With the massive deployment of converter-based power generation, low-frequency oscillations (LFOs) such as sub-synchronous oscillations (SSOs) caused by converter-grid interactions have garnered widespread attention. Accurate field-measurement of low-frequency impedance for grid-connected converters is essential for the stability assessment. However, it is challenging for high-voltage and power converter systems, due to the high cost of impedance measurement equipment. To address the issue, a three-step equivalent transformation of the external voltage excitation source to a virtual excitation source is proposed, which facilitates imposing the excitation signals directly into the control system of the converter under test. Such equivalence is mathematically proven, demonstrating that the virtual excitation source can ensure an accurate self-measurement of the low-frequency impedance of grid-connected converters. Further, the impacts of filter parameter shifts and the side-band harmonics of switching modulation on the impedance measurement accuracy are analyzed in detail. Finally, the effectiveness of the virtual excitation source injection method is validated through experimental tests.