AC and DC impedances of switching power converters are used for the stability analysis of modern power electronics systems at three-phase AC and single-phase DC interfaces. Therefore, a small-signal characterization algorithm for switching power converter, which is based on FFT, will be presented and explained. The presented extraction algorithm is general and can be used to obtain other small-signal transfer functions of arbitrary power converter switching simulation models. Furthermore, FFT algorithm is improved by using cross power spectral density functions for identification, resulting in an algorithm, which is more noise immune. Both small-signal identification algorithms are validated in simulations, and CPSD algorithm is used in experimental measurement procedure. Several wide bandwidth injection signals, among which are chirp, multi-tone, pulse and white noise, are compared and theoretically analyzed. Several hardware examples are included in the analysis. The second part of the dissertation will focus on the modeling of small-signal input dq admittance of multi-pulse diode rectifiers, providing comparison between well-known averaged value models (AVMs), parametric averaged value models (PAVM), the switching simulation model and hardware measurements. Analytical expressions for all four admittances present in the dq matrix are derived and analyzed in depth, revealing the accuracy range of the averaged models. Furthermore, a hardware set-up is built, measured and modeled, showing that the switching simulation model captures nonlinear sideband effects accurately. In the end, a multi-pulse diode rectifier feeding a constant power load is analyzed with modified AVM and through detailed simulations of switching model, proving effectiveness of the proposed modifications. The third part describes implementation and design of a single-phase multi-level single-phase shunt current injection converter based on cascaded H-bridge topology. Special attention is given toward the selection of inductors and capacitors, trying to optimize the selected component values and fully utilize operating range of the converter. The proposed control is extensively treated, including inner current, outer voltage loop and voltage balancing loops. The designed converter is constructed and integrated with measurement system, providing experimental verification. The proposed multi-level single-phase converter is a natural solution for single-phase shunt current injection with the following properties: modular design, capacitor energy distribution, reactive element minimization, higher equivalent switching frequency, capability to inject higher frequency signals, suitable to perturb higher voltage power systems and capable of generating cleaner injection signals. Finally, a modular interleaved single-phase series voltage injection converter, consisting of multiple paralleled H-bridges is designed and presented. The decoupling control is proposed to regulate ac injection voltage, providing robust and reliable strategy for series voltage injection. The designed converter is simulated using detailed switching simulation model and excellent agreement between theory and simulation results are obtained. The presented control analysis treats different loads, examining robustness of the circuit to load variations. Simulation model and hardware prototype results verify the effectiveness of the proposed wide-bandwidth identification of small-signal dq impedances via single-phase injections. / Ph. D.