WO2013079250A1

WO2013079250A1 - Method for the inspection of dielectric properties in electrical insulators

Info

Publication number: WO2013079250A1
Application number: PCT/EP2012/070358
Authority: WO
Inventors: Henning Fuhrmann
Original assignee: Abb Research Ltd
Priority date: 2011-11-28
Filing date: 2012-10-15
Publication date: 2013-06-06

Abstract

A method for the inspection of dielectric properties in components used for electrical insulation is provided. The method comprises as a first step the provision of a the form of a component used for electrical insulation. This sample (4) is irradiated with an electromagnetic wave having a frequency between 10GHz and 10 THz. For a plurality of positions and/or directions,the attenuation of the electromagnetic wave is determined after having at least partially penetrated the sample (4). From the attenuation determined at different positions and/or directions,a two-dimensional spatial distribution of dielectric property values of the sample (4) is derived.

Description

TITLE

Method for the Inspection of Dielectric Properties in Electrical Insulators

TECHNICAL FIELD

The present invention concerns a method for the inspection of dielectric properties in components used for electrical insulation, such as in high voltage insulators. In particular, the method is used for the detection of defects in such components. The present invention also concerns an apparatus being used for carrying out such a method.

PRIOR ART

Insulation components are commonly used in high voltage electrical equipment, such as for example capacitors, transformers or switchgear barriers. Dielectrics of the insulation component can, however, exhibit regions of high electric field intensity which may cause partial discharges that are characterized in that they do not completely bridge the electrodes being connected by the insulation component. The partial discharges quite often start, for example, within inhomogeneities such as voids or cracks within a solid insulation component or bubbles within a liquid insulation component. For simplicity of presentation, the inhomogeneities are referred to as "voids" in the following. The voids are typically gas- filled. If the voltage stress across a void exceeds the inception voltage for the gas within the void, the gas ionizes and partial discharges start to occur within the void. The partial discharges will then cause progressive deterioration of the material of the insulation component, which ultimately might lead to an electrical breakdown of the insulation component.

Such an electrical breakdown of the insulation component often occurs several years after the component has been installed in an electrical circuit. A replacement of the insulation component due to the deterioration of the material becomes necessary, which is usually costly and often involves a complete shutdown of at least the subsystem in which the defective component is integrated. Therefore, there is a need for testing insulation components with regard to possible voids and other defects before installing them in an electrical circuit, in order to ensure a correct functioning of these components.

Due to these reasons, insulating components of electrical insulation, especially for high voltage applications, are subjected to incoming inspection and factory tests. Known methods are for example optical inspection, X-ray imaging, and partial discharge testing.

For example, document EP 2 157 439 discloses a method in which an alternate current (AC) voltage is applied to insulation components, while at the same time partial discharges are induced by means of X-ray pulses.

The problem of known methods is, however, that for example a metal particle enclosed in an insulator might be too small to be detected by X-ray (e.g. an Al-particle in an AI₂O₃- filled insulator), but might still cause a very high field distortion and lead to the failure of the entire insulator. On the other hand, a particle identified by means of X-ray and estimated to represent a defect might be irrelevant as concerns the quality of the insulator, because the particle has a similar or even the same dielectric constant as the surrounding insulation material. Optical methods, on the other hand, are of course limited to the inspection of the insulator surface, if the insulator is made from an intransparent material.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for the inspection of components used for electrical insulation, which allows a safe detection of relevant defects in these components. Relevant defects are defects which are likely to cause a failure of the entire insulating component.

This object is solved by a method as claimed in claim 1. Further embodiments of the method are provided in dependent claims 2 to 11. An apparatus used for carrying out such a method is claimed in claim 12, and further embodiments of this apparatus are provided in dependent claims 13 to 16. The present invention provides a method for the inspection of dielectric properties in components used for electrical insulation, the method comprising the following steps:

- providing, as a sample, a component used for electrical insulation;

- irradiating the sample with an electromagnetic wave having a frequency between 10 GHz and 10 THz, more preferably between 50 GHz and 5 THz, and most preferably between about 100 GHz and 1 THz;

- determining, for a plurality of positions and/or directions, the attenuation of the electromagnetic wave after the electromagnetic wave has at least partially penetrated the sample; and

- deriving a two-dimensional spatial distribution of dielectric property values of the sample from the attenuation determined at different positions and/or directions.

Electromagnetic waves in this range of frequencies are attributed to the (upper) microwave or the (lower) terahertz (THz) range. Electromagnetic waves in these ranges propagate through insulators and are reflected or refracted at interfaces of materials with different dielectric constants. Metal impurities have very high dielectric constants and can therefore easily be detected by measuring the reflected electromagnetic radiation. Because of the strong reflection, even the presence of metal objects that are much smaller than the wavelength of the electromagnetic wave can be detected. Non-metallic particles, however, are not detected, if they have a dielectric constant similar to the one of the insulator material. Since such particles do not influence the physical properties of the insulator with regard to its function, these particles do not represent a relevant defect of the insulator.

Since dielectric inhomogeneities are the main cause for defects occurring in components used for electrical insulation, the method according to the invention allows a direct detection of the physical quantity being responsible for the defects. A dielectric inhomogeneity within the sample causes a field distortion, which in turn influences the propagation of the electromagnetic wave. Thus, the contrast of the obtained dielectric property values directly reflects the relevant physical quantity. Accordingly, the method allows a safe detection of exactly those relevant defects that will very likely cause a failure of the entire insulating component.

The component used for electrical insulation, which is used as the sample in the method according to the invention, is preferably a high voltage (HV)-insulator. HV-insulators are suited for the insulation of alternate current (AC) voltages of more than 1000 V or for the insulation of direct current (DC) voltages of at least 1500 V. The component used for electrical insulation can be solid and be made for example from a ceramic, such as porcelain, from glass or from composite polymer materials. The component used for electrical insulation can, however, also be a liquid insulation component.

Preferably, the dielectric property value measured using the described method is the dielectric constant or a quantity which is directly related to the dielectric constant, because this value is the essential value regarding the insulation properties of the inspected component. The dielectric constant is also the most relevant value with regard to defects occurring in the lifetime of an electric insulator. The method preferably comprises the further step of deciding, based on the measured dielectric property values, whether the component used for electrical insulation fulfils certain quality requirements or not. Based on this decision, the component can for example be excluded from further production, or the method by which these components are produced can be optimized.

Preferably, the attenuation of the electromagnetic wave is determined after the electromagnetic wave has been reflected by the sample. Thus, the electromagnetic wave is preferably emitted and received at essentially the same positions with regard to the sample.

Alternatively or in addition, the attenuation of the electromagnetic wave is determined after transmission of the electromagnetic wave through the sample. In this case, the electromagnetic wave is emitted and received at opposite sides of the sample. Furthermore, it is conceivable to also determine the attenuation of the diffracted portion of the electromagnetic wave.

In a preferred embodiment, the method additionally comprises the step of providing an emitter for emitting the electromagnetic wave, wherein the emitter and/or the sample are moved in two dimensions relative to each other, in order to irradiate the sample at different positions and/or from different directions. It is also preferred to have more than one emitter, such that the sample can be irradiated at more than one position simultaneously. If more than one emitter is provided, the emitters and/or the sample are preferably moved relative to each other in such a way, that the emitters are always aligned in parallel to each other. It is also possible to rotate the sample for example about its longitudinal axis or to rotate the emitter around the sample, in order to irradiate the sample at different positions and/or from different directions. In addition to this rotation, the sample and/or the emitter can of course also be displaced relative to each other. The method usually also comprises the step of providing a receiver for receiving the electromagnetic wave. If the emitter and/or the sample are moved relative to each other, the receiver and/or the sample are preferably also moved relative to each other, such that the emitter and the receiver are always aligned relative to each other.

In a particularly preferred embodiment, the method for the inspection of dielectric properties in components used for electrical insulation further comprises the following steps:

- continuously varying the frequency of the electromagnetic wave;

- determining the frequency of the electromagnetic wave after the electromagnetic wave has at least partially penetrated the sample;

- calculating the travel time of the electromagnetic wave based on the determined frequency; and

- identifying the penetration depth of the electromagnetic wave in the sample based on the calculated travel time of the electromagnetic wave.

By continuously varying the frequency, i.e. modulating the frequency of the electromagnetic wave, and using the information of the measured frequency after the electromagnetic wave has penetrated the sample, the location of the reflection inside the sample can be determined. This allows assigning dielectric property values to spatial locations having different distances to the surface of the sample. Thereby, a three- dimensional distribution of dielectric property values inside the sample can be obtained. For the modulation of the frequency of the electromagnetic wave, for example a sine, sawtooth or triangle modulation can be used. Particularly preferred is emitting an electromagnetic wave which is a frequency modulated continuous-wave (FMCW).

In a preferred embodiment of the invention, the dielectric property values of the sample are determined at several depths and at several positions of the sample, such that a three- dimensional spatial distribution of dielectric property values inside the sample is obtained. Thereby, the assignment of a defect to a particular location inside the sample is possible. Preferably, dielectric property values are determined for a volume covering essentially the entire component used for electrical insulation.

Preferably, the obtained dielectric property values of the sample are represented in the form of an image, which reflects a spatial distribution of the dielectric property values. In such an image, the dielectric property values can for example be color-coded. If a three- dimensional spatial distribution of dielectric property values inside the sample is obtained, the dielectric property values can of course be represented in the form of a plurality of images.

Furthermore, an apparatus is provided for the inspection of dielectric properties in components used for electrical insulation with a method as described. The apparatus comprises:

a holder for holding a sample, the sample being a component used for electrical insulation,

at least one emitter for emitting an electromagnetic wave propagating at frequencies between 10 GHz and 10 THz, more preferably between 50 GHz and 5 THz, and most preferably between about 100 GHz and 1 THz, the emitter being designed and arranged such, that the emitted electromagnetic wave at least partially penetrates a sample at different positions and/or from different directions,

at least one receiver for receiving the electromagnetic wave after the electromagnetic wave has at least partially penetrated the sample held by the holder,

a module for measuring the attenuation of the received electromagnetic relative to the emitted electromagnetic wave, and

a module for assigning, for each position and/or direction, a dielectric property value to the sample based on the determined attenuation, such that a two-dimensional spatial distribution of dielectric property values inside the sample is obtained.

Such an apparatus serves to carry out the described method for the inspection of dielectric properties in components used for electrical insulation.

Preferably, at least one receiver is, with regard to the holder, arranged on the side of the emitter, in order to receive the electromagnetic wave reflected by the sample. Advantageously, the receiver is even part of the same unit as the emitter. For example, an emitter and receiver head can be provided, in which both the receiver and the emitter are integrated.

Alternatively or in addition, at least one receiver is, with regard to the holder, arranged on the opposite side of the emitter, in order to receive the electromagnetic wave after having been transmitted through the sample.

It is preferred that the apparatus comprises more than one emitter and more than one receiver. If this is the case, all emitters and receivers are advantageously arranged in parallel to each other. Preferably, the apparatus further comprises a drive motor for moving the emitter or the emitters relative to the sample. The receiver or the receivers are then preferably also moved correspondingly by the drive motor.

In a particularly preferred embodiment, the emitter is designed to emit a frequency- modulated continuous wave (FMCW), and the apparatus further comprises a module for measuring the frequency of the received electromagnetic wave and a module for calculating the penetration depth of the electromagnetic wave in the sample based on the determined frequency. By providing such an apparatus, it is possible to obtain a three- dimensional distribution of dielectric property values inside the sample.

SHORT DESCRIPTION OF THE FIGURES

Preferred embodiments of the invention are described in the following with reference to the drawings, which only serve for illustration purposes, but have no limiting effects. In the drawings it is shown:

Fig. 1 shows a perspective view of a part of an apparatus for carrying out the method according to a first inventive embodiment, a first receiver unit being provided for receiving the reflected electromagnetic wave and a second receiver unit being provided for receiving the transmitted electromagnetic wave; and

Fig. 2 shows a perspective view of a part of an apparatus for carrying out the method according to a second inventive embodiment, a single receiver unit being provided for receiving the reflected electromagnetic wave.

DESCRIPTION OF PREFERRED EMBODIMENTS In figure 1, a first embodiment of an apparatus is shown which serves to carry out the method for the inspection of dielectric properties in components used for electrical insulation according to the invention. The apparatus comprises a sample holder 1, an emitter and receiver unit 2 and a receiver unit 3. The sample 4, which is a component used for electrical insulation, such as a high voltage (HV)-insulator, is mounted on the sample holder 1. The component for electrical insulation can for example be made from ceramic, such as porcelain, from glass or from composite polymer materials. Of course, the sample 4 can also be a liquid insulation component, whose dielectric properties are to be examined.

The sample holder 1 has a common design being sufficiently known to a person skilled in the art. It basically serves to fixedly hold the sample in a releasable manner such, that an inspection of dielectric properties by means of electromagnetic radiation is possible. The emitter and receiver unit 2 and the receiver unit 3 are each mounted on horizontal guide rails 52. The horizontal guide rails 52 are connected to vertical guide rails 51, such that the emitter and receiver unit 2 and the receiver unit 3 are able to be moved either by hand or by means of a drive motor in at least two perpendicular directions relative to the sample 4. Thereby, the emitter and receiver unit 2 and the receiver unit 3 are always moved in parallel, such that they are always mutually aligned.

The emitter and receiver unit 2 of the present embodiment comprises three emitter and receiver heads 21, which are each suited to both emit and receive electromagnetic waves propagating in the microwave and/or the terahertz (THz) range. In the present embodiment, the emitter and receiver heads 21 operate at frequencies between 230 GHz and 320 GHz. In different embodiments, emitter and receiver heads 21 can for example be provided which operate at frequencies between 75 GHz and 110 GHz or between 840 GHz and 870 GHz. Thereby, the frequency band chosen mainly depends on the material and the quality requirements of the sample 4.

The emitter and receiver heads 21 are arranged vertically and in parallel to each other.

Thus, each emitter and receiver head 21 is suited to emit an electromagnetic wave in a downwardly oriented, vertical direction. By means of optical devices specifically designed for this purpose, the emitted electromagnetic waves are deflected from the vertical into a horizontal direction right after exiting the lower end of each emitter and receiver head 21.

From there, the electromagnetic radiation travels in a horizontal direction through optical lenses 22 and towards the sample 4, which is arranged at the end of the optical lenses 22.

The optical lenses 22 having a preferred focal distance between 50 mm and 200 mm serve to focus the electromagnetic waves on the sample 4.

Advantageously, the frequency of the emitted electromagnetic waves is continuously varied by the emitter and receiver heads 21. Even more advantageously, each emitter and receiver head 21 emits an electromagnetic wave in the form of a frequency modulated continuous-wave (FMCW). Thereby, the frequency is periodically varied. For example, a sine, sawtooth or triangle modulation can be used for this purpose.

The emitter and receiver heads 21 are arranged such, that the emitted electromagnetic wave at least partially penetrates the sample 4. In the embodiment shown in figure 1, the portion of the electromagnetic radiation, which is reflected by dielectric inhomogeneities inside the sample 4, reaches again the optical lenses 22 and is received by the emitter and receiver heads 21. A module comprised for example in a separate post-processing unit, which is not shown in figure 1, measures the intensity of the received electromagnetic wave and compares this measured intensity with the intensity of the originally emitted electromagnetic wave. The difference between these intensities, i.e. the intensities of the electromagnetic wave before and after penetration of sample 4, is then used for the calculation of a dielectric property, particularly the dielectric constant, inside the sample 4. The amount of electromagnetic radiation being reflected at a certain location inside the sample 4 is directly dependent on the change of the dielectric constant at the same location. Thus, large amounts of electromagnetic radiation are reflected at interfaces of dielectric inhomogeneities, such as at interfaces between voids and the normal, homogeneous insulator material. Only small amounts of electromagnetic radiation, however, are reflected in regions which are homogeneous with regard to the dielectric constant.

Since more than one emitter and receiver head 21 is provided in the present embodiment, information concerning the dielectric constant can be obtained from the sample 4 at more than one position simultaneously. By additionally moving the emitter and receiver unit 2 in horizontal and vertical directions relative to the sample 4, dielectric property values can be acquired from a plurality of spatial positions of the sample 4. This allows assigning, for each acquisition position of an emitter and receiver head 21, a corresponding dielectric property value to the corresponding location of the sample 4 based on the determined attenuation of the received electromagnetic wave. In this way, a plurality of dielectric property values is obtained for different locations of the sample 4, such that a two- dimensional spatial distribution of dielectric property values of the sample can be obtained. This distribution can of course be represented in the form of an image, in which the dielectric property values can be for example be color-coded. The information concerning dielectric inhomogeneities of the sample 4 obtained in the way described can then be used in a straightforward manner, in order to decide, whether the sample, i.e. the electrical insulation component being examined, fulfils the quality requirements or not.

If the emitter and receiver heads 21 emit the electromagnetic wave at varying frequencies, such as in the form of a frequency modulated continuous-wave (FMCW), the apparatus preferably further comprises a module for measuring the frequencies of the received electromagnetic waves as well as a module for calculating the penetration depths of the electromagnetic waves in the sample 4 based on these determined frequencies. Both of these modules can be part of a separate post-processing unit, which is not shown in figure 1. Based on the knowledge of the frequency function of the emitted electromagnetic wave, the frequency of the received electromagnetic radiation allows assigning a travel period to an electromagnetic wave of a certain frequency, which can then be used for the calculation of the penetration depth and accordingly for the determination of the location of the reflection of this electromagnetic wave inside the sample 4. The bandwidth of the electromagnetic radiation emitted by the emitter and receiver heads 21 depends on the thickness of the sample 4 and the required spatial resolution. A person skilled in the art is well acquainted with such calculations and the assignment of the location of reflection inside the sample 4 based on the measured frequency is straightforward.

By means of varying the frequency of the emitted electromagnetic wave, different reflexions originating from various locations inside the sample can be allocated to their respective origins. In this way, information concerning the dielectric properties at different penetration depths inside the sample can be obtained. If the emitter and receiver unit 2 is moved relative to the sample 4 during such a data acquisition, information concerning the three-dimensional distribution of dielectric properties of the entire sample 4 can be obtained. This information, i.e. the obtained dielectric values, can then for example be represented in the form of one or several images, which can for example be color-coded. Based on the obtained dielectric values, it can then be decided, whether a specific sample 4 fulfils the quality requirements or not.

In the present embodiment of figure 1, three emitter and receiver heads 21 are provided, which, in order to cover the entire sample 4, are moved along vertical and horizontal guide rails 51 and 52. Instead of moving the emitter and receiver unit 2 relative to the sample 4, in order to acquire information of dielectric properties of the entire sample 4, it would of course also be possible to provide even more emitter and receiver heads 21 in such a way, that the entire sample 4 can be irradiated simultaneously at a sufficient number of positions. The embodiment shown in figure 4 also comprises an additional receiver unit 3 having receiver heads 31, which serve to receive the portion of the electromagnetic wave having been transmitted through the sample 4. Similar as the emitter and receiver unit 2, the receiver unit 3 also comprises optical lenses 32, which serve to focus the received electromagnetic waves on the corresponding receiver heads 31. By also receiving and measuring the transmitted electromagnetic wave, further information can be obtained concerning dielectric properties inside the sample 4. Of course it would also be possible to only measure the transmitted electromagnetic waves, in order to obtain information about the dielectric properties of the sample 4.

In the present embodiment, the commercially available SynViewScan™ imaging system was used including SynViewHeads™ as emitter and as receiver units 2 and receiver units 3. Of course, the system had to be adapted to the described method for the inspection of insulating components. The system components are produced by SynView™, Germany.

Figure 2 shows a second embodiment of an apparatus for carrying out the method according to the invention. In this embodiment, an emitter and receiver unit 2 having only a single emitter and receiver head 21 is used for data acquisition, and no additional receiver unit is provided. The basic method of how information concerning dielectric properties is obtained from the sample 4 is the same as explained with regard to the embodiment shown in figure 1. In the present embodiment, however, only the reflected portion of the electromagnetic radiation is received and used for the determination of dielectric properties of the sample 4. The emitter and receiver unit 2 is movably installed in a horizontal position on a vertical guide rail 51, such that the electromagnetic wave exits the emitter and receiver head 21 in a horizontal direction. Since the electromagnetic wave also propagates through the optical lenses 22 in a horizontal direction, it does not need to be deflected inside the emitter and receiver unit 2. The sample 4, which can for example be a high voltage (HV)-insulator, is mounted on a sample holder 1, which is here rotatable around its vertically oriented longitudinal axis by means of a drive motor 6. By rotating the sample holder 1 and vertically moving the emitter and receiver unit 2, it is possible to irradiate the sample 4 at various positions and from various directions, such that a complete three-dimensional distribution of dielectric properties of the entire sample 4 can be obtained.

In the embodiment of figure 2, SynViewHead™ produced by SynView™, Germany, was used as emitter and receiver unit 2. The invention is of course not limited to the preceding presented embodiments and a plurality of modifications is possible. For example, in order to obtain a two-dimensional spatial distribution of dielectric property values, the emitter does not necessarily have to be moved relative to the sample, if the emitter or a plurality of emitters is designed such, that the sample is irradiated at more than only one position or from more than only one direction. Of course, the receiver needs to be adapted to the manner, by which the sample is irradiated by the emitter. The emitter could even be designed such, that electromagnetic radiation is emitted in a way, that a large portion or even the entire sample is irradiated simultaneously in a single step. The receiver in such a system could for example be provided in the form of a charge-coupled device (CCD)-sensor. A plurality of further modifications is possible.

REFERENCE NUMERALS

1 Sample holder 32 Optical lens

2 Emitter and receiver unit 4 Sample

21 Emitter and receiver head 51 Vertical guide rail

22 Optical lens 52 Horizontal guide rail

3 Receiver unit 6 Drive motor

31 Receiver head

Claims

1. A method for the inspection of dielectric properties in components used for electrical insulation, the method comprising the following steps:

- providing, as a sample (4), a component used for electrical insulation;

- irradiating the sample (4) with an electromagnetic wave having a frequency between 10 GHz and 10 THz;

- determining, for a plurality of positions and/or directions, the attenuation of the electromagnetic wave after the electromagnetic wave has at least partially penetrated the sample (4); and

- deriving a two-dimensional spatial distribution of dielectric property values of the sample (4) from the attenuation determined at different positions and/or directions.

2. The method as claimed in claim 1, wherein the attenuation of the electromagnetic wave is determined after the electromagnetic wave has been reflected by the sample (4).

3. The method as claimed one of the preceding claims, wherein the attenuation of the electromagnetic wave is determined after transmission of the electromagnetic wave through the sample (4).

4. The method as claimed in one of the preceding claims, further comprising the step of providing an emitter (21) for emitting the electromagnetic wave, wherein the emitter (21) and/or the sample (4) are moved in two dimensions relative to each other, in order to irradiate the sample (4) at different positions and/or from different directions.

5. The method as claimed in one of the preceding claims, wherein the component used for electrical insulation is a high voltage (HV)-insulator.

6. The method as claimed in one of the preceding claims, further comprising the following steps:

- continuously varying the frequency of the electromagnetic wave;

- identifying the penetration depth of the electromagnetic wave in the sample (4) based on the calculated travel time of the electromagnetic wave.

7. The method as claimed in claim 6, wherein the electromagnetic wave is a frequency modulated continuous-wave (FMCW).

8. The method as claimed in one of the preceding claims, wherein the dielectric property values of the sample (4) are determined at several depths and at several positions of the sample (4), such that a three-dimensional spatial distribution of dielectric property values inside the sample (4) is obtained.

9. The method as claimed in one of the preceding claims, wherein the obtained dielectric property values of the sample (4) are represented in the form of an image, which reflects a spatial distribution of the dielectric property values.

10. The method as claimed in one of the preceding claims, wherein the obtained dielectric property values represent the dielectric constant or a quantity which is directly related to the dielectric constant.

11. The method as claimed in one of the preceding claims, further comprising the step of deciding, based on the derived dielectric property values, whether the component used for electrical insulation fulfils certain quality requirements or not.

12. An apparatus for the inspection of dielectric properties in components used for electrical insulation with a method as claimed in one of the preceding claims, comprising

a holder (1) for holding a sample (4), the sample (4) being a component used for electrical insulation,

at least one emitter (21) for emitting an electromagnetic wave propagating at frequencies at frequencies between 10 GHz and 10 THz, the emitter (21) being designed and arranged such, that the emitted electromagnetic wave at least partially penetrates a sample (4) at different positions and/or from different directions,

at least one receiver (21, 31) for receiving the electromagnetic wave after the electromagnetic wave has at least partially penetrated the sample (4) held by the holder (1),

a module for assigning, for each position and/or direction, a dielectric property value to the sample (4) based on the determined attenuation, such that a two-dimensional spatial distribution of dielectric property values inside the sample (4) is obtained.

13. The apparatus as claimed in claim 12, wherein, with regard to the holder (1), at least one receiver (21) is arranged on the side of the emitter (21), in order to receive the electromagnetic wave reflected by the sample (4).

14. The apparatus as claimed in one of claims 12 or 13, wherein, with regard to the holder (1), at least one receiver (31) is arranged on the opposite side of the emitter (21), in order to receive the electromagnetic wave after having been transmitted through the sample (4).

15. The apparatus as claimed in one of claims 12 to 14, further comprising a drive motor (6) for moving the emitter (21) relative to the sample (4).

16. The apparatus as claimed in one of claims 12 to 15, wherein the emitter (21) is designed to emit a frequency-modulated continuous wave (FMCW), and wherein the apparatus further comprises a module for measuring the frequency of the received electromagnetic wave and a module for calculating the penetration depth of the electromagnetic wave in the sample (4) based on the determined frequency.