Abstract:
An active damping switching system includes an active damping switching apparatus, including a damping capacitor, a damping resistor coupled to the damping capacitor, an input switch coupled to the damping capacitor, an oscillator coupled to the input switch and configured to open and close the input switch at a frequency, a direct current power source coupled to the active damping switching apparatus, a constant power load and an input filter disposed between the constant power load and the active damping switching apparatus.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of the legally related U.S. Ser. No. 13/218,166 filed Aug. 25, 2011, the contents of which are incorporated by reference herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to electric power generation and distribution, and more particularly to systems and methods for active damping with a switched capacitor. 
         [0003]    Electrical power systems in hybrid vehicles, such as military hybrid vehicles, can include high voltage direct current power generation and distribution systems. Such electrical systems, however, can experience stability problems. Constant power loads, such as a switched mode power converter may introduce a destabilizing effect on a DC bus, causing significant voltage oscillation. The source ripple filter must attenuate rectification ripple and current harmonics resulting from active rectifier switching. The input filter of a switching power converter must provide forward voltage attenuation of alternating current (AC) voltage superimposed on a DC bus voltage, attenuate current harmonics resulting from power converter switching and those injected into DC bus to allowed levels, and have a low output impedance so as not to adversely affect the stability of switched-mode power converter. A power converter&#39;s input LC filter without a damper introduces possible instability in the presence of constant power (i.e., negative impedance) loads. Traditionally LC or RC damping networks are used to stabilize unstable loads. The LC damper is connected in series with the inductor of the input LC filter, while an RC damper is connected in parallel with the capacitor of the input LC filter. The size of damping networks is considerably larger than the input LC filter itself, and can therefore increase system size and weight. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    Exemplary embodiments include an active damping switching system, including an active damping switching apparatus, including a damping capacitor, a damping resistor coupled to the damping capacitor, an input switch coupled to the damping capacitor, an oscillator coupled to the input switch and configured to open and close the input switch at a frequency, a direct current power source coupled to the active damping switching apparatus, a constant power load and an input filter disposed between the constant power load and the active damping switching apparatus. 
         [0005]    Additional exemplary embodiments include an active damping switching system, including an active damping switching apparatus, including a damping capacitor, a damping resistor coupled to the damping capacitor, an input switch coupled to the damping capacitor, a zero cross detector coupled to the input switch, a direct current power source coupled to the active damping switching apparatus, a constant power load and an input filter disposed between the constant power load and the active damping switching apparatus. 
         [0006]    Additional exemplary embodiments include an active damping switching system, including an active damping switching apparatus, including a first input switch, a first damping resistor coupled to the first input switch, a second input switch, a damping capacitor coupled to the second input switch, a second damping resistor coupled to the damping capacitor, a first gate drive coupled to the first input switch and a second gate drive coupled to the second input switch an to the first gate drive, a direct current power source coupled to the active damping switching apparatus, a constant power load and an input filter disposed between the constant power load and the active damping switching apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  illustrates a DC electric power system with a switched capacitor stabilization network with an external oscillator; 
           [0009]      FIG. 2  illustrates DC electric power system with a switched capacitor stabilization network with a combination of external oscillator and filter capacitor feedback current; 
           [0010]      FIG. 3  illustrates a DC electric power system with a switched capacitor stabilization; and 
           [0011]      FIG. 4  illustrates DC electric power system with dual control active damping. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Exemplary embodiments include systems and methods for active damping by implementing a switched RC stabilization network. In response to current ripple on the DC bus, the RC stabilization network can be switched on to dampen the ripple.  FIG. 1  illustrates an active damping system  100 . In one embodiment, the system  100  can include a DC power source  105  electrically coupled to a constant power load  120  via input filter  115 . In one embodiment, the DC source  105  can be an AC generator rectified to a DC voltage that includes voltage/current ripples. The system  100  can further include a switched capacitor stabilization network  110 . In one embodiment, the input filter  115  is an LC filter having an input capacitor  125  and an input inductor  130  having values selected to filter out certain frequencies in voltage/current that exists between the dc power source  105  and the constant power load  120 . 
         [0013]    In one embodiment, the switched capacitor stabilization network  110  includes an input switch  135  electrically coupled to the damping capacitor  140 . The damping capacitor  140  is coupled to a damping resistor  145  that is coupled to a point between the input inductor  130  and the input capacitor  125  of the input filter  115 . The switched capacitor stabilization network  110  can further include an oscillator  150  tuned at frequency equal or close to the resonant frequency of the LC input filter  115   
         [0000]    
       
         
           
             
               ( 
               
                 f 
                 = 
                 
                   1 
                   
                     2 
                      
                     
                         
                     
                      
                     π 
                      
                     
                       LC 
                     
                   
                 
               
               ) 
             
             . 
           
         
       
     
         [0000]    In one embodiment, if the system  100  becomes unstable, the switched capacitor stabilization network  110  stabilizes the system  100  by closing switch  135  which places the damping capacitor  140  and the damping resistor  145  in parallel with the input capacitor  125  of the input filter  115  for a period of time, by directly controlling the input switch  135 . In one embodiment, the oscillator  150  switches the damping capacitor  140  and the damping resistor  145  into parallel with the input capacitor  125  at a frequency approximately equal to the resonant frequency of the LC input filter  115 . By matching the frequency of the oscillator  150  to the resonant frequency of the input filter  115 , system oscillations can be eliminated. 
         [0014]      FIG. 2  illustrates an active damping system  200 . In one embodiment, the system  200  can include a DC power source  205  electrically coupled to a constant power load  220  via input filter  215 . The system  200  can further include the switched capacitor stabilization network  210 . In one embodiment, the input filter  215  can be an LC filter having an input capacitor  225  and an input inductor  230  tuned to filter out certain frequencies in voltage/current that exists between the dc power source  205  and the constant power load  220 . The input filter  215  further includes a current sensor  275  coupled to the input capacitor  225  as further described herein. 
         [0015]    In one embodiment, the switched capacitor stabilization network  210  includes an input switch  235  electrically coupled to the return path of DC power source  205 . The input switch  235  is coupled to a damping capacitor  240 . The damping capacitor  240  is coupled to a damping resistor  245  that is coupled to a point between the input inductor  230  and the input capacitor  225  of the input filter  215 . The switched capacitor stabilization network  210  can further include an oscillator  250  tuned at frequency equal or close to the resonant frequency of the LC input filter  215   
         [0000]    
       
         
           
             
               ( 
               
                 f 
                 = 
                 
                   1 
                   
                     2 
                      
                     
                         
                     
                      
                     π 
                      
                     
                       LC 
                     
                   
                 
               
               ) 
             
             . 
           
         
       
     
         [0000]    In one embodiment, if the system  200  becomes unstable, the switched capacitor stabilization network  210  stabilizes the system  200  by closing switch  235  which places the damping capacitor  240  and the damping resistor  245  in parallel with the input capacitor  225  of the input filter  215  for a period of time, by directly controlling the input switch  235 . In one embodiment, the oscillator  250  switches the damping capacitor  240  and the damping resistor  245  into parallel with the input capacitor  225  at a frequency approximately equal to the resonant frequency of the LC input filter  215 . By matching the frequency of the oscillator  250  to the resonant frequency of the input filter  215 , system oscillations can be eliminated. 
         [0016]    In one embodiment, the switched capacitor stabilization network  210  can further include filter capacitor feedback current to control the damping in the system  200 . As such, the switched capacitor stabilization network  210  can further include a transition conditions controller  255  coupled to the oscillator  250  via a second switch  260 . In one embodiment, the transition conditions controller  255  can switch between the damping provided by the oscillator  250  and damping provided by the filter capacitor feedback current. The switched capacitor stabilization network  210  can therefore further includes a low pass filter  265  to select frequency due to system instability. The filter  265  is coupled to a zero cross detector  270 . Transition condition block  255  controls switch  260  in response to the magnitude of the current ripple detected by the current sensor  275 . During initial detection of unstable system  200  operation, current magnitude is above pre-determined level and switch  260  is connected to the output of zero-cross detector  270  forcing stabilization switch  235  to respond to the frequency detected by the current sensor  275 . When current magnitude falls below pre-determined level, the transition condition block  255  reconnects control input of switch  235  to the output of the oscillator  250  via switch  260 . This approach benefits system stabilization by providing fast reduction of voltage/current ripple during initial detection of system instability and reduction of voltage/current ripple during steady-state operation. The filter capacitor feedback current from the input capacitor  225  in the input filter  215  is detected by a current sensor  275  coupled between the input capacitor  225  and the filter  265 , and disposed in the input filter  215 . In one embodiment, the current sensor  275  can detect the filter capacitor feedback current from the input capacitor  225  and pass the current to the filter  265 . 
         [0017]    The transition conditions controller  255  can be any suitable microcontroller or microprocessor for executing the instructions (e.g., on/off commands) described herein. As such, the suitable microcontroller or microprocessor can be any custom made or commercially available processor, a central processing unit (CPU), an auxiliary processor among several processors, a semiconductor based microprocessor (in the form of a microchip or chip set), a microprocessor, or generally any device for executing software instructions. 
         [0018]    In another embodiment, active damping is also attained by synchronizing switch control with the frequency due to system instability.  FIG. 3  illustrates an active damping system  300 . In one embodiment, the system  300  can include a DC power source  305  electrically coupled to a constant power load  320  via input filter  315 . In one embodiment, the DC source  305  can be an AC generator which output is rectified to a DC voltage that includes voltage ripple. The system  300  can further include the switched capacitor stabilization network  310 . In one embodiment, the input filter  315  can be an LC filter having an input capacitor  325  and an input inductor  330  having values selected to filter out certain frequencies between the switched capacitor stabilization network  310  and the constant power load  320 . In one embodiment, the switched capacitor stabilization network  310  further includes an input switch  335  electrically coupled to a damping capacitor  340 , which is further coupled to a damping resistor  345 , which is coupled to the input filter  315 . The switched capacitor stabilization network  310  can further include filter capacitor feedback current to control the damping in the system  300 . The switched capacitor stabilization network  310  can therefore further include a low pass filter  365  to select frequency due to system instability. The filter  365  is coupled to a zero cross detector  370 . In one embodiment, the zero cross detector  370  is implemented such that the input switch  335  is turned on/off as the filter capacitor feedback current crosses zero. The input filter  315  further includes a current sensor  375  coupled between the input capacitor  325  and the filter  365 . The filter capacitor feedback current from the input capacitor  325  in the input filter  315  is detected by the current sensor  375 . In one embodiment, the current sensor  375  can detect the filter capacitor feedback current from the input capacitor  325  and pass the current to the low pass filter  365 . When the zero cross detector  370  detects the filter capacitor feedback current crossing zero, the input switch  335  is closed placing the damping capacitor  340  and the damping resistor  345  in parallel with the filter capacitor  325  thereby providing active damping based on the filter capacitor feedback current. 
         [0019]    In another embodiment active damping can also be attained by synchronizing multiple switches within a switched capacitor stabilization network for dual control.  FIG. 4  illustrates an active damping system  400 . In one embodiment, the system  400  can include a DC power source  405  electrically coupled to a constant power load  420  via input filter  415 . In one embodiment, the DC source  405  can be an AC generator which output is rectified to a DC voltage that includes voltage ripple. The system  400  can further include the switched capacitor stabilization network  410 , coupled to a constant power load  420 . In one embodiment, the input filter  415  can be an LC filter having an input capacitor  425  and an input inductor  430  having values selected to filter out certain frequencies between the dc power source  405  and the constant power load  420 . The input filter  415  further includes a current sensor  475  described further herein. 
         [0020]    The system  400  includes a first input switch  434  coupled to the DC power source  405  and a first damping resistor  444 . The first damping resistor  444  is further coupled to the input filter  415 . The system  400  further includes a second input switch  435  coupled to the damping capacitor  440  and to a second damping resistor  445 , which is coupled to the input filter  415 . Each of the first and second switches  434 ,  435  is respectively coupled to a first and second gate drive  480 ,  481 . The first gate drive  480  is coupled to an “and” function  472  and a first zero cross detector  470 . The second gate drive  481  is coupled to a second zero cross detector  471 . As described further herein, the first gate drive  480  activates the first switch  434  when current inputs to both the first and second zero cross detectors  470 ,  471  pass zero, and the second gate drive  481  activates the second switch  435  when current input into the second zero cross detector  471  passes zero. Dual control of stabilization network  410  via switch  434  and switch  435  allows size reduction of damping capacitor  440  and minimize power losses in damping resistor  444 . The second zero cross detector  471  is also coupled to a band pass filter  463  that is coupled to the current sensor  475  in the input filter  415 . The band pass filter  463  is coupled to an absolute value unit  464 , which is coupled to a low pass filter  465 , which filters our any undesirable high frequencies, such as high frequency harmonics. The band pass filter  463  is tuned to the frequency defined by the input filter  415   
         [0000]    
       
         
           
             
               ( 
               
                 f 
                 = 
                 
                   1 
                   
                     2 
                      
                     
                         
                     
                      
                     π 
                      
                     
                       LC 
                     
                   
                 
               
               ) 
             
             . 
           
         
       
     
         [0000]    The absolute value unit  464  provides a positive number for later comparison to a current ripple reference  466 . The low pass filter  465  and the current ripple reference  466  are coupled to a comparator  467  that is coupled to the first zero cross detector  470 . Current is input into the first zero cross detector  470  if the current from the low pass filter  465  exceeds the current ripple reference  466 . 
         [0021]    On one embodiment, the system  400  attains active damping by closing and synchronizing the input switches  434 ,  435 . Technical effects include the improvement of power quality of the system bus by providing active damping. In addition, the systems and methods described herein reduce system weight, size, and cost by reducing damping capacitor by approximately three times in comparison the capacitor size required using passive techniques. 
         [0022]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments 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.