Sequence Impedance Measurement of Utility-Scale Wind Turbines and Inverters - Reference Frame, Frequency Coupling, and MIMO/SISO Forms

Sequence impedance responses are increasingly used for the stability analysis of converter-grid systems; however, many aspects of the sequence impedance measurement process, particularly those resulting from the frequency coupling between the positive- and negative-sequence impedances, are not yet fully explored. Existing methods for measuring sequence impedance with frequency coupling are complicated, not suitable for large wind turbines and inverters, and they can measure the sequence impedance in only one of the MIMO and SISO forms. This paper shows that the sequence impedance has a reference frame similar to the dq impedance and presents a method for measuring the sequence impedance with frequency coupling. The proposed method aligns the sequence impedance reference frame based on the estimation of the grid voltage angle and obtains the impedance response in both MIMO and SISO forms. The paper also demonstrates the impact of the fundamental frequency on the accuracy of the impedance measurements. The proposed method and practical issues associated with the impedance measurement of utility-scale wind turbines and inverters are demonstrated on a 1.9-MW Type III wind turbine and a 2.2-MVA inverter using an impedance measurement system built around a 7-MW/13.8-kV grid simulator and a 5-MW dynamometer.