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This paper proposes a new multivariable dynamic model and a control approach for a voltage-source converter (VSC) based on adopting instantaneous real- and reactive-power components as the VSC dynamic variable. Using power components as the dynamic variables reduces the degree of nonlinearities of the VSC model in comparison with the conventional VSC model that uses qd current components as variables. Furthermore, since waveforms of power components are independent of the selected qd coordinates, the proposed control is more robust to the conventionally unmodelled dynamics such as dynamics of the VSC phase-locked loop system. The proposed control system regulates the VSC ac-side real- and reactive-power components, and the dc-side voltage. This paper also introduces an overcurrent limiting mechanism, based on limiting the VSC power exchange during abnormal conditions, for the proposed control system. The proposed model and control are applied to a VSC-based reactive power compensator and simulation results based on PSCAD/EMTDC software are presented. Index Terms--Dynamic modelling, multivariable control, sequential loop closing, voltage-source converter (VSC).

This paper introduces a small-signal dynamic model of a fixed-frequency (induction machine based) wind farm connected to an electrical power system. The model represents the system small-signal dynamics in the frequency range of a fraction of Hz to 50/60 Hz. The model includes the dynamics of the wind energy capturing mechanism, the rotating shaft system, the generator electrical system of each wind energy unit, the wind farm collector system, and the utility grid. Based on the proposed model, two performance indices are introduced to i) investigate sensitivity of the system modes, e.g., torsional modes, to the system parameters, and ii) evaluate the system capability to reject electrical and mechanical disturbances, e.g., a wind gust. The application of the model to the analysis of a system with two wind units is presented and the study results are validated based on comparison with the time-domain simulation results obtained in the PSCAD/EMTDC environment. Index Terms--Fixed-speed wind farm, sensitivity analysis, small-signal dynamics, torsional phenomenon.

This paper presents a systematic approach to develop a dynamic model for a matrix converter. The matrix converter interfaces two three-phase ac systems that can be at different voltages and/or frequencies, e.g., a high-speed micro-turbine generator and a utility distribution system. The model is developed by (i) transferring the abc frame equations to a newly defined switching reference frame and then (ii) transferring the resultant equations to a rotating dqo frame. The model is then linearized and the concept of 'sequential loop closing' is used to design a controller for the matrix converter as a multi-input multi-output system. The accuracy of the developed model and performance of the controller are evaluated based on comparing the study results with those obtained from digital time-domain simulations in the PSCAD/EMTDC software environment. Index Terms--Control design, dynamic model, matrix converter, small-signal dynamics, switching reference frame.

The complex torque coefficients method has been widely accepted for the analysis of the phenomenon of torsional interaction of turbine-generator units in power systems. This paper shows that, depending on the system parameters and the operating point, the complex torque coefficients method may exhibit limitations and not accurately and fully predict the system behavior in the frequency range of interest. These shortcomings consist of inability to i) predict monotonic instability due to real poles, ii) identify all electromechanical oscillatory modes, and iii) accurately predict damping (and consequently stability) of the oscillatory modes. This paper develops mathematical expressions to highlight the limitations of the complex torque coefficients method. Quantitative results based on three case studies, including a study on the first IEEE Benchmark System, are reported and results from eigenvalue analysis method, complex torque coefficients method, and time-domain simulation are presented and compared. This paper concludes that the complex torque coefficients method can be used only as a preliminary method for the investigation of torsional interactions and the results must be verified by other methods. Index Terms--Complex torque coefficients method, frequency response method, small-signal dynamics, subsynchronous resonance, torsional interactions.

This paper introduces a frequency-domain approach for investigation of the phenomenon of torsional dynamics. The Nyquist criterion is adopted by the proposed method to identify stability regions with respect to torsional oscillations. This paper also introduces two performance indices to evaluate torsional dampings and propensity of the system to experience torsional oscillations. The proposed method is an alternative approach to the eigenvalue analysis method and the complex torque method for investigation of torsional dynamics. The salient features of the method as compared with the aforementioned two methods are:1) its computational efficiency since it utilizes the open-loop transfer-function matrix of the system and inherently more advantages for large size systems; 2) its performance indices that readily reveal 'damping of' and 'propensity to' a torsional oscillatory mode; and 3) its capability to formulate a multimachine system which includes both induction and synchronous machines. The proposed method is applied to the First IEEE Benchmark Systems for SSR studies and the results are verified based on comparison with those obtained from eigenvalue studies and digital-computer time-domain simulation. Index Terms--Complex torque coefficients, frequency-response method, small-signal stability, subsynchronous resonance, torsional interactions.

This paper presents a topology for wideband piezoelectric wind energy harvesting and a design optimization method for capturing maximum available energy considering the wind speed distribution model. The proposed device is useful for powering of wireless sensor nodes in smart monitoring applications. The device includes a piezoelectric beam that vibrates due to the interactions between miniaturized permanent magnets (PMs) embedded in a small turbine and a magnet at the tip of the beam. The design method determines the resonance frequency of the beam with respect to the wind speed distribution model. Compared to the conventional wideband topologies that include multiple beams for resonating at different frequencies, the suggested topology uses a single beam and achieves the wideband feature via the arrangement of PMs within the turbine. This feature enhances the power density of the piezoelectric wind energy harvester. The wideband feature of the device is experimentally verified using a device prototype in which the beam resonance occurs at the fan rotational speeds of 155.0, 193.5, 257.9, and 387.0 rad/s. The maximum power density, maximum efficiency, and cut-in wind speed of the device are 0.59 mW/\textcm^3, 24%, and 2.1 m/s, respectively. [ABSTRACT FROM AUTHOR]

This paper presents a topology for micropower piezoelectric wind energy harvesting useful for developing self-powered wireless sensor nodes. The features of the proposed topology, as compared with the existing piezoelectric/electromagnetic topologies, are as follows: 1) delivering power at high voltage levels, particularly at low-speed air flows; 2) starting operation at low cut-in speeds (about 1 m/s); and 3) robust structure for operating at high-speed wind flows practically tested up to 20 m/s. The proposed topology consists of a small fan with embedded permanent magnets (PMs) and a piezoelectric beam with a PM proof mass, which interacts with the PMs in the fan to harvest wind power. This paper also presents an analytical model and a design procedure to determine the number of PMs in the fan and their arrangements to maximize the captured power and minimize the cut-in speed. Using a prototype of the proposed topology, it is shown that the device starts capturing wind power at the wind speeds above 0.9 m/s. It is also shown that the suggested topology is at least 10% more efficient than the existing topologies in using piezoelectric materials and that its total volume power density is higher than those of the other topologies. [ABSTRACT FROM PUBLISHER]

An adaptive energy-harvesting circuit with low power dissipation is presented and demonstrated, which is useful for efficient ac/dc voltage conversion of a piezoelectric micropower generator. The circuit operates stand-alone, and it extracts the piezoelectric strain energy independent of the load and piezoelectric parameters without using any external sensor. The circuit consists of a voltage-doubler rectifier, a step-down switching con- verter, and an analog controller operating with a single supply voltage in the range of 2.5-15 V. The controller uses the piezoelectric voltage as a feedback and regulates the rectified voltage to adaptively improve the extracted power. The nonscalable power dissipation of the controller unit is less than 0.05 mW, and the efficiency of the circuit is about 60% for output power levels above 0.5 mW. Experimental verifications of the circuit show the following: 1) the circuit notably increases the extracted power from a piezoelectric element compared to a simple full-bridge diode rectifier without control circuitry, and 2) the efficiency of the circuit is dominantly determined by its switching converter. The simplicity of the circuit facilitates the development of efficient piezoelectric energy harvesters for low-power applications such as wireless sensors and portable devices. [ABSTRACT FROM AUTHOR]