ELECTRICITY GENERATION SYSTEM THAT WITHSTANDS VOLTAGE DIPS
The present invention describes a power generation system resistant to voltage dips and a method for operating said generator. The existence of disturbances in the power grid requires power generation plants to contribute to its stability, for example by injecting reactive current in the case of voltage dips. Until recently, these requirements have not become extensive to power plants based on renewable energy sources, as these represented a small percentage of the total power generated. However, in view of the spectacular increase in these types of power generation plants in recent years, the level of exigency of these types of power generation plants has increased considerably. In the specific case of voltage dips, there are regulations that require aerogenerators to fulfill stringent requirements with regard to the reactive current injected into the grid during the voltage dip and it is envisaged that, once the voltage dip that required disconnection of the generator has been overcome, it can be quickly reconnected after the dip if voltage returns after a few seconds. In this manner, the active power that was being generated before the disturbance will be injected as quickly as possible. These requirements are usually expressed in the form of the so-called dip profile, which defines temporal and amplitude limits for the voltage dips that the aerogenerators must be capable of bearing. The state of the art proposes different courses of action in the event of voltage dips, such as for example:
Therefore, neither of these systems allows fulfillment of grid requirements and fast reconnection once the voltage has been re-established. The proposed invention resolves the aforementioned drawbacks through a system that combines the advantages of the two known systems of the prior art. According to a first aspect, the voltage dip-resistant electrical power generation system of the invention comprises a doubly-fed generator, the rotor or which is connected to the power grid by a back-to-back converter and the stator thereof is connected to the power grid, in addition to comprising:
That is, the system of the invention simultaneously comprises the two additional impedances disclosed by the systems of the prior art and at least one control unit that manages the activation/deactivation of said additional impedances according to the method described later in the text; said method makes them function in a coordinated manner, different to that of independent systems. The control unit(s) may be dependent or independent from the control unit of the converter. While not explicitly mentioned herein, it is understood that the described system also comprises all the usual auxiliary elements of power generation systems based on doubly-fed generators known to a person skilled in the art. For example, it is evident that the connection between the stator and the grid will have a switch that allows disconnection of the generator, in addition to the existence of means, such as switches or similar, to activate the additional impedances at the times that will be defined later in the text. It is understood that the generator rotor of the system can be moved by any type of renewable energy source, for example sea currents or tides. According to a preferred embodiment, however, the generator rotor is mechanically coupled to a wind turbine, thereby forming an aerogenerator assembly. A second aspect of the invention is aimed at a method for operating the previously described system which combines the advantages of each of the systems known in the prior art, while avoiding the individual drawbacks of each. The power generation system detects a voltage dip and injects the required reactive current into the grid while the system is operating within the established dip profile. To this end, the first additional impedance is activated if necessary, even though the invention also comprises the case wherein activation is not required. When it detects that a maximum period operating under minimum voltage conditions has elapsed or when re-establishing control over aerogenerator load torque is deemed necessary, the stator is disconnected from the grid and the second impedance is activated, absorbing the power, which allows control of the generator through the load torque. Therefore when the grid recovers, the system is capable of synchronizing the voltage generated and that of the grid and coupling much faster than in the case of the systems known to date, thereby increasing system availability. Therefore, the method of the invention comprises injecting reactive current into the grid without disconnecting the back-to-back converter when a voltage dip is detected (provided that the voltage exceeds the limits marked by the dip profile). The injection of reactive current can be carried out during a part or throughout the duration of this stage. In this manner, it contributes to grid recovery. Additionally, after the maximum permitted period of operation under minimum voltage conditions has elapsed (this period is the time elapsed between the start of the voltage dip and the moment in which grid voltage drops below a dip profile imposed by a grid operator) or when re-establishing control over the load torque in the aerogenerator is deemed necessary, the stator is disconnected from the grid and the second additional impedance is activated. In this manner, rapid reconnection is allowed when grid voltage returns to its nominal values. When the end of the voltage dip is detected, stator voltage is synchronized with grid voltage and the stator is reconnected to the grid, subsequently deactivating the second additional impedance. The proposed invention also envisages that the order in which this last phase is carried out comprises firstly deactivating the second additional impedance and, subsequently, reconnecting the stator to the grid. The invention is described below making reference to the attached figures. In particular, This system (100) comprises additional impedance (105) in parallel between the rotor and the back-to-back converter (104), which is activated in the event of voltage dips (103) to protect the rotor converter (104 The invention comprises a doubly-fed generator (2), the rotor thereof is connected to the power grid (3) by a back-to-back converter (4) and the stator thereof is connected to the power grid (3), essentially standing out for additionally comprising at least one first additional impedance (5) connected in parallel between the generator (2) rotor and the back-to-back converter (4); at least one second additional impedance (6) connected to the generator stator (2); and at least one control unit capable of governing the additional impedances (5, 6). 1-3. (canceled) 4. Electrical power generation system (1) resistant to voltage dips which comprises a doubly-fed generator (2), the rotor thereof is connected to the power grid (3) by a back-to-back converter (4) and the stator thereof is connected to the power grid (3), characterized in that it comprises:
at least one first additional impedance (5) connected in parallel between the generator (2) rotor and the back-to-back converter (4); at least one second additional impedance (6) connected to the generator stator (2), adapted to be activated only once the grid voltage falls below a dip profile and at least one control unit capable of governing the additional impedances (5, 6). 5. System (1), according to 6. Method for operating a system (1), according to detecting the voltage dip, remaining connected to the grid (3) and making use, for the required period of time, of the first additional impedance (5), injecting the required reactive current, detecting that a maximum operating period elapses under these conditions, disconnecting the stator of the grid (3) stator and activating the second additional impedance (6) once the grid voltage falls below a dip profile, thereby controlling the generator (2) load torque, detecting the re-establishment of grid (3) voltage to values within the operating range, synchronizing stator voltage and grid (3) voltage, connecting the stator to the grid (3) and deactivating the second additional impedance (6). 7. Method for operating a system (1), according to detecting the voltage dip, remaining connected to the grid (3) and making use, for the required period of time, of the first additional impedance (5), injecting the required reactive current, detecting that a maximum operating period elapses under these conditions, disconnecting the stator of the grid (3) stator and activating the second additional impedance (6) once the grid voltage falls below a dip profile, thereby controlling the generator (2) load torque, detecting the re-establishment of grid (3) voltage to values within the operating range, synchronizing stator voltage and grid (3) voltage, connecting the stator to the grid (3) and deactivating the second additional impedance (6).OBJECT OF THE INVENTION
BACKGROUND OF THE INVENTION
DESCRIPTION
BRIEF DESCRIPTION OF THE FIGURES
DESCRIPTION OF A PARTICULAR EMBODIMENT