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BackgroundIntravascular ultrasound (IVUS) images vary in intensity because of the angle of the transducer relative to the plaque. The purpose of this study was to determine the angle dependence of ultrasound backscatter when the IVUS transducer is aligned coaxially in atherosclerotic arteries and to examine its feasibility in tissue characterization of human atherosclerotic tissue.Methods and resultsThirty-nine noncalcified regions of interest (ROI, 0.4 to 0. 6 mm in diameter) within cross sections of formalin-fixed human iliac arterial plaque were imaged with a 3.9F, 25-MHz IVUS catheter in saline at room temperature. The catheter was moved coaxially from 8 to 16 positions and spanned 50 to 122 degrees relative to the ROI and the lumen center. Echo intensity for each ROI was defined as the videointensity relative to a standard reflector. The angle dependence of echo intensity was defined as the slope of the regression line between the angle of incidence and echo intensity. Each ROI was histologically classified into 4 groups: fibro-acellular (fibrous cap, n=7), fibro-cellular (n=9), fibro-fatty (n=13), or fatty tissue (n=10). The echo intensity of the majority (72%) of plaque components in IVUS images are significantly affected by the angle of incidence of the transducer. The angle dependence of fibro-acellular samples was significantly greater than that of the other 3 groups (4.69 +/- 3.29 x 10(-3) x echo intensity/degree vs 1.06 +/- 1.10 in fibro-cellular area, 2.09 +/- 1.75 in fibro-fatty area, and 2.16 +/- 1.92 in fatty area, P

To examine the accuracy of intravascular ultrasound (IVUS) in assessing the biophysical properties of atherosclerotic plaque, 33 human iliac arteries were imaged with a 25 MHz IVUS transducer and classified into four groups on the basis of IVUS appearance: minimally diseased arterial wall, bright echogenic plaque with acoustic shadowing, bright echogenic plaque without shadowing, and hypoechogenic plaque (so-called "soft echoes"). The hardness of each plaque was assessed with an ultrasensitive compression ergonometer. The radial static stress-strain relations fit well (r > 0.98) to exponential curves, providing a compression stiffness constant (K) defined as the coefficient of the exponential power. K for bright echogenic plaque with shadowing was significantly greater than that of the other tissues. However, K among minimally diseased entire arterial wall, hypoechogenic plaque, and bright echogenic plaque without shadowing was not significantly different, but these tissues are not physically soft compared with adipose tissue. Therefore, tissue characterization by IVUS distinguishes calcified from noncalcified plaque and accurately predicts its biomechanical hardness. However, soft echoes, although less firm than calcium, do not necessarily correspond to soft tissue.

This article presents a new current controller in the synchronous reference frame and its associated design for enhancing the performance of three-phase grid-connected inverters, especially against weak-grid conditions. The existing controllers do not perform strongly during high-impedance grid conditions and lead to oscillations and instability issues due to the interactions between the synchronization and control units. The proposed controller addresses this issue by 1) deriving a linear model of the three-phase phase-locked loop (PLL), 2) integrating the PLL model into the current controller design, 3) using a multivariable control design for multi-input multi-output systems, and 4) designing the controller gains using optimal linear quadratic theory. The proposed controller has superior performance over a substantially wider range of weak-grid conditions compared to conventional controllers. Extensive simulation and experimental results are presented in order to validate and reveal the desirable performance of the proposed controller. [ABSTRACT FROM AUTHOR]

This paper presents an active islanding detection method for a distributed resource (DR) unit which is coupled to a utility grid through a three-phase voltage-sourced converter (VSC). The method is based on injecting a negative-sequence current through the VSC controller and detecting and quantifying the corresponding negative-sequence voltage at the point of common coupling of the VSC by means of a unified three-phase signal processor (UTSP). UTSP is an enhanced phase-locked loop system which provides high degree of immunity to noise, and thus enable islanding detection based on injecting a small (< 3%) negative-sequence current. The negative-sequence current is injected by a negative-sequence controller which is adopted as the complementary of the conventional VSC current controller. Based on simulation studies in the PSCAD/EMTDC environment, performance of the islanding detection method under UL1741 anti-islanding test is evaluated, and its sensitivity to noise, grid short-circuit ratio, grid voltage imbalance, and deviations in the UL1741 test parameters are presented. The studies show that based on negative-sequence current injection of about 2% to 3%, islanding can be detected within 60 ms even for the worst case scenario. Index Terms--Distributed resource (DR), islanding detection, UL1741 test.

This paper presents a new control strategy for islanded (autonomous) operation of an electronically coupled distributed generation (DG) unit and its local load. The DG unit utilizes a voltage-sourced converter (VSC) as the coupling medium. In a grid-connected mode, based on the conventional dq-current control strategy, the VSC controls real- and reactive-power components of the DG unit. Subsequent to an islanding detection and confirmation, the dq-current controller is disabled and the proposed controller is activated. The proposed controller utilizes I) an internal oscillator for frequency control and 2) a voltage feedback signal to regulate the island voltage. Despite uncertainty of load parameters, the proposed controller guarantees robust stability and prespecified performance criteria (e.g., fast transient response and zero steady-state error). The performance of the proposed controller, based on time-domain simulation studies in the PSCAD/EMTDC software environment, is also presented. Index Terms--Autonomous operation, control, distributed generation (DG), distributed resource, dynamic model.

A novel system for decomposing a set of three-phase signals into its constituting symmetrical components is proposed. The system also estimates the frequency, the magnitudes and phase-angles of the sequence components. The system is useful for online estimation of instantaneous symmetrical components and their attributes in the presence of frequency variations. The proposed system can be considered as a fundamentally improved version of the conventional three-phase Phase-Locked Loop (PLL) system which is widely used in power system applications. From this viewpoint, the proposed system obviates the inherent shortcoming of the conventional PLL system which is its erroneous response in the presence of negative-sequence component. Mathematical derivations and computer simulations are presented to describe the principles of operation and the performance of the proposed system. Examples of applications of the proposed system are in Flexible ac Transmission Systems (FACTS), HVDC systems, and Custom Power Controllers. Index Terms--Dynamic phasors, Flexible ac Transmission Systems (FACTS), frequency estimation, phase-angle detection, Phase-Locked Loop (PLL), synchronization, time-domain symmetrical components.

This paper introduces a new phase-locked loop (PLL) system. The proposed system provides the dominant frequency component of the input signal and estimates its frequency. The mechanism of the proposed PLL is based on estimating in-phase and quadrature-phase amplitudes of the desired signal and, hence, has application advantages for communication systems which employ quadrature modulation techniques. The studies demonstrate that the proposed PLL also provides a superior performance for power system applications. Derivation of the mathematical model and theoretical stability analysis of the proposed PLL are carried out using dynamical systems theory. Advantages of the proposed PLL over the conventional PLLs are its capability of providing the fundamental component of the input signal which is not only locked in phase but also in amplitude to the actual signal while providing an estimate of its frequency. Computer simulation is used to evaluate its performance. Evaluations confirm structural robustness of the proposed PLL with respect to noise and distortions. Index Terms--Frequency estimation, peak detectors, phase detection, phase-locked loops (PLLs), power system state estimation, quadrature amplitude modulation, synchronization.

A new method for estimation of power frequency and its rate of change is presented. Unlike conventional methods which are based on the concept of linearization, the proposed scheme accommodates the inherent nonlinearity of the frequency estimation problem. This makes the method capable of providing a fast and accurate estimate of the frequency when its deviation from the nominal value is incremental or large. The estimator is based on a newly developed quadrature phase-locked loop concept. The method is highly immune to noise and distortions. The estimator performance is robust with respect to the parameters of its structure. Structural simplicity and performance robustness are other salient features of the method. Index Terms--Frequency estimation, nonlinear systems, notch filters, phase-locked loops.

This paper provides an adaptive filter for synchronous detection and extraction of harmonics. The filter can he used as integral part of the control system of a power electronic apparatus (e.g., STATCOM, APF, and UPFC) to generate the desired control signals. Stability and convergence analyses of the adaptive filter are presented based on dynamical systems theory. Performance of the filter is verified as a means for reference signal generation in a shunt active power filter. Index Terms--Active power filter, adaptive filter, averaging theorem, synchronous harmonic detection.

The proliferation of phasor measurement units (PMUs) presents new challenges in archiving and processing large amounts of synchrophasor data which necessitates advanced data compression methods. This paper proposes a singular value decomposition (SVD)-based method for compression of synchrophasor data, including magnitude, phase-angle, and complex phasor. The proposed method includes a dimensionality evaluation and reduction technique and a real-time progressive partitioning algorithm. The proposed dimensionality reduction technique employs the measurement uncertainty of PMUs and introduces a threshold criterion on the signal-to-noise ratio (SNR) of SVD modes. Singular modes with high SNR are retained, and those dominated by measurement error are discarded to achieve a high compression ratio (CR) while preserving the critical information with adequate accuracy. The proposed progressive partitioning separates the data corresponding to normal and disturbance conditions by monitoring the dimensionality variations in real-time. The partitions containing the data of similar dimensionality are separately compressed to further improve the accuracy and CR. The performance of the proposed method is evaluated and benchmarked against state-of-the-art methods using both field and simulated PMU data. The results show that the proposed method provides high CR while accurately preserving the critical information of events and disturbances. [ABSTRACT FROM AUTHOR]