# Observations about Propagation, Attenuation, and Radiation of EMI from HVDC Converter Stations
## 3 Results
### The Finite-Difference Time-Domain Method: Initial Insights from Simple Geometries
Figure 3a shows the log magnitude (which is twenty times the log base 10 of the magnitude) of the Fourier transform of the \(E_{z}\) component of the electric field, measured at the points inside and outside the hall over a range of frequencies from 0 to 35 MHz. For reference, the Fourier transform of the source function is also provided. One can see that peak energy occurs at 10 MHz. Note that below approximately 14 MHz, there is very little difference between the fields inside and outside the hall and, at several frequencies, the field outside the hall is greater than that inside the hall. For frequencies greater than 14 MHz, the field inside the hall is generally larger than that outside the hall. This appears to indicate that the valve hall is serving as a filter. However, such an assertion can be called into question, as discussed below. Note that the lowest-order resonant frequency for a valve hall of this size is 4.20 MHz. The strongest resonances generally occur below this frequency. Thus, these resonances must be associated with the overall structure of the station, including the conductors in the AC yard and DC hall, and not merely with the valve hall itself. As will be seen, these \"low-frequency\" resonances remain rather persistent regardless of what is done to the valve hall. Consider the fields normalized by the spectral content of the source function, as shown in Figure 3b. Here, the similarity of the fields below 14 MHz and their apparent filtering  
Figure 3: The log magnitude of the spectral content of the \(E_{z}\) component of the field, measured at two points when (**a**) the observation points are inside and outside the valve hall. The reference curve shows the Fourier transform of the source function. (**b**) Fields at the observations points normalized by the source function. (**c**) The valve hall has been removed, with no other changes to the simulation. above 14 MHz are somewhat easier to discern. However, also consider the results shown in Figure 3c, which are identical to those of Figure 3b, except that now the valve hall has been removed. (These curves will be labeled \"Inside\" and \"Outside\", although in Figure 3c there is no valve hall present to distinguish inside from outside while the location of the observation points have not changed.) Several of the peaks, especially those below about 7 MHz, remain nearly unchanged. The persistence of these resonances, independent of the existence of the valve hall, indicates that they are associated with the conductors throughout the model. For frequencies above this, the results in Figure 3c are much \"quieter\" than they are when the valve hall is present. Consider the \"outside\" curves in Figure 3b,c; although the curve in Figure 3b is much noisier than that of Figure 3c, the fluctuations in Figure 3b appear to be roughly about the \"baseline\" shown in Figure 3c. The same cannot be said of the results for the \"inside\" curves. It is certainly true that the results for the inside point in Figure 3c are quieter than those for Figure 3b, but one would not say that the fluctuations in Figure 3b are roughly about the curve present in Figure 3c. Instead, when the valve hall is present, rather than filtering the outside field, its presence tends to elevate the interior field. Putting these observations together, we can say that the valve hall does not diminish the exterior field but rather enhances the interior field. Note, however, that this claim pertains to these two particular observation points, and both are in relatively close proximity to the line on which the source pulse exists. Were one to consider an exterior point located close to one of the two walls with no conductors passing through them, the field measured with the presence of the valve hall would certainly be reduced relative to having no hall present. Nevertheless, the interior field is enhanced by the presence of the valve hall (independent of where the field is observed in the interior). This enhancement may not be evident at a particular time or at a particular frequency.