Experimental Validation of Harmonic Impedance Measurement and LTP Nyquist Criterion for Stability Analysis in Power Converter Networks

Abstract

This paper presents the first experimental validation of the stability analysis based on the online measurement of harmonic impedances exploiting the Linear Time Periodic (LTP) approach, applied to AC networks of power converters. Previous publications have provided the theoretical framework for the method, enabling the stability assessment of an unknown system adopting a blackbox approach, relying only on injected perturbations and local measurements. The experimental case study considered in this paper comprises two single-phase converters, one acting as source subsystem and the other as load subsystem. A third converter, the Stability Measurement Unit (SMU), is controlled to inject small current perturbations at the point of common coupling (PCC).From the measured small-signal perturbations of PCC voltage, source current and load current, the harmonic impedances of source and load subsystems are calculated. The LTP Nyquist Criterion is then applied to the ratio of the two harmonic impedances in order to assess the stability of the whole system. Theoretical and experimental results from a 5 kW laboratory prototype are provided and confirm the effectiveness of the method. In addition, the measurements do not require sophisticated equipment or control boards and can be easily performed from data sampled by commercial micro-controllers.

Measurement-Based Black-Box Harmonic Stability Analysis of Commercial Single-Phase Inverters in Public Low-Voltage Networks

“…However, the inductance values in these studies are unrealistically high with regard to public LV networks (e.g., 12.8 mH [34]). Furthermore, existing black-box studies only indicate that the inverters will eventually become instable [38], but provide no experimental proof of the minimal required phase margin in practical applications. In contrast, this study accurately calculates the specific critical conditions for a commercially available PV inverter and validates the preciseness of the fully measurementbased black-box stability assessment.…”

Vector-Norm Based Truncation of Harmonic Transfer Functions in Black-Box Electronic Power Systems

“…Comparing Case II and Case III, it is indicated that the converter control on the harmonic mitigation can result in a reduced-order modeling of the converter-based system. That is to say, truncation relying only on the system's steady-state harmonics like in [9]- [12] may result in a more conservative truncation order.…”

“…HTFs are thus more feasible than the HSS models, since they can directly characterize the system's dynamics from the input-output perspectives, even without knowing the internal control details. Moreover, the HTFs can be obtained easily by the frequency-scan measurement [8], [9] and further used for stability analysis based on the Nyquist stability criterion [9]- [12].…”

Small-Signal Modeling of AC Power Electronic Systems: Critical Review and Unified Modeling

“…Since the admittance model is truncated by a 6-by-6 matrix, the frequency scan measurement should inject perturbations 6 times to measure all the elements of ( 27) [46]. The frequency is scanned within the fundamental frequency interval of the HTF model, i.e., (0, 100) Hz.…”