Isolation monitoring device and method

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;
• and
• in the event of an estimate a value of an isolation impedance falling below a predetermined threshold, annunciating the event to a human user;
• the method 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;
• wherein the power source is a battery and the at least first reference point in the electrical system is a terminal of the battery;
• 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;
• wherein the mathematical function stored in the storage medium is a least square estimator which produces a least squared error estimate;
• 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;
• wherein the method is performed iteratively;
• wherein the present value estimates are expressed as a vector and the associated uncertainties are expressed as a covariance matrix for the vector;
• 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.