Design and Development of Advanced Control strategies for Power Quality Enhancement at Distribution Level

In recent times, power quality (PQ) issues such as current and voltage harmonics, voltage sag/swell, voltage unbalances have become the important causes for malfunctioning and degradation of the quality of power. Poor power quality severely affects on electrical equipment and finally results in significant economic losses. Hence, installation of the custom power devices to improve the power quality issues becomes an important consideration. Therefore, this thesis considers the enhancement of power quality for power distribution systems by utilizing unified power quality conditioner (UPQC). An UPQC can adequately handle several power quality problems such as load current harmonics, supply voltage distortions, voltage sags/swells and voltage unbalance. Therefore, the main focus behind this thesis is to develop advanced control strategies that improve the compensation capability of the UPQC so that power quality issues of distribution network are efficiently improved. Firstly, the current harmonics are considered and are compensated by using the shunt active power filter (SAPF). Therefore, two control strategies such as Hysteresis current control (HCC) and Sliding Mode Control (SMC) based control algorithms are implemented to compensate current harmonics in the power distribution network. Furthermore, both the current control techniques utilize the Coulon oscillator based PLL (CO-PLL) for extraction of positive sequence signal from the supply voltage and generate the three-phase reference currents by employing PI-controller based DC-link voltage regulation. The performances of both current control techniques for SAPF are evaluated under different source voltage conditions such as balanced, unbalanced and non-sinusoidal. The SAPF effectively compensates currents harmonic, however, it is unable to compensate voltage related problems. To overcome this drawback, this thesis considers the UPQC, which comprises with shunt APF and series APF, can be utilized to compensate both current and voltage related problems. The research on UPQC is carried out progressively by considering different advanced control strategies. Each progress in the research enhances the compensation capabilities of the previous UPQC control system. The simulation and realtime Opal-RT studies are carried out to verify the operating performance of each design concept of UPQC. At first, operating principle and design of UPQC is presented and then a novel control algorithm is introduced with the aid of nonlinear DC-link voltage controller such as nonlinear variable gain fuzzy (NVGF) controller and nonlinear sliding mode controller (NLSMC) with modified synchronous reference frame (SRF) control strategy for improvement of both current and voltage compensation performance of the UPQC. However, existence of large settling time in dc voltage leads to poor dynamic performance of NVGF control technique and hence current harmonics, voltage distortions and voltage disturbance such as voltage sag/swell as well as voltage unbalance compensation capability of this technique is not quite effective in comparison to the NLSMC technique. Moreover, NLSMC is very sensitive to model mismatch and noise. It is quite sluggish in rejecting long drifting grid disturbances. Hence, a suitable control strategy has to be developed in UPQC, which has improved DC-link voltage regulation as well as tracking performance through load and grid perturbations. To overcome this drawback a resistive optimization technique (ROT) incorporated with enhanced phase-locked loop (EPLL) based NVGF hysteresis control strategy and an optimum active power (OAP) technique combined with enhanced phase-locked loop (EPLL) based fuzzy sliding mode (FSM) pulse-width modulation (PWM) control strategy for UPQC have been discussed. ROT-NVGF and OAP-FSMC based UPQC control strategies are adaptive as well as robust and able to mitigate the PQ problems satisfactorily during all dynamic conditions of power system perturbation. However, performances of these controllers are not effective when there is a variation occurring either in the nonlinear load parameter or supply voltage parameter. Thus, UPQC may not be able to compensate PQ problems satisfactorily. Considering aforesaid problems, this thesis proposes a command generator tracker (CGT) based direct adaptive control (DAC) applied to a three-phase three-wire UPQC to improve the current and voltage harmonics, sag/swell and voltage unbalance in the power system distribution network. CGT is a model reference control law for a linear timeinvariant system with known coefficients and is formulated for the generation of reference signal for both shunt and series inverter. The main advantage of the proposed control algorithm is that no online extraction is needed to perceive the UPQC parameters. Moreover, IV the adaptive control law is designed to track a linear reference model to reduce the tracking error between model reference output and measured signal to be controlled. Additionally, this proposed algorithm adaptively regulates the DC-link capacitor voltage without utilizing additional controller circuit. As a result, the proposed algorithm provides more robustness, flexibility and adaptability in all operating conditions of the power system network. At last, model reference robust adaptive control (MRRAC) technique is proposed for single phase UPQC system. This control strategy is designed with the purpose of achieving high stability, high disturbance rejection and high level of harmonics cancellation. From simulation results, it is not only found to be robust against PI-controller, but also satisfactory THD results have been achieved in UPQC system. This has motivated to develop a prototype experimental set up in the Laboratory using FPGA (Field Programmable Gate Array) based NI (National Instruments) cRIO-9014. From both the simulation and experimentation, it is observed that the proposed MRRAC approach to design a UPQC system is found to be more effective as compared to the conventional PI-controller

Fuzzy Logic-Based Direct Power Control Method for PV Inverter of Grid-Tied AC Microgrid without Phase-Locked Loop

A voltage source inverter (VSI) is the key component of grid-tied AC Microgrid (MG) which requires a fast response, and stable, robust controllers to ensure efficient operation. In this paper, a fuzzy logic controller (FLC)-based direct power control (DPC) method for photovoltaic (PV) VSI was proposed, which was modelled by modulating MG’s point of common coupling (PCC) voltage. This paper also introduces a modified grid synchronization method through the direct power calculation of PCC voltage and current, instead of using a conventional phase-locked loop (PLL) system. FLC is used to minimize the errors between the calculated and reference powers to generate the required control signals for the VSI through sinusoidal pulse width modulation (SPWM). The proposed FLC-based DPC (FLDPC) method has shown better tracking performance with less computational time, compared with the conventional MG power control methods, due to the elimination of PLL and the use of a single power control loop. In addition, due to the use of FLC, the proposed FLDPC exhibited negligible steady-state oscillations in the output power of MG’s PV-VSI. The proposed FLDPC method performance was validated by conducting real-time simulations through real time digital simulator (RTDS). The results have demonstrated that the proposed FLDPC method has a better reference power tracking time of 0.03 s along with reduction in power ripples and less current total harmonic distortion (THD) of 1.59%.© 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).fi=vertaisarvioitu|en=peerReviewed

Controlled Power Point Tracking for Grid Connected and Autonomous Operation of PMSG based Wind Energy Conversion System

With continuous depletion of conventional sources of energy, Wind Energy Conversion Systems (WECS) are turning out to be one of the major players with immense potential to meet the future energy demands. It is one of the most preferable source, as it can be installed onshore as well as offshore. But with the increasing penetration of wind energy into power system, wind energy conversion systems (WECSs) should be able to control the power flow for limited as well as maximum power point tracking. Apart from tracking desired power, there are some other issues which needs to be addressed for stable and reliable operation of WECS in grid connected as well as islanded mode. In the grid connected mode synchronization of the system to the grid and maintenance of dc-link voltage in absence of ESS are the main control requirements apart from controlled power extraction from the wind turbine. Unlike the grid connected mode, where most of the system level dynamics are imposed by the grid and hence load voltage magnitude an frequency are dictated by the grid itself, in the autonomous operation of WECS the load voltage magnitude and frequency control comes in as additional control requirements other than controlled power extraction from Wind Turbine. However the usage of batteries in the system is unavoidable due to stability and reliability issues. In contrast to the traditional pitch angle control, this work focusses on field oriented speed control of permanent magnet synchronous generator (PMSG) for controlling the active power flow based on the wind turbine characteristics. A back to back AC/DC/AC topology is implemented for interfacing the WECS to the distribution network with various power electronic interfaces providing the necessary control over the power flow. By maintaining the dclink voltage constant and by deploying PLL, power balance and grid v synchronization are attained respectively in grid connected operation of WECS. For the standalone operation of WECS, however the ideology for controlled power extraction from WECS remains same but the load voltage magnitude and frequency control are attained by carrying out the analysis and design exercise in synchronously rotating reference frame so that linear control techniques can be employed easily and sinusoidal command following problem gets transformed to an equivalent dc command tracking thus yielding desired performance with zero steady state error. The motive behind using batteries in the system is to facilitate transient stability and enhance reliability. Proper decoupling and feed forward techniques have been deployed to eliminate crosscoupling and mitigate the effect of load side disturbances. Simulations are carried out under varying load demand as well as changing weather conditions to demonstrate the applicability and effectiveness of the proposed control strategies for grid connected as well as standalone WECSs. Overall, the project work involves study, design, modelling and simulation of grid connected as well as standalone Wind Energy Conversion System

Power Quality

Electrical power is becoming one of the most dominant factors in our society. Power generation, transmission, distribution and usage are undergoing signifi cant changes that will aff ect the electrical quality and performance needs of our 21st century industry. One major aspect of electrical power is its quality and stability – or so called Power Quality. The view on Power Quality did change over the past few years. It seems that Power Quality is becoming a more important term in the academic world dealing with electrical power, and it is becoming more visible in all areas of commerce and industry, because of the ever increasing industry automation using sensitive electrical equipment on one hand and due to the dramatic change of our global electrical infrastructure on the other. For the past century, grid stability was maintained with a limited amount of major generators that have a large amount of rotational inertia. And the rate of change of phase angle is slow. Unfortunately, this does not work anymore with renewable energy sources adding their share to the grid like wind turbines or PV modules. Although the basic idea to use renewable energies is great and will be our path into the next century, it comes with a curse for the power grid as power fl ow stability will suff er. It is not only the source side that is about to change. We have also seen signifi cant changes on the load side as well. Industry is using machines and electrical products such as AC drives or PLCs that are sensitive to the slightest change of power quality, and we at home use more and more electrical products with switching power supplies or starting to plug in our electric cars to charge batt eries. In addition, many of us have begun installing our own distributed generation systems on our rooft ops using the latest solar panels. So we did look for a way to address this severe impact on our distribution network. To match supply and demand, we are about to create a new, intelligent and self-healing electric power infrastructure. The Smart Grid. The basic idea is to maintain the necessary balance between generators and loads on a grid. In other words, to make sure we have a good grid balance at all times. But the key question that you should ask yourself is: Does it also improve Power Quality? Probably not! Further on, the way how Power Quality is measured is going to be changed. Traditionally, each country had its own Power Quality standards and defi ned its own power quality instrument requirements. But more and more international harmonization efforts can be seen. Such as IEC 61000-4-30, which is an excellent standard that ensures that all compliant power quality instruments, regardless of manufacturer, will produce of measurement instruments so that they can also be used in volume applications and even directly embedded into sensitive loads. But work still has to be done. We still use Power Quality standards that have been writt en decades ago and don’t match today’s technology any more, such as fl icker standards that use parameters that have been defi ned by the behavior of 60-watt incandescent light bulbs, which are becoming extinct. Almost all experts are in agreement - although we will see an improvement in metering and control of the power fl ow, Power Quality will suff er. This book will give an overview of how power quality might impact our lives today and tomorrow, introduce new ways to monitor power quality and inform us about interesting possibilities to mitigate power quality problems. Regardless of any enhancements of the power grid, “Power Quality is just compatibility” like my good old friend and teacher Alex McEachern used to say. Power Quality will always remain an economic compromise between supply and load. The power available on the grid must be suffi ciently clean for the loads to operate correctly, and the loads must be suffi ciently strong to tolerate normal disturbances on the grid

New Control Algorithms for the Robust Operation and Stabilization of Active Distribution Networks

The integration of renewable distributed generation units (DGs) alters distribution systems so that rather than having passive structures, with unidirectional power flow, they become active distribution networks (ADNs), with multi-directional power flow. While numerous technical, economic, and environmental benefits are associated with the shift toward ADNs, this transition also represents important control challenges from the perspective of both the supervisory and primary control of DGs. Voltage regulation is considered one of the main operational control challenges that accompany a high penetration of renewable DGs. The intermittent nature of renewable energy sources, such as wind and solar energy, can significantly change the voltage profile of the system and can interact negatively with conventional schemes for controlling on-load tap changers (OLTCs). Another factor is the growing penetration of plug-in electric vehicles (PEVs), which creates additional stress on voltage control devices due to their stochastic and concentrated power profiles. These combined generation and load power profiles can lead to overvoltages, undervoltages, increases in system losses, excessive tap operation, infeasible solutions (hunting) with respect to OLTCs, and/or limits on the penetration of either PEVs or DGs. With regard to the dynamic control level, DG interfaces are typically applied using power electronic converters, which lack physical inertia and are thus sensitive to variations and uncertainties in the system parameters. Grid impedance (or admittance), which has a substantial effect on the performance and stability of primary DG controllers, is nonlinear, time-varying, and not passive in nature. In addition, constant-power loads (CPLs), such as those interfaced through power electronic converters, are also characterized by inherited negative impedance that results in destabilizing effects, creating instability and damping issues. Motivated by these challenges, the research presented in this thesis was conducted with the primary goal of proposing new control algorithms for both the supervisory and primary control of DGs, and ultimately of developing robust and stable ADNs. Achieve this objective entailed the completion of four studies: Study#1: Development of a coordinated fuzzy-based voltage regulation scheme with reduced communication requirements Study#2: Integration of PEVs into the voltage regulation scheme through the implementation of a vehicle-to-grid reactive power (V2GQ) support strategy Study#3: Creation of an estimation tool for multivariable grid admittance that can be used to develop adaptive controllers for DGs Study#4: Development of self-tuning primary DG controllers based on the estimated grid admittance so that stable performance is guaranteed under time-varying DG operating points (dispatched by the schemes developed in Study#1 and Study#2) and under changing grid impedance (created by network reconfiguration and load variations). As the first research component, a coordinated fuzzy-based voltage regulation scheme for OLTCs and DGs has been proposed. The primary reason for applying fuzzy logic is that it provides the ability to address the challenges associated with imperfect information environments, and can thus reduce communication requirements. The proposed regulation scheme consists of three fuzzy-based control algorithms. The first control algorithm was designed to enable the OLTC to mitigate the effects of DGs on the voltage profile. The second algorithm was created to provide reactive power sharing among DGs, which will relax OLTC tap operation. The third algorithm is aimed at partially curtailing active power levels in DGs so as to restore a feasible solution that will satisfy OLTC requirements. The proposed fuzzy algorithms offer the advantage of effective voltage regulation with relaxed tap operation and with utilization of only the estimated minimum and maximum system voltages. Because no optimization algorithm is required, it also avoids the numerical instability and convergence problems associated with centralized approaches. OPAL real-time simulators (RTS) were employed to run test simulations in order to demonstrate the success of the proposed fuzzy algorithms in a typical distribution network. The second element, a V2GQ strategy, has been developed as a means of offering optimal coordinated voltage regulation in distribution networks with high DG and PEV penetration. The proposed algorithm employs PEVs, DGs, and OLTCs in order to satisfy the PEV charging demand and grid voltage requirements while maintaining relaxed tap operation and minimum curtailment of DG active power. The voltage regulation problem is formulated as nonlinear programming and consists of three consecutive stages, with each successive stage applying the output from the preceding stage as constraints. The task of the first stage is to maximize the energy delivered to PEVs in order to ensure PEV owner satisfaction. The second stage maximizes the active power extracted from the DGs, and the third stage minimizes any deviation of the voltage from its nominal value through the use of available PEV and DG reactive power. The primary implicit objective of the third stage problem is the relaxation of OLTC tap operation. This objective is addressed by replacing conventional OLTC control with a proposed centralized controller that utilizes the output of the third stage to set its tap position. The effectiveness of the proposed algorithm in a typical distribution network has been validated in real time using an OPAL RTS in a hardware-in-the loop (HiL) application. The third part of the research has resulted in the proposal of a new multivariable grid admittance identification algorithm with adaptive model order selection as an ancillary function to be applied in inverter-based DG controllers. Cross-coupling between the and grid admittance necessitates multivariable estimation. To ensure persistence of excitation (PE) for the grid admittance, sensitivity analysis is first employed as a means of determining the injection of controlled voltage pulses by the DG. Grid admittance is then estimated based on the processing of the extracted grid dynamics by the refined instrumental variable for continuous-time identification (RIVC) algorithm. Unlike nonparametric identification algorithms, the proposed RIVC algorithm provides a parametric multivariable model of grid admittance, which is essential for designing adaptive controllers for DGs. HiL applications using OPAL RTS have been utilized for validating the proposed algorithm for both grid-connected and isolated ADNs. The final section of the research is a proposed adaptive control algorithm for optimally reshaping DG output impedance so that system damping and bandwidth are maximized. Such adaptation is essential for managing variations in grid impedance and changes in DG operating conditions. The proposed algorithm is generic so that it can be applied for both grid-connected and islanded DGs. It involves three design stages. First, the multivariable DG output impedance is derived mathematically and verified using a frequency sweep identification method. The grid impedance is also estimated so that the impedance stability criteria can be formulated. In the second stage, multi-objective programming is formulated using the -constraint method in order to maximize system damping and bandwidth. As a final stage, the solutions provided by the optimization stage are employed for training an adaptation scheme based on a neural network (NN) that tunes the DG control parameters online. The proposed algorithm has been validated in both grid-connected and isolated distribution networks, with the use of OPAL RTS and HiL applications.1 yea

Analysis and Modeling of Advanced Power Control and Protection Requirements for Integrating Renewable Energy Sources in Smart Grid,

Attempts to reduce greenhouse gas emissions are promising with the recent dramatic increase of installed renewable energy sources (RES) capacity. Integration of large intermittent renewable resources affects smart grid systems in several significant ways, such as transient and voltage stability, existing protection scheme, and power leveling and energy balancing. To protect the grid from threats related to these issues, utilities impose rigorous technical requirements, more importantly, focusing on fault ride through requirements and active/reactive power responses following disturbances. This dissertation is aimed at developing and verifying the advanced and algorithmic methods for specification of protection schemes, reactive power capability and power control requirements for interconnection of the RESs to the smart grid systems. The first findings of this dissertation verified that the integration of large RESs become more promising from the energy-saving, and downsizing perspective by introducing a resistive superconducting fault current limiter (SFCL) as a self-healing equipment. The proposed SFCL decreased the activation of the conventional control scheme for the wind power plant (WPP), such as dc braking chopper and fast pitch angle control systems, thereby increased the reliability of the system. A static synchronous compensator (STATCOM) has been proposed to assist with the uninterrupted operation of the doubly-fed induction generators (DFIGs)-based WTs during grid disturbances. The key motivation of this study was to design a new computational intelligence technique based on a multi-objective optimization problem (MOP), for the online coordinated reactive power control between the DFIG and the STATCOM in order to improve the low voltage ride-through (LVRT) capability of the WT during the fault, and to smooth low-frequency oscillations of the active power during the recovery. Furthermore, the application of a three-phase single-stage module-integrated converter (MIC) incorporated into a grid-tied photovoltaic (PV) system was investigated in this dissertation. A new current control scheme based on multivariable PI controller, with a faster dynamic and superior axis decoupling capability compared with the conventional PI control method, was developed and experimentally evaluated for three-phase PV MIC system. Finally, a study was conducted based on the framework of stochastic game theory to enable a power system to dynamically survive concurrent severe multi-failure events, before such failures turn into a full blown cascading failure. This effort provides reliable strategies in the form of insightful guidelines on how to deploy limited budgets for protecting critical components of the smart grid systems

Model Predictive Control Based Wind-Solar Hybrid Energy Conversion System

Presently a lot of work is being carried in the field of distributed renewable generation. Many distributed generation systems are being designed and connected to the electric grid. At the time when the conventional sources of energy such as coal, oil, gas etc. are fast disappearing, a study of distributed renewable generation systems becomes very important