# Observations about Propagation, Attenuation, and Radiation of EMI from HVDC Converter Stations
## 3 Results
### The Method of Moments: Attributes and Limitations
Another way of thinking of one percent is as two orders of magnitude or 40 dB. But in the scenario described above, with a source in a perfect valve hall, where the exterior field should be zero, the difference between any non-zero field and the correct value is infinite on a dB scale. Alternatively, one might compare the \"numerically leaked\" field at the exterior to the field when no valve hall is present. This was carried out for excitation frequencies of 200 kHz, 1 MHz, 5 MHz, and 25 MHz. Table 1 shows the reduction of the measured field caused by the valve hall compared to that when the valve hall is not present (a finitely conducting ground beneath a perfect valve hall is present for all calculations with \(\epsilon_{r}=15\) and \(\sigma=10\) mS/m). Because the hall has no apertures, the correct change is \(-\infty\) on a dB scale, i.e., the lower the value the better. At 200 kHz, we note that the reduction is in the order of 40 dB, which is the value often given as the effect of the valve hall. Recall that a 40 dB reduction appeared in [5; 12] and was based on measurement. However, the approximate 40 dB reduction observed at 200 kHz with this NEC-5 simulation is purely a numeric artifact. The fact that this may agree with the measurements should be considered coincidental and not indicative of this software capturing the full physics of the effect of the valve hall. In [25], the authors also report a \"shielding effectiveness\" at 15 MHz of \(-59.6\) dB. That matches the value of 5 MHz in Table 1 remarkably closely. But, again, this is mere coincidence. If one continues to higher frequencies (up to 100 MHz), ref. [12] reports that the shielding effectiveness further increases to \(-74.54\) dB, whereas the MoM results mysteriously drop to \(-7.96\) dB at 25 MHz (the highest frequency that was considered). In contrast to the behavior at these frequencies, the results obtained at 25 MHz are unacceptably high (predicting only about \(-8\) dB of shielding from the perfect valve hall) and apparently nearly immune to an increase in the discretization of the valve hall walls. This is a cause for concern and at present we have no suitable explanation for this behavior. The results in Table 1 were obtained using a discretization that adheres to the recommendations in the NEC-5 user's manual [25]. Nevertheless, keeping in mind that any non-zero field represents an error and observing that the results ran into difficulties at \begin{table}
\begin{tabular}{c c} \hline \hline
**Frequency** & **Change Caused** \\  & **by Valve Hall** \\ \hline
200 kHz & \(-41.94\) dB \\
1 MHz & \(-49.12\) dB \\
5 MHz & \(-58.84\) dB \\
25 MHz & \(-7.96\) dB \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The shielding effectiveness of a “perfect” valve hall as determined by NEC-5. The measurement point was 200 m from the center of a \(10\times 10\times 10\) m cubic hall. 25 MHz, the test was run using a finer discretization. The results in Table 1 were obtained using 400 patches per face of the cube (i.e., 20 patches along a side of the cube for \(20\times 20=400\) patches). There was no significant difference in the results.