Magnetic resonance (MR) brain section images are segmented and then synthetically colored to give visual representations of the original data with three approaches: the literal and approximate fuzzy c-means unsupervised clustering algorithms, and a supervised computational neural network. Initial clinical results are presented on normal volunteers and selected patients with brain tumors surrounded by edema. Supervised and unsupervised segmentation techniques provide broadly similar results. Unsupervised fuzzy algorithms were visually observed to show better segmentation when compared with raw image data for volunteer studies. For a more complex segmentation problem with tumor/edema or cerebrospinal fluid boundary, where the tissues have similar MR relaxation behavior, inconsistency in rating among experts was observed, with fuzz-c-means approaches being slightly preferred over feedforward cascade correlation results. Various facets of both approaches, such as supervised versus unsupervised learning, time complexity, and utility for the diagnostic process, are compared. >

A comparison of neural network and fuzzy clustering techniques in segmenting magnetic resonance images of the brain

This paper explores the interaction between dynamic loads and power systems. Based on a generic model of dynamic loads, the frequency response of such loads is discussed. Also, the frequency response of the system is investigated. It is shown that the dynamics of the load provide a feedback path which can influence the damping of the modal oscillations of the system. This influence is very dependent on load parameters as well as the system configuration. Under some circumstances damping can be improved, but under other conditions dynamic load may cause a decrease in damping. >

Effects of load dynamics on power system damping

This paper considers the significance of load voltage dynamics in studies of power system damping. A generic model of dynamic loads is used to investigate the influence of active and reactive power dynamics on the damping of oscillations in a multimachine power system. The interaction between the load and the power system is explored in terms of load and system transfer functions. It is shown that the power system transfer function is composed of a static part and a dynamic part. The static part is derived from the power flow Jacobian. The investigations indicate that load voltage dynamics can significantly influence the damping of modal oscillations. Static load models can give quite misleading predictions of damping when loads actually exhibit dynamic behaviour.

Load modelling in studies of power system damping

This study presents a control scheme using a flywheel energy-storage system (FESS) to simultaneously achieve power-fluctuation mitigation and dynamic-stability enhancement of an offshore wind farm (OWF) and marine-current farm (MCF) connected to a power grid. The performance of the studied OWF is simulated by an equivalent aggregated 80-MW doubly-fed induction generator (DFIG), while the characteristics of the studied MCF are simulated by an equivalent aggregated 40-MW squirrel-cage induction generator. A proportional–integral–derivative (PID) damping controller of the proposed FESS which is connected to the common AC bus of the OWF and MCF is designed by using modal control theory to contribute effective damping characteristics to the studied OWF and MCF under different operating conditions. A frequency-domain approach based on a linearised system model using eigenvalue analysis is performed. A time-domain scheme based on a non-linear system model subject to disturbance conditions is also carried out. It can be concluded from the simulation results that the proposed FESS combined with the designed PID damping controller can effectively stabilise the studied OWF and MCF under various disturbance conditions. The inherent power fluctuations injected to the power grid can also be effectively mitigated by the proposed control scheme.

Dynamic stability improvement of an integrated offshore wind and marine-current farm using a flywheel energy-storage system

A method to regulate the unified power flow controller (UPFC) to improve the dynamic performance of power systems is proposed in the paper. The series voltage responds to the power variations of the transmission line, while the shunt compensation is controlled to maintain the power system bus voltage and to stabilise the DC link of the UPFC. Decoupled control algorithms for active and reactive power are developed. A nonlinear dynamic model of the power network is established for transient studies. The small-signal model is derived using linearisation and network reduction. An integrated formulation can be achieved which represents the dynamics of the generators, loads and the UPFC. The effectiveness of the UPFC and the accuracy of the analytical models are examined using numerical examples.

Dynamic modelling of a unified power flow controller

Results of a comparative study of the application of three different compensators, the power system stabilizer (PSS), the static VAR compensator (SVC), and the rectifier current regulator (RCR), for the damping enhancement of generator oscillations in a power system are presented. In order to enhance the dampings of both the mechanical mode and the exciter mode in the system, a unified approach based on modal control theory is proposed for the design of the PSS, the SVC, and the RCR. A proportional-integral-derivative (PID) type controller using generator speed deviation as a modulated signal to generate the desired damping is proposed, and it is shown that both affected system modes can be exactly located at the prespecified positions on the complex plane by the proposed damping schemes. To demonstrate the effectiveness of the proposed PID controllers and their relative merits, a frequency-domain study based on eigenvalue analysis under different operating conditions and a time-domain study based on nonlinear model simulations under disturbance conditions are performed. >

Li Wang

Papers

A comparative study of damping schemes on damping generator oscillations