One of the main technical challenges in the energy transition is the seamless integration of renewable energies into the grid ensuring, among others, system stability and acceptable power quality levels. This thesis explores the effect that voltage imbalance has on the small-signal stability and harmonic performance of the grid-side converter in grid-following type 4 wind turbine generators (full converter). Within the scope of this thesis is voltage imbalance that appears in steady-state (typically, a low value limited by grid codes and international power quality standards) and also voltage imbalance that appears during transient situations (typically, a higher value due to temporary conditions like faults).@en

In this article, the Linear Time Invariant (LTI) and Linear Time Periodic (LTP) models of two different implementations of the DDSRF-PLL in the presence of voltage imbalance are derived analytically. The accuracy of the models is investigated with time domain simulations, frequency scans, and stability analysis. On top of this, a guideline for properly choosing between LTI and LTP models for stability assessment of the DDSRF-PLL according to the degree of grid voltage imbalance is proposed. Furthermore, it is revealed that, depending on the DDSRF-PLL implementation, the positive-sequence voltage might also cause LTP dynamics, rendering the LTI model inaccurate even when the imbalance is low.@en

To accurately simulate electric vehicle DC fast chargers' (DCFCs') harmonic emission, a small time step, i.e., typically smaller than 10 μs, is required owing to switching dynamics. However, in practice, harmonics should be continuously assessed with a long duration, e.g., a day. A trade-off between accuracy and time efficiency thus exists. To address this issue, a multi-time scale modeling framework of fast-charging stations (FCSs) is proposed. In the presented framework, the DCFCs' input impedance and harmonic current emission in the ideal grid condition, that is, zero grid impedance and no background harmonic voltage, are obtained based on a converter switching model with a small timescale simulation. Since a DCFC's input impedance and harmonic current source are functions of the DCFC's load, the input impedance and harmonic emission at different loads are obtained. Thereafter, they are used in the fast-charging charging station modeling, where the DCFCs are simplified as Norton equivalent circuits. In the station level simulation, a large time step, i.e., one minute, is used because the DCFCs' operating power can be assumed as a constant over a minute. With this co-simulation, the FCSs' long-term power quality performance can be assessed time-efficiently, without losing much accuracy.@en

To accurately simulate the harmonic emission of EV DC fast chargers (DCFCs) and the harmonic voltage of the power grid to which the chargers are connected, a small time-step, i.e., typically smaller than 10μs, is required. However, for harmonic assessment, a long timescale, typically a day, is required. A conflict between accuracy and time efficiency exists. To address this issue, a multitimescale modeling framework of fast charging stations (FCSs) is proposed in this paper. In the presented framework, the DCFCs’ input impedance and harmonic current emission in the ideal grid condition, i.e., the grid impedance is zero and there are no background harmonic voltages, is obtained firstly through a converter switch model with a small timescale. Since the DCFC’s input impedance and harmonic current source change in the charging course, the input impedance and harmonic emission at different input power should be obtained. Then, the DCFCs’ input impedance and harmonic emission will be used in the fast-charging station modeling, where the DCFCs are simplified as their Norton equivalent circuits. In the station level modeling, a bigger time step, i.e., 1 minute, is used, since the DCFCs’ operating power can be assumed as a constant in one minute. With this framework, the FCSs’ long-term power quality performance can be assessed efficiently without neglecting the DCFCs’ small timescale dynamics. @en

This article presents a small-signal model for power-electronics converters that use a typical control structure in wind energy applications: the double synchronous reference frame current control. The article considers the presence of unbalanced currents and voltages, and analyzes their impact on the frequency couplings of the converter. In addition, it is revealed that, in the presence of negative-sequence voltage synchronization, the converter presents an additional coupling at -2f_1-f_p.

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A voltage imbalance at the AC terminals of a three-phase inverter creates a ripple in the power signal on the DC side. In order to minimize this ripple, several techniques can be applied, in which a double Synchronous Reference Frame (SRF) current control structure is very typical. In this approach, both the positive and negative sequence currents are controlled. This technique has been shown to have an adequate response against imbalances; however, this paper shows that in the typical implementation of the double SRF control, the output AC pq instantaneous powers will have a constant error due to the phase-angle misalignment of the negative sequence with the positive sequence. Based on mathematical formulations and simulation results, this paper shows that this AC-power error exists, and that it is due to the above reason. In order to overcome this, this paper proposes to have a phase-tracking system that specifically follows the negative sequence phase-angle. The results show that this implementation is able to properly control the output AC power.

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In this decade, a significant amount of additional wind and solar power will be integrated in the Dutch high voltage grid. The 383 MW nearshore Wind Park Fryslân is foressen to be connected to the Dutch 110kV grid in 2020 through two 55km cable circuits. To quantify the risks of exceeding emission levels for both the TSO and the wind farm developer, harmonic voltage distortion studies must already be performed at an early phase of the project. Relevant data of wind farm components may not yet be available then and must therefore be estimated including data of the wind turbine generator (WTG). The objective of this paper is to assess the impact of a WTG converter impedance model on the harmonic impedance of the windfarm and subsequent harmonic voltage amplification. For this purpose, an analytical converter harmonic model with tunable parameters has been developed. A sensitivity study was performed in a case study of Wind Park Fryslân to analyse the impact of these parameters on the harmonic amplification at the Point of Connection with the 110 kV grid. The length and amount of array cables were also included in the study. The paper follows a step-wise approach where subsequently wind farm impedance, grid impedance and amplification factor have been calculated. It has been found that the WTG converter impedance model has a significant impact on the harmonic amplification of low harmonic orders (h=2 to h=10). Notably, not taking the converter impedance into account (infinite impedance) will result in both over- as well as underestimation of the harmonic amplification over the mentioned frequency range. It is shown that, when no WTG manufacturer has been selected yet, the analytical converter model provides a valid alternative for performing early-stage harmonic studies. @en

There is an increasing focus on integrating flexible dc links for bulk power routing in medium voltage distribution grids. In such applications, the ac-dc Modular Multilevel Converter (MMC) devised for medium voltage and high-power ratings can be an interesting choice. This paper highlights some less explored design trade-offs arising due to the limitation on N in relation to modulation frequency and arm inductance. Specifically, the study intends to describe the interdependent influence of each degree of freedom on several aspects such as capacitor voltage balancing, circulating currents and harmonic performance. Finally, the importance of considering interhar-monies in the performance assessment of the MMC instead of the conventional interpretation of distortion calculation is highlighted.

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In this paper, the output impedance of a three-phase inverter based on a dual current control (also called double synchronous reference frame current control) is modelled, which includes: the current loop gain, the control delay, and the Phase-
Locked Loop (PLL). The impact of these parameters on the impedance is then analysed by a sensitivity study. The model is derived using transfer matrices and complex transfer functions, and it results in a compact impedance formulation that can be used in harmonic small-signal stability studies and system-wide steady-state harmonic calculations.@en

Several efforts are being done nowadays to improve the modelling of Wind Turbine Systems (WTSs) for harmonic analysis in Offshore Wind Power Plants (OWPPs). Due to the high influence of the different control structures in the Power Electronic Converters (PECs) on the dynamic response of a WTS, each structure needs to be modelled specifically. Following these lines, this paper addresses the impedance part of the harmonic model in the case of a double Synchronous Reference Frame control structure. The main focus lies on the correct modelling of one of the main elements of this structure: the notch filter tuned at twice the fundamental frequency. The inclusion or disregard of this notch filter is very important because, as shown in works by other authors, this filter can have a big influence in the shaping of the WTS output impedance and such of the OWPP. However, the modelling procedure for the notch filter followed previously ignores the cross-couplings that this element creates in the αβ frame, which leads to a wrong calculation of the WTS impedance in the lower frequency range. The proper modelling of this notch filter and its implications are detailed in the paper first theoretically and then by numerical simulations. @en

The concern for power quality has been on the rise in recent years for all kinds of electric power systems. Harmonic analysis is especially interesting in the case of Offshore Wind Power Plants (OWPPs) due to the special susceptibility of these systems to harmonic issues and to the inherent differences in their behavior with respect to other electric networks. The aim of this article is to provide an overview of the basic concepts for harmonic analysis in OWPPs and to outline the typical assessment procedure used to this day. Some of the limitations of current industry practice will be outlined, as well as some possible complementary approaches for its improvement.@en