Abstract:
A pre-charge circuit limits in-rush currents on a direct current (DC) link that includes a first DC link bus and a second DC link bus. The pre-charge circuit includes a switching device connected in series with the first DC link bus. The switching device has an ON state in which power flow is enabled on the DC link and an OFF state in which power is disabled on the DC link. A controller selectively modulates the state of the switching device to limit in-rush currents on the DC link.

Description:
BACKGROUND 
     The present invention is related generally to electrical power systems, and in particular to starting/generating systems. 
     Starting/generating systems refer to systems capable of operating in either a starting mode in which the system operates as a motor to accelerate a rotor portion to a desired speed or in a generating mode in which the system operates as a generator to convert mechanical energy provided by the rotor portion into electrical energy for distribution to attached loads. 
     Depending on the mode, various electronic circuits are required to provide the desired functionality. During starting, a pre-charging or soft-start circuit may be employed to prevent large in-rush currents from damaging a DC link capacitor(s). For example, prior art pre-charge circuits may employ a switching device and resistor (connected in parallel with one another) connected in series on the DC link bus between the power supply and the inverter/rectifier circuit. The switching device is turned OFF in order to force current through the resistor connected in parallel with the switching device, thereby limiting the current provided to the inverter/rectifier circuit and DC link capacitor. However, this topology does not provide functionality beyond pre-charge operations. Alternatively, the switching device and resistor can be placed in series with the DC link capacitor, which is connected between the DC link buses in parallel with the inverter/rectifier circuit. In this way, the switching device is not required to be capable of carrying the full inverter/rectifier current, but the presence of the switching device in series with the capacitor decreases the performance of the DC link capacitor, due to the resistance of the switching device. 
     In addition to circuits or components employed to provide pre-charging functionality, starting/generating systems employ additional hardware/circuits to implement functions such as battery charging, power flow enablement, and fault protection. These additional hardware/circuit components add to the overall cost and weight of starting/generating systems, reduction of which is desirable. 
     SUMMARY 
     A pre-charge circuit limits in-rush currents on a direct current (DC) link that includes a first DC link bus and a second DC link bus. The pre-charge circuit includes a switching device connected in series with the first DC link bus. The switching device has an ON state in which power flow is enabled on the DC link and an OFF state in which power is disabled on the DC link. A controller selectively modulates the state of the switching device to limit in-rush currents during pre-charge on the DC link. In addition, the pre-charge circuit can be used subsequent to pre-charge of the DC link to implement additional functionality with respect to power flow on the DC link. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a starting/generating system according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a circuit diagram of starting/generating system  10  according to an embodiment of the present invention. System  10  includes rotor portion  12  and stator portion  14 . Rotor portion  12  includes motive power shaft  16 , main generator portion field winding  18 , rotating rectifier  20 , three-phase armature windings  22 , permanent magnet generator (PMG) magnets  24 , and prime mover  26 . Stator portion  14  includes filter circuit  28  (which includes capacitor C 1 , inductor L 1 , and diode D 1 ), solid state circuit breaker  30 , controller  32 , DC link buses  34   a  and  34   b  (collectively DC link  34 ), DC link capacitor C 2 , inverter/rectifier  36 , main generator portion armature winding  38 , exciter field winding  40 , H-bridge  42 , three-phase (PMG) stator windings  44 , rectifier  46  and control electronics  48 . Power source  50  represents a generic power source for providing DC power to starting/generating system  10  during starting operations and DC load  52  represents a generic load that receives power from starting/generating system  10  during generating operations. The embodiment shown in  FIG. 1  represents the system employed with respect to a wound field synchronous generator, but would be applicable to other generator systems such as flux regulated permanent magnet generators and other well-known starting/generating systems. 
     In the starting mode, electrical energy provided by DC power source  50  is converted to alternating current (AC) power by inverter/rectifier  36  (operating as an inverter). Further, the exciter power converter H-bridge  42  delivers AC power to the exciter field winding  40 . The exciter acts as a rotary transformer having a primary winding comprising the field winding  40  and secondary windings comprising the armature windings  22  so that AC power is induced in the armature windings  22 . The AC power is rectified by the rotating rectifier  20  and applied as DC power to the main generator portion field winding  18 . The AC power is provided to main generator portion armature winding  38 , which interacts with main generator portion field winding  18  to generate motive force that causes rotor portion to rotate. 
     In the generating mode, mechanical energy provided by prime mover  26  is converted to electrical energy. In particular, rotation of PMG magnets  24  generates electrical energy in three-phase PMG stator windings  44 . Rectifier  46  converts the AC voltage to a DC voltage that is selectively supplied to exciter field winding  40  via H-bridge  42 . The DC excitation provided by exciter field winding  40  interacts with three-phase armature windings  22 . The DC current in exciter field winding  40  is controlled in response to the output DC voltage applied to DC load  52  by a voltage regulator located within control electronics  48 . The AC voltage generated by three-phase armature windings  22  is converted to DC by rotating rectifier  20  and supplied to main generator portion field winding  18 . The rotating field generated by field winding  18  interacts with main generator portion armature winding  38  to generate AC voltage. Inverter/rectifier  36  (operating as a rectifier) converts the AC voltage to DC voltage that is supplied to DC load  52 . In addition, the DC voltage may be used to charge an attached battery (for example, DC power source  50 ). The dual functionality of starting/generating system  10  is illustrated visually by switch S 1 , which indicates that starting/generating system may receive power from DC source  50  (starting mode) and may supply power to a DC load  52  (generating mode). Although in some embodiments, the DC power source (i.e., battery) may also act as a DC load during re-charging of the battery from power generated by starting/generating system  10 . 
     Solid-state circuit breaker  30  is connected on DC link bus  34   a  in series between inverter/rectifier  36  and DC power source  50  (or DC load  52 , depending on the mode of operation). Solid-state circuit breaker  30  combines functionality previously provided by a plurality of individual circuits. During pre-charge (i.e., soft-starting) of DC link capacitor C 2 , the state of solid-state circuit breaker  30  is selectively modulated (i.e., turned ON and OFF) to control in-rush currents. During a starting mode (subsequent to pre-charge), solid-state circuit breaker  30  is selectively controlled to enable power flow from DC power source  50  to inverter/rectifier  36  and to disable power flow from inverter/rectifier  36  to DC power source  50 . During a generating mode, solid-state circuit breaker  30  enables power provided by the generator to be supplied to DC load  52 , and is selectively controlled (i.e., turned OFF) in response to fault conditions such as short-circuit conditions, overload conditions, etc., to prevent damage to the generator and/or DC load  52 . Also during the generating modes, solid-state circuit breaker  30  is selectively modulated to provide a desired current profile for battery charging operations. 
     Pre-charging (i.e., soft-starting) of DC link capacitor C 2  prevents large currents from damaging DC link capacitor C 2  during an initial application of power from DC power source  50 . Pre-charging functionality is provided by selectively modulating solid-state circuit breaker  30  (i.e., turning it ON and OFF). In-rush current is a function of the voltage applied to the capacitor and the characteristics of the capacitor. By selectively modulating solid-state circuit breaker  30 , the voltage applied to DC link capacitor C 2  can be controlled, thereby limiting the in-rush current provided to DC link capacitor C 2 . 
     In one embodiment, controller  32  monitors one or more parameters and based on the monitored parameters selectively controls the modulation of solid-state circuit breaker  30 . The operation of controller  32  may be closed-loop or open-loop, depending on the application. In an open-loop application, the duty cycle of solid-state circuit breaker  30  is selectively controlled without feedback regarding the voltage or current provided to DC link capacitor C 2 . For instance, controller  32  may control the duty cycle based on the length of time from application of power from DC power source  50 , with the duty cycle increasing based on some function (linearly or non-linearly) until the expiration of the pre-charge cycle. At the end of the pre-charge cycle solid-state circuit breaker  30  is turned ON (i.e., maintained in the ON state continuously) such that DC power source  50  supplies power to inverter/rectifier  36  for starting operations. 
     In closed-loop applications, controller  32  monitors one or more parameters and in response selectively controls the modulation (i.e., duty cycle) of solid-state circuit breaker  30 . Examples of parameters used to determine the modulation of solid-state circuit breaker  30  include the monitored DC link voltage, the monitored DC link capacitor current, and/or the monitored DC link current. Based on these parameters controller  32  can selectively control in-rush currents during pre-charge of DC link capacitor C 2 . For example, because the in-rush current is dependent on the voltage supplied to DC link capacitor C 2 , the monitored DC link voltage may be used as feedback to selectively control the in-rush current. Controller  32  monitors the voltage across DC link and in response selectively modulates solid-state circuit breaker  30  to provide the desired pre-charge of capacitor C 2 . As the voltage across DC link increases, the duty cycle of solid-state circuit breaker  30  is selectively increased until some pre-charge threshold, at which time solid-state circuit breaker is maintained in the ON state (continuously) to provide starting power to inverter/rectifier  36 . In other embodiments, the monitored DC link current and/or DC link capacitor current can be used as feedback to selectively control the in-rush current. Once again, the duty cycle is increased until at the end of the pre-charge cycle solid-state transistor  30  is maintained in the ON state (continuously) to provide starting power to inverter/rectifier  36 . Monitoring the in-rush current directly provides feedback regarding the output to be controlled, but requires additional hardware (e.g., current sensors) to implement. 
     Solid-state circuit breaker  30  may also be used to selectively enable power flow during starting operations and may be used to disable power flow in response to the voltage generated by the generator exceeding the voltage provided by DC power source  50  (prior to supplying voltage from the generator to DC load  52 ). For example, having pre-charged DC link capacitor C 2 , solid-state circuit breaker  30  is selectively controlled (i.e., turned ON) to enable power flow from DC power source  50  to inverter/rectifier  36  to operate in a starting mode that may include accelerating rotor portion  12 , igniting a combustor (for gas turbine engines) and assisting in accelerating the rotor portion  12  to a desired speed following successful ignition. Subsequent to these stages, solid-state circuit breaker  30  is turned OFF to prevent power generated by starting/generating system  10  from flowing into DC power source  50 . For example, when the voltage provided by starting/generating system  10  (i.e., the DC power provided by inverter/rectifier  36 ) exceeds the magnitude of the DC voltage provided by DC power source  50 , then controller  32  turns solid-state circuit breaker  30  OFF to disable power flow from DC power source to starting/generating system  10  (or vice versa). 
     Circuit breaker  30  may also be employed during the generating mode to provide the desired current profile for optimal battery charging. For example, DC power source  50  may be a battery that requires re-charging after each starting operation. Rather than employ a separate circuit for monitoring and controlling the current profile provided to the battery (i.e., DC power source  50 ), controller  32  monitors the current provided to the battery and selectively modulates solid-state circuit breaker  30  to provide the desired current profile for charging. Typically, the current provided to the battery is sensed and provided as feedback to controller  32 , although in other embodiments other parameters may be monitored and used in feedback to control the current profile during battery charging operations. 
     Solid-state circuit breaker may also be employed to provide fault protection during the generator mode by selectively disabling power flow from starting/generating system  10  to DC load  30  in response to a detected fault condition. For example, controller  32  may monitor one or more parameters, such as DC link voltage, DC link current and/or DC link capacitor current to detect faults such as short-circuits. In response to a detected fault, controller  32  causes solid-state circuit breaker  30  to turn OFF to prevent excessive currents from being provided to DC load  52 . In one embodiment, the fault protection provided by solid-state circuit breaker is not activated until a detected fault has existed on the link for a predetermined period of time, to prevent transient conditions from initiating fault protection. Additional parameters well-known in the art for detecting fault conditions may also be monitored by controller  32 . In addition, other controllers, such as control electronics  48 , may provide input to controller  32  regarding detected fault conditions. In response to these inputs, controller  32  selectively activates fault protection by turning OFF solid-state circuit breaker  30 . 
     The present invention provides a starting/generating circuit topology in which a solid-state circuit breaker is employed to implement a number of functions required at various stages starting/generating system operation. The solid-state circuit breaker is connected in series on a DC link bus and is selectively controlled (e.g., turned ON and OFF) to provide the desired functionality. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.