WO2020079059A1

WO2020079059A1 - Method for operating a wind turbine in the event of a fault

Info

Publication number: WO2020079059A1
Application number: PCT/EP2019/078053
Authority: WO
Inventors: Marc Stevens
Original assignee: Senvion Gmbh
Priority date: 2018-10-17
Filing date: 2019-10-16
Publication date: 2020-04-23
Also published as: DE102018008195A1

Abstract

The invention relates to a method and to a converter controller (25) for operating a wind turbine (1) in the event of a fault. The wind turbine (1) comprises a generator (14) which is driven by a wind rotor (12), wherein the generator (14) is embodied as an asynchronous generator which has a double feed and interacts with the converter (15) in order to generate electrical power. In order to feed the electrical power into a power grid (9, 99), a stator (16) is connected directly to a connecting line (17), and a rotor (18) is connected to the connecting line (17) via the converter (15). The converter (15) has a machine-side inverter (15b) and a power-grid-side inverter (15a) which are connected via an intermediate circuit (20). According to the invention, in the event of a fault the wind turbine (1) is disconnected from the power grid (9, 99) and the power grid-side inverter (25a) is connected to the stator (16) in order to produce a generative braking torque. The generator (14) of the wind turbine (1) can be braked in a controlled fashion by selectively increasing the generator torque by means of suitable actuation of the inverters (15a, 15b). The present invention makes it possible to reduce design-relevant overloading of a wind turbine.

Description

Method for operating a wind turbine in the event of an accident

The invention relates to a method and a converter control for operating a wind energy installation with a double-fed asynchronous generator (DFIG) in the event of a fault.

The operation of wind turbines is critical with regard to their design and dimensioning, especially with regard to safety aspects. In the event of a power failure, in which the torque generated by the wind in the wind rotor does not counteract a braking generator torque, the rotating part of the wind energy installation can become overspeed and the associated increased or even extreme loads. Due to the failure of the network or more generally due to a load drop on the generator, the wind rotor begins to accelerate due to the wind because the braking generator torque is missing.

As a result of the reduction or elimination of the generator torque generated by the generator as a counter-load, the wind rotor of the wind energy installation is accelerated and speeds occur which move a control device to intervene. As soon as the control device detects a deviation of the speed of the wind rotor or generator from the target speed, it reacts with a change in the blade pitch angle. However, the mechanical control is too slow in the event of a sudden drop in load. This can lead to damaging overspeeds of the wind rotor or generator.

As a result of a sudden drop in load, the wind turbine suddenly leaps out of balance between the drive torque generated by the wind on the one hand and the braking torque effective in the generator on the other. The mechanical power supplied to the wind turbine by the air flow can no longer be dissipated to the network in the form of electrical power. Wind rotor, including hub and drive train are accelerated and the excess power is converted into kinetic energy of the rotating masses. If the speeds of the rotating parts exceed design limits, the components may be damaged. Damage to rotating parts represents an enormous safety risk for the wind turbine.

In order to protect the wind turbine from overspeed even in the event of a sudden load drop, the generator of the wind turbine must be braked. With a load waste, the generator of the wind turbine runs suddenly without load with neglected own requirements), which results in an immediate increase in speed. This is associated with much greater loads on all parts of the system compared to normal operation. Depending on the strength of the braking torque, heavy loads can also occur on the wind turbine.

It is known to remove excess power from a chopper in the intermediate circuit in wind turbines with a full converter. The chopper converts the excess power into heat. This is possible with wind turbines with full converters, since the generator is separated from the grid by the DC link converter and thus the entire electrical power generated passes the DC link converter. With this full converter generator system, loads can be easily reduced using the DC link chopper.

In the case of a double-fed asynchronous generator (DFIG), however, this is not possible because only one part of the generator, namely the rotor, is connected to the mains by the converter, with the other part (the stator) directly - i.e. without an intermediate converter the network.

It is the object of the present invention to present a method and a converter control for operating a wind energy plant in the event of a malfunction which overcome the disadvantages known from the prior art.

The object is achieved with the features of the independent claims. Advantageous embodiments are the subject of the dependent claims.

First, some terms are explained.

A load case with overspeed is referred to as a malfunction of the wind turbine, in this operating state there can be extreme loads on the wind turbine. In general, all load cases with overspeed are considered here, e.g. B. can be caused by a power failure or a century gust. In the event of a malfunction, for example, the wind turbine generates more power than it can deliver (for example, by feeding it into a network to which it is connected). This is the case, for example, if the network is in an error state. The network can be a transmission network, a distribution network or an internal wind farm network. Network faults or network faults occur, for example, after lightning strikes or short circuits and make z. B. noticeable as brief voltage drops in the network. A technical problem or a technical defect in the wind turbine itself can also be considered as an accident.

In wind turbines with a so-called double-fed asynchronous generator (double-fed induction generator, DFIG), the converter is connected to the rotor windings. A major advantage of using a double-fed asynchronous generator in a wind turbine is the associated smaller dimensioning of the converter.

The method according to the invention for operating a wind power plant in the event of a fault relates to a wind power plant with a generator driven by a wind rotor, the generator being designed as a double-fed asynchronous generator which interacts with a converter for generating electrical power. To feed the electrical power into a network, the stator of the generator is connected directly to a connecting line and a rotor is connected to the connecting line by the converter, the converter having a machine-side inverter and a network-side inverter which are connected to one another via an intermediate circuit are connected.

According to the invention, the wind power installation is disconnected from the grid in the event of a malfunction and the grid-side inverter is connected to the stator in order to generate a regenerative braking torque.

The separation from the grid and the connection of the stator to the grid-side inverter results in a circuit consisting of a (generator) stator, a grid-side inverter, an intermediate circuit, a machine-side inverter and a (generator) rotor. The generator and thus the wind rotor of the wind energy installation can be braked in a controlled manner by means of a suitable control of the inverters by specifically and controlledly increasing the generator torque. The generator torque acts, possibly via a gearbox, to counteract the torque absorbed by the wind rotor from the wind. The grid-side inverter thus controls the stator for the targeted setting of a generator braking torque.

The method according to the invention thus enables the generator, and thus the mechanical drive train of the wind energy installation, to be braked by the converter in the event of a fault. Due to the good controllability of the converter, the generator motor the DFIG can be precisely controlled and the wind turbine can be braked precisely and specifically.

This can reduce the extreme loads (e.g. operating cases with overspeed and the associated overloads) of the wind turbine, which can be taken into account positively in the dimensioning and design of the wind turbine. With the present invention, design-relevant overloads of a wind power plant can be reduced.

In an advantageous embodiment, the wind turbine is separated from the grid by opening a switching element. The switching element is preferably a circuit breaker or a contactor. The switching element is preferably arranged on the connection line on the network side of a connection point between the rotor branch and the stator branch. The rotor or stator branch is used to denote the connection from the rotor or stator to the connection point. At the connection point, the rotor and stator branches meet the connecting cable. In other words, the stator and the grid-side inverter are connected at the connection point. Due to the practical arrangement of the circuit breaker or contactor on the grid side from the connection point, the stator is electrically connected to the grid-side inverter and the rotor to the machine-side inverter when the circuit breaker or contactor is open. The generator is therefore only connected to the converter by opening the circuit breaker or contactor. By opening the circuit breaker or contactor, a braking device is created and a regenerative braking torque is generated.

It is advantageous if the power generated by the stator is fed into the converter.

When the switching device is open, the power generated by the stator flows through the line-side converter into the intermediate circuit. The machine-side converter in turn draws power from the intermediate circuit and controls the rotor of the generator so that the desired braking torque is set.

According to a preferred embodiment of the invention, an excess power is converted into heat in the intermediate circuit via a current sink, in particular a chopper. By disconnecting the wind turbine from the grid and the associated load drop, there is an excess of power. In particular, transport via the stator circuit (which usually accounts for approximately 2/3 of the power generated by a wind turbine) breaks down. By connecting the grid-side inverter to the The stator circuit is now connected to the DC link (via the grid-side inverter). In order to reduce the excess power, a current sink is arranged in the intermediate circuit between the grid-side and machine-side inverters. The excess power can be partially or completely converted into heat via the current sink in the DC link. The current sink is preferably a chopper. The connection between the stator and the grid-side inverter makes it possible to use an already existing chopper arranged in the intermediate circuit. Except for a somewhat larger dimensioning of the chopper, no additional effort is therefore necessary. The power generated by the stator flows through the line-side converter into the chopper (equivalent load). In particular, the use of a chopper makes the traditional crowbar solution for protection against surges superfluous. When the crowbar is activated, the wind turbine is sometimes subjected to extreme loads, which creates additional problems for the operating behavior of the wind turbine.

The regenerative braking torque can be modulated via the grid-side and the machine-side inverter. The generator of the wind energy installation can thus be braked in a controlled manner by suitable control of the inverters by targeted and controlled increase in the generator torque.

Immediately after a load drop, the electrical power of the generator that is still generated can be converted into heat via the chopper. The chopper is designed in such a way that it dissipates the active power during the fault state, so that the system can continue to operate without impairment. The method according to the invention can cover, in particular, that period of time (or the electrical power generated in this period of time) up to which the pitch angle of the rotor blades can be changed and the power generated by the electrical generator can thereby be reduced.

The invention further relates to a converter control for a wind energy installation, the wind energy installation comprising a generator driven by a wind rotor, where the generator is designed as a double-fed asynchronous generator which interacts with a converter for generating electrical power, with the supply of the electrical power in a network, a stator is connected directly to a connecting line and a rotor is connected to the connecting line via the converter, the converter having a machine-side inverter and a network-side inverter which are connected via an intermediate circuit. According to the invention, the converter control comprises a brake control module which disconnects the wind power installation from the grid in the event of a fault and controls the grid-side inverter in such a way that a generator braking torque is generated despite the separation from the grid.

The grid-side inverter controls the stator for the targeted setting of a regenerative braking torque. The wind turbine can be braked by increasing the generator torque. The regenerative braking torque can be modulated via the control of the grid-side inverter.

The converter control according to the invention can be continued with further features which have been described in connection with the method according to the invention. The method according to the invention can be continued with further features which have been described in connection with the converter control according to the invention.

The invention is described below by way of example with reference to the accompanying drawings using advantageous embodiments. Show it:

FIG. 1: a wind energy plant with a double-fed asynchronous generator and a converter control according to the invention with a brake control module; and

Figure 2: a topology of a wind turbine with double-fed

Asynchronous generator to illustrate the principle of the method according to the invention for operating the wind power plant in the event of a fault.

The basic features of the wind turbine 1 shown in FIG. 1 are of conventional design. The wind energy installation 1 comprises a tower 10, at the upper end of which a machine house 11 is arranged to be pivotable in the azimuth direction. A wind rotor 12 with a plurality of rotor blades 13 which are adjustable with respect to their setting angle is rotatably mounted on an end face of the machine house 11. The wind rotor 12 drives the generator 14 via a rotor shaft and a gear, which may be arranged between the wind rotor 12 and the generator 14, but is not shown here. Together with a converter 15 connected to it, this generates electrical energy. The generator 14 is designed as a double-fed asynchronous generator (DFIG), to the stator 16 (see FIG. 2) of which a connecting line 17 for dissipating the electrical energy is connected directly, the connecting line 17 also being connected to the converter 15, which in turn is connected to the rotor 18 of the generator 14. As can be seen in FIG. 2, a rotor branch 17b and a stator branch 17a are connected to the connecting line 17 at a connection point 28. The converter 15 comprises a grid-side inverter 15a and a machine-side inverter 15b, which are connected to one another via an intermediate circuit 20.

The wind energy installation 1 further comprises a converter control 25 arranged in the nacelle 11, which is connected to the converter 15 via means (not shown) and acts thereon. The converter control 25 also includes a brake control module 25a for controlling the inverters for generating a regenerative braking torque.

The connecting line 17 of the wind energy installation 1 is connected via a transformer 98, for example, to an internal wind farm 9. A large number of wind energy plants 1, T can be connected to the wind farm internal collecting network 9. However, the wind turbine 1 can also be connected directly to a medium or high voltage network 99 via a transformer.

The method according to the invention intervenes when the wind energy installation 1 is in a fault state. This is the case in a load case with overspeed, for example when the network (9, 99) is in an error state.

The principle of the method is described below on the basis of the topology of the wind turbine with DFIG shown in FIG. 2. In the event of a grid fault, the wind energy installation 1 can no longer (completely) deliver the generated power to the grid 9, 99. In the event of a malfunction, the wind power installation 1 is disconnected from the network 9, 99 by opening a switching device 30. The switching element 30 is, for example, a circuit breaker.

As a result of this sudden drop in load, the wind energy installation 1 suddenly leaps out of balance between the drive torque generated by the wind on the one hand and the braking torque effective in the generator on the other hand. The mechanical power supplied to the wind energy installation 1 by the wind can no longer be dissipated to the network 9, 99 in the form of electrical power, and there is an excess of power. this leads to inevitably an acceleration of the wind rotor 12 (including the hub and the drive train), since the excess power is converted into kinetic energy of the rotating masses.

In the event of a load drop, the generator 14 of the wind energy installation 1 runs suddenly without load, which also results in an abrupt increase in the speed. This results in much greater loads on all parts of the system compared to normal operation. The method according to the invention counteracts this.

When the switching element 30 is opened, the current transport in the stator circuit breaks down. The power generated in the stator 16 can no longer be fed into the network 9, 99. Due to the arrangement of the switching element 30 on the connection line 17 on the grid side from the junction point 28 at which the stator branch 17a and the rotor branch 17b meet, the stator 16 remains connected to the grid-side inverter 15a when the switching element 30 is opened. The invention uses this to generate a regenerative braking torque.

The generator torque counteracts the torque absorbed by the wind rotor 12 from the wind, possibly via a gear. The wind rotor 12 is thus braked by a regenerative braking torque. The grid-side inverter 15a now acts directly on the stator 16 and controls the stator 16 for the targeted setting of the regenerative braking torque. The generator 14 can consequently be braked via the converter 15 in the event of a fault. As a result, the generator 14 of the wind energy installation 1 is braked in a controlled manner even in the event of a sudden load drop (disconnection of the wind energy installation 1 from the network 9, 99) and is therefore protected against overspeeds.

Due to the grid separation and the connection of the stator 16 to the grid-side inverter 15a, a circuit results from (generator) stator 16, grid-side inverter 15a, intermediate circuit 20, machine-side inverter 15b and (generator) rotor 18 the power generated by the stator 16 through the network-side inverter 15a into a chopper 22. The arrows in FIG. 2 illustrate the action of the network-side inverter 15a on the stator 16 and the action of the machine-side inverter 15b on the rotor 18 .

The rotor 18 is still connected to the intermediate circuit 20 via the machine-side inverter 15b, while the stator circuit, which previously had no effect on the intermediate circuit 20, is now also connected via the network-side inverter 15a acts on the intermediate circuit 20. The power generated by the stator 16 is thus directed in the converter 15. For this purpose, a current flows into the intermediate circuit 20.

In the embodiment of the invention shown in FIG. 2, the excess power is converted into heat via a current sink, namely a chopper 22 in the intermediate circuit 20. The intermediate circuit 20 in FIG. 2 is a direct voltage intermediate circuit, in which a capacitor is also arranged in parallel with the chopper 22. The chopper 22 in the intermediate circuit 20 is designed so that the DC voltage in the intermediate circuit can be stabilized in as many operating states as possible. This makes the use of a so-called crowbar in particular superfluous. The active power is dissipated by the chopper 22 during the fault state, so that the system can continue to operate without being exposed to extreme loads.

The brake control module 25a, which is designed as part of the converter control 25, controls the grid-side inverter 15a in the event of a fault in such a way that a regenerative braking torque is not only generated, but can also be modulated. Due to the good controllability of the converter 15, the generator torque or braking torque of the DFIG can be precisely controlled and modulated, as a result of which the wind rotor 12 or the rotating components of the wind energy installation 1 are braked precisely and in a targeted manner and thus effectively protected against overload.

Claims

1. A method for operating a wind power plant (1) in the event of a fault, the wind power plant (1) comprising a generator (14) driven by a wind rotor (12), the generator (14) being designed as a double-fed asynchronous generator which interacts with a converter (15) to generate electrical power, a stator (16) being connected directly to a connecting line (17) and a rotor (18) via the. to feed the electrical power into a network (9, 99) Converter (15) is connected to the connecting line (17), the converter (15) having a machine-side inverter (15b) and a network-side inverter (15a), which are connected via an intermediate circuit (20), characterized that, in the event of a malfunction, the wind energy installation (1) is disconnected from the grid (9, 99) and the grid-side inverter (15a) is connected to the stator (16) in order to generate a regenerative braking torque.

2. The method according to claim 1, wherein the separation of the wind energy installation (1) from the network (9, 99) takes place by opening a switching element (30), in particular a circuit breaker or a contactor.

3. The method according to claim 2, wherein the switching element (30) is arranged on the connection line (17) on the network side from a connection point (28) between a stator branch (17a) and a rotor branch (17b).

4. The method according to any one of the preceding claims, wherein the power generated by the stator is conducted into the converter (15), wherein preferably current flows into the intermediate circuit (20).

5. The method according to any one of the preceding claims, wherein the excess power is converted into heat via a current sink in the intermediate circuit (20), in particular via a chopper (22).

6. The method according to any one of the preceding claims, wherein the regenerative braking torque is modulated via the grid-side inverter (15a).

7. The method according to any one of the preceding claims, wherein the malfunction of the wind turbine is a load case with overspeed, preferably a voltage drop in the network (9, 99).

8. converter control (25) for a wind power plant (1), the wind power plant (1) comprising a generator (14) driven by a wind rotor (12), the generator (14) being designed as a double-fed asynchronous generator, which is provided with a converter (15) cooperates to generate electrical power, a stator (16) being connected directly to a connecting line (17) and the rotor (18) via the converter for feeding the electrical power into a network (9, 99) (15) is connected to the connecting line (17), the converter (15) having a machine-side inverter (15b) and a network-side inverter (15a) which are connected via an intermediate circuit (20), characterized in that that the converter control (25) comprises a brake control module (25a), which disconnects the wind turbine (1) from the grid (9, 99) in the event of a fault, and which separates the grid-side inverter (15a) to generate a generator controls braking torque.

9. converter control (25) for a wind power plant (1) according to claim 8, characterized in that the wind power plant (1) is further developed according to one of claims 2 to 6.