FIELD OF THE INVENTION

The present invention relates to a wind turbine comprising an overvoltage protection device being arranged to protect an auxiliary system of the wind turbine against overvoltages. Furthermore, the present invention relates to a method for protecting an auxiliary system of a wind turbine, using such an overvoltage protection device.

BACKGROUND OF THE INVENTION

A wind turbine connected to a power grid may experience variations in voltage. These variations in voltage may, e.g., be in the form of transient voltages or overvoltages. Transient voltages are characterised by having a short duration, typically no more than few milliseconds, and a high magnitude. Lightning strikes and switching transients are two examples of transient voltages. Switching transients occur through switching operations of inductive loads such as transformers. Overvoltages are characterised by having a longer duration and a lower magnitude compared to transient voltages. Such overvoltages may typically arise from faults and/or contingencies in the power grid and/or in the wind turbine.

In order for a wind turbine to function appropriately, a number of auxiliary functions must be operational. The auxiliary functions may include pitching of the wind turbine blades, yawing of the nacelle, heating and/or cooling of various parts of the wind turbine, hydraulic systems, sensors, etc. The auxiliary functions may be handled by an auxiliary system comprising one or more electrical components. The electrical components may have a specific voltage rating which is lower than voltage levels present during a transient voltage and/or an overvoltage. Thus, transient voltages and/or overvoltages may influence the working of one or more electrical components of the auxiliary system of the wind turbine, and in worst case damage, degrade, or destroy the electrical components.

A way of protecting the auxiliary system of the wind turbine is using a resistive component, such as a dump load, which may be used for dissipating the energy of the transient voltage and/or overvoltage, via the resistive component.

However, the high voltage level on transient voltages and overvoltages require that a large resistive component is used, in order to properly protect the electrical components of the auxiliary system. Such a large resistive component may be costly and take up space inside the wind turbine. DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a wind turbine, which is capable of protecting electrical components of an auxiliary system against overvoltages using smaller components as compared to prior art wind turbines.

It is a further object of embodiments of the invention to provide a wind turbine, which is capable of protecting electrical components of an auxiliary system against overvoltages in a fast and reliable manner.

It is an even further object of embodiments of the invention to provide a method for protecting an auxiliary system of a wind turbine in a fast and reliable manner. According to a first aspect the invention provides a wind turbine comprising:

- a generator being electrically connected to a power grid,

- an auxiliary system comprising one or more electrical components, and

- an auxiliary transformer being electrically connected on a primary side to the generator and to the power grid, and being electrically connected on a secondary side to the auxiliary system, the auxiliary transformer having an internal impedance, wherein the wind turbine further comprises an overvoltage protection device, being electrically arranged on the secondary side of the auxiliary transformer, the overvoltage protection device being arranged to be triggered by a voltage exceeding an overvoltage threshold, and wherein triggering of the overvoltage protection device, due to a voltage being above the overvoltage threshold, causes a current exceeding a nominal current through the auxiliary transformer to flow into the overvoltage protection device, thereby causing a secondary side voltage of the auxiliary transformer to be limited due to the internal impedance of the auxiliary transformer.

Thus, according to the first aspect, the invention provides a wind turbine comprising a generator, an auxiliary system and an auxiliary transformer. A wind turbine is a structure which converts energy of the wind into electrical power, via mechanical rotation of a rotor. The wind turbine will normally comprise a tower carrying a nacelle, the nacelle carrying a rotor and a set of wind turbine blades mounted thereon. The nacelle may accommodate electrical components, such as the generator, power electronics, etc. The nacelle may further be mounted on top of the tower, but may also be mounted on other parts of the tower, e.g. in the case that the wind turbine is a multirotor wind turbine comprising two or more rotors. The generator is electrically connected to a power grid, and may absorb or provide power to the power grid.

In the present context the term 'power grid' should be interpreted to mean an interconnected electrical network for delivering electrical power from producers to consumers. Furthermore, the power grid may have different topologies. Examples of such topologies are, e.g., radial power grids and meshed power grids.

As described above, a wind turbine may be able to perform auxiliary functions, which may be handled by the auxiliary system comprising one or more electrical components. The auxiliary system, including the electrical component(s) of the auxiliary system, must therefore be operational in order to allow the wind turbine to perform these auxiliary functions.

The auxiliary transformer is electrically connected on a primary side to the generator and to the power grid, and electrically connected on a secondary side to the auxiliary system. The auxiliary transformer transforms a voltage from the primary side of the auxiliary transformer to the secondary side of the auxiliary transformer. The transformed voltage on the secondary side of the auxiliary transformer may be lower than the voltage on the primary side of the auxiliary transformer, in which case the auxiliary transformer transforms the voltage to a lower level. Alternatively, the transformed voltage on the secondary side of the auxiliary transformer may be higher than the voltage on the primary side of the auxiliary transformer, in which case the auxiliary transformer transforms the voltage to a higher level.

The electrical component(s) of the auxiliary system may be supplied by a voltage from the power grid and/or the generator of the wind turbine. However, the electrical component(s) of the auxiliary system may have a specific voltage rating, which is lower or higher than a voltage level of the rest of the wind turbine, i.e. the electrical component(s) of the auxiliary system may run at a lower or higher voltage compared to the rest of the wind turbine. Thus, in order to comply with the voltage requirement of the electrical component(s), the voltage from the power grid and/or the generator of the wind turbine is transformed from the primary side of the auxiliary transformer to the secondary side of the auxiliary transformer to a lower or a higher voltage level before being supplied to the electrical components of the auxiliary system. Thereby the voltage which is supplied to the electrical components is in accordance with the voltage required by these.

The auxiliary transformer further has an internal impedance, Z. The internal impedance, Z, is of the complex form Z=R+jX, wherein R is a real part of the internal impedance, Z, representing a resistive part of the internal impedance,

Z, and X is an imaginary part of the internal impedance, Z, representing a reactive part of the internal impedance, Z. The real part, R, may be negligible, in which case the internal impedance, Z, is almost purely reactive. This could, e.g., be the case for larger transformers. As an alternative, the real part, R, as well as the imaginary part, X, may be non-negligible, in which case the resistive part as well as the reactive part of the impedance, Z, needs to be taken into account.

The resistance, R, may represent the losses in the windings of the transformer, and the reactance, X, may represent the leakage flux of the windings of the transformer. The leakage flux is the component of the flux that does not link both windings of the transformer. The internal impedance, Z, causes a voltage drop across the auxiliary transformer, which may be proportional to the current through the auxiliary transformer, and may be in the form:

V _drop = IZ = I (R + j X) = IR + I(jX ) wherein the voltage drop across the auxiliary transformer is the sum of the voltage drop across the resistance, R, and the reactance, X, respectively.

The wind turbine further comprises an overvoltage protection device for protecting the electrical components of the auxiliary system of the wind turbine against overvoltages. An overvoltage may arise from faults and/or contingencies in the power grid and/or in the wind turbine, and is characterised by having a duration spanning from few milliseconds to hundreds of milliseconds. Furthermore, an overvoltage may, e.g., be 1.1-1.8 times larger than a nominal voltage, i.e. the voltage at normal operating conditions.

The overvoltage protection device is electrically arranged on the secondary side of the auxiliary transformer. The overvoltage protection device may, e.g., be electrically arranged between the auxiliary transformer and the auxiliary system. Alternatively, the overvoltage protection device may be electrically arranged at any other position on the secondary side of the auxiliary transformer. For instance, the overvoltage protection device may form part of the auxiliary system, or it may even be positioned on an opposite side of the auxiliary system. In the case that the voltage on the secondary side of the auxiliary transformer is lower than the primary side of the auxiliary transformer, a lower voltage is seen by the overvoltage protection device. Such a lower voltage on the secondary side of the auxiliary transformer may in itself enable the use of an overvoltage protection device, which has a smaller size, compared to a case where an overvoltage protection device is electrically arranged on the primary side of the auxiliary transformer. However, the size of the overvoltage protection device may be mainly determined by the voltage drop due to the internal impedance,

Z, of the auxiliary transformer, as will be described in further detail below.

The overvoltage protection device is arranged to be triggered by a voltage exceeding an overvoltage threshold. The overvoltage threshold may, e.g., be approximately 1.1 times larger than the nominal voltage, such as 1.1-1.4 times the nominal voltage. Thereby the overvoltage protection device may be triggered for all overvoltages in the range of 1.1-1.8 times the nominal voltage, i.e. the range which normally applies for overvoltages.

The triggering of the overvoltage protection device, due to a voltage being above the overvoltage threshold, causes a current exceeding a nominal current through the transformer to flow into the overvoltage protection device. The current is preferably significantly larger than the nominal current, e.g., 2-10 times the nominal current or 5-10 times the nominal current, i.e. the current at normal operating conditions. As described above, the internal impedance, Z, of the auxiliary transformer causes a voltage drop across the auxiliary transformer, which may be proportional to the current through the auxiliary transformer. Thus, for a current larger than the nominal current, the voltage drop due to the internal impedance, Z, of the auxiliary transformer will accordingly be larger.

The voltage drop due to the internal impedance, Z, of the auxiliary transformer will in turn reduce the voltage on the secondary side of the auxiliary transformer to a level, which is lower than the secondary side voltage of the auxiliary transformer, when the voltage drop due to the internal impedance, Z, of the auxiliary transformer is not taken into account. Furthermore, the larger the current through the auxiliary transformer, the larger the voltage drop due to the internal impedance, Z, will be. Thus, the voltage drop due to the internal impedance, Z, of the auxiliary transformer results in a voltage level on the secondary side of the auxiliary transformer which is lower than the secondary side voltage level without the voltage drop due to the internal impedance, Z, of the auxiliary transformer, and the reduction in voltage increases as the current flowing into the auxiliary transformer increases.

Accordingly, the voltage drop due to the internal impedance, Z, of the auxiliary transformer reduces the voltage on the secondary side of the auxiliary transformer, thus enabling the use of an overvoltage protection device, which has a smaller size. Therefore, due to the internal impedance, Z, of the auxiliary transformer, the wind turbine may be protected using an overvoltage protection device which is smaller in size, thereby decreasing the costs of the wind turbine as well as the space required for accommodating the overvoltage protection device. In summary, by electrically arranging the overvoltage protection device on the secondary side of the auxiliary transformer, a smaller size overvoltage protection device may be used.

It is an advantage that the overvoltage protection device is electrically arranged on the secondary side of the auxiliary transformer. Thereby it may be possible to use an overvoltage protection device which is smaller in size, due to the voltage drop across the auxiliary transformer. The smaller overvoltage protection device may take up less space in the wind turbine. Thereby, the electrical component(s) of the auxiliary system of the wind turbine may be protected against overvoltages using a smaller and less costly overvoltage protection device.

After the overvoltage protection device is triggered, it is ensured that a current exceeding a nominal current flows through the auxiliary transformer, and into the overvoltage protection device, after the occurrence of the overvoltage, thus limiting the secondary side voltage of the auxiliary transformer. The overvoltage protection device offers a fast and reliable manner of protecting the auxiliary system of the wind turbine by ensuring that the current exceeding a nominal current flows into the overvoltage protection device, after the overvoltage protection device is being triggered. Thereby the electrical components of the auxiliary system may be protected in a fast and reliable manner.

The secondary side voltage may further be limited by a saturation in the auxiliary transformer. Saturation is reached when an increase in applied external magnetic field, H, cannot increase the magnetisation of the auxiliary transformer, so the total magnetic flux density, B, approximately stays unchanged. The saturation of the auxiliary transformer may be a limit determined by the material and core dimensions of the auxiliary transformer.

Thus, when the auxiliary transformer is saturated, an increase in the voltage on the primary side of the auxiliary transformer due to an overvoltage will not result in a further increase of the voltage on the secondary side of the auxiliary transformer. Thus, the saturation of the auxiliary transformer sets a limit on the voltage that may be transformed from the primary side of the auxiliary transformer to the secondary side of the auxiliary transformer. Thereby, it is sufficient to apply an overvoltage protection device which is capable of handling a voltage level which corresponds to the saturated voltage level, because the voltage on the secondary side of the auxiliary transformer will never exceed this level, and therefore the overvoltage protection device will never be subjected to a higher voltage level. The scenario described above may occur for a fully saturated auxiliary transformer. However, it may also, at least to some extend, occur for a partly saturated auxiliary transformer.

A first electrical breaker may be electrically arranged on the primary side of the auxiliary transformer between the auxiliary transformer, and the generator and the power grid, respectively. The first electrical breaker may, e.g., be an electrical switch that may be operated automatically. Since the first electrical breaker is arranged between the auxiliary transformer and the generator/power grid, opening the first electrical breaker will result in the auxiliary transformer, and consequently the auxiliary system, being disconnected from generator/power grid, and thereby from its power source. During normal operating conditions, the first electrical breaker will typically be closed, thus ensuring a power supply to the auxiliary system. However, in the case of an overvoltage, the first electrical breaker may be opened, thereby disconnecting the auxiliary system from its normal power source. This will be described in further detail below.

The first electrical breaker may be arranged to open, thereby disconnecting the auxiliary transformer from the generator and the power grid, in response to a current through the first electrical breaker exceeding a predefined current threshold value and/or a predefined duration of a current through the first electrical breaker which exceeds a predefined energy threshold value, as a result of the overvoltage protection device being triggered.

According to this embodiment, when a current through the first electrical breaker results in an energy dissipation in the first electrical breaker which exceeds a predefined energy threshold value, the first electrical breaker is arranged to open, and thereby interrupt the current flow from the power grid and the generator of the wind turbine to the auxiliary transformer.

The predefined energy threshold value may define an upper limit on the energy that may be dissipated in the first electrical breaker. The first electrical breaker may have an internal resistance, R, in which the current may flow through. Thus, for a current, I, with a duration, t, the energy dissipated in the first electrical breaker may be calculated in the following manner:

Thus, the energy dissipated in the first electrical breaker depends on the current level as well as on the duration of the current. An energy dissipation which exceeds the predefined energy threshold value may be the result of a sufficiently high current level, even if the duration is very short. However, overvoltage events with a somewhat lower current level will also result in the predefined energy threshold value being reached, if the duration is sufficiently long. Thus, by applying the energy threshold value, it is ensured that events with a very high current level as well as events with a lower current level, but a longer duration, will result in opening of the first electrical breaker.

The overvoltage protection device may comprise a voltage dependent resistor.

A voltage dependent resistor has a variable resistance, which depends on the voltage applied. Typically, the resistance of the voltage dependent resistor decreases when the voltage increases, and vice versa. The voltage dependent resistor may preferably be in the form of a varistor, such as a metal oxide varistor (MOV). MOVs are especially applicable for high power applications, such as in wind turbines.

In case of an excessive voltage increase, such as in case of an overvoltage, the resistance of the voltage dependent resistor drops immediately. Thereby, it may be ensured that a large current, such as the current exceeding a nominal current, flows into the voltage dependent resistor, and thereby protect the auxiliary system of the wind turbine. Thus, the voltage dependent resistor may offer a fast and reliable manner of protecting the auxiliary system of the wind turbine.

Alternatively or additionally, the overvoltage protection device may comprise a solid state switch and a resistive element, the solid state switch being closed when the voltage exceeds the overvoltage threshold. According to this embodiment, a resistive element is coupled to the secondary side of the auxiliary transformer via a solid state switch. When the voltage level is above the overvoltage threshold the solid state switch closes, thereby enabling the current to flow into the resistive element and protecting the auxiliary system of the wind turbine.

Solid state switches are characterised by having a high switching speed, and have no mechanical parts that may wear out, compared to an electromechanical switch. Thus, solid state switches offer a fast and reliable manner of switching. Solid state switches may, e.g., be based on one or more metal-oxide- semiconductor field-effect transistors (MOSFETs). The resistive element may, e.g., be a dump load or any other suitable resistive element that may be arranged to dissipate the energy from the overvoltage.

Thus, the solid state switch and the resistive element may protect the auxiliary system of the wind turbine by closing the solid state switch, when the voltage exceeds the overvoltage threshold, and thereby lead the current to the resistive element, where it may be dissipated. Thereby, the auxiliary system of the wind turbine may be protected in a fast and reliable manner.

The overvoltage protection device may be controlled by means of an electrical circuit, sensing a voltage level at the overvoltage protection device. According to this embodiment, an electrical circuit controls the overvoltage protection device based on the applied voltage across the terminals of the overvoltage protection device.

The electric circuit may comprise voltage sensing means, such as a voltage sensing circuit for measuring the voltage at the overvoltage protection device, and may further send a signal indicating whether the overvoltage protection device should be triggered or not. The electrical circuit may further control the overvoltage protection device based on a voltage exceeding the overvoltage threshold. For instance, in the case that the electrical circuit senses a voltage exceeding the overvoltage threshold, the electrical circuit may trigger the overvoltage protection device. However, in the case that the electrical circuit senses a voltage below the overvoltage threshold, the electrical circuit may not trigger the overvoltage protection device. Thus, the electrical circuit may enable controlling the overvoltage protection device in a reliable manner.

The overvoltage protection device may further be arranged to provide transient voltage protection. As described above, transient voltages are characterised by having a short duration and a high magnitude. Thus, transient voltages may be in the form of fast voltage spikes. These voltage spikes may cause a high current through the auxiliary transformer to flow into the overvoltage protection device, and at the same time cause a large voltage drop across the auxiliary transformer. However, since the duration of the transient voltage is significantly lower than the duration of the overvoltage, the auxiliary transformer and the overvoltage protection device may be able to protect the auxiliary system against such transient voltages. Thereby, the auxiliary system may be protected against overvoltages, as well as transient voltages.

The wind turbine may further comprise a battery being electrically connected to the electrical components of the auxiliary system, the battery being arranged to supply power to the electrical components during an overvoltage event.

According to this embodiment, the auxiliary system of the wind turbine is supplied with power from an external power source, i.e. a battery, when the voltage level in the wind turbine exceeds the overvoltage threshold, such as in the case of an overvoltage or a transient voltage.

When an overvoltage event occurs, it may no longer be possible to supply the auxiliary system with power from the power grid and/or from the generator of the wind turbine, since the overvoltage may damage, degrade, or destroy the electrical component(s) of the auxiliary system. Furthermore, in the case that the wind turbine is provided with an electrical breaker arranged on the primary side of the auxiliary transformer between the auxiliary transformer, and the generator and the power grid, respectively, this breaker may further be opened, thereby disconnecting the auxiliary system from the power grid. Thereby, it may be required to supply power to the auxiliary system from another source in order to ensure that the wind turbine can continue performing auxiliary functions throughout the duration of the overvoltage event.

According to this embodiment, the auxiliary system is supplied with power from the battery. Accordingly, the battery ensures that the wind turbine is able to perform auxiliary functions throughout the duration of an overvoltage event.

The battery may supply power to the auxiliary system when the voltage level in the wind turbine exceeds the overvoltage threshold and/or when a possible first electrical breaker is opened. During normal operating conditions, the battery may be charged by the power supplied from the power grid and/or from the generator of the wind turbine, thereby ensuring that the battery is able to supply power to the auxiliary system in the case of an overvoltage event.

The wind turbine may further comprise a second electrical breaker being electrically arranged on the secondary side of the auxiliary transformer between the overvoltage protection device and the auxiliary system, the second electrical breaker being configured to open in response to an overvoltage triggering the overvoltage protection device. According to this embodiment, the auxiliary system of the wind turbine is further protected by a second electrical breaker that opens as soon as the overvoltage protection device is triggered by an overvoltage, thus disconnecting the auxiliary system from the overvoltage protection device and the auxiliary transformer.

The second electrical breaker may be an electrical breaker of the same type as the first electrical breaker described above, or it may be another type of an electrical breaker, such as a solid state switch.

As described above, when an overvoltage occurs, a current exceeding the nominal current flows through the auxiliary transformer and into the overvoltage protection device. The overvoltage on the secondary side of the auxiliary transformer drives the current towards the overvoltage protection device. However, the entire current may not flow into the overvoltage protection device, and may instead flow into the auxiliary system of the wind turbine, i.e. a part of the overvoltage propagates towards the auxiliary system. Although it is only a part of the current that flows into the auxiliary system, it may still damage, degrade, or destroy the electrical component(s) of the auxiliary system, due to being higher than the nominal current. Thus, the current flow to the auxiliary system has to be interrupted in order to fully protect the auxiliary system of the wind turbine.

By electrically arranging the second electrical breaker between the overvoltage protection device and the auxiliary system, it is ensured that the current from the overvoltage flows into the overvoltage protection device, but does not flow towards the auxiliary system, when the second electrical breaker is opened. Thereby, the current from the overvoltage is dissipated in the overvoltage protection device, and the auxiliary system is protected against currents higher than the nominal current.

According to a second aspect, the invention provides a method for protecting an auxiliary system of a wind turbine against overvoltage, the wind turbine comprising a generator being electrically connected to a power grid, an auxiliary system comprising one or more electrical components, an auxiliary transformer being electrically connected on a primary side to the generator and to the power grid, and being electrically connected on a secondary side to the auxiliary system, the auxiliary transformer having an internal impedance, and an overvoltage protection device being electrically arranged on the secondary side of the auxiliary transformer, the method comprising the steps of:

- in response to an overvoltage originating from a power grid and/or from the generator of the wind turbine and causing a voltage which exceeds an overvoltage threshold, triggering the overvoltage protection device, thereby causing a current exceeding a nominal current through the auxiliary transformer, and dissipating energy of the current on the secondary side via the overvoltage protection device, and

- transforming the overvoltage from the primary side of the auxiliary transformer to the secondary side of the auxiliary transformer, the current through the auxiliary transformer thereby causing a secondary side voltage of the auxiliary transformer to be limited due to the internal impedance of the auxiliary transformer.

It should be understood, that a skilled person would readily recognise that any feature described in combination with the first aspect of the invention could also be combined with the second aspect of the invention, and vice versa.

Thus, according to the second aspect, the invention provides a method for protecting the auxiliary system of a wind turbine against overvoltage. The wind turbine could, e.g., be the wind turbine of the first aspect of the invention. In the method according to the second aspect of the invention, an overvoltage originating from the power grid and/or from the generator of the wind turbine occurs. The overvoltage causes a voltage which exceeds an overvoltage threshold, in the manner described above with reference to the first aspect of the invention.

In response to the voltage, the overvoltage protection device of the wind turbine is triggered. The triggering of the overvoltage protection device results in a current exceeding a nominal current to flow through the auxiliary transformer and into the overvoltage protection device. The current is preferably significantly larger than the nominal current, e.g., 2-10 times the nominal current or 5-10 times the nominal current. The energy of the current on the secondary side of the auxiliary transformer is dissipated via the overvoltage protection device. The overvoltage protection device may comprise a resistive element, such as a dumpload and/or a varistor, in which energy from the current may be dissipated. Thereby, it may be ensured that at least a part or the full current may flow into the overvoltage protection device, and thus enabling the protection of the auxiliary system of the wind turbine.

The auxiliary transformer has an internal impedance, Z, as defined above with reference to the first aspect of the invention. The internal impedance, Z, causes a voltage drop across the auxiliary transformer, which may be proportional to the current through the auxiliary transformer. Thus, when the overvoltage from the power grid and/or from the generator of the wind turbine is transformed from the primary side of the auxiliary transformer to the secondary side of the auxiliary transformer, a voltage drop occurs due to the current through the auxiliary transformer. This voltage drop reduces the voltage on the secondary side of the auxiliary transformer compared to the case where there is no voltage drop across the auxiliary transformer, and thereby also the voltage seen by the overvoltage protection device. Since the voltage on the secondary side of the auxiliary transformer is reduced, an overvoltage protection device, which is smaller in size, may be used. Thereby, the auxiliary system of the wind turbine may be protected using a smaller overvoltage protection device, thereby decreasing the costs of the wind turbine as well as the space required for accommodating the overvoltage protection device.

The step of triggering the overvoltage protection device may further comprise the step of opening a first electrical breaker being electrically arranged on the primary side of the auxiliary transformer between the auxiliary transformer, and the generator and the power grid, respectively, thereby disconnecting the auxiliary transformer from the generator and the power grid, in response to a current through the first electrical breaker exceeding a predefined current threshold value and/or a predefined duration of a current through the first electrical breaker which exceeds a predefined energy threshold value, as a result of the overvoltage protection device being triggered.

According to this embodiment, the first electrical breaker is opened in response to an energy dissipation in the first electrical breaker reaching a threshold, essentially as described above with reference to the first aspect of the invention. As described above, the energy threshold may be reached as a consequence of a sufficient high current and/or as a consequence of a sufficiently long duration of the overvoltage event.

According to an alternative embodiment, the first electrical breaker may be opened by at least one of the electrical components of the auxiliary system. The electrical component(s) may measure the energy dissipation in the first electrical breaker, e.g. using sensing equipment, such as an undervoltage coil, and open the first electrical breaker in response to an energy dissipation in the first electrical breaker reaching a threshold.

The method may further comprise the step of at least one of the electrical components of the auxiliary system providing a closing command to the first electrical breaker causing the first electrical breaker to close, in response to ending of an overvoltage event. According to this embodiment, when the voltage is below a certain level, e.g., the overvoltage threshold, one of the electrical components of the auxiliary system causes the first electrical breaker to close. Thus, when the voltage in the wind turbine reaches a level where it is no longer considered damaging for the electrical components of the auxiliary system, one of the electrical components sends a closing command to the first electrical breaker, thereby causing it to close and restore the electrical connection between the generator and the power grid, on the one hand, and the auxiliary system, on the other hand. Thus, at least one of the electrical components may be in communicative connection with the first electrical breaker.

By sending a closing command to the first electrical breaker at the end of the overvoltage event, the auxiliary system can be automatically reconnected to the power grid and the generator of wind turbine, without manually interacting with first electrical breaker. Thereby, the wind turbine can be put into production again without requiring service personnel to access the wind turbine, thereby reducing costs related to the service personnel, as well as production loss due to down time. This is particularly relevant for offshore wind turbines, where access by service personnel is difficult and costly.

In order to send a closing command to the first electrical breaker, the at least one of the electrical components needs to be operational during the overvoltage event. Since the first electrical breaker is open due to the overvoltage event, it may not be possible to supply the auxiliary system with power from the power grid and/or from the generator of the wind turbine. Thus, the auxiliary system has to be supplied with power from an external source, such as a battery, in order to be able to send a closing command to the first electrical breaker.

The method may further comprise the step of supplying power to the electrical components during an overvoltage event, using a battery being electrically connected to the electrical components of the auxiliary system. According to this embodiment, it is ensured that power is supplied to the electrical components of the auxiliary system throughout an overvoltage event, thereby ensuring that the wind turbine is capable of performing relevant auxiliary functions, as described above with reference to the first aspect of the invention. Furthermore, in the case that the wind turbine is of a kind which comprises a first electrical breaker arranged between the generator/power grid and the auxiliary transformer, it may be ensured that one of the components of the auxiliary system is capable of sending a closing command to the first electrical breaker when the overvoltage event has ended, thereby avoiding the requirement of service personnel to access the wind turbine in order to put it into production, as described above.

The overvoltage may be in a range from 110% to 180% of a nominal voltage level. Thus, in the case of an overvoltage event, the voltage on the primary side of the auxiliary transformer may be in this range. Thereby, the overvoltage protection device may be arranged to be triggered and protect the auxiliary system of the wind turbine against overvoltages in this range.

The overvoltage protection device may further protect against voltages above 180% of a nominal voltage. In this case, the voltage on the primary side of the auxiliary transformer may be a transient voltage. In this case, the overvoltage protection device may further provide transient voltage protection.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further details with reference to the accompanying drawings, in which

Fig. 1 is a single line diagram of a wind turbine according to an embodiment of the invention, and

Fig. 2 is a flow chart illustrating a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a single line diagram of a wind turbine 1 according to an embodiment of the invention. The wind turbine 1 comprises a generator 2 being electrically connected to a power grid 3 via a transformer 4. The generator 2 and the power grid 3 are electrically connected on a primary side of an auxiliary transformer 5. The generator 2 may be connected to the power grid 3 through a power converter. The auxiliary transformer 5 is on a secondary side electrically connected to an auxiliary system 6 which comprises one or more electrical components 7. The one or more electrical components 7 may be responsible for handling one or more auxiliary functions performed by the wind turbine 1, such as yawing, pitching, heating, cooling, control of hydraulic systems, etc.

The wind turbine 1 further comprises an overvoltage protection device 8 which is electrically arranged on the secondary side of the auxiliary transformer 5, between the auxiliary transformer 5 and the auxiliary system 6. The overvoltage protection device 8 of Fig. 1 is grounded for each phase of the wind turbine 1. Alternatively, the overvoltage protection device 8 may be electrically arranged in between the phases of the wind turbine.

A first electrical breaker 9 is electrically arranged between the auxiliary transformer 5, and the generator 2 and the power grid 3, respectively. Thus, opening the first electrical breaker 9 disconnects the generator 2 and the power grid 3, on the one hand, from the auxiliary system 6 and the overvoltage protection device 8, on the other hand.

Finally, a second electrical breaker 10 is electrically arranged between the overvoltage protection device 8 and the auxiliary system 6. Thus, opening the second electrical breaker 10 disconnects the auxiliary system 6 from the generator 2 and the power grid 3, as well as from the overvoltage protection device 8.

When an overvoltage originating from the generator 2 and/or from the power grid 3 occurs, the voltage level in the wind turbine 1 can increase to a level which exceeds an overvoltage threshold. The overvoltage is typically 1.1-1.8 times larger than a nominal voltage, i.e. the voltage at normal operating conditions, for instance in the range of 1.1-1.4 times the nominal voltage. Thereby the overvoltage protection device can be triggered for all overvoltages in the range of 1.1-1.8 times the nominal voltage. An overvoltage exceeding the overvoltage threshold causes the overvoltage protection device 8 to trigger. The triggering of the overvoltage protection device 8, due to the voltage being above the overvoltage threshold, causes a current exceeding a nominal current through the auxiliary transformer 5 to flow into the overvoltage protection device 8. At the same time, the second electrical breaker 10 may be opened, thus disconnecting the auxiliary system 6 from the rest of the wind turbine. When the second electrical breaker 10 is opened, it is ensured that the current from the overvoltage flows into the overvoltage protection device 8, but does not flow towards the auxiliary system 6. As an alternative, the second electrical circuit breaker 10 may remain closed, e.g. in the case that the majority of the current flows into the overvoltage protection device 8, and the current has a level, which is not considered damaging for the auxiliary system.

In order for the auxiliary system 6 to be operational throughout the duration of the overvoltage event, the auxiliary system 6 may be supplied with power from an external power source (not shown), such as a battery. Thereby, the one or more electrical components 7 may appropriately handle the auxiliary functions of performed by the wind turbine 1 during the overvoltage event.

The voltage received at the overvoltage protection device 6 is reduced, due to a voltage drop across an internal impedance of the auxiliary transformer 5. The voltage drop results in a voltage level on the secondary side of the auxiliary transformer 5 which is lower than the secondary side voltage level without the voltage drop. Thereby a small overvoltage protection device 8 can be used without risking that the overvoltage protection device 8 is incapable of handling any overvoltage event.

The voltage on the secondary side of the auxiliary transformer 5 may further be limited by a saturation in the auxiliary transformer 5. When the auxiliary transformer 5 is saturated, an increase in the voltage on the primary side of the auxiliary transformer 5 due to an overvoltage will not result in a further increase of the voltage on the secondary side of the auxiliary transformer 5. Thereby, it is sufficient to apply an overvoltage protection device 6 which is capable of handling a voltage level which corresponds to the saturated voltage level, since voltage on the secondary side of the auxiliary transformer 5 will never exceed this level, and therefore the overvoltage protection device 6 will never be subjected to a higher voltage level.

When a current through the first electrical breaker 9 results in an energy dissipation in the first electrical breaker 9 which exceeds a predefined energy threshold value, the first electrical breaker 9 opens, thereby interrupting the current flow from the power grid 3 and the generator 2 towards the auxiliary transformer 5. The predefined energy threshold value depends on the current level through the first electrical breaker 9 as well as on the duration of the current through the first electrical breaker 9. Thus, a high current level with a short duration as well as a somewhat lower current level with a longer duration may result in opening of the first electrical breaker 9.

When the overvoltage event has ended, e.g. when the voltage level is again approaching the nominal voltage, one of the electrical components 7 of the auxiliary system 6 may cause the first electrical breaker 9 to close by sending a closing command. Thereby, the wind turbine 1 can be automatically put into production again, without requiring service personnel to access the wind turbine.

Fig. 2 is a flow chart illustrating a method according to an embodiment of the invention. The method is initiated in step 11, in which a wind turbine is operated at normal operating conditions

In step 12, it is investigated whether a voltage level in the wind turbine is above an overvoltage threshold or not. In the case that the voltage level is below the overvoltage threshold, the wind turbine continues to operate at normal operating conditions.

In the case that step 12 reveals that the voltage level in the wind turbine is above the overvoltage threshold, this is an indication that an overvoltage event is occurring. Therefore, the process is forwarded to step 13, where an overvoltage protection device arranged on a secondary side of an auxiliary transformer is triggered. The triggering of the overvoltage protection device causes a current higher than a nominal current to flow into the overvoltage protection device, in the manner described above with reference to Fig. 1. Furthermore, a second electrical breaker arranged between the overvoltage protection device and the auxiliary system is opened. The second electrical breaker disconnects the auxiliary system from a power grid and a generator of the wind turbine, and ensures that a current from the overvoltage flows into the overvoltage protection device, but does not flow towards the auxiliary system.

Alternatively, in step 13, the overvoltage protection device may be triggered without opening the second electrical breaker.

In step 14, the energy dissipated in a first electrical breaker arranged between a primary side of the auxiliary transformer between the auxiliary transformer, and the generator and the power grid, respectively, is monitored. The energy dissipated in the first electrical breaker depends on the current level as well as on the duration of the current, as described above. The monitoring may be performed autonomously by measuring the current level as well as the duration of the current or by electromagnetic disconnection.

In step 15, it is investigated whether the energy dissipated in the first electrical breaker is above an energy threshold. In the case that the dissipated energy is below the energy threshold, the process is returned step 15, and monitoring of the energy dissipation in the first electrical breaker is continued.

In the case that step 15 reveals that, the energy dissipation in the first electrical breaker is above the energy threshold, the process is forwarded to step 16, where the first electrical breaker is opened, thereby disconnecting the auxiliary transformer from the power grid and the generator of the wind turbine, in the manner described above with reference to Fig. 1.

In step 17, the voltage level in the wind turbine is monitored, and in step 18, it is investigated whether the overvoltage event has ended. In the case that the overvoltage event has not ended, the process is returned to step 17, and monitoring of the voltage level in the wind turbine is continued.

In the case that step 18 reveals that, the overvoltage event has ended, the process is forwarded to step 19, where the first and second electrical breakers are closed, in the manner described above with reference to Fig. 1, thereby putting the wind turbine back into production, and the wind turbine is again operated at normal operating conditions. When the overvoltage event has ended, and the wind turbine operated at normal operating conditions, the process is returned to step 12, and the voltage level is checked for new overvoltage events.