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Naval ships as well as aerospace power systems are incorporating a greater degree of power electronic switching sources and loads. Although these components provide exceptional performance, they are prone to instability due to their high efficiency and constant power characteristics that can exhibit negative impedance nature at certain frequencies. When designing these systems, integrators must consider the impedance versus frequency at an interface (which designates source and load). Stability criteria have been developed in terms of source and load impedances for both dc and ac systems, and it is often helpful to have techniques for impedance measurement. For dc systems, the measurement techniques have been well established. This paper introduces a new method of impedance measurement for three-phase ac systems. By injecting an unbalanced line-to-line current between two lines of the ac system, all impedance information in the traditional synchronous reference frame d-q model can be determined. For medium-voltage systems, the proposed technique is simpler and less costly than having an injection circuit for each phase. Since the current injection is between only two phase lines, the proposed measurement device can be used for both ac and dc interfaces. Simulation and laboratory measurements demonstrate the effectiveness of this new technique.