Nonlinear Modeling of Power Electronics-based Power Systems for Control Design and Harmonic Studies

The massive integration of power electronics devices in the modern electric grid marked a turning point in the concept of stability, power quality and control in power systems. The evolution of the grid toward a converter-dominated network motivates a deep renovation of the classical power system theory developed for machine-dominated networks. The high degree of controllability of power electronics converters, furthermore, paves the way to the investigation of advanced control strategies to enhance the grid stability, resiliency and sustainability. This doctoral dissertation explores four cardinal topics in the field of power electronics-based power systems: dynamic modeling, stability analysis, converters control, and power quality with particular focus on harmonic distortion. In all four research areas, a particular attention is given to the implications of the nonlinearity of the converter models on the power system

Integration and Optimization of Voltage Active Filtering Functionality in a PV Park

The stringent regulations on the power quality declared in the standard IEEE 519-2014 push the companies and the power producers to install active filters to compensate the voltage harmonics distortion in the point of common coupling (PCC). However, in the case of a Photovoltaic (PV) park, the cost for an additional active filter converter can be saved by using the PV converters themselves as active filter. This solution is very attractive, but reserves several challenges. In fact, the harmonic current injection by the PV converters can generate ripples in the DC link which increases the stress on the converter components and affects the MPPT. Moreover, the overcurrent protection of the PV converter must be taken into account. In this paper a centralized optimized strategy to share the harmonic current injection among all the converters in a PV park is investigated. The optimization is formulated as a quadratic programming (QP) problem: the active power consumed by the PV park for the active filtering and the DC link ripple of the PV converters caused by the harmonic currents injection are minimized. The limit on the maximum injectable harmonic current by each PV converter are respected

State-feedback-based Low-Frequency Active Damping for VSC Operating in Weak-Grid Conditions

Voltage source converters (VSCs) are nowadays widely integrated in the power grid, nevertheless they can induce low frequency stability problems under weak grid conditions. The interaction of PLL, dc-link voltage control, and ac voltage control generates a positive feedback which threatens the power system stability. The existing researches mainly focus on modeling strategies and stability analyses tools, however still few studies dealt with active damping in the low frequency range. In this paper, a nonlinear state space model of a VSC is presented and linearized around the operating point. From the model, a linear state feedback control law is designed and incorporated in the dc-link and ac voltage control in order to increase the system damping. Eigenvalue analysis is used to investigate the performance of the proposed controller. The simulation results based on a 2 MW grid connected wind generation unit, clearly show the effectiveness of the proposed solution. Experimental results with a 4 kW scaled-down setup validate the analytic and simulation results

Analysis of Overmodulation in Power Synchronization-based Voltage Source Converters

Low power system inertia in power electronics-dominated grids is a widely discussed problem. Power synchronization allows to emulate inertia by means of the active power low-pass filter, slowing down the grid frequency excursions. Nevertheless, inertia emulation during grid disturbances results in high energy exchange with the dc-link, which brings large dc voltage excursions. When the dc voltage dips below the ac line voltage peak, the modulation index can become too high and make the converter unable to produce the requested ac voltage. The consequent saturation of the voltage can cause current control wind-up, harmonic distortion and loss of controllability. Moreover, when the reference voltage is saturated, the converter does not behave anymore according to the power synchronization control law, making the conventional stability analyses not valid. Despite the relevance of these issues, a systematic analysis about the voltage saturation induced by grid disturbances in power converters is still missing. For that, this paper proposes a mathematical tool based on Bode plots. The target of this tool is to properly chose the converter parameters in order to tolerate grid disturbances without reaching the voltage saturation. Simulations and experiments are provided

Robustness Analysis of Voltage Control Strategies of Smart Transformer

The increasing penetration of Distributed Generators (DG) in the modern electric distribution network poses high priority on the problem of the stability. In this article the Harmonic Stability of a Smart Transformer-fed microgrid is investigated under different control strategies. The considered microgrid is composed by a Smart Transformer and three Distributed Generators, considering the bandwidth of the DGs unknown. The robustness is evaluated analysing the eigenvalues as a consequence of a variation of the DGs bandwidth. The system is modelled as a Multi Input Multi Output System (MIMO); the eigenvalue based analysis is carried out to assess the stability and compare the robustness of the traditional double-loop PI and a state-feedback (SF) integral controller. The results show that the SF controller ensures a higher robustness than the traditional PI controller with respect to increasing bandwidths of the DGs

Design Oriented Analysis of Control Loops Interaction in Power Synchronization-based Voltage Source Converter

Power synchronization mechanism is coupled with the dc-link, since both synchronization and dc-link dynamics depend on the active power. The active power filter used in the frequency droop is correlated with inertia emulation, which affects the dc-link voltage. Furthermore, the coupling between active and reactive power in the low-voltage grid suggests a possible interaction between dc-link and reactive power control. The strong coupling between all the control loops and the dc-link in power synchronization-based converters represents a challenge both for the stability and for the controller tuning. This paper addresses the tuning of power synchronization-based converter with consideration of all the control loops interactions. A complete converter state-space model is derived, and a step-by-step tuning procedure based on eigenvalue analysis is proposed. Experimental results are provided to demonstrate the method

Scalable State-Space Model of Voltage Source Converter for Low-Frequency Stability Analysis

Low frequency instability phenomena in power electronic based power systems can originate both at converter and at power system level. At the converter level, the interaction between PLL, dc-link and ac voltage control in voltage source converters (VSCs) connected to a weak grid can lead to instability. At the power system level, the interactions among different parallel VSCs can produce oscillatory phenomena, and even result in instability. However, a model to study the instability phenomena at both levels is still under development. In this paper, a scalable VSC state space model, which captures the interactions among PLL, dc-link and ac voltage control is proposed. The proposed model is suitable for interconnection, and an example of power system modeling is shown. The model is then validated through simulations and experimental tests. Eigenvalue analysis is carried out to investigate the influence of the control parameters on the stability

State-Feedback Reshaping Control of Voltage Source Converter

Admittance reshaping is a widely used strategy to address the converters low-frequency stability issues in weak grid, caused by the PLL and its interaction with the dc and ac voltage control. However, the asymmetric control of the d- and q-axis current references and the coupling between the converter ac and dc side restricts the damping capability of Single-Input-Single-Output feedbacks. This phenomena gets even worse in presence of nearby converters. This paper extends the concept of admittance reshaping to Multi-Input-Multi-Output (MIMO) control. A full state-feedback is added to the current reference of the converter to increase the damping of the conventional multi-loop control. A systematic offline algorithm is delegated to design the feedback, and a scalar coefficient is employed to activate/deactivate online the reshaping feedback, making the proposed solution user-friendly. The proposed control is analyzed both in time- and frequency-domain and tested in parallel-operation with other converters, and shows higher damping capability than conventional solutions and good robustness with respect to grid impedance and operating point variations. Experimental tests under ac and dc disturbances are conducted both in lab setup and in Hardware-In-the-Loop

Modelling of AC/DC Interactions of Converter-Interfaced Resources for Harmonic Power-Flow Studies in Microgrids

Modern power distribution systems experience a large-scale integration of Converter-Interfaced Distributed Energy Resources (CIDERs). As acknowledged by recent literature, the interaction of individual CIDER components and different CIDERs through the grid can lead to undesirable amplification of harmonic frequencies and, ultimately, compromise the distribution system stability. In this context, the interaction of the DC and AC sides of CIDERs has been shown to have a significant impact. In order to analyze and support the mitigation of such phenomena, the authors of this paper recently proposed a Harmonic Power-Flow (HPF) framework for polyphase grids with a high share of CIDERs. The framework considers the coupling between harmonics, but ignores the DC-side response of the CIDERs. Modelling the DC side and the AC/DC converter introduces a nonlinearity into the CIDER model that needs to be approximated for the numerical solution of the HPF. This paper extends the CIDER model and HPF framework to address this aspect, whose inclusion is non-trivial. The extended HPF method is applied to a modified version of the CIGRÉ low-voltage benchmark microgrid. The results are compared to (i) timedomain simulations with Simulink, (ii) the predecessor of the extended HPF which neglects the DC side, and (iii) a classical decoupled HPF