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This paper presents a new operational strategy for a small scale wind farm which is composed of both fixed and variable speed wind turbine generator systems (WTGS). Fixed speed wind generators suffer greatly from meeting the requirements of new wind farm grid code, because they are largely dependent on reactive power. Integration of flexible ac transmission systems (FACTS) devices is a solution to overcome that problem, though it definitely increases the overall cost. Therefore, in this paper, we focuses on a new wind farm topology, where series or parallel connected fixed speed WTGSs are installed with variable speed wind turbine (VSWT) driven permanent magnet synchronous generators (PMSG). VSWT-PMSG uses a fully controlled frequency converter for grid interfacing and it has abilities to control its reactive power as well as to provide maximum power to the grid. Suitable control strategy is developed in this paper for the multilevel frequency converter of VSWT-PMSG. A real grid code defined in the power system is considered to analyze the low voltage ride through (LVRT) characteristic of both fixed and variable speed WTGSs. Moreover, dynamic performance of the system is also evaluated using real wind speed data. Simulation results clearly show that the proposed topology can be a cost effective solution to augment the LVRT requirement as well as to minimize voltage fluctuation of both fixed and variable speed WTGSs.

A Variable Speed Wind Turbine Control Strategy to Meet Wind Farm Grid Code Requirements

This paper presents an advanced control strategy for the rotor and grid side converters of the doubly fed induction generator (DFIG) based wind turbine (WT) to enhance the low-voltage ride-through (LVRT) capability according to the grid connection requirement. Within the new control strategy, the rotor side controller can convert the imbalanced power into the kinetic energy of the WT by increasing its rotor speed, when a low voltage due to a grid fault occurs at, e.g., the point of common coupling (PCC). The proposed grid side control scheme introduces a compensation term reflecting the instantaneous DC-link current of the rotor side converter in order to smooth the DC-link voltage fluctuations during the grid fault. A major difference from other methods is that the proposed control strategy can absorb the additional kinetic energy during the fault conditions, and significantly reduce the oscillations in the stator and rotor currents and the DC bus voltage. The effectiveness of the proposed control strategy has been demonstrated through various simulation cases. Compared with conventional crowbar protection, the proposed control method can not only improve the LVRT capability of the DFIG WT, but also help maintaining continuous active and reactive power control of the DFIG during the grid faults.

/pdf/advanced-control-strategy-of-dfig-wind-turbines-for-power-2g6s3w6n4r.pdf

Advanced Control Strategy of DFIG Wind Turbines for Power System Fault Ride Through

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This review paper, prepared for this second special issue on lightning of the IEEE Transactions on Electromagnetic Compatibility, summarizes major publications on lightning and lightning protection since the first special issue published in November 1998, i.e., during the last decade. The review is organized in the following five sections: lightning discharge-observations, lightning discharge-modeling, lightning occurrence characteristics/lightning locating systems, lightning electromagnetic pulse and lightning-induced effects, and protection against lightning-induced effects.

/pdf/overview-of-recent-progress-in-lightning-research-and-3nshvbrq5k.pdf

Overview of Recent Progress in Lightning Research and Lightning Protection

In the last 15 years, the use of doubly fed induction machines in modern variable-speed wind turbines has increased rapidly. This development has been driven by the cost reduction as well as the low-loss generation of Insulated Gate Bipolar Transistors (IGBT). According to new grid code requirements, wind turbines must remain connected to the grid during grid disturbances. Moreover, they must also contribute to voltage support during and after grid faults. The crowbar system is essential to avoid the disconnection of the doubly fed induction wind generators from the network during faults. The insertion of the crowbar in the rotor circuits for a short period of time enables a more efficient terminal voltage control. As a general rule, the activation and the deactivation of the crowbar system is based only on the DC-link voltage level of the back-to-back converters. In this context, the authors discuss the critical rotor speed to analyze the instability of doubly fed induction generators during grid faults.

/pdf/crowbar-system-in-doubly-fed-induction-wind-generators-uvned2lk2z.pdf

Crowbar System in Doubly Fed Induction Wind Generators

The use of wind power has been increasing very fast in the last 10 years. Many new projects for the next 10 years including offshore and onshore wind farms are been developed and planned. The fast growing of the use of wind power has brought new challenges to the Transmission System Operators (TSO) in regions where wind power has reached significant penetration levels like Denmark, United Kingdom, Spain and Germany. According to new grid code requirements wind turbines must remain connected to the grid during grid disturbances and, moreover, they must also contribute to voltage support during and after grid faults. Dynamic models of doubly fed induction generator (DFIG) were developed to investigate the behavior off different converter control and protection strategies of the back-to-back IGBT-based converters during grid fault. The results have showed that reactive power injection by DFIG-based wind farms is limited when the rotor side converter is blocked.

Control strategies of doubly fed induction generators to support grid voltage

The paper presents a hybrid method for evaluating the electromagnetic field in the vicinity of the lightning channel base. Several models have been proposed for this purpose, but, in spite of general agreement on the magnetic field results, the literature shows quite different results concerning the intensity and waveshape of the near electric field. The proposed method is an alternative approach that takes advantage of this fact to calculate, first, the magnetic field, and, secondly, based partially on the FDTD method, the electric field. The suitability of this procedure is checked through the comparison with another method, for a particular configuration proposed in the literature.

An analytical-FDTD method for near LEMP calculation

The five parameters of the Jiles–Atherton (J–A) model are identified using a simple program based on the Matlab platform which identifies the J–A parameters automatically from experimental B – H hysteresis curves of magnetic cores. This computational tool is based on adaptive adjustment of the J–A model parameters and conjugates its parametric non-linear coupled differential equations with techniques of simulated annealing.

Identification of the Jiles–Atherton model parameters using random and deterministic searches

In a railway line, trains are constantly changing their positions and loads. At each specific time, trains, substations, contact line and earthing system establish a different DC circuit configuration. To solve those circuits, several approaches were tested. These approaches may be divided into linear systems and nonlinear systems. The ICCG (incomplete cholesky conjugate gradient) solver for DC traction load flow is proposed in the paper. This method is described and applied to a test circuit. To verify the ICCG effectiveness applied to DC traction load flow, a comparison with other commonly used methods is made.

José Roberto Cardoso

Papers

Crowbar System in Doubly Fed Induction Wind Generators

Control strategies of doubly fed induction generators to support grid voltage

An analytical-FDTD method for near LEMP calculation

Identification of the Jiles–Atherton model parameters using random and deterministic searches

ICCG method applied to solve DC traction load flow including earthing models