Adaptive Control of Grid-Connected Inverters Based on Online Grid Impedance Measurements

Abstract

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.

Observer-Based Pulsed Signal Injection for Grid Impedance Estimation in Three-Phase Systems

“…The results in [24] are related to the ones presented in this paper, but they are only focused on a pure inductive three phase balanced impedance. The method proposed in [23] is based on parameter identification using the pulse response and an adaptive model approach.…”

“…The existing literature approaches, as discussed before, rely on the injection of a voltage/current signal and measuring the resulting current/voltage [18], [28]; the use of the closed-loop current response for implementing a MRAS strategy [12]; or the use of observers or estimators [27]. In this paper, the estimation of the system parameters is carried out by using an RLS approach [24], [44]. For that purpose, the differential voltage equation for the equivalent grid impedance in the stationary reference frame is discretized using Tustin method.…”

Frequency-dependent source and load impedances in power systems based on power electronic converters

“…Additionally, the multi-tone principle is not limited to sinusoidal signals. Binary signal injection is performed in [5], while impulse-response is conducted in [6]. It is remarked that a challenge associated with multi-tone injection is the possible cross-coupling between different frequency.…”

“…Several alternative methods exist with ability to estimate impedance closer to real-time, and with low or zero additional hardware requirements. Examples are binary sequence injection [5], impulse response [6] and recurrent neural networks [7].…”

“…One is analyzing the system in the phase domain through symmetric components [1][5] [6][9] [13], while the other applies the synchronous (dq) reference frame [2][3] [4][8] [12]. Both domains have certain advantages and disadvantages, but neither a rigorous comparison nor an attempt to bridge them are conducted.…”