Harmonic reduction methods for electrical generation: a review

This paper provides a comprehensive literature review of techniques for harmonic related power quality improvement of electrical generation systems. An increasing interest in these aspects is due to the ever more stringent power quality requirements, deriving from new grid codes and compliancy standards, aimed at limiting waveform harmonic distortion at all points of the distribution network. Although a wealth of literature is available for such techniques, it has never been compiled into a handbook incorporating all the solutions aimed at both electrical machine and power systems engineers

Improvement of Stability of a Grid-Connected Inverter with an LCL filter by Robust Strong Active Damping and Model Predictive Control

This study addresses development and implementation of robust control methods for a three-phase grid-connected voltage source inverter (VSI) accompanied by an inductive-capacitive-inductive (LCL) filter. A challenge of current control for the VSI is LCL filter resonance near to the control stability boundary, which interacts with the inverter control switching actions and creates the possibility of instability. In general, active damping is needed to stabilize the system and ensure robust performance in steady-state and dynamic responses. While many active damping methods have been proposed to resolve this issue, capacitor-current-feedback active damping has been most widely used for its simple implementation. There has been no clear consensus regarding design of a control system including capacitor-current-feedback active damping. This is due to the fact that simulation/experiment results are not congruent with the design analyses on which the control is designed. This study explains the incoherence between theory and practice when it comes to a capacitor-currents-feedback active damping system. Proposed capacitor-current-estimate active damping utilizing a developed posteriori Kalman estimator gives coherent simulation results as expected from the design analyses. This reveals that the highly oscillatory capacitor currents containing the inverter switching effects bring about uncertainty in the system performance. The switching effects are not incorporated in the analyses and control system design. Therefore, it is required to remove the switching noise from the capacitor currents in order to yield consistent results. It has been confirmed that the proportional-negative feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor. In a digitally controlled system, the computation delay causes the equivalent resistance of the virtual impedance to become negative in the frequency range of fs/6 to fs/2, which produces a pair of open-loop unstable poles in RHP. This happens when the displaced resonance peak by active damping is in that region. Thus, an a priori Kalman estimator has been developed to generate one-sample-ahead state variable estimates to reconstruct the capacitor currents for active damping, which can compensate for the delay. The one-sample-ahead capacitor-current estimates are computed from the inverter-side and grid-side current estimates. The proposed method provides extended limits of the active damping gain that improve robustness against system parameter variation. It also allows strong active damping which can sufficiently attenuate the resonance. Grid condition is another significant factor affecting the stability of the system. In particular, a weak grid tends to provide high impedance. The system employing the proposed active damping method stably operates in a weak grid, ensuring robustness under grid impedance variation. The developed Kalman estimators offer an effective and easy way of determining the stability status of a system in addition to the functions of filtering and estimation. Stability analysis can be easily made since state variable estimates go to infinity when a system is unstable. As a promising approach, model predictive control (MPC) has been designed for the system. This study suggests that MPC including active damping can be employed for a grid-connected VSI with an LCL filter with good dynamic performance

Ultra-low losses SiC based shunt active power filter for harmonics mitigation and harmonics power recovery in industrial power systems

The classical method to suppress resonances in power systems is by installing passive dampers in parallel to the loads. However, observations indicate significant power losses due to harmonic currents flowing over passive dampers. Certainly, passive dampers absorb harmonic active power and dissipate this power as heat on their resistive elements leading to energy waste. On the other hand, the passive damper counterpart is the active damper. The latter is also known in technical literature as Voltage driven shunt active power filter (VSAPF). The active damper is a power-electronics-based system that emulates a virtual resistance at harmonic frequencies. Truly, very little was known about the harmonic power absorption on active dampers. Therefore, this dissertation delves into a profound analysis of the capability of an ultra-low losses active damper based on SiC semiconductor technology to process the harmonic power intake and perform harmonic power recovery. Harmonic power recovery in this context is understood as the process of transforming the harmonic active power absorbed into fundamental power that is injected back into the power system. The next topic that is addressed is the reduction in the fundamental power demanded by an industrial facility due to the recovery of harmonic active power. To this end, this dissertation analyzes the power balance flow of a distribution power system (e.g., industrial grid) that includes an ultra-low losses active damper. Arising out of the power balance flow analysis, it was found that the active damper with harmonic recovery function achieves a 1.4% reduction on the fundamental power demanded compared to a passive damper. Naturally, the lower the active damper´s power losses, the higher will be the amount of harmonic active power\ud that can be recovered from the power system. Therefore, during this research work, various power electronics converters topologies are analysed to find the best possible design for the active damper with harmonic power recovery functionality. Arising out of this investigation, it was found that the conventional three-level neutral point piloted converter (3L-TNPC) and the asymmetrical three-level converter (3L-ASYM) are the most suitable power circuit topologies for the ultra-low losses active damper. The former topology, the 3L-TNPC, exhibits the lowest power losses for switching frequencies up to 60 kHz. And then, the 3L-ASYM topology presents the lowest losses among all the studied power circuits for switching frequencies beyond 70 kHz. Furthermore, as an active damper forms a closed loop between harmonic voltages and compensating currents, its stability must be ensured. Thus, a careful design of the VSAPF control system and its inner current controllers is essential. On account of this, this dissertation proposes using the Ragazzini method to design the VSAPF’s inner current controllers. Furthermore, the direct design of the inner current controllers on the discrete domain using the Ragazzini method increases the current controllers’ bandwidth by a factor of three compared to the controllers’ design with conventional methods. Consequently, the increased current controller’s bandwidth achieved through the Ragazzini method pushes the stability limit of the active damper forward compared with traditional current controller designs

Eingangsadmittanz-Modellierung und passivitätsbasierte Stabilisierung von digital-stromgeregelten, netzgebundenen Umrichtern

Due to the ever increasing number of renewable energy systems in the electrical power grid, the application of power electronic-based circuits is gaining more and more importance. It has however been known for a while that interactions of one or multiple converters with resonances in the grid can lead to poorly damped oscillations, and thus, may threaten the stability of parts of the power system. The passivity theory has proven to be particularly powerful in preventing such situations. Accordingly, the stability of the power grid can be guaranteed by design if all components act passive. This means that all active loads and energy feeding converters have an input admittance with a non-negative real part. This can theoretically be achieved using passive or active damping strategies, but most research neglects real-world effects, which arise from the sampling of high-frequency switching harmonics. The aim of this dissertation is therefore to review the complete modeling and analysis of digitally current-controlled grid-connected converters and to extend the controller as well as filter design. On the basis of typical single-input single-output models of the converter’s input admittance, methods for the design of a passive damping or an active feed-forward are proposed and it is discussed which aspects have to be considered when implementing the filters. However, since the used models cannot reproduce all alias effects, in the further part of the thesis a multiple-input multiple-output converter model is developed. It is shown that the mirroring of high-frequency signal components onto low-frequency components can in principle be described by a dynamic uncertainty that affects the behavior of the converters' baseband dynamics. Due to this new insight it becomes clear which criteria passive or active filters should fulfill in order to specifically counteract the often negative mirroring effects of digital control. Finally, it is demonstrated that a robust passivation of the converter input admittance can prevent a destabilization of the power system by harmonics for a large number of grid impedances. The presented theory and the developed controller design are illustrated and verified by various simulations of an exemplary converter system.Aufgrund der immer größer werdenden Anzahl von erneuerbaren Energieanlagen im elektrischen Energieversorgungsnetz gewinnt der Einsatz von leistungselektronischen Schaltungen immer mehr an Bedeutung. Es ist jedoch seit längerem bekannt, dass Wechselwirkungen von einem oder mehreren Umrichtern mit Resonanzen im Netz zu schlecht gedämpften Schwingungen führen und damit die Stabilität von Teilen des Energienetzes gefährden können. Die Passivitätstheorie hat sich als besonders wirkungsvoll erwiesen, um solche Situationen zu verhindern. Demnach kann die Stabilität des Stromnetzes bereits in der Designphase gewährleistet werden, indem alle Komponenten passiv wirken. Das bedeutet, dass alle aktiven Verbraucher und einspeisenden Umrichter eine Eingangsadmittanz mit nicht negativem Realteil besitzen. Dies ist theoretisch mit Hilfe von passiven oder aktiven Dämpfungsstrategien zu erreichen. Die meisten Forschungsarbeiten vernachlässigen jedoch reale Effekte, die bei der Abtastung von hochfrequenten Harmonischen entstehen. Ziel dieser Dissertation ist es daher, den kompletten Modellierungs-, Analyse- und Regler- sowie Filterentwurfsprozess von digital-stromgeregelten, netzgebundenen Umrichtern zu überprüfen und zu erweitern. Auf der Basis typischer Eingrößenmodelle der Umrichter-Eingangsadmittanz werden Verfahren für die Auslegung einer passiven Dämpfung bzw. einer aktiven Vorsteuerung vorgeschlagen und es wird diskutiert, welche Aspekte bei der Implementierung der Filter zu berücksichtigen sind. Da sich mit den Modellen jedoch nicht alle Alias-Effekte abbilden lassen, wird im weiteren Teil der Arbeit ein Mehrgrößen-Umrichtermodell entwickelt. Es zeigt sich, dass die Spiegelung hochfrequenter Signalanteile auf niederfrequente Anteile prinzipiell durch eine dynamische Unsicherheit beschrieben werden kann, die das Grundfrequenzverhalten der Umrichter beeinflusst. Dank dieser neuen Erkenntnisse wird deutlich, welche Kriterien passive oder aktive Filter erfüllen sollten, um den oft negativen Spiegeleffekten der digitalen Regelung gezielt entgegenzuwirken. Es wird demonstriert, dass eine robuste Passivierung der Umrichter-Eingangsadmittanz eine Destabilisierung des Energienetzes durch Harmonische für eine Vielzahl von Netzimpedanzen verhindern kann. Die vorgestellte Theorie und der erarbeitete Reglerentwurf werden anhand diverser Simulationen eines beispielhaften Umrichtersystems verdeutlicht und validiert

Artificial Intelligence-Based Methods for Power System Security Assessment with Limited Dataset

This thesis concerns the relationship between the load, load model, and power system stability. It investigates the possibility of developing a dynamic load model to represent the power system load characteristic during system faults when the power system operates at a high percentage of the power generation from wind farms, solar power, and vehicle-to-grid technology. Additionally, with artificial intelligence supporting the seamless integration of an increasingly distributed and multi-directional power system to unlock the vast potential of renewables, new approaches are proposed to improve the training performance for the applications of artificial neural networks in non-intrusive load monitoring and dynamic security assessment. An improved hybrid load model is proposed to represent the load characteristics in the above power system operation. Genetic algorithms and the multi-curve identification method are applied to determine the parameters of the load model, aiming to minimize the error between the estimated and measured values. The results indicate that the proposed hybrid load model has a reasonably low fitting error to represent the load dynamics. In addition, new approaches are proposed to tackle the challenges posed by limited data when training artificial neural networks (ANNs) for their application in power systems. The knowledge transfer approach is utilized to support the ANN training to generate synthetic data for non-intrusive load monitoring. The results indicate that this approach improves the issue of mode collapse and reduces the need for lengthy training iterations, making the ANN effective for generating synthetic data from limited data. Moreover, the knowledge transfer approach also supports ANN training with limited data for dynamic security assessment. Kernel principal component analysis is employed to eliminate the dimensionality reduction step. The results indicate an improvement in the training performance