Co-Siting Criteria for Wind Turbine Generators and Transmitter Antennas

Transcription

1 CONFTELE '99 ISBN Pro. CONFTELE II Conf. de Teleomuniações, Sesimbra, Portugal, , Abr Co-Siting Criteria for Wind Turbine Generators and Transmitter Antennas Carlos Salema, Carlos Fernandes Instituto Superior Ténio and Instituto de Teleomuniações, Lisboa, Portugal Abstrat Transmitter antennas, for VHF and higher frequenies, are usually installed in hill-tops and mountaintops to improve overage. These sites tend to be windy and thus are favorable loations for wind turbine generators, partiularly if alone or in very small wind farms. Modern wind turbine generators make use of large solid metalli pylons whih, if loated too lose to the antennas, will ause severe modifiations to the antenna radiation pattern and thus to the quality of servie. Using WIPL-D we have modeled a typial antenna and a wind turbine at 100 MHz in order to provide riteria for o-siting. The inrease in the number of unknowns with inreasing operating frequeny prevented the analysis to be extended to the GSM band 900 MHz. An alternative formulation based on the Kirhhoff-Huygens vetor formulas was used to over from 100 MHz to 1000 MHz, enabling both to onfirm the previous results and to extend them. Keywords wind turbine generator, WIPL-D, ellular radio I. Introdution Transmitter antennas, for VHF and higher frequenies, are usually installed in isolated hill-tops and mountaintops to improve overage. Favorable sites are often rather rowded but mutual interferene is kept to aeptably low levels due to one or more of the following fators: tower siting, distane between towers, small tower ross-setion and high antenna diretivity. Good antenna sites tend to be windy and thus are favorable loations for wind turbine generators, partiularly if alone or in very small wind farms. However modern wind turbine generators employ large solid metalli pylons whih have a muh larger ross-setion and thus will ause severe blokage to the antenna radiation pattern and to the quality of servie if loated too lose to the antennas. This problem arises mostly at frequenies between 100 MHz and 1000 MHz used both for radio and TV broadasting and for mobile (ellular) radio. In any ase the presene of the wind turbine generator is felt as a modifiation of the signal intensity on the area of overage whih beomes objetionable if pereived by the user. There seems to be preiously little published data on this subjet from whih design riteria might be derived. For analog transmission (FM radio and TV) average user awareness threshold varies with the intensity of the reeived signal but may roughly be stated as being about ±3 db, exept very lose to the transmitter, when the reeived signal is likely to saturate the reeiver. In the latter ase a larger reeived signal derease (possibly up to - 6 db) may well be aeptable. For digital transmission, suh as used in GSM ellular radio, the situation is quite different. The bit error rate inreases as the reeived signal dereases but, up to a minimum signal level, the user is likely to be unaware of the fat. In this ase the riteria for o-sitting should be the minimum signal level, rather than a hange in the signal level due to the presene of the wind turbine generator. To further ompliate matters radio ell radius may vary from less than 1 km up to about 25 km (aording to the expeted traffi) and ells often overlap so that if the signal from a transmitter falls below the threshold level the servie may be guaranteed by another transmitter. Sine wind turbine generators are always loated in fairly isolated plaes we may reasonably assume that ell radius would be large, from 10 to 20 km, and with little or no overlapping. II. Modeling using WIPL-D We assumed the wind turbine generator to be made up of a solid ylindri pylon (perfet ondutor), 3 m in diameter and 40 m high, with 3 (loss-less) dieletri blades, eah 20 m long and 1 m wide. The transmitter antenna was taken as a half wavelength dipole mounted at a distane d t from the axis of the pylon and at a height of 30 m, and the reeiver antenna was a short dipole at the height of 1.5 m. Due to the distanes involved (up to 20 km) Earth urvature was negleted. We started by modeling the transmitter antenna and the wind turbine generator, shown in figure 1, using WIPL-D 1], 2]. Even if this program provided a very good model, up to the inlusion of the blades, it beomes unusable when trying to extend the analysis up to 1000 MHz due to the large number of unknowns involved. Disregarding ground effets, the far field radiation pattern of the transmitter antenna in the horizontal plane is onstant, with a diretivity of 2.16 db. The presene of the wind turbine generator introdues perturbations in this pattern at 100 MHz as shown in figures 2, 3 and 4, for a transmitter antenna height of 30 m and distanes to the pylon axis d t equal to 50 m, 100 m and 200 m respetively. From these figures we onfirm that, as expeted, the influene of the wind generator turbine in the transmitter antenna radiation pattern dereases as the distane d t inreases. For distanes greater than about 50 m the presene of the wind turbine generator is not likely to be felt by most radio listeners and VHF TV viewers. The height of the transmitter antenna does not have a signifiant impat on the results (ompare figures 5 and 3) exept when it beomes higher than the wind turbine

2 2 Fig. 4. Far field radiation pattern for a pylon axis distane of 200 m at 100 MHz. Fig. 5. Far field radiation pattern for a transmitter antenna height of 20 m and a distane of 100 m to the pylon axis at 100 MHz. Fig. 1. Shemati representation of the wind turbine generator as simulated using WIPL-D. Fig. 2. Far field radiation pattern for a pylon axis distane of 50 m at 100 MHz. pylon in whih ase the disturbane dereases very rapidly (figures 3 and 6). Stati blades, even when assumed to be metalli, have very little influene in the results. It is instrutive to plot the exess obstale attenuation lose to the pylon. Figure 7, shows the obstale attenuation for d t = 200 m and a transmitter antenna height of 30 m at 100 MHz, where the darker areas orrespond to the higher attenuation. The dynami range is rather small, between 3.4 and +2.3 db. Negative values of attenuation imply that field intensities are below their free-spae value. For higher frequenies, typial of ellular radio, WIPL- D quikly beomes impratial due to the inrease in the number of unknowns. An alternative method must thus be used. Fig. 3. Far field radiation pattern for a pylon axis distane of 100 m at 100 MHz. Fig. 6. Far field radiation pattern for a transmitter antenna height of 50 m and a distane of 100 m to the pylon axis at 100 Mhz.

3 3 Fig. 8. Finite width plate geometry. Fig. 7. Obstale attenuation for d = 200 m alulated using WIPL- D at 100 MHz. III. Modeling with a finite width plate In radio link design it is ommon pratie to replae obstales by equivalent knife-edges. This approah provided good results at large distanes from the obstale, where its attenuation is reasonably independent from obstale shape aording to obstale theory. Sine finite ylindrial obstales are notoriously hard to deal with, we replae the pylon by a retangular plate with an area equal to the ylinder ross-setion. Under these onditions the lassial Kirhhoff-Huygens formulation an be used to derive the exess attenuation A in db aused by a retangular opaque plate: ( a ) A = 10 log 10 (1) 4 with: where: { a = C(w 2 ) C(w 1 )] C(h e ) 1 ] 2 S(w 2 ) S(w 1 )] S(h e ) ]} { 2 + S(w 2 ) S(w 1 )] C(h e ) 1 ] + 2 C(w 2 ) C(w 1 )] S(h e ) 1 2 (2) 2]} = πd t d r (d t + d r ) (3) h e = x rd t + x t d r (4) w 1 = (d t + d r )y ob y r d t w 2 = (d t + d r )y ob + y r d t and d r, d t, x r, x t, y t and y ob are obtained from figure 8 after normalization, that is, multipliation by 2π λ where λ is the free-spae wavelength. C(x) and S(x) are the Fresnel integrals defined in 3]. Figure 9 shows the obstale attenuation alulated using 2 for the same onditions as before. Diffration effets at the sharp edges of the finite width knife-edge are expeted to underestimate the attenuation for ertain regions but omparison with ylinder obstale results from WIPL-D at 100 MHz shows that the differenes are not signifiant (Figures 7 and 9), that is usually under about 1 db, partiularly in the shadow region. A loser examination of the results shows that the maximum and minimum values of attenuation in both methods are mostly within ±0.5 db, although their loation may be slightly displaed. We expet the finite width knife-edge to beome more aurate as the frequeny inreases. Using the finite plate approximation we have also alulated the effet of the obstale in the far field radiation pattern of the transmitter antenna for d t in the range 50 to 200 m. The results math very losely, well within ±0.5 db, those obtained using WIPL-D, partiularly in the shadow region. The only exeption is when the distane between the obstale and the transmitter antenna falls below about 75 m, where differenes between the two methods may reah about 1.5 db. One the retangular plate approximation has been established we an now proeed to alulate the obstale attenuation for 1000 MHz (λ = 0.3 m). Figure 10 shows the results for a pylon 200 m away from the transmitter antenna. Now the shadow is better defined and the (5) (6)

4 4 Fig. 9. Obstale attenuation for d = 200 m alulated using the Kirhhoff-Huygens formulation at 100 MHz. Fig. 10. Obstale attenuation for d = 200 m alulated using the Kirhhoff-Huygens formulation. attenuation is higher than before ( 6.4 db) just behind the pylon. As expeted the higher attenuation region approahes the geometrial shadow region. For smaller distanes between the obstale and the transmitter antenna, the width of the shadow region and the attenuation within it both inrease. The latter behaviour is displayed in Figure 11 whih represents the maximum attenuation in the same 100 m by 900 m region as before, alulated using the finite width plate for different values of d t and frequenies of 100 and 1000 MHz. Figure 11 shows that pylon attenuation is not signifiant at 100 MHz sine the alulated worst ase values are not likely to affet typial appliations at this frequeny. A different onlusion is obtained at 1000 MHz. Now, the influene of the obstale annot be negleted even for a transmitter antenna to pylon distane as large as 200 m. However worst ase values orrespond to rather small areas (tens of metres aross) in the shadow region, whih may be aeptable for ellular radio planning. The effet of the obstale in the far field radiation pattern of the transmitter antenna is learly more pronouned at 1000 MHz than at 100 MHz. Figure 12 shows the far field radiation pattern at 1000 MHz for d t equal to 50, 100 and 200 m. Now for the lower distanes (d t = 50 m) the effet on the transmitter antenna radiation pattern is likely to affet the quality of servie in a rather narrow region, orresponding to the geometrial shadow region ( 1.7 degrees). Fig. 11. Maximum attenuation as a funtion of transmitter antenna to obstale distane d t. Fig. 12. Far field radiation pattern for transmitter antenna to pylon axis distanes of 50, 100 and 200 m, at 100 MHz.

5 5 IV. Disussion From the results presented for a single obstale we may derive the following tentative rules-of-thumb for o-siting of transmitter antennas and wind turbine generators: For frequenies around 100 MHz, the minimum distane between the transmitter antenna and the pylon of the wind turbine generator should not be less than about 50 m in most ases, unless the transmitter antenna is plaed onsiderably higher than the top of the pylon. Inreasing this distane to 100 m dereases the perturbations whih are unlikely to be felt exept very lose to the wind turbine generator. For frequenies around 1000 MHz, the minimum distane between the transmitter antenna and the pylon of the wind turbine generator has to inrease onsiderably, from 50 to 100 or preferably 200 m, the shorter distanes (around 50 m) displaying a signifiant dip in the radiation pattern, losely orresponding to the geometrial shadow region. For frequenies above 1000 MHz the shadowing effet of the pylon will inrease and thus even larger distanes beome neessary. The results presented in this paper refer to a single obstale. Simulations using WIPL-D for ombinations of two similar obstales show that the exess attenuation is only slightly higher in the immediate viinity (less than 100 m) of the obstales, within its geometrial shadow region. It is thus expeted that attenuation aused by wind farms may be alulated using models for isolated wind generators. All the disussion refers to the diffration effet of the obstale, whih is expeted to be the main phenomena in typial ases. However, when either the base station antenna or the mobile antenna are well above the obstale tip, refletions on the turning blades may also affet the reeived signal. These were not onsidered here. Referenes 1] B. Kolundzija and B. Relji, Plate modeling of wire strutures, Proeedings of the IEEE APS International Symposium, vol. 3, pp , ] B. Kolundzija, Eletomagneti modeling of omposite metalli and dieletri strutures, Proeedings of the IEEE APS International Symposium, vol. 3, pp , ] M. Abramowitz and I. Stegun, Handbook of Mathematial Funtions, Dover Publiations, In., New York, 1968.