Pablo Ledesma

Doubly fed induction generator model for transient stability analysis

This paper proposes a model of the doubly fed induction generator (DFIG) suitable for transient stability studies. The main assumption adopted in the model is that the current control loops, which are much faster than the electromechanic transients under study, do not have a significant influence on the transient stability of the power system and may be considered instantaneous. The proposed DFIG model is a set of algebraic equations which are solved using an iterative procedure. A method is also proposed to calculate the DFIG initial conditions. A detailed variable-speed windmill model has been developed using the proposed DFIG model. This windmill model has been integrated in a transient stability simulation program in order to demonstrate its feasibility. Several simulations have been performed using a base case which includes a small grid, a wind farm represented by a single windmill, and different operation points. The evolution of several electric variables during the simulations is shown and discussed.

Transient stability of a fixed speed wind farm

A typical fixed speed wind farm connected to a simple grid is modelled. Using this model, a three-phase fault is applied close to the wind farm, and cleared by disconnecting the affected line. The effect of several electric, mechanical and operational parameters on the critical fault-clearing time of this base case is evaluated and discussed. The studied parameters are the short-circuit power at the connection bus, the reactive power compensation, the distance to the fault, the rotor inertia, the hub-generator resonant frequency, the wind speed and the power output. For each parameter, the relationship between its value and the critical fault-clearing time is shown graphically. The results help to understand the transient stability phenomena in fixed speed wind farms, and could help to design fixed speed wind farms attending to transient stability requirements.

Incidence on power system dynamics of high penetration of fixed speed and doubly fed wind energy systems: study of the Spanish case

In this paper, a preliminary analysis of the impact of high wind power penetration in the planning and operation of the Spanish power system is presented. The problems studied in the paper are those related to the stability of the power system. This key subject could be deeply influenced by the installation of up to 15000 MW of wind power generation as it has been planned for the next six years. The analysis presented here is based on the results of dynamic simulations. Dynamic models of the induction generator (squirrel cage and doubly fed) and models of the wind farms have been developed to make possible the simulations. From the simulation results some conclusions and recommendations have been extracted, which would contribute to the appropriate integration of the new wind generation foreseen for the Spanish power system.

Contribution of Variable-Speed Wind Turbines to Voltage Control

Variable speed, grid connected, wind turbines open new possibilities for voltage control, because they use electronic converters, which may regulate the reactive power interchange with the grid. This paper proposes two voltage control schemes for variable speed wind turbines with double-fed induction generator. The first scheme acts on the wind-turbine power factor, while the latter acts directly on the converter current. Advantages and drawbacks of both techniques are discussed. Both control techniques have been tested by simulations of a base case, which represent a synchronous generator, a wind farm and a local load, and several disturbances such as the loss of compensator capacitors.

Function approximation with neural networks for obtaining an operating point sufficiently small signal stable in power systems including wind parks

This paper shows a simple approach to obtain an operating point sufficiently small signal stable, using function approximation with neural networks. The idea is to use a neural network to predict systems eigenvalues, taking as input data voltage at buses, generated power, reactive load, and the output data are the eigenvalues. Unstable and Poorly damped modes are identified and then these modes will be damped. The system modifies the parameters until reach a stable operating point. In the case of a stable operating point with a poorly damped oscillatory mode, the objective is to increase the damping of that mode. That is, the power system linearization at the operating point is modified. Operator actions such as redispatch, varying load, varying reactive power (voltage) often modify the operating point to do this; the effect of this is that transients near enough to the operating point will decay more quickly. However, the analysis does not attempt the more difficult study of large signal transients. The existence of a stable operating point is of course necessary for system security, but there is no guarantee that large signal transients will result in operation at that operating point.

Modification of power system linearization at the operating point to improve the stability in power systems including wind parks

This paper shows a simple approach to modify power system linearization to improve stability in power systems including wind parks. In the case of a stable operating point with a poorly damped oscillatory mode, the objective is to increase the damping of that mode. That is, the power system linearization at the operating point is modified. Operator actions such as redispatch, varying load, varying reactive power (voltage) often modify the operating point to do this; the effect of this is that transients near enough to the operating point will decay more quickly. However, the analysis does not attempt the more difficult study of large signal transients. The existence of a stable operating point is of course necessary for system security, but there is no guarantee that large signal transients will result in operation at that operating point.