Patent ID: 7746084

Claim:
A method for measuring impedance in a three phase alternating current (ac) circuit, the three phase ac circuit having a source portion and a load portion that are integrally connected at each phase of the three phase ac circuit, the method comprising: injecting a first unbalanced line-to-line shunt current at an injected current frequency in the three phase ac circuit; obtaining a first set of line voltages and a first set of phase currents of the three phase ac circuit; determining a first fundamental electrical frequency for the first set of line voltages and the first set of phase currents; determining a first set of transformed voltages and a first set of transformed currents by applying a fast Fourier transform to the first set of line voltages and the first set of phase currents at a frequency of interest and transforming the first set of line voltages and the first set of phase currents to a q-d synchronous reference frame using the first fundamental electrical frequency; injecting a second unbalanced line-to-line shunt current at a frequency linearly independent of the injected current frequency in the three phase ac circuit; obtaining a second set of line voltages and a second set of phase currents of the three phase ac circuit; determining a second fundamental electrical frequency for the second set of line voltages and the second set of phase currents; determining a second set of transformed voltages and a second set of transformed currents by applying a fast Fourier transform to the second set of line voltages and the second set of phase currents at a frequency of interest and transforming the second set of line voltages and the second set of phase currents to a q-d synchronous reference frame using the second fundamental electrical frequency; and calculating an impedance for one of the set consisting of the source portion of the three phase ac circuit and the load portion of the three phase ac circuit using the first set of transformed voltages, the first set of transformed currents, the second set of transformed voltages, and the second set of transformed currents.