Robust Control of Wide Bandgap Power Electronics Device Enabled Smart Grid

abstract: In recent years, wide bandgap (WBG) devices enable power converters with higher power density and higher efficiency. On the other hand, smart grid technologies are getting mature due to new battery technology and computer technology. In the near future, the two technologies will form the next generation of smart grid enabled by WBG devices. This dissertation deals with two applications: silicon carbide (SiC) device used for medium voltage level interface (7.2 kV to 240 V) and gallium nitride (GaN) device used for low voltage level interface (240 V/120 V). A 20 kW solid state transformer (SST) is designed with 6 kHz switching frequency SiC rectifier. Then three robust control design methods are proposed for each of its smart grid operation modes. In grid connected mode, a new LCL filter design method is proposed considering grid voltage THD, grid current THD and current regulation loop robust stability with respect to the grid impedance change. In grid islanded mode, µ synthesis method combined with variable structure control is used to design a robust controller for grid voltage regulation. For grid emergency mode, multivariable controller designed using H infinity synthesis method is proposed for accurate power sharing. Controller-hardware-in-the-loop (CHIL) testbed considering 7-SST system is setup with Real Time Digital Simulator (RTDS). The real TMS320F28335 DSP and Spartan 6 FPGA control board is used to interface a switching model SST in RTDS. And the proposed control methods are tested. For low voltage level application, a 3.3 kW smart grid hardware is built with 3 GaN inverters. The inverters are designed with the GaN device characterized using the proposed multi-function double pulse tester. The inverter is controlled by onboard TMS320F28379D dual core DSP with 200 kHz sampling frequency. Each inverter is tested to process 2.2 kW power with overall efficiency of 96.5 % at room temperature. The smart grid monitor system and fault interrupt devices (FID) based on Arduino Mega2560 are built and tested. The smart grid cooperates with GaN inverters through CAN bus communication. At last, the three GaN inverters smart grid achieved the function of grid connected to islanded mode smooth transitionDissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Control of an active rectifier with an inductive-capacitive-inductive filter using a Twisting based algorithm

This paper presents a novel controller for an active rectifier with an inductive-capacitive-inductive filter. The proposed control scheme comprises two levels. The internal level, is a current controller based on the second order sliding mode Twisting algorithm, which robustly ensures an unity power factor at the connection point. The slower external level is a PI controller in charge of regulating the output DC voltage to a desired reference. The controller setup also includes a finite-time observer of the current derivative that can be used to avoid direct (and sometimes problematic) numerical differentiation. Finally, simulation results are presented to validate the proposed control scheme.Instituto de Investigaciones en Electrónica, Control y Procesamiento de SeñalesConsejo Nacional de Investigaciones Científicas y Técnica

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

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

Comprehensive analysis and comparison of digital current control techniques for active rectifiers

This paper presents a comprehensive analysis and comparison of digital current control techniques for active rectifiers. These rectifiers are connect-ed to the power grid and are controlled aiming to obtain sinusoidal grid currents and unitary power factor. In this context this paper presents the principle of operation of a full-bridge full-controlled active rectifier, which is controlled by different digital current control techniques, namely, proportional-integral (PI) in stationary frame, PI in synchronous frame, PI sinewave (PIS), feedforward, sliding mode, and predictive. These digital current control techniques are explained in detail and is established a comparison in terms of their current errors in steady-state, as well as in terms of their digital implementation using the digital signal processor (DSP) TMS320F28335 from Texas Instruments.FCTCOMPETE: POCI-01-0145-FEDER-00704

Model-based active damping control for three-phase voltage source inverters with LCL filter

(c) 2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.This paper presents a robust model-based active damping control in natural frame for a three-phase voltage source inverter with LCL filter. The presence of the LCL filter complicates the design of the control scheme, particularly when system parameters deviations are considered. The proposed control method is addressed to overcome such difficulties and uses a modified converter model in an state observer. In this proposal, the converter model is modified by introducing a virtual damping resistor. Then, a Kalman filter makes use of this model to estimate the system state-space variables. Although the state estimates do not obviously match the real world system variables, they permit designing three current sliding-mode controllers that provide the following features to the closed loop system: a) robust ande active damping capability like in the case of using a physical damping resistor, b) robustness because the control specifications are met independently of variation in the system parameters, c) noise immunity due to the application of the Kalman filter, and d) power loss minimization because the system losses caused by the physical damping resistor are avoided. An interesting side effect of the proposed control scheme is that the sliding surfaces for each controller are independent. This decoupling property for the three controllers allows using a fixed switching frequency algorithm that ensures perfect current control. To complete the control scheme, a theoretical stability analysis is developed. Finally, selected experimental results validate the proposed control strategy and permit illustrating all its appealing features.Peer ReviewedPostprint (author's final draft

A Novel Control Method For Grid Side Inverters Under Generalized Unbalanced Operating Conditions

This thesis provides a summary on renewable energy sources integration into the grid, using an inverter, along with a comprehensive literature research on variety of available control methods. A new generalized method for grid side inverter control under unbalanced operating conditions is also proposed. The presented control method provides complete harmonic elimination in line currents and DC link voltage with adjustable power factor. The method is general, and can be used for all levels of imbalance in grid voltages and line impedances. The control algorithm proposed in this work has been implemented by using MATLAB Simulink and dSPACE RT1104 control system. Simulation and experimental results presented in this thesis are in excellent agreement

Sliding-mode and proportional-resonant based control strategy for three-phase two-leg T-type grid-connected inverters with LCL filter

In this study, sliding-mode and proportional-resonant based control strategy is proposed for three-phase two-leg T-type grid-connected inverter with LCL filter. The sliding surface function is formed by using the inverter current and capacitor voltage errors. When the inverter current and capacitor voltage feedbacks are included into the control loop, the active damping requirement is automatically resolved. The PR controllers are employed in cascaded manner to generate the references for inverter current and capacitor voltage. The use of PR controllers ensures zero steady-state error in the inverter current, capacitor voltage and grid current. In addition, since the proposed three-phase inverter has only two legs, the total switch count is reduced resulting in cheaper and reliable topology. The proposed system is validated through computer simulations which show that proposed control algorithm can achieve the control of grid currents. The total harmonic distortion level of the grid currents is in the limits of international standards

DFIG Based Wind Turbine System For Clemson Micro-grid

As an important part of the smart grid, the micro-grid interfaces with distributed energy sources, loads and control devices. A doubly fed induction generator (DFIG) based wind turbine (WT) is the main power source of the presented project. The DFIG system is connected to the three phase AC grid via back-to-back power converter and an LCL filter. Decoupled q-d control strategies are investigated for the DFIG system. Matlab/Simulink results will show the performance of the proposed system. Hardware validation results are also presented and discussed. As a rapidly increasing research interest area the dc micro-grid has been extensively investigated. A topology is proposed to connect the DFIG based WT system to a dc link using a diode bridge and a three phase power converter. The rotor side of the DFIG is connected to the dc link through a converter while the stator is connecting to a three phase diode bridge with the dc side connected to a dc link. The control method is developed to regulate the stator frequency and the d-q axis voltage of the diode bridge to operate the DFIG at a desired stator frequency and generate the required power. Undesired harmonics in the three phase system will lead to excessive THD, a decrease the power quality and an increase the power loss of the system. An novel methods to compensate the current harmonics by controlling the power converter of the DFIG system is also proposed. With the DFIG connected to the three phase AC gird, the focus has been put into a scenario: a nonlinear load connected to the same node of the DFIG point of common coupling (PCC) to the gird, to draw the harmonics to the system. In the proposed dc link system, the diode bridge will introduce harmonics to the stator current of the DFIG. In both cases, the selected low-order harmonics are detected and calculated by a multiple reference frame estimator. The control methods of how to regulate the harmonics are developed for both the grid-side converter and the rotor-side converter based on multiple reference frame theory. A hybrid state observer for speed-sensorless motor drives of induction machines is also proposed. The hybrid observer comprises of a Luenberger observer and a sliding mode observer. For a conventional induction motor with shorted rotor, the stator currents and rotor flux linkages are estimating following a Luenberger observer. While, for a DFIG the similar approach will apply to the stator currents and rotor currents. The rotor speed is estimated using a sliding mode observer. The combination of two observers takes advantage of both approaches. The Luenberger observer is easy to realize and the computational burden is small. The sliding mode observer is known for its robustness with respect to model parameter errors and it will also provide a fast convergence rate. The chattering of the sliding mode observer is addressed by applying a boundary layer