Accurate Measurement of Converter Sequence Impedance Responses by Active Cancellation of Coupling over Frequency

Impedance-based analysis plays important role in assessing the stability and dynamics of grid-connected inverters and related systems. Measuring the inverter impedance is, however, not straightforward due to the load effect caused by the power grid. The power grid interacts with the inverter and produces additional (mirrored) frequency harmonics thus distorting the impedance measurements when applying conventional measurement techniques. This paper proposes an active-power filtering method to minimize the undesired load effect caused by the power grid in the measured impedance. Applying the method, the inverter output impedance can be reliably measured under non-ideal grid conditions.