DE10105892A1 - Wind power plant and method for operating it has overvoltage limiter during momentary interruption in power supply - Google Patents

Abstract

An asynchronous generator (ASM) supplies the distribution network through a generator converter (GSR), a dc link circuit (ZK), a network converter (NSR) and transformer (MST). In the event of a generator overvoltage an overvoltage limiter (BC) connected to the link circuit converts the excess energy into heat.

Description

The present invention relates to the field of electrical energy generation, in particular special wind turbines for generating and feeding electrical energy into electricity Supply network and a method for operating a wind turbine.

Conventionally, variable-speed drives of all kinds, such as industrial drives, traction, etc., use self-commutated converters for speed and / or power control. In the exclusively generator mode, for example in a wind energy installation, systems with a double-fed asynchronous machine DASM are currently predominantly used, as is shown in FIG. 6. Here, a rotor R, in which the setting angle of the rotor blades can be set via a system controller AS, is driven by wind. Its rotation is used via a gearbox G with a transmission ratio of 1: n (where n assumes values between 50 and 100) to drive the double-fed asynchronous machine DASM. The actual speed n _Gen (actual) of the generator DASM is recorded and fed to the system controller AS. The double-fed DASM asynchronous machine is connected on the one hand directly to a medium-voltage transformer MST and a medium-voltage switchgear MSS with a voltage of 6 to 30 kV, this connection being switchable. During operation, the wind turbine is always connected to the medium voltage. The medium voltage network is used to feed in the energy produced. Another branch is formed parallel to this connection, in which a generator converter GSR, an intermediate circuit ZK and a line converter NSR are formed. The generator converter GSR and the line converter NSR are each controlled by means of a control system device LGSR or LNSR, the setpoint power and the setpoint speed being used as setpoints. The control technology device LGSR and LNSR as well as the system control AS are each supplied with energy via the medium voltage transformer MST and the medium voltage switchgear MSS.

Also common in wind turbines, ie for generator operation, the so-called "Danish concept" is very widespread, in which a slip ring-free asynchronous machine ASM without a converter is directly connected to the mains voltage by means of a synchronization or run-up device HL, as shown in FIG. 7 is shown. Here, a rotor R is connected to a 4- or 6-pole asynchronous machine ASM via a gearbox G with a transmission ratio of 1: n (n taking values between 50 and 100). The three output phases of this asynchronous machine ASM can either be connected via a first branch with a medium voltage transformer MST and a medium voltage switchgear MSS with a voltage of b to 30 kV or alternatively via a second branch with the ramp device HL, the ramp device HL being a switching device S1 controls for opening and closing the first branch. A system controller AS is supplied with an actual generator speed n _Gen (ist) and this controls the ramp-up device HL and a further switching device S2 in response thereto, so that a connection with the medium-voltage transformer MST and the medium-voltage switchgear MSS can also be formed via the second branch ,

DE 196 51 364 A1, for example, describes a device for improving the network ver Inertia of wind turbines with asynchronous generators connected to a power supply are connected, known. In this device there is a computer controlled converter formed with a DC intermediate circuit and a buffer. The spit cher essentially serves to create short-term positive ones that deviate from the mean performance Record fluctuations in performance and deliver them again after a delay. It is also revealed that short-term power peaks of higher order with the buffer loaded by means of a passive component can be derived, d. H. be converted into heat, the Component as a heating element, d. H. ohmic resistance can be formed.

Furthermore, EP 0 884 833 A1 describes a wind energy installation with a generator control direction with a converter with active switches known. The generator rule is off Direction for regulating the energy output of a multi-phase generator of the wind turbine can be actuated to re the torque of the rotor over the setting angle of the rotor blades rules.

In addition, WO 00/19094 discloses a wind power plant with a wind-driven one Rotor with adjustable rotor blades and one directly or indirectly connected to the rotor Generator for generating electrical energy. There is a power output from the generator possible with variable rotor speed. In addition, the wind turbine has control logic by means of which the power output and the rotor speed below a predetermined limit speed, as far as possible, constant to a nominal power / nominal speed and above the limit speed down to the switch-off speed essentially linearly decreasing is regulated.  

These conventional wind turbines with self-commutated converters each exist however, there is a problem in that the entire sys system must be switched off and re-synchronized when it is switched on again and with the mains failure the load cutoff occurs abruptly.

It is therefore an object of the present invention to provide a wind turbine in which at short time no interruption of the entire system and resynchronization due to network interruptions required when switching on again and no abrupt load cut-off in the event of a power failure follows, as well as to develop a method for operating a wind energy installation.

This object is achieved by a wind power plant with the features of patent claim 1 and a method for operating a wind turbine with the features of the patent Proposition 7 solved. Advantageous developments of the invention are set forth in the subclaims give.

As a result of the configuration according to the invention, the wind energy installation can be used for short-term networks interruptions due to the operation of a generator converter and a brake chopper be kept at the nominal speed. In addition, the Mains voltage energy can be fed back into the mains. In the event of a power failure, the rotation moment linearly reduced to zero in the range of seconds, d. H. there is a soft shutdown, the energy generated during this time by the brake chopper in Thermal energy is implemented. In particular, by eliminating a direct, switchable Connection between asynchronous motor and medium voltage transformer and medium voltage switch there was a faster response to changing network conditions, especially network interruptions and a network connection can be restored.

In the case of the wind power installation according to the invention, in the event of brief network interruptions a shutdown completely avoided while in the event of prolonged network interruptions load shocks are avoided by gently switching off the torque and vibrations conditions on the entire drive train can be reduced. This increases among other things the lifespan of the mechanical system is also considerable.

These and other objects, advantages and features of the invention will follow from the the description of preferred embodiments can be seen in connection with the drawing.

Show it:

Fig. 1 is a block diagram of an inventive wind turbine with asynchronous machine,

Fig. 2 is a speed-power and speed-torque control characteristic curve in the erfindungsge MAESSEN wind power plant,

Fig. 3 shows a course of the supply power and the maximum power in the inventive wind power plant,

Fig. 4 shows the behavior of the inventive wind power plant during brief Netzunterbre Chung,

Fig. 5 shows the behavior of the inventive wind power plant with prolonged power failure,

Fig. 6 is a block diagram of a conventional wind turbine with double-fed Asyn chron machine and

Fig. 7 is a block diagram of a conventional wind turbine according to the "Danish concept".

In the following, a wind energy installation according to the invention, in particular one, is invented wind turbine according to the invention with asynchronous machine described in more detail.

A wind power installation according to the invention, as shown in the block diagram in FIG. 1, comprises a rotor R (not shown) driven by the wind, which is mechanically coupled to an asynchronous machine ASM via a shaft and a transmission. The asynchronous machine ASM is connected to the generator converter GSR. The power converter NSR is also connected to the same DC link ZK. The line converter NSR is connected to a public power supply network (medium voltage network) via a medium voltage transformer MST and a medium voltage switchgear MSS. The wind power installation according to the invention is coordinated as a whole with regard to protection, grid effects and grid connection (short-circuit power of the grid to be fed) in accordance with specified requirements.

In contrast to conventional wind turbines, the winch according to the invention has However, the system does not have a branch via which a direct connection is made by means of a switching device between the asynchronous machine ASM and the medium voltage transformer MST and medium voltage switchgear MSS can be manufactured.

In its single branch, the wind power installation according to the invention comprises the network converter NSR with the medium voltage transformer MST and the medium voltage system MSS ver is bound, a (high-performance) converter with a line converter NSR, a motor or generator converter GSR, a brake chopper BC in an intermediate circuit ZK and associated control systems LNSR and LGSR and a connection control device VSt as an interface to an AS control system and to the medium-voltage (compact) MSS system and allows a variable-speed operation of a slip ringless asynchronous machine ASM (Ka figläufer).

For example, a ventilated asynchronous machine can be used as the generator ASM become. Here, the rotor is advantageously designed such that a simultaneous, for cated ventilation of the stator and rotor is possible. By such a compact design of the Generator ASM, the weight of the drive train can be significantly reduced and therefore in contrast to conventional wind turbines, installation of the MST medium voltage transformer possible in the gondola. With the concept of a mains converter NSR and a generator converter GSR is also a variable-speed operation over the entire speed range of the generator ASM possible.

The (high-performance) converter has high-performance switches and, for example, as shown in FIG. 1, is constructed in IGBT technology with high-blocking integrated power modules (IPM) or integrated high-performance switches, for example as a modular power converter (MPC) or modular high-performance converter. An integrated high-performance switch is designed, for example, for a maximum reverse voltage of 4.5 kV. The maximum switching current is 1.5 kA with an intermediate circuit voltage of 3.4 kV. The IGBT is controlled via gate electronics or a gate drive, which controls integrated high-performance switches connected in parallel, detects short circuits, automatically shorts off short circuits (soft turn-off), outputs and receives control signals and feedback via optical fibers, detects undervoltage, reports and maintains states and monitors the collector-emitter voltage (Uce) and protection times and limits them to a maximum value.

The power consumption of the control is, for example, approx. 6 W. Integrated parallel High-performance switches are controlled by a gate adapter device, which one Gate surge protection and a gate termination resistor included.

In the preferred exemplary embodiment according to FIG. 1, the power section of the wind power installation according to the invention essentially consists of several phase modules which are equipped with integrated high-power switches and in each case a freewheeling diode which is parallel thereto. Furthermore, the generator and the mains converter GSR, NSR DC link capacitors, current and voltage sensors, a low-inductance DC link busbar and coolant piping. In addition, a disconnect and pre-charge contactor is designed, which is required for a smooth connection of the self-commutated converter to the supply network.

The main power converter NSR and the medium voltage transformer MST are the main components of the Grid connection. The medium voltage transformer MST has a relatively high short circuit voltage, which limits the short-circuit current on the converter side and the harmonic currents reduced. It also has a combined auxiliary operation and filter winding. This wick is used to supply the wind turbine with auxiliary energy (3,400 Veff / 50 Hz) and a connection of filter capacitors to reduce harmonics.

The medium-voltage (compact) switchgear MSS has remote triggering and enables a safe and quick disconnection of the entire electrical equipment of the wind turbine from the medium voltage network. The medium voltage (compact) switchgear MSS is usually in the Tower base housed.

The wind power plant according to the invention is shown in FIG. 1 as a block diagram. This wind energy installation according to the invention is equipped with three rotor blades which drive an asynchronous motor or generator ASM via a three-stage gearbox G. The generator ASM, a self-guided (high-performance) converter and a medium-voltage transformer MST are located in a preferred embodiment in a nacelle of the wind power plant. A medium-voltage (compact) switchgear MSS is housed in the tower base.

This exemplary construction according to the invention is adapted with regard to the installation location (e.g. vibration of the parts installed in the nacelle) and ambient conditions and consists of the following components:

• - Generator, for example slip ring-free asynchronous machine (squirrel-cage rotor), ASM
• - Self-guided (high-performance) converter consisting of generator converter GSR and Mains converter NSR each with control technology LGSR, LNSR and an intermediate circuit ZK
• - Cooling systems for the self-controlled (high-performance) converter, the medium voltage trafo MST and the generator ASM, without water-air heat exchanger
• - Medium voltage transformer MST for adapting the high power converter output voltage to the desired medium voltage level
• - Auxiliary power supply 3 .400 Veff / 50 Hz from the medium voltage transformer MST (possibly from a small transformer)
• - Fans, cyclone filters and air ducts for ventilation of the ASM generator and the (high performance) converter
• - Wiring generator-converter and converter medium-voltage transformer
• - Medium voltage (compact) switchgear MSS (in the tower base), completely encapsulated
• - Fully insulated medium voltage cable for mounting inside the tower

The wind power plant according to the invention allows variable-speed operation. Through the Its variable-speed operation is a complete elimination or at least a significant one Reduction of mechanical load impacts on the drive train, the rotor blades, the machine door ger and reachable to the tower. In the wind power plant according to the invention is in contrast uninterrupted operation of the wind power plant can be implemented in relation to the prior art, if there are short interruptions in the range of approximately 20 to 200 ms, this value being determined by the maximum energy that can be absorbed by the brake chopper. In the event of prolonged power interruptions or power failures, turning is gently switched off moments to avoid vibrations due to an overlay of possible natural frequencies to avoid the generator-converter system with components of the wind turbine the. In addition, in the wind energy installation according to the invention, the rotor blade natural frequency actively damped by approx. 3 kHz.

The following is now more precise on the control and regulation of the wind according to the invention energy system received.  

The control and regulation of the wind turbine according to the invention is based on a higher-level speed control. The speed can be adjusted depending on the wind speed by adjusting the rotor blades (pitch control). This function is taken over by the AS control system. A subordinate control of a converter tracks the output power of the generator converter GSR according to a control characteristic stored in the control system according to a current speed. Such a control characteristic is shown in FIG. 2 for continuous and short-term operation (<10 s). The speed is recorded independently for the AS system controller (external) and the GSR generator converter.

A speed or power exceeding a nominal point Pn is only briefly (<10 s) hazards. The time period is defined by the adjustment time of the blade adjustment.

The internal torque control achieves an accuracy of ± 5%. For the feed Solution in the network, however, higher accuracy is required. This is achieved by the Be calculation of the line converter power is used for correction. In addition, a DC or DC offset of all current and voltage transformers when switched off (Clock lock for line converter NSR and generator converter GSR) automatically to zero adjusted.

Given that the determination of the control characteristic shown in FIG. 2 as the rest of parameterization of the generator converter software from externally via a personal computer. The parameterization of the control characteristic can in principle be done with n-line segments or with a mathematically closed formula.

The regulation of the generator converter GSR ensures the active damping of sys natural frequencies in the range of 2 to 10 Hz (e.g. rotor blade natural frequencies). In contrast The stator flow-oriented regulation used here is essential for field-oriented regulation Lich more dynamic and thus leads to an inherent damping of the system in the above Frequency range. The frequency range mentioned is a guideline. With this high Dynamic control configuration has already been used in other applications at frequencies up to 50 Hz attenuated.

The line converter NSR converts the DC or DC voltage of the DC link ZK (e.g. 2800 V) into a 3-phase AC voltage. The main control variable for the NSR line converter is the DC link voltage or its maintenance. As soon as a power fed into the grid, ie a power of the line converter NSR exceeds a value of P _max1 , as shown in FIG. 3, the brake chopper or instantaneous surge limiter BC is activated and a differential power P - P _max1 in a resistor in heat implemented. P _max2 is the upper limit of the maximum power that can be converted into heat in the instantaneous _{surge limiter} BC. The behavior described above results in an excellent short-term flicker behavior for the entire wind turbine. The specified value P _max1 , which defines the maximum feed-in power, can also be shifted towards greater power depending on the application.

However, in the event of a network interruption, regardless of its duration, even with the above regulation, there is still the problem that power would then be fed into the network since the value P _{max1 is} undershot, and this problem has hitherto only been achieved by hard shutdown of the Wind turbine was eliminated when a grid interruption was detected. The associated disadvantages already discussed above were accepted.

In order to eliminate these disadvantages of conventional wind energy plants, a distinction is therefore made in the wind energy plant according to the invention in the event of grid interruptions by the plant control system AS between two different cases:

• 1. Brief interruption in the network for a period of time t ₁ ≦ t _max with t _max = 200 ms.
• 2. Longer grid interruption or power failure, t ₁ <t _max .

In the event of brief network interruptions, the wind power installation according to the invention can be operated without interruption. In this case, the wind energy installation is regulated in such a way that the generator-generated energy is converted into heat by the brake chopper or instantaneous surge limiter BC by setting the maximum feed power P _max1 to zero. In this case, the instantaneous surge limiter BC is designed in such a way that it allows operation with nominal power, that is to say the nominal power P _n is absorbed by the resistor, up to network interruptions lasting 200 ms.

The behavior of the wind power installation according to the invention with such a brief interruption in the network is now explained in more detail below with reference to FIG. 4. In Fig. 4, U denotes the mains voltage, P _NSR the power of the mains converter NSR, P _chopper the power of the brake chopper or instantaneous surge limiter BC and P _d the power in the intermediate circuit.

In the event of a brief network interruption with a duration t ≦ t _max , the system controller AS continues to strive to keep the speed of the rotor R constant. As a result, the power Pd in the DC link ZK remains constant, since the wind turbine continues to generate energy despite a power failure. So that the voltage in the DC link ZK does not rise inadmissibly, this energy is converted into heat in the resistance of the instantaneous surge limiter BC. Of course, such an "energy conversion" is not possible any number of times within a short time. The maximum permissible number of short-term network interruptions is, for example, 3x per 5 min, but it depends on the dimensioning of the resistance and its heating due to the "energy conversion". Exceeding this exemplary, predetermined value can lead to a shutdown of the wind energy installation due to overheating of the instantaneous surge limiter resistance. In this case, the AS control system can automatically restart the wind turbine after the resistance has cooled down.

From Fig. 4 it can be seen that during the failure of the mains voltage U over a period of time t ₁ , while the power P _d in the intermediate circuit ZK remains constant, the power P _{NSR of} the mains power converter NSR goes to zero, while the power Pchopper of the instantaneous surge limiter increases to the value of P _NSR before the start of t ₁ . After the failure has ended, the power P _NSR gradually increases again, while P _Chopper decreases to the same extent until the state before the start of t _{1 is} restored.

The (high-performance) converter of the wind power installation according to the invention is also such that, despite the cooling failure (the auxiliary systems such as fans, coolant pumps, etc. are connected to the auxiliary winding of the medium-voltage transformer MST), for a period of at least t _max with the nominal power P _{n can} work (generator converter GSR and momentary surge limiter BC).

In this way, a shutdown can take place in the wind power installation according to the invention a short-term network interruption can be avoided. Now you have to take the Case in which a network interruption lasts longer, causing an energy conversion through the instantaneous surge limiter BC is no longer possible due to the heating. This case is discussed in more detail below.  

Typically, a network failure, i.e. H. a prolonged network interruption during which generator operation of an asynchronous machine, for example in a conventional wind energy system, to an abrupt load cut-off. This load cut-off can cause due to the rapid change in torque, vibrations in the mechanical part of the wind cause energy system and thereby put considerable strain on the mechanical system. Therefore takes place in the wind power plant according to the invention in the event of a power failure by the plant gene control AS a linear reduction of the torque in the second range to zero, d. H. a soft shutdown. The energy generated as a generator during this time becomes converted into heat by the instantaneous surge limiter BC and vibrations in the mechanical parts are prevented or at least significantly reduced.

If a power failure lasts longer than, for example, 200 ms in the exemplary embodiment described, the system controller AS is caused to drive the rotor R into a spin mode via a feedback signal "signaling power failure". The (high-performance) converter itself drives the torque in a controlled and ramped manner to zero, as shown in FIG. 6 in the event of a prolonged power failure. The time t ₂ until the torque and thus the power P _d in the DC link ZK and the power P _Chopper in the current overvoltage limiter BC is reduced to zero can be set, for example, to a maximum of 3 seconds. The energy generated is in turn converted into heat in the resistance of the momentary overvoltage limiter BC. This inventive, gentle reduction of the torque on the generator shaft helps to brake the rotor more quickly and prevents excitation of resonances, as can occur with a 100% load shedding.

In summary, the present invention thus discloses a wind turbine in which it is ensured that a performance up to a predetermined threshold is given during a power failure in the event of a short-term Grid interruption the wind turbine remains in operation, but no energy to the grid issues and in the event of a long-term network interruption the wind turbine softly is switched. For this purpose, only one branch is formed in the wind turbine connects the asynchronous motor or generator to the network, so that a reliable and quick reaction to changing network conditions is possible. This branch has a genera gate converter, an intermediate circuit and a line converter. To convert from  excess energy that cannot be fed into the grid is to shutdown to prevent in the event of brief interruptions in the network or a soft shutdown in the event of a to allow longer-term network interruption, a momentary surge limiter in the Intermediate circuit is formed in which this is converted into thermal energy. Beyond that discloses a method for operating a wind turbine.

Claims (9)

1. Wind turbine, with:
a rotor with adjustable rotor blades,
an asynchronous machine (ASM) driven by the rotor via a shaft,
A self-commutated converter, which has a generator converter (GSR), an intermediate circuit (ZK) with an instantaneous overvoltage limiting device (BC) and a network converter (NSR), which is located between the asynchronous machine (ASM) and a medium-voltage transformer (MST) and a medium-voltage switchgear downstream (MSS) is switched,
a control and regulating device (LGSR, LNSR, VSt), for detecting a power failure and for determining whether a power failure lasts longer than a predetermined first time period (t _max ), and for outputting a signal indicating this information to a plant control device ( AS)
wherein the system control device (AS) is designed to control energy generation by the asynchronous machine (ASM), to control the self-commutated power converter and to control the instantaneous overvoltage limiting device (BC) and,
when the signal from the control and regulating device (LGSR, LNSR, VSt) indicates a power failure that is shorter than the predetermined first time period (t _max ), the power in the intermediate circuit (ZK) keeps constant and the instantaneous overvoltage limiting device (BC) Art controls that the power in the intermediate circuit (ZK) instead of the line converter (NSR) is fed to the instantaneous overvoltage limiting device (BC) and the electrical energy is converted there into thermal energy, or
if the signal from the control and regulating device (LGSR, LNSR, VSt) indicates a power failure that is longer than the predetermined first time period (t _max ), actuates an adjusting device for the rotor blades in such a way that the rotor is adjusted by adjusting the rotor blades in is operated in a spin mode, drives the self-commutated converter in such a way that the torque of the asynchronous motor (ASM) is controlled with a ramp function and slowly brought down to zero. 2. Wind turbine according to claim 1, wherein the instantaneous surge limiting device (BC) has a resistance in  where the electrical energy is converted into thermal energy. 3. Wind power plant according to claim 1 or 2, wherein when the control and regulating device (LGSR, LNSR, VSt) detects an exceeding of a _maximum power (P _max1 ) by the current output power (P) at the output of the mains power converter (NSR) , a corresponding signal is output to the system control device (AS), and the system control device (AS) in response to this signal _supplies the differential power (P - P _max1 ) of the instantaneous _{overvoltage limiting device} (BC) for converting electrical energy into thermal energy. 4. Wind turbine according to one of the preceding claims 1 to 3, wherein in the self-commutated power converter high-performance switch with parallel clearance run diode are used. 5. Wind turbine according to claim 4, wherein the high-performance switches built in IGBT technology Integrated Power Modules (IPM) are. 6. Wind turbine according to one of the preceding claims 1 to 5, wherein the instantaneous overvoltage limiting device (BC) is dimensioned such that in a predetermined number of times a conversion within a predetermined period tion of electrical energy in thermal energy is possible. 7.Method for operating a wind power plant, with a rotor with adjustable rotor blades, an asynchronous machine (ASM) driven by the rotor via a shaft, a self-commutated converter, a generator converter (GSR), an intermediate circuit (ZK) with an instantaneous overvoltage limiting device ( BC) and a power converter (NSR), which is connected between the asynchronous machine (ASM) and a medium-voltage transformer (MST) and a medium-voltage switchgear downstream (MSS), and a control and regulating device (LGSR, LNSR, VSt) Detection of a power failure and for determining whether a power failure lasts longer than a predetermined first time period (t _max ) and for outputting a signal indicating this information to a system control device (AS),
wherein the system control device (AS) is designed to control energy generation by the asynchronous machine (ASM), to control the self-commutated converter and to control the instantaneous overvoltage limiting device (BC) with the steps
before switching on or after switching off the wind turbine operating a control system in an operating state without a network via a backup battery, after switching on the medium voltage supplying all necessary auxiliary systems with the mains voltage,
Charging an intermediate circuit of the self-commutated converter via a precharging device to a peak value of the current line voltage and switching on a main contactor of the converter, a line and a generator converter not being clocked, detecting a wind speed by the system controller (AS) and when a lower limit value is exceeded the wind speed, giving a start command to a connection control device and contacting the generator and mains converter (GSR, NSR), so that an intermediate circuit voltage (Zk) is set to a nominal value and the wind power plant is in normal operation,
during normal operation of the wind energy installation, the control and regulating device (LGSR, LNSR, VSt) detect whether there is a power failure, and,

• - If the power failure is shorter than the predetermined first time period (t _max ), keep the power in the DC link (ZK) constant and control the instantaneous overvoltage limiter (BC) such that the power in the DC link (ZK) instead of the power converter (NSR) the instantaneous overvoltage limiting device (BC) is supplied and the electrical energy is converted there into thermal energy, or
• - If the Netzausfakk is longer than the predetermined first time period (t _max ), driving an adjusting device for the rotor blades in such a way that the rotor is driven in a spin mode by adjusting the rotor blades, and driving the self-commutated converter in such a way that the torque of the Asynchronous motor (ASM) controlled with a ram function and slowly brought down to zero.

8. The method according to claim 7, characterized by the further step of detecting a current output power (P) at the output of the line converter (NSR) and when a maximum power (P _max1 ) is _exceeded by the current output power (P) at the output of the line converter (NSR) Outputting a corresponding signal to the plant control device (AS), and in response to this signal by the plant control _device (AS) supplying the differential power (P - P _max1 ) to the instantaneous _{overvoltage limiting device} (BC) for converting electrical energy into thermal energy. 9. The method according to claim 7 or 8, wherein in the event that a network interruption with a time period shorter than the predetermined first time period (t _max ) occurs within a predetermined second time period more than a predetermined number of times, a shutdown as in a network interruption the predetermined first time period (t _max ) takes place until the resistance of the instantaneous overvoltage limiting device (BC) has cooled below a predetermined temperature.