Study of new vector-control algorithms for 3-phase inverters used in renewable agents connected to the low-voltage utility grid with disturbances

[ESP] La demanda de energía eléctrica se ha ido incrementando a través de los años debido al desarrollo que han tenido el sector industrial y de transporte, sumándose además el aumento de la población mundial y el desarrollo de nuevas tecnologías que requieren mayor cantidad de energía. Por ello, y con el propósito de generar la energía eléctrica necesaria para suplir a estos sectores, el consumo de combustible ha presentado un aumento significativo. Así, la energía consumida en el año 2010 fue de unos 153,000 TWh y se prevé que para el año 2020 esta cifra ascienda a 184,000 TWh, siendo la mayor parte de esta energía proveniente de combustibles fósiles, aunque el futuro de esta tendencia es incierto. Además, la población mundial se está concienciando cada vez más de los efectos negativos medioambientales que está provocando el llamado “efecto invernadero” y, como consecuencia, se están creando una serie de políticas energéticas con el fin de reducir la generación de gases y partículas contaminantes. Una alternativa para reducir la dependencia de los combustibles fósiles y, a la vez, reducir las emisiones de los gases tóxicos causantes del efecto invernadero, es el uso de fuentes de energías renovables como la solar fotovoltaica y la eólica, así como el uso de pilas de combustibles para almacenamiento de energía, todas ellas a instalar en el mix energético. En este sentido, los nuevos agentes renovables que se conecten a la red eléctrica trifásica de baja tensión deben controlarse adecuadamente y cumpliendo con las legislaciones energéticas vigentes. En este sentido, deben diseñarse nuevas y sofisticadas estrategias de control con el propósito de controlar adecuadamente las corrientes de línea de los inversores de conexión a red utilizados en los agentes renovables cuando existan perturbaciones en la red eléctrica de baja tensión, tales como las variaciones de su frecuencia nominal, los desequilibrios en las tensiones trifásicas y la presencia de contaminación armónica de baja frecuencia. Por todo lo anteriormente mencionado, esta tesis está enfocada en el estudio de varios algoritmos de control y sincronización utilizados en inversores en fuente de tensión (VSI) para conexión a red que operan como los acondicionadores de potencia para los sistemas renovables. Los estudios realizados se aplican a un sistema fotovoltaico, pero pueden extenderse a cualquier tipo de agente renovable utilizado en un sistema de Generación Distribuida. [ENG] Throughout decades, the electric power demand has been rising due to the growth of the industrial sector and transportation, and the development of new technologies that require more energy together with the increase of the global population have led to a higher fuel demand needed for the electric energy generation. The global energy consumption in 2010 was 153,000 TWh and it is expected an increment to 184,600 TWh by 2020, the majority provided by fossil fuels, although the future of these trend is uncertain. Besides, greenhouse effect is causing environmental changes that concern mankind and the creation of new energetic policies is a fact. An alternative for reducing the fossil fuel dependence and the reduction of the greenhouse gas emission is the use of clean and infinite renewable energy sources such as photovoltaic, wind, as well as fuel cells for energy storage, which have been installed in the energetic mix. In this context, new renewable agents are connected to the 3-phase utility grid and must be properly controlled according to power electrical legislations. For this, new and sophisticated control algorithms are to be designed in order to control properly the line currents of the grid-connected inverter when variations of the nominal frequency, voltage unbalances and low-order harmonics are present in the 3-phase utility grid voltage. This thesis is focused in the study of several control and synchronization algorithms used in grid-connected Voltage Source Inverters (VSI) working as the power conditioner circuits for renewable energy systems. The study of these algorithms is carried out using a grid-connected photovoltaic system, but they can be extended to any renewable agent in any distributed generation system.Universidad Politécnica de Cartagen

Power quality improvements of single-phase grid-connected photovoltaic systems

PhD ThesisThe number of distributed power generation systems (DPGSs), mostly based on photovoltaic (PV) energy sources is increasing exponentially. These systems must conform to grid codes to ensure appropriate power quality and to contribute to grid stability. A robust and reliable synchronization to the grid is an important consideration in such systems. This is due to the fact that, fast and accurate detection of the grid voltage parameters is essential in order to implement stable control strategies under a broad range of grid conditions. The second-order generalized integrator (SOGI) based phase-locked loop (PLL) is widely used for grid synchronization of single-phase power converters. This is because it offers a simple, robust and flexible solution for grid synchronization. However, the SOGI-PLL is affected by the presence of a dc offset in the measured grid voltage. This dc voltage offset is typically introduced by the measurements and data conversion process, and causes fundamental-frequency ripple in the estimated parameters of the grid voltage (i.e. voltage amplitude, phase angle and frequency). In addition to this ripple, the unit amplitude sine and cosine signals of the estimated phase angle (i.e. unit vectors), that are used to generate reference signals in the closed-loop control of grid-connected PV converters will contain dc offset. This is highly undesirable since it can cause dc current injection to the grid, and as a consequence, the quality of the power provided by the DPGSs can be degraded. To overcome this drawback, a modified SOGI-PLL with dc offset rejection capability is proposed. The steady-state, transient and harmonic attenuation performance of the proposed PLL scheme are validated through simulation and experimental tests. The overall performance demonstrates the capability of the proposed PLL to fully reject such dc current injection as well as to provide a superior harmonic attenuation when compared with the SOGIPLL and two other existing offset rejection approaches. It is shown that, the proposed PLL scheme can enhance the overall total harmonic distortion (THD%) of the injected power by 15% when compared to the conventional SOGI-PLL. In addition to the synchronization, grid-connected PV systems require a current control scheme to regulate the output current. Due to the simple implementation, proportional-integral (PI) controllers in the stationary reference frame are commonly used for current controlled inverters. However, these PI-controllers exhibit a major drawback of failure to track a sinusoidal reference Abstract ii without steady-state error, which may result in low-order harmonics. This drawback can be overcome if the PI-controllers are implemented in direct-quadrature (dq) rotating reference frame. In single-phase systems, the common approach is to create a synthesized phase signal orthogonal to the fundamental of the real single-phase system so as to obtain dc quantities by means of a stationary-to-rotating reference frame. The orthogonal synthesized signal in conventional approaches is obtained by phase shifting the real signal by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. This thesis proposes an alternative way of implementing such PI-controllers in the dq reference frame without the need of creating such orthogonal signals. The proposed approach, effectively improves the poor dynamic of the conventional approaches while not adding excessive complexity to the controller structure. The results show that, in addition to its ability to regulate the current and achieve zero steady-state error, the proposed approach shows superior dynamic response when compared with that of conventional delay-based approach.Libyan Governmen

Zero-voltage ride-through capability of single-phase grid-connected photovoltaic systems

Distributed renewable energy systems play an increasing role in today’s energy paradigm. Thus, intensive research activities have been centered on improving the performance of renewable energy systems, including photovoltaic (PV) systems, which should be of multiple-functionality. That is, the PV systems should be more intelligent in the consideration of grid stability, reliability, and fault protection. Therefore, in this paper, the performance of single-phase grid-connected PV systems under an extreme grid fault (i.e., when the grid voltage dips to zero) is explored. It has been revealed that combining a fast and accurate synchronization mechanism with appropriate control strategies for the zero-voltage ride-through (ZVRT) operation is mandatory. Accordingly, the representative synchronization techniques (i.e., the phase-locked loop (PLL) methods) in the ZVRT operation are compared in terms of detection precision and dynamic response. It shows that the second-order generalized integrator (SOGI-PLL) is a promising solution for single-phase systems in the case of fault ride-through. A control strategy by modifying the SOGI-PLL scheme is then introduced to single-phase grid-connected PV systems for ZVRT operation. Simulations are performed to verify the discussions. The results have demonstrated that the proposed method can help single-phase PV systems to temporarily ride through zero-voltage faults with good dynamics

Flexible operation of grid-interfacing converters in distribution networks : bottom-up solutions to voltage quality enhancement

Due to the emerging application of distributed generation (DG), large numbers of DG systems are expected to deliver electricity into the distribution network in the near future. For the most part these systems are not ready for riding through grid disturbances and cannot mitigate unwanted influences on the grid. On the one hand, with the increasing use of sensitive and critical equipment by customers, the electricity network is required to serve high voltage quality. On the other hand, more and more unbalanced and nonlinear equipment, including DG units, is negatively affecting the power quality of distribution networks. To adapt to the future distribution network, the tendency for grid-interfacing converters will be to integrate voltage quality enhancement with DG functionality. In this thesis, the flexible operation of grid-interfacing converters in distribution networks is investigated for the purpose of voltage quality enhancement at both the grid and user sides. The research is carried out in a bottom-up fashion, from the low-level power electronics control, through the realization of individual system functionality, finally arriving at system-level concepts and implementation. Being essential to the control of grid-interfacing converters, both stationaryframe techniques for voltage detection and synchronization in disturbed grids, and asymmetrical current regulation are investigated. Firstly, a group of high performance filters for the detection of fundamental symmetrical sequences and harmonics under various grid conditions is proposed. The robustness of the proposed filters to small grid-frequency variation and their adaptability to large frequency change are discussed. Secondly, multiple reference frame current regulation is explored for dealing with unbalanced grid conditions. As a complement to the existing proportional resonant (PR) controllers, sequence-decoupled resonant (SDR) controllers are proposed for regulating individual symmetric sequences. Based on the modeling of a four-leg grid-connected system in different reference frames, three types of controllers, i.e. PI, PR, and proportional plus SDR controllers are compared. Grid-interactive control of distributed power generation, i.e. voltage unbalance compensation, grid-fault ride-through control and flexible power transfer, as well as the modeling of harmonic interaction, are all investigated. The in-depth study and analysis of these grid interactions show the grid-support possibilities and potential negative impact on the grid of inverter-based DG units, beyond their primary goal of power delivery. In order to achieve a co-operative voltage unbalance compensation based on distributed DG systems, two control schemes, namely voltage unbalance factor based control and negative-sequence admittance control, are proposed. The negativesequence voltages at the grid connection point can be compensated and mitigated by regulating the negative-sequence currents flowing between the grid and DG converters. Flexible active and reactive power control during unbalanced voltage dips is proposed that enables DG systems to enhance grid-fault ride-through capability and to adapt to various requirements for grid voltage support. By changing adaptable weighting factors, the compensation of oscillating power and the regulation of grid currents can be easily implemented. Two joint strategies for the simultaneous control of active and reactive power are derived, which maintain the adaptive controllability that can cope with multiple constraints in practical applications. The contribution of zero-sequence currents to active power control is also analyzed as a complement to the proposed control, which is based on positive- and negative-sequence components. Harmonic interaction between DG inverters and the grid is modeled and analyzed with an impedance-based approach. In order to mitigate the harmonic distortion in a polluted grid, it is proposed to specify output impedance limits as a design constraint for DG inverters. Results obtained from modeling, analysis, and simulations of a distribution network with aggregated DG inverters, show that the proposed method is a simple and effective way for estimating harmonic quasi-resonance problems. By integrating these proposed control strategies in a modified conventional series-parallel structure, we arrived at a group of grid-interfacing system topologies that is suitable for DG applications, voltage quality improvement, and flexible power transfer. A concrete laboratory system details the proposed concepts and specifies the practical problems related to control design. The introduction of multi-level control objectives illustrates that the proposed system can ride through voltage disturbances, can enhance the grid locally, and can continue the power transfer to and from the grid while high voltage quality is maintained for the local loads within the system module. A dual-converter laboratory set-up was built, with which the proposed concepts and practical implementation have been fully demonstrated

Control Strategies for Power Electronic Interfaces in Unbalanced Diesel Hybrid Mini-Grids with Renewable Sources and Storage

Traditionally, remote communities worldwide consist of autonomous power systems (mini-grids) supplied almost exclusively by diesel-engine generator sets (gensets) at relatively high costs. Integration of renewable energy sources (RESs), such as, photovoltaic (PV) and wind, can substantially reduce the cost of electricity generation and emissions in these remote communities. However, the highly variable load profile typical of mini-grids and the fluctuating characteristics of the RESs, cause frequent operation of the diesel genset at low loading condition, at low efficiency points and subject to carbon build up, which can significantly affect the maintenance costs and even the life time of the genset. Another important issue that is frequently overlooked in small (< 100 kVA) mini-grids, which usually present a low number of loads thus reducing the averaging effect, is load unbalance. Diesel gensets supplying unbalanced loads experience overheating in the synchronous generator and vibration in the shaft. For efficient operation, the genset should be operated near its full capacity and also in balanced mode. In order to address the above mentioned issues, a fast and reliable multi-mode battery energy storage system (BESS) employing voltage source inverter (VSI), is proposed in this thesis. In the genset support mode, as a basic feature, it can provide minimum loading for the genset and supplement it under peak load conditions. In addition, it can also provide load balancing and reactive power compensation for the mini-grid system. Therefore, the genset operates in balanced condition and within its ideal power range. In cases when the power demand from the genset is low, due to high supply of RESs and/or low load consumption, the genset can be shut-down and the BESS forms the grid, regulating voltage and frequency of the mini-grid system in the grid forming mode. Besides, the logic for defining the operating mode of the BESS and for achieving smooth transitions between modes are also presented in this thesis. The conventional approach for the control of three-phase VSI with unbalanced loads uses three-phase vector (dq) control and symmetrical components calculator which usually results in slow dynamic responses. Besides, the common power (P) vs. frequency (f) droop characteristic of the genset results in the mini-grid operating with variable frequency what further complicates the design of the controller for the VSI. Therefore, a new frequency adaptive per-phase dq-control scheme for three-wire and four-wire three-phase VSI based on the concept of fictive axis emulation is presented. It enables the control of current/voltage of each phase separately to achieve better dynamic performance in the variable frequency diesel hybrid mini-grid system. The effectiveness of the proposed techniques is demonstrated by means of simulation and experimental results

Virtually synchronous power plant control

During the last century, the electrical energy infrastructures have been governed by synchronous generators, producing electrical energy to the vast majority of the population worldwide. However, power systems are no longer what they used to be. During the last two decades of this new millennium the classical, centralized and hierarchical networks have experienced an intense integration of renewable energy sources, mainly wind and solar, thanks also to the evolution and development of power conversion and power electronics industry. Although the current electrical system was designed to have a core of generation power plants, responsible of producing the necessary energy to supply end users and a clear power flow, divided mainly into transmission and distribution networks, as well as scalable consumers connected at different levels, this scenario has dramatically changed with the addition of renewable generation units. The massive installation of wind and solar farms, connected at medium voltage networks, as well as the proliferation of small distributed generators interfaced by power converters in low voltage systems is changing the paradigm of energy generation, distribution and consumption. Despite the feasibility of this integration in the existing electrical network, the addition of these distributed generators made grid operators face new challenges, especially considering the stochastic profile of such energy producers. Furthermore, the replacement of traditional generation units for renewable energy sources has harmed the stability and the reliable response during grid contingencies. In order to cope with the difficult task of operating the electrical network, transmission system operators have increased the requirements and modified the grid codes for the newly integrated devices. In an effort to enable a more natural behavior of the renewable systems into the electrical grid, advanced control strategies were presented in the literature to emulate the behavior of traditional synchronous generators. These approaches focused mainly on the power converter relying on their local measurement points to resemble the operation of a traditional generating unit. However, the integration of those units into bigger systems, such as power plants, is still not clear as the effect of accumulating hundreds or thousands of units has not been properly addressed. In this regard, the work of this thesis deals with the study of the so-called virtual synchronous machine (VSM) in three control layers. Furthermore, an in-depth analysis of the general structure used for the different virtual synchronous machine approaches is presented, which constitutes the base implementation tree for all existent strategies of virtual synchronous generation. In a first stage, the most inner control loop is studied and analyzed regarding the current control on the power converter. This internal regulator is in charge of the current injection and the tracking of all external power reference. Afterward, the synchronous control is oriented to the device, where the generating unit relies on its local measurements to emulate a synchronous machine in the power converter. In this regard, a sensorless approach to the virtual synchronous machine is introduced, increasing the stability of the power converter and reducing the voltage measurements used. Finally, the model of the synchronous control is extrapolated into a power plant control layer to be able to regulate multiple units in a coordinated manner, thus emulating the behavior of a unique synchronous machine. In this regard, the local measurements are not used for the emulation of the virtual machine, but they are switched to PCC measurements, allowing to set the desired dynamic response at the power plant level.Postprint (published version

A Review of Control Techniques in Photovoltaic Systems

Complex control structures are required for the operation of photovoltaic electrical energy systems. In this paper, a general review of the controllers used for photovoltaic systems is presented. This review is based on the most recent papers presented in the literature. The control architectures considered are complex hybrid systems that combine classical and modern techniques, such as artificial intelligence and statistical models. The main contribution of this paper is the synthesis of a generalized control structure and the identification of the latest trends. The main findings are summarized in the development of increasingly robust controllers for operation with improved efficiency, power quality, stability, safety, and economics