Method for damping harmonics

Abstract

Harmonics of a power output of a power plant at a point of common coupling between the power plant and a utility grid, wherein the power plant comprises a plurality of energy production units. The method comprises determining an electrical characteristic at the point of common coupling; determining the electrical characteristic at an output terminal of each of the energy production units and dispatching a control signal to at least one of the energy production units to control the electrical characteristic at an output terminal of the respective energy production units. The control signal is based on the measurement of the electrical characteristic at the point of common coupling and arranged for damping the harmonic of the power output of the power plant at the point of common coupling, wherein the control signal is determined on the basis of a predetermined prioritizing sequence of said electrical characteristic.

Claims

1. A method for damping harmonics of a power output of a power plant at a point of common coupling between the power plant and a utility grid, wherein the power plant comprises a plurality of energy production units, the method comprising: determining a total harmonic distortion of the power plant at the point of common coupling, wherein the total harmonic distortion is a sum of harmonic root mean square (RMS) voltage or harmonic RMS current of the plurality of energy production units; determining harmonics of the output powers of the plurality of energy production units at output terminals of the energy production units; determining a control signal, based on the total harmonic distortion of the power plant, the harmonics of the output powers of the plurality of energy production units, and a predetermined prioritizing sequence that prioritizes one of active power, reactive power, or harmonics; and dispatching the control signal to at least one of the energy production units to control the harmonics of the output powers of the plurality of energy production units at the output terminals of the plurality of energy production units such that the total harmonic distortion of the power plant at the point of common coupling is reduced.

2. The method for damping harmonics according to claim 1, wherein the prioritizing sequence comprise prioritizing active power over reactive power and harmonics of the power output at the point of common coupling.

3. The method for damping harmonics according to claim 1, wherein the prioritizing sequence comprise prioritizing reactive power over active power and harmonics of the power output at the point of common coupling.

4. The method for damping harmonics according to claim 1, wherein the prioritizing sequence comprise prioritizing harmonics of the power output over active power and reactive power at the point of common coupling.

5. The method for damping harmonics according to claim 1, wherein the control signal is further determined on the basis of a harmonic current reference.

6. The method for damping harmonics according to claim 5, wherein the control signal is further determined on the basis of a measured harmonic current reference.

7. The method for damping harmonics according to claim 1, wherein the control signal is determined by use of a PI controller.

8. The method for damping harmonics according to claim 1, wherein the control signal comprises a current set point, so as to control one or more harmonic orders of the harmonics, at an output terminal of at least one of said energy production units.

9. The method for damping harmonics according to claim 8, wherein a dispatching block is configured for generating said control signal, said control signal comprising said current set point, so as to control one or more harmonic orders of the harmonics, at an output terminal of at least one of said energy production units.

10. The method for damping harmonics according to claim 9, wherein said dispatching block is configured for receiving input from a dispatching controller and from said prioritizing sequence.

11. The method for damping harmonics according to claim 1, wherein the energy productions units comprise at least one of wind turbine generators, or photovoltaic cells.

12. The method for damping harmonics according to claim 1, further comprising measuring at least one of reactive power, active power, voltage, current, or power factor at the point of common coupling or at the output terminal of each of the energy production units.

13. The method for damping harmonics according to claim 12, wherein the total harmonic distortion is based on an analysis of the measured voltage or current at the point of common coupling or at the output terminal of the energy production units.

14. A wind farm, comprising: a plurality of wind turbine generators; a power plant controller communicatively coupled to the plurality of wind turbine generators and configured to perform an operation for damping harmonics of a power output of the wind farm at a point of common coupling between the wind farm and a utility grid, wherein the operation, comprises: determining a total harmonic distortion of the wind farm at the point of common coupling, wherein the total harmonic distortion is a sum of harmonic root mean square (RMS) voltage or harmonic RMS current of the plurality of wind turbine generators; determining harmonics of the output powers of the plurality of wind turbine generators at output terminals of each of the wind turbine generators; determining a control signal, based on the total harmonic distortion of the wind farm, the harmonics of the output powers of the plurality of wind turbine generators, and a predetermined prioritizing sequence that prioritizes one of active power, reactive power, or harmonics; and dispatching the control signal to at least one of the wind turbine generators to control the harmonics of the output powers of the plurality of wind turbine generators at the output terminals of the plurality of wind turbine generators such that the total harmonic distortion of the wind farm at the point of common coupling is reduced.

15. The wind farm of claim 14, wherein the prioritizing sequence comprise prioritizing active power over reactive power and harmonics of the power output at the point of common coupling.

16. The wind farm of claim 14, wherein the prioritizing sequence comprises prioritizing reactive power over active power and harmonics of the power output at the point of common coupling.

17. The wind farm of claim 14, wherein the prioritizing sequence comprise prioritizing harmonics of the power output over active power and reactive power at the point of common coupling.

18. The wind farm of claim 14, wherein the control signal is further determined on the basis of a harmonic current reference.

19. The wind farm of claim 18, wherein the control signal is further determined on the basis of a measured harmonic current reference.

20. The wind farm of claim 14, wherein the control signal is determined by use of a PI controller.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

(2) FIG. 1 schematically illustrates a wind turbine.

(3) FIG. 2 schematically illustrates a generic wind power plant architecture.

(4) FIG. 3 schematically illustrates an embodiment of a method for damping harmonics at the output of a power plant comprising a plurality of wind turbines.

(5) FIG. 4 schematically illustrates a prioritizing scheme according to an embodiment.

(6) FIG. 5 schematically illustrates the calculation of a Harmonic Order Current Reference according to an embodiment.

(7) FIG. 6 schematically illustrates the calculation of the Harmonic Order Current according to another embodiment.

(8) FIG. 7 schematically illustrates a Harmonic Current H Controller according to an embodiment.

(9) FIG. 8 schematically illustrates a PPC according to an embodiment.

DETAILED DESCRIPTION OF AN EMBODIMENT

(10) FIG. 1 shows a wind turbine generator WTG 1 comprising a tower 2 and a rotor 3. The rotor comprises three rotor blades 4. However, the number of blades may vary, and there may be two, four or even more blades. The rotor 3 is connected to a nacelle 5, which is mounted on top of the tower 2, and is arranged to drive an electrical generator situated inside the nacelle. The rotor 3 is rotatable by action of the wind. The wind-induced rotational energy of the rotor blades 4 is transferred via a shaft to the electrical generator. Thus, the WTG 1 is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades 4 and, subsequently, into electric power by means of the electrical generator. The electrical layout of the WTG 1 may in addition to the electrical generator include a power converter. The power converter is connected in series between the electrical generator and the electrical grid for converting the variable frequency generator AC power into a grid frequency AC power to be injected into the utility/electrical grid. The electrical generator is via the power converter controllable to produce a power corresponding to a power request. Here the WTG can be, but not limited to, a full scale turbine or a doubly-fed induction generator turbine (DFIG).

(11) The blades 4 can be pitched in order to alter the aerodynamic properties of the blades, e.g. in order to maximize uptake of the wind energy. The blades 4 are pitched by a pitch system, which includes actuators for pitching the blades dependent on a pitch request.

(12) A WTG is, in normal operation, set to capture as much power from the wind, at any given wind speed. This works as long as the power production is below the rated power limit for the wind turbine, i.e. partial load operation. When the wind speed increases above rated wind speed, often designed at 10-12 m/s, the WTG has to pitch the blades 4, so the energy captured is stable at rated power, even if the wind is well above rated wind speed.

(13) A wind power plant WPP comprises a plurality of WTGs controlled by a power plant controller PPC and interconnection infrastructure. FIG. 2 shows an example of a generic WPP architecture with a plurality of WTGs, a collection grid with a MV collection bus, a transformer TRF. At the high voltage side of the transformer there is a Point of Measurement Poll, close to the Point of Common Coupling PCC. Between the PCC and the TRF a power plant circuit breaker or a switch gear is installed and is operated by the PPC, in order for system operators to disconnect the WPP from the grid.

(14) From the WTGs to the PCC there may be several electrical infrastructure components, e.g. power cables etc. All the components are needed, but they contribute to losses from the WTGs to the PCC. Losses which have to be taken into account when controlling the WPP.

(15) The measurements obtained at the PoM are communicated to the PPC and optionally also to a SCADA system. The SCADA is optional and is not necessarily interacting with embodiments of the present invention. Based on the measurements, the PPC controls the WTGs accordingly. Further optional equipment is also shown, such as a STATCOM, MSU (Mechanically Switched Unit, wherein the unit can be either capacitors or inductors), ES (Energy Storage) all used for improving power quality and stability.

(16) In an embodiment the Power Plant Controller PPC has the responsibility to control active power P and reactive power Q at the point of common coupling with the utility grid. The P and Q quantities are the means by which other system parameters can be influenced, such as the grid frequency f and voltage V. The controller structure has as inner loops the P and Q control, and has as outer loops the f and V control.

(17) Besides the core functionalities described above, the PPC may also be responsible for other WPP functionalities, required either by the Transmission System Operator TSO or by the WPP owner.

(18) The active power control loop is responsible for controlling P at the point of common coupling. This inner loop can be used to influence the grid frequency, by adding appropriate external control loops (e.g. primary frequency regulation and fast frequency response). Power oscillation damping can be achieved as well by adding an appropriate external control loop.

(19) FIG. 3 shows the general concept of PPC harmonic control and dispatching method according to the present invention.

(20) A power meter measures the harmonic voltage U.sub.Hi_MEAS and Harmonic Current I.sub.Hi_MEAS at the PCC and the PPC internally dispatches the signals to the Current Reference Generator 9 and to the Harmonic Current H Controller 14. The method determines the electrical characteristic EC_PCC, which may be the harmonics H of the output power of the power plant, based on an analysis, such as a Fourier analysis, of the measured voltage and/or current at the PCC. The method determines the electrical characteristics EC_WTGi, which may be the harmonics of the output power of the plurality of the WTGs, based on an analysis, such as a Fourier analysis, of the measured voltage and/or current at the output terminal of the WTGs.

(21) The TSO 7 sends (P, Q, H) references and (P, Q, H) priorities to the P, Q, H reference Generator 8. Based on different grid demands, the PPC select different priorities among P, Q and H. Disregarding which quantity has the highest or second or lowest priority, the three quantities P, Q and H respects the relationship P.sup.2+Q.sup.2+H.sup.2=S.sup.2. The PPC may be set up with a prioritizing sequence as requested by the TSO or as needed by operations. The prioritizing scheme is illustrated in FIG. 4. The prioritizing scheme 4 discloses situations “a”-“f” in the first column, each situation corresponding to a specific prioritization of the three quantities P, Q and H. The second column discloses each situation's prioritizing of the three quantities P, Q and H. The third column illustrates the calculation of the H.sub.ref, which is either calculated based on the P reference and Q reference or provided by the ISO or operator. The fourth column discloses information on the origin of two of the three quantities and the calculation of the third quantity.

(22) The P, Q, H Reference Generator 8 calculates the H reference H.sub.REF on the basis of a prioritizing sequence scheme. According to different orders of priority of the quantities P, Q and H, the H.sub.REF is either calculated by the P, Q, H Reference Generator 8 or given by the TSO. For example, if P is prioritized over Q and over H (situation “a” in the prioritizing scheme), the P and Q references are given by the TSO or operator and H.sub.REF is calculated with H=√{square root over (S.sup.2−P.sup.2−Q.sup.2)}. In situation “e”, the H and P references are given by the TSO or operator, and the Q reference is calculated with Q=√{square root over (S.sup.2−P.sup.2−H.sup.2)}. The output in any of the situations “a-f” in the prioritizing scheme is the H reference H.sub.REF on plant level.

(23) In situations “b”, “d”, “e” and “f” where H.sub.REF is given from the TSO, the P, Q, H Reference Generator 8 calculates the Q reference or P reference, so as to fulfill the H.sub.REF requirement from the TSO and respect the relationship P.sup.2+Q.sup.2+H.sup.2=S.sup.2. The P reference and Q references are dispatched to the P Controller 10 and Q Controller 11 and subsequently to the P.sub.WTG Dispatching Controller 12 and Q.sub.WTG Dispatching Controller 13 for dispatching P and Q set points to the WTGs. The H.sub.REF signal is dispatched to the Current Reference Generator 9.

(24) In situations “a” and “c”, the calculation of H.sub.REF is based on the P reference and Q reference e.g. provided by the TSO or operator. The H.sub.REF signal is dispatched to the Current Reference Generator 9.

(25) The input of the Current Reference Generator 9 is the H.sub.REF, which is the output from any of the situations in the prioritizing scheme. The control signal, or set points, sent to the WTGs are Harmonic Current Set Points: I.sub.Hx,WTG1, I.sub.Hx,WTG2 . . . I.sub.Hx,WTGi, (x=3,5,7 . . . ). Methods of defining/calculating the Harmonic Order Current Reference ΔI.sub.Hi are required to generate the Plant Level Harmonics Current Reference I.sub.H1 WTG; I.sub.H2 WTG . . . I.sub.Hi WTG.

(26) The Current Reference Generator 9 may use two methods for calculating the Harmonic Order Current Reference ΔI.sub.Hi.

(27) Method I:

(28) The Current Reference Generator 9 receives Harmonic Current Reference command I.sub.Hi_REF from the TSO. The I.sub.Hi_REF is by default set as 0, meaning that the PPC ideally controls the total output harmonic current at the PCC to 0 √{square root over (Σ.sub.iH.sub.WTGi)}=0. In method I, the H.sub.REF acts as a saturation factor to limit the Harmonic Order Current Reference ΔI.sub.Hi.

(29) FIG. 5 schematically illustrates the calculation of the Harmonic Order Current Reference ΔI.sub.Hi according to method I.

(30) Method II:

(31) The Current Reference Generator 9 compares the H.sub.REF with the Total Measured Harmonics H.sub.MEAS. The Total Measured Harmonics H.sub.MEAS is calculated by: H.sub.MEAS=√{square root over (Σ.sub.iI.sub.Hi_Meas.sup.2)}*U.sub.1; U1 being the fundamental voltage component and I.sub.Hi_MEAS being the i th harmonic current measured. The difference between H.sub.REF and H.sub.MEAS, (ΔH), is used to generate the Harmonic Order Current Reference ΔI.sub.Hi, in which the ratio among all harmonics are used to split the ΔH. FIG. 6 schematically illustrates the calculation of ΔI.sub.Hi according to method II.

(32) FIG. 7 schematically illustrates the functionality of the Harmonic Current H Controller 14. The input to the H Controller 14 is the ΔI.sub.Hi from the Current Reference Generator 9. PI controllers (Proportional-Integral Controller) or alternatively a PID (Proportional-Integral-Derivative Controller) may be used for each harmonic, in order to eliminate the ΔI.sub.Hi. The output of the H Controller 14 is the Plant Level Harmonics Current Reference I.sub.H1 WTG; I.sub.H2 WTG . . . I.sub.Hi WTG, which is subsequently dispatched to the WTG Dispatching Block 15.

(33) The WTG dispatching block 15 is schematically illustrated in FIG. 3. The WTG dispatching block 15 is used to generate the control signal to the WTGs, comprising Harmonics Current Set Points I.sub.Hx,WTG1, I.sub.Hx,WTG2 . . . I.sub.Hx,WTGi, (x=3,5,7 . . . ) The WTG dispatching block 15 receives input P.sub.WTG1; P.sub.WTG2 . . . P.sub.WTGi and Q.sub.WTG1; Q.sub.WTG2 . . . Q.sub.WTGi from the P.sub.WTG Dispatching Controller 12 and Q.sub.WTG Dispatching Controller 13 respectively, and (P, Q, H) priorities from the TSO. The dispatching can be performed according to each wind turbines operating condition and (P, Q, H) prioritizing order. As an example, if TSO requires P over Q, and Q over H (situation “a” in the prioritizing scheme), the Plant Level Harmonics Current Reference I.sub.H1 WTG; I.sub.H2 WTG . . . I.sub.Hi WTG is calculated and prioritized in accordance with the remaining capability of each wind turbine after regulating the P and Q quantities (electrical characteristic EC_WTGi) of the wind turbines. One principle is that the same WTG should not be requested to generate certain order of harmonics all the time. The output of the WTG dispatching block 15 is the control signal, comprising Harmonic Current Set Points I.sub.Hx,WTG1, I.sub.Hx,WTG2 . . . I.sub.Hx,WTGi, (x=3,5,7 . . . ), which are dispatched to the WTGs.

(34) The WTG Harmonics Compensation block 16 represent the inside of the WTGs, when the WTGs receives the Harmonic Current Set Points I.sub.Hx,WTG1, I.sub.Hx,WTG2 . . . I.sub.Hx,WTGi, (x=3,5,7 . . . ). The WTGs executes the command by using its own compensation algorithm.

(35) The method according to present invention can be implemented in the PPC software. In FIG. 8 the “PPC existing inputs” and “PPC existing outputs” schematically illustrates the existing signals in the PCC, which includes active and reactive power set points dispatched to the WTGs. The H.sub.REF, P, Q, H priority, I.sub.H2_MEAS, U.sub.H2_MEAS, I.sub.Hi_MEAS and U.sub.Hi_MEAS schematically illustrates the new inputs and outputs of the PPC. In the illustrative example in FIG. 8 there are n WTGs in a power plant.

(36) It is noted that the wind turbines should be adapted to handle the received Harmonics Current Set Points and regulate the harmonics accordingly. Thus software for doing so should be present in the WTGs. Preferably, the wind turbines' grid side converter controller are adapted to execute a harmonics compensation algorithm.

(37) A power meter may be used to execute the harmonics calculation in order to obtain the harmonics measurements for the PPC. Implemented software in the PPC may be used to execute the H.sub.REF calculation and control algorithm and dispatching algorithm.

(38) Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.