Method and control arrangement for controlling a reactive power source

Abstract

The present invention relates to a method for controlling a reactive power source in a wind power plant, the method comprising the steps of providing a wind turbine reactive power control signal and providing an active power reference signal, said active power reference signal being a measure of an active power production of the wind power plant. A control signal for the reactive power source is generated by combining the wind turbine reactive power control signal and the active power reference signal in such a way that the control signal for the reactive power source becomes a weighted signal of the wind turbine reactive power control signal. Moreover, the present invention relates to control units and wind power plants suitable for carrying out the present invention.

Claims

1. A method for controlling power output of a wind power plant (WPP) comprising one or more wind turbines (WTs) and a non-WT reactive power source, the method comprising: calculating an active power control signal for a first WT of the one or more WTs, wherein the active power control signal indicates increasing an active power production of the first WT from a current active power production level; determining that increasing the active power production of the first WT from the current active production level requires decreasing a reactive power production of the first WT from a current reactive power production level; generating a reactive power control signal for the first WT corresponding to decreasing the reactive power production of the first WT; and activating the non-WT reactive power source in conjunction with decreasing the reactive power production of the first WT responsive to determining that decreasing the reactive power production of the first WT is required, wherein activating the non-WT reactive power source comprises generating a control signal for the non-WT reactive power source using (i) the reactive power control signal and (ii) an active power reference signal representing an amount of active power production of the WPP.

2. The method of claim 1, wherein the non-WT reactive power source comprises a capacitive reactive power source.

3. The method of claim 2, wherein the non-WT reactive power source comprises a static compensator (STATCOM).

4. The method of claim 1, wherein the non-WT reactive power source comprises an inductive reactive power source.

5. The method of claim 1, wherein the reactive power control signal comprises a voltage reference for the first WT, wherein generating the reactive power control signal is performed responsive to determining that the first WT is unable to produce, at an increased active power production level specified by the active power control signal, sufficient reactive power to meet the voltage reference.

6. The method of claim 1, wherein the active power reference signal comprises a factor K that is determined using a measurement of the active power production of the WPP.

7. The method of claim 6, wherein the reactive power control signal for the first WT comprises a voltage reference V.sub.ref, wherein the control signal for the non-WT reactive power source is generated using a multiplicative product (KV.sub.ref).

8. The method of claim 6, wherein a value of the control signal for the non-WT reactive power source is controlled within a range between zero and a value of the reactive power control signal for the first WT.

9. The method of claim 1, wherein the active power reference signal is a measure for a wind speed at a site of the WPP.

10. A control arrangement for a wind power plant (WPP) comprising one or more wind turbines (WTs) and a non-WT reactive power source, the WPP having an amount of active power production, the control arrangement comprising: a controller comprising a processor configured to: calculate an active power control signal for a first WT of the one or more WTs, wherein the active power control signal indicates increasing an active power production of the first WT from a current active power production level; determine that increasing the active power production of the first WT from the current active power production level requires decreasing a reactive power production of the first WT from a current reactive power production level; generate a reactive power control signal for the first WT corresponding to decreasing the reactive power production of the first WT; and activating the non-WT reactive power source in conjunction with decreasing the reactive power production of the first WT responsive to determining that decreasing the reactive power production of the first WT is required, wherein activating the non-WT reactive power source comprises generating a control signal for the non-WT reactive power source using (i) the reactive power control signal and (ii) an active power reference signal representing the amount of active power production of the WPP.

11. The control arrangement of claim 10, wherein the reactive power source comprises a capacitive reactive power source.

12. The control arrangement of claim 11, wherein the reactive power source comprises a static compensator (STATCOM).

13. The control arrangement of claim 10, wherein the reactive power source comprises an inductive reactive power source.

14. The control arrangement of claim 10, wherein the reactive power control signal comprises a voltage reference for the first WT, wherein generating the reactive power control signal is performed responsive to determining that the first WT is unable to produce, at an increased active power production level specified by the active power control signal, sufficient reactive power to meet the voltage reference.

15. A wind power plant comprising the control arrangement of claim 10.

16. The control arrangement of claim 10, wherein the active power reference signal is a measure for a wind speed at a site of the WPP.

17. A computer program product comprising a computer-readable device having computer-readable program code embodied therewith, the computer-readable program code configured to perform an operation for controlling power output of a wind power plant (WPP) comprising one or more wind turbines (WTs) and a non-WT reactive power source, the operation comprising: calculating an active power control signal for a first WT of the one or more WTs, wherein the active power control signal indicates increasing an active power production of the first WT from a current active power production level; determining that increasing the active power production of the first WT from the current active production level requires decreasing a reactive power production of the first WT from a current reactive power production level; generating a reactive power control signal for the first WT corresponding to decreasing the reactive power production of the first WT; and activating the non-WT reactive power source in conjunction with decreasing the reactive power production of the first WT responsive to determining that decreasing the reactive power production of the first WT is required, wherein activating the non-WT reactive power source comprises generating a control signal for the non-WT reactive power source using (i) the reactive power control signal and (ii) an active power reference signal representing an amount of active power production of the WPP.

18. The computer program product of claim 17, wherein the reactive power control signal comprises a voltage reference for the first WT, wherein generating the control signal for the reactive power source is performed responsive to determining that the first wind turbine is unable to produce, at an increased active power production level specified by the active power control signal, sufficient reactive power to meet the voltage reference.

19. The computer program product of claim 17, the operation further comprising: generating the active power reference signal by filtering a measurement of the amount of active production of the wind power plant.

20. The computer program product of claim 17, wherein the active power reference signal is a measure for a wind speed at a site of the WPP.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be explained in further details with reference to the accompanying drawings, wherein

(2) FIG. 1 shows a) a simplified doubly-fed diagram, and b) a simplified doubly-fed controller diagram,

(3) FIG. 2 shows a P-Q doubly-fed chart,

(4) FIG. 3 shows a wind power plant diagram, and

(5) FIG. 4 shows a control dispatcher diagram.

(6) While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail 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

(7) In principle the present invention relates to various types of wind turbine configurations including full scale wind turbine facility and in particular DFIG configurations. Since the present invention is of particular relevance for DFIG configurations the invention will, in the following, be described with reference to such configurations.

(8) The variable speed doubly-fed generator, cf. FIG. 1a, allows full control of generator active and reactive power using the rotor-connected frequency converter. Its rating is typically in the order of 0.3 pu. Operating both with sub- and super-synchronous speed, the power can be fed both in and out of the rotor circuit. The rotor-connected converter can employ various power dissipation solutions during severe transients. These solutions may involve an active crowbar, which is located at the rotor terminals, or a chopper in the DC link, R.sub.chcf. FIG. 1a. The grid converter is used to regulate the voltage level of the DC link.

(9) A simplified control diagram of the DFIG controller is depicted in FIG. 1b where the active power, P, and reactive power, Q, are controlled using the d and q axis, respectively. The DFIG controller calculates or receives power references, P.sub.ref, Q.sub.ref, from an external controller. These power references are processed using two PI-controllers in cascade; and they will generate the needed voltage references, V.sub.d ref, V.sub.q ref, which are translated by the PWM to pulse the rotor converter. Finally, the rotor is fed with a voltage that produces the desired P and Q at the stator terminals.

(10) FIG. 2 shows a P-Q chart obtained from the stator terminals of a DFIG machine. The asymmetry with respect to the Q injection is due to the excitation of the generator. All P-Q combinations inside the plotted area can be injected. Clearly, the maximum Q injection is dependent on the current P value, thus Q.sub.max=f(P). The ratio Q/P gets its maximum value around 10% active power injection.

(11) The P-Q DFIG chart coordinates are listed in Table 1:

(12) TABLE-US-00001 TABLE 1 Unit in Point P Q WPP base 0 0 10 [%] 1 10 60 [%] 2 70 60 [%] 3 100 20 [%] 4 100 30 [%] 5 85 60 [%] 6 10 60 [%] 7 0 15 [%]

(13) Thus, from FIG. 2 and Table 1 it is clear that the nature of a DFIG configuration sets an upper limit to the available reactive power in the high active power regime. For example, going from working point (P.sub.2, Q.sub.2) to working point (P.sub.3, Q.sub.3) the amount of reactive power, Q, needs to be reduced from 60% to 20% of nominal power in order to increase the active power, P, from 70% to 100% of nominal power.

(14) In wind power plant reactive power support can be provided by power generating unit other than wind turbines. Thus, it may be advantageous to activate such other reactive power generating unit when the wind turbines of the wind power plant are operated at or near their nominal power levels. A reactive power generating unit can be a STATCOM.

(15) The STATCOM based on a voltage source converter is one of the most used devices for reactive power support. STATCOMs are found increasing utilization in power systems because of their ability to provide improved performance compared with conventional thyristor-based converters. The primary purpose of a STATCOM is to support busbar voltage by providing appropriate capacitive and inductive reactive power into the system. It is also capable of improving the transient- and steady-state stability of a power system. Therefore, STATCOM systems have been initially used for improving flexibility and reliability of power transmission systems.

(16) Referring now to FIG. 3 a WPP controller is required to control the characteristics of the power injected at the PCC. Therefore, a centralized plant controller (WPP ctrl.) is needed to supervise the power injected at PCC. The plant controller receives the references and feedback (measurements) and outputs the turbine set-points. The plant controller is formed by a measurement device, which senses the currents and voltages at the PCC, a dedicated computer which allocates the control algorithms, and a communication hub. The communication hub will exchange control references and other signals with a large amount of WTGs (WTG ctrl.) and other devices located in the substation.

(17) Still referring to FIG. 3 the dispatcher of the WPP controller has the functionality of splitting the reference calculated by the WPP controller into the different power generating units constituting the WPP. The way of splitting the reference can be done following several strategies, e.g. minimization of lost energy production. The strategy suggested in view of the present invention is based on using the STATCOM as a reactive power back-up for the system in case the reactive power injected by the WTGs is not sufficient for grid code fulfillment.

(18) In FIG. 4 the dispatcher block is illustrated in a schematic way. The active power, P.sub.m, measured at PCC is filtered with a low-pass filter and used for calculating a K-factor with a look-up table. P.sub.m can be replaced accordingly with the wind speed. The K-factor varies between 0 and 1, and multiplies with the reference V.sub.ref which is calculated by the WPP controller thereby obtaining the reference for the STATCOM, V.sub.ref.sub._.sub.STATCOM.

(19) As depicted in FIG. 4 the number of WTGs on-line, N, is a control parameter as well. The value of K thus depends on the number on-line WTGs within the WPP. Alternatively or in combination therewith, it is possible to use the amount of available power from the on-line WTGS, P.sub.ava, as a control parameter. The available amount of power from the WPP is the sum of the available power from the individual WTGs and it is given by:
P.sub.ava=P.sub.ratedP.sub.actual

(20) Moreover, the wind speed signal and the active power production can be used to set the disconnection of the external reactive power compensation device in order to avoid electrical losses, e.g. if the production of active power and the wind speed have been below some range during a certain amount of time a disconnection command/signal will be sent to the reactive power compensation unit. When the wind speed and production level conditions are re-established a connection command/signal will be send to the reactive power compensation unit.

(21) As shown in FIG. 4 the reference for the WTGs, V.sub.ref.sub._.sub.WTG, is the same as the one calculated by the WPP controller, V.sub.ref.

(22) The dispatcher look-up table can be implemented as shown in table 2.

(23) TABLE-US-00002 TABLE 2 Unit in P K WPP base 0 0.0 [%] 10 0.1 [%] 20 0.2 [%] 30 0.3 [%] 40 0.4 [%] 50 0.5 [%] 60 0.6 [%] 70 0.7 [%] 80 0.8 [%] 90 0.9 [%] 100 1.0 [%]

(24) Thus, by following the control strategy suggested by the present invention reactive power support to a power supply grid, from for example a STATCOM, can be provided as a weighted value of WTG references.