Isolation monitoring device and method

Abstract

Wide deployment of high voltage battery systems in traction, industrial and renewable energy installations is raising the concerns for human safety. Exposure to hazardous high voltages may occur due to deterioration of insulation materials or by accidental events. It is thus important to monitor for such faults and being able to provide timely warnings to affected persons. For this purpose it has become mandatory for electrified passenger vehicles (CFR 571.305) to maintain high isolation values which can be continuously monitored by electrical isolation monitoring devices. The task of monitoring isolation resistance within the electrically noisy car environment is not a trivial task and the solution to this problem has become quickly a field of research and innovation for all affected industries.

1 . A method to estimate a change in values of isolation impedance in an isolated ground (IT) electrical system comprising a power source, the method comprising:
 modeling a first isolation path between a first reference point and a second reference point and modeling a second isolation path between a third reference point and a fourth reference point, thereby creating a theoretical model of the isolated ground electrical system;   providing an initial value of a first isolation resistance for the first isolation path and an initial value of a second isolation resistance for the second isolation path;   measuring an initial value of a voltage between the first reference point and the second reference point and storing the measured initial value in a storage medium;   measuring an initial value of a voltage between the third reference point and the fourth reference point and storing the measured initial value in the storage medium;   measuring a subsequent different value of the voltage between the first reference point and the second reference point and storing the measured subsequent value in the storage medium;   measuring a subsequent value of the voltage between the third reference point and the fourth reference point and storing the measured subsequent value in the storage medium;   entering the measured initial values of the voltages, the measured subsequent values of the voltages, the provided values of the isolation impedances and an elapsed amount of time between the initial measurements and the subsequent measurements into a mathematical function stored in the storage medium;   wherein the mathematical function minimizes the discrepancy between the measured change in values of the voltages and the modeled theoretical values by adjusting values of modeled isolation impedances associated with the isolation paths in the electrical system;   extracting estimated values of isolation impedances associated with the isolation paths in the electrical system by application of the mathematical function; and   storing the estimated values in the storage medium.   
     
     
         2 . The method of  claim 1 , wherein the second reference point is a chassis ground of the electrical system. 
     
     
         3 . The method of  claim 2 , wherein the fourth reference point is the chassis ground of the electrical system. 
     
     
         4 . The method of  claim 1 , further comprising: comparing the estimated values of isolation resistance with a range of acceptable values and communicating that the estimated value of resistance for an isolation path is outside the range of acceptable values. 
     
     
         5 . The method of  claim 1 , further comprising: communicating an amount of estimated energy stored in the isolation impedances. 
     
     
         6 . The method of  claim 1 , wherein the power source is a battery and wherein the first reference point and the third reference point are positive and negative terminals of the battery. 
     
     
         7 . The method of  claim 1 , wherein the power source is a supercapacitor. 
     
     
         8 . The method of  claim 1 , wherein the power source is a DC charger. 
     
     
         9 . The method of  claim 1 , further comprising: identifying a minimum resistance path from the estimated values of isolation resistance. 
     
     
         10 . The method of  claim 9 , further comprising: communicating a value of resistance for the minimum resistance path in the electrical system. 
     
     
         11 . The method of  claim 9 , further comprising: associating the minimum resistance path with one of the power source terminals. 
     
     
         12 . The method of  claim 1 , wherein the theoretical model of the electrical system is an equivalent circuit model. 
     
     
         13 . The method of  claim 1 , further comprising: extracting an estimated value of at least a first capacitance associated with an isolation path by application of the mathematical function and storing the estimated value in the storage medium. 
     
     
         14 . The method of  claim 13 , further comprising: comparing the estimated values of capacitance with a range of acceptable values and communicating that the estimated value of the at least first capacitance is outside a range of acceptable values. 
     
     
         15 . The method of  claim 13 , wherein the power source is a battery and the at least first reference point in the electrical system is a terminal of the battery. 
     
     
         16 . The method of  claim 15 , wherein the measured voltage values are measurements of a varying voltage within the electrical system while the electrical system is operating and measurements of a voltage signal source while the electrical system is idle. 
     
     
         17 . The method of  claim 16 , wherein the mathematical function stored in the storage medium is a least square estimator which produces a least squared error estimate. 
     
     
         18 . The method of  claim 17 , wherein the least squared error estimate is performed over a predetermined number of voltage measurements and corresponding voltage predictions, thereby minimizing a deviation between the measured voltage values and the estimated voltage values, and thereby producing a corresponding number of present value estimates and associated uncertainties for the present value estimates. 
     
     
         19 . The method of  claim 18 , wherein the method steps are performed iteratively. 
     
     
         20 . The method of  claim 19 , wherein the present value estimates are expressed as a vector and the associated uncertainties are expressed as a covariance matrix for the vector. 
     
     
         21 . The method of  claim 20 , further comprising a stochastic filter, wherein the extracted estimated values are fed to the filter and the filter maintains the most likely present value estimates and associated uncertainties for the present value estimates. 
     
     
         22 . The method of  claim 21 , wherein the stochastic filter is a Kalman filter. 
     
     
         23 . The method of  claim 22 , further comprising:
 receiving as inputs to the Kalman filter a set of previous present value estimates and associated uncertainties;   receiving as inputs to the Kalman filter a set of estimated values, including the estimated value of the resistance change and the estimated value of the capacitance change;   outputting a new set of values for the most likely present value estimates and associated uncertainties by application of the filter; and   updating the present value estimates and associated uncertainties stored in the storage medium.   
     
     
         24 . The method of  claim 23 , wherein the method steps are performed iteratively. 
     
     
         25 . An apparatus for estimating unknown values of isolation impedance in an isolated ground (IT) electrical power system, comprising:
 a power source having a positive terminal and a negative terminal, said terminals connected in circuit to at least one additional electrical component and isolated from a chassis ground within the electrical system;   wherein the electrical system contains an isolation impedance between each of the terminals and the chassis ground;   a storage medium;   means measuring an initial value and a subsequent different value of a voltage between the chassis ground and a first reference point and between a second reference point and a third reference point in the electrical system;   means storing the measured initial values and the subsequent different values in the storage medium;   a mathematical function stored in the storage medium, whereby application of the mathematical function extracts estimated values of isolation impedances associated with the voltage measurements by using a model of the electrical system and minimizing an error function.   
     
     
         26 . The apparatus of  claim 25 , further comprising: wherein the electrical system contains at least one capacitance between each of the terminals and the chassis ground, and wherein the mathematical function extracts an estimated value of the capacitance associated with the voltage measurement. 
     
     
         27 . The apparatus of  claim 25 , wherein the power source is a battery. 
     
     
         28 . The apparatus of  claim 25 , wherein the power source is a power conversion system. 
     
     
         29 . An apparatus for estimating a change in values of isolation impedance in an isolated ground (IT) electrical power system, comprising:
 a power source having a positive terminal and a negative terminal, said terminals connected in circuit to at least one additional electrical component and isolated from a chassis ground within the electrical system;   wherein the electrical system contains an isolation impedance between each of the terminals and the chassis ground;   a storage medium;   means measuring an initial value and a subsequent different value of a voltage between the chassis ground and a first reference point and between a second reference point and a third reference point in the electrical system;   means storing the measured initial values and the subsequent different values in the storage medium;   a mathematical function stored in the storage medium, whereby application of the mathematical function extracts an estimated change in values of isolation impedances associated with the voltage measurements by using a model of the electrical system and minimizing an error function.   
     
     
         30 . A method to estimate a change in values of isolation impedance in an isolated ground (IT) electrical system comprising a power source and a load, the method comprising:
 modeling a first isolation path between a first reference point and a second reference point and modeling a second isolation path between a third reference point and a fourth reference point, thereby creating a theoretical model of the isolated ground electrical system;   at a time when power from the power source is being dissipated in the load, measuring an initial value of a voltage between the first reference point and the second reference point and storing the measured initial value in a storage medium;   at a time when power from the power source is being dissipated in the load, measuring an initial value of a voltage between the third reference point and the fourth reference point and storing the measured initial value in the storage medium;   at a time when power from the power source is being dissipated in the load, measuring a subsequent different value of the voltage between the first reference point and the second reference point and storing the measured subsequent value in the storage medium;   at a time when power from the power source is being dissipated in the load, measuring a subsequent value of the voltage between the third reference point and the fourth reference point and storing the measured subsequent value in the storage medium;   entering the measured initial values of the voltages, the measured subsequent values of the voltages and an elapsed amount of time between the initial measurements and the subsequent measurements into a mathematical function stored in the storage medium;   wherein the mathematical function minimizes the discrepancy between the measured initial values of the voltages, the measured subsequent values of the voltages and the modeled theoretical values by adjusting values of modeled isolation impedances associated with the isolation paths in the electrical system;   extracting estimated values of isolation impedances associated with the isolation paths in the electrical system by application of the mathematical function; and   storing the estimated values in the storage medium.   
     
     
         31 . The method of  claim 30 , wherein the second reference point is a chassis ground of the electrical system. 
     
     
         32 . The method of  claim 31 , wherein the fourth reference point is the chassis ground of the electrical system. 
     
     
         33 . The method of  claim 30 , further comprising: comparing the estimated values of isolation resistance with a range of acceptable values and communicating that the estimated value of resistance for an isolation path is outside the range of acceptable values. 
     
     
         34 . The method of  claim 30 , further comprising: communicating an amount of estimated energy stored in the isolation impedances. 
     
     
         35 . The method of  claim 30 , wherein the power source is a battery and wherein the first reference point and the third reference point are positive and negative terminals of the battery. 
     
     
         36 . The method of  claim 30 , wherein the power source is a supercapacitor. 
     
     
         37 . The method of  claim 30 , wherein the power source is a DC charger. 
     
     
         38 . The method of  claim 37 , further comprising: identifying a minimum resistance path from the estimated values of isolation resistance. 
     
     
         39 . The method of  claim 38 , further comprising: communicating a value of resistance for the minimum resistance path in the electrical system. 
     
     
         40 . The method of  claim 38 , further comprising: associating the minimum resistance path with one of the power source terminals. 
     
     
         41 . The method of  claim 30 , wherein the theoretical model of the electrical system is an equivalent circuit model. 
     
     
         42 . The method of  claim 30 , further comprising: extracting an estimated value of at least a first capacitance associated with an isolation path by application of the mathematical function and storing the estimated value in the storage medium. 
     
     
         43 . The method of  claim 42 , further comprising: comparing the estimated values of capacitance with a range of acceptable values and communicating that the estimated value of the at least first capacitance is outside a range of acceptable values. 
     
     
         44 . The method of  claim 42 , wherein the power source is a battery and the at least first reference point in the electrical system is a terminal of the battery. 
     
     
         45 . The method of  claim 44 , wherein the measured voltage values are measurements of a varying voltage within the electrical system while the electrical system is operating and measurements of a voltage signal source while the electrical system is idle. 
     
     
         46 . The method of  claim 45 , wherein the mathematical function stored in the storage medium is a least square estimator which produces a least squared error estimate. 
     
     
         47 . The method of  claim 46 , wherein the least squared error estimate is performed over a predetermined number of voltage measurements and corresponding voltage predictions, thereby minimizing a deviation between the measured voltage values and the estimated voltage values, and thereby producing a corresponding number of present value estimates and associated uncertainties for the present value estimates. 
     
     
         48 . The method of  claim 47 , wherein the method steps are performed iteratively. 
     
     
         49 . The method of  claim 48 , wherein the present value estimates are expressed as a vector and the associated uncertainties are expressed as a covariance matrix for the vector. 
     
     
         50 . The method of  claim 49 , further comprising a stochastic filter, wherein the extracted estimated values are fed to the filter and the filter maintains the most likely present value estimates and associated uncertainties for the present value estimates. 
     
     
         51 . The method of  claim 50 , wherein the stochastic filter is a Kalman filter. 
     
     
         52 . The method of  claim 51 , further comprising:
 receiving as inputs to the Kalman filter a set of previous present value estimates and associated uncertainties;   receiving as inputs to the Kalman filter a set of estimated values, including the estimated value of the resistance change and the estimated value of the capacitance change;   outputting a new set of values for the most likely present value estimates and associated uncertainties by application of the filter; and   updating the present value estimates and associated uncertainties stored in the storage medium.   
     
     
         53 . The method of  claim 52 , wherein the method steps are performed iteratively.