WO2012005635A1

WO2012005635A1 - Device and method for measuring ice thickness

Info

Publication number: WO2012005635A1
Application number: PCT/SE2010/050775
Authority: WO
Inventors: Eduardo FIGUEROA-KARLSTRÖM
Original assignee: Saab Ab
Priority date: 2010-07-05
Filing date: 2010-07-05
Publication date: 2012-01-12
Also published as: BR112013000307A2; EP2591307A4; EP2591307A1

Abstract

The present invention relates to a device (100) and method for measuring ice thickness on a first surface (110) of a construction element (120) arranged in an asp of a wind turbine. The device comprises a sensor arrangement (130) arranged in connection with said construction element and arranged to provide impedance values, and a processing unit (140) coupled to the sensor arrangement (130) and arranged to determine the presence of ice based on the provided impedance values. The device comprises further a reference sensor arrangement (150) arranged to generate reference impedance values. The processing unit (140) is further arranged to determine the ice thickness based on determinations of a relation between the impedance values determined by the sensor arrangement and the reference impedance values generated by the reference sensor arrangement.

Description

Device and method for measuring ice thickness

TECHNICAL FIELD

The present invention relates to a device and method for measuring ice thickness on a surface of wind power plant asps.

BACKGROUND ART

It is known that water/ice exhibit a phase transition while passing through 0°C. In particular, the values of the dielectric properties exhibit a dramatic change as the temperature is lowered across the freezing point. Furthermore, the conductivity of liquid water changes dramatically with its ionic content. This allows for use of total impedance measurements to the presence of either water or ice.

In wind power applications, it is important that the rotating moment acting on the main shaft is as homogenous as possible. The wind turbine drives an axle which is connected to a generator often via a transmission. High latitude wind power plants will be exposed to ice accretion resulting in unevenly distributed weight changes in the asps. This uneven distribution of weight results in moment of inertia that leads to dangerous inhomogeneities on the moment excerpted on the main shaft of the wind power plant. This has an adverse impact on the transmission which in worst case leads to damage and a stop in the power production. Therefore, there is a need for a reliable measurement of ice accretion on the surfaces of the asps. Today, ice measurement systems rely on indirect measurements performed in the vicinity of the wind power plant, said measurements providing information about the risk of ice accretion.

SUMMARY OF THE INVENTION

It is one object to obviate at least some of the above disadvantages so as to increase the time when the wind power plant is operating. This has in one example been achieved by means of a device for measuring ice thickness on a first surface of a construction element arranged in an asp of a wind turbine The device comprises a sensor arrangement arranged in connection with said construction element and arranged to provide

impedance values, a reference sensor arrangement and a processing unit coupled to the sensor arrangement and the reference sensor arrangement. The reference sensor arrangement is arranged to generate reference impedance values. The processing unit is arranged to determine the presence of ice based on determination of a relation between the impedance values determined by the sensor arrangement and the reference impedance values generated by the reference sensor arrangement.

The use of a differential detection where the output of the sensor is compared to a reference value provides for accuracy and a short response time, i. e. ice accretion can be detected fast. Further, the accuracy of the measurements can be unchanged during the life time of the device due to the fact that the reference sensor arrangement does not have to be exposed to the environment.

In one option, the sensor arrangement comprises an electrode separated from the construction element by an insulating gap, wherein the electrode has a second surface. In accordance with this example, the device is so designed that it can be accommodated to any surface such that it is does not introduce roughness and geometrical variation or protrusions. Thereby, the mounting of the sensor arrangement does not alter laminar air flow. Thus, the mounting of the sensor does not affect the aerodynamic of the wind turbine.

In one option, the construction element forms a second electrode of the sensor arrangement. The construction element may be connected to a grounding plane. The construction element is for example connected to the grounding plane by means of a conductive path running through the asp. In one option, the processing unit comprises a calculation unit arranged to calculate a thickness of the ice based on consecutive determinations of the relation between the impedance value and the reference impedance value. The calculation unit is then for example arranged to repeatedly calculate a mean value and a standard deviation value for the ice thickness.

In one option, the processing unit comprises a lock-in amplifier arranged to output the relation between the impedance value and the reference impedance value. The lock-in amplifier is arranged to receive the impedance value and the reference impedance value on its inputs. The lock-in amplifier is phase locked at the same frequency as the operating frequency of the sensor arrangement and the reference sensor arrangement. The use of a lock-in amplifier allows for high precision, resolution and accuracy. The fact that the lock-in amplifier only operates at one frequency provides for a high level of noise rejection. Further, the use of a lock-in amplifier enables measurements of accretion thickness with discrimination down to fraction of millimetres. Thereby, ice rejection actuators can be activated at the right moment in time, i.e. before any mechanical perturbation is introduced to the balance of the asps.

Further options are set out in the dependent claims.

The invention also relates to a wind turbine having a plurality of asps, wherein each asp is formed in a plastic material and wherein at least one of the asps being provided with at least one device for measuring ice thickness as described above. The invention also relates to a wind power plant comprising said wind turbine. The invention also comprises a method for measuring ice thickness. The method comprises the steps of providing an impedance signal partly caused by an insulating gap between a construction element in the asp and an electrode formed therein, providing a reference impedance signal, and determining the presence of ice based on a relation between the provided impedance signal and the provided reference impedance signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows one example of a device for measuring ice thickness.

Fig 2a is a top view of a sensor arrangement mounted at a first surface in accordance with one example of the device for measuring ice thickness of Fig 1 .

Fig 2b is a side view of the sensor arrangement of the device for measuring ice thickness in Fig 1 .

Fig 3 is a top view of an alternative design of a sensor arrangement mounted at a first surface in accordance with one example of the device for measuring ice thickness of Fig 1 . Fig 4 shows one example of a reference sensor arrangement of the device for measuring ice thickness in Fig 1 .

Fig 5 is an electrical scheme schematically illustrating the measurements of the sensor arrangement and the reference sensor arrangement.

Fig 6 shows one example of a processing unit in the device for measuring ice thickness in Fig 1 .

Fig 7 shows one example of a lock-in amplifier of the processing unit of Fig 6. Fig 8 shows one example of a device for measuring ice thickness having a plurality of sensor arrangements and mounted in the asp of a wind turbine in accordance with a first embodiment. Fig 9 shows one example of a device for measuring ice thickness having a plurality of sensor arrangements and mounted in the asp of a wind turbine in accordance with a second embodiment.

Fig 10 is a flow chart showing an example of a method for measuring ice thickness.

Fig 1 1 is a flow chart showing a method for initiating activation of a de-icing system based on the ice thickness measured in accordance with the method of Fig 10.

DETAILED DESCRIPTION

In Fig 1 , a device for measuring ice thickness 100 on a surface 135 of an asp 180 of a wind turbine is depicted. The wind turbine is in one example mounted on a land based or sea based power plant. The operation of the device for measuring ice thickness 100 is based on the temperature dependence of the dielectric property tensor of ice while the detection of water, water with ionic additives (environmental impurities, glycols or the like), or water and ice is based on the electrical conductivity of the mixed liquid phase. The real part of the dielectric constant shows a linear behaviour on temperature. Furthermore, at the liquid to solid state phase transition point, i.e. at 0°C, the dielectric constant of water exhibits a discontinuity. This discontinuity is used to detect the inception of ice formation. The asps 190 have a skin 131 formed in a plastic material 131 . In one example, the plastic material is an epoxy such as a fibre reinforced epoxy. The asps 190 are in one example hollow. A metallic structure 133 is formed in internal of the asp 190. The metallic structure 133 can provide reinforcement and support to the polymeric skin of the asps. The metallic structure 133 can also act as a conducting grounding path. Thus, the metallic structure 133 can act as a conductor for lightning. The device for measuring ice thickness 100 comprises a construction element 120 formed in a cavity in the skin 131 of the asp 190. The

construction element 120 comprises in one example a metal such as aluminium or an alloy thereof. The construction element 120 is so arranged in relation to the asp such that a surface 1 10 of the construction element form an integrated surface with the surface 135 of the asp. A plurality of

construction elements 120 can be formed in each asp 190 so as to form devices for measuring ice thickness 100. In one example, the size of the surface 1 10 of the construction element 120 is 20x20 cm. The device for determining the presence of ice on the asp surface 135 in fact determines the presence of ice at the construction element surface 1 10. The device 100 comprises at least one sensor arrangement 130, at least one processing unit 140 and at least one reference sensor arrangement 150. The sensor arrangement 130 is arranged to measure impedance. The sensor arrangement comprises said construction element 120 and an electrode 134. The electrode 134 is separated from the construction element by an insulating gap 160. In one example, the electrode is formed in the same material as the construction element 120. The electrode 134 has a second surface 132. The construction element is connected to ground. In one example, the metal structure 133 forms a grounding plane of the sensor arrangement, i.e. the construction element is electrically connected to the metallic structure 133. Alternatively, the construction element is

connected to a grounding point anywhere in the asp. The electrode 134 and the grounding plane are electrically connected to inputs of the processing unit 140.

The reference sensor arrangement 150 is arranged to generate reference impedance. In detail, the reference sensor arrangement comprises an electrode and grounding plane geometrically identical to the sensor arrangement 130. The electrode and grounding plane of the reference sensor arrangement are electrically connected to inputs of the processing unit 140. The impedance of the reference sensor arrangement is designed so as to be the same as for the sensor arrangement 130, when the first and second surfaces of the sensor arrangement are clean and new. Due to ageing and wear of the sensor arrangement 130, slight erosion of the gap will likely occur. Although this can be compensated for with regular compensation or control in processing of the signals, it is estimated to be negligible while measuring in differential mode.

The processing unit 140 has, as stated above inputs for the electrode 134 and the grounding plane of the sensor arrangement 130 and an electrode and the grounding plane of the reference sensor arrangement 150. The processing unit 140 is arranged to generate an output value indicative of ice thickness based on the signals received on the inputs. The processing unit 140 is arranged to determine the ice thickness based on consecutive measurements of a relation between the impedance values determined by the sensor arrangement and by the reference sensor arrangement 150. In the shown example, a presentation unit 170 is arranged to receive

information related to the determination of the ice thickness and to present that information. In one example, the information is visually presented. In an alternative or complementing example, the information is presented by means of a sound signal. The visual presentation can for example be obtained by means of a display or lamp(s) or diode(s) in a control room. In the shown example, a unit for automatic control is arranged to receive information related to the ice thickness and to actuate de-icing equipment based on the received information.

In one example (not shown) a heating element is arranged in relation to the sensor arrangement 130. The heating element can be activated so as to clean the surface from remaining ice for example after ice rejection attempts. This will allow the zeroing of the process after a successful de-icing attempt of the aircraft. Heating say one of the sensors among a plurality of them could serve as a control of the cleaning degree of the others after exposure to ice accretion or any other wetting surfactant. In Fig 2, a second surface 232 of an electrode 234 and a first surface 210 of a construction element 220 form an integrated surface. Thus, the second surface 232 forming an electrode 234 is arranged in relation to the first surface 210 and formed such that there are no discontinuities between the first surface 210 and the second surface 232. In the shown example, the integrated surface is flat. Accordingly, the second surface 232 is flat. In another example, the integrated surface is curved. Accordingly, the second surface 232 is adjusted to fit the curvature of the first surface 210. In the example of Fig 2 (see Fig 2a), the electrode 234 has a circular surface. The area of the electrode 234 can be freely adjusted to fit into a wanted specific application. The area of the electrode 234 can also be adjusted for optimizing signals; i.e. larger areas yield larger signal amplitudes while smaller areas yield smaller signal amplitudes. In one example the area of the electrode surface 232 is up to 30cm². In one example, the area of the electrode surface 232 is larger than 5 cm².

An insulation gap 260 insolates the electrode from the construction element 220. The insulating gap 260 is in one example filled with a material of known dielectric properties. In one example, the gap is filled with a polymeric material. For example, the gap is filled with a polyolefin or any other polymer withstanding the current application. Alternatively the gap could be filled with a ceramic material. The gap filling material has in one example properties compatible with the application in which the ice thickness measurement is used. The gap filling material can for example be selected to have properties withstanding erosion action. The material is in one example provided as a ring mounted in the gap. In one example, the thickness t of the gap is 10 mm or less. In one example, the thickness t of the gap is within the range 1 to 3mm. For larger values for the gap thickness t, the resolution in ice thickness measurements is lower and linearity problems occur at thicker ice levels of accretion.

The dimensions of the gap 260 can be chosen so as to optimize performance of the measurements and/or to fit aerodynamic applications. Factors which can be considered when choosing the geometrical dimension of the insulating gap comprise the rate of erosion (rain, sand, etc.) and the amount of insects which can be accumulated on the surface eventually bridging the electrode to the rest of the construction element surface due to accumulation of proteinic or amino acid accumulation. Further, a to wide gap lead to departure from linearity in thickness measurements much earlier compared to a narrow gap. Certainly, in practice, it has to be considered mounting and installation constrains that might suggest a different gap than any ideally optimized thickness value for the gap.

The electrode 234 could be chosen with any arbitrary shape providing that it is isolated from construction element 220 by a homogeneous isolating gap 260. The construction element 220, filled gap 260 and electrode 234 are formed in a cavity or opening in the plastic skin 231 of the asp. Furthermore, the surface 232 of the electrode 234 and the surface 210 of the construction element 220 could be equally adapted to curved asp surfaces 235. In Fig 3, an electrode 334 as described above is mounted in a construction element separated from said construction element by an insulating gap 360. The electrode 234 has a complex shape. The construction element with the electrode 334 and the insulating gap 360 are formed in a cavity of the plastic skin 331 of the asp. The asp surface 335 forms an integrated surface with the surfaces of the construction element, insulating gap and electrode.

In Fig 4, a reference sensor arrangement 450 comprises a reference sensor arrangement 455 mounted in the hollow internal of the asp such that the reference sensor arrangement 450 is at least partly enclosed by the metallic layer 433. The reference sensor arrangement 450 comprises a reference electrode 454 arranged in relation to a reference construction element 452 so that a second reference surface 453 of the reference electrode 454 and a first reference surface 451 of the reference construction part 452 form an integrated surface. An insulation gap 458 insulates the reference electrode from the reference construction element 452. In the shown example, the reference construction element 452 is connected to a metallic structure 433, which forms a conducting grounding path for the reference sensor

arrangement 455. Alternatively, the reference construction element is connected to a grounding point anywhere in the asp.

In ideal conditions, the sensor arrangement and the reference sensor arrangement is geometrically substantially identical, however, under careful controls the reference sensor arrangement could be selected such that its total impedance is comparable to the total impedance of a clean sensor at the same temperature.

As stated above, the reference sensor arrangement 450 is mounted in a close environment representing a clean and not worn first surface, second surface and gap filling material. In the above described example, the reference sensor arrangement set-up with the insulating gap 458 and construction element is designed to be identical to the measuring sensor arrangement 130.

The size of the hollow internal of the asp is characteristically spacious enough so that the electrical field generated by the reference sensor arrangement is substantially the same as that generated at the sensor arrangement.

In one example, the reference sensor arrangement is allocated as close as possible to the sensor arrangement 130 to ensure thermal equilibrium between them. A thermal sensor such as a PTC sensor could be attached to the device for measuring ice thickness for the sake of completion of the measurements performed and as redundancy. The purpose of the thermal sensor is mainly to call for attention when the temperature is closing 0°C. However, in spite of its numerous advantages, it is not mandatory to have it, although a Pt100 reliable PTC sensor can easily be accommodate in the electronic.

Fig 5 is an electrical scheme schematically illustrating the measurements of the sensor arrangement and the reference sensor arrangement. A voltage is applied between the electrode in the sensor arrangement and ground. An impedance Z_g is then provided due to the insulating gap between the electrode and the construction element. If ice exists over-bridging the insulating gap, an additional impedance Z_ice is also provided due to the ice accretion between the first and second surfaces. Thus, the insulating gap impedance Z_g and the ice impedance Z_ice form electrically a parallel coupling with a source for the applied voltage.

Further, the very same voltage is also applied to the reference sensor arrangement of the reference sensor arrangement. An impedance∑ref is then provided due to the insulating gap between the reference electrode and the reference construction element. The reference sensor arrangement has the purpose of providing reference impedance. This can be achieved in numerous ways. In the herein described example, it has been achieved by forming a reference sensor arrangement substantially identical to the sensor arrangement 130. The important thing is that the insulating gap impedance Z_g and the free space impedance between the electrode and the earthed rest of the surface across the gap is equal to the reference impedance Z_ref which encompass both a gap and a free space from the reference electrode to the earthed reference construction element.

In one example the source for the voltage is arranged to operate in the radio frequency range. In one example the source for voltage is arranged to operate in the kHz-range. The voltage source can for example be arranged to operate at a frequency below 100 kHz.

In Figure 6, a processing unit 640 is arranged to determine the presence of ice based on the provided resulting outputs from a sensor arrangement and a reference sensor arrangement. In detail, the processing unit 640 comprises a comparator 641 arranged to determine the relation between the impedance value and the reference impedance value and a calculation unit 642 arranged to calculate a thickness of the ice based on consecutive determinations of the relation between output values from the sensor arrangement and the reference sensor arrangement. In one example, the calculation unit 642 is arranged to repeatedly calculate a mean value and a standard deviation value for the ice thickness. In one example, the calculating unit 642 is arranged to calculate the mean value and standard deviation value based on the relation between the sensor arrangement output and the reference sensor arrangement output measured at time intervals of 5 - 15 seconds. In one example the time intervals are 10 seconds. The number of measurements performed during each time interval depends on the performance of the comparator 641 and the calculating unit 642. In one example, about 10 measurements are performed during each time interval; in another example about 20 measurements are performed during said time interval. Time intervals can be decided upon comparison of consecutive readings. A fast growing ice accretion demands low time intervals and consequently lower significance of statistics. The key issue is to deliver reliable information on the growing rate of accreted ice.

The calculation unit 642 is then arranged to evaluate the obtained mean value and standard deviation value. If the obtained standard deviation value is smaller than a preset standard deviation value, the ice thickness is determined to be the mean value. In one example the preset standard deviation value for clear ice lies within the region 0.1 -0.3 mm, for example 0.2 mm. If the obtained standard deviation value is larger than the preset standard deviation value, the mean thickness value is unreliable and there is a risk that ice accretion has started to build up fast. The thickness can for example be determined as the mean value plus the standard deviation value.

The mean value for the last ice thickness determination possibly corrected with the standard deviation value is in one example compared to a preset ice thickness value. The unit for automatic control of de-icing 180 is activated once the preset ice thickness value has been exceeded. In one example, the unit for control of de-icing 180 is also activated if the standard deviation value is too high after one or more measurements of the mean value and the standard deviation value. In one example, information about the ice thickness, the standard deviation value and/or information that the de-icing has been activated is fed to the presentation unit of a control room. In one example the preset ice thickness value is for example 2-5mm. As the device for measuring ice thickness is fast, updated thickness measurements can be performed with time intervals of seconds. The calculation unit 642 is for example implemented in a microprocessor. Furthermore, the processing unit herein described could be miniaturized. At the initial stage, i.e., at very thin levels of ice accretion, say at fractions of a millimeter, the surface of ice accreted is not homogenous and thereby detection is affected by high level of uncertainty. However, laboratory trial shows that thickness down to about 0.6 - 0.8 millimeters could still be measured.

In Fig 7, the comparator comprises a lock-in amplifier 747 arranged to output the relation between the impedance value and the reference impedance value. The lock-in amplifier 747 comprises an internal oscillator that generates a reference voltage that can be used with or without amplification to excite the sensor arrangement and the reference sensor arrangement. The reference voltage is provided by means of an output Ref of the lock-in amplifier. The reference voltage has a predetermined amplitude U and a predetermined frequency ! In one example the amplitude is about 1 Volt. The frequency selection has been discussed above. The lock-in amplifier 747 further comprises a first input A and a second input B. The inputs A, B are phase locked. The lock-in amplifier is arranged to receive the sensor arrangement output and the reference sensor arrangement output on the first input A and the second input B, respectively.

An optional possibility of the set-up is to use the reference signal to obtain an output from the reference sensor arrangement while the signal from the sensor arrangement could be phase sh ifted by 1 80° (ττ) . Thereafter, by means of a low impedance voltage divider (ideally negligible impedance) a fraction could be selected from the reference signal as to cancel the one from the sensor whereby the lock-in amplifier works rather like a precision phase- locked zero detector.

The lock-in amplifier has further a device 744 for setting the measuring range. I ce meas u reme nts a re perform ed with i n a voltag e ra ng e corresponding to impedances measured when ice is coating the construction elements. It should be mentioned that whenever a low noise, high resolution and sensitivity, etc., is wanted, the methods hereby described could be of grate advantage and outstanding performance. Such could be the case of any sensor delivering a low voltage output.

In Fig 8, an asp of a wind turbine is shown, wherein a plurality of devices for ice thickness measurement are formed, wherein each device for ice thickness measurement has an individual reference sensor arrangement 850 and processing unit 840. In the shown example, a metallic structure 833 formed in the asp connects to ground for both sensor arrangement and the reference sensor arrangement 850.

In Fig 9, an asp of a wind turbine is shown, wherein a plurality of devices for ice thickness measurement are formed , wherein the devices share one reference sensor arrangement 950 and processing unit 940. A metallic structure 933 formed in the asp forms connection to ground for both sensor arrangement and the reference sensor arrangement 950 In Fig 10, a method 1000 for measuring ice thickness on a first surface of a construction element comprises the following steps: measuring 1010 an impedance caused by an insulating gap between the construction element and an electrode formed therein, measuring 1020 a reference impedance, and determining 1030 ice thickness based on a relation between the measured impedance and the measured reference impedance. The measuring steps 1010, 1020 are performed in parallel. The method is in one example applied to ice thickness measurements on one or a plurality of asps of a wind turbine. In Fig 1 1 , method 1 100 for initiating activation of a de-icing system comprises the following steps. In a first step 1 1 10, the ice thickness is determined. If the ice thickness is thinner than let us say 0.6 to 0.8 mm, ice will not be detected. This is due to the fact that the second surface of the sensor arrangement is not completely covered with ice. Once ice can be detected (step 1 120), the measured ice thickness is compared 1 130 to a critical value tcriticai- In one example, t_criticai= 5mm. If the ice thickness is determined to be bigger than the critical value t_criticai in step 1 140, de-icing is activated automatically 1 180. The method is in one example applied to initiating activation of a de-icing system of one or a plurality of asps of a wind turbine

Claims

A device (100) for measuring ice thickness on a first surface (1 10; 210; 310) of a construction element (120;220) arranged in an asp of a wind turbine, characterized in that the device comprises a sensor arrangement (130) arranged in connection with said construction element and arranged to provide impedance values, a reference impedance sensor arrangement (150; 450) arranged to provide reference impedance values, and a processing unit (140; 640) coupled to the sensor arrangement (130) and the reference sensor arrangement (150; 450), said processing unit being arranged to determine the ice thickness based on determinations of a relation between the impedance values determined by the sensor arrangement and the reference impedance values generated by the reference sensor.

A device according to claim 1 , wherein the sensor arrangement (130) comprises one first electrode (134, 234, 334) separated from the construction element (120, 220) by an insulating gap (160, 260, 360), wherein the first electrode (134, 234, 334) has a second surface (132, 232) and wherein the construction element forming a second electrode of the sensor arrangement.

A device according to claim 2, wherein the construction element is attached to a grounding plane.

4. A device according to claim 3, wherein the construction element is connected to the grounding plane by means of a conductive path running through the asp.

5. A device according to claim 2, 3 or 4, characterized in that the first electrode (134, 234, 334), construction element and an asp surface (135) are arranged in relation to each other so that they form an integrated surface.

6. A device according to any of the preceding claims, characterized in that the reference sensor arrangement (150; 450) is arranged to generate substantially the same impedance values as the sensor arrangement when the first and second surfaces are clean.

7. A device according to any of the preceding claims, characterized in that the sensor arrangement (130) and the reference sensor

arrangement (150; 450) are operated at the same frequency (AC). 8. A device according to any of the preceding claims, characterized in that the processing unit (140; 640) comprises a calculation unit (642) arranged to calculate a thickness of the ice based on a plurality of determinations of the relation between the impedance value and the reference impedance value.

9. A device according to claim 8, characterized in that the calculation unit (642) is arranged to repeatedly calculate a mean value and a standard deviation value for the ice thickness. 10. A device according to claim 8 or 9, characterized in that the calculation unit (642) is implemented in a microprocessor.

1 1 . Device according to any of the preceding claims, characterized in that the processing unit (140; 640) comprises a lock-in amplifier (641 ) arranged to output the relation between the impedance value and the reference impedance value, said lock-in amplifier being arranged to receive the impedance value and the reference impedance value on its inputs.

12. Wind turbine having a plurality of asps, wherein each asp is formed in a plastic skin wherein at least one of the asps being provided with at least one device according to any of the claims 1 -1 1 .

13. Wind power plant comprising a wind turbine according to claim 12.

Method (1000) for measuring ice thickness on a surface of an asp of a wind turbine, said method comprising the steps of measuring (1010) an impedance change caused by ice accretion as added load to an insulating gap between a construction element (120;220) arranged in the asp and an electrode formed in the construction element measuring (1020) a reference impedance, and

- determining (1030) the ice thickness based on a correlation between the measured impedance change and the measured reference impedance.