Ali Keyhani

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.

Identification of Photovoltaic Source Models

This paper presents a photovoltaic (PV) model estimation method from the PV module data. The model is based on a single-diode model of a PV cell. The cell model of a module is extended to estimate the parameters of arrays. The variation of the parameters with change in temperature and irradiance is also studied. Finally, estimation of the maximum power point from the estimated parameters under varying environmental conditions is presented.

Analytical model for permanent magnet motors with surface mounted magnets

This paper presents an analytical method of modeling permanent magnet (PM) motors. The model is dependent only on geometrical and materials data which makes it suitable for insertion into design programs, avoiding long finite element analysis (FEA) calculations. The modeling procedure is based on the calculation of the air gap field density waveform at every time instant. The waveform is the solution of the Laplacian/quasi-Poissonian field equations in polar coordinates in the air gap and takes into account slotting. The model allows the rated performance calculation but also such effects as cogging torque, ripple torque, back-EMF form prediction, some of which are neglected in commonly used analytical models.

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.

Sliding-Mode Flux Observer With Online Rotor Parameter Estimation for Induction Motors

Field orientation techniques without flux measurements depend on the parameters of the motor, particularly on the rotor resistance or rotor time constant (for rotor field orientation). Since these parameters change continuously as a function of temperature, it is important that the value of rotor resistance is continuously estimated online. A fourth-order sliding-mode flux observer is developed in this paper. Two sliding surfaces representing combinations of estimated flux and current errors are used to enforce the flux and current estimates to their real values. Switching functions are used to drive the sliding surfaces to zero. The equivalent values of the switching functions (low-frequency components) are proven to be the rotor resistance and the inverse of the rotor time constant. This property is used to simultaneously estimate the rotor resistance and the inverse of the time constant without prior knowledge of either the rotor resistance or the magnetizing inductance. Simulations and experimental results prove the validity of the proposed approach

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.

Estimation of induction machine parameters from standstill time domain data

A step-by-step approach to identifying the parameters of an induction machine from standstill time-domain test data is presented. A step voltage disturbance is applied across the stator terminals while the machine is in standstill condition, and the resulting stator voltage and current responses are measured. The initialization of the transfer function model parameters is achieved by the Laplace transformation of the recorded time-response data, and the maximum likelihood method is used for both the transfer function and the equivalent circuit model parameter estimation. Based on the identified models, simulation studies are performed and compared to the measured standstill time-domain response, the standstill frequency response, and the free acceleration response to validate the identified models. A three-phase wound rotor induction machine rated at 5 hp and 220 V was tested at standstill, and its parameters were estimated.<<ETX>>

Control of a Fuel Cell Based Z-Source Converter

This paper presents system modeling, modified space vector PWM implementation (MSVPWM), and control system design of a Z-source converter. For an electrical analysis of the fuel cell powered systems, the fuel cell system is modeled by an R-C circuit including its voltage-current polarization characteristics. A discrete-time state space equation is given to implement digital control and a space vector pulse-width modulation technique is modified to realize the shoot-through zero vectors that boost the dc-link voltage. Three discrete-time controllers and an asymptotic observer are designed.

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.