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

Practical implementation of long-horizon direct model predictive control

Thesis (MEng)--Stellenbosch University, 2018.ENGLISH ABSTRACT: The use of model predictive control in power electronics has increased significantly in recent years. More specifically, the so-called direct model predictive control methods are primarily considered for power electronic converters due to their switching nature. In direct control methods the output of the controller directly manipulates the converter inputs, which are restricted to integers, without the use of a modulator. However, predominantly, only a short horizon of one prediction step is considered. This can be attributed to two reasons. Firstly, it has been previously regarded that longer horizons do not provide any performance benefits in power electronics. Secondly, the computational burden associated with prediction horizon increases exponentially, discouraging practical consideration. Recently it was shown that the stigma that longer horizons do not provide performance benefits is false, and that long horizons do indeed increase the harmonic performance of a converter. In fact, if the prediction horizon is long enough, model predictive control can compete with the highly regarded optimised pulse patterns in terms of harmonic distortion. Furthermore, it was shown that the optimization problem of direct model predictive control with long horizons can be reformulated as an integer least-squares. A branch-and-bound method, known as sphere decoding, can solve the reformulated optimization problem in a time-efficient manner, enabling practical considerations. The primary contribution of this thesis is the practical implementation of long-horizon direct model predictive control. A detailed description of the implementation of the controller within a field programmable gate-array is given. It is shown that, for almost 90% of the cases, only 8:4 μs are required to calculate the optimal inputs for a three-phase neutral-point-clamped inverter when using a prediction horizon of 5 with a sampling interval of 25 μs. Continuing on the practical implementation of long-horizon direct model predictive control, experimental results are captured and analysed for prediction horizons one to five. The claim that longer horizons do provide a performance increase is validated through experimental results. A decrease of roughly 8:5% in total total harmonic distortion at a switching frequency of 250 Hz is achieved when adopting a prediction horizon of five instead of one. The secondary contribution of this thesis is the proposal of a method to selectively suppress selected harmonics. The formulation of the method is explained, and simulations are used to verify the suppression of harmonics.AFRIKAANSE OPSOMMING: Die toepassing van modelvoorspellendebeheer vir drywsingselektronika het aansienlik toegeneem in die afgelope paar jaar. Die sogenaamde direkte voorspellende beheer tegnieke is veral van toepassing tot die veld van drywingselektronika as gevolg van die skakelnatuur van die toerusting. ’n Modulator is afwesig wanneer direkte beheer metodes gebruik word, omdat die beheersein direk aan die intree van die omsetter gekoppel word. Die beheersein is daarom beperk tot heelgetalle. Oor die algemeen word ’n kort voorspellings horison van een gebruik. Daar is hoofsaaklik twee redes hiervoor. Eerstens, in die verlede was daar verneem dat langer horison geen voordelige bydra tot die optrede van ’n drywsingelektroniese toestel in hou nie. Tweedens, berekeninge wat verband hou met die voorspellings horison verhoog eksponensieel en ontmoedig daarom die gebruik daarvan. Dit was redelik onlangs bewys dat langer horisonne wel ’n beduidende positiewe bydrae tot die werking van drywingselektroniese toestelle maak. Indien die horison lank genoeg is, kan voorspellende beheer redelik goed kompeteer met die hoog aangeskrewe optimalepulspatrone. Verder was dit bewys dat die direkte-voorspellendebeheer optimeringsprobleem herformuleer kan word as ’n heelgetal-kwadratiese optimeringsprobleem. ’n Metode wat bekend staan as sfeerdekodering kan gebruik word om die herformuleerde optimeringsprobleem effektief op te los, wat die praktiese gebruik daarvan bemoedig. Die primêre bydrae van die tesis is die praktiese implementering van direkte modelvoorspellendebeheer met lang horisonne. ’n Gedetailleerde beskrywing om die beheerder binne ’n veldprogrammeerbare hekskikking te implementeer word aangebied. Dit word bewys dat in byna 90% van die gevalle slegs 8:4 μs benodig word om die optimale intree te bereken vir ’n driefase neutralepuntgeklampte omsetter, wanneer ’n voorspellings horison van vyf gebruik word met ’n monsterperiode van 25 μs. Verder word praktiese resultate aangebied vir voorspellendehorisonne vanaf een tot vyf. Dit word bewys dat langer horisonne verbetering rakende die harmoniese gedrag van drywingselektroniese toestelle teweeg bring. ’n Afname van 8:5% in stroomharmoniesedistorsie teen ’n skakelfrekwensie van 250 Hz word waargeneem, wanneer ’n voorspellendehorison van vyf in plaas van een gebruik word. Die sekondêre bydrae van die tesis is die bekendstelling van ’n metode om harmonieke selektief te onderdruk. Die formulering van die metode word verduidelik en simulasies word uitgevoer om te bevestig dat harmoieke wel onderdruk word

Facilitating the transition to an inverter dominated power system : experimental evaluation of a non-intrusive add-on predictive controller

The transition to an inverter-dominated power system is expected with the large-scale integration of distributed energy resources (DER). To improve the dynamic response of DERs already installed within such a system, a non-intrusive add-on controller referred to as SPAACE (set point automatic adjustment with correction enabled), has been proposed in the literature. Extensive simulation-based analysis and supporting mathematical foundations have helped establish its theoretical prevalence. This paper establishes the practical real-world relevance of SPAACE via a rigorous performance evaluation utilizing a high fidelity hardware-in-the-loop systems test bed. A comprehensive methodological approach to the evaluation with several practical measures has been undertaken and the performance of SPAACE subject to representative scenarios assessed. With the evaluation undertaken, the fundamental hypothesis of SPAACE for real-world applications has been proven, i.e., improvements in dynamic performance can be achieved without access to the internal controller. Furthermore, based on the quantitative analysis, observations, and recommendations are reported. These provide guidance for future potential users of the approach in their efforts to accelerate the transition to an inverter-dominated power system

Design and Control of Power Converters 2019

In this book, 20 papers focused on different fields of power electronics are gathered. Approximately half of the papers are focused on different control issues and techniques, ranging from the computer-aided design of digital compensators to more specific approaches such as fuzzy or sliding control techniques. The rest of the papers are focused on the design of novel topologies. The fields in which these controls and topologies are applied are varied: MMCs, photovoltaic systems, supercapacitors and traction systems, LEDs, wireless power transfer, etc

A modified neutral-point balancing space vector modulation technique for three-level neutral point clamped converters in high speed drives

This paper describes a high performance neutral point voltage balancing technique for a Neutral point clamped (NPC) Converter. Conventional neutral point voltage balancing methods do not function well under low power factor, low pulse ratio and near-unity modulation index operation conditions. These conditions are essentially dominant operation conditions for aircraft starter/generator systems. This paper introduces an alternative space vector modulation technique for three-level NPC converters in an aircraft starter generator system. The selection of voltage space vectors is optimized for high modulation index and low power factor operation. Disturbances caused by low pulse ratio is also compensated. The proposed method maintains neutral point voltage balance and ripple minimization over the full range of operating conditions. The paper also provides a detailed analysis into the sources of neutral point voltage imbalances and ripples in high speed drives with deep flux weakening. Simulation results obtained from a Simulink/PLECS model and experimental results obtained from a 45kVA, 32krpm aircraft starter generator test rig proves the proposed method eliminates the neutral point voltage imbalance and significantly reduces the neutral point voltage ripple

System-Wide Harmonic Mitigation in a Diesel Electric Ship by Model Predictive Control

Email Print Request Permissions This paper proposes a system-oriented approach for mitigating harmonic distortions by utilizing a single Active Power Filter (APF) in an electrical grid with multiple buses. Common practice for control of APFs is to locally compensate the load current harmonics or to mitigate voltage harmonics at a single bus. However, the operation of an APF in a multi-bus system will influence the voltages at neighboring buses. It is therefore possible to optimize the APF operation from a system perspective instead of considering only conventional local filtering strategies. For such purposes, Model Predictive Control (MPC) is proposed in this paper as a framework for generating APF current references that will minimize the harmonic distortions of the overall system within a given APF rating. A diesel-electric ship, with two buses supplying separate harmonic loads, with an APF located at one of the buses, is used as study case. The operation with on-line MPCbased optimization of the APF current references is compared to two benchmark methods based on conventional approaches for APF control. The results demonstrate that the MPC generates current references that better utilize the APF current capability for system-wide harmonic mitigation.2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other work

Power Converters in Power Electronics

In recent years, power converters have played an important role in power electronics technology for different applications, such as renewable energy systems, electric vehicles, pulsed power generation, and biomedical sciences. Power converters, in the realm of power electronics, are becoming essential for generating electrical power energy in various ways. This Special Issue focuses on the development of novel power converter topologies in power electronics. The topics of interest include, but are not limited to: Z-source converters; multilevel power converter topologies; switched-capacitor-based power converters; power converters for battery management systems; power converters in wireless power transfer techniques; the reliability of power conversion systems; and modulation techniques for advanced power converters