# 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 2b shows the field at the same two points, using the same excitation and the same geometry, but with the conductors that transition through the holes in the valve hall walls now intact. Again, the field arrives at the exterior point after arriving at the interior point. However, in this case, the fields are almost comparable in magnitude. The inside field is generally larger, as would be anticipated, but only slightly so, and there are instances  
Figure 2: The log magnitude of the electric field measured inside and outside the valve hall when (**a**) 2 m segments of the conductors that nominally pass through the valve hall walls are removed and when (**b**) the conductors that pass through the walls are intact. where the field at the point outside the hall is greater than inside the hall. Another way to think about the contrast between Figure 2a,b is that having intact conductors raises the field exterior to the hall by approximately 50 dB. The simulation in Figure 2b merely had the conductors passing through holes surrounded by free space. Another simulation was performed that modeled the bushings associated with each hole. These bushing were assumed to be cylindrical, extend 1 m to either side of the wall, and have a relative permittivity, \(\epsilon_{r}\), of 3. The radius of the bushings was such that they filled the holes through which the conductors passed. The bushings made essentially no difference to the overall observed behavior seen without the bushings. Another simulation was performed where, in addition to having the bushings present, lumped-element inductors were incorporated into the FDTD grid to represent the smoothing reactors found in the DC hall. The inductance used was 0.98 mH (in keeping with the inductance provided in [21]; its implementation followed the work described in [41]). Again, this made effectively no difference to the roughly similar field observed inside and outside the valve hall. In Figure 2a, note that after about 1 us, the fields decrease very little. Contrast this to the slight, but clearly evident, decrease in the field as time progresses in Figure 2b. In both simulations there are nine holes in the valve hall walls, but only in the simulation for Figure 2b do conductors pass through the holes. In the case of Figure 2a, given the relatively small size of the holes compared to the wavelength, the fields escaping the hall are primarily evanescent. These fields can couple onto the conductors that are present outside the hall, but this represents a nearly vanishingly small radiation of the fields, which explains why there is seemingly no decay (with time) of the fields in Figure 2a. (Figure 2 only displays the first 5 us of the simulation, but this persistence of the field is evident for the entire 66.03 simulation.) On the other hand, when the conductors are intact, they serve to create what is effectively a transverse electromagnetic (TEM) transmission line for the fields to pass from the interior to the exterior of the valve hall. There is no cut-off frequency for this transmission line (which needs to be the case to move DC and low-frequency energy in and out of the hall). A conductor above the ground plane serves as a TEM transmission line. The lines in the AC yard can be thought of as segments of such lines. As a line passes through the valve hall wall, one might envision this transit as a very short co-axial line, where the conductor is the inner core of the co-axial line and the wall is the outer conductor. There is an impedance mismatch going from one form of transmission line to another, but given the short extent of the intervening \"co-axial\" line, this mismatch does not cause significant reflections, i.e., fields directed out of the hall largely continue to exit the hall, a point which will be returned to when the currents in the conductors are considered. The \(E_{z}\) component of the electric field at these observation points when the conductors are intact will now be considered.