To meet the requirements of harmonic impedance measurement for ports with different impedance characteristics in power systems, a modular impedance measurement device is studied. The proposed impedance measurement device consists of a front-end rectifier circuit and a cascaded modular H-bridge converter. To realize harmonic impedance measurements across a wide frequency range with varying frequency, magnitude, and characteristics, the rectifier’s output voltage variation is used to adjust the amplitude of the injected disturbance, while a wideband reference signal is introduced into the H-bridge converter control to adjust the disturbance frequency. A load resistor is also introduced at the device output, enabling switching between harmonic voltage or current signal injection according to the impedance of the tested port, thereby improving measurement accuracy. In addition, current feedforward control is applied in the converter to avoid interference from background harmonics on the output disturbance. Finally, real-time RTDS simulations verify that the measurement device can flexibly perform harmonic impedance measurement at different voltage levels, with a measurement range of 1~5000 Hz and a single harmonic injection bandwidth of 200 Hz covering 21 harmonic frequency points. Compared with non-intrusive and conventional intrusive methods, the impedance measurement error of the proposed device is less than 0.5%, satisfying the measurement accuracy requirements.