The electromagnetic (EM) emissions of wind energy conversion systems (WECS) are evaluated in situ. Results of in situ tests, however, are only valid for the examined equipment under test (EUT) and cannot be applied to series production as samples, as the measurement uncertainty for in situ environment is not characterized. Currently measurements must be performed on each WECS separately, this is associated with significant costs and time requirement to complete. Therefore, in this work, based on the standard procedure according to the “Guide to the Expression of Uncertainty” (GUM, 2008) the measurement uncertainty is characterized. From current normative situation obtained influences on the measurement uncertainty: wind velocity and undefined ground are evaluated. The influence of increased wind velocity on the measurement uncertainty is evaluated with an analytical approach making use of the dipole characteristic. A numerically evaluated model provides information about the expected uncertainty due to reflection on different textures and varying values of relative ground moisture. Using a classical reflection law based approach, the simulation results are validated. Thanks to the presented methods, it is possible to successfully characterize the measurement uncertainty of in situ measurements of WECS's EM emissions.
In order to meet the goal of the 2015 Paris agreement, it is necessary to reduce the amount of carbon dioxide emissions currently being produced (UNFCC, 2015). The reduction of carbon dioxide emissions is a movement towards power generation systems with renewable energy sources instead of fossil ones. One approach is to operate wind energy conversion systems (WECS). Like all other industrial, scientific and medical (ISM) devices, WECS must be assessed and evaluated regarding their radiated electromagnetic (EM) emissions based on international standards (CISPR 11, 2015). Due to their geometrical size, WECS cannot simply be installed and tested at a defined test site such as an open area test site (OATS). Instead, they need to be tested in situ. The problem is, that for equipment under test (EUT), evaluated in situ only, compliance with these standards can be proven for this specific EUT, but not for the whole product line. In order to reduce the effort and costs, it is always aimed for a series release. However, a series release is only possible with the knowledge of the measurement uncertainty, determined according to the “Guide to the Expression of Uncertainty in Measurement” (GUM, 2008). The measurement uncertainty for in situ tests of WECS is not specified yet. Therefore, the goal of this work is to define and characterize possible contributions to uncertainty during in situ measurements of EM emissions from WECS. The normatively given limit values as well as the causes of the EM emissions are not focus of this article. The latter are discussed e.g. in Koj et al. (2017) and Fisahn et al. (2017).
This paper explains the measurement of EM emissions of WECS according to the normative situation in the second Chapter. Subsequently, Sect. 3 describes the standard procedure for the determination of the measurement uncertainty according to GUM. In Sect. 4 the normative situation on determination of the uncertainty in EM emission tests is analysed. The comparison between OATS and in situ leads to two main emphases: the deflection of the antenna due to wind velocity and the reflection of EM waves on different grounds. The numerical and analytical evaluation of the antenna deflection is presented in Sect. 5 and the influence of the ground reflections is discussed in Sect. 6 respectively. A conclusion of the significant results completes this article.
In this chapter the determination of the EM emissions of WECS according to the normative situation is roughly sketched. Further, more detailed considerations on this topic can be found in e.g. Koj et al. (2016a, b).
WECS, also known as wind turbines (WTs), have to be evaluated with respect to their radio frequency (RF) disturbances respectively as well as the radiated emissions from other ISM (industrial, scientific and medical) electrical and electronic equipment. Therefore, the proof of compliance according to the Directive 2014/30/EU with the national implementation of this directive (e.g. the EMC Law, 2016 in Germany), must be provided. The standard CISPR 11 deals with the measurement methods and limits for the RF disturbances of ISM equipment. With respect to this standard, WTs must be classified into Group 1 and Class A, since the WT forms an unintentional radiator (Group 1) that is always located and operated out of living areas (Class A). Because of the enormous geometrical dimensions of a WT with a typical height of more than 100 m, the only practical way to assess the device is to perform an in situ measurement, even though equipment that is classified into Group 1 could be tested either on a test site or in situ. Thus, the radiated emissions have to be determined solitarily in order to be evaluated. Therefore, the emission measurements have to be carried out with an antenna and an EMI receiver according to CISPR 16-1-1 (2015), CISPR 16-1-4 (2010), CISPR 16-2-3 (2010), CISPR 16-4-2 (2011) and CISPR/TR 16-2-5 (2008) so as to measure the magnetic field strength in the frequency range from 150 kHz to 30 MHz (CISPR Band B) respectively the electric field strength in the range from 30 MHz to 1 GHz. The latter frequency range corresponds to the CISPR Bands C (30–300 MHz) and D (300 MHz–1 GHz). Figure 1 shows examples of standard compliant antennas. Using the loop antenna, the recorded magnetic field strength and the biconical antenna, the electric field strength can be measured. The above mentioned standards also allow the use of a logarithmic periodic dipole antenna (LPDA) for the electric field strength measurement, whereby both polarizations of the electric field strength shall be measured, the horizontal and the vertical.
Example antennas for measuring the radiated RF emissions of wind turbines.
Further information about a measurement campaign at a WT can be found in the technical guideline FGW/TR 9, 2014. This guideline describes the procedure for the uniform definition of measuring positions, where the RF emissions shall be measured. An example of this is shown in Fig. 2. The field measurements must be carried out on at least four measuring positions, whereby the distance between the outer tower wall and the measuring position should be 30 m each. In order to ensure the reproducibility of the measuring positions, there should be a fixed reference point chosen, e. g. the tower door of the WT. At each of the measuring positions, the WT must be measured in at least two modes: when the WT is in power harvesting mode, and when the WT is turned off. Further modes can be found in the FGW/TR 9. The measured field strength values of each measuring position should be rated using the limit values given by CISPR 11. If the measured radiated emissions are below these frequency dependent limits, the wind turbine will pass the test, otherwise it will fail.
Top view on a wind turbine (WT). The measuring positions (MP) are located around the WT (FGW/TR 9, 2014). The tower door sets the orientation of the reference frame.
Procedure for determining the measurement uncertainty according to GUM (Sommer and Siebert, 2004).
A general explanation of the uncertainty of measurement will be discussed in the following chapter.
A measurement result is only complete with inclusion of the associated
measurement uncertainty. In order to determine the measurement uncertainty,
the method according to GUM, which has been established in recent years, is
utilized. This method is described in detail in various publications, for
example in Sommer and Siebert (2004). Therefore, in this chapter, the
standard GUM procedure is only succinctly explained. The procedure requires
a model equation for the measurand
In most cases it is not possible to specify the input values exactly. With
knowledge of the limitation of the possible input values
For a complete measurement result
This work focuses in the definition and on the characterization of the input values according to Fig. 3. For this purpose, the following chapter considers the normative situation for measurement uncertainty in other test sites.
Overview of standards with information on measurement uncertainty in EM emission measurements.
As shown in Fig. 4, CISPR 11 refers to the CISPR 16-2-3, CISPR/TR 16-2-5 and
CISPR 16-4-2 standards in terms of measurement uncertainty. CISPR 16-2-3
contains general advice about performance of measurement campaign. In the
annexes informations and specifications for measurements in the presence of
environmental interferences can be found.
While CISPR/TR 16-2-5 defines requirements to in situ measurement in
general, of interest is in particular the demand to carry out in situ
measurement campaign at wind velocities below 10 m s Measuring equipment, Test setup, Measurement environment.
The “measuring equipment” (receiver display, antenna factor) used on OATS
and in situ is the same, thus all input values are conform and can be
adopted to in situ measurement.
For the category “test setup” the influence of the wind velocity has to be
investigated (the other measurement uncertainty aspects can be adopted). The
requirement of the CISPR/TR 16 2-5 to carry out the measurements at wind
velocity below 10 m s
The category “measurement environment” shows the biggest differences between the OATS and in situ environment. OATS should be placed (theoretically) far away from sources of interference and the ground should be (perfectly) conductive in order to enable reproducible measurement results. In contrast, WECS are often installed in industrial environments, where the presence of other sources of interference must be expected. Furthermore, the electromagnetic properties of the soil around a WT depends on the local texture (clay, sand) and also varies in time due to weather conditions (moisture). In order to deal with environmental EM disturbances in the vicinity of WECS, the instructions of CISPR 16-2-3 can be consulted. Approaches to deal with uncertainties due to undefined ground conditions around a WT are presented in Sect. 6.
The impact of wind velocity on the measurement uncertainty is evaluated in two steps. First, numerical simulations and an analytical approach relate the antenna deflection with the field deviation. Second, the wind related uncertainty is estimated relating measurement deviation and the wind velocity.
Taking into account the CISPR 11 Bands the evaluation is divided in three
parts: Band B, C and D. As CISPR 16-2-3 instructs to use a loop antenna to
measure the magnetic field component at a height
Tilted LPDA in plain wave field.
Dipole characteristic.
Simulated deviation of feed point voltage for different antenna tilting angles.
By approximating the behavior of the antennas by an adapted dipole of which
the directional pattern has a well-known torus geometry
Analytically calculated deviation over the tilting angle of the dipole.
Antenna tripod modelled as a bending beam strained by wind
force
The tilting angle of the antenna is caused by the deflection of the tripod
due to wind force, therefore the tripod is modeled as a bending beam, shown
in Fig. 9. In order to evaluate the deviation of antenna directivity by wind
velocity the tilting angle is related to the wind force
The following Eq. (9)
By inserting Eqs. (8) and (9) in Eq. (7) and taking into account the
parameters of a typical wooden antenna tripod, the wind caused deviation
Analytically calculated deviation over the wind velocity. At wind
velocity of 27 m s
The evaluation of the reflected EM field on undefined ground is approached in two ways. A numerical one – explained in Sect. 6.1 – uses a simplified model of a WT to calculate the EM fields above the ground. The ground properties are simulated by varying EM parameters of electrical conductivity and relative permittivity. And a conservative one – explained in Sect. 6.2 – known from the law of reflection. The grounds chosen are electrically neutral (free space) and PEC. The obtained standard uncertainty follows a worst case scenario, thus used for validation of the simulation results.
Of all the possible grounds WECS can be built on, sand has the lowest
conductivity while clay has the highest; therefore, those textures are
considered the two extremes (Hippel, 1995). In order to evaluate the
reflection of WT's EM emission on undefined ground a simulation is set-up in
FEKO, a field simulator by Altair. As shown in Fig. 11, a
The ground relative permittivity
Bottom loaded monopole as a simplified model of a WT.
Exemplary parameters for clay at 10 MHz (Hippel, 1995).
Carrying out the simulation the field is detected corresponding to CISPR
16-2-3 in
Validation of the simulation model. Electric field strength calculated in free space and over PEC.
Magnetic field strength calculated over sand ground with various relative moisture rate.
Magnetic field strength calculated over clay ground with various relative moisture rate.
In order to evaluate the impact on measurement uncertainty due to undefined ground the spectrum is divided into three parts based on the CISPR Bands B, C and D. Shown as an example, for the magnetic field with sand in Fig. 13 and with clay in Fig. 14 it is observed that the amplitude increases with increasing moisture rate, resonances occur proportional to the monopole`s length and the curve progression is similar for all moisture rates and both textures.
In order to differ between the texture's and the moisture rate's influence
they are considered separately in the following. For the texture the
deviation
Standard measurement uncertainties caused by field reflection on undefined ground.
Deviation of the magnetic field strength.
Deviation of the electric field strength.
Supposing rectangular distribution, the uncertainty due to different
moisture rate can be calculated by Eq. (13) taking each maximum deviation
As shown in Sect. 6.1, the relative moisture rate is a significant
factor for the measurement uncertainty. In consideration of extreme EM
properties, ground with the relative humidity of zero percent, is assumed EM
neutral behaviour, i.e. the WT is in free space. The field strength detected
at this state corresponds to the value
Field strength PDF according to the conservative approach.
Table 2 summarizes the values of the standard measurement uncertainties due to undefined ground, derived in this work. It can be seen that the numerically obtained standard uncertainties are always smaller than the conservative uncertainty, validating the simulation results. However, it should be noted that the simulation results are only applicable to onshore WECS, but the conservative standard uncertainty is a general valid assumption, hence applicable to both, onshore and offshore installations.
The electromagnetic (EM) emissions of wind energy conversion systems (WECS) are evaluated in situ. Results of in situ tests, however, are only valid for the examined equipment under test (EUT) and cannot be applied to series production as samples, as the measurement uncertainty for in situ environments is not characterized. Currently measurements must be performed on each WECS separately, that is associated with significant costs and time requirement to complete.
Therefore, this work, explains the measurement of EM emissions of WECS. Evaluating the normative situation on the determination of EM emissions of WECS results in the definition of the magnetic (CISPR Band B) and the electric field strength (CISPR Bands C and D) as measurand.
In order to evaluate the measurement uncertainty, the measurands are described using model equations, based on the standard procedure according to the “Guide to the expression of uncertainty” (GUM). To determine the expected value and the associated measurement uncertainty of the measurand, knowledge of the so-called input values of the model equation is necessary.
Evaluation of the normative situation on measurement uncertainty of EM emission measurements leads to the result that open area test site (OATS) is the most similar test-site to in situ environment. Model equations for the measured field strength and input values specified for OATS can be adopted. Taking a closer look on the input values, two aspects occur with the need to further evaluation: the deflection of the antenna caused by wind velocity, and the reflection of EM fields on undefined ground.
In order to evaluate the deflection of the antenna tripod, caused by wind velocity, common antennas are simulated under different tilting angles in a plain wave field. The antenna foot point voltage, which directly relates to the EM field, is observed. By calculating the force necessary to tilt a common antenna tripod up to a certain angle and using the analytical characteristic of a dipole, the relation between wind velocity and deviation of the EM field is established. The EM field of tilted and not tilted antenna is compared and the impact on the standard measurement uncertainty is presented.
The reflection of the EM waves on undefined grounds is evaluated in two approaches. In the first one, a simplyfied model of a WECS is simulated above infinite extended ground with different EM characteristics. The observed EM fields of the extremes in texture and moisture are compared. This shows that the influence of varying ground moisture has a much higher influence on the measurement uncertainty than the variation of the texture.
Therefore, a second, conservative approach assessing the measurement uncertainty is derived from the law of radiation, taking the relative soil moisture into account. In summary, it can be said that the numerically obtained standard uncertainties are always smaller than the conservative uncertainty, validating the simulation results. However, it should be noted that the simulation results are only applicable to onshore WECS, but the conservative standard uncertainty is a general valid assumption, hence applicable to both, onshore and offshore installations. Thanks to the achievements made in this contribution it is possible to determine the measurement uncertainty of radiated EM emissions during WECS evaluation.
No data sets were used in this article.
The authors declare that they have no conflict of interest.The publication of this article was funded by the open-access fund of Leibniz Universität Hannover. Edited by: Thorsten Schrader Reviewed by: Kevin Herrling and one anonymous referee