert the DC power provided by the DC power source 116 at a relatively low voltage into a DC power for the load 108 at a relatively higher voltage. Alternatively, or in addition, the DC-DC converter 112 may be configured to change a number of other characteristics of the DC power provided by the DC power source 116 to provide a DC power having a number of characteristics that may be suitable for the load 108.The DC-DC converter 112 may be implemented in any suitable manner. Without limitation, the DC-DC converter 112 may be implemented as, for example, a switching type AC converter including an inverter 120, a transformer 121, a rectifier 122, a filter circuit 124, and a voltage regulator 126. The inverter 120 may convert a direct current from the direct current power source 116 into an alternating current using a suitable switching circuit. The alternating current generated by inverter 120 drives the primary winding of transformer 121. The transformer 121 may be configured to convert a voltage up or down to the required output level at the secondary winding of the transformer 121. The rectifier 122 may be configured to convert the alternating current at the secondary winding of the transformer 121 into a direct current using a suitable diode circuit or in any other suitable manner.The output of rectifier 122 is provided to a filter circuit 124 at a filter circuit input 127. The filter circuit 124 is configured to smooth the output of the rectifier 122. The filter circuit 124 may include an inductor 128 and a capacitor 130. The smoothed DC power is provided by the filter circuit 124 at a filter circuit output 131. The filter circuit output 131 may also include the DC-DC converter output 115 of the DC-DC converter 112.The voltage regulator 126 may include a feedback circuit configured to monitor the output voltage of the DC-DC converter 112 and compare the output voltage of the DC-DC converter 112 with an output voltage desired for the DC-DC converter 112. The voltage regulator 126 may be configured to control switching devices in the inverter 120 in a suitable manner such that the monitored output voltage of the DC-DC converter 112 is controlled to match the desired output voltage for the DC-DC converter 112.The AC-DC converter 114 is configured to provide DC power to the load 108 via the AC-DC converter output 132 from AC power provided by an AC power source 133. The AC power source 133 may include any suitable AC power source. Without limitation, the AC power source 133 may include, for example, a generator 134. Without limitation, the generator 134 may be powered onboard the aircraft 102, for example, by an engine 135 of the aircraft 102.The AC power source 133 may provide single phase or polyphase AC power. Without limitation, AC power source 133 may provide three-phase 136 AC power, for example. Alternatively, the AC power source 133 may provide polyphase AC power with less or more than three phases.The AC-DC converter 114 may be implemented in any suitable manner. Preferably, the AC-DC converter 114 is configured to provide DC power with a relatively low voltage ripple at the AC-DC converter output 132. Without limitation, the AC-DC converter 114 may be implemented, for example, as a transformer rectifier unit 137 including a transformer 138 and a rectifier 140.In this case, AC power may be provided from AC power source 133 to the input of transformer 138. The transformer 138 may be configured to convert the AC power provided by the AC power source 133 at one voltage to an AC power at another voltage at the output of the transformer 138. Without limitation, the transformer 138 may be configured to convert an AC power provided by the AC power source 133 having a relatively high voltage into an AC power having a relatively lower voltage, for example. Alternatively, or additionally, the transformer 138 may be configured to convert an AC power provided by the AC power source 133 having a relatively low voltage into an AC power having a relatively higher voltage. Alternatively, or in addition, the transformer 138 may be configured to change a number of other characteristics of the AC power provided by the AC power source 133. Without limitation, the transformer 138 may be configured to convert a number of phases, such as provided by the AC power source 133, to an AC power having a different number of phases at the output of the transformer 138, for example.The output of the transformer 138 is provided to the rectifier 140. The rectifier 140 may be configured to convert the alternating current at the output of the transformer 138 to a direct current using a suitable diode circuit or in any other suitable manner. The DC power of the rectifier 140 may be provided as the output of the AC-DC converter 114 to the AC-DC converter output 132.According to an illustrative embodiment, the AC-DC converter output 132 is connected via a connector via a connector 141 to the filter circuit input 127 in the DC-DC converter 112. The connector 141 may have any suitable structure for connecting the DC power provided by the AC-DC converter 114 at the AC-DC converter output 132 to the filter circuit input 127 in the DC-DC converter 112. Without limitation, the connector 141 may include, for example, a suitable cable.The connector 141 may or may not include a switch 142. The switch 142 may be implemented in any suitable manner to connect or disconnect the AC-DC converter output 132 to the filter circuit input 127 in the DC-DC converter 112 via the connector 141. Without limitation, the switch 142 may include, for example, a suitable electronic, mechanical, or electromechanical switching device.Operation of one or more components of the electrical power system 100 may be controlled by a controller 146. For example, the controller 146 may be implemented in any suitable manner to control the DC-DC converter 112, the AC-DC converter 114, the AC power source 133, the switch 142, other components of the electrical power system 100, or various combinations of components of the electrical power system 100 in any suitable manner. Without limitation, the controller 146 may be configured to control a number of components of the electrical power system 100 to change from providing power to the load 108 by the AC-DC converter 114 to providing power to the load 108 by the DC-DC converter 112, and vice versa.By connecting the AC-DC converter output 132 to the filter circuit input 127 in the DC-DC converter 112, an uninterrupted power change occurs when switching between providing power to the load 108 by the AC-DC converter 114 and providing power to the load 108 by the DC-DC converter 112. An uninterrupted power change may be defined as a change between providing power to the load 108 by the AC-DC converter 114 and providing power to the load 108 by the DC-DC converter 112 that does not drop the voltage on the DC bus 110 out of the DC voltage range 148. DC voltage range 148 may represent a desired or allowed range of voltage for the DC power provided to load 108 via DC bus 110. The DC voltage range 148 may be defined based on, for example, the range of a voltage on the DC bus 110 at which the load 108 receiving power from the DC bus 110 may be properly operated. Without limitation, the DC voltage range 148 on the DC bus 110 for the aircraft 102 may be, for example, about 28 volts to about 32 volts, or any other suitable voltage range.The DC voltage range 148 may include a first voltage range 150 and a second voltage range 152. Preferably, the first voltage range 150 and the second voltage range 152 are within the DC voltage range 148 and do not overlap. In other words, the first voltage range 150 and the second voltage range 152 are preferably separated by a gap 154 within the DC voltage range 148. Without limitation, the first voltage range 150 on the DC bus 110 for the aircraft 102 may be, for example, about 30.5 volts to about 32 volts, or any other suitable voltage range. The second voltage range 152 on the DC bus 110 for the aircraft 102 may be about 28 volts to about 29.5 volts or any other suitable voltage range.According to an illustrative embodiment, the AC-DC converter 114 may be configured to provide power via the DC bus 110 within the first voltage range 150 in the DC voltage range 148. The DC-DC converter 112 may be configured to provide power via the DC bus 110 within the second voltage range 152 in the DC voltage range 148. In this case, the source of DC power provided via the DC bus 110 at a particular time may be determined relatively easily by determining the voltage on the DC bus 110 at that time and by determining whether the identified voltage on the DC bus 110 is in the first voltage range 150 or the second voltage range 152.Turning now to FIG. 2, an illustration of a circuit diagram for an electrical power system according to an illustrative embodiment is shown. An electrical power system 200 may represent an example of an implementation of the electrical power system 100 of FIG. 1.The electrical power system 200 is configured to provide DC power to a load via a DC bus 202. The electrical power system 200 is configured to alternatively provide DC power via the DC bus 202 through a DC-DC converter 204 and a transformer rectifier unit 206.The DC-DC converter 204 is configured to provide the DC power at a DC-DC converter output 207 from the DC power provided by a DC power source 208. The DC-DC converter 204 may be configured in any suitable manner to provide the DC power provided by the DC power source 208 at one voltage to a DC power at a different voltage at the DC-DC converter output 207. The DC-DC converter output 207 may be connected to the DC bus 202 in any suitable manner to provide the DC power from the DC-DC converter 204 via the DC bus 202.The DC-DC converter 204 may be implemented as a switching AC converter including an inverter 210, a transformer 212, a rectifier 213, a filter circuit 214, and a voltage regulator 223. The inverter 210 may be configured to convert a direct current from the direct current power source 208 into an alternating current using switching devices 215, 216, 217, and 218 arranged in a suitable bridge configuration. Without limitation, switching devices 215, 216, 217, and 218 may include, for example, suitable transistors or other solid state switching devices or other suitable switching devices. The alternating current generated by inverter 210 drives a primary winding 219 of transformer 212. The number of turns and an arrangement of a primary winding 219 and secondary windings 220 and 221 of the transformer 212 may be selected to provide a desired output voltage at the secondary windings 220 and 221 of the transformer 212 for a predetermined input voltage provided at the primary winding 219 of the transformer 212. The rectifier 213 may be configured to convert the alternating current at the secondary windings 220 and 221 of the transformer 212 into a direct current using diodes 222 and 224.The output of rectifier 213 is provided to filter circuit 214 at filter circuit input 225. The filter circuit 214 is configured to provide a smoothed DC power at a filter circuit output 226. The filter circuit output 226 may also include the DC-DC converter output 207 for the DC-DC converter 204. In this example, the filter circuit includes an inductor 227 and a capacitor 228. The inductor 227 is connected in series between the filter circuit input 225 and the filter circuit output 226. Capacitor 228 is connected between filter circuit output 226 and ground 229. The inductor 227 and capacitor 228 may be selected appropriately to smooth the output of the rectifier 213, as desired.The voltage regulator 223 may include a feedback circuit configured to monitor the output voltage of the DC-DC converter 204 at the DC-DC converter output 207 and compare the output voltage of the DC-DC converter 204 at the DC-DC converter output 207 with a desired output voltage for the DC-DC converter 204. The voltage regulator 223 may be configured to control the switching devices 215, 216, 217, and 218 in the inverter 210 in a suitable manner to drive the monitored output voltage of the DC-DC converter 204 at the DC-DC converter output 207 to match the desired output voltage for the DC-DC converter 204.The transformer rectifier unit 206 is configured to provide DC power at the AC-to-DC converter output 230 from AC power provided by the AC power source 232. The transformer rectifier unit 206 may be implemented in any suitable manner. Preferably, the transformer rectifier unit 206 is configured to provide a DC power with a relatively low voltage ripple at the AC-to-DC converter output 230.The transformer rectifier unit 206 includes a transformer 234 and a rectifier 236. Without limitation, the three-phase AC power of AC power source 232 may be provided to the input of transformer 234 in this example. The transformer 234 may be configured to convert the three-phase AC power provided by the AC power source 232 to a 12-phase AC power having a desired voltage at the output of the transformer 234. Each of the 12 AC signals provided at the output of the transformer 234 has alternating positive and negative pulses. Therefore, the transformer 234 may be referred to as a 24-pulse transformer.The 12-phase AC output of transformer 234 is provided to rectifier 236. In this example, rectifier 236 includes a 24-pulse rectifier configured to convert the 24-pulse output of transformer 234 to AC energy at AC-DC converter output 230.The AC-DC converter output 230 is connected via a connector 238 to the filter circuit input 225 in the DC-DC converter 204. By connecting the AC-DC converter output 230 to the filter circuit input 225 in the DC-DC converter 204, an uninterrupted power change occurs when switching between providing power via the DC bus 202 by the transformer rectifier unit 206 and providing power via the DC bus 202 by the DC-DC converter 204. The connector 238 may include a switch 240. The switch 240 may be implemented in any suitable manner to connect or disconnect the AC-DC converter output 230 to or from the filter circuit input 225 in the DC-DC converter 204.The illustrations of FIGS. 1 and 2 are not intended to include physical or architectural limitations of the manner in which illustrative embodiments may be implemented. Other components may be used in addition to or in place of those illustrated. Some components may be optional. Also, blocks are depicted in the figures to illustrate some functional components. One or more of these blocks may be combined, shared, or combined, and divided into different blocks when implemented in an illustrative embodiment.Turning now to FIG. 3, an illustration of a block diagram of a transformer rectifier unit according to an illustrative embodiment is shown. A transformer rectifier unit 300 may represent an example of an implementation of the transformer rectifier unit 137 in FIG. 1 and the transformer rectifier unit 206 in FIG. 2. The transformer rectifier unit 300 is configured to convert an AC power provided by an AC power source 301 into a DC power 302.The transformer rectifier unit 300 includes a transformer 303 and a rectifier 304. In this example, the transformer 303 is configured to receive three phase AC power from the AC power source 301 at a number of transformer inputs 305. The transformer 303 is configured to convert the three-phase AC power at the number of transformer inputs 305 into a 12-phase AC power at a number of transformer outputs 306. Each of the 12 AC signals provided at the number of transformer outputs 306 alternately has positive and negative pulses. Therefore, the transformer 303 may be referred to as a 24-pulse transformer. In this case, the rectifier 304 comprises a 24-pulse rectifier configured to convert the 24-pulse output provided at the number of transformer outputs 306 into the DC power 302. The transformer rectifier unit 300 may provide the DC power 302 with a lower voltage ripple than may be provided by the transformer rectifier units providing the DC power with less than 24 pulses.The transformer 303 has a core 307. The core 307 may comprise one or more different suitable types of materials. Without limitation, the core 307 may include, for example, steel, iron, a metal alloy, some other types of metals, or a combination thereof.In this example, core 307 includes legs 308, 309, and 310. Each of the legs 308, 309, and 310 may include an elongated portion of the core 307. In this case, the legs 308, 309, and 310 may be formed integrally with the core 307. As used herein, a first object formed "uniformly" with a second object may be considered part of the second object.The transformer 303 also includes a number of winding sets. In this example, windings in winding sets 311 are wound around the leg 308 of the core 307. Windings in a winding set 312 are wound around the leg 309 of the core 307. Windings in a winding set 313 are wound around the leg 310 of the core 307.Each of the windings in the winding sets 311, 312, and 313 may include a number of turns of a conductive material. Without limitation, each of the windings in winding sets 311, 312, and 313 may include, for example, a coil of a conductive wire including aluminum, copper, a metal alloy, any other conductive material, or a combination of conductive materials.In this example, each winding set 311, 312, and 313 may have seven windings. Therefore, the transformer 303 is implemented with a total of 21 windings in this example. The transformer 303 provides a 24-pulse transformer that can be less complex, simpler, and less expensive to manufacture and maintain, and that has higher reliability than 24-pulse transformers that can be implemented with more than 21 windings.The winding set 311 includes the primary winding 314 and secondary windings 315. The primary winding 314 in the winding set 311 has a number of turns 316. The primary winding 314 may be connected between the transformer input 317 in the number of transformer inputs 305 and a transformer input 318 in the number of transformer inputs 305.The secondary windings 315 in the winding set 311 include a first winding 319, a second winding 320, a third winding 321, a fourth winding 322, a fifth winding 323, and a sixth winding 324. The first winding 319 has a first number of turns 325. The second winding 320 has a second number of turns 326. The third winding 321 has a third number of turns 327. The fourth winding 322 has a fourth number of turns 328. The fifth winding 323 has a fifth number of turns 329. The sixth winding 324 has a sixth number of turns 330.The first winding 319 is connected between a first point 331 and a second point 332. A transformer output 333 in the number of transformer outputs 306 is connected to the first point 331. The second winding 320 is connected between the second point 332 and a third point 334. The third winding 321 is connected between the third point 334 and a fourth point 335. The fourth winding 322 is connected between the second point 332 and the transformer output 336 in the number of transformer outputs 306. The fifth winding 323 is connected between the second point 332 and a transformer output 337 in the number of transformer outputs 306. The sixth winding 324 is connected between the third point 334 and a transformer output 338 in the plurality of transformer outputs 306.The first winding 319, the second winding 320, and the third winding 321 may include a single winding 339. In other words, the single winding 339 wound around the leg 308 may be used to function as the first winding 319, the second winding 320, and the third winding 321. In this case, the second point 332 may have a first tap point on the single winding 339 and the third point 334 may have a second tap point on the single winding 339.The winding set 312 includes a primary winding 340 and secondary windings 341. The primary winding 340 in the winding set 312 has a number of turns 342. The primary winding 340 may be connected between the transformer input 318 in the number of transformer inputs 305 and a transformer input 343 in the number of transformer inputs 305.The secondary windings 341 in the winding set 312 include a first winding 344, a second winding 345, a third winding 346, a fourth winding 347, a fifth winding 348, and a sixth winding 349. The first winding 344 has a first number of turns 350. The second winding 345 includes a second number of turns 351. The third winding 346 has a third number of turns 352. The fourth winding 347 has a fourth number of turns 353. The fifth winding 348 has a fifth number of turns 354. The sixth winding 349 has a sixth number of turns 355.The first winding 344 is connected between a first point 356 and a second point 357. A transformer output 358 in the plurality of transformer outputs 306 is connected to the first point 356. The first point 356 is also connected to the fourth point 335 in the secondary windings 315 in the winding set 311. The second winding 345 is connected between the second point 357 and a third point 359. The third winding 346 is connected between the third point 359 and a fourth point 360. The fourth winding 347 is connected between the second point 357 and a transformer output 361 in the number of transformer outputs 306. The fifth winding 348 is connected between the second point 357 and a transformer output 362 in the plurality of transformer outputs 306. The sixth winding 349 is connected between the third point 359 and a transformer output 363 in the number of transformer outputs 306.The first winding 344, the second winding 345, and the third winding 346 may include a single winding 364. In other words, the single winding 364 wound around the leg 309 may be used to function as the first winding 344, the second winding 345, and the third winding 346. In this case, the second point 357 may have a first tap point on the single winding 364 and the third point 359 may have a second tap point on the single winding 364.The winding set 313 includes a primary winding 365 and secondary windings 366. The primary winding 365 in the winding set 313 has a number of turns 367. The primary winding 365 may be connected between the transformer input 343 in the number of transformer inputs 305 and the transformer input 317 in the number of transformer inputs 305.The secondary windings 366 in the winding set 313 include a first winding 368, a second winding 369, a third winding 370, a fourth winding 371, a fifth winding 372, and a sixth winding 373. The first winding 368 includes a first number of turns 374. The second winding 369 includes a second number of turns 375. The third winding 370 includes a third number of turns 376. The fourth winding 371 has a fourth number of turns 377. The fifth winding 372 has a fifth number of turns 378. The sixth winding 373 has a sixth number of turns 379.The first winding 368 is connected between a first point 380 and a second point 381. A transformer output 382 in the plurality of transformer outputs 306 is connected to the first point 380. The first point 380 is also connected to the fourth point 360 in the secondary windings 341 in the winding set 312. The second winding 369 is connected between the second point 381 and a third point 383. The third winding 370 is connected between the third point 383 and a fourth point 384. The fourth point 384 is also connected to the first point 331 in the secondary windings 315 in the winding set 311. The fourth winding 371 is connected between the second point 381 and a transformer output 385 in the number of transformer outputs 306. The fifth winding 372 is connected between the second point 381 and a transformer output 386 in the plurality of transformer outputs 306. The sixth winding 373 is connected between the third point 383 and a transformer output 387 in the number of transformer outputs 306.The first winding 368, the second winding 369, and the third winding 370 may include a single winding 388. In other words, the single winding 388 wound around the leg 310 may be used to function as the first winding 368, the second winding 369, and the third winding 370. In this case, the second point 381 may have a first tap point on the single winding 388 and the third point 383 may have a second tap point on the single winding 388.Each of the primary windings 314, 340, and 365 may have the same number of turns Np. The first winding 319, the second winding 320, and the third winding 321 in the secondary windings 315, the first winding 344, the second winding 345, and the third winding 346 in the secondary windings 341, and the first winding 368, the second winding 369, and the third winding 370 in the secondary windings 366 may all have the same number of turns n 1. The fourth winding 322 and the sixth winding 324 in the secondary windings 315, the fourth winding 347 and the sixth winding 349 in the secondary windings 341, and the fourth winding 371 and the sixth winding 373 in the secondary windings 366 may all have the same number of turns n 2. The fifth winding 323 in the secondary windings 315, the fifth winding 348 in the secondary windings 341, and the fifth winding 372 in the secondary windings 366 may all have the same number of turns n 3.Ratios between the number of turns Np in the primary windings 314, 340, and 365 and the number of turns n 1, n 2, and n 3 in the secondary windings 315, 341, and 366 may be selected based on the desired voltage magnitude of the DC power 302 provided by the transformer rectifier unit 300 for the voltage magnitude of the AC power provided to the transformer rectifier unit 300 by the AC power source 301. Ratios between the number of turns n1, n2, and n3in the secondary windings 315, 341, and 366 may be selected such that when the phases of the AC input signals at the number of transformer inputs 305 are separated by about 120 degrees, the phases of the AC output signals at the number of transformer outputs 306 are separated by about 30 degrees. Without limitation, the ratio of n1to n2may be, for example, about 1:0.772 or another suitable ratio, and the ratio of n1to n3may be about 1:1.033 or another suitable ratio.In other words, if the phases of the AC input signals at the number of transformer inputs 305 are separated by about 120 degrees, the phase of the AC output signal at each of the number of transformer outputs 306 preferably captures the phase of a first different one of the AC output signals at a first different one of the number of transformer outputs 306 by about 30 degrees and responds to the phase of a second one of the AC output signals at a different second one of the number of transformer outputs 306 by about 30 degrees. As another example, the phases of the AC input signals at the number of transformer inputs 305 may be separated by about 110 degrees to about 130 degrees, and the phases of the AC output signals at the number of transformer outputs 306 may be separated by about 25 degrees to about 35 degrees. As another example, the phases of the AC input signals at the plurality of transformer inputs 305 may be separated by about 100 degrees to about 140 degrees, and the phases of the AC output signals at the plurality of transformer outputs 306 may be separated by about 20 degrees to about 40 degrees.Turning to FIG. 4, an illustration of a circuit diagram of a transformer rectifier unit according to an illustrative embodiment is shown. A transformer rectifier unit 400 may illustrate an example of an implementation of the transformer rectifier unit 300 in FIG. 3. The transformer rectifier unit 400 is configured to convert an AC power provided by the AC power source 401 into a DC power at an AC-to-DC converter output 402.The transformer rectifier unit 400 comprises a transformer 403 and a rectifier 404. In this example, the transformer 403 is configured to receive three-phase AC power from the AC power source 401 via transformer inputs 405, 406 and 407. The transformer 403 is configured to convert the three-phase AC power at the transformer inputs 405, 406 and 407 to 12-phase AC power at a number of transformer output lines 408. Each of the 12 AC signals provided to the plurality of transformer output lines 408 has alternating positive and negative pulses. Therefore, the transformer 403 may be referred to as a 24-pulse transformer. In this case, the rectifier 404 includes a 24-pulse rectifier configured to convert the 24-pulse output provided at the plurality of transformer output lines 408 into DC power at the AC-DC converter output 402. The transformer rectifier unit 400 may provide the DC power with a lower voltage ripple than may be provided by the transformer rectifier units providing DC power with less than 24 pulses.The transformer 403 includes three winding sets 411, 412, and 413. Each winding set 411, 412, and 413 may be wound around a separate leg of the core of the transformer 403. The winding sets 411, 412, and 413 may correspond to the winding sets 311, 312, and 313, respectively, in FIG. 3, for example.In this example, each winding set 411, 412, and 413 may have seven windings. Therefore, the transformer 403 is implemented with a total of 21 windings in this example. The transformer 403 provides a 24-pulse transformer that is less complex, simpler, and less expensive to manufacture and maintain, and that has higher reliability than the 24-pulse transformers that may be implemented with more than 21 windings.The winding set 411 includes a primary winding 414 and six secondary windings 415. The winding set 412 includes a primary winding 440 and six secondary windings 441. The winding set 413 includes a primary winding 465 and six secondary windings 466.For example, primary windings 414, 440, and 465 may correspond to primary windings 314, 340, and 365, respectively, in FIG. 3. The primary winding 414 may be connected between the transformer input 405 and a transformer input 406. The primary winding 440 may be connected between the transformer input 406 and a transformer input 407. The primary winding 465 may be connected between the transformer input 407 and the transformer input 405. The primary windings 414, 440, and 465 may be arranged in a delta configuration. The primary windings 414, 440, and 465 may each have Np turns.The secondary windings 415, 441, and 466 may correspond to the secondary windings 315, 341, and 366, respectively, in FIG. 3, for example. The secondary windings 415, 441, and 466 may also be arranged in a delta configuration. Each winding in the secondary windings 415, 441, and 466 may have n1, n2, or n3turns depending on the connections of the winding to other windings and to transformer output lines 408, as shown.Ratios between the number Npof turns in the primary windings 414, 440, and 465 and the number n1, n2, and n3of turns in the secondary windings 415, 441, and 466 may be selected based on the desired voltage magnitude of the DC power provided by transformer rectifier unit 400 at the AC-to-DC converter output 402 for the voltage magnitude of the AC power provided by transformer rectifier unit 400 via the AC power source 401. Ratios between the number n1, n2, and n3of turns in the secondary windings 415, 441, and 466 may be selected such that when the phases of the AC input signals at the transformer inputs 405, 406, and 407 are separated by about 120 degrees, the phases of the AC output signals at the transformer output lines 408 are separated by about 30 degrees.Turning now to FIG. 5, an illustration of a pointer diagram of a transformer according to an illustrative embodiment is shown. A pointer diagram 500 illustrates an example of relative phases of signals at various points in the transformer 403 in FIG. 4.Relative phases of signals in the primary windings of the transformer are indicated by the relative positions of points on a circle 502. Relative phases of signals in the secondary windings of the transformer are indicated by relative positions of points on a circle 504.Points 510, 512, and 514 may correspond to transformer inputs 405, 406, and 407 into primary windings 414, 440, and 465 in FIG. 4, for example. In this example, the AC input signals at transformer input points 510, 512, and 514 are separated at the primary windings by 120 degrees.Points 522 may correspond to points in the secondary windings 415, for example, that are connected to the transformer output lines 408 in FIG. 4. Points 524 may correspond to points in the secondary windings 441, for example, that are connected to the transformer output lines 408 in FIG. 4. Points 526 may correspond to points in the secondary windings 466 connected to the transformer output lines 408 in FIG. 4, for example. In this example, AC output signals at transformer output points 522, 524, and 526 are separated at the secondary windings by 30 degrees.The illustrations of FIGS. 3, 4, and 5 are not intended to mean physical or architectural constraints for a manner in which illustrative embodiments may be implemented. Other components may be used in addition to or in place of those illustrated. Some components may be optional. Also, blocks are depicted in the figures to illustrate some functional components. One or more of these blocks may be combined, shared, or combined, and divided into different blocks when implemented in an illustrative embodiment.Illustrative embodiments may include, for example, a transformer rectifier unit configured to receive as input an AC power having less or more than three phases. Illustrative embodiments may include a transformer configured to provide AC signals as outputs having more or less than 24 phases. Illustrative embodiments may include a rectifier configured to rectify AC signals having more or less than 24 phases.Turning now to FIG. 6, an illustration of a flow chart of a process for switching power according to an illustrative embodiment is shown. The process 600 may be implemented in the electrical power system 100 in FIG. 1, for example.Process 600 may begin with an AC-DC converter output being connected to a filter circuit input in a DC-DC converter (operation 602). The AC-DC converter output may or may not be connected to the filter circuit input in the DC-DC converter via a switch. DC power may then be provided to a load by the AC-DC converter (operation 604). The DC-DC converter output may be connected to the load, for example, via a DC bus or in any other suitable manner, such that the AC-DC converter provides the DC power to the load via the filter circuit in the DC-DC converter and via the DC bus.It may then be determined whether a condition is identified to change from providing energy to the load by the AC-DC converter to providing energy to the load by the DC-DC converter (operation 606). Without limitation, the state may be triggered at operation 606, or may be manually, automatically, and automatically identified, for example. In response to a determination that such a condition is not identified, process 600 returns to operation 604, and the DC power to the load continues to be provided by the AC-DC converter until such a condition is identified.In response to a determination that the condition is identified in operation 606, an uninterrupted transition may be made from providing energy to the load by the AC-DC converter to providing energy to the load by the DC-DC converter (operation 608). Without limitation, operation 608 may include, for example, turning off the AC-DC converter and simultaneously turning on the DC-DC converter. Alternatively, or additionally, operation 608 can include opening a switch in the connection between the AC-DC converter and the filter circuit input in the DC-DC converter. The DC power for the load may then be provided by the DC-DC converter (operation 610).It may then be determined whether a state to transition from providing power to the load by the DC-DC converter to providing power to the load by the AC-DC converter is identified (operation 612). Without limitation, the state to be identified may be triggered in operation 612, or may be manually, automatically, and automatically identified, respectively. In response to a determination that a condition is not identified, operation 600 returns to operation 610 and the DC power to the load continues to be provided by the DC-DC converter until such condition is identified.In response to a determination that the condition is identified in operation 612, an uninterrupted transition from providing energy to the load by the DC-DC converter to providing energy to the load by an AC-DC converter may be performed (operation 614). Without limitation, operation 614 may include, for example, turning off the DC-DC converter and simultaneously turning on the AC-DC converter. Alternatively, or additionally, operation 614 may include closing a switch in the connection between the AC-DC converter and the filter circuit input in the DC-DC converter. The process 600 may then return to operation 604, where the DC power is provided to the load by the AC-DC converter.Turning now to FIG. 7, an illustration of a graph of voltage during a conventional power change is shown. Graph 700 shows voltage on a y-axis 702 and time on an x-axis 704. Line 706 indicates a voltage provided to a load by an AC-DC converter connected in parallel with a DC-DC converter in a conventional manner. In this example, a changeover occurs between supplying power to the load by the AC-DC converter to supplying power to the load by the DC-DC converter at a time t=4.0 m. Line 706 shows a relatively large voltage drop when the energy change occurs. This relatively large voltage drop may result in an interruption of the energy supplied to the load.Turning now to FIG. 8, an illustration of a graph of voltage during a power change is shown according to an illustrative embodiment. Graph 800 shows voltage on y-axis 802 and time on x-axis 804.A line 806 indicates a voltage provided to a load by the AC-to-DC converter and the DC-to-DC converter used to provide power to the load in the example of FIG. 7. However, in this example, the AC-DC converter is connected to a filter circuit input in the DC-DC converter according to an illustrative embodiment, such as illustrated in FIG. 1 and as described with reference to FIG. 1. In this example, a changeover occurs between supplying power to the load by the AC-DC converter to supplying power to the load by the DC-DC converter at a time t=4.0 m. Line 806 shows a relatively small voltage drop when the power change occurs in this case. This relatively low voltage drop results in no interruption of the energy provided to the load. Therefore, line 806 indicates an uninterrupted power change according to an illustrative embodiment.Turning now to FIG. 9, an illustration of a graph of an output voltage provided by a conventional transformer rectifier unit is shown. Graph 900 shows voltage on y-axis 902 and time on x-axis 904.Line 906 shows the voltage ripple at the output of a 12-pulse transformer rectifier unit. A line 908 shows the average voltage at the output of the transformer rectifier unit. The voltage ripple is relatively large in this case.Turning now to FIG. 10, an illustration of a graph of an output voltage provided by a transformer rectifier unit according to an illustrative embodiment is shown. A graph 1000 shows voltage on a y-axis 1002 and time on an x-axis 1004.A line 1006 shows voltage ripple at the output of a 24-pulse transformer rectifier unit according to an illustrative embodiment, such as the transformer rectifier unit 300 in FIG. 3. A line 1008 shows an average voltage for the transformer rectifier unit. The voltage ripple at the output of a 24-pulse transformer rectifier unit according to an illustrative embodiment is less than the voltage ripple generated by the 12-pulse transformer rectifier unit shown in FIG. 9.Furthermore, the disclosure includes embodiments according to the following clause:Clause 1. Apparatus comprising:a DC-DC converter (112) having a filter circuit (124) and a DC-DC converter output (115), the filter circuit (124) having a filter circuit input (127) and a filter circuit output (131) having the DC-DC converter output (115);an AC-DC converter (114) having an AC-DC converter output (132); anda connector (141) connecting the AC-DC converter output (132) to the filter circuit input (127) in the DC-DC converter (112).Clause 2nd Apparatus of Clause 1, further comprising:a load (108) connected to the DC-DC converter output (115); andwherein the connector (141) connects the AC-DC converter output (132) to the filter circuit input (127) in the DC-DC converter (112) such that a seamless power change occurs when switching between supplying power to the load (108) by the AC-DC converter (114) and supplying power to the load (108) by the DC-DC converter (112).Clause 3rd apparatus according to Clause 1 or 2, wherein the AC-DC converter (114) comprises a transformer rectifier unit (137) comprising a transformer (138) connected to a rectifier (140).Clause 4th apparatus of Clause 3, wherein the transformer (138) comprises:a number of transformer inputs (305);a number of winding sets (311, 312, 313), each winding set in the number of winding sets (311, 312, 313) comprising a primary winding (314, 340, 365) and secondary windings (315, 341, 366), the primary winding (314, 340, 365) in each winding set (311, 312, 313) being connected between two of the number of transformer inputs (317, 318, 343) and the secondary windings (315, 341, 366) in each winding set (311, 312, 313) comprising six secondary windings (319, 320, 321, 322, 323, 324); anda number of transformer outputs (306) connected to the rectifier (304) and having four transformer outputs (306) for each set of windings in the number of sets of windings, each of the four transformer outputs (306) for a set of windings in the number of sets of windings (311, 312, 313) being connected to a number of the secondary windings (314, 341, 366) in the set of windings (311, 312, 313).Clause 5. The apparatus of Clause 1, 2, 3, or 4, wherein the connector (141) includes a switch (142) configured to connect or disconnect the AC-DC converter output (132) to or from the filter circuit input (127) in the DC-DC converter (112) via the connector (141).Clause 6th The apparatus of Clause 1, 2, 3, 4, or 5, wherein the filter circuit (124) comprises:an inductor (128) connected in series between the filter circuit input (127) and the filter circuit output (131); anda capacitor (130) connected between the filter circuit output (131) and ground (229).Clause 7. the apparatus of Clause 1, 2, 3, 4, 5, or 6, wherein:the DC-DC converter (112) is configured to provide a DC energy at the DC-DC converter output (115) within a first voltage range (150) in a DC voltage range (148);the AC-DC converter (114) is configured to provide a DC power at the DC-DC converter output (115) within a second voltage range (152) in the DC voltage range (148); andthere is a gap (154) between the first voltage range (150) and the second voltage range (152).Clause 8th Method for transferring energy to a load (108), comprising:connecting an AC-to-DC converter output (132) for an AC-to-DC converter (114) to a filter circuit input (127) in a DC-to-DC converter (112) via a connector (141), such that an uninterrupted power change occurs when switching between supplying power to the load (108) by the AC-to-DC converter (114) and supplying power to the load (108) by the DC-to-DC converter (112).Clause 9. method according to Clause 8, wherein:the DC-DC converter (112) has a filter circuit (124) and a DC-DC converter output (115);the filter circuit (124) includes the filter circuit input (127), a filter circuit output (131) having the DC-DC converter output (115), an inductor (128) connected in series between the filter circuit input (127) and the filter circuit output (131), and a capacitor (130) connected between the filter circuit output (131) and ground (229); andthe load (108) has a DC bus (110) connected to the DC-DC converter output (115).Clause 10th The method of Clause 8 or 9, wherein the AC-DC converter (114) comprises a transformer rectifier unit (137) comprising a transformer (138) connected to a rectifier (140), the transformer (138) comprising:a number of transformer inputs (305);a number of winding sets (311, 312, 313), each winding set in the number of winding sets (311, 312, 313) comprising a primary winding (314, 340, 365) and secondary windings (315, 341, 366), the primary winding (314, 340, 365) in each winding set (311, 312, 313) being connected between two of the number of transformer inputs (317, 318, 343) and the secondary windings (315, 341, 366) in each winding set (311, 312, 313) comprising six secondary windings (319, 320, 321, 322, 323, 324); anda number of transformer outputs (306) connected to the rectifier (304) and having four transformer outputs (306) for each set of windings in the plurality of sets of windings (311, 312, 313), each of the four transformer outputs (306) for a set of windings in the number of sets of windings (311, 312, 313) connecting a number of the secondary windings (315, 341, 366) in the set of windings (311, 312, 313) to the rectifier (304).Clause 11 The method of Clause 8, 9, or 10, wherein:the connector (141) includes a switch (142); andthe method further comprises controlling the switch (142) to connect or disconnect the AC-DC converter output (132) to or from the filter circuit input (127) in the DC-DC converter (112) via the connector (141).Clause 12th Method of Clause 8, 9, 10, or 11, further comprising:providing energy to the load (108) by the DC-DC converter (112) within a first voltage range (150) in a DC voltage range (148); andproviding energy to the load (108) by the AC-DC converter (114) within a second voltage range (152) in the DC voltage range (148), wherein there is a gap (154) between the first voltage range (150) and the second voltage range (152).Clause 13 Apparatus comprising:a number of transformer inputs (305);a number of winding sets (311, 312, 313), each of the winding sets in the number of winding sets (311, 312, 313) including a primary winding (314, 340, 365) and secondary windings (315, 341, 366), the primary winding (314, 340, 365) in each winding set (311, 312, 313) being connected between two of the plurality of transformer inputs (317, 318, 343) and the secondary windings (315, 341, 366) in each winding set (311, 312, 313) including six secondary windings (319, 320, 321, 322, 323, 324); anda plurality of transformer outputs (306) having four transformer outputs (306) for each of the sets of windings in the number of sets of windings (311, 312, 313), each of the four transformer outputs (306) for a set of windings in the number of sets of windings (311, 312, 313) being connected to a number of the secondary windings (315, 341, 366) in the set of windings.Clause 14. The apparatus of Clause 13, wherein:a transformer (303) having three of the transformer inputs (317, 318, 343), three of the winding sets (311, 312, 313) and twelve of the transformer outputs (333, 336, 337, 338, 358, 361, 362, 363, 382, 385, 386, 387); anda number of turns of each of the secondary windings (315, 341, 366) is selected such that a phase of an AC output signal at each of the transformer outputs (333, 336, 337, 338, 358, 361, 362, 363, 382, 385, 386, 387) introduces a phase of an AC output signal at a first of the transformer outputs by about 30 degrees and a phase of an AC output signal at a second other of the transformer outputs by about 30 degrees when phases of the AC input signals at the transformer inputs are separated by about 120 degrees.Clause 15th The apparatus of Clause 13 or 14, wherein the secondary windings (315, 341, 366) in each winding set include:a first winding having a first number of turns connected between a first point and a second point, a first transformer output connected to the first point;a second winding having a second number of turns connected between the second point and a third point;a third winding having a third number of turns connected between said third point and a fourth point;a fourth winding having a fourth number of turns connected between the second point and a second transformer output;a fifth winding having a fifth number of turns connected between the second point and a third transformer output; anda sixth winding having a sixth number of turns connected between the third point and a fourth transformer output.Clause 16th Apparatus according to Clause 15, wherein:the first number of turns, the second number of turns, and the third number of turns are approximately equal; andthe fourth number of turns and the sixth number of turns are approximately equal.Clause 17: The apparatus of Clause 16, wherein:a ratio of the first number of turns to the fourth number of turns is about 1:0.772; anda ratio of the first number of turns to the fifth number of turns is about 1:1.033.Clause 18th Apparatus of Clause 15, 16, or 17, wherein:the first winding, the second winding, and the third winding comprise a single winding having a number of turns;the second point has a first tap point on the single winding such that about one third of the number of turns of the single winding is between the first point and the second point; andthe third point has a second tap point on the single winding such that about one third of the number of turns of the single winding is between the third point and the fourth point.Clause 19th Device according to Clause 13, 14, 15, 16, 17 or 18, further comprising:a DC-DC converter (112) having a filter circuit (124) and a DC-DC converter output (115), the filter circuit (124) having a filter circuit input (127) and a filter circuit output (131) having the DC-DC converter output (115);a rectifier (304) connected to the plurality of transformer outputs (306) to form an AC-DC converter (144) having an AC-DC converter output (132); anda connector (141) connecting the AC-DC converter output (132) to the filter circuit input (127) in the DC-DC converter (112).Clause 20th Device according to Clause 13, 14, 15, 16, 17, 18 or 19, further comprising:a DC-DC converter (112) connected to a DC bus (110) and configured to provide a DC power across the DC bus (110) within a first voltage range (150) in a DC voltage range (148); anda rectifier (304) connected to the plurality of transformer outputs (306) and forming an AC-DC converter (114) configured to provide DC power across the DC bus (110) within a second voltage range (152) in the DC voltage range (148), wherein there is a gap (154) between the first voltage range (150) and the second voltage range (152).The description of the various illustrative embodiments has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the embodiments to the form disclosed. Many modifications and variations will occur to those skilled in the art. Furthermore, various illustrative embodiments may provide different features compared to other desired embodiments. The selected embodiment or embodiments are chosen and have been described in order to best describe the principles of the embodiments and the practical application, and to enable one skilled in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

An apparatus comprising: a DC-DC converter (112) comprising a filter circuit (124) and a DC-DC converter output (115), the filter circuit (124) comprising a filter circuit input (127) and a filter circuit output (131) comprising the DC-DC converter output (115); an AC-DC converter (114) comprising an AC-DC converter output (132); and a connector (141) connecting the AC-DC converter output (132) to the filter circuit input (127) in the DC-DC converter (112), the AC-DC converter (114) comprising a transformer rectifier unit (137) comprising a transformer (138) connected to a rectifier (140), the transformer (138) comprising: a number of transformer inputs (305); a number of winding sets (311, 312, 313), each winding set in the number of winding sets (311, 312, 313) comprising a primary winding (314, 340, 365) and secondary windings (315, 341, 366), the primary winding (314, 340, 365) in each winding set (311, 312 or 313) being connected between two of the number of transformer inputs (317, 318, 343) and the secondary windings (315, 341, 366) in each winding set (311, 312, 313) consisting of six secondary windings (319, 320, 321, 322, 323, 324); and a number of transformer outputs (306) connected to the rectifier (304) and having the four transformer outputs (306) for each set of windings in the plurality of sets of windings, each of the four transformer outputs (306) for one set of windings in the number of sets of windings (311, 312, 313) being connected to a number of the secondary windings (315, 341, 366) in the set of windings (311, 312, 313).The apparatus of claim 1, further comprising: a load (108) connected to the DC-DC converter output (115); and wherein the connector (141) connects the AC-DC converter output (132) to the filter circuit input (127) in the DC-DC converter (112) such that an uninterrupted power change occurs when switching between providing power to the load (108) by the AC-DC converter (114) and providing power to the load (108) by the DC-DC converter (112).The apparatus of claim 1 or 2, wherein the connector (141) comprises a switch (142) configured to connect or disconnect the AC-DC converter output (132) to or from the filter circuit input (127) in the DC-DC converter (112) via the connector (141).The apparatus of claim 1, 2 or 3, wherein the filter circuit (124) comprises: an inductor (128) connected in series between the filter circuit input (127) and the filter circuit output (131); and a capacitor (130) connected between the filter circuit output (131) and ground (229).The apparatus of claim 1, 2, 3 or 4, wherein: the DC-DC converter (112) is configured to provide DC power at the DC-DC converter output (115) within a first voltage range (150) in a DC voltage range (148); the AC-DC converter (114) is configured to provide DC power at the DC-DC converter output (115) within a second voltage range (152) in the DC voltage range (148); and there is a gap (154) between the first voltage range (150) and the second voltage range (152).A method of transferring power to a load (108), comprising: connecting an AC-to-DC converter output (132) for an AC-to-DC converter (114) to a filter circuit input (127) in a DC-to-DC converter (112) via a connector (141) such that an uninterrupted power change occurs when switching between providing power to the load (108) by the AC-to-DC converter (114) and providing power to the load (108) by the DC-to-DC converter (112), wherein the AC-to-DC converter (114) comprises a transformer rectifier unit (137) comprising a transformer (138) connected to a rectifier (140), wherein the transformer (138) comprises: a number of transformer inputs (305); a number of winding sets (311, 312, 313), each winding set in the number of winding sets (311, 312, 313) comprising a primary winding (314, 340, 365) and secondary windings (315, 341, 366), the primary winding (314, 340, 365) in each winding set (311, 312, 313) being connected between two of the number of transformer inputs (317, 318, 343) and the secondary windings (315, 341, 366) in each winding set (311, 312, 313) consisting of six secondary windings (319, 320, 321, 322, 323, 324); and a plurality of transformer outputs (306) connected to the rectifier (304) and having four transformer outputs (306) for each set of windings in the number of sets of windings (311, 312, 313), each of the four transformer outputs (306) for a set of windings in the number of sets of windings (311, 312, 313) connecting a number of the secondary windings (315, 341, 366) in the set of windings (311, 312, 313) to the rectifier (304).The method of claim 6, wherein: the DC-DC converter (112) comprises a filter circuit (124) and a DC-DC converter output (115); the filter circuit (124) comprises the filter circuit input (127), a filter circuit output (131) comprising the DC-DC converter output (115), an inductor (128) serially connected between the filter circuit input (127) and the filter circuit output (131), and a capacitor (130) connected between the filter circuit output (131) and ground (229); and the load (108) comprises a DC bus (110) connected to the DC-DC converter output (115).The method of claim 6 or 7, wherein: a connector (141) comprises a switch (142); and the method further comprises controlling the switch (142) to connect or disconnect the AC-DC converter output (132) to or from the filter circuit input (127) in the DC-DC converter (112) via the connector (141).The method of claim 6, 7 or 8, further comprising: providing energy to the load (108) via the DC-DC converter (112) within a first voltage range (150) in a DC voltage range (148); and providing energy to the load (108) by the AC-DC converter (114) within a second voltage range (152) in the DC voltage range (148), wherein there is a gap (154) between the first voltage range (150) and the second voltage range (152).

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