Study on Voltage Controlling Techniques In Grid Connected PV System

The energy is the very important parameter for survival or today’s growth we can transfer the energy from one form to other. The mainly wind and solar energies are the most available among other renewable energy sources in all over the world. In the present years, because of the rapid advances of power electronic systems the production of electricity from wind and photovoltaic energy sources have increased significantly. This paper proposed hybrid system is using of controlling power

Reliability Enhancing Control Algorithms for Two-Stage Grid-Tied Inverters

In the photovoltaic (PV) generation system, two types of grid-tied inverter systems are usually deployed: the single-stage grid-tied inverter system and the two-stage grid-tied inverter system. In the single-stage grid-tied inverter system, the input of the inverter is directly connected to the PV arrays, while an additional dc-dc stage is inserted between the PV arrays and the dc-ac inverter in the two-stage design. The additional dc-dc stage could provide a stable dc-link voltage to the inverter, which also enables new design possibilities, including the multi-MPPT operation and solar-plus-storage application. Thus, the two-stage grid-tied inverter has been widely used in the PV generation system.As the core component of the PV generation system, the reliability of the grid-tied inverter determines the overall robustness of the system. The two-stage grid-tied inverter system includes three parts: the dc-dc stage, dc-link capacitor, and dc-ac inverter. Thus, the reliability of the two-stage grid-tied inverter relies on the reliability of each part. The dc-dc stage is used to provide a stable dc-link voltage to the inverter. However, when the inverter stage provides constant power to the grid, the load of the dc-dc stage becomes the constant power load (CPL), which will deteriorate the stability of the dc-dc stage. The dc-link capacitor is used to attenuate the voltage ripple on the dc-link and balance the transient power mismatch between the dc-dc stage and the dc-ac stage. However, during the operation of the inverter system, the degradation of the capacitor will reduce the converter reliability, and even result in system failure. The inverter stage is connected to the grid through the output filter, and the LCL type filter has been commonly used due to its superior performance. The resonance of the LCL filter must be properly damped to enhance the inverter stability. However, the grid-side impedance will lead to the resonant frequency drifting of the LCL filter, which will worsen the stability margin of the inverter. Thus, the control design of the two-stage grid-tied inverter system must consider those reliability challenges. In this work, three control algorithms are proposed to solve the reliability challenges. For the dc-dc stage, an uncertainty and disturbance estimator (UDE) based robust voltage control scheme is proposed. The proposed voltage control scheme can actively estimate and compensate for the disturbance of the dc-dc stage. Both the disturbance rejection performance and the stability margin of the dc-dc stage, especially under the CPL, could be enhanced. For the dc-link capacitor, a high-frequency (HF) signal injection based capacitance estimation scheme is proposed. The proposed estimation scheme can monitor the actual dc-link capacitance in real-time. For the inverter stage, an adaptive extremum seeking control (AESC) based LCL filter resonant frequency estimation scheme is proposed. The AESC-based estimation scheme can estimate the resonant frequency of the LCL filter online. All the proposed reliability enhancing control algorithms could enhance the reliability of the two-stage grid-tied inverter system. Detailed theoretical analysis, simulation studies, and comprehensive experimental studies have been performed to validate the effectiveness

Reliability Enhancing Control Algorithms for Two-Stage Grid-Tied Inverters

In the photovoltaic (PV) generation system, two types of grid-tied inverter systems are usually deployed: the single-stage grid-tied inverter system and the two-stage grid-tied inverter system. In the single-stage grid-tied inverter system, the input of the inverter is directly connected to the PV arrays, while an additional dc-dc stage is inserted between the PV arrays and the dc-ac inverter in the two-stage design. The additional dc-dc stage could provide a stable dc-link voltage to the inverter, which also enables new design possibilities, including the multi-MPPT operation and solar-plus-storage application. Thus, the two-stage grid-tied inverter has been widely used in the PV generation system.As the core component of the PV generation system, the reliability of the grid-tied inverter determines the overall robustness of the system. The two-stage grid-tied inverter system includes three parts: the dc-dc stage, dc-link capacitor, and dc-ac inverter. Thus, the reliability of the two-stage grid-tied inverter relies on the reliability of each part. The dc-dc stage is used to provide a stable dc-link voltage to the inverter. However, when the inverter stage provides constant power to the grid, the load of the dc-dc stage becomes the constant power load (CPL), which will deteriorate the stability of the dc-dc stage. The dc-link capacitor is used to attenuate the voltage ripple on the dc-link and balance the transient power mismatch between the dc-dc stage and the dc-ac stage. However, during the operation of the inverter system, the degradation of the capacitor will reduce the converter reliability, and even result in system failure. The inverter stage is connected to the grid through the output filter, and the LCL type filter has been commonly used due to its superior performance. The resonance of the LCL filter must be properly damped to enhance the inverter stability. However, the grid-side impedance will lead to the resonant frequency drifting of the LCL filter, which will worsen the stability margin of the inverter. Thus, the control design of the two-stage grid-tied inverter system must consider those reliability challenges. In this work, three control algorithms are proposed to solve the reliability challenges. For the dc-dc stage, an uncertainty and disturbance estimator (UDE) based robust voltage control scheme is proposed. The proposed voltage control scheme can actively estimate and compensate for the disturbance of the dc-dc stage. Both the disturbance rejection performance and the stability margin of the dc-dc stage, especially under the CPL, could be enhanced. For the dc-link capacitor, a high-frequency (HF) signal injection based capacitance estimation scheme is proposed. The proposed estimation scheme can monitor the actual dc-link capacitance in real-time. For the inverter stage, an adaptive extremum seeking control (AESC) based LCL filter resonant frequency estimation scheme is proposed. The AESC-based estimation scheme can estimate the resonant frequency of the LCL filter online. All the proposed reliability enhancing control algorithms could enhance the reliability of the two-stage grid-tied inverter system. Detailed theoretical analysis, simulation studies, and comprehensive experimental studies have been performed to validate the effectiveness

Uncertainty and disturbance estimator design to shape and reduce the output impedance of inverter

Power inverters are becoming more and more common in the modern grid. Due to their switching nature, a passive filter is installed at the inverter output. This generates high output impedance which limits the inverter ability to maintain high power quality at the inverter output. This thesis deals with an impedance shaping approach to the design of power inverter control. The Uncertainty and Disturbance Estimator (UDE) is proposed as a candidate for direct formation of the inverter output impedance. The selection of UDE is motivated by the desire for the disturbance rejection control and the tracking controller to be decoupled. It is demonstrated in the thesis that due to this fact the UDE filter design directly influences the inverter output impedance and the reference model determines the inverter internal electromotive force. It was recently shown in the literature and further emphasized in this thesis that the classic low pass frequency design of the UDE cannot estimate periodical disturbances under the constraint of finite control bandwidth. Since for a power inverter both the reference signal and the disturbance signal are of periodical nature, the classic UDE lowpass filter design does not give optimal results. A new design approach is therefore needed. The thesis develops four novel designs of the UDE filter to significantly reduce the inverter output impedance and maintain low Total Harmonic Distortion (THD) of the inverter output voltage. The first design is the based on a frequency selective filter. This filter design shows superiority in both observing and rejecting periodical disturbances over the classic low pass filter design. The second design uses a multi-band stop design to reject periodical disturbances with some uncertainty in the frequency. The third solution uses a classic low pass filter design combined with a time delay to match zero phase estimation of the disturbance at the relevant spectrum. Furthermore, this solution is combined with a resonant tracking controller to reduce the tracking steady-state error in the output voltage. The fourth solution utilizes a low-pass filter combined with multiple delays to increase the frequency robustness. This method shows superior performance over the multi-band-stop and the time delayed filter in steady-state. All the proposed methods are validated through extensive simulation and experimental results

Model-based decentralized optimal control of a microgrid

Power networks have experienced dramatic changes with the growth of renewable energy and `smart' grids. To accommodate the challenges posed to traditional power system control architectures, the microgrid concept has gained traction. Microgrids are small-scale power networks that can disconnect from the main grid and operate autonomously if necessary. These systems add robustness and facilitate the incorporation of renewable power, but they face control challenges of their own due to the lack of significant inertial generation. Without the main grid to provide balance, the high proportion of electrically-interfaced power resources can cause significant deterioration in microgrid stability. This dissertation proposes designs to improve decentralized control in microgrids; model based information is incorporated into controllers and estimators to more optimally guide control signals, while still only using local data for real-time computation. We outline the role that microgrid topology can have on stability, and how judicious power injection can mitigate instabilities. These results are extended to a decentralized H-infinity control design for microgrid frequency; even with limited model-based information and controller distribution, the design offers significant improvements over traditional controllers. Building upon the idea of microgrid stabilization, we also present a control method by which a wind turbine can be coordinated for microgrid support. The wind turbine is used as a controllable power source by utilizing the rotational energy stored in its rotor; this design incorporates an aerodynamic wind turbine model and a novel optimal blade pitch angle controller to ensure stable turbine operation. This allows for rapid power injection for grid support. This theme concludes with a decentralized estimation scheme to facilitate coordinated control across a microgrid using only local data. We leverage the frequency synchronization and load-sharing intrinsic to the microgrid so that local measurements can provide insight about grid-wide conditions. This allows for effective implementation of optimal filtering techniques so that remote conditions can be estimated using only local data; this allows for grid-wide coordination and optimization. Together these ideas represent the concept that the microgrid model, even in a limited and inaccurate sense, can be manipulated to provide significant benefits for decentralized control across the network.Mechanical Engineerin

Control of power converter in modern power systems

A la portada consta el nom del programa interuniversitari: Joint Doctoral Programme in Electric Energy Systems [by the] Universidad de Málaga, Universidad de Sevilla, Universidad del País Vasco/Euskal Erriko Unibertsitatea i Universitat Politècnica de CatalunyaPower system is undergoing an unpreceded paradigm shift: from centralized to distributed generation. As the renewable-based generations and battery storage systems are increasingly displacing conventional generations, it becomes more and. more difficult to maintain the stability and reliability of the grid by using only conventional generations. The main reason for the degradation of grid stability is the rapid penetration of nonconventional sources. These new generations interface with the grids through power electronics converters which are conventionally designed to maximize conversion efficiency and resource utilization. Indeed, these power converters only focus on their internal operation despite the grid conditions, which often worsens the grid operation. To overcome such a drawback, the grid-forming concept has been proposed for power converters, aiming to redesign the control of the power converters to enforce more grid-friendly behaviours such as inertia response and power oscillation damping to name a few. Despite the rich literature, actual adaptation of grid-forming controller in real-world applications is still rare because incentives for renewable power plants to provide services based on such advanced grid-forming functions were at best scarce. In the last years, however, several system operators have imposed new requirements and markets for grid-supporting services. In addition, the existing grid-forming controllers require modification to low-level control firmware of a power converter, which is often unrealistic due to the control hardware limitations as well as necessary testing and certifications. To ensure a stable operation of a grid-forming converter under adverse operating conditions, a robust voltage sensorless current controller is developed in this PhD thesis. The proposed controller is able to handle most of the possible abnormal conditions of the grid such as impedance variations, unbalanced voltage; harmonics distortion. These abnormalities of the grid are mathematically represented using equivalent linear models such that they can be used for calculating the controller gains. Linear matrix inequality techniques are also used to facilitate parameter tuning. In fact, the performance and stability of the current control loop can be determined through only two tuning parameters instead of eight parameters for a controller of a similar structure. The existing grid-forming implementations are designed considering that the control firmware of the power converter can be upgraded at will. However, modifications of the control firmware are not straightforward and cost-effective at mass scale. To overcome such a limitation, an external synchronous controller is presented in this PhD thesis. The external synchronous controller uses measurements, which are either provided by the power converter or a dedicated measurement unit, to calculate the actual active and reactive power that should be injected by the power converters in a way that the power plant acts as an aggregated grid­forming converter. As a result, any conventional power converters can be utilized for providing grid-supporting services with minimal modification to the existing infrastructure. Power converters can provide even better performance than a synchronous generator if a proper control scheme is used. In this regard, the final chapter of this PhD thesis presents the multi-rotor virtual machine implementation for grid-forming converter to boost their damping performance to power oscillations. The multi-rotor virtual machine-controller implements several virtual rotors instead of only one rotor as in typical grid-forming strategies. Since each of the virtual rotors is tuned to target a specific critical mode, the damping participation to such a mode can be increased and adjusted individually. The controllers presented in this PhD thesis are validated through simulators and experiments in the framework of the H2020 FlexiTranstore project. The results are throughout analysed to assess the control performance as well as to highlight possible implications.A medida que las generaciones basadas en energías renovables y los sistemas de almacenamiento de baterías desplazan la generación convencional, se vuelve cada vez más difícil mantener la estabilidad y confiabilidad de la red. Estas nuevas generaciones interactúan con las redes a través de convertidores de electrónica de potencia que están diseñados tradicionalmente para maximizar la eficiencia de conversión y la utilización de recursos. Estos convertidores centran su funcionamiento interno independientemente de las condiciones de la red, lo que a menudo empeora el funcionamiento de la red. Para esto, se ha propuesto el concepto de convertidores de potencia formadores de red (grid-forming), con el objetivo de rediseñar el control de los convertidores de potencia para imponer comportamientos más favorables a la red, por ejemplo, la respuesta inercial y la amortiguación de oscilaciones de potencia. No en tanto, la adaptación real del controlador grid-forming en aplicaciones del mundo real todavía es escasa debido a los pocos incentivos para que las plantas de energía renovable proporcionen servicios basados en funciones de formación de red tan avanzadas. Aunque en los últimos años, operadores de sistemas han impuesto nuevos requisitos y mercados para servicios auxiliares, los controladores grid-forming existentes requieren cambios en el firmware de control de bajo nivel de un convertidor de potencia, algo poco realista debido a las limitaciones del hardware de control, así como a las pruebas y certificaciones necesarias. En esta tesis se desarrolla un controlador de corriente robusto, sin sensor de tensión, para garantizar el funcionamiento estable de un convertidor grid-forming en condiciones de operación adversas. Este controlador es capaz de manejar la mayoría de las condiciones anormales de red, como variaciones de impedancia, tensión desequilibrada y distorsión de armónicos. Estas anomalías de la red se representan matemáticamente mediante modelos lineales equivalentes, utilizados para calcular las ganancias del controlador. También, usando técnicas de desigualdad matricial lineal para facilitar el ajuste de parámetros. De hecho, el rendimiento y la estabilidad del bucle de control de la corriente pueden determinarse mediante sólo dos parámetros de sintonización. Las implementaciones de formación de red existentes están diseñadas considerando que el firmware de control del convertidor de potencia puede actualizarse a voluntad. Sin embargo, las modificaciones del firmware de control no son sencillas ni rentables a gran escala. Por tanto, esta tesis presenta un controlador síncrono externo que utiliza las mediciones proporcionadas por el convertidor de potencia o por una unidad de medición dedicada para calcular la potencia activa y reactiva real que deben inyectar los convertidores de potencia, de forma que la central eléctrica actúe como un convertidor grid-forming agregado. Como resultado, cualquier convertidor de potencia convencional puede utilizarse para proporcionar servicios de apoyo a la red con una modificación mínima de la infraestructura existente. Los convertidores de potencia pueden ofrecer mejor rendimiento que un generador síncrono utilizando un esquema de control adecuado. El último capítulo de esta tesis presenta la implementación de una máquina virtual multirrotor para que los convertidores de red aumenten su rendimiento de amortiguación de las oscilaciones de potencia. El controlador de la máquina virtual multirrotor implementa varios rotores virtuales en lugar de un solo rotor como en las estrategias típicas de grid-forming. Dado que cada uno de los rotores virtuales está sintonizado para dirigirse a un modo crítico específico, la participación de la amortiguación a dicho modo puede aumentarse y ajustarse individualmente. Los controladores presentados en esta tesis doctoral han sido validados mediante simulaciones y experimentos en el marco del proyecto H2020 FlexiTranstore.Postprint (published version

Microgrids/Nanogrids Implementation, Planning, and Operation

Today’s power system is facing the challenges of increasing global demand for electricity, high-reliability requirements, the need for clean energy and environmental protection, and planning restrictions. To move towards a green and smart electric power system, centralized generation facilities are being transformed into smaller and more distributed ones. As a result, the microgrid concept is emerging, where a microgrid can operate as a single controllable system and can be viewed as a group of distributed energy loads and resources, which can include many renewable energy sources and energy storage systems. The energy management of a large number of distributed energy resources is required for the reliable operation of the microgrid. Microgrids and nanogrids can allow for better integration of distributed energy storage capacity and renewable energy sources into the power grid, therefore increasing its efficiency and resilience to natural and technical disruptive events. Microgrid networking with optimal energy management will lead to a sort of smart grid with numerous benefits such as reduced cost and enhanced reliability and resiliency. They include small-scale renewable energy harvesters and fixed energy storage units typically installed in commercial and residential buildings. In this challenging context, the objective of this book is to address and disseminate state-of-the-art research and development results on the implementation, planning, and operation of microgrids/nanogrids, where energy management is one of the core issues

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

Control of voltage source converters connected to variable impedance grids

The increase in new renewable energy resources is key to achieving carbon reduction targets, however it also introduces new grid integration challenges. The best renewable resource in Scotland is found in remote parts of the country, and as a result new renewable based generation is increasingly subjected to high and variable levels of impedance. Impedances that cause resonances are also increasingly common, given the higher order characteristics of impedance when transformers, filters, subsea cables, compensators and so on are present in the network. For a better understanding of impedance related stability issues, the estimation of the grid impedance using both Thévenin equivalent and wide spectrum techniques is studied in this thesis and integrated into the converter’s control. These estimations inform the controller of the grid conditions, allowing for controller adaptation. In instances where weak grid conditions are severe and the local grid impedance is dominant, a disturbance rejection mechanism called the pre-emptive voltage decoupler (PVD) is proposed. The PVD feeds forward the active current reference and measured voltage, and adapts the reactive current reference as a function of the impedance estimation, to pre-emptively compensate the local voltage for changes in active power transfer. This is justified through small signal analysis using linearised state space models and validated in the laboratory using large inductors and a converter. The control is also made more resilient with an instability detector, proposed to prevent instability when significant grid disturbances occur. Through early detection of sudden power angle changes, stability can be maintained. This is achieved by momentarily reducing the power reference and re-establishing grid parameters. The implementation of the proposed changes improves the steady state stability region from -0.75 – 0.55 pu to -0.85 – 0.75 pu. Further, the nonlinear transient performance is much more resilient, and uninterrupted power flow can be maintained. When the local grid is not dominant, and higher order grid impedances cause undesired resonances, a detection of the resonant frequency allows for an adaptation of the outer loop gains, thus damping the resonances and improving stability. Such grids are also prone to instability, but a reduction of the power reference does not improve stability, on the contrary the reduction of the power reference shifts eigenvalues into the right hand plane. A better preventative measure is to reduce the outer loop gains, and once the frequency of the problematic resonances is identified, final decisions on outer loop tuning can be taken. With this implementation, the stability of the system is maintained and the power output can be recovered within about 1 second.The increase in new renewable energy resources is key to achieving carbon reduction targets, however it also introduces new grid integration challenges. The best renewable resource in Scotland is found in remote parts of the country, and as a result new renewable based generation is increasingly subjected to high and variable levels of impedance. Impedances that cause resonances are also increasingly common, given the higher order characteristics of impedance when transformers, filters, subsea cables, compensators and so on are present in the network. For a better understanding of impedance related stability issues, the estimation of the grid impedance using both Thévenin equivalent and wide spectrum techniques is studied in this thesis and integrated into the converter’s control. These estimations inform the controller of the grid conditions, allowing for controller adaptation. In instances where weak grid conditions are severe and the local grid impedance is dominant, a disturbance rejection mechanism called the pre-emptive voltage decoupler (PVD) is proposed. The PVD feeds forward the active current reference and measured voltage, and adapts the reactive current reference as a function of the impedance estimation, to pre-emptively compensate the local voltage for changes in active power transfer. This is justified through small signal analysis using linearised state space models and validated in the laboratory using large inductors and a converter. The control is also made more resilient with an instability detector, proposed to prevent instability when significant grid disturbances occur. Through early detection of sudden power angle changes, stability can be maintained. This is achieved by momentarily reducing the power reference and re-establishing grid parameters. The implementation of the proposed changes improves the steady state stability region from -0.75 – 0.55 pu to -0.85 – 0.75 pu. Further, the nonlinear transient performance is much more resilient, and uninterrupted power flow can be maintained. When the local grid is not dominant, and higher order grid impedances cause undesired resonances, a detection of the resonant frequency allows for an adaptation of the outer loop gains, thus damping the resonances and improving stability. Such grids are also prone to instability, but a reduction of the power reference does not improve stability, on the contrary the reduction of the power reference shifts eigenvalues into the right hand plane. A better preventative measure is to reduce the outer loop gains, and once the frequency of the problematic resonances is identified, final decisions on outer loop tuning can be taken. With this implementation, the stability of the system is maintained and the power output can be recovered within about 1 second

Control and estimation techniques applied to smart microgrids : a review

DATA AVAILABILITY : No data was used for the research described in the article.The performance of microgrid operation requires hierarchical control and estimation schemes that coordinate and monitor the system dynamics within the expected manipulated and control variables. Smart grid technologies possess innovative tools and frameworks to model the dynamic behaviour of microgrids regardless of their types, structures, etc. Various control and estimation technologies are reviewed for developing dynamic models of smart microgrids. The hierarchical system of a microgrid control consists of three architectural layers, primary, secondary and tertiary, which need to be supported by real-time monitoring and measurement environment of the system variables and parameters. Various control and estimation schemes have been devised to handle the dynamic performance of microgrids in the function of control layers requirement. Firstly, control schemes in the innovative grid environment are evaluated to understand the dynamics of the developed technologies. Six control technologies, linear, non-linear, robust, predictive, intelligent and adaptive, are mainly used to model the control design within the layer(s) regardless of the types of microgrids. Secondly, the estimation technologies are evaluated based on the state of variables, locations and modelling of microgrids that can efficiently support the performance of the controllers and operating microgrids. Finally, a future vision for designing hierarchical and architectural control techniques for the optimal operation of intelligent microgrids is also provided. Therefore, this study will serve as a fundamental conceptual framework to select a perfect optimal design modelling strategy and policy-making decisions to control, monitor and protect the innovative electrical network.http://www.elsevier.com/locate/rserhj2023Electrical, Electronic and Computer Engineerin