Subbaraya Yuvarajan

Control of DFIG-Based Wind Generation to Improve Interarea Oscillation Damping

Power systems with high penetration of wind power usually require long-distance transmission to export wind power to the market. Interarea oscillation is an issue faced in long-distance transmission. Can wind generation based on doubly fed induction generator (DFIG) help to damp oscillations and how? In this paper, a control scheme is developed for the DFIG with rotor-side converter to damp interarea oscillations. The DFIG is modeled in MATLABreg/Simulink utilizing its vector control scheme feature, and inner current control and outer active/reactive power control loops are modeled and designed. A two-area system that suffers from poor interarea oscillation damping along with a wind farm in the area that exports power is investigated. A damping controller is designed and time-domain simulations are used to demonstrate the effectiveness of the controller. The major contributions of the paper are as follows: 1) built a wind farm interarea oscillation study system based on the classical two-area four-machine system, 2) established that in vector control scheme, active power modulation can best help to damp oscillations, 3) successfully designed a feedback controller using remote signals with good interarea oscillation observability.

Quasi-Active Power Factor Correction Circuit for HB LED Driver

High brightness light emitting diodes (HB LEDs) are likely to be used for general lighting applications due to their high efficiency and longer life. The paper presents a quasi-active power factor corrector (PFC) for driving a string of HB LEDs. The single-stage PFC circuit has a high efficiency, and it does not increase the voltage/current stress on the active switch used in the switching converter due to PFC. The circuit has two operating modes based on the input voltage level and its features, like power factor correction and power balance, are explained. The experimental results obtained on a prototype converter along with waveforms are presented.

Harmonic Analysis of a DFIG for a Wind Energy Conversion System

This paper develops a framework for analysis of harmonics in a doubly fed induction generator (DFIG) caused by nonsinusoidal conditions in rotor and unbalance in stator. Nonsinusoidal rotor voltages are decomposed into harmonic components and their corresponding sequences are identified. Induced harmonics in stator are analyzed and computed, from which the torques produced by these interactions between stator and rotor harmonic components can be found. During unbalanced stator conditions, symmetric component theory is applied to the stator voltage to get positive-, negative-, and zero-sequence components of stator and rotor currents. The steady-state negative-sequence equivalent circuit for a DFIG is derived based on the reference frame theory. Harmonic currents in the rotor are computed based on the sequence circuits. In both scenarios, the harmonic components of the electromagnetic torque are calculated from the interactions of the harmonic components of the stator and rotor currents. Three case studies are considered, namely: 1) nonsinusoidal rotor injection; 2) an isolated unbalanced stator load scenario; and 3) unbalanced grid-connected operation. The analysis is verified with results from numerical simulations in Matlab/Simulink. For illustration, the second case is verified using experiments. The simulation results and experimental results agree well with the results from analysis.

Photo-voltaic power converter with a simple maximum-power-point-tracker

A simple power conditioner to convert the power available from solar panels for feeding into 60 Hz AC mains is described. The output from the solar panels will be converted into a regulated DC voltage using a boost converter and a large capacitor. The DC output will then be converted to 60 Hz AC using a bridge inverter. The ratio between the load current and the short-circuit current of a PV panel at maximum power point is nearly constant for different insolation (light) levels and this property is utilized in designing a simple maximum power point tracking (MPPT) controller. The results obtained on an experimental converter are presented.

Wind Farms With HVdc Delivery in Inertial Response and Primary Frequency Control

This paper develops a coordination control strategy for wind farms with line commutated converter-based HVdc delivery for participating in inertial response and primary frequency control. The coordination philosophy is to feedback the grid frequency and its derivative and adjust the delivery power of the HVdc link according to the feedback signals. The feedback loop employing the derivative of the grid frequency aims to improve the inertial response while the feedback loop employing the grid frequency deviation introduces a droop at the rectifier control loop. When the grid frequency is too high or too low, active power flow through the HVdc link will be ramped down or up. In turn, the wind generation will increase or decrease the blade angles to reduce or increase the captured wind power through pitch control. A case study demonstrates the effectiveness of the inertial enhancement and frequency droop in HVdc control. Simulation results in transient security assessment tool are given.

Control of DFIG based wind generation to improve inter-area oscillation damping

In this paper, control of inter-area oscillation in power systems with high wind power penetration is achieved via doubly fed induction generator (DFIG). The DFIG is modeled in Matlabreg/Simulink. A two-area system is investigated with a wind farm connected in the area that exports power. Control schemes for active power and reactive power regulation are designed firstly. The paper finds that without active power and reactive power regulation, it is impossible to design an effective inter-area oscillation damping controller through DFIG machine side inverter. An auxiliary control loop for oscillation damping that adjusts the active power command can help damp the inter-area oscillation. Time domain simulations demonstrate the effectiveness of the control.

A novel circuit model for PEM fuel cells

The proton exchange membrane (PEM) fuel cell is being investigated as an alternate power source for various applications like transportation and emergency power supplies. The paper presents a novel circuit model for a PEM fuel cell that can be used to design and analyze fuel-cell power system. The PSPICE-based model uses BJTs and LC elements available in the PSPICE library with some modification. The model includes the phenomena like activation polarization, ohmic polarization, and mass transport effect present in a PEM fuel cell. The static and dynamic characteristics obtained through simulation are compared with experimental results obtained on a commercial fuel-cell module. The performance of the fuel cell module when feeding a boost converter is studied and the waveforms are presented.

Constant-switching-frequency AC-DC converter using second-harmonic-injected PWM

This paper describes the operation of an AC-DC converter employing second-harmonic-injected PWM. The boost converter operating on the rectified output uses a constant switching frequency PWM and a discontinuous current mode to reduce the total harmonic distortion in the input current. The implementation and the characteristics of the converter are presented.

Single-phase AC-AC converters using power MOSFETs

The authors analyze the performance of a single-phase AC-AC converter with particular emphasis on the output harmonic content and input voltage utilization. A complete power circuit that makes use of power MOSFETs is given. An analysis of the output waveform shows that its harmonic content is very low. It is also found that the ratio of the switching frequency to the modulating triangular wave, has negligible effect on the harmonic content. Hence, the ratio can be chosen at nominal value of 4 and it is mainly used to control the amplitude of the fundamental component in the output. The only disadvantage of the single-phase AC-AC converter is that the amplitudes of certain harmonic frequencies become abnormally high. >

A fast and accurate maximum power point tracker for PV systems

The paper presents a fast and accurate maximum power point tracking (MPPT) algorithm for a photovoltaic (PV) panel that uses the open circuit voltage and the short circuit current of the PV panel. The mathematical equations describing the nonlinear V-I characteristics of the PV panel are used in developing the algorithm. The MPPT algorithm is valid under different insolation, temperature, and level of degradation. The algorithm is verified using MATLAB and it is found that the results obtained using the algorithm are very close to the theoretical values over a wide range of temperature and illumination levels. The maximum deviation in the maximum power is less than 1.5% for the illumination levels and temperatures normally encountered by a commercial PV panel. The complete derivation of this MPPT algorithm is presented. It is seen that the algorithm is faster than other MPPT algorithms like perturbation and observation (P&Q) and more accurate than approximate methods that use the linearity between voltage (current) at maximum power point and open-circuit voltage (short-circuit current). The block diagram of a buck-boost PV battery charger with the proposed maximum power point (MPP) tracker is given.