Abstract:
An improved circuit and method for detecting dielectric breakdown and ground fault conditions is provided. The circuitry and method of the present invention include taking a continuous voltage reading of the high voltage battery and sampling the continuous voltage reading of the high voltage battery at a fixed time interval. The circuitry and method calculate a change in the continuous voltage reading of the high voltage battery over the change in time and repeatedly calculate an optimum fixed time interval and an optimum change in voltage over time. Storage of the optimum fixed time interval and optimum change in voltage over time provides for repeatedly comparing the optimum change in voltage over the fixed time interval to the constant voltage of the high voltage battery to calculate the resistance of the dielectric breakdown fault. The calculation of the resistance of the dielectric breakdown fault is carried out independently of the capacitance of the electric circuit. The circuit and method provide adjustment of the optimum fixed time interval to improve the speed of the comparison of the optimum change in voltage over time to the constant voltage of the high voltage battery to calculate the resistance of the dielectric breakdown fault.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to detecting dielectric breakdown faults, and more particularly to an improved circuit and method for detecting dielectric breakdown and ground fault conditions 
       BACKGROUND OF THE INVENTION 
       [0002]    High voltage applications, particularly electrically powered and hybrid (conventional fossil fuel power in combination with electrical power) vehicles require relatively large capacity battery system to deliver a relatively large amount of power compared to a 12 volt automobile storage battery. Since power is directly proportional to battery voltage and system current, the high power delivery requirements which must be satisfied by such batteries mean that higher electrical voltages will be present in electric and hybrid vehicles than in conventionally powered vehicles. Such vehicles are becoming increasingly attractive alternatives to fossil fuel powered cars. However, because of the high voltage requirements of its traction battery an electric or hybrid electric vehicle (HEV) raises significant electrical safety concerns. 
         [0003]    For example, unwanted electric current flow outside of the intended circuit path (i.e. dielectric breakdown faults, ground fault conditions and the like) may cause significant damage to electronic components within a system (such as an electric vehicle or hybrid electric vehicle propulsion system), thereby disabling or even destroying the electronic equipment. In addition, such ground fault conditions may result in an electric shock, which can have more serious safety consequences when the shock is caused by contact with a high voltage battery system, as compared to a conventional, relatively low voltage automotive storage battery system. To reduce the likelihood of such shock, many high voltage battery systems are not grounded to the frame of the machine or vehicle chassis. 
         [0004]    Instead, high voltage battery systems have a closed loop return path, so that the negative power conductor of the system (i.e., the electrical current return path) is isolated from the frame or chassis of the machine, electric vehicle or HEV. 
         [0005]    While such isolated systems may minimize the likelihood of a significant electric shock to a person in the event of a short circuit or low impedance connection (i.e. dielectric breakdown fault), certain electronic components typically in electrical communication with the positive and negative power conductors (bus lines or rails) that supply high voltage power are subject to damage resulting from extreme voltage or current swings occurring thereon. 
         [0006]    Existing high voltage standards relating to ground fault detection, including Federal Motor Vehicle Safety Standard (FMVSS) 305, require a minimum response time for detection under constant monitoring of the isolation parameters in both DC and AC circuits. In addition, these standards require detection of an isolation fault within 100 milliseconds and report of any such fault within 50 milliseconds of detection. The minimum isolation resistance recommended by the SAE is 500 ohms per volt and it is commonly preferred to set this measurement to at least twice the SAE minimum or 1000 ohms per volt. 
         [0007]    Typically previously known fault detection circuits typically use resistor/capacitor networks requiring multiple measurement circuits to provide detection of dielectric breakdown resistance. This configuration results in greater expense due to the multiple measurement circuits required and slower than desired detection times due to the time constant created by the resistor/capacitor network. In addition, these circuits must reach steady state to obtain an accurate measurement which is an undesirable operational limitation. Furthermore these known detection circuits must pulse or switch high voltage to the chassis during measurement causing additional noise to be created in addition to the dangers associated with such a high voltage pulse. Moreover, prior art systems are not capable of measuring the ground fault resistance in both DC and AC circuits which provides an advantage in circuit operation, reducing circuit construction costs and meeting the standards of ground fault detection noted above. 
         [0008]    Such a prior art fault detection circuit is shown in  FIG. 1  and is indicated generally by reference number  10 . Prior art circuit  10  includes an isolated high voltage battery  12  with voltage V pack. As shown in  FIG. 1 , a leakage path is depicted by reference numeral  16  through resistance R 1   eg    18 . Typically, battery  12  is grounded along with the line  20  to the vehicle chassis  22  through capacitors Cy  24 ,  26 . As noted above, prior art system  10  must reach a steady state wherein no current is flowing through capacitors Cy  24 ,  26  to provide an accurate measurement of the dielectric breakdown resistance. To achieve this steady state condition, both R-C loops  28 ,  30  must be in a steady state condition before taking the Va and Vb readings necessary to calculate the dielectric breakdown resistance R 1   eg . Such a steady state requirement introduces less than desirable response time in detecting a dielectric breakdown fault. In addition, this prior art detection circuit must charge and discharge Vpack through the chassis of the vehicle which creates the potential for noise and electric shock through the chassis. Furthermore, this circuit varies and is dependent on the capacitance of the circuit which creates difficulty is accurately detecting and measuring the resistance of the dielectric breakdown fault. Importantly, this prior art circuit does not meet the detection time requirement of 100 ms as noted above in the Federal Motor Vehicle Safety Standard (FMVSS) 305 specification. 
         [0009]    Accordingly, it is an object of the present invention to provide a system and method for detecting faults in high voltage battery systems which provide quick, accurate and cost effective fault detection in both DC and AC circuits, is safe and which does not unduly cause system battery drain. Another object of the present invention is to provide a system and method for detecting faults in high voltage, electric vehicle and hybrid vehicle battery systems which measures the dielectric breakdown system (DBS) resistance and detects the DBS fault to the chassis or frame when the DBS resistance is 35,000 ohms or less. Further, it is an object of the present invention to provide a system and method for detecting faults in high voltage, electric vehicle and hybrid vehicle battery circuits which detects the DBS fault to the chassis or frame and measures the dielectric breakdown system (DBS) resistance which is independent of the capacitance of the circuit. Still another object of the present invention is to provide a system for detecting faults in high voltage battery systems which is simple in construction, quick in detection response, does not introduce external current into the circuit to obtain a measurement, is easy to use and is cost effective. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention discloses a detection circuit for detecting a dielectric breakdown fault in an electric circuit. The circuit includes a high voltage battery and circuitry for continuously monitoring the voltage of the high voltage battery. The voltage across the high voltage battery is sampled by taking a first voltage reading of the high voltage battery at a fixed time interval and subsequently sampled again by taking a second voltage reading of the high voltage battery at a fixed time interval. The change between the first and second voltage readings over the change in time is calculated and then compared to the voltage of the high voltage battery thereby generating a first output signal which is stored for later use and comparison. A third voltage reading of the high voltage battery is taken at a subsequent fixed time interval while the change between the second and third voltage readings over the change in time is calculated and compared to the voltage of the high voltage battery thereby generating a second output signal which is stored for later use and comparison to detect the dielectric breakdown fault and measure the resistance of such a fault. Circuitry is included for adjusting the sampling time interval to estimate the steady state voltage and using the output signals to determine the resistance of the dielectric breakdown fault. 
         [0011]    The electric circuit may include capacitance but the detection circuit of the present invention detects a dielectric breakdown fault and measures the resistance of such a fault independent of any such capacitance included in the circuit. The detection circuit includes the ability to adjust the time interval by a minimum of a 10 percent variation in the change in voltage over the change in time. The detection circuit can be positioned near the center of the positive and negative terminals of the high voltage battery and can be switched into and out of the measuring circuit across the high voltage battery to conserve energy. The circuit measures the steady state DC voltage and resistance of the dielectric breakdown fault when the change in voltage over time of the circuit is below a predetermined threshold voltage or when the slope of the curve of the change in voltage over time approaches 0. The detection circuit also measures the value of the voltage reading of the high voltage battery to determine if the voltage is greater than or less than 0. In an aspect of the invention an amplifier is included in the detection circuit to amplify the signal which is representative of the value of the voltage reading of the high voltage battery when the signal is less than a threshold voltage. This threshold voltage level can be less than 1 volt but may be as low as 0.75 volts. 
         [0012]    The detection circuit for detecting a dielectric breakdown fault in an electric circuit having a high voltage battery of the present invention includes circuitry which takes a continuous voltage reading of the high voltage battery and samples the continuous voltage reading of the high voltage battery at a fixed time interval. The circuitry calculates a change in the continuous voltage reading of the high voltage battery over the change in time and repeatedly calculates an optimum fixed time interval and an optimum change in voltage over time. Storage of the optimum fixed time interval and optimum change in voltage over time provides for repeatedly comparing the optimum change in voltage over the fixed time interval to the constant voltage of the high voltage battery to calculate the resistance of the dielectric breakdown fault. The calculation of the resistance of the dielectric breakdown fault is carried out independently of the capacitance of the electric circuit. The circuit provides adjustment of the optimum fixed time interval to improve the speed of the comparison of the optimum change in voltage over time to the constant voltage of the high voltage battery to calculate the resistance of the dielectric breakdown fault. 
         [0013]    The detection circuit of the present invention embodies a method of detecting a dielectric breakdown fault in an electric circuit having a high voltage battery including the steps of measuring a continuous voltage reading of the high voltage battery then sampling the continuous voltage reading of the high voltage battery at a fixed time interval. The method includes the step of calculating a change in the continuous voltage reading of the high voltage battery over the fixed time interval and repeatedly calculating an optimum fixed time interval and an optimum change in voltage over time. The optimum fixed time interval and optimum change in voltage over time are stored and then repeatedly compared to the constant voltage of the high voltage battery to calculate the resistance of the dielectric breakdown fault. The method includes the step of calculating the resistance of the dielectric breakdown fault independent of the capacitance of the electric circuit. The method includes the step of adjusting the optimum fixed time interval to reduce the time to compare the optimum change in voltage over time to the constant voltage of the high voltage battery thereby reducing the time to calculate the resistance of the dielectric breakdown fault. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The details, advantages, structure, operation and various additional features of the present invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with accompanying drawings where like reference numerals identify like elements throughout the drawings: 
           [0015]      FIG. 1  is a schematic circuit diagram of a prior art fault detection circuit and the equation for calculating the resistance of the fault; 
           [0016]      FIG. 2  shows a schematic illustration of a ground fault detection system according to an aspect of the present invention. 
           [0017]      FIG. 3  is a flow chart which illustrates the steps according to an aspect of the present invention. 
           [0018]      FIG. 4  shows another schematic illustration of a ground fault detection system according to yet another aspect of the present invention. 
           [0019]      FIG. 5  is a flow chart which illustrates yet another set of steps according to an aspect of the present invention. 
           [0020]      FIG. 6  illustrates a series of timing diagrams associated with the detection and measurement of a dielectric breakdown fault in a high voltage battery circuit. 
           [0021]      FIGS. 7A-7B  show the calculations detailing the proposed method of detecting a dielectric breakdown fault of the present invention. 
           [0022]      FIGS. 8A-8B  show an example of the calculations carried out using the method of the present invention on a set of predetermined component values. 
           [0023]      FIG. 9  shows a plot of the isolation resistance fault detection time of the prior art and the present invention. 
           [0024]      FIG. 10  shows a plot of the error of isolation resistance measurement of the prior art and the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 2  shows a ground fault detection system  34  according to an aspect of the invention. Detection circuit VR 1   36  continuously reads the voltage across a resistor network including R 1   38  and R 2   40  for sensing an AC signal indicative of an unintended electric path such as a short circuit or very low impedance connection between a battery  12  with V 1  and V 2 . This short circuit or low impedance path is shown as the dielectric breakdown fault R(DBS_Fault)  42 . Capacitance in the range of 0.5 to 10 microfarads is inherently included in such circuits and is shown as capacitors Cy(Neg)  24  and Cy(POS)  26 . 
         [0026]      FIG. 3  illustrates the steps necessary to accurately and timely measure the dielectric breakdown resistance. Specifically, at power up (block  44 ) voltage VR 1   36 A is taken continuously as indicated in block  46 . In this preferred embodiment, the chosen stepping time of the detection circuit (dt step) is 10 milliseconds, but it should be understood that this initial stepping time will be chosen by the circuit parameters and component values and will change based on the application. The change in the voltage (dVR 1 ) over the change in time (dt) is continuously calculated by the circuitry of the present invention such that the 3 required points are calculated and stored by the circuitry. In addition, at block  48 , the circuitry repeatedly adjusts for and estimates the optimum time interval for detection and measurement. As indicated in block  50  the circuit is initially calibrated based on 100 k ohms as indicated in  FIG. 1  resistors R 1   38  and R 2   40 . If the circuit is in steady state mode where dVR 1 /dt equals 0 volts (block  52 ), then the circuit dielectric breakdown voltage is calculated at block  54  using the steady state DC method as shown in equation 2 in  FIG. 7A . If a steady state has not been achieved, then the dielectric breakdown resistance is detected and measured using the change in voltage over the change in time method as shown in block  56  of  FIG. 3 . As can be seen, the detection and measurement of the resistance of the dielectric breakdown fault is carried out independent of any capacitance that may be present in the electric circuit. 
         [0027]      FIG. 4  shows a detailed schematic drawing of an embodiment of the ground fault detection system  34  of the present invention. As in  FIG. 2  the circuit includes R 1   38  and R 2   40  for sensing the signal indicative of an unintended short circuit or low impedance path R(DBS_Fault)  42  between battery  12  shown with V 1  of 250V and V 2  of 250V. Capacitance in the range of 0.5 to 10 microfarads is shown as capacitors Cy(Neg)  24  and Cy(POS)  26 . Detection circuit VR 1   36  continuously reads the voltage across a resistor network including R 1   38  and R 2   40  and includes a switch U 3   58  which connects and enables measurement by detection circuit VR 1   36 . It is important to note that switch U 3   58  can be eliminated and detection circuit VR 1   36  can be continuously connected for measurement without significant power drain on the system. 
         [0028]    Detection circuit VR 1   36  also includes switches U 1   60  and U 2   62  for alternately sampling the voltage across the battery  12 . Resistors  64 ,  66  and  68  set up the appropriate resistor networks upon the closing of switches U 1   60  and U 2   62  for measuring the voltage across resistor R(DBS_Fault)  42  and allowing the subsequent calculation of its value, the dielectric breakdown fault resistance R(DBS_Fault)  42 . Capacitors C 3   70  and C 4   72  are common to the floating ground connection  74  of the detection circuit  34 . The absolute values of the signals are used to properly calculate the voltages and are provided by voltage converters/inverters  76  and  78 . It is understood that a variety of commercially available converters and inverters can be chosen to perform this inversion/conversion function. 
         [0029]    A to D converter  80  accepts input of signals P 1  and the appropriate signals of P 2 , P 3 , P 4  and P 5  for comparison and calculation of the dielectric breakdown fault resistance R(DBS_Fault)  42 . A to D converter  80  will be chosen based on the necessary resolution determined by the component values of detection circuit  36 . As illustrated in  FIG. 4 , A to D converter  80  is shown as a 12 bit device using the depicted values. However, it will be understood that A to D converter  80  is preferably chosen to be in the range of 8 to 14 bits. Depending on the application, an A to D converter of higher resolution as is necessary to increase the accuracy of the dielectric breakdown resistance calculation will be chosen. An amplifier  82  is provided to amplify the signal VR 1   36 A when the voltage of signal  36 A is less than a threshold voltage, in this case shown as 0.75 volts. 
         [0030]    As shown in  FIG. 5 , the circuit begins detection at power up block  44  where voltage VR 1   36 A is taken continuously as indicated in block  46 . The change in the voltage (dVR 1 ) over the change in time (dt) is calculated by the circuitry of the present invention such that the circuitry repeatedly adjusts for and estimates the optimum time interval for detection as indicated in block  48 . As indicated in block  50 , the circuit is initially calibrated based on 100 k ohms as indicated by resistors R 1   38  and R 2   40  in  FIG. 2 . If the circuit is in steady state mode where dVR 1 /dt equals 0 volts (block  52 ), then the circuit dielectric breakdown voltage is calculated at block  54  using the steady state DC method as shown in equation 2 in  FIG. 7A . If a steady state has not been achieved, then the dielectric breakdown resistance is detected and measured using the change in voltage over the change in time or slope method as illustrated in blocks  56 A,  56 B,  56 C and  56 D. These measurements are carried out using the combination of the AC equation and equation 1 in  FIG. 7A  and such measurements are independent of the capacitance of the circuit. 
         [0031]      FIG. 6  illustrates the timing of the operation of the detection circuit of  FIG. 4 . Measurement is enabled and switch U 3   58  is closed at power up shown in block  44  of  FIGS. 3 and 5  as shown in diagrams  6 A and  6 B. Voltage readings VR 1 , signal  36 A are taken continuously at the illustrated VR 1  sample rate of 10 milliseconds (diagram  6 C). This sample rate will be chosen based on the circuit particulars and application. Switches U 1   60  and U 2   62  close at a chosen sampling rate (dt) interval of 30 milliseconds (diagram  6 D) to measure voltages VR_ 0 , VR_ 1  and VR_ 2  which determine the slope of the curve as shown in diagram  6 E. 
         [0032]    In operation as shown in  FIG. 6 , voltage measurements VR_ 0 , VR_ 1  and VR_ 2  are used to determine the change in voltage over time (dVR 1 _ 1 /dt and dVR 1 _ 2 /dt) which provide the intervals necessary to calculate the dielectric breakdown resistance using equation 1 of  FIG. 7  when the detection circuit  36  is not in a steady state mode. This is shown as the AC measurement step in diagram  6 F. When the change in the voltages over time (dVR 1 _ 1 /dt-dVR 1 _ 2 /dt) approaches 0 (and accordingly the slope of the curve in diagram  6 E approaches 0) the detection circuit is considered to be in a steady state. At this time during circuit operation, the voltages can be measured using a DC measurement (diagram  6 E) allowing the dielectric breakdown resistance to be calculated using equation 2 of  FIG. 7A . While these voltage readings may be a negative value, the circuit will use absolute values by converting the signals through voltage converters/inverters  76  and  78 . As shown in  FIG. 6 , when a fault occurs, the change in voltages over time (dVR 1 _ 1 /dt-dVR 1 _ 2 /dt) begins to change (diagram  6 E). At this point the sampling time (dt) is adjusted in an adaptive manner to a minimum of 10% of the change in the dVR 1 _ 1 /dt-dVR 1 _ 2 /dt reading compared to the previous reading. This adaptive adjustment will ensure that the data points will be on the expected curve (as shown in diagram  6 E) with enough change in slope to accurately estimate the steady state voltage VR 1   36 A of the circuit. 
         [0033]    Referring to  FIG. 7B , the exponential response using a steady state assumption (steady state value of VR 1   ss ) and the transient value of VR 1   t  allows the derivation of the three point equation. This derivation is shown in  FIG. 7B  resulting in equation 1 of  FIG. 7A . As previously described, one of the advantages of using the three point delta measurement for VR 1  to calculate the R(DBS_Fault)  42  is that the capacitance is canceled out as shown in equation 1 of  FIG. 7A . Therefore, the R(DBS_Fault) resistance  42  can be calculated independent of the capacitance of the dielectric breakdown detection circuit  34 . 
         [0034]      FIG. 8  shows an example wherein using the method of the present invention, the detection time can be reduced to 30% of the time constant achieved using the prior art detection method. Specifically, as shown in the  FIG. 8  example, for a 50K R(DBS_Fault)  42 , with the total capacitance of 2uF, the time constant is 100 milliseconds. Using the steady state detection method of the prior art ( FIG. 1 ), it will take 300 milliseconds for the steady state calculation. However measuring three (3) points of the curve and using the dV/dt slope method of the present invention, the R(DBS_Fault)  42  can be detected in a time period of 30 milliseconds. Once the fault is detected, it can be verified by the subsequent 3 point data with the dV/dt slope method until the R(DBS_Fault)  42  can be verified with steady state method when dV/dt=0. Using this dV/dt method, it is possible to report a potential fault within 100 milliseconds and continuously monitor the fault until it has reached a steady state value. As shown in the example ( FIG. 8 ) and in equation 1 ( FIG. 7A ), longer dt time will provide better measurement accuracy. 
         [0035]    The advantages of the detection method and circuit of the present invention, the results of the calculations shown in  FIGS. 7   a  and  7 B and the example of  FIG. 8  are shown in  FIGS. 9 and 10 . In  FIG. 9  the results of the detection time of equation 1 ( FIG. 7A ) are plotted versus the detection time of the prior art circuit of  FIG. 1 . This plot shows that the detection times of the circuit and method of the present invention are faster than those of the prior art circuit of  FIG. 1  and significantly faster when isolation resistance (R(DBS_Fault)  42 ) is over 100,000 Ohms. As shown in  FIG. 10 , the error in measurement of R(DBS_Fault)  42  is considerably lower using the detection circuit and method of the present invention at isolation resistances R(DBS_Fault)  42  smaller than 100,000 Ohms. 
         [0036]    It is to be understood that several of the steps disclosed in the flow charts of  FIGS. 3 and 5 , the specific logic of the circuits illustrated in  FIGS. 2 and 4  or the details of the calculations of  FIGS. 7A ,  7 B and  8 , including but not limited to the adjustment of the circuit parameters or timing, could be performed by software programmed to carry out such steps. These steps could be performed, by way of example only, through software or a program storage device which may be part of a digital computer or computer network. In accordance with the present invention, the program or storage device may be implemented by a processor within a computer that executes a series of computer-executable instructions. These instructions may reside, for example, in RAM, ROM or other storage media of the computer. Alternatively, the instructions may be contained on a data storage medium, such as a computer cd, DVD, ROM, RAM or diskette. Furthermore, the instructions may be stored on a DASD array, magnetic tape, conventional hard disk drive, electronic read-only memory, optical storage device, or other appropriate data storage device. In such an alternate embodiment, the computer-executable instructions may be lines of compiled executable code as available in any computer executable code, steps or language. 
         [0037]    While the present invention has been described above in terms of specific embodiments, it is to be understood that the invention is not intended to be confined or limited to the preferred embodiments disclosed herein and that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art. In addition, while specific component values have been show for ease of illustration and description, it should be understood that a variety of combination of values is possible and contemplated by the present invention. Further, while specific connections have been used and shown for ease of description, it should also be understood that a variety of connection points are possible and may vary depending on the specifics of the application and circuit used. These and all other such modifications and changes are considered to be within the scope of the appended claims and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims.