Ahda P. Grilo

Control strategies of doubly fed induction generators to support grid voltage

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.

The influence of the applied rotor voltage on ride-through capability of doubly fed induction generator

This paper presents a discussion about the large disturbance instability phenomenon of grid-connected doubly fed induction generators. The behavior of squirrel cage induction generator (SCIG) on large disturbance stability depends basically on the ability of the blade pitch angle control and the injection of reactive power during contingency on the network. However, the behavior of the doubly fed induction generator (DFIG) is also affected by the applied rotor voltage. The instability behavior of the SCIG is well documented in the literature, on the other hand this behavior is not well described for the case of DFIG. The analyses performed in this paper evaluate the effect of different rotor voltage applied during grid fault on the DFIG stability. A sensitive study using dynamics simulations was performed to determine the critical fault tolerance time. The results have shown that when enlarging the direct axis voltage the DFIG has better stability performance.

A study on the rotor side control of DFIG-based wind turbine during voltage sags without crowbar system

The connection of wind farms to the electrical power system has increased very fast in the last years requiring the ability of the wind turbines to assist network during voltage sags. Most of the modern wind turbines are based on doubly fed induction generator with a back-to-back power converter connecting the rotor windings to the grid. One limitation of doubly fed induction generator is the operation during grid faults. The rotor side converter can be damage by high currents induced by the stator windings. This article investigates the behavior of the wind farm during voltage sags using 2 different control strategies. The results have indicated that the strategy which the reference currents are calculated to compensate the stator flux variation during voltage sags can avoid overcurrents in the rotor windings, in some cases, without the use of a crowbar system.

Protection strategies for rotor side converter of DFIG-based wind turbine during voltage dips

Modern wind turbines operate with variable speed and most of them are based on doubly fed induction generators (DFIG), with a back-to-back power converter. During voltage dips, the stator terminals can cause overcurrents in the rotor windings, which could threaten the converter integrity. The use of crowbar systems is the most common technique to avoid this situation. The crowbar activation disables the control of the rotor side converter during a short period of time. Once the rotor side converter is blocked, the DFIG operates like a typical induction generator and thus consumes reactive power. In this paper, the performance of the well-known crowbar protection is compared with a current compensation strategy. Results show that the current compensation strategy can have positive effects on the terminal voltages during the fault without disconnection of the rotor side converter.

Methodology for grid current unbalance compensation appling DFIG as a parallel active filter

This paper presents a new control strategy for the DFIG working under grid voltage unbalance, intending not only the ideal machine operation, but also to compensate the current unbalance in the point of common coupling (PCC). Models that allow the DFIG to inject negative sequence current in the grid to compensate unbalances in the PCC have been already proposed, but they are limited by the residual power of the Grid Side Converter (GSC) or by the oscillations in the electromagnetic torque when the Rotor Side Converter (RSC) control is used. To overcome these limitations, this paper proposes a control that not only injects negative sequence current by the machine stator but also sets the stator negative sequence voltage, reducing the oscillations in the electromagnetic torque. A third converter is coupled to the machine terminals to allow the stator voltage control. Therefore, the use of the proposed control reaches two objectives simultaneously, the support to the grid power quality, and the ideal operation of the machine.

Method for adaptive overcurrent protection of distribution systems with Distributed synchronous Generators

The increase of Distributed Generators (DG) has encouraged the operation of these generators in islanded mode or intentional islanding. Such operation mode can increase the system reliability, once in the event of network failure the supply of critical consumers will be maintained by the DGs. However, it is necessary to consider that the fault current is considerably reduced when the system is in intentional islanding, and the overcurrent protection setting must be adapted in order to take into account the new fault current level. This paper proposes the use of adaptive protection to change the overcurrent protection setting based on the identification of the system operating state. The system operating state is identified by the use of an active measurement technique, which identifies its operating condition by the impedance level of the grid. Simulation results show that the proposed scheme can be used to update the protection settings for the islanded operation condition.

Methodology for grid voltage unbalance compensation appling a two-converter series DFIG topology

This paper presents a new control strategy for the DFIG replacing the conventional Grid Side Converter for a series one, namely series grid side converter (SGSC). The control strategy for operation under grid voltage unbalance intends not only the ideal machine operation, but also to attenuate the grid voltage unbalance. This compensation is performed injecting unbalanced currents in the point of common coupling (PCC). The SGSC promotes a higher operational capacity during grid disturbances as voltage unbalances. The proposed methodology uses the machine electromagnetic power equation decomposed by the symmetrical components to obtain a relationship between the rotor negative sequence current and the stator negative sequence voltage, where the oscillating components of the electromagnetic power are zero. Appling this relationship is possible to reduce the electromagnetic torque oscillations when injecting negative sequence current by the machine stator.

Analysis of the Doubly Fed Induction Generator performance on frequency support of microgrids

Doubly Fed Induction Generators (DFIG), represent one the of the most employed technologies, for wind conversion systems. Due to the intermittent nature of wind, the conventional control used by these generators has the objective to operate at a point that ensures maximum power extraction of the wind turbine. However, using only this control philosophy, these generators do not participate in frequency control of the power system, since the mechanical system is decoupled from the electrical system due to the action of the back-to-back converter. Consequently, the conventional DFIG does not present inertia response. However, in power systems with high penetration of wind power generation, or systems that operate islanded, as microgrids, the wind power generator inertia response allows a smother behavior of the electrical frequency. In this context, this work studies, analyzes and compares the Doubly Fed Induction Generator behavior contributing to the frequency support of a microgrid formed due an unintentional islanding through the implementation of three control methods in the rotor side converter of the DFIG. In order to evaluate this behavior, computer simulations were performed simulating the occurrence of islanding, where a part of the distribution network shall operate islanded with power supplied by a DFIG and by a synchronous generator. The results allows analyzing the performance of DFIG in the grid frequency support considering different methodologies.