Wind turbines employing double-fed induction generators (DFIG) use two converters, one for the rotor side and one for the generator side. To improve the performance during severe grid failures, in [1] the advantages of using alternative configurations to the two-level converter, such as the parallel interleaved 2-level inverter, and the 3-level inverter, have been investigated. It has also been investigated to replace the classical dq-PLL with a new PLL, and to include a series dynamic braking resistor (SDBR) between the converters and the threephase connections.Wind turbines must meet strict requirements, in terms of their behavior, in the event of grid failures, which are regulated by the LVRT regulations in each country. These regulations indicate, by means of voltage and time graphs, how long the wind turbines must remain connected depending on the depth of the faults. In addition, the limits of active and reactive power that can be exchanged during faults are established. The aim is to avoid cascading disconnections of wind turbines that would compromise the stability of the grid. In [2], the effect of various elements in improving the behaviour of a DFIG against grid faults has been investigated. The first of these elements is the parameters of the IGBTs, concluding that the on-resistance has the greatest influence. The second is the use of a new PLL, and the third is the use of a SDBR during a grid failure. It was found that all of them could improve the performance of the generator in the event of a grid failure.When a wind turbine uses a permanent magnet synchronous generator (PMSG), 100% of the energy generated passes through both converters. In [3], the control systems of the generatorside and grid-side converters have been considered; several scenarios regarding the turn on resistance of the IGBTs have been considered, and their behaviour during grid faults has been analysed. Generator performance has also been studied with and without the use of a DC-DC converter for overvoltage protection.The main generators used in wind turbines are DFIG and PMSG. In [4], a comparison of the behaviour of both wind turbines against grid faults has been carried out in various scenarios with different values of generator parameters.The number of converters, associated with renewable generation, connected to the grid has increased significantly lately. This can affect the dynamic response, especially during disturbances, but it can also provide new grid support functionalities if information on the oscillation characteristics is available. Through the use of artificial intelligence, in [5] the abilities to predict and damp electromechanical oscillations have been improved.With the constant increase in the number of wind turbines connected to the grid, it is very important to have the ability to maintain grid frequency control. In [6], a work has been presented to stabilise the wind farm during periods of wind speed change by using capacitors connected via a DC-DC converter and a grid-connected DC-AC converter. It was concluded that higher values of the DC-DC converter time constant lead to better performances during load transients. A system including two steam turbines and two squirrel cage induction generators was used in the experiments.Microgrids bring power generation closer to the places of consumption to reduce the saturation of distribution lines. They consist of renewable generation, energy storage and fossil fuel generation. They have three levels of control, where the primary level is the closest to the converters, and the tertiary level, the most external and slowest, performs general monitoring functions. The paper [7] focuses on the secondary control functions related to grid failure performance. It proposes a distributed control between inverters and is based on fast detection techniques (fast Delayed Signal Cancellation, DSC), with the objective of a fast control of active and reactive power. A robust transition method between fault mode and normal mode is proposed. Accurate coordination and power sharing between distributed energy resources is achieved.Some energy conversion systems are so complex that they are very difficult to build and test in the laboratory. These include the study of high voltage direct current (HVDC) transmission when several modular multilevel converters (MMC) are involved together with DC grid failure protection elements. In [8], a system including a simulated part (two digitally simulated MMC) and a physical part (two MMC) has been experimented with. The coupling between the two parts has been carried out by means of A/D and D/A converters and power amplifiers.[
Editorial06 October 2021
Editorial: Grid Connection of Converters in Renewable Applications
This paper investigates the transient performance of the two Variable Speed Wind Turbines (VSWTs) that are widely employed in wind energy conversion, considering their machine parameters. The two wind turbines are the Doubly Fed Induction Generator (DFIG) and the Permanent Magnet Synchronous Generator (PMSG). The machine parameters of both wind turbines were varied considering different scenarios, while keeping other parameters constant, in order to study the behavior of the wind generators. The study was carried out using the same operation conditions for both wind turbines technology. The wind turbines were subjected to a severe three-phase to ground bolted fault, in order to test the robustness of their controllers during grid fault conditions. Efforts were made to carry out an extensive comparative study to investigate the machine parameters that have more influence on the stability of the different wind turbines considered in this study. Simulations were carried out in Power System Computer Aided Design and Electromagnetic Transient Including DC (PSCAD/EMTDC). Effective machine parameter selection could help solve Fault Ride Through (FRT) problems in a cost effective way for both VSWTs, without considering external circuitry and changing of the original architecture of the wind turbines.
ORIGINAL RESEARCH
21 April 2021
One of the ways of generating electrical power from wind energy is by employing the promising technology of the Permanent Magnet Synchronous Generator (PMSG) Variable Speed Wind Turbine (VSWT). With the daily increase and integration of wind farms into traditional power grids, it is imperative to carry out transient stability studies of wind generators in wind farms, in order to fulfil the operational grid codes. To solve the transient stability intricacies posed by the stochastic nature of wind energy during transient states or grid faults, this paper presents the enhancement of PMSG wind turbine considering the excitation parameters of the Insulated Gate Bipolar Transistors (IGBTs) of the wind generator. The investigation was carried out using the turn on and turn off resistances of the IGBTs of the power converters of the PMSG wind turbine, considering different scenarios, with and without overvoltage protection scheme. A severe three-line to ground fault was used to test the robustness and rigidity of the controllers of the wind generator during transient state. Furthermore, the results obtained using the PMSG wind turbine were compared to those using the Doubly Fed Induction Generator (DFIG) wind turbine. The evaluation of the system performance was done using Power System Computer Design and Electromagnetic Transient Including DC (PSCAD/EMTDC) platform. The same conditions of operation were used in investigating the various scenarios considered in this study.
ORIGINAL RESEARCH
27 January 2021
The major aim of successful synchronization of a wind turbine system to the grid, is to mitigate electrical and mechanical stresses on the wind generator. During transient, the gearbox, shaft and rotor of the wind generator could be damaged due to mechanical stress. Also, the rotor and stator windings of the wind generator, including its insulation could be affected. In this paper, an extensive analysis of the effects of the excitation parameters of the power converter Insulated Gate Bipolar Transistors (IGBTs), on the transient performance of the Doubly Fed Induction Generator (DFIG) was carried out considering different scenarios. The salient part of this work is that, the best excitation parameters of the IGBTs were used for further analysis of the wind generator, considering a new Phase-Locked-Loop (PLL) scheme. The PLL computes phase displacement of the grid required to achieve orientation and synchronization control. Consequently, it helps in preventing power system distortion due to stator-grid interphase. This paper proposes a new PLL control strategy and Series Dynamic Braking Resistor (SDBR) combined approach, to augment the fault ride through capability of a variable speed wind turbine DFIG-based. The SDBR helps in the post fault recovery of the wind generator. Simulations were run in Power System Computer Aided Design and Electromagnetic Transient Including DC (PSCAD/EMTDC) for severe fault conditions, to test the robustness of the controllers employed. The results show that the proposed hybrid control strategy help in the fast recovery of the DFIG wind generator variables, during fault conditions.