Abstract:
A method of detecting a ground fault condition between a direct current power system and the chassis ground of an electric or hybrid-electric vehicle is provided. The method includes sequentially opening and closing a first switch connected between a positive node of the direct current power system and the chassis ground of the vehicle and a second switch connected between a negative node of direct current power system and the chassis ground. The sequential opening and closing of the first and second switches charges and discharges an inherent capacitance present between the metal components of the direct current power system and the chassis. First and second currents are created as the inherent capacitance is charged and discharged. Measurements of the created first and second currents are then used to determine whether a ground fault condition exists between the direct current power system and the vehicle chassis ground.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/152,905, filed on Feb. 16, 2009, and entitled FAULT DETECTION METHOD FOR DETECTING LEAKAGE PATHS BETWEEN POWER SOURCES AND CHASSIS, the specification of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The following disclosure relates to electrical fault detection systems and methods for high voltage DC systems. 
     BACKGROUND 
     A number of factors including ever-increasing energy costs, environmental concerns and the development of new battery technology has fueled interest in electrically powered automobiles. “Plug-in” type electric vehicles rely solely on a battery powered electric motor to propel the vehicle. “Hybrid” vehicles use a combination of an electric drive motor in combination with a gasoline or diesel fueled engine to achieve higher mileage. 
     Plug-in and hybrid vehicles typically use a high voltage DC drive powered with large batteries or battery packs. Voltages present in these systems may range from 100 to 1000 or greater. Consequently, the use of such systems can present a danger if the high voltage system is not effectively isolated from the vehicle chassis. The threshold voltage where DC becomes dangerous can be as low as 55 to 60 volts and contact with a high voltage DC source can cause serious injuries. Contact with direct current tends to cause continuous muscular contractions that make the victim hold on to a live conductor, increasing the risk of burns and other injuries. Current leakage from the high voltage system to the chassis (a fault) may result from frayed wires contacting chassis components and component failure. Corrosion and/or infiltration of salt, dirt and other debris may provide a current path. Consequently it is important to identify potentially dangerous faults. 
     One presently proposed fault detectors utilizes a capacitively coupled signal injected into an isolated ground. However, large amounts of parasitic and inherent capacitance in electrically powered vehicles tend to make such devices too “noisy” for reliable use of a capacitively coupled signal for fault detection. Another proposed approach is the use of a wheatstone bridge. However, a short across the detection nodes of a wheatstone bridge may be undetectable. Thus, there exists a need for a more reliable fault detection system for use with high voltage DC systems such as those used in electric vehicles and other applications. 
     SUMMARY 
     In one embodiment, an apparatus for monitoring a direct current system for ground faults in a device having inherent capacitance between the direct current system and a chassis ground is provided. This embodiment includes a fault detection module connected between the chassis ground and a first switch and a second switch. The first switch is also connected between a positive node of the direct current system and the fault detection module. The second switch is connected between a negative node of the direct current system and the fault detection module. A switch driver is provided to sequentially open and close the first and second switches such that the inherent capacitance is charged and discharged. 
     In one aspect, the fault detection module includes a controller for controlling the switch driver and a current sensor. The current sensor senses a first current when the first switch is closed and a second current when the second switch is closed. The current sensor then transmits a signal to the controller indicating an amperage or measurement of the first and second currents. A data interface connected to the controller outputs a signal in response to the amperage or measurement of the first and second currents. The current sensor may include a programmable gain amplifier and an analog to digital signal converter wherein the programmable gain amplifier transmits a signal to the analog to digital signal converter and the analog to digital signal converter transmits a signal to the controller. 
     In another aspect, a first resistor is connected in series between the positive node of the direct current system and the first switch. And, a second resistor is connected between the negative node of the direct current system and the second switch. Preferably, the resistances of the first and second resistors are substantially equal. The resistances of the first and second resistors is typically high, on the order of a mega ohm. 
     The apparatus may include a non-volatile memory connected to the controller, with preprogrammed instructions that are utilized by the controller for detecting a fault based on changes in the first and second currents. The non-volatile memory may also include or be programmed to include instructions, utilized by the controller, for determining a parasitic resistance of the direct current system signal in response to changes in the first and second currents as well as instructions, utilized by the controller, for determining a leakage current of the direct current system signal based on the first and second currents. 
     In another embodiment, a fault detection apparatus for an electrically powered vehicle having a direct current power system with an inherent capacitance between the direct current power system and the vehicle&#39;s chassis is provided that includes a fault detection module connected to a chassis ground of the vehicle. The fault detection module is also connected to a first switch and a second switch, which are in turn connected to positive and negative nodes of the direct current power system respectively. The fault detection module includes a switch output driver control that sequentially or in an alternating manner opens and closes the first and second switches to charge and discharge the inherent capacitance at predetermined intervals. A current sensor senses a first current when the first switch is closed while the inherent capacitance is charged and senses a second current when the second switch is closed while the inherent capacitance is discharged. Based on the sensed values of the first and second currents, a digital controller, which may include a signal processor, utilizes preprogrammed instructions stored in a non-volatile memory to determine if a ground fault condition exists between the direct current power system and the vehicle&#39;s chassis. A data interface connected to the digital controller sends a signal, indicative of whether a fault condition is detected, to an on board computer or other controller or processor of the vehicle. 
     In yet another embodiment, a method of detecting a ground fault condition in a direct current power system of an electric vehicle or hybrid-electric vehicle, includes sequentially opening and closing a first switch connected between a positive node of the direct current power system and a chassis ground of the electric vehicle and a second switch connected between a negative node the of direct current power system and the chassis ground of the electric vehicle. As the first and second switches are opened and closed, an inherent capacitance between a metal or other conductive component of the power system and the chassis is charged and discharged. A first current through the first switch and a second current through the second switch are each measured as the inherent capacitance is charged and discharged. The measured values of the currents are used to determine if a ground fault condition exists between the direct current power system and the chassis ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding, reference is now made to the following description taken in conjunction with the accompanying Drawings in which: 
         FIG. 1  illustrates a schematic representation of a high-voltage DC electrical system employing a fault detection system according to the disclosure; 
         FIG. 2  is a graph illustrating the simulated operation of the fault detection system; 
         FIG. 3  is a graph illustrating the simulated operation of the fault detection system of  FIG. 1  in the case of a fault; 
         FIG. 4  is a graph representing the simulated operation of the fault detection system of  FIG. 1  in a case of a second fault; 
         FIG. 5  is a graph further illustrating the simulated operation of the fault detection system of  FIG. 1 ; and 
         FIG. 6  is a block diagram illustrating one configuration of the fault detection module of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic representation of a high-voltage DC electrical system employing an exemplary fault detection system  100 . For example, system  100  may be employed in an electrically-powered vehicle  102 , such as a golf cart or automobile, driven by an electrical motor  104  and powered with a battery or battery pack  106 . System  100  may also be employed in a gasoline-electric hybrid or other type of hybrid electric device or machine. Such systems typically operate at relatively high voltages, e.g., 100-1000 volts or higher. As illustrated, a detection module  108 , which will be described in greater detail below, is connected to the positive terminal of battery  106  through a first switch  110  and a first resistor  112 . Detection module  108  is also connected to the negative terminal of battery  106  through a second switch  116  and a second resistor  118 . The first switch  110  and the second switch  116  may be combined as a single pole and double throw style switch or relay  113  as optionally shown. 
     In operation, switches  110  and  116  are sequentially opened and closed under the control of fault detection module  108 . First and second switches  110 ,  116  are high-voltage, solid-state switches, for example, optically-coupled relays. First and second resistors  112 ,  118  are large-value resistors, typically on the order of 1 mega ohm or greater. Preferably, the resistance values of resistors  112 ,  118  are equal or substantially equal. In one embodiment, wherein the battery&#39;s ground and the chassis ground  122  are floating relevant to each other, the system  100  includes an over-voltage protection device that may include zener diodes  120 . Other devices may be used to provide over-voltage protection for this system. As illustrated zener diodes  120  are connected between the chassis ground  122  and the high voltage system and have a breakdown voltage slightly greater than the potential across the direct current supply system, battery or battery pack  106 . 
     In  FIG. 1 , parasitic resistance in the system is represented by resistor  124 . Ideally, the value of resistance  124  is very large, on the order of mega ohms. Capacitor  126  represents the inherent capacitance in the system existing between the battery section  106  and the vehicle chassis ground  122 . In the case of an electrically-powered vehicle, an inherent capacitance  126  is typically large due to the number and size of metal chassis components positioned near or adjacent to metal portions of the high-voltage system. 
       FIG. 2  is a graph illustrating the simulated operation of exemplary fault detection system  100 . For the purpose of this example, the value of resistor  112  is assumed to be 1 mega ohm, the value of inherent capacitance  126  is assumed to be 1 nanofarad and the value of parasitic resistance  124  is assumed to be 10 mega ohms. This represents an ideal condition where there is little or no current leakage from the high voltage system to the chassis ground. As illustrated, when switch  110  is closed at time T 0 , current flows through resistor  112  and switch  110  until the inherent capacitance  126  is charged. As inherent capacitance  126  is charged, the value of I 1  declines, approaching zero, reflecting the large value of resistor  112 . At T 2 , switch  110  is opened and switch  116  is closed. Inherent capacitance  126  is discharged through resistor  118  and switch  116  resulting in current flow I 2 . As illustrated, as capacitance  126  is discharged, the value of I 2  approaches zero. As will be appreciated,  FIG. 2  represents a system having a very high parasitic resistance and, consequently, a low leakage current designated as I lk  in  FIG. 1 . 
       FIG. 3  is a graph illustrating the simulated operation of the exemplary fault detection system  100  of  FIG. 1  in the case of a fault. For the purposes of  FIG. 3 , a 300 volt battery system  106  is assumed with the values of resistors  112  and  118  set at 1 mega ohms and with the value of capacitance  126  being 1 nanofarad. In this case, a leakage current of 10 milliamps (I lk ) is also assumed. At T 0 , switch  110  is closed. Current I 1  flows through the resistor  112  and switch  110 . However, in this example the current flow I 1  does not decline due to leakage current flowing through parasitic resistance  124 . At time T 2 , switch  110  is opened and switch  116  is closed. Current I 2  flows through resistor  118  and switch  116 . Again, the value of I 2  does not decline due to leakage I lk  across parasitic resistance  124 . 
       FIG. 4  is a graph representing the simulated operation of the fault detection system of  FIG. 1  in a case where the value of parasitic resistance  124  is assumed to be 300 kilohms. Again, the values of resistors  112  and  118  are assumed to be 1 mega ohm and the value of inherent capacitance  126  is assumed to be 1 nanofarad. At T 0 , switch  110  is closed and current flows through resistor  112  and the switch. As illustrated, the value of current I 1  declines rapidly, however, the value of I 1  does not approach zero due to leakage across parasitic resistance  124 . 
       FIG. 5  is a graph illustrating the simulated operation of the fault detection system of  FIG. 1  wherein resistor  112  has failed open. As in the case of  FIG. 4 , a 300 volt battery system  106  is assumed wherein the value of resistor  118  is 1 mega ohm and the value of parasitic resistance  124  is 300 kilohms with the value of inherent capacitance  126  being 1 nanofarad. At T 0 , switch  110  is closed. However, because resistor  112  has failed open, no I 1  current flows through resistor  112 . The lack of an I 1  current is indicative and can be measured as a failed open resistor  112 . At time T 1 , switch  116  is closed and switch  110  is opened. As is illustrated, current I 2  flows through switch  116  and resistor  118  peaking and then declining to a level reflecting the relatively low value of parasitic resistance  124 . At time T 2 , switch  116  is opened and switch  110  is closed. However, the value of I 1  goes to zero because of the failure of resistor  112 . Furthermore, the circled portion  200  identifies a further detectable and measurable indication that resistor  112  is failed open. The further indication of an open resistor  112  is seen, for example, at time T 4  in the circled portion  200 , wherein the overshoot  202  normally seen due to the occurrence of a capacitor/resistor charge or discharge, is not present. Thus, in some embodiments each leading or overshoot portion  200 ,  202  of the I 1 , I 2  wave form can also be monitored or compared by the controller (as discussed below) in order to determine if there is a system resistor failure. In similar fashion, a failure of resistor  118  can also be monitored. In other words, multiple current measurements taken at different locations of the I 1 , I 2  current wave form can be used to determine whether a ground fault occurs or whether the ground fault detection circuit or detection system has failed. 
       FIG. 6  is a block diagram illustrating an exemplary configuration of fault detection module  108  of  FIG. 1 . As illustrated, module  108  includes a digital controller or a microprocessor  600  and an associated non-volatile memory  602 . Non-volatile memory  602  may be programmed or provided with instructions for storage. The stored instructions are utilized by the operating module  108  for specific applications. For example, non-volatile memory  602  may be programmed with instructions dictating the frequency at which switches  110  and  116  are sequentially cycled, the frequency at which currents I 1  and I 2  are measured or sampled and threshold values for indicating a fault. As illustrated, microprocessor  600  controls the operation of switches  110  and  116  via a switch output driver control  604 . 
     The values of I 1  and I 2  are measured using a current sensor circuit  605  that may include a transimpedance programmable gain amplifier or a programmable/adjustable current sensor  606 . The transimpedance programmable amplifier  606  essentially represents a current measurement of an input (IN) as a voltage output (V OUT ). In other embodiments, other devices or circuits that provide an output signal that represents a measurement of a current flow or that performs essentially the same function may be used. Referring again to  FIG. 2 , values of I 1  and I 2 , may be determined or measured at predetermined intervals. For example, the value of I 1  may be sampled one, two, three or more times between T 0  and T 2 . Other sampling frequencies may be used. 
     The output of programmable gain amplifier  606  is received by an analog to digital converter  608 , which converts the voltage output signal to a digital output. In one embodiment, the digital output is sampled at predetermined intervals under the control of microprocessor  600  and the samples are stored in storage registers  610 . In some embodiments, selected stored values may be averaged in order to smooth the current measurements over a predetermined period of time and/or over a predetermined number of stored value samples. In various embodiments, the digital output of the current sensor circuit  605  or of the analog to digital converter  608  is provided directly, substantially directly or indirectly to the controller  600 . Analog to digital converter  608  may be supplied with a reference voltage from one of an internal reference voltage circuit  614  or from an external reference voltage  612 . Module  108  may be powered with the output of a voltage regulator circuit  616 , which may be integral to the module  108  or may be a separate external device. 
     Module  108  also includes a data or communication interface  618  allowing microprocessor  600  to communicate with an external device. Various standard communication interfaces can be employed as the communications interface  618 , including, but not limited to, I 2 C, CAN, SAA, one-wire or other communication interfaces including custom communication interfaces as well as analog signal interfaces. For example, in the case where fault detection system  100  is employed in an electrical vehicle, communications interface  618  may transmit a signal indicative of whether a fault is detected to a controller or computer system of the electrical vehicle. In one embodiment, microprocessor  600  may transmit a signal indicating the presence of a fault through interface  618  to an electric vehicle&#39;s computer or control system. In this variation, a number of actions may be taken. For example, an audible or visual alarm may be sounded or a relay may be opened when the vehicle is turned off and placed in “park” to disable the vehicle&#39;s electrical system. In other embodiments, the output of the signal from microprocessor  600  through communications interface  618  is utilized for diagnostic purposes with a test device. 
     By way of further illustration, measuring the values I 1  and I 2  enables the value of any parasitic resistance  124  to be determined. Turning again to  FIG. 1 , the value of parasitic resistance  124  may be determined as follows. When switch  112  is closed:
 
 I   1 =(( V 1 −VR ))/( RS   1   +R   lk )
 
     Where RS 1  is the resistance of first resistor  112 , R lk  is the value of parasitic resistance  124  and V 1  and VR are the indicated voltages in  FIG. 1 . 
     When switch  112  is opened and switch  116  closed:
 
 I   2 =( V 2− VR )/( RS   2   +R   lk )
 
     Where RS 2  is the resistance of the second resistor  116 . 
     Combining the currents and setting RS 1 =RS 2 :
 
( I   1   −I   2 )=( V 1 −VR +( V 2 +VR ))/( RS   2   +R   lk )= V BATT/( RS   2   +R   lk )
 
     Where VBATT is the voltage of battery  106 . 
     Since I 1  and I 2  are measured using the detection module  108  and VBATT and RS 2  are known quantities, the foregoing equation can be solved using the digital controller  600  and instruction from the non-volatile memory  602 , for R lk . With the value of R lk  known, the leakage current I lk  may be determined as I lk =VBATT/R lk . If the digital controller determines that I lk  is greater than a predetermined value, such as 1 milliamp, the digital controller  600  will instruct the communication interface  618  to communicate a fault signal. 
     Referring again to  FIG. 1 , although system  100  is illustrated as being connected directly to the terminals of battery pack  106 , in other versions it may be connected across different nodes of the high voltage system. In different embodiments, system  100  may be incorporated into high voltage battery packs for vehicles, utilized in battery powered backup power systems, cordless power tools or equipment and similar devices. Furthermore, the fault detection module  108  may be incorporated into the direct current power supply side or onto the chassis side of the vehicle, device, or machine. 
     Another embodiment of the invention may comprise a hand or user held high voltage ground fault tester that incorporates the module  108  and the switches  110  and  116 , while having a three pronged lead connected thereto. One of the three leads could be made to contact a chassis or device ground, the second lead may incorporate a resistor RS 1   112  and contact a high side of a high voltage DC power source, and the third lead may incorporate a resistor RS 2   118  and contact a low or ground side of the high voltage DC power source. With such an exemplary hand held test device, ground faults comprising potentially deadly current to humans can be easily tested in electrically noisy high voltage electrical environments between electric or hybrid-electric machinery and high voltage DC power sources or AC power sources having an associated large DC offset voltage. 
     Although the preferred embodiment and other embodiments have been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the concepts and scope of the invention.