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This paper addresses stability problems in power systems with loads that exhibit constant-power behavior. Instability may occur in such systems due to the negative incremental impedance of constant-power loads (CPLs). Existing approaches to stabilizing such systems require modification of the source and/or the load control characteristics, or isolating the CPL from the rest of the system by additional active devices, which are difficult to implement and often conflict with other system requirements such as control bandwidth, size, weight, and cost. In this paper, we investigate passive damping as a general method to stabilize power systems with CPL. Using a representative system model consisting of a voltage source, an LC filter, and an ideal CPL, we demonstrate that a CPL system can be stabilized by a simple passive damping circuit added to one of the filter elements. Three different damping methods are considered and analytical models are developed for each method to define damping parameters required for stabilizing the system. Time-and frequency-domain measurements from an experimental system are presented to validate the methods.

Stability of a grid-connected inverter depends on the ratio of the grid impedance to the inverter impedance. Since the grid impedance changes during normal power system conditions, this paper proposes a gain-scheduling adaptive control system that uses online grid impedance measurements. For grid impedance measurement, an impulse-response analysis method is programmed in the digital-signal processor (DSP) of the grid-connected inverter. For adaptation, a Routh-Hurwitz stability analysis approach is used to derive, analytically, the stable operation boundaries of the interconnected system. To simplify the analytical derivations, the grid impedance is assumed inductive at low frequencies and curve fitted to the online impedance measurements. Experimental measurements demonstrate the improvement in system stability, when the impedance identification and adaptive control algorithms are programmed together in the DSP of a three-phase inverter, which is connected to a grid with a variable feeder impedance.

Harmonic resonance can happen in traditional power systems between power factor correction (PFC) capacitors and transformer leakage inductance. In a distribution network with high penetration of renewable generation sources, inverter PWM harmonic currents can be a source of harmonic excitation for system resonance. This paper discusses harmonic resonance problems caused by impedance interactions between the wind inverter and the grid. An impedance-based analysis approach is used to characterize such resonance. Modeling of inverter output impedance directly in the phase domain to enable such analysis is presented. Possible mitigation methods by active damping and grid synchronization design are presented, and their effectiveness demonstrated by simulations and experiments.

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