The number of wind generators installed in many power systems worldwide has relevantly increased in the last years. As a consequence conventional generators are being progressively substituted by others than, due to the nature of its primary energy, have limited capacity to modify the generated active power. Furthermore, many of these generators operate at variable speed and are not able to add their inertia to the power system. These circumstances have a clear negative effect on the frequency variations that take place after a sudden loss of generation or load.
Several transmission systems operators have designed operating procedures forcing wind generators to modify their active power in terms of the system frequency. In the majority of the cases these directives focus in the obligation of reducing the generated power in case of high frequency situations.
However new alternatives have been proposed in the technical literature in order to take advantage of the flexibility and fast response of variable speed wind generators allowing the regulation of the generated power and being able to control to a larger extent frequency oscillations rates.
In this Doctoral Thesis a complete analysis regarding the contribution of variable speed wind generators to the frequency control of power systems has been performed. First, a numerical model of a doubly-fed induction generator specific for the performance of load-frequency control analyses has been implemented and introduced as an aggregated unit representing the wind energy generation in the power system model. In the complete model the effect of the following elements has also been considered: i) three types of conventional generation based on steam, hydraulic and combined cycle turbines; ii) an automatic load-shedding system; and iii) a system for automatic disconnection of intermittent generators.
Using the previously described model several hypothetical generation scenarios and incidents have been simulated obtaining the frequency variations experimented by the system when wind generators do not contribute to the frequency control. Under severe incidents the presence of the automatic load-shedding system ensures limited frequency variations in most scenarios. Only in the case of valley scenarios, when conventional generation has been uncoupled due to an excessive wind power generation, a bad performance is detected. In these cases fast frequency variations associated to the low inertia of the system lead to an excessive load-shedding. As a consequence the frequency undergoes severe oscillation levels activating the disconnection of intermittent generation units. In extreme cases this can lead to the collapse of the power system. In these cases it has been proven that the contribution of wind farms to the frequency control achieves a proper automatic load-shedding and, consequently, a fast stabilization of the frequency with a very small quasi-static deviation.
For the remaining scenarios, including valley situations in which inertia levels are higher, as conventional generation are at the minimum operating level, special attention has been paid to how the contribution of wind farms to the frequency control affects the automatic load-shedding schemes.
It has been proven that when small incidents take place, as can be compensated with conventional generation effects, wind farms may reduce the amount of load shed. On the other hand, under the appearance of relevant incidents as load-shedding is unavoidable the presence of wind farms in the power system may be detrimental. While the wind generators maintain its power increment, the power system behaves as if this increment came from conventional generation or as if the incident causing the frequency variation had a smaller magnitude. In some scenarios this leads to a smaller amount of load shed at first. If this is not sufficient for the frequency stabilization the load-shedding process will continue until lower frequency loads are shed. As a result the frequency deviation increases and the load-shedding process does not correspond to the incident magnitude.
In many cases the power increment induced by the frequency control of the wind generators leads to a reduction of its rotating velocity lowering the generated power. Additional control algorithms have been proposed with the aim of recovering the initial velocity stable regime.
Finally, the effect of wind energy generation in electricity balancing markets has been evaluated in the particular case of Spain and compared with other regions. From these comparative analyses several improvement measurements have been proposed with the aim of progressively improving the integration of wind energy in electricity markets.