Abstract:
A method of startup of an electric start electrical power generating system (EPGS) is provided. The EPGS includes a generator configured to power a direct current (DC) load via a DC bus. The method includes: disconnecting the DC load from the DC bus; connecting a battery to a boost converter, the boost converter being connected to the generator; powering the generator using the battery via the boost converter; when the generator reaches a minimum speed: disconnecting the battery from the boost converter; deactivating the boost converter; and activating a synchronous active filter, the synchronous active filter being connected to the DC bus; bringing up a voltage on the DC bus by the generator; and when the voltage on the DC bus reaches a predetermined level, connecting the DC load to the DC bus.

Description:
FIELD OF INVENTION 
     The subject matter disclosed herein relates generally to the field of electrical power generating systems. 
     DESCRIPTION OF RELATED ART 
     Vehicles, such as military hybrid vehicles or aircraft, may include electric power generating systems (EPGS) that utilize a synchronous generator to power a DC load in the vehicle. The synchronous generator may comprise a permanent magnet (PM) or wound field (WF) generator. A voltage ripple on the direct current (DC) bus exists after rectification of the generator output. To reduce the DC bus voltage ripple to levels that are appropriate to meet specification requirements for the DC load, a relatively large DC bus capacitor may be required in the EPGS, adding weight and size to the EPGS. The DC bus capacitor is sized based on the ripple frequency, and may need to be significantly increased when the generator operates at a relatively low speed. Another approach to reduce DC bus voltage ripple includes increasing the frequency bandwidth of an active rectifier that rectifies the generator output; however, this approach may increase system noise. 
     BRIEF SUMMARY 
     According to one aspect of the invention, a method of startup of an electric start electrical power generating system (EPGS), the EPGS comprising a generator configured to power a direct current (DC) load via a DC bus includes: disconnecting the DC load from the DC bus; connecting a battery to a boost converter, the boost converter being connected to the generator; powering the generator using the battery via the boost converter; when the generator reaches a minimum speed, disconnecting the battery from the boost converter; deactivating the boost converter; and activating a synchronous active filter, the synchronous active filter being connected to the DC bus; bringing up a voltage on the DC bus by the generator; and when the voltage on the DC bus reaches a predetermined level, connecting the DC load to the DC bus. 
     According to another aspect of the invention, an electrical power generating system (EPGS) includes a generator connected to a direct current (DC) bus; a boost converter connected to the generator; a battery, the battery configured to power the generator during startup via the boost converter, wherein when the generator reaches a minimum speed, the battery is configured to be disconnected from the boost converter; and a synchronous active filter (SAF), the SAF being connected to the DC bus; wherein when a voltage on the DC bus is brought up to a predetermined level by the generator, the generator is configured to power a DC load, the boost converter is configured to be deactivated, and the SAF is configured to be activated. 
     Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  illustrates an embodiment of an EPGS with a WF generator comprising a reconfigurable power converter configured as a boost converter during start mode. 
         FIG. 2  illustrates an embodiment of the EPGS with a WF generator comprising a reconfigurable power converter configured as a synchronous active filter during generate mode. 
         FIG. 3  illustrates an embodiment of an EPGS with a WF generator comprising a buck-boost converter and a synchronous active filter. 
         FIG. 4  illustrates a detailed view of an embodiment of a synchronous active filter. 
         FIG. 5  illustrates an embodiment of an EPGS with a PM generator comprising a reconfigurable power converter. 
         FIG. 6  illustrates an embodiment of a method of operating an EPGS comprising a boost converter and a synchronous active filter. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of systems and methods for an EPGS with a boost converter and a synchronous active filter (SAF) are provided, with exemplary embodiments being discussed below in detail. A boost converter is a DC-DC power converter that has an output voltage that is greater than its input voltage. The boost converter may be connected to a battery that powers a generator via the boost converter and start inverter during startup. The generator is operated as a motor in the startup to convert electrical power supplied by a start inverter into motive power, which is provided to the prime mover to bring it up to self-sustaining speed. In the case of a WF generator, AC power is provided to the armature windings of the main portion of the WF generator and to AC exciter field windings, so that the motive power may be developed. This may be accomplished by using two separate inverters. During generate mode, the DC bus is connected to a DC load, and the power converter is reconfigured as an SAF. The SAF acts to reduce DC bus voltage ripple on the DC bus during generate mode. Reduction in DC bus voltage ripple allows for reduction in size of the DC bus capacitor. In some embodiments, the boost converter may comprise a buck/boost converter, which may output a voltage that is either higher or lower than the input voltage. In some embodiments, the boost converter and the SAF may be embodied in a reconfigurable power converter. 
       FIG. 1  illustrates an embodiment of an electric start EPGS  100  comprising a reconfigurable power converter  113  and a WF generator  101 . Reconfigurable power converter  113  may function as a boost converter during startup, and as an SAF during normal operation. The WF synchronous generator  101  generates AC power through the rotation of rotating portion  103 , which comprises an exciter armature winding  104 , rotating rectifier  105 , and main field winding  106 . Rotating portion  103  rotates in proximity to exciter field winding  102  and main armature winding  107 . Exciter field winding  102  is connected to exciter inverter  108 , and main armature winding  107  is connected to start inverter  110 . Exciter inverter  108  and start inverter  110  are both on high voltage DC (HVDC) bus  109 . Battery  112  is connected to reconfigurable power converter  113  via switches  111   a - b . DC bus capacitor  116  is connected across reconfigurable power converter  113 , and DC load  115  is connected across reconfigurable power converter  113  via switches  114   a - b.    
     Reconfigurable power converter  113  is configured as a three-phase interleave boost converter during startup. During startup, the DC load  115  is disconnected from the HVDC bus  109  by opening switches  114   a - b , and switches  111   a - b  are closed, connecting battery  112  to the reconfigurable power converter  113 . Then the reconfigurable power converter  113  increases the voltage on the HVDC bus  109  using power from battery  112 , increasing the voltage to exciter inverter  108  and start inverter  110 . Exciter inverter  108  provides AC constant frequency (about 400 Hz in some embodiments) power from HVDC system bus  109  to the three-phase exciter field windings  102 , and start inverter  110  provides variable voltage variable frequency (VVVF) power from HVDC bus  109  to the main armature windings  107 . Then, upon achieving generate mode speed (about 800 rpm in some embodiments) by WF synchronous generator  101 , the battery  112  is disconnected from the reconfigurable power converter  113  by opening switches  111   a - b , and the reconfigurable power converter  113  is reconfigured as an SAF for normal operation of EPGS  100 . Also, after startup is completed, the exciter inverter  108  is reconfigured as DC exciter by disabling one of the phase legs, and all switches comprising start inverter  110  are turned-off, reconfiguring start inverter  110  into a 6-pulse rectifier. 
       FIG. 2  illustrates an embodiment of an EPGS  200  comprising a WF generator  201  and a reconfigurable power converter  213  during generate mode. Reconfigurable power converter  213  may function as a boost converter during startup and as an SAF during normal operation. Start mode is not supported in the embodiment of  FIG. 2 . The EPGS  200  of  FIG. 2  also includes a  6 -pulse rectifier  210  and DC exciter inverter  208 . The WF synchronous generator  201  generates AC power through the rotation of rotating portion  203 , which comprises an exciter armature winding  204 , rotating rectifier  205 , and main field winding  206 . Rotating portion  203  rotates in proximity to exciter field winding  202  and main armature winding  207 . Exciter field winding  202  is connected to DC exciter inverter  208 , and main armature winding  207  is connected to rectifier  210 . DC exciter inverter  208  and rectifier  210  are both on HVDC bus  209 . 
     To enable power generation, the battery  212  provides a DC voltage to the DC exciter inverter  208  that controls the DC exciter current in response to the voltage on DC bus  209 . During this time, the DC load  215  is disconnected from the DC bus  209  by opening switches  214   a - b , and switches  211   a - b  are closed. When the DC bus voltage exceeds the battery voltage, the battery  212  is disconnected from the DC bus  209  by opening switches  211   a - b . The DC exciter inverter  208  gradually increases DC exciter field current to provide soft start of the voltage on HVDC bus  209  to its specified value (about 270 Vdc-800 Vdc in some embodiments). When the voltage on HVDC bus  209  reaches the specified value, a power quality monitor (not depicted) may detect that DC power at no-load on HVDC bus  209  is within specification levels for DC load  215 . At this point, reconfigurable power converter  213  is reconfigured from a boost converter to an SAF, and the DC load  215  is connected to the HVDC bus  209  by closing switches  214   a - b . DC exciter inverter  208  controls the DC exciter current to achieve gradual increase of the voltage on DC bus  209 , which is commonly referred as a soft start of the DC bus voltage. The DC exciter inverter  208  powers exciter field winding  202 . Rectifier  210  may comprise a 6-pulse rectifier in some embodiments. Reconfigurable power converter  213  acts to reduce the ripple on HVDC bus  209  during generate mode by acting as an SAF. 
       FIG.3  illustrates another embodiment of an EPGS  300  comprising a buck/boost converter and an SAF during generate mode. Start mode is not supported in the embodiment shown in  FIG. 3 . EPGS  300  comprises a WF synchronous generator  301  comprising a rotating portion  303 , which comprises an exciter armature winding  304 , rotating rectifier  305 , and main field winding  306 , exciter field winding  302 , main armature winding  307 . Exciter field winding  302  is connected to DC exciter  308 , and main field winding  306  is connected to rectifier  310 . Battery  312  is connected to exciter inverter  308  via buck/boost converter  317  and switches  311   a - b . DC load  315  connected to HVDC bus  309  via switches  318   a - b . At low generator speed, the battery  312  is connected to buck/boost converter  317  by closing switches  311   a - b , switch  314  is opened, and switches  318 a-b are opened to disconnect DC load  315  from HVDC bus  309 . The buck/boost converter  317  raises the voltage on HVDC bus  309  available to exciter inverter  308  to accommodate for any residual load on the bus, or, in the case when the voltage on HVDC bus  309  is relatively high (above 200 Vdc), the buck/boost converter  317  may reduce the voltage available to exciter inverter  308  to a level appropriate to avoid exciter inverter operation with a very low duty cycle. When generate mode speed (about 800 rpm in some embodiments) is achieved by WF synchronous generator  301 , battery  312  is disconnected by opening switches  311   a - b , and the SAF  313  is enabled to reduce the ripple on HVDC bus  309  during generate mode. The DC exciter  308  gradually increases exciter field current to achieve soft start of the voltage on HVDC bus  309  to its specified value (about 270 Vdc-800 Vdc in some embodiments). Then, a power quality monitor may detect that DC power at no-load on HVDC bus  309  is within the specification levels for DC load  315 . At this point, the DC load  315  is connected to the bus by closing switches  318   a - b , and switch  314  is also closed. The voltage applied to the DC exciter inverter  308  is controlled by operating the buck/boost converter  313  in buck mode during operation. 
       FIG.4  illustrates a detailed view of an embodiment of a synchronous active filter (SAF)  414  for an EPGS in generate mode, which may comprise either of SAFs embodied in reconfigurable power converters  113  or  213 , or SAF  313 . SAF  414  may exhibit a single phase topology. Synchronous active filter  414  comprises capacitor  415 , inductor  416 , and switches  418   a - b  connected in series. The gate drive  413  of switches  418   a - b  is controlled using data from current sensor  417 . Main armature winding  401  (which may comprise any of main armature windings  107 ,  207 , or  307 ) and voltage sensor  402  are connected via a phase locked loop  403  to the multiplier  404 . The multiplier  404  provides synchronization frequency to the synchronous compensators  405 ,  406 , and  407  by multiplying signal from the phase locked loop  403  by  6 . This synchronization frequency is the dominant frequency of the voltage ripple on DC bus after  6 -pulse rectification of the generator voltage. Synchronous compensators also receive input from voltage sensor  420 , which is connected across HVDC bus  421  and DC bus capacitor  419  (which may comprise any of HVDC buses  209  or  309 , and DC bus capacitors  216  or  316 , respectively). The outputs of synchronous compensators  406  and  407  are added by adder  408 , and the output of adder  408  is added to the output of synchronous compensator  405  by adder  409 . The output of adder  409  after a limit function becomes a current reference (I_ref) to the current loop that comprises current feedback signal (I fdbk) from the current sensor  417 , error summer  410  and amplifier  411 . Pulse width modulator  412  converts controlled voltage out the amplifier  411  output to modulate SAF  414  power switches  418   a  and  418   b  via gate drive  413 . Synchronous compensator  405  is tuned to cancel or reduce the dominant frequency of the DC bus voltage ripple, while synchronous compensators  406  and  407  are tuned to cancel or reduce the  2   nd  and  4   th  harmonics of the DC bus voltage ripple respectively. Synchronous active filter  414  modulates the DC bus in response to the current reference I_ref to cancel or reduce the voltage ripple on DC bus which is the product of 6-pulse rectification of the generator voltages. 
       FIG. 5  illustrates an embodiment of an EPGS  500  with a permanent magnet (PM) generator  502 , start inverter/active rectifier  505  and reconfigurable power converter  509 . Reconfigurable power converter  509  may function as a boost converter during startup, and as an SAF during normal operation. PM generator  502  is powered by prime mover  501 , which maybe any portion of a vehicle that moves in a manner appropriate to be harnessed for power generation. During startup, reconfigurable power converter  509  is configured as a boost converter, and start inverter/active rectifier  505  acts as a start inverter. During startup, battery  506  is connected to boost converter  505  by closing switches  507 a-b, and DC load  511  is disconnected from HVDC bus  504  by opening switches  510   a - b . Battery  506  then powers armature winding  503  during startup via reconfigurable power converter  509  and start inverter  505 . When generate mode speed is reached by PM generator  502 , battery  506  is disconnected by opening switches  507   a - b , reconfigurable power converter  509  is reconfigured to act as an SAF, and start inverter/active rectifier  505  is reconfigured to act as an active rectifier  505 . Voltage on HVDC bus  504  is then brought up to an appropriate level DC load  511 , and a power quality monitor may detect that DC power at no-load is within the specification levels. At this point, the DC load  511  is connected to the bus by closing switches  510   a - b . Power is then transferred from armature winding  503  of PM generator  502  to active rectifier  505 , which powers DC load  511  via to HVDC bus  504  and SAF  509 . The SAF portion of buck/boost converter/SAF  509  helps to reduce the necessary size ofthe DC bus capacitor  508  by reducing ripple on HVDC bus  504 , without increasing the frequency bandwidth of the active rectifier  505  during generate mode. 
       FIG. 6  illustrates an embodiment of a method  600  of operating an EPGS comprising a boost converter/SAF. Method  600  may be implemented in any of EPGSs  100 ,  200 ,  300 , or  500 . In block  601 , a battery is connected to a generator of the EPGS via the boost converter in start mode. In block  602 , the generator reaches a predetermined generate speed using the power from the boost converter and battery. In block  603 , the battery is disconnected from the boost converter, the boost converter is deactivated, and the SAF is activated. In embodiments in which the boost converter comprises a buck/boost converter (for example,  FIG. 3 ), deactivating the boost converter may comprise operating the buck/boost converter in buck mode. In block  604 , the voltage on the HVDC bus reaches an appropriate level for the DC load. In block  605 , the DC load is connected to the HVDC bus via the SAF in generate mode. The boost converter functions to increase voltage to power start inverter during startup, and the SAF functions to reduce the ripple that is experienced by the DC load during generate mode. 
     The technical effects and benefits of exemplary embodiments include reduction of weight and size of DC bus capacitance due to introduction of an SAF that reduces voltage ripple experienced by a DC load connected to an HVDC bus of an EPGS. A reconfigurable power converter, which may be configured as a boost converter or an SAF, allows reduction of power electronic components by utilizing multiple functions via software configuration of a three-phase interleave power converter. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while various embodiment of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.