Variable Frequency Applications Research Articles

Integration of IEC 60287 in Power System Load Flow for Variable Frequency and Long Cable Applications

Integration of IEC 60287 in Power System Load Flow for Variable Frequency and Long Cable Applications

Generic Reliability Evaluation Method for Industrial Grids with Variable Frequency Drives

The paper presents a reliability evaluation method based on fault tree analysis with set theory and minimal cut set as core algorithm, which can be used to evaluate the reliability for industrial grids with wide application of variable frequency drives. The working principle is introduced firstly, based on which the method development considering different system topology designs, backup solutions and redundancy mechanisms are analyzed in details. In the end the proposed method is applied to two cases to show the reliability performance of system with variable frequency drives. The proposed method is also suitable for analyzing the reliability performance of industrial grids with other types of power electronic converter technology.

Three-Dimensional Flux Vector Modulation of Four-Leg Sine-Wave Output Inverters

The time integral of the output voltage vector of a three-phase inverter is often termed the inverter flux vector. This paper addresses the control of a three-phase four-leg sine-wave output inverter having an LC filter at its output, by controlling the flux vector in three dimensions. Flux vector control has the property that an output filter resonance is actively damped by an output voltage control loop alone. Furthermore, an inverter switching action inherently regulates the output voltage rapidly against dc-bus voltage variations. The flux vector control of sine-wave output inverters finds several applications in three-phase four-wire systems. This paper presents a flux modulation method for three-phase four-leg inverters feeding unbalanced and nonlinear loads. All the necessary steps for the digital implementation of the flux modulator are presented. The switching behavior of the modulator has been evaluated, which is useful for the variable fundamental frequency applications of the inverters. To provide experimental validation, the modulator is implemented as a part of the control system for a stand-alone three-phase four-leg inverter with an LC filter at its output. Control system details are also provided. Experimental results indicate the effectiveness of the modulator and control system in providing balanced voltages at the output of the LC filter even under highly unbalanced conditions with nonlinear loads. The resonance damping and voltage regulation properties of the modulator are also apparent from the experimental results.

Application of variable-frequency electric drives for controlling the drain pumps of low-pressure heaters used in turbine units at cogeneration stations

Results obtained from equipping the drain pumps of low-pressure heaters used in a T-250/300-23.8 turbine unit with a standard adjustable drive are presented. It is shown that the use of this drive allows the level in the high-pressure heater to be efficiently controlled in different modes of operation with smaller consumption of energy for station auxiliaries.

Controlador repetitivo para inversores PWM com referência de freqüência variável

Controladores repetitivos têm se mostrado eficazes na rejeição de distúrbios cíclicos em inversores de tensão modulados por largura de pulso. Contudo, observa-se que estes controladores têm seu desempenho comprometido caso seu período não seja sincronizado com o período do sinal de referência, o que restringe o seu uso em aplicações de freqüência variável. Para mitigar este problema, é proposta uma modificação nestes controladores para adequá-los à aplicações com referência senoidal de freqüência variável. O algoritmo proposto é simples, sendo apenas ajustados o comprimento das memórias circulares e os ponteiros empregados para endereçá-las nos controladores repetitivos convencionais. Resultados experimentais, obtidos em dois protótipos com especifiações distintas, são apresentados para validar a técnica proposta.

Energy Conservation Opportunities in an Industrial Refrigeration System

Industrial refrigeration can be found in many types of applications such as food processing, product preservation, and more. In many cases, it can represent one of the largest energy consumers in a facility. Studies show that it is possible to reduce energy consumption by up to 40 percent in industrial refrigeration systems. This article presents an analysis of the energy conservation opportunities identified in an ammonia-based industrial refrigeration system in a Canadian warehouse. Some of the energy conservation opportunities included in the analysis are: application of a floating head pressure control; application of variable-frequency drives in evaporator fans; optimization of hot gas defrost; heat recovery opportunities from the evaporative condensers; reduction of infiltration of air in cold storage; application of bi-level lighting control in the lamps in the cold storage; and application of screw compressor VFD controls.

An Improved Robust Predictive Current Regulation Algorithm

Current regulation techniques for pulsewidth-modulated (PWM) voltage source inverters (VSIs) can be classified as either linear or nonlinear. Linear techniques consist principally of either a proportional-integral (PI) or a predictive current control strategy, while nonlinear schemes are usually based on a hysteresis strategy. Of the two linear strategies, predictive current control offers the advantages of precise current tracking with minimal distortion and can also be fully implemented on a digital platform. However, the conventional implementation of the predictive current regulation (PCR) algorithm is sensitive to noise and errors in the load inductance estimate, particularly when the back EMF is also estimated. This paper presents an improved PCR algorithm that retains all the benefits associated with PCR while achieving significantly increased robustness to load parameter mismatch and reduced zero current clamped oscillation effects. It is also relatively insensitive to noise in the sampled current measurements. The algorithm is equally applicable to variable fundamental frequency applications such as variable speed drives and to fixed fundamental frequency applications such as PWM rectifier systems or active filters. Simulation and experimental results are presented to confirm the improved robustness of the new algorithm.

Variable frequency microwave (VFM) processing facilities and application in processing thermoplastic matrix composites

Microwave processing of materials is a relatively new technology advancement alternative that provides new approaches for enhancing material properties as well as economic advantages through energy savings and accelerated product development. Factors that hinder the use of microwaves in materials processing are declining, so that prospect for the development of this technology seem to be very promising [1]. The two mechanisms of orientation polarisation and interfacial space charge polarisation, together with dc conductivity, form the basis of high frequency heating. Clearly, advantages in utilising microwave technologies for processing materials include penetration radiation, controlled electric field distribution and selective and volumetric heating. However, the most commonly used facilities for microwave processing materials are of fixed frequency, e.g. 2.45 GHz. This paper presents a state-of-the-art review of microwave technologies, processing methods and industrial applications, using variable frequency microwave (VFM) facilities. This is a new alternative for microwave processing.

Gate-drive circuit for zero-voltage-switching half-and full-bridge converters

Resonant techniques like zero-voltage switching (ZVS) and zero-current switching (ZCS) are widely applied to raise the switching frequency and efficiency of converters. An important subclass of converters using resonant techniques is formed by ZVS half- and full-bridge converters. While different varieties of this subclass of converters have been reported, the gate-drive circuit is seldom described. In this paper, a new gate-drive circuit applicable in ZVS half- and full-bridge converters is proposed. The circuit shows excellent features such as transformer isolation, no ringing of the gate-source voltage of the switches, programmable dead time, no active components besides one low-side driver, insensitivity to component value variation, negative gate-source voltage at switch-off, and low drive power. Moreover, the circuit can be used in variable-frequency applications. The theoretical results obtained are verified by experiments.

Harmonic interaction between 1500 kVA supply transformer and VFD load at an industrial plant

The application of variable-frequency drives (VFDs) in industrial and commercial facilities has seen a tremendous increase. However, when multiple VFDs are fed from one dedicated power transformer, care should be taken not to drive the transformer into saturation. At one industrial location, nine VFDs rated at 200 HP each, and fed from a single 1500 kVA air-cooled transformer, showed saturation phenomenon, which had the potential of causing serious damage to the transformer. This paper discusses the input AC current waveform asymmetry caused by the interaction of the transformer and the nine 200 HP VFD load fed from it. The term waveform asymmetry is used when the positive half of the input AC current cycle is not equal to the negative half of the input AC current cycle. This results in the existence of DC components along with even harmonics in the input at current waveform. The various tests and the eventual solution to the waveform asymmetry caused by the VFDs is presented. The value of the inductance in the DC smoothing inductor (DC-link choke), which is included as part of the VFD, was found to contribute significantly to the waveform asymmetry. Experimental results showing the performance of a reconfigured DC-link inductor to reduce its interaction with the power transformer are presented.

Dynamic behavior of cycloconverter system

The dynamic behavior of a multiprocessor-based 12-pulse circulating current cycloconverter is discussed. This type of cycloconverter can provide a high output to input frequency ratio for fixed and variable-frequency applications. Waveform level simulation of the cycloconverter indicates that both feedback and feedforward techniques can be used to minimize the tracking error of the circulating current controller. The lab prototype is used to test the response of the 12-pulse, single-phase cycloconverter system. The control is implemented by a total of six 16-b microprocessor cards and additional input and output cards resident in a single card cage. >

High performance frequency-to-voltage converter

ABSTRACT An analogue frequency-to-voltage converter, suitable for low and variable-frequency applications is presented. Circuit advantages include nominally zero output ripple, fast dynamic response, good accuracy and linearity in a wide frequency range.

Analysis and Design of Forced Commutated Cycloconverter Structures with Improved Transfer Characteristics

Forced Commutated Cycloconverters (FCC's) are converter structures capable of providing simultaneous voltage and frequency transformation without the need of intermediate stage reactive components. Consequently, they are well-suited for high-power density variable-frequency applications.

The Influence of Saturation on Induction Machine Drive Dynamics

A small signal model which employs two different magnetizing reactances to incorporate the effect of main flux saturation on induction machine dynamic response is presented. Three different types of experiments which confirm the improved accuracy of the saturated model are discussed, and the model is used to examine the effect of saturation on dynamic response in variable frequency applications. The results indicate that whereas saturation has a relatively small effect under normal operating conditions, it does have an important stabilizing effect when the damping is poor. It is also shown that neglecting saturation leads to significant errors in both pole and zero locations when the operating frequency is low.

Harmonic Analysis of a Synchronized Pulse-Width-Modulated Three-Phase Inverter

In pulsewidth-modulated inverters used for variable-frequency applications, a number of harmonic control policies can be used. Most of these policies are complex to implement by hardware. The most simple and yet effective technique is synchronizing or the synchronized harmonic control policy. This paper uses double Fourier analysis techniques to obtain the output harmonics as a function of pulsewidth, pulsing frequency, and output frequency. The method obtains the harmonics as analytical functions and thus provides the necessary relationships for control studies and system optimization. The more significant theoretical results are provided and theoretical and experimental results obtained from a 25-kW three-phase pulsewidth-modulated inverter, are compared.

Sleeve-Rotor Induction Motors for Control Applications

The development of the drag-cup construction of induction motors of ratings of the order of 5 hp for control-system applications is described. Consideration is given to the suitability of these machines for the conventional two-phase method of control and the variable-frequency application.