Mohammad N. Marwali

Control of distributed generation systems - Part II: Load sharing control

This work is concerned with the control strategy for the parallel operation of distributed generation systems (DGS) in a standalone ac power supply. The proposed control method uses only low-bandwidth data communication signals between each generation system in addition to the locally measurable feedback signals. This is achieved by combining two control methods: droop control method and average power control method. The average power method with slow update rate is used in order to overcome the sensitivity about voltage and current measurement errors. In addition, a harmonic droop scheme for sharing harmonic content of the load currents is proposed based on the voltages and currents control algorithm. Experimental and simulation studies using two parallel three-phase pulsewidth modulation (PWM) inverters are presented to show the effectiveness of the proposed control.

Power Flow Control of a Single Distributed Generation Unit

This research addresses power flow control problem of a grid-connected inverter in distributed generation applications. A real and reactive power control solution is proposed on the basis of an existing voltage control strategy developed for island operations. The power control solution takes advantage of a newly designed system parameter identification method and a nonlinear feedforward algorithm, both of which are based on Newton-Raphson iteration method and implemented in real time. The proposed power control solution also performs grid-line current conditioning and yields harmonic free grid-line current. A phase locked loop based algorithm is developed as a part of the solution to handle possible harmonic distorted grid-line voltage and maintain harmonic free line current. The effectiveness of the proposed techniques is demonstrated by both simulation and experimental results.

Integration of green and renewable energy in electric power systems

PREFACE. ACKNOWLEDGMENTS. 1 SMART GRID DISTRIBUTED GENERATION SYSTEMS. 2 INVERTER CONTROL VOLTAGE AND CURRENT IN DISTRIBUTED GENERATION SYSTEMS. 3 PARALLEL OPERATION OF INVERTERS IN DISTRIBUTED GENERATION SYSTEMS. 4 POWER CONVERTER TOPOLOGIES FOR DISTRIBUTED GENERATION SYSTEMS. 5 VOLTAGE AND CURRENT CONTROL OF A THREE-PHASE FOUR-WIRE DISTRIBUTED GENERATION (DG) INVERTER IN ISLAND MODE. 6 POWER FLOW CONTROL OF A SINGLE DISTRIBUTED GENERATION UNIT. 7 ROBUST STABILITY ANALYSIS OF VOLTAGE AND CURRENT CONTROL FOR DISTRIBUTED GENERATION SYSTEMS. 8 PWM RECTIFIER CONTROL FOR THREE-PHASE DISTRIBUTED GENERATION SYSTEM. 9 MATLAB SIMULINK SIMULATION TESTBED. APPENDIX A: SIMULINK MODEL DSIMSERVO.MDL. APPENDIX B: FILE SSMODE.M. BIBLIOGRAPHY. INDEX.

Stability Analysis of Load Sharing Control for Distributed Generation Systems

This paper discusses the small-signal stability analysis of the combined droop and average power method for load sharing control of multiple distributed generation systems (DGS) in a stand-alone ac supply mode. The small-signal model is developed and its accuracy is verified from simulations of the original nonlinear model. It is shown that the small-signal model accurately predicts the stability of the parallel systems.

A Three-Phase Four-Wire Inverter Control Technique for a Single Distributed Generation Unit in Island Mode

A control technique is developed for a three-phase four-wire split DC bus inverter of a single distributed generation unit working in island mode. The control technique combines an inner discrete-time sliding mode controlled (DSMC) current loop and an outer robust servomechanism controlled voltage loop. The control algorithms are developed under stationary alphabeta0 (Clarkes) reference frame and a modified space vector pulsewidth modulation (MSVPWM) is proposed to implement the algorithm under Clarkes reference frame. The proposed technique achieves voltage regulation with low steady state error and low total harmonic distortion and fast transient response under various load disturbances. Meanwhile the usage of MSVPWM in a stationary alphabeta0 reference frame yields better transient performance under limited DC bus voltage compared to conventional uniformly sampled sine wave modulation in ABC reference frame. In this paper, besides the development and description of the algorithms, a series of discussions, analysis and studies are performed on the proposed control technique, including the L-C filter design issue, frequency domain closed-current-loop and closed-voltage-loop responses, and time domain simulations and experiments under various load conditions. All the analysis, simulations, and experiments demonstrate the effectiveness of the proposed control solution.

Robust stability analysis of voltage and current control for distributed generation systems

This paper analyzes the robust stability of a voltage and current control solution for a stand-alone distributed generation (DG) unit using the structured singular value or /spl mu/-framework-based method. The voltage and current control solution consists of a discrete-time sliding mode current controller and a perfect robust servomechanism problem voltage controller, which has been shown effective in transient response and harmonic minimization in previous study without considering parametric uncertainty. The stability robustness of the system and its transient performance are investigated under various tuning parameters of the controller. The analysis results presented in this paper demonstrate that the controller parameters can be tuned and verified to satisfy a certain transient performance requirement and at the same time guarantee robust stability under system parameter uncertainties and load variations.

An Integrated Virtual Learning System for the Development of Motor Drive Systems

This paper describes the basic concept of an integrated virtual learning system as an instructional tool for the development of DSP-based control schemes for motor drive applications. The system is comprised of a graphical user interface (GUI) front-end and a hardware-inthe-loop custom digital signal processor (DSP) for rapid prototyping and efficient testing of digital control algorithms. The circuit design and control algorithm development of a pulse width modulation (PWM) voltage source inverter (VSI) for three-phase brushless dc (BLDC) motor control applications is used to describe the functions of the system. It is shown that the virtual learning system provides a cost-effective learning tool for students or engineers in training and can serve as a supplement to a conventional laboratory based system.

Power flow control of a single distributed generation unit with nonlinear local load

Distributed generation units with small energy sources, such as fuel cells, micro-turbines, and photovoltaic devices, can be connected to utility grid as alternative energy sources besides providing power to their local loads. The distributed generation units are interfaced with utility grid using three phase inverters. With inverter control, both active and reactive power pumped into the utility grid from the distributed generation units can be controlled. Reactive power flow control allows the distributed generation units to be used as static VAr compensation units besides energy sources. This work presents a distributed generation unit control technique which provides robust voltage regulation with harmonic elimination under island running mode and decoupled active and reactive power flow control under grid-connected mode. The control technique, which combines discrete-time sliding mode current control, robust servomechanism voltage control, and integral power control, enables seamless switching between island mode and grid-connected mode and guarantees sinusoidal line current waveform such nonlinear local load. The P Q coupling issue is addressed and the stability of the power control loop is proved using Lyapunov direct method.

A PWM rectifier control technique for three-phase double conversion UPS under unbalanced load

PWM rectifiers are widely used in three-phase ac-dc-ac systems due to its capability in dc voltage boost and regulation, input power factor correction, and input current harmonic control. However, with the conventional rectifier control technique, the input current tends to be unbalanced under unbalanced inverter load, which contaminates input power source and is therefore undesirable. In this paper, the cause of the unbalancing is disclosed by evaluating the spectra of the switching functions of the full bridge three-phase inverter analytically using Bessel function under standard space vector PWM switching scheme, which relates the dc link current and voltage ripples to the inverter load balancing. The analysis shows that the dc link voltage contains significant second order harmonic component which affects the voltage loop of the rectifier controller, especially when the control gain is high. A notch filter based voltage control loop is proposed to eliminate the second harmonic component in the dc-link voltage feedback signal and achieve balanced three-phase input currents. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed control technique in decoupling the rectifier and the inverter under unbalanced load.

Implementation of indirect vector control on an integrated digital signal processor-based system

This paper presents an implementation of indirect vector control of an induction machine on an integrated DSP (digital signal processor) system manufactured by dSPACE GmbH. The system integrates into a single board the computational power of a TMS320C31 DSP with extra peripherals needed in vector control application, and therefore requires minimal hardware development. The induction machine parameters required for the vector control operation are obtained through an offline parameters identification using the maximum likelihood estimation technique with DC voltage source excitation. It is shown through extensive experimental study that the offline identified parameters yield in a reliable field orientation of the induction machine.