# 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 5a shows the electric field magnitude over the initial 5 \(\upmu\)s of two simulations, one when the valve hall is present and the other where all the conductors are unchanged but the valve hall has been removed. Unsurprisingly, when the valve hall is not present, the initial fields are stronger than when the hall is present (please refer to the interval between 1 and 2 \(\upmu\)s). This is a consequence of the fields being free to directly radiate in all directions (other than \"down\" into the perfectly conducting ground) from the commutation event. However, after this initial burst, the fields without the valve hall drop below those observed when the hall is present. This is because without the hall present much of the energy is quickly lost to radiation throughout the surrounding environment. On the other hand, with the hall present (and there being no loss mechanisms contained within the hall) and the energy unable to radiate in all directions, the energy confined to the hall can only escape slowly via the conductors which, in turn, serve as antennas to radiate the field to the surrounding environment. As previously mentioned, including inductances (which model the smoothing reactors) and bushings in the simulation has little effect on the radiated field (affecting mostly its phase but not its magnitude). Two other scenarios were considered. In one, the walls of the valve hall remained in place, but the roof was removed. Potentially, this could have directed significant amounts of energy \"up\" so that this energy would not be present for terrestrial measurements, such  
Figure 5: The electric field magnitude measured approximately 200 m from the converter station. (**a**) Field observed when the valve hall is and is not present. (**b**) Fields observed when there is and is not a lossy dielectric slab placed in the valve hall. as at the observation point being considered. However, at the frequencies being considered, it appears that the height of the valve hall walls was not sufficient to direct the energy up. Instead, the fields largely diffract over the walls so that the observed fields, in this case, are similar to having no valve hall present at all, and thus these results are not shown here. The other scenario we considered introduced absorption loss into the valve hall. This took the form of a 2 m thick (in the \(z\) direction) dielectric slab adjacent to the ceiling of the intact valve hall. The slab nearly spanned the \(x\) and \(y\) extents of the hall and was displaced from the walls and roof by 0.5 m. The slab had a relative permittivity, \(\epsilon_{r}\), of 1.5 and a conductivity, \(\sigma\), of 13.8 mS/m. Figure 5b again shows the electric field magnitude at the observation point over the initial 5 us of two simulations. The results are again shown for an intact valve hall (the same as in Figure 5a) but also for the case in which the lossy dielectric slab is in the valve hall. In this case, the initial fields in the interval from 1 to 1.5 us are nearly identical, but with the lossless case having slightly higher peaks. However, beyond that, the field for the lossy hall decreases significantly relative to the lossless case. Figure 6 shows the log magnitude of the normalized spectrum of the \(x\) component of the magnetic field for the three scenarios considered in Figure 5. (If \(H_{x}\) is scaled by the impedance of free space, nearly identical results are obtained compared to plotting the spectrum of \(E_{z}\). In practice, the horizontal component of the magnetic field is measured using a loop antenna, which is what motivated the plotting of \(H_{x}\) in Figure 6). Across much of the spectrum, when the lossy slab is present, the field is smaller than in the other two scenarios and, over certain ranges of frequencies, significantly so. Above approximately 14 MHz, the lossy results are below those of the intact hall by roughly 10 dB. Between 7 MHz and 14 MHz, the difference between these results varies, but the lossy hall consistently out-performs the intact hall in terms of having lower fields.