The application of high-power voltage-source converters (VSCs) to multiterminal dc networks is attracting research interest. The development of VSC-based dc networks is constrained by the lack of operational experience, the immaturity of appropriate protective devices and the lack of appropriate fault analysis techniques. VSCs are vulnerable to dc cable short-circuits and ground faults due to the high discharge current from the dclink capacitance. However, faults occurring along the interconnecting dc cables are most likely to threaten system operation. In this paper, cable faults in VSC-based dc networks are analyzed in detail with the identification and definition of the most serious stages of the fault that need to be avoided. A fault location method is proposed because this is a prerequisite for effective design of a fault protection scheme. It is demonstrated that it is relatively easy to evaluate the distance to a short-circuit fault using voltage reference comparison. For the more difficult challenge of locating ground faults, a method of estimating both the ground resistance and the distance to the fault is proposed by analyzing the initial stage of the fault transient. Analysis of the proposed method is provided and is based on simulation results, with a range of fault resistances, distances and operational conditions considered.
The multi-terminal DC wind farm is a promising topology with a voltage source inverter (VSI) connection at the onshore grid. Voltage source converters (VSCs) are robust to AC side fault conditions. However, they are vulnerable to DC faults on the DC side of the converter. This paper analyses DC faults, their transients and the resulting protection issues. Overcurrent faults are analysed in detail and provide an insight into protection system design. The radial wind farm topology with star or string connection is considered. The outcomes may be applicable for VSCs in both the multi-VSC DC wind farm collection grid and VSC-based high voltage direct current (HVDC) offshore transmission systems. Index Terms-Voltage source converter (VSC), fault overcurrent, multi-terminal DC wind farm, wind power generation.
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A B S T R A C T (C o n t in u e on r e v e r se s id e if n e c e s s a r y and id e n tify by b lo c k num ber)Recent progress in the use of singular perturbation and two-time-scale methods of modeling and design for control systems is reviewed. Over 400 references are organized into major problem areas. Representative issues and results are disucssed with a view to outlining research directions and indicating potential areas of application. The survey is aimed at engineers and applied mathema ticians interested in model-order reduction, separation of time scales and allied simplified methods of control system analysis and design.The exposition does not assume prior knowledge of singular perturbation methods.
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SINGULAR PERTURBATIONS AND TIME-SCALE METHODS
IN CONTROL THEORY
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