Grid-adaptive control and active-filter functionality of grid-connected PWM converters in renewable energy generation

Die hohe Durchdringung öffentlicher Energienetze mit zumeist leistungselektronisch angebundenen regenerativen Energieerzeugungsanlagen (kurz: EZAs) führt zu einem Wandel der Energieerzeugung. Die Netzimpedanz stellt dabei einen Schlüsselparameter bei der Erzeugung und Verteilung elektrischer Energie dar. In dieser Arbeit wird ein Gesamtkonzept für die Erweiterung der Regelung eines Netzpulsstromrichters einer dezentralen EZA um die Adaption variierender ohmsch-induktiver Netzimpedanzen (kurz: netzadaptive Regelung) und einer zusätzlichen Aktiv-Filter Funktionalität (kurz: AFF) entwickelt, untersucht und unter Laborbedingungen validiert. Für die Detektion der äquivalenten Netzparameter werden zwei Verfahren berücksichtigt: (1) Die Messung der Netzimpedanz durch die Einspeisung eines interharmonischen Oberschwingungsstromes und (2) die Schätzung der Netzimpedanz mittels eines Erweiterten Kalman-Filters. Die Analysen dieser Detektionsverfahren zeigen, dass sich die Messung der Netzimpedanz durch eine hohe Detektionsgenauigkeit und die Schätzung der Netzimpedanz durch eine hohe Detektionsgeschwindigkeit auszeichnet. Diese Erkenntnis führt zu der Entwicklung eines kombinierten Identifikationskonzeptes, welches die Detektion der Netzparameter in Echtzeit und mit minimaler Systemanregung ermöglicht. Das kombinierte Identifikationskonzept dient als Grundlage für die Entwicklung eines netzadaptiven Regelungskonzeptes eines Netzpulsstromrichters mit netzseitigem LCL-Filter. Auf der Grundlage einer detaillierten Modellierung der Regelstrecke in einem rotierenden (dq) Koordinatensystem wird ein zeitdiskreter Stromregler mit idealerweise vollständiger Entkopplung der nieder- und hochfrequenten Verkopplungsdynamiken von LCL-Filtern entworfen. Es wird ein auf einer Sollwertfilterung basierendes aktives Dämpfungsnetzwerk entwickelt und um eine Adaption variierender Netzparameter erweitert. Die experimentellen Untersuchungen zeigen, dass die entwickelte netzadaptive Regelung insbesondere bei sprunghaften Änderungen der Netzimpedanz zu einer Erhöhung des Stabilitätsbereiches führt ohne die Dynamik der gesamten Stromregelung reduzieren zu müssen. Zudem wird die netzadaptive Regelung um eine AFF erweitert. Die zusätzliche AFF führt zu einer Reduzierung bzw. vollständigen Kompensation von Spannungsunsymmetrien und -oberschwingungen am Anschlusspunkt des Netzpulsstromrichters einer dezentralen EZA. Die theorischen und praktischen Analysen zu der AFF offenbaren, dass eine lastadaptive Betriebsführung für die Ausnutzung der maximalen Betriebsgrenzen eines Netzpulsstromrichters zielführend ist. Hierfür wird ein lastadaptiver Begrenzungsregler entwickelt. Das neu entwickelte lastadaptive Regelungskonzept zur Umsetzung der AFF führt zu einer deutlichen Verbesserung der Spannungsqualität am Anschlusspunkt einer regenerativen und denzentralen EZA.The high penetration of public energy distribution networks with distributed energy production units mainly connected through power electronic converters leads to a high utilization of the existing grid structures. This trend is mainly driven by the efforts to push electrical power generation toward green and sustainable energy production. In this context the grid impedance is a key parameter for the generation and distribution of electrical energy. In this work an overall control concept for the grid impedance adaptive control and an additional active-filter functionality of grid-connected PWM converters is proposed, developed and validated with experimental results. The detection of equivalent grid parameters is examined using two different methods: (1) the measurement of the grid impedance by means of inter-harmonic current injection and (2) the estimation of the grid impedance using an Extended Kalman-Filter. The analysis of these two methods reveals that measurement of the grid impedance leads to a high detection precision whereas the estimation of the grid impedance leads to high detection dynamics. This insight motivates the development of a combined identification method that is able to detect the grid impedance conditions in a real-time manner with minimal system excitation. The combined identification method is the basis for the development of a grid impedance adaptive control concept for grid connected PWM converters with LCL-filters. The control is based on a detailed model of the control plant in the rotating (dq) reference frame whereas a discrete current controller with improved decoupling dynamics of both low and high frequency coupling dynamics of a LCL-filter is proposed. An active damping of the LCL-filter resonance is achieved using a grid impedance adaptive reference value filter. The measurement analysis shows that the proposed grid impedance adaptive control is able to guarantee stable converter operation with high control performance in the presence of step-wise grid impedance changes without the need of decreasing the current control dynamics. In addition to high performance current control and grid impedance detection capability, the grid impedance adaptive control is superimposed with an active-filter functionality. The active-filter functionality leads to a mitigation or compensation of voltage-unbalances and lower-order voltage-harmonics at the grid-connection point of the renewable energy system. The analysis of the active-filter functionality motivated the development of the load adaptive operation point controller to utilize the maximum capability limits of the grid-connected converter. A measurement analysis validates the proposed control concept and demonstrates that a considerable voltage quality improvement is achieved by the additional active-filter functionality

Higher harmonics compensation in grid-connected PWM converters for renewable energy interface and active filtering

The paper presents overview of high-order harmonics compensation methods applied for control of grid-connected converter. Two harmonic compensation methods are presented. One based on band-pass filers cooperating with Direct Power Control with Space Vector Modulation (DPC-SVM) is dedicated for renewable energy interface. Second method based on resonant controllers applied in Voltage Oriented Control (VOC) adds an active filtering function to PWM rectifier. Simulation and preliminary experimental results for these two methods are presented

Control proposals for the operation of power converters in wind power systems

The wind turbines based on Doubly Fed Induction Generators (DFIG) represent almost the 60% of the installed windpower worldwide. Nowadays most of the existing algorithms, used for regulating the production of active and reactive power in such systems, are based on control techniques that use rotational reference frames theories, where the dq0 is maybe the most popular. Despite of its good operation under balanced conditions its behaviour depends on a good phase synchronization, otherwise the different transformations cannot be performed with enough guarantees. Of course this kind of response is becomming insuficient, as far as the windpower penetration is increasing. This paper will present a new control topology for DFIG, that are connected using back to back inverters, that is able to behave better in case of faulty grids. Besides some proposals for improving the operation of the DFIG will be presented as well. The aim of that part is to extend the possibilities that the back to back inverters are able to offer to the generator system by means of including new functionalities, as the active filtering capability or the reduction of the flicker emission that will be later discussed.Postprint (published version

Electric Vehicles Charging Technology Review and Optimal Size Estimation

AbstractMany different types of electric vehicle (EV) charging technologies are described in literature and implemented in practical applications. This paper presents an overview of the existing and proposed EV charging technologies in terms of converter topologies, power levels, power flow directions and charging control strategies. An overview of the main charging methods is presented as well, particularly the goal is to highlight an effective and fast charging technique for lithium ions batteries concerning prolonging cell cycle life and retaining high charging efficiency. Once presented the main important aspects of charging technologies and strategies, in the last part of this paper, through the use of genetic algorithm, the optimal size of the charging systems is estimated and, on the base of a sensitive analysis, the possible future trends in this field are finally valued

Three-phase switchmode direct conversion type welding power supplies with active power factor correction

Бурлака, В. В. Трехфазные сварочные источники прямого преобразования с активной коррекцией коэффициента мощности / В. В. Бурлака // Вісник Приазовського державного технічного університету : зб. наукових праць / ПДТУ. – Маріуполь, 2013. – Вип. 26. – С. 210–215. – (Серія : Технічні науки).Представлены оригинальные схемные решения инверторных источников питания с трехфазным входом, имеющих трансформаторную развязку выхода и использующих технику прямого преобразования трехфазного напряжения в высокочастотное. Применение принципа непосредственного преобразования позволяет уменьшить число элементов в силовой цепи инвертора, повысив тем самым его КПД. Кроме этого, путем применения специального алгоритма управления ключами, возможно достижение входного коэффициента мощности, близкого к единице.Представлені оригінальні схемні рішення інверторних джерел живлення з трифазним входом, які мають трансформаторну розв'язку виходу та використовують техніку прямого перетворення трифазної напруги в високочастотну. Застосування принципу безпосереднього перетворення дозволяє зменшити число елементів в силовому ланцюзі інвертора, підвищивши тим самим його ККД. Крім цього, шляхом застосування спеціального алгоритму керування ключами, можливе досягнення вхідного коефіцієнта потужності, близького до одиниці.A topology of inverter-type power supply with threephase input and transformer-isolated output is presented. The power supply is built using the technique of direct conversion of three-phase mains voltage to high-frequency voltage. Applying the principle of direct conversion allows reducing the number of elements in the inverter power path, thus increasing its efficiency. In addition, by using an appropriate control algorithm, it is possible to maintain close to unity input power factor

Study of a Symmetrical LLC Dual-Active Bridge Resonant Converter Topology for Battery Storage Systems

A symmetrical LLC resonant converter topology with a fixed-frequency quasi-triple phase-shift modulation method is proposed for battery-powered electric traction systems with extensions to other battery storage systems. Operation of the converter with these methods yields two unique transfer characteristics and is dependent on the switching frequency. The converter exhibits several desirable features: 1) load-independent buck-boost voltage conversion when operated at the low-impedance resonant frequency, allowing for dc-link voltage regulation, zero-voltage switching across a wide load range, and intrinsic load transient resilience; 2) power flow control when operated outside the low-impedance resonance for integrated battery charging; 3) and simple operational mode selection based on needed functionality with only a single control variable per mode. Derivation of the transfer characteristics for three operation cases using exponential Fourier series coefficients is presented. Pre-design evaluation of the S-LLC converter is presented using these analytical methods and corroborated through simulation. Furthermore, the construction of a rapid-prototyping magnetics design tool developed for high-frequency transformer designs inclusive of leakage inductance, which is leveraged to create the magnetic elements needed for this work. Two 2kW prototypes of the proposed topology are constructed to validate the analysis, with one prototype having a transformer incorporating the series resonant inductance and secondary clamp inductance into the transformer leakage and magnetizing inductance, respectively. A test bench is presented to validate the analysis methods and proposed multi-operational control scheme. Theoretical and experimental results are compared, thus demonstrating the feasibility of the new multi-mode operation scheme of the S-LLC converter topology

Performance Analysis of Photovoltaic Fed Distributed Static Compensator for Power Quality Improvement

Owing to rising demand for electricity, shortage of fossil fuels, reliability issues, high transmission and distribution losses, presently many countries are looking forward to integrate the renewable energy sources into existing electricity grid. This kind of distributed generation provides power at a location close to the residential or commercial consumers with low transmission and distribution costs. Among other micro sources, solar photovoltaic (PV) systems are penetrating rapidly due to its ability to provide necessary dc voltage and decreasing capital cost. On the other hand, the distribution systems are confronting serious power quality issues because of various nonlinear loads and impromptu expansion. The power quality issues incorporate harmonic currents, high reactive power burden, and load unbalance and so on. The custom power device widely used to improve these power quality issues is the distributed static compensator (DSTATCOM). For continuous and effective compensation of power quality issues in a grid connected solar photovoltaic distribution system, the solar inverters are designed to operate as a DSTATCOM thus by increasing the efficiency and reducing the cost of the system. The solar inverters are interfaced with grid through an L-type or LCL-type ac passive filters. Due to the voltage drop across these passive filters a high amount of voltage is maintained across the dc-link of the solar inverter so that the power can flow from PV source to grid and an effective compensation can be achieved. So in the thesis a new topology has been proposed for PV-DSTATCOM to reduce the dc-link voltage which inherently reduces the cost and rating of the solar inverter. The new LCLC-type PV-DSTATCOM is implemented both in simulation and hardware for extensive study. From the obtained results, the LCLC-type PV-DSTATCOM found to be more effective than L-type and LCL-type PV-DSTATCOM. Selection of proper reference compensation current extraction scheme plays the most crucial role in DSTATCOM performance. This thesis describes three time-domain schemes viz. Instantaneous active and reactive power (p-q), modified p-q, and IcosΦ schemes. The objective is to bring down the source current THD below 5%, to satisfy the IEEE-519 Standard recommendations on harmonic limits. Comparative evaluation shows that, IcosΦ scheme is the best PV-DSTATCOM control scheme irrespective of supply and load conditions. In the view of the fact that the filtering parameters of the PV-DSTATCOM and gains of the PI controller are designed using a linearized mathematical model of the system. Such a design may not yield satisfactory results under changing operating conditions due to the complex, nonlinear and time-varying nature of power system networks. To overcome this, evolutionary algorithms have been adopted and an algorithm-specific control parameter independent optimization tool (JAYA) is proposed. The JAYA optimization algorithm overcomes the drawbacks of both grenade explosion method (GEM) and teaching learning based optimization (TLBO), and accelerate the convergence of optimization problem. Extensive simulation studies and real-time investigations are performed for comparative assessment of proposed implementation of GEM, TLBO and JAYA optimization on PV-DSTATCOM. This validates that, the PV-DSTATCOM employing JAYA offers superior harmonic compensation compared to other alternatives, by lowering down the source current THD to drastically small values. Another indispensable aspect of PV-DSTATCOM is that due to parameter variation and nonlinearity present in the system, the reference current generated by the reference compensation current extraction scheme get altered for a changing operating conditions. So a sliding mode controller (SMC) based p-q theory is proposed in the dissertation to reduce these effects. To validate the efficacy of the implemented sliding mode controller for the power quality improvement, the performance of the proposed system with both linear and non-linear controller are observed and compared by taking total harmonic distortion as performance index. From the obtained simulation and experimentation results it is concluded that the SMC based LCLC-type PV-DSTATCOM performs better in all critical operating conditions

Real-time power system impedance estimation for DG applications: Using PV-inverter based harmonic injection method

On-line power system (PS) Thévenin equivalent impedance (TEI) estimation involves the reduction of the PS's complex circuit into a simple form that provides valuable insight into its state and behaviour. It finds application in numerous areas such as voltage stability monitoring and islanding detection. In the context of distributed generation (DG), on-line TEI estimation can be easily implemented in existing hardware to add functionality and improve the operation of power converters – the key components of DG systems. Two distinct methods of on-line PS TEI estimation exist. The passive method involves only measurement of voltage and current, whereas the active method involves injection of current into the PS and measurement of the response. This work is focused on the active method. Through a review of the available literature, limitations of past work are highlighted. It is shown that the nature of current injection varies greatly in different works and that evaluation of implementation performance is generally not thorough. Little consideration has been made of the effect of injection current level and frequency on the performance of on-line TEI estimation. Furthermore, the behaviour of the grid and its impact has not been thoroughly investigated. In this work, the active method is implemented in a three-phase PV-inverter and thoroughly tested in terms of its TEI estimation accuracy. Dependence of said accuracy on parameters such as the level of injected current and its frequency is shown to be high through tests performed on the live PS at two locations. These parameters are optimised such that TEI accuracy is maximised and the performance of the device is shown to be good compared to calibration equipment. The accuracy of PS TEI tracking is evaluated and quantified. Considerations are also made of the device's hardware limitations and their effect. A process by which a device's TEI estimation accuracy can be thoroughly evaluated is developed through this work. The behaviour of the PS's TEI is also investigated over long periods and characterised. It is found that the TEI remains steady around an average level in both test locations, with a low standard deviation. Consistency in results is found to be high between the two tests

Hybrid multimodule DC-DC converters for ultrafast electric vehicle chargers

To increase the adoption of electric vehicles (EVs), significant efforts in terms of reducing the charging time are required. Consequently, ultrafast charging (UFC) stations require extensive investigation, particularly considering their higher power level requirements. Accordingly, this paper introduces a hybrid multimodule DC-DC converter-based dual-active bridge (DAB) topology for EV-UFC to achieve high-efficiency and high-power density. The hybrid concept is achieved through employing two different groups of multimodule converters. The first is designed to be in charge of a high fraction of the total required power, operating at a relatively low switching frequency, while the second is designed for a small fraction of the total power, operating at a relatively high switching frequency. To support the power converter controller design, a generalized small-signal model for the hybrid converter is studied. Also, cross feedback output current sharing (CFOCS) control for the hybrid input-series output-parallel (ISOP) converters is examined to ensure uniform power-sharing and ensure the desired fraction of power handled by each multimodule group. The control scheme for a hybrid eight-module ISOP converter of 200 kW is investigated using a reflex charging scheme. The power loss analysis of the hybrid converter is provided and compared to conventional multimodule DC-DC converters. It has been shown that the presented converter can achieve both high efficiency (99.6%) and high power density (10.3 kW/L), compromising between the two other conventional converters. Simulation results are provided using the MatLab/Simulink software to elucidate the presented concept considering parameter mismatches.Qatar Foundation; Qatar National Research FundScopu