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:

• • A first structure is disclosed in WO2007057480A1, wherein the aerogenerator comprises variable impedance connected in parallel between the back-to-back converter and the generator rotor. When a voltage dip occurs, said impedance is activated to protect the back-to-back converter from the voltage surges that appear in the rotor during the dip, thereby allowing the generator stator to remain connected to the grid for a certain time interval from the start of the voltage dip, injecting reactive current during that time to contribute to grid recovery. If the duration of the voltage dip exceeds the limits marked within the dip profile, the generator will eventually become disconnected from the grid. The main advantage of this system is that it allows fulfillment of grid reactive current injection requirements during the time in which the generator remains connected to the grid. The main disadvantage is that the disconnection of the generator implies stopping the wind turbine, due to which a long time is required to couple it to the grid after the voltage dip.
  • WO2009156540A1 and U.S. Pat. No. 7,332,827 disclose a second solution known in the art which is based on impedance connected to the generator stator. In this case, the operating method consists of activating said impedance at the time the dip is detected, which allows disconnection of the generator from the grid while controlling generator load torque and evacuating the power generated towards said impedance. The main advantage of this system is that, if the voltage recovers within a certain time interval, the stator can be coupled to the grid, provided that it does not exceed the maximum limits established by regulations. The drawback is that the generator is disconnected from the grid from the start of the voltage dip, due to which it cannot inject the reactive current required by grid operators to contribute to recovery thereof.

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:

• • at least one first additional impedance connected in parallel between the generator rotor and the back-to-back converter;
  • at least one second additional impedance connected to the generator stator; and
  • at least one control unit capable of governing the additional impedances (5, 6).

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, FIGS. 1 and 2a-d show a system (100) according to the prior art specifically applied to wind power generation. This system comprises a doubly-fed generator (102), the rotor thereof is connected to the power grid (103) by a back-to-back converter (104) formed by a rotor converter (104a), a grid converter (104b) and a direct current link (104c). The rotor is also mechanically coupled to a wind turbine (107). The stator, on the other hand, is connected to the grid (103) by means of a switch (108).

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 (104a) from the transient surges generated during the dip. FIGS. 2a-d respectively show grid voltage (U) behavior, activation (C_ZR) of the additional impedance (105), assuming that activation thereof has been required, the reactive intensity (i_q) injected into the grid (103) during the dip and the coupling status (C_on) of the generator (102) to the grid (103). It can be observed how, upon detecting the voltage dip, the additional impedance (105) is immediately activated for a short period of time (FIG. 2b), whereupon reactive current is injected into the grid (103) (FIG. 2c). Once grid (103) voltage (U) falls below the dip profile imposed by regulations, represented herein by a broken line in FIG. 2a, the generator (102) becomes disconnected (FIG. 2d).

FIG. 3 shows a second system (300), according to the prior art, where parts equivalent to those of the system (100) of FIG. 1 have been referenced using the same reference number but substituting the original 1 for a 3. The system (300), however, has additional impedance (306) connected to the generator stator (302). FIGS. 4a-d show some characteristic system (300) magnitudes during operation thereof. Specifically, FIG. 4a shows the shape of the voltage (U) dip in relation to the dip profile imposed by regulations (dip profile represented herein by a broken line). From the time the dip is detected, the additional impedance (306) is activated (C_ZS) (FIG. 4b) and the generator (302) is decoupled (C_on) from the grid (303) (FIG. 4d). When grid (303) voltage (U) falls below the dip profile, the additional impedance is deactivated (306) (FIG. 4b). As can be observed in FIG. 4b, reactive intensity (i_q) is not injected into the grid (303) at any point.

FIG. 5 shows the power generation system (1) of the invention that comprises an electric generator (2) mechanically coupled to a wind turbine (7), the stator thereof is connected to the grid (3) by means of a switch (8) and the rotor thereof is connected to a back-to-back converter (4) which is in turn connected to the grid (3). The back-to-back converter is formed by the rotor converter (4a) and the stator converter (4b) joined by a direct current link (4c). The system (1) also comprises a first additional impedance (5) connected in parallel between the generator rotor (2) and the rotor converter (4a) and a second additional impedance (6) connected to the stator. The instants of activation/deactivation of the additional impedances (5, 6) are controlled by means of a control unit (not shown).

FIGS. 6a-e show some graphics that illustrate the operation of the system (1) of the invention when a voltage dip occurs, the duration thereof requires the use of both additional impedances (5, 6). FIG. 6a shows the voltage dip in relation to the dip profile imposed by regulations (the dip profile is represented herein by means of a broken line). Firstly, as can be observed in FIG. 6b, the first additional impedance (5) is activated (C_ZR) and, shortly afterwards, reactive current (i_q) is injected into the grid (3) (FIG. 6d). In this embodiment, once grid (3) voltage (U) falls below the dip profile, the generator (2) is disconnected from the grid (3) (FIG. 6e) and the second additional impedance (6) is activated (C_ZS) (FIG. 6c). Evidently, as shown in FIG. 6d, the injection of reactive current (i_q) into the grid (3) ends at that moment and the power generated is dissipated in the second additional impedance (6). When grid (3) voltage (U) returns to its nominal values, as shown in FIG. 6a, stator voltage and grid (3) voltage are synchronized and the generator (2) is reconnected to the grid (3), whereupon the second additional impedance (6) is deactivated.