Decoupled dq-Current Control of Grid-Tied Voltage Source Converters Using Nonparametric Models

This paper presents a vector control strategy for regulating the current of grid-tied voltage source converters (VSCs) in a rotating reference frame. The proposed approach is based on shaping the open-loop and closed-loop transfer matrices of the system. Solving a constrained convex optimization problem, the shaping is achieved, which guarantees the stability of the closed-loop system. The designed controller results in the desired dynamic performance and decouples the direct and quadrature (dq) current axes. The structure of the proposed controller is similar to that of its predecessors and consists of four proportional-integral controllers. The performance of the method is evaluated based on simulation and experimental results. It is confirmed that its dynamic performance is better than that of the previously proposed approaches, and it results in the decoupled current axes

Disturbance-Observer-Based PBC for Static Synchronous Compensator under System Disturbances

© 2019 IEEE. Passivity-based control (PBC) relies on an accurate mathematical model and thus its performance will be degraded by the weak robustness against parameters uncertainties, modeling error, and external disturbances. Moreover, it cannot achieve zero tracking error of the steady-state current under parameter uncertainties and modeling error. This paper proposes a novel disturbance-observer-(DO) based PBC (DO-PBC) for static synchronous compensator (STATCOM) to achieve better stability and dynamic performances against disturbances. A DO that has been introduced into the PBC current loop is used to compensate system disturbances, which can improve the robustness of the control system and eliminate the steady-state tracking error. Moreover, the proposed DO-PBC provides faster responses in handling various kinds of disturbances. Then, the detail design process, stability and robustness analysis, and parameters tuning method are investigated and presented. Also, the proposed method is simple to be implemented by the separation principle. The performance comparisons among the proportional integral, the conventional PBC, and the proposed DO-PBC are carried out to show the effectiveness of the proposed method against disturbances and the precise current tracking, via simulation tests and experimental tests based on a down-scale laboratory prototype experiment of 380 V STATCOM

Real-time modeling and simulation of distribution feeder and distributed resources

Includes bibliographical references.2015 Fall.The analysis of the electrical system dates back to the days when analog network analyzers were used. With the advent of digital computers, many programs were written for power-flow and short circuit analysis for the improvement of the electrical system. Real-time computer simulations can answer many what-if scenarios in the existing or the proposed power system. In this thesis, the standard IEEE 13-Node distribution feeder is developed and validated on a real-time platform OPAL-RT™. The concept and the challenges of the real-time simulation are studied and addressed. Distributed energy resources include some of the commonly used distributed generation and storage devices like diesel engine, solar photovoltaic array, and battery storage system are modeled and simulated on a real-time platform. A microgrid encompasses a portion of an electric power distribution which is located downstream of the distribution substation. Normally, the microgrid operates in paralleled mode with the grid; however, scheduled or forced isolation can take place. In such conditions, the microgrid must have the ability to operate stably and autonomously. The microgrid can operate in grid connected and islanded mode, both the operating modes are studied in the last chapter. Towards the end, a simple microgrid controller modeled and simulated on the real-time platform is developed for energy management and protection for the microgrid

Analysis and simulation of a MMCC-SSBC converter

Battery energy storage systems (BESS) are the most versatile type of energy storage. With an increasing share of renewable energy, they could prove to be essential to provide the much needed flexibility. The MMCC-SSBC might be the most suitable converter for modern BESS. It is modular, and allows for an individualized treatment of the connected battery modules. The main objective of this thesis is to develop a tool which simulates the behavior of a MMCC-SSBC converter. This objective is fulfilled by the core deliverable of the thesis: a Matlab implementation of a dynamic model of the converter. As a secondary objective, this thesis aims to demonstrate the usefulness of this tool. It applies the tool to a specific use case, and analyzes three key characteristics based on the simulations: efficiency, power quality and reliability. This leads to some concise design guidelines, and continued operation under a battery short-circuit fault

Operation of Grid-Connected Inverter under Unbalanced Grid Conditions Using Indirect Voltage Sensoring

Abstract The grid connected voltage source inverter is now the most widely used interface for connecting renewable power generation to the grid. Control of this device is a key aspect to ensure the performance, reliability and life span of the renewable power generation system. Conventionally, the current control of the grid connected inverter is based on the measured grid side voltage. The power and the power factor at the receiving end, which is usually defined as the point of common coupling, can be controlled accurately. This controller topology has been widely used and many control methods have been developed aiming at objectives such as increasing system stability, decreasing harmonic injection, and improving transient response of the system. However, in case of the voltage measurement is not available, i.e. a faulty voltage sensor, the conventional current control topology will be disabled for lack of information of the grid voltage. This would decrease the reliability and efficiency of the system thus should be improved. voltage-sensor-less In this research, a current control system for the grid connected inverter system not relying on the information provided by the a.c. side voltage sensors will be developed with compliance to the recommendations issued to the performances of the distribution generations such as the harmonic limitations and the fault-ride-through capabilities. Three problem will be addressed and solved. Firstly, the a.c. side voltage should be acquired without the use of a.c. side voltage sensors. This is achieved by adopting an a.c. side voltage estimation algorithm. Secondly, the grid connected inverter should be able to start-up without synchronising to the grid while keep the current injected in a safe range. This is achieved by the newly designed start-up process. Thirdly, the grid connected inverter should be able to ride-through grid faults and providing support to the grid. The transient response of the grid connected inverter is the key measure to define the performance. In this study, a faster symmetrical component decomposition method is proposed to improve the transient response of the current control, without relying on grid voltage sensors. The proposed system is verified by both simulation and experimental tests, with analyses and insight aiming at general applications of the proposed method and algorithms

Frequency-Domain Control Design in Power Systems

The scope of this thesis encompasses two main subjects: fixed-structure data-driven control design on one side, and control design in power systems on the other. The overall goal is to identify challenging and relevant problems in power systems, to express them as rigorous specifications from the viewpoint of control systems, and to solve them by developing and applying advanced methods in robust control. This work aims to combine expertise from both fields to open up a holistic perspective and bridge the gap between control and power systems. First, the derivation of a novel fixed-structure, data-driven frequency-domain control design method for multivariable systems is described. A key feature of the method is that only the frequency response of the plant is required for the design, and no parametric model is required. The designed controllers are fully parametrized in terms of matrix polynomial functions and can take a centralized, decentralized or distributed structure. The controller performance is formulated as H_2 and H_infinity constraints on any loop transfer function. A convex formulation of the optimization problem is derived, and it is shown that the solution converges to a locally optimal solution of the original problem. The versatility of the design method is demonstrated in various simulation examples, as well as in experiments on two electromechanical setups. Next, a frequency-domain modeling approach for power grids is discussed. A model based on dynamic phasors is developed that represents the electromagnetic and electromechanic dynamics of lines, inverters, synchronous machines and constant power loads. It also offers a modular structure that makes it straightforward to combine white-, grey- and blackbox models in a single framework. Then, the control design method and dynamic phasor model are applied in two relevant power systems case studies. First, the design of a decentralized current controller for parallel, grid-connected voltage source inverters in a typical distribution grid is considered. It is shown how performance specifications can be formulated as frequency-domain constraints in order to attenuate the resonances introduced by the output filters and coupling effects, and to provide robustness against model uncertainties and grid topology changes. The controllers for all VSIs are designed in a single step, and stability and performance is guaranteed by design. Furthermore, an approach for plug-and-play control design is presented. The results are validated in numerical simulation as well as in power-hardware-in-the-loop experiments. The second study concerns the design of a distributed controller that combines primary and secondary frequency and voltage control for an islanded, meshed low-voltage grid with any number of voltage source inverters and synchronous generators in a single framework. No assumption on the R/X-ratio of the lines is made, and it is shown how advanced control specifications such as proportional active power sharing, zero frequency steady-state error and decoupling can be formulated as constraints on the norm of weighted sensitivity functions. Furthermore, the communication delays of the distributed controller are considered exactly during the design. The controller is implemented in numerical simulation, and results show significantly improved performance as compared to the classical hierarchical structure

A Linear Parameter Varying Controller for Grid-tied Converters under Unbalanced Voltage Network Conditions

This thesis focuses on the development and practical assessment of a contemporary Linear Parameter Varying (LPV) controller for grid-tied converters. The increasing popularity of renewable energy resources necessitates intelligent power converters to interface with utility network. The proposed control methodology can effectively regulate converter powers/currents under highly unbalanced voltage conditions. The methodology can be easily applied to rotating electrical machines that have similar dynamic models. A LPV model of grid-tied converter with filters are derived in synchronous positive and negative rotating frames and a detailed controller design procedure is then carried out using Matrix Linear Inequality technique. The proposed controller uses network frequency as a reference and it has the capability to handle the system frequency variations. Off-line controller design stage is computed by Matlab software while on-line controller calculations are dealt by a Digital Signal Processor (DSP). The highly distorted voltage at the point of common coupling between Voltage Source Inverter (VSI) and utility network may degrade the outputs of the phase locked loop (PLL) module and overall controller performance. An enhanced version of PLL technique is proposed to overcome the voltage distortions and a significant reduction of Total Harmonic Distortion has been recorded. The harmonic issue is successfully treated further with an additional harmonic observer supporting the main controller. To verify the proposed control approach, studies are carried out using Matlab/SIMULINK platform with the code-based simulation. This simulation method can ensure the results close to a real DSP system and enables the user to transfer the simulation studies effectively to the experimental setup without major modifications. A prototype of a three phase VSI with a DSP controller is then investigated using dSPACE DS1104 development board. Experimental results from this system validate the proposed control technique and its benefits