METHOD FOR CONTROLLING A POWER PLANT

Abstract

The present invention relates to a method for operating a power plant comprising a plurality of power producing units, the power plant being operatively connected to an associated power grid having a nominal grid frequency, the method comprising the steps of determining grid frequency variations of the associated power grid, and providing, in response to the determined grid frequency variations, an active power set-point to each of the plurality of power producing units, said active power set-point causing each of the plurality of power producing units to produce active power in response to said active power set-point, said produced active power comprising, in a frequency domain, one or more active power frequency components being different from one or more undesired frequency modes. The present invention also relates to a power plant controller for performing the method.

Claims

1. A method for operating a power plant comprising a plurality of power producing units, the power plant being operatively connected to an associated power grid having a nominal grid frequency, the method comprising: a) determining grid frequency variations of the associated power grid, and b) providing, in response to the determined grid frequency variations, an active power set-point to each of the plurality of power producing units, said active power set-point causing each of the plurality of power producing units to produce active power in response to said active power set-point, said produced active power comprising, in a frequency domain, one or more active power frequency components being different from one or more undesired frequency modes.

2. The method of claim 1, wherein the one or more undesired frequency modes comprise one or more preselected frequency modes.

3. The method of claim 2, wherein the one or more preselected frequency modes comprise one or more known natural frequency modes of the power producing units, such as one or more tower natural frequency modes.

4. The method of claim 1, wherein the one or more undesired frequency modes comprise one or more dynamic frequency modes being based on measured parameters, such as a measured side-to-side tower acceleration and/or a measured active power.

5. The method of claim 4, wherein the one or more dynamic frequency modes are determined in real time.

6. The method of claim 1, wherein the one or more active power frequency components, in the frequency domain, are different form the one or more undesired frequency modes by a certain margin.

7. The method of claim 1, wherein each active power set-point is specifically tailored to the power producing unit to which it is intended to be provided.

8. The method of claim 1, wherein the power producing units are selected from the group consisting of: wind turbine generators, photovoltaic units and batteries.

9. (canceled)

10. A power plant controller for controlling a power plant comprising a plurality of power producing units, the power plant being operatively connected to an associated power grid having a nominal grid frequency, the power plant controller comprising: a) means for determining grid frequency variations of the associated power grid, and b) a dispatcher arrangement for providing, in response to the determined grid frequency variations, an active power set-point to each of the plurality of power producing units, said active power set-point causing each of the plurality of power producing units to produce active power in response to said active power set-point, said produced active power comprising, in a frequency domain, one or more active power frequency components being different from one or more undesired frequency modes.

11. A power plant controller according to claim 10, wherein the one or more undesired frequency modes comprise one or more preselected frequency modes.

12. A power plant controller according to claim 11, wherein the one or more preselected frequency modes comprise one or more known natural frequency modes of the power producing units, such as one or more tower natural frequency modes.

13. A power plant controller according to claim 10, wherein the one or more undesired frequency modes comprise one or more dynamic frequency modes being based on measured parameters, such as a measured side-to-side tower acceleration and/or a measured active power.

14. A power plant controller according to claim 13, wherein the one or more dynamic frequency modes are determined in real time.

15. A power plant controller according to claim 10, wherein the one or more active power frequency components, in the frequency domain, are different form the one or more undesired frequency modes by a certain margin.

16. A power plant controller according to claim 10, wherein each active power set-point is specifically tailored to the power producing unit to which it is intended to be provided.

17. A power plant controller according to claim 10, wherein the power producing units are selected from the group consisting of: wind turbine generators, photovoltaic units and batteries.

18. (canceled)

19. A computer program product for performing an operation when said computer program product is executed on one or more processors, the operation, comprising: determining grid frequency variations of a power grid having a nominal frequency and associated with a power plant comprising a plurality of power producing units, and providing, in response to the determined grid frequency variations, an active power set-point to each of the plurality of power producing units, said active power set-point causing each of the plurality of power producing units to produce active power in response to said active power set-point, said produced active power comprising, in a frequency domain, one or more active power frequency components being different from one or more undesired frequency modes.

20. A power plant, comprising: a plurality of power producing units associated with a power grid having a nominal frequency; the plurality of power producing units selected from one or more of: wind turbine generators, photovoltaic units and batteries; and a controller communicatively coupled to the plurality of power producing units and configured to perform an operation, comprising: determining grid frequency variations of the power grid, and providing, in response to the determined grid frequency variations, an active power set-point to each of the plurality of power producing units, said active power set-point causing each of the plurality of power producing units to produce active power in response to said active power set-point, said produced active power comprising, in a frequency domain, one or more active power frequency components being different from one or more undesired frequency modes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will now be explained in further details with reference to the accompanying figures, wherein

[0031] FIG. 1 illustrates a grid frequency response with a narrow deadband,

[0032] FIG. 2 shows a block diagram of a wind power plant,

[0033] FIG. 3 shows a block diagram of a control scheme without real time feedback,

[0034] FIG. 4 shows a block diagram of a control scheme with real time feedback, and

[0035] FIG. 5 shows a block diagram of a control scheme with pre-dispatcher and real time feedback.

[0036] While the invention is susceptible to various modifications and alternative forms specific embodiments have been shown by way of examples in the drawings and will be described in details herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

[0037] In a general aspect the present invention relates to a method and a power plant controller for providing a safe grid frequency response in environments where there might be grid frequency oscillation modes close to, for example, the natural frequency oscillation modes of the power production units of the power plant. This includes conditions where the deadband has been reduced and/or where the power production units respond to under-frequency events via, for example, power curtailment.

[0038] Moreover, the present invention may facilitate safe operation in connection with small signal damping operation using active power using for example existing frequency response modules of power plants. Moreover, the present invention is not detrimental to the lifetime of the power producing units.

[0039] Generally, the present invention aims to decouple grid frequency oscillation modes caused by for example grid angle variations from a power production unit response during grid frequency response operation in order to allow for a more safe and controlled operation at the power production unit level. According to the present invention the output from an existing frequency control module in a power plant controller is distributed as active power set-points to a plurality of power production units in the power plant in a selective manner so that the active power set-points to the individual power producing units have their respective local frequency oscillation modes decoupled from the grid frequency oscillation modes at the power plant level.

[0040] As it will be disclosed in further details below the present invention may be implemented using the output of a power plant controller frequency control module and distribute separated active power set-points to the individual power production units based on frequency modes. According to the present invention the active power set-points including frequency oscillation modes close to undesired power production unit modes, e.g. natural tower frequency modes or other modes, will be distributed accordingly so that the individual active power set-points have a safety margin away from unwanted or undesired operation modes.

[0041] The present invention may also be implemented on the basis of real time feedback information from each power production unit. The feedback information may relate to current operational status and may thus provide information such as side-to-side tower accelerations, active power output of the power production units etc. The overall objective of providing real time feedback information is to provide an optimized dispatcher strategy in order to establish an optimized decision process when dispatching active power set-points to each of the power production units.

[0042] Referring now to FIG. 1 a typical grid frequency response function, i.e. P.sub.ref vs. f.sub.grid, is depicted where P.sub.ref is the active power set-point and f.sub.grid is the grid frequency. The active power set-point, P.sub.ref, typically relates to an entire power plant, but it may in principle also relate to a single power producing unit. As depicted in FIG. 1 a change or oscillation in the grid frequency of Δf.sub.grid will result in an active power set-point change of ΔP.sub.ref in order to, on for example a power plant level, counteract the grid frequency change or oscillation, Δf.sub.grid. A narrow deadband 100 exist around the nominal grid frequency which may be 50 Hz or 60 Hz.

[0043] Turning now to FIG. 2 a block diagram of a wind power plant connected to an associated power grid 201 is depicted. In general, and as illustrated in lower left portion of FIG. 2, detected grid frequency variations 208 cause an active power response 209 from the wind power plant, i.e. on a plant level, in order to counteract the detected grid frequency variations 208.

[0044] The wind power plant comprises a plurality wind turbine generators (WTG.sub.1-WTG.sub.x) being connected to an internal power grid 207 which is connected to the associated power grid 201 via an optional transformer 204. A point of common coupling (PCC) 205 separates the internal power grid 207 from the associated power grid 201. Moreover, a point of measurement (PoM) 206 is provided between the PCC 205 and the transformer 204. Various rneasureable parameters like for example active power, reactive power, grid frequency, phase angle, grid voltage etc. may be measured at the PoM 206 and provided to the power plant controller (PPC) 202 for further processing. It should be noted that the various measureable parameters may also be provided by other plant or turbine controllers, synchrophasors etc.

[0045] In case of an active power response from the wind power plant due to detected grid frequency variations the dispatcher 203 provides individual active power set-points (P.sub.SP1-P.sub.SPx) to the plurality wind turbine generators (WTG.sub.1-WTG.sub.x) which, in response thereto, provides respective active power distributions (P.sub.1-P.sub.x) to the internal power grid 207 as depicted in FIG. 2.

[0046] According to the present invention the active power produced by wind turbine generators (WTG.sub.1-WTG.sub.x) in response to the dispatched active power set-points (P.sub.SP1-P.sub.SPx) comprises, in the frequency domain, frequency components that do not coincide with undesired frequency modes, such as natural frequency oscillation modes of the wind turbine generators. Thus, according to the present invention the power plant dispatcher 203 tailors each of the active power set-points so that none of the wind turbine generators (WTG.sub.1-WTG.sub.x) produce active power having, in the frequency domain, frequency components at or near undesired frequency modes. The undesired frequency modes are typically below 5 Hz. In the present context “at or near” should be taken to mean that there is a frequency margin or gap between the frequency components of the produced active power distributions (P.sub.1-P.sub.x) and the respective undesired frequency modes. The frequency margin or gap may be in the range of pre-defined stability margin.

[0047] This approach is advantageous in that the method according to the present invention provides a safe grid frequency response although the grid frequency oscillation modes may be close to for example the natural frequency oscillation modes of one or more of the wind turbine generators (WTG.sub.1-WTG.sub.x).

[0048] In general, the undesired frequency modes comprise one or more preselected frequency modes, i.e. fixed frequency modes, and/or one or more dynamic frequency modes, i.e. variable frequency modes, being based on measured parameters. The one or more preselected frequency modes may, as already mentioned, comprise one or more known natural frequency modes of the wind turbine generators (WTG.sub.1-WTG.sub.x), such as one or more tower natural frequency modes. The one or more dynamic frequency modes may be determined in real time and may comprise measurements of side-to-side tower accelerations and/or measurements of active power. The one or more dynamic frequency modes may however also be based on measurements of other parameters.

[0049] The wind turbine generators (WTG.sub.1-WTG.sub.x), or at least a number thereof, may be operated in a curtailed mode of operation in order to establish an available power reserve that may be used for grid frequency response during under frequency events in particular in connection with narrow deadbands.

[0050] Although FIG. 2 shows a wind power plant it should be noted that the method according to the present invention is also applicable for other types of power plants comprising other types of power producing units, such as photovoltaic (PV) units, batteries as well as other types of power producing units.

[0051] Referring now to FIGS. 3-5 more detailed block diagrams are depicted. Starting from the top in FIG. 3 the controller 301 provides a signal, Pd, to the processor 302. The processor 302 is configured to analyze the signal, Pd, from the controller 301 and thus detect any frequency oscillation modes originating from the associated power grid (not shown). Potentially detected frequency oscillation modes are compared against any undesired (target) frequency modes, Ft. As previously discussed the undesired frequency modes may involve one or more preselected frequency modes, i.e. fixed frequency modes, and/or one or more dynamic frequency modes, i.e. variable frequency modes, being based on measured parameters.

[0052] In the processor 302 signal responses (for active power) to non-target frequency oscillation modes, Pdnot, are selected, separated from the response target frequency oscillation modes, Pdt, and sent directly to a dispatcher 303. Signal responses to the target oscillation modes, Pdt, are sent to a shifter 304 where they are modified as disclosed in the following. The target signal responses to the target oscillation modes, Pdt, are modified in such a way that any single power producing unit response provides a response that is, in the frequency domain, shifted away from target frequency oscillation modes, whereas the overall/total summated response of the power producing units will match the required frequency response and will be in counter phase with the frequency oscillation modes originating from the associated power grid. In the shifter 304 the signal responses may be modified in various ways including, but not limited to, frequency shifting, phase shifting, partial cycle slicing, delay, etc.

[0053] The response signals to the non-target oscillation modes, Pdnot, and the shifted response signals to the target oscillation modes, Pdt′, are provided as inputs to the power plant dispatchers 303, 305 whereafter the individual response signals, Pdnot1-n, Pdt1-n, are combined in combiner 306 and sent to each of the respective power producing units 307.

[0054] In another embodiment, cf. FIG. 4, the processor 302 may furthermore be configured to analyze the signal, Pd, from the controller 301 by taking into consideration measured real time information relating to the overall power plant active power output and/or individual active power output, Pout, from the power producing units. Such real time information is provided to the processor 302 via a pre-processor 308. Other control feedback signals 309, such as sensor signals from for example accelerometers, synchrophasors, etc. may also be provided to the pre-processor 308.

[0055] In yet another embodiment, cf. FIG. 5, a pre-dispatcher 502 is inserted between the controller 501 and the processor 503/pre-processor 504. The functioning of the shifter 505, dispatcher 506 and the combiner 507 are similar as disclosed above. Thus, the processor 503 is configured to take into consideration measured real time information relating to the overall power plant active power output and/or individual active power output, Pout, from the power producing units 508. Such measured real time information is provided via a pre-processor 504. Again, other control feedback signals 509, such as sensor signals from for example accelerometers, synchrophasors, etc. may also be provided to the pre-processor 504. The pre-dispatcher 502 is advantageous in that it facilitates that signal detection can be performed on individual power producing unit set-points after they have been allocated which allows flexibility in cases where it has been decided not to perform any action if the required active power response for a given mode per WTG is below a certain threshold.