METHOD FOR RESPONDING TO A GRID EVENT

Abstract

A method is provided for controlling a wind power plant, in particular in case of a frequency drop in a utility grid to which the wind turbines are connected, the method including: Combining demand response, inertial response and spinning reserve for given wind speeds in order for wind power plants to deliver fast aggregate under frequency response for a wide wind speed range with minimal recovery time and minimal production loss at each wind speed. In case of the frequency drop the utility grid is additional stabilized by active power, which is provided from a static VAR-compensator and which is fed into the grid. The static VAR-compensator is connected with the wind turbine via a transmission system. The static VAR-compensator is based on using super-capacitors thus it can provide an amount of active power for grid support in case of the frequency drop.

Claims

1. A method for controlling a wind turbine of a wind power plant in case of a frequency drop in a utility grid to which the wind turbine is connected, the method comprising: increasing power extraction from a rotating rotor of the wind turbine; simultaneously decreasing a power demand of the wind turbine from a nominal power demand, thereby increasing a net power output of the wind turbine to the utility grid, wherein the increase of the net power output of the wind turbine to the utility grid is assisted by an amount of active power, which is provided by a static VAR-compensator, while the static VAR-compensator is connected with the wind turbine via a transmission system and while the static VAR-compensator is based on using super-capacitors.

2. The method according to claim 1, further comprising: receiving a command to increase the net power output of the wind turbine by a required amount; determining that a maximal decrease of the power demand is smaller than the required amount; decreasing the power demand of the wind turbine by the maximal decrease of the power demand; and increasing the power extraction from the rotating rotor such that the required amount equals the increased net power output.

3. The method according to claim 1, wherein increasing the power extraction from the rotating rotor resulting in decreasing a rotational speed of the rotor from a nominal rotational speed to a rotational speed lower than the nominal rotational speed.

4. The method according to claim 1, further comprising: operating the wind turbine at a first wind speed range, between 5 m/s and 12 m/s, or at a second wind speed range, between 23 m/s and 30 m/s, extracting power from the rotating rotor to an amount being 5% to 10% below an available power extraction for the given wind speed, receiving a command to increase the net power output of the wind turbine by a required amount; determining that a maximal decrease of the power demand is smaller than the required amount; decreasing the power demand of the wind turbine, from the grid, by the maximal decrease of the power demand; and increasing the extracting the power from the rotating rotor such that the required amount equals the increased net power output.

5. The method according to claim 1, wherein demand reduction capacity information indicative of the maximal decrease of the power demand for at least one operational condition, for different external conditions, is accessed and is processed for controlling the wind turbine.

6. The method according to claim 1, wherein inertial capacity information indicative of a maximal increase of power extraction, between 5% and 10% of an actual power extraction, from the rotating rotor for at least one operational condition, for different external conditions, is accessed and is processed for controlling the wind turbine.

7. The method according to claim 1, wherein recovery time information about a recovery time to recover from a rotational speed loss to the nominal rotational speed, for at least one operation condition, is accessed and is processed for controlling the wind turbine.

8. The method according to claim 1, wherein, depending on a required increase of the net power output, an amount of decreasing the power demand of the wind turbine and an amount of increasing the power extraction from the rotating rotor is determined based on the recovery time information for controlling the wind turbine, in order to minimize the recovery time.

9. The method according to claim 1, wherein the controlling is based on a combination of the demand reduction capacity information, the inertial response capacity information and the recovery time information.

10. The method according to claim 1, wherein the decreasing the power demand of the wind turbine, from the grid, comprises decreasing power demand of at least one electrical wind turbine device comprising at least one of: a de-icing system for de-icing a rotor blade of the wind turbine, a heating system, in particular for de-icing a rotor blade of the wind turbine, a heating system, in particular for heating up controllers enclosures, a cooling system for cooling a mechanical and/or electrical component of the wind turbine, a lighting system, a yaw motor for orienting the wind turbine relative to the wind direction, hydraulics for moving a component of the wind turbine, an uninterruptible power supply system, a pump circulating cooling liquids or cooling liquids.

11. The method according to claim 1, further comprising: supplying a control signal to a converter connected to a generator of the wind turbine causing the converter to increase a torque exerted on the rotating rotor to increase the power extraction from a rotating rotor.

12. The method according to claim 1, adapted for controlling a plurality of wind turbines.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] FIG. 1 schematically illustrates a flow diagram of a method for controlling a wind turbine according to an embodiment of the present invention;

[0071] FIG. 2 illustrates a graph considered in a method according to an embodiment of the present invention;

[0072] FIG. 3 illustrates a recovery delay achieved when performing a method according to an embodiment of the present invention,

[0073] FIG. 4 illustrates a graph showing contribution of particular energy portions as considered in a method according to an embodiment of the present invention; and

[0074] FIG. 5 illustrates a recovery time of a conventional method.

[0075] FIG. 6 illustrates a selection of a respective active power energy source, which is done in dependency of specific time periods.

[0076] FIG. 7 illustrates a respective wind turbine of a wind power plant, which is connected via a transmission system with the utility grid while a SVC-system is connected with the transmission system additionally.

DETAILED DESCRIPTION

[0077] FIG. 1 illustrates a flow diagram of a method 100 for controlling a wind turbine according to an embodiment of the present invention. Thereby, a number of wind turbines 101 are providing information 103 regarding a demand reduction capacity indicative of the maximal decrease of the power demand. Further, the individual wind turbines 101 provide each an inertial capacity information 104 to the controller 105.

[0078] The information 103, i.e. the plural demand reduction capacity information of each turbine 101 is provided to a controller 105 which may receive the particular demand capacities 103 and the particular inertial capacities 104 in order to obtain a sum of demand capacities 107 and a sum of inertial capacities 108 which are then provided to a decision block 109.

[0079] The decision block 109 further receives a command 111 which defines a required amount 113 as to how much increase the net power output of the plural wind turbines. In the decision block 109 it is tested, whether the sum of the demand reduction capacity (sum(P_demand)) is larger than the required amount (P_required) of the additional power required or needed. If the sum of the demand reduction capacity (sum (P_demand)) is larger than the required power increase (P_required), then it is branched to the method step 115, in which the power demand of one or more wind turbines is decreased such as to effectively increase the net power output of the wind turbine being equal to the required power P_required.

[0080] If the sum of the demand reduction capacity (sum (P_demand)) is not greater than the required additional power (P_required), it is branched to a method step 117 in which it is tested whether the sum of the demand reduction capacity (sum (P_demand)) and the sum of the inertial capacity (sum (P_inertial)) is larger than the required additional power P_required. If this is the case, it is branched to the method step 119, in which the power demand of one or more of the wind turbine is decreased and in which further the power extraction from one or more rotating rotors of one or more wind turbines is increased, in order to need the required additional power P_required.

[0081] If on the other hand this is not the case, it is branched to the method step 121, in which additionally to the measures taken in the method step 119 a spinning reserve of one or more of the wind turbines is exploited which allows extracting additional power from the rotating rotor of one or more of the wind turbines which rotating rotor rotates at a rotational speed greater than the nominal speed.

[0082] This logic may also be used to set the spinning reserve reference dynamically, so that only the required amount of spinning reserve is available at any given time.

[0083] Further, a not illustrated arrangement for controlling one or more wind turbines 101, in particular in case of a frequency drop in a utility grid to which the wind turbine is connected, may perform the method 100 according to an embodiment of the present invention.

[0084] FIG. 2 illustrates a graph, wherein on an ordinate 201 the percentage of the power relative to the actual power is indicated, while on the abscissa 203 the power in percent of the nominal power is indicated. The curve 205 indicates the demand reduction capacity of a single wind turbine having a particular number of electrical devices supplied with electric energy and the curve 207 indicates another example of a wind turbine which has even more electrical devices which are supplied with electrical energy from the utility grid.

[0085] As can be seen from curve 205 by switching off one or more of the electrical devices above 8% of the total power produced at about 10% power output may be saved by switching off these electrical devices. The curve 207 indicates that even almost 18% of the energy can be saved by switching off the electrical devices, when the wind turbine is operated according to a power output of 10% of the nominal power output. Further, for larger power output the relative contribution of the energy which may be saved by switching off or turning down the devices of the wind turbine decreases. In particular, the demand respond in the case of curve 205 is able to contribute 9% or around 20% (curve 207) of the production level.

[0086] FIG. 3 illustrates on an abscissa 301 a wind speed in m/s and on an ordinate 303 a recovery delay in seconds, when a method for controlling a wind turbine according to an embodiment of the present invention is performed.

[0087] As can be appreciated from FIG. 3 the recovery delay as indicated by curve 305 is zero for the wind speed range between 0 m/s and 6 m/s and increases relatively rapidly from 6 m/s to 9 m/s where a maximum is achieved. From there the recovery delay decreases to become zero at 11 m/s and for higher wind speed values between 11 m/s and 24 m/s the recovery delay stays at zero. A small increase is observed between 24 m/s and 26 m/s. Beyond 26 m/s the recovery delay stays at zero.

[0088] The recovery delay is in particular advantageously decreased compared to a recovery time of a conventional method, as is illustrated in FIG. 5. In particular, on the abscissa 501 of FIG. 5 the wind speed is indicated in m/s and on the ordinate 503 the recovery time is indicated in seconds. As can be appreciated from FIG. 5, the recovery time 505 is much higher than the recovery time illustrated in FIG. 3. In particular, the recovery time 505 is above 25 s in a wind speed range between 4 m/s and 10 m/s, while the recovery time of the curve 305 according to an embodiment of the present invention is zero in this wind speed range. Further, at a wind speed of 9 m/s the recovery time of the conventional method is at about 25 m/s, while the recovery time according to an embodiment of the present invention (curve 305 of FIG. 3) is only at around 15 s.

[0089] In particular, as can be taken from FIG. 3 the recovery delay is 0 up to 6 m/s followed by a proportional increase in the recovery delay up to a wind speed for nominal production of around 11 m/s and finally a drop to again 0 response delay up to approximately 25 m/s is observed. In particular, for turbine models with a fixed high wind stop limit there will be no response delay above the nominal wind speed all the way up to 40 m/s.

[0090] For wind turbines with a high wind ramp down function there may be a small response delay around 25 m/s where the main portion of the response would have to come from the converter and not from the demand response function.

[0091] A plant controller may calculate a recovery delay indication in a numeric format what the plant operator can expect of a recovery delay at any given time for the actual operation condition for the plant. Thereby, the index may include a time and a capacity designation, such as a demand reduction capacity information and/or inertial capacity information either as a total megawatt indication or a percentage of installed capacity or a percentage of actual capacity. Further, the central controller may also aggregate value for what portion of the simulated inertial response will be performed by the demand response function and what portion will be performed by a temporarily converting additional kinetic energy and exporting it to the public utility grid, thereby distributing between demand response-inertial response and spinning reserve. Thus, the central controller may calculate the portion of the increasing the power extractor from the rotating rotor and also the portion of the decreasing the power demand of the wind turbine in order to meet the additionally required power or energy.

[0092] Further, a slight spinning reserve may be kept by one or more wind turbines. However, this may be applied only during particular wind conditions, where the recovery delay response falls outside the desired criteria.

[0093] FIG. 4 illustrates a graph showing the different contributions for the demand reduction capacity, the inertial capacity and the spinning reserve according to an embodiment of the present invention.

[0094] An abscissa 401 indicates the wind speed in m/s, while an ordinate 403 indicates the percentage of the actual power output. In the area 405 the demand reduction capacity is indicted. As can be appreciated from FIG. 4 the demand reduction capacity is especially high at low wind, i.e. between 1 m/s and 3 m/s, wherein it decreases from 3 m/s to about 13 m/s where it reaches zero. Further, the demand reduction capacity increases from 24 m/s to very high values for higher wind speeds.

[0095] In the areas 407 the contribution of the spinning reserve is indicated. It can be appreciated that the spinning reserve is only provided in two different wind speed ranges, in particular in the range 409 ranging from about 7 m/s to 9 m/s and in the wind range 411 ranging from about 27 m/s to 29 m/s. Thus, in these wind regions the wind power plant is operated with spinning reserve (in particular the rotational speed of the rotor is increased above its nominal level) so that in case of a frequency drop or voltage drop in the utility grid the amount of spinning reserve (in particular the rotational speed) can be reduced in order to extract more energy from the rotating rotor, in order to stabilize the utility grid. Thereby, the spinning reserve may mean a rotor speed change but it may also be just a pitch change or a combination of both.

[0096] In other embodiments spinning reserve is provided across a larger wind speed range from e.g. about 4 m/s to e.g. about 29 m/s by extending the areas 407 vertically upwards in FIG. 4.

[0097] Further, in the speed range 413, i.e. in the range between about 4 m/s and 28 m/s an inertial response is available as indicated by the area 415.

[0098] For example, if the external condition corresponds to a wind speed of 9 m/s and if a grid even occurs, such as a frequency drop, the demand reduction capacity would contribute about 3% of the actual power output as indicated by the double arrow 417. Further, the inertial response would contribute about 6% as is indicated by the double arrow 419. Furthermore, the spinning reserve would contribute with about 1 to 2% as indicated by the double arrow 421. As will be understood by the skilled person at other wind speeds the relative contributions of the demand response, the inertial capacity and the spinning reserve may assume different relative values.

[0099] In a different implementation the contribution from the spinning reserve may be provided by a local turbine or a plant storage system further reducing the need for the wind turbine plant to spill power in order to provide this ancillary service of inertial response to the grid.

[0100] Inter alia it is described a method for controlling a power plant, in particular wind power plant, in particular in case of a frequency drop in a utility grid to which the turbines of the plant are connected, the method comprising: Combining demand response, inertial response and spinning reserve for given wind speeds in order for wind power plants to deliver fast aggregate under frequency response for a wide wind speed range with minimal recovery time and minimal production loss at each wind speed.

[0101] FIG. 6 illustrates a selection of a respective active power energy source, which is done in dependency of specific time periods.

[0102] For the stabilisation of the utility grid a certain amount of active power is provided and fed as actual power into the grid.

[0103] This active power is provided by the wind turbine and by the SVC as described below.

[0104] The time periods for the respective power contributions addresses that point of time, which marks that the grid frequency dropped.

[0105] As shown below the wind turbines of the wind power plant are connected via a transmission system with a super-capacitor based SVC.

[0106] This SVC contributes a level of active power in case of a needed “Transient Under-Frequency-Response, TUFR” (i.e. a simulated inertial response).

[0107] Thus the net power output of the wind turbine to the utility grid is increased.

[0108] The SVC delivers a kind of “fast inertial response” for grid support without having a negative impact on the mechanical structure of the wind turbine.

[0109] As illustrated in FIG. 6 the supply of active power P_act preferably shows different patterns in dependency of certain periods of time t.

[0110] In a first period of time t the active power P_act is supplied and delivered by the SVC. This first period is shown on the left side of FIG. 6 and is denoted there with “SVC_TUFR”. By this supply the under-frequency-response contributes in a more aggressive way to restore the grid frequency.

[0111] In a second period of time t the active power P_act is supplied and delivered by the wind turbine(s) of the wind power plant. This second period is shown in the middle of FIG. 6 and is denoted there with “Turbine TUFR”.

[0112] In a preferred configuration the levels of the active power P_act of the first and of the second period of time t are equal.

[0113] In a third period of time t the active power P_act is supplied and delivered by the SVC again. This third period is shown on the right side of FIG. 6 and is denoted there with “SVC_TUFR”.

[0114] In a preferred configuration the active power P_act, which is provided in the third period of time t, is ramped down from a level, which is different to the levels before.

[0115] Thus a predictable combined TUFR with a minimum recovery time from a transmission company perspective is provided.

[0116] The shape of the respective active power contribution, provided by SVC and/or by the turbine(s), is varied in dependency of the actual needs of the grid.

[0117] The shape may even be varied in dependency of the given specific site location and architecture.

[0118] As shown in FIG. 6 respective power can be pulled out from a single source at a time or can be pulled out from combined sources in parallel at a time.

[0119] Thus an optimal magnitude and duration of the respective overall power in view to specific needs of a project is possible.

[0120] The magnitude of the TUFR-contribution can also be defined in relation to the severity of a given frequency deviation.

[0121] Thus the provided power level is designed in view to the frequency deviation and its deviation level from the nominal grid frequency.

[0122] The magnitude of the TUFR-contribution can be designed as a proportion of the actual power level at that point of time, when the frequency drop is detected.

[0123] The magnitude of the TUFR-contribution can be designed as a proportion of the installed capacity of the wind power plant or some other correlation to the actual power delivered to the grid.

[0124] The detection of when to activate the TUFR function can be determined by a delta f (df) detection rate of change (df/dt) or an external state signal.

[0125] FIG. 7 illustrates a wind power plant WP, which comprises a number of wind turbines WT1-WTn.

[0126] The wind turbines WT1-WTn are connected via a transmission system TS with a static VAR compensator system SVC.

[0127] The static VAR compensator system SVC is based on using super-capacitors, thus the system is able to provide an amount of active power for grid support.

[0128] The transmission system TS might be a high voltage transmission system thus it might comprise a transformer capability denoted as TF1.

[0129] The transmission system TS is connected with the utility grid of a grid operator.

[0130] It should be noted that the term “comprising” does not exclude other elements or steps and “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.