Power generation system of a multi-converter wind turbine and control method thereof

Abstract

Wind turbine comprising a plurality of converters, which are dynamically switched between at least a first standby state, a second running state, and a third state with an active direct current link. Converters are switched from the first standby state to the third state when a required reactive power is higher than a reactive power capability of converters on the second running state and when a voltage transient occurs.

Claims

1. Power generation system comprising control means configured to manage a plurality of converters of a wind turbine, each of the plurality of converters further comprising first power electronics between a generator and a capacitor and second power electronics between the capacitor and an electric grid, wherein the control means are configured to switch the plurality of converters between at least: a first standby state with no reactive power injection to the electric grid; a second running state wherein reactive power injection into the electric grid is regulated by a commutation of the first power electronics and the second power electronics; and a third state wherein reactive power injection into the electric grid is regulated by the commutation of the second power electronics, while the first power electronics are not commutating nor regulating a voltage of the capacitor, wherein the control means are further configured to switch some of the plurality of converters from the first standby state to the third state by applying a ramp to a current direct current link voltage up to a target direct current link voltage in order to activate a direct current link, wherein converters in the first standby state are configured to charge the direct current link through the first power electronics.

2. Power generation system according to claim 1 wherein the control means are further configured to switch at least a converter of the plurality of converters from the first standby state to the third state when a required reactive power is higher than a reactive power capability of converters on the second running state and the third state.

3. Power generation system according to claim 1 wherein the control means are further configured to switch at least a converter of the plurality of converters from the first standby state to the third state when a voltage transient occurs.

4. Power generation system according to claim 3 wherein the control means are further configured to, when the voltage transient occurs, set the target direct current link voltage of the converters in the third state to a value prior to the voltage transient.

5. Power generation system according to claim 1 wherein the plurality of converters are configured to dynamically send reactive power information to the control means.

6. Method of controlling power generation of a wind turbine comprising a plurality of converters, each of the plurality of converters further comprising: first power electronics between a generator and a capacitor and second power electronics between the capacitor and an electric grid, wherein the method comprises dynamically switching a plurality of converters between at least: a first standby state with no reactive power injection to the electric grid, a second running state wherein reactive power injection into the electric grid is regulated by a commutation of the first power electronics and the second power electronics, and a third state wherein reactive power injection into the electric grid is regulated by the commutation of the second power electronics, while the first power electronics are not commutating nor regulating a voltage of the capacitor; and switching some of the plurality of converters from the first standby state to the third state by applying a ramp to a current direct current link voltage up to a target direct current link voltage in order to activate a direct current link, the method further comprising, when a converter is in the first standby state, charging the direct current link through the first power electronics.

7. Method according to claim 6 further comprising switching at least a converter of the plurality of converters from the first standby state to the third state when a required reactive power is higher than a reactive power capability of converters on the second running state and the third state.

8. Method according to claim 6 further comprising switching at least a converter of the plurality of converters from the first standby state to the third state when a voltage transient occurs.

9. Method according to claim 6 further comprising dynamically sending reactive power information from the plurality of converters to centralized control means.

10. Multi-converter wind turbine comprising a plurality of blades connected to a rotor and a plurality of converters rotationally coupled to said rotor, wherein the wind turbine further comprises a power generation system according to claim 1.

11. A non-transitory computer-readable medium storing a computer program comprising computer program code which, when executed by a processor, causes the processor to perform the steps of the method according to claim 6 when said program is run on a computer, a digital signal processor, a field-programmable gate array, an application-specific integrated circuit, a micro-processor, a micro-controller, or any other form of programmable hardware.

12. Power generation system according to claim 1, wherein each of the first power electronics and the second power electronics comprises two or more insulated gate bipolar transistor (IGBT) inverters in a back-to-back configuration.

13. Method according to claim 6, wherein each of the first power electronics and the second power electronics comprises two or more insulated gate bipolar transistor (IGBT) inverters in a back-to-back configuration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For the purpose of aiding the understanding of the characteristics of the invention, according to a preferred practical embodiment thereof and in order to complement this description, the following figures are attached as an integral part thereof, having an illustrative and non-limiting character:

(2) FIG. 1 shows a schematic view of the multi-converter wind turbine of the invention, according to a particular embodiment thereof, as well as of the electric grid connected to said turbine.

(3) FIG. 2 exemplifies a particular embodiment of the electronic components of the multi-converter wind turbine of the invention.

(4) FIG. 3 presents a schematic representations of the operation states of each converter of the wind turbine according to a particular embodiment of the invention.

(5) FIG. 4 shows a flow diagram of a particular embodiment of the direct current link voltage control mechanism of the invention.

(6) FIGS. 5 and 6 illustrate state transitions caused by voltage faults according to particular embodiments of the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) The matters defined in this detailed description are provided to assist in a comprehensive understanding of the invention. Accordingly, those of ordinary skill in the art will recognize that variation changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, description of well-known functions and elements are omitted for clarity and conciseness.

(8) Note that in this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

(9) FIG. 1 shows a preferred embodiment of a multi-converter wind turbine (100), which comprises a preferred embodiment of the power generation system of the invention, and which implements a preferred embodiment of the method of the invention. In particular, the wind turbine (100) comprises a plurality of converters (101), controlled by control means (102). The reactive power provided by the plurality of converters is combined at a power output (103) which feeds an electric grid (104).

(10) FIG. 2 displays in further detail the components of a preferred embodiment of the multi-converter wind turbine (100). In particular, each converter (101) comprises first power electronics (105) between the generator (106) and a capacitor (107); and second power electronics (108) between the capacitor (107) and the electric grid (104). First power electronics (105) and second power electronics (108) comprise IGBT inverters in back-to-back configuration. The multi-converter wind turbine (100) further comprises inductances (109) on both the generator side and the grid side of the converters (101), as well as a transformer (110) to connect the converters (101) and the electric grid (104).

(11) FIG. 3 presents a schematic diagram of the states of each converter (101) and the transitions between said states. In particular, each converter (101) of the wind turbine (100) can be dynamically and individually switched between the following states, depending on wind conditions and grid requirements: A first standby state (201), also named READY state (201) in this text for clarity. While in READY state, power electronics remain disconnected and anti-parallel diodes are configured to charge the direct current link up to a value of V.sub.f×√2; where Vf is the phase voltage. A second running state (202), also named RUN state (202) in this text for clarity. A third state (203) with an active DC link, also named DCLINKON state (203) in this text for clarity. When a converter (101) changes to the DCLINKON state (203) from the READY state (201), the controlled voltage in the DC-link is increased to the working point with a predefined ramp.

(12) Operation of the wind turbine (100) power generation system has two main stages, namely, computing the number of required active converters (101) and sending DC link voltage information to control means (102). In the first stage, the number of required active converters (101) can be performed periodically or as a consequence of a change in the wind conditions or electric grid requirements. Accordingly, control means (102) determine a target number of converters (101) in RUN state (202) and DCLINKON state (203) as a function of current number of converters (101) in RUN state (202) and demanded reactive power.

(13) In particular embodiments, the computation of required active converters (101) may use a predefined hysteresis. This hysteresis consists on activating (that is, sending order to pass from STEADY to DCLINKON state) a new converter when the global reactive power reference reaches a first predefined percentage of the addition of the capability of converters in RUN or DCLINKON mode (for example the addition of the capability of all converters subtracted by a 15% of the capability of one converter). In addition, a converter in DCLINKON mode is deactivated if the global reactive power reference reaches a second predefined percentage of the addition of the capability of converters in RUN or DCLINKON mode (for example the addition of the capability of all converters subtracted by a 40% of the capability of one converter)

(14) The percentage of capability that determines the number of converters (101) in RUN state (202) and DCLINKON state (203) is preferably modified using a random number in order to avoid that the converter (101) switching operation does not happen simultaneously in a plant with several multi-converter wind turbines (101) and then the effects of the transients are added in the wind power plant reactive power production. This correction is for example a random value between 0% to 5%.

(15) In the second stage, the target reactive power for each converter (101) is calculated by control means (102) based on the state of each converter (101) and the total demanded reactive power. Said target reactive power is progressively reached through a predefined ramp. FIG. 4 presents a more detailed flow diagram of the steps followed by a preferred embodiment of the method of the invention in order to share the direct current link voltage between the converters when a change between states occurs. i. The method starts (301) using as input the actual value or the reactive power reference to converters (101) and the global reactive power demand to wind turbine. ii. The reactive power of converters (101) which were defined to be switched from RUN state (202) and DCLINKON state (203) to READY state (201) are calculated (302) and ramped to zero progressively with a predefined ramp (303). For this ramping purpose the actual value or the reactive power reference to converters is employed. iii. A loop is initiated (304) by calculating target reactive powers of converters (101) in RUN state (202) and DCLINKON state (203). The reference is calculated as the subtraction of the global reactive power demand to wind turbine by the reactive power reference to the converters which are being ramped to zero (from 1) and by the addition of the reactive power references to converters already defined. Notice that the value of converters already defined starts at zero and is updated in step v. This result is divided by the number of converters which are not referenced yet in this loop. Notice that this value is also updated in step v, but when the process starts, is defined as the number of converters in RUN state and DCLINK on state. This result is the target reactive power of a converter. iv. The converter (101) with the largest difference between current reactive power and target reactive power is selected (305). v. The target reactive power is sent (306) to the converter (101) chosen in step iv once limited by maximum and minimum values and by maximum positive and negative ramp. vi. The method verifies (307) if the current reactive powers of all converters (101) in RUN state (202) and DCLINKON state (203) have been set to match their target reactive power. If they have all been set, the method ends (308), otherwise, steps iii to vi are repeated.

(16) FIG. 5 shows the state evolution of the wind turbine (100) power generation system and method when the converter is in READY state (201) and a voltage fault message (401) is received. Before the voltage fault (401) is received, no switching is performed and the DC link is charged, as defined by the READY state (201). After the voltage fault message (401) is received, the converter is switched to DC-LINKON state (203), causing the grind inverter to start switching and inject reactive current (402). When a voltage fault finalization message (403) is received, the converter is switched back to READY state (201).

(17) Likewise, FIG. 6 shows the state evolution of the wind turbine (100) power generation system and method when the converter is in RUN state (202) and a voltage fault message (401) is received. Before the voltage fault (401) is received, inverters both on the grid side and the generator side are switching, as defined by the RUN state (203). After the voltage fault message (401) is received, the converter is switched to DC-LINKON state (203), causing a dip that adapts the injection of active/reactive current (404). When a voltage fault finalization message (403) is received, the converter is switched back to RUN state (202).