Abstract:
In order to detect deterioration of a DC link capacitor between a DC source and an inverter load in an electric propulsion vehicle, capacitance of the link capacitor must be accurately measured during in-service use. A charge is established on the link capacitor. The capacitor is isolated from the source and the inverter load. A constant current circuit is activated to discharge the capacitor. A first voltage is measured across the capacitor at a first time during the discharging. A second voltage is measured across the capacitor at a second time during the discharging. A discharge current flowing from the capacitor is measured during the discharging. The capacitance is calculated in response to the discharge current multiplied by a ratio of a difference between the second and first times to a difference between the first and second voltages. The calculated capacitance is monitored for a decline indicative of a failure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to electric drive circuits for road vehicles, and, more specifically, to measuring the capacitance of a DC link capacitor to support determination of a state of health of the link capacitor. 
     Electric drive systems for traction motors in electric and/or hybrid road vehicles typically require conversion of electrical power from a DC source to an alternating current signal at a variable frequency and power for driving an AC traction motor. The DC source itself may include a storage device such as a battery or fuel cell and a DC-to-DC converter for stepping up the DC voltage supplied to the AC inverter. A necessary component for coupling the DC source (e.g., the battery or an intermediate DC-to-DC converter) to the DC-to-AC inverter is a DC link capacitor. 
     In automotive vehicle applications, the DC link capacitor is typically comprised of a film capacitor such as a metalized polypropylene film to take advantage of their relatively low cost and robustness. The DC link capacitor performance is vital to the functioning of the inverter system, and any degradation in its properties can lead to a loss of proper functioning of the inverter. 
     During extended use, film capacitors in particular may be subject to deterioration as a result of high operating temperatures, high operating voltages (e.g., voltage spikes), high humidity, and manufacturing defects such as impurities or film defects. To help ensure that DC link capacitors properly perform their intended functions for the desired lifetime of a vehicle, it is a common practice to select a capacitor design with performance specifications greater than what is normally encountered during use. This allows a higher confidence in withstanding worst case conditions but results in increased costs. In order to reduce cost and improve overall operation using a capacitor design that is better matched to normal conditions, it becomes necessary to monitor the state of health of the link capacitor so that it can be replaced in the event of a degradation which could lead to failure. 
     Degradation of the link capacitor is typically specified in terms of a defined loss in its capacitance, e.g., a 5% loss from its initial value. The change of capacitance is an indicator of physical processes that take place inside the capacitor on an ongoing basis. In one particular method for predicting failure of the DC link capacitor, its capacitance can be determined automatically and periodically during its lifetime. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a method is provided for detecting deterioration of a DC link capacitor between a DC source and an inverter load. A charge is established on the link capacitor. The link capacitor is isolated from the source and the inverter load. A constant current circuit is activated to discharge the link capacitor. A first voltage is measured across the link capacitor at a first time during the discharging. A second voltage is measured across the link capacitor at a second time during the discharging. A discharge current flowing from the link capacitor is measured during the discharging. The capacitance is calculated in response to the discharge current multiplied by a ratio of a difference between the second and first times to a difference between the first and second voltages. The calculated capacitance is monitored for a decline indicative of a failure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing an electric drive system having a DC to DC converter coupled to a battery and a DC to AC inverter coupled to a motor. 
         FIG. 2  is a schematic diagram showing one embodiment of a capacitance measuring circuit of the present invention. 
         FIG. 3  is a plot showing voltage and current changes for the circuit of  FIG. 2 . 
         FIG. 4  is a schematic diagram showing another embodiment of the invention. 
         FIG. 5  is a flowchart showing one preferred method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a vehicle  10  having an electric drivetrain with a traction motor  11  and a battery  12 . A battery voltage V B  from battery  12  is converted by a boost converter  13  to a higher voltage V DC  across a DC link capacitor  14 . The converted DC voltage is inverted in a PWM inverter  15  for supplying an alternating voltage to traction motor  11 . A vehicle system controller (VSC)  16  is connected to inverter  15  for implementing a known type of control strategy when the electric drivetrain is activated. In the case where vehicle  10  is a hybrid electric vehicle, VSC  16  is also coupled with a combustion engine  17  for operating vehicle  10  in gas-powered modes as known in the art. 
     In the illustrated embodiment, boost converter  13  includes a capacitor  18  and an inductor  19  connected to battery  12 . A pair of power transistors  20  and  21  are selectably switched on and off at predetermined time intervals as known in the art in order to generate the desired voltage V DC . Each power switch includes a freewheeling diode in parallel with the transistor as known in the art. 
     Inverter  15  has a first phase leg including power transistors  22   a  and  22   b , a second phase leg having transistors  23   a  and  23   b , and a third phase leg having transistors  24   a  and  24   b . Transistors  20 - 24  may be controlled by VSC  16  or by another dedicated controller as known in the art. 
     DC link capacitor  14  possesses a relatively high capacitance. Failure of link capacitor  14  would prevent successful operation of the electric drivetrain. 
       FIG. 2  shows a first embodiment of the present invention for monitoring the DC link capacitance. A voltage V B  from a DC source such as a battery  30  is boosted in a boost converter comprised of capacitor  31 , inductor  32 , and power switches  33  and  34 . The converted DC voltage V DC  is applied across a DC link capacitor  35  and is input to an inverter  36 . The invention employs a constant current discharge circuit  40  which is active only during predetermined measuring periods to evaluate an instantaneous capacitance of link capacitor  35 . 
     Discharge circuit  40  is coupled across link capacitor  35 . Discharge circuit  40  provides an open circuit until it is activated, so that it does not interrupt normal operation of inverter  36 . In this embodiment, discharge circuit  40  includes a bipolar junction transistor  41  with its collector coupled to V DC  via a resistor  42  and has its emitter coupled to ground via a resistor  43 . The base of transistor  41  is connected through a base resistor  44  to a switch  45  controlled by a microcontroller  46  to selectably provide a voltage V ON  via switch  45  to cause transistor  41  to conduct during the measuring period. The voltage applied to the base of transistor  41  and the resistance of emitter resistor  43  are selected to ensure that transistor  41  operates in a constant current region so that during a measuring period link capacitor  35  will be discharged at a fixed, predetermined rate. 
     Other semiconductor devices besides a bipolar transistor can be used to create a constant current draw, such as a MOSFET or an IGBT. The semiconductor device should have a voltage rating sufficiently high to withstand (i.e., block) the highest voltages which may appear along the positive DC bus when the device is not turned on. 
     During the constant current draw, the voltage appearing on link capacitor  35  will ramp down at a rate which depends on the instantaneous capacitance. Thus, in order to calculate the capacitance, the present invention employs a current-sensing circuit  47  across emitter resistor  43  and a voltage-sensing circuit  48  across link capacitor  35 . Each sensing circuit  47  and  48  may be comprised of an op-amp providing an output to microcontroller  46  for indicating the desired values of the constant discharge current i dis  and the link capacitor voltage V CAP . 
     If the current i dis  is the only current discharging the link capacitor, the capacitor voltage V CAP  at time T2, or V2 can be described by the following equation: 
                     V   2     =       V   1     +       1   C     ⁢       ∫     t   ⁢           ⁢   1       t   ⁢           ⁢   2       ⁢       i   dis     ⁢           ⁢     ⅆ   t                     Eq   .           ⁢   1               
Where V1 is the capacitor voltage at time T1. This equation can also be expressed by a differential equation:
 
                     i   dis     =     C   ⁢       ⅆ     V   cap         ⅆ   t                 Eq   .           ⁢   2               
If the discharge current is maintained constant, the equation can be simplified to:
 
     
       
         
           
             
               
                 
                   
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       FIG. 3  shows a plot of the DC link capacitor voltage V CAP  along a line  50  and discharge current i dis  along a line  51 . Before initiating a measurement, a charge is established on the link capacitor corresponding to an initial voltage that insures V CAP  remains above a voltage margin V m  described in more detail below. When the constant current discharge circuit is activated, discharge current i dis  rises to a substantially fixed value at  52  which is then maintained during an entire measuring period. In response, the link capacitor voltage begins to decline at  53 . During the discharge, the value of the link capacitor voltage is sampled at times T1 and T2 providing measurements V 1  and V 2 . With the collected samples, the capacitance of the DC link capacitor  35  is found using the following formula: 
                   C   =         ∫     t   ⁢           ⁢   1       t   ⁢           ⁢   2       ⁢       i   dis     ⁢           ⁢     ⅆ   t             V   2     -   V               Eq   .           ⁢   5               
If the discharge current is maintained constant, the above equation can be simplified to:
 
                   C   =       i   dis     ⁢         t   2     -     t   1           V   2     -     V   1                   Eq   .           ⁢   6               
Once calculated by the controller, the capacitance C is stored for diagnostic purposes. It is compared with previous capacitance measurements and/or an initial specified value for the link capacitor so that any decline can be detected which is indicative of an imminent or existing failure of the link capacitor.
 
     Returning to  FIG. 2 , it can be seen that link capacitor  35  must be isolated from the inverter load of inverter  36  and from the DC source (e.g., battery) so that the discharging of link capacitor  35  is determined solely by and through discharge circuit  40 . To initiate a measuring period, controller  46  deactivates the phase leg switches in inverter  36 . Isolation from the DC source may be provided by a switch (not shown) or by ensuring that the bus voltage V DC  is sufficiently high to maintain reverse bias of any freewheeling diode contained in power switch  33 . In other words, it is not sufficient to have power switch  33  turned off since the freewheeling diode could conduct from battery  30  if V CAP  is not high enough to reverse bias the diode. The required voltage is designated V m  which is equal to the battery voltage plus a standard diode drop. In the event that a boost converter is not used and link capacitor  35  is driven directly by battery  30 , then a battery contactor switch would be required to provide the isolation. 
     In order to conduct a capacitance measurement, link capacitor  35  must first be charged to a voltage sufficient to maintain a discharge over the required period of time. The step of establishing a charge on link capacitor  35  may also succeed in isolating capacitor  35  from the DC source as explained above. 
     Voltage sensing of the op-amps used for current and voltage measurements may include inherent time delays due to filtering. The delays can be compensated for, as known in the art. 
     In order to avoid the presence of high voltages stored on the link capacitor, it should be discharged during times when a vehicle is not in use. Discharge circuit  40  can be used to perform such a function. If a faster discharge rate is desired, then an additional passive discharge resistor can be connected across the link capacitor in order to bleed off the charge more quickly when the electric drive is turned off. The resistance of the passive discharge resistor must be sufficiently high to avoid any significant effect on drive performance during normal operation and sufficiently low to discharge the link capacitor within a reasonable time after deactivating the drive. Whenever a passive discharge resistor is present, it becomes necessary to isolate the link capacitor from the passive discharge resistor during the measurement period. However, the passive nature of the operation for the passive discharge must be maintained. As shown a further embodiment of the invention in  FIG. 4 , a passive discharge resistor  55  is connected in series across link capacitor  35  together with a normally-on switching device  56  which is capable of being switched off by microcontroller  46  simultaneously with the turning on of the constant current discharge circuit  40 . In the illustrated example, a depletion-mode MOSFET. Resistor  55  performs the desired discharge function except during a measuring period when switch  56  is actively being driven. 
     One preferred method of the invention is summarized in  FIG. 5 . At an appropriate time (e.g., during vehicle idling, during a start-up sequence, or during a shut-down sequence), the controller of the present invention implements a measuring period which begins by establishing a charge on the DC link capacitor in step  60 . In step  61 , the DC link capacitor is isolated from the load and battery. In step  62 , the constant current discharge circuit is turned on. At any suitable time during the measurement period, the discharge current i dis  is measured in step  63 . In step  64 , a first value of the link capacitor voltage v 1  is measured at a time t 1 . A second sample of the capacitor voltage v 2  is collected at a subsequent time t 2  in step  65 . In step  66 , the capacitance of the link capacitor is calculated using the formula given above. Using the calculated capacitance, a decline in the capacitance can be detected in step  67  and used to indicate an impending or actual failure of the link capacitor.