This paper introduces a new black-box approach for time domain modeling of commercially available single-phase photovoltaic (PV) inverters in low voltage networks. An artificial neural network is used as a nonlinear autoregressive exogenous model to represent the steady state behavior as well as dynamic changes of the PV inverter in the frequency range up to 2 kHz. The data for the training and the validation are generated by laboratory measurements of a commercially available inverter for low power applications, i.e., 4.6 kW. The state of the art modeling approaches are explained and the constraints are addressed. The appropriate set of data for training is proposed and the results show the suitability of the trained network as a black-box model in time domain. Such models are required, i.e., for dynamic simulations since they are able to represent the transition between two steady states, which is not possible with classical frequency-domain models (i.e., Norton models). The demonstrated results show that the trained model is able to represent the transition between two steady states and furthermore reflect the frequency coupling characteristic of the grid-side current.

This paper presents a probabilistic stability analysis of commercial inverters. The set of chosen inverters consists of common types of rooftop inverters for photovoltaic (PV)-system applications in the low voltage grid for low power applications below 10 kW. The main aim is to assess the stability according to the Nyquist criterion based on a comprehensive number of real measured impedances in public distribution grids. This provides an enhanced and more general approach for the assessment and the analysis of the behavior of individual inverter designs with regard to possible instabilities. The state of the art assessment that uses only one single hypothetical grid configuration has the problem which grid parameters to choose. This paper proposes a probabilistic approach covering a wide range of typical grid impedances, which allows a more general statement on grid compatibility of particular inverters and its possibly problematic frequency ranges. This provides a more comprehensive tool for inverter manufacturers to analyze the robustness of their own inverter designs. The paper proposes a first assessment index to quantify the "robustness" of an inverter. In addition, the paper evaluates the impact of the power level on the stability, since inverters operate usually at lower than rated power.

This article presents a non-destructive measurement-based identification of the dc-link capacitance in single-phase power electronic devices for modeling purposes. These devices are typically found in public low voltage networks for low power applications. A large number of manufacturers of these devices with individual approaches to the design of the components of the power electronic devices challenges the modeling and thus the analysis of their interaction with other devices and the low voltage network. Since the manufacturers typically do not disclose their design, only little information based on the assumptions can be made with regard to an individual device. This article provides a new method to identify the dc-link capacitance of single-phase power electronic devices based on analyzing the ripple part of the dc-link voltage, which contributes to the present research activities toward realistic modeling of such devices for power systems studies. The analytical background is provided and a simulative validation is performed. Finally, a laboratory application to a commercially available single-phase inverter for photovoltaic systems is demonstrated in terms of real measurement.

This paper presents a modular model for single-phase photovoltaic (PV) systems for low-voltage studies. The main aim of the model is to assess the harmonic behavior of these systems. Besides steady state analysis, the model allows also dynamic studies, e.g. of harmonic instabilities. In order to identify and assess possible reasons for such instabilities on the inverter side, an easy interchangeability of the model components is provided based on a modular concept. The modular model can be applied to simulate various PV-system implementations in time-domain simulations. An impedance-based assessment based on the fingerprint method is used to assess the different behavior of the PV-systems regarding their topology and the choice of parameters. A comparison between different Phase Locked Loops (PLLs) and different designs of the grid-side filter is drawn to illustrate the different behavior of the PV-system in a steady state operating point. The model is implemented in MATLAB/Simulink and the Simscape Power Systems library.

This paper studies the behavior of grid-connected single-phase photovoltaic inverters in low voltage grids. The interaction of the inverter control, the grid-side filter and the power grid can lead to harmonic instabilities. The term is used, since its origin is not depending on the active or reactive power at fundamental frequency but an interaction of the inverter control with the impedance of the power grid at frequencies in the harmonic range. In power electronics, the power grid impedance is typically assumed as an RL- equivalent so that a critical RL- combination above which the inverter becomes instable can be calculated by using the Nyquist criterion. Neglecting resonances in a power grid can result in an erroneous assessment of stability based on the Nyquist criterion. This is of importance since recent field measurements indicate that the origin of harmonic instabilities of PV-inverters is primarily related to resonances in the grid impedance. This paper studies the behavior of a single-phase PV-inverter based on a simulation model for different impedance conditions of the power grid to illustrate the impact of the grid impedance and its resonances. The results are compared with laboratory measurements of commercially available PV-inverters.