1、CCIR RECMN*805 92 = 4855212 0520115 356 136 Rec. 805 RECOMMENDATION 805 ASSESSMENT OF IMPAIRMENT CAUSED TO TELEVISION RECEiTON BY A WIND TURBINE (Question 611 I) ( 1992) The CCIR, co mide ring that severe degradation of television reception can be caused by reflections from moving objects such as th
2、e that these effects are particularly serious because the impairment caused can be quasi-permanent, being a) blades of a wind turbine; b) reduced only during periods when the wind turbine is not rotating; 4 be caused by the installation of any proposed wind turbine; d) impairment caused by wind turb
3、ines, that it is important to bave available a simple method for calculating the potential impairments which could that ghost cancellation techniques are being investigated and that these may offer some amelioration of the recommends that the method given in Annex 1 be used to assess the potential i
4、nterference from a proposed wind turbine 1. installation consisting of a single machine; 2. 3. installation; 4. that further work be carried out to refine the simplified model given in Annex 1; that further work be carried out to investigate the impairment caused by a multiple-machine wind turbine t
5、hat the temporal nature of the impairment caused by a wind turbine be investigated. ANNEX I Simplified model of impairment caused to television reception by a wind turbine Figure 1 shows the plan view of the general wind turbine problem. At any receiving location, R the wanted field strength is FSR.
6、 At the wind turbine site, WT, the field strength is FSWT. A “reflection factor”, RF, may be defined which includes the free-space path loss for the first lan of the path from the wind turbine site to R. Thus, FSWT + RF gives the maximum amplitude, at a distance of I km from the wind turbine, of the
7、 signai scattered from the wind turbine. The maximum value of this reflection factor due to the wind turbine blades is 20 log (AA) - 60 dB. The relative amplitude, RA, in the forward scatter region is given by: sin (n . Wlh . sin a) n . Wlh . sin a RA = 20log where: A : blade area (m2) W : width of
8、the blade (m) h : wavelength (m). The relative amplitude in the general scatter area is taken to be -10 dB. CCIR RECMN*805 92 4855212 0520116 292 General scatter region Rec. 805 FIGURE 1 k I hTX I I CI ). 0 .( Jl Forward scatter region R 137 Tx: television transmitter WT: wind turbine R: receiving l
9、ocation In the case of a free-space path, of length d (km), between the wind turbine and the receiving location, the unwanted field strength may be calculated as: FSWT + RF + max (-10, RA) - 20 log d where: max (-10, RA) : larger of -10 dB and the relative amplitude of the forward iobe. If this path
10、 is obstructed, the field strength may be calculated by other means, with the first three tem giving the field strength at 1 km on an unobstructed path. The receiving antenna directivity discrimination as a function of is given in CCIR Recommendation 419 and this should be applied to determine the r
11、atio of the wanted to unwanted signai for any specific receiving location. The required wanted to unwanted signai ratio as a function of the time difference between the wanted and unwanted signals is given i0 Fig. 2. FIGURE 2 An example of the use of this method is given in Appendix 1. CCIR RECMN*A0
12、5 72 4B552i12 05201i57 129 138 Rec. $05 APPENDIX 1 TO ANNEX 1 Example of use of slmpllfied assessment method In Fig. 1 of Annex 1, the point marked R is a receiver location, near the site of a proposed wind turbine WT. As a first step, calculate or, preferably, measure the field strength values, FSR
13、, at the various receiver locations. Experience suggests that in the c,ase where the terrain is fairly flat and reception locations are not screened from the wanted transmitter, it is unlikely that a wind turbine installation will cause significant impairment to reception at distances of more than a
14、bout 0.5 km from the wind turbine site. Experience also suggests that it is unlikely to be necessary to extend the investigation area to more than about 5 km from the proposed wind turbine site (or sites, if there are multiple turbines). However, if there are special CirCUmstanCeS, for example build
15、ings which are screened from the wanted transmitter but which are line-of-site to the wind turbine, then the area may need to be extendcd. Calculate or, preferably, measure the field strength, FSiW, at the wind turbine site, near the height of the centre of rotation of the blades. Calculate the maxi
16、mum amplitude of the reflection factor: RF = 20 log (AA) - 60 dB For each of the receiving points, R: - calculate the unwanted field strength using details of the path between the wind turbine and the receiver using FSWT -1- RF as the field strength at 1 km for an unobstructed path; - calculate the
17、larger of (-10, relative amplitude of forward Isbe) where the relative amplitude, RA, of the forward lobe is given by: (For convenience, the -10 dB half width of the forward lobe is given approximately by: sin- (0.75 WW); - calculate the wanted to unwanted signai ratio, taking account of the receivi
18、ng antenna directivity discrimination; - using the curve given in Fig. 2, determine if the impairment at the receiving point will be worse than grade 4. The results of the study may then be presented in the form of a map showing the areasflocations where worse than grade 4 impairment may be expected
19、. It should be noted that the process is more complicated if there are multiple wind turbines on a given site as there are then several possible sources of impairment at each receiving location. It is desirable to carry out further investigations in order to derive a suitable calculation process for this case.
