# Observations about Propagation, Attenuation, and Radiation of EMI from HVDC Converter Stations
## 4 Discussion  
The electromagnetic interference (EMI) generated by switching operations within high-voltage DC (HVDC) converter stations is an issue that has been addressed by CIGRE, and methods to manage EMI should be considered in the design phase using electromagnetic modeling techniques. More specifically, CIGRE has proposed guidelines for the maximum values of EMI fields. Investigations into the modeling of high-frequency (e.g., 9 kHz-30 MHz) EMI emissions from an HVDC converter station leads to the conclusion that one must use a full-wave model to be able to capture the complicated physics of the problem over the broad spectrum  
Figure 6: The normalized spectrum of the \(H_{x}\) field (in dB), measured approximately 200 m from the valve hall. The results are shown for an intact valve hall, when no valve hall is present, and when the valve hall is intact but contains a lossy dielectric slab. of excitation that exists. However, work with MoM and the FDTD method points to the fact that there are numerous ways in which such a full-wave model might lead one astray or be rather limited in the information it can provide. For example, it is shown here that MoM calculations of the small fields outside a shielded valve hall may be numerical artifacts rather than accurate results. Although there are various papers that purport to characterize the radiation from or into a shielding structure (e.g., [42; 43]), to our knowledge there does not appear, in the open literature, to be a thorough analysis in which one can be confident that the effects of a valve hall are fully captured. Here, a realistic model of an HVDC converter station has been developed and analyzed using the FDTD method. The model consists of a (shielded or unshielded) valve hall as well as the conductors within it, the AC yard, the DC hall, and the territory surrounding the station so that the EMI can be predicted at distant points. It is often claimed that a valve hall provides perfect (or near-perfect) shielding because it is assumed to be a Faraday cage. This is shown here to be true even if there are holes (small compared to a wavelength) in the shield, despite the claims that small (compared to a wavelength) seams from valve hall construction can cause significant leakage [20]. It is also shown that the spectrum of the field inside the valve hall is greatly influenced by the cavity resonances associated with the cuboid geometry of the valve hall shield. If, however, conductors penetrate the valve hall wall, significant leakage of the electromagnetic fields within the valve hall occurs. And, in practice, there must be conductors that pass through bushings uninterrupted, from the interior to the exterior, to carry the AC and DC currents into and out of the valve hall. Hence, the valve hall must be considered \"leaky\". It is shown here that the difference between the fields inside and outside the valve hall with and without conductors that make holes in the shield can be to the order of 50 dB. To understand what happens to the high-frequency electromagnetic energy generated by commutation events in a valve hall, it must first be noted that this energy is dissipated only by its absorption into lossy materials or radiation to infinity. A shielded valve hall by itself (i.e., without absorbing material or conductors penetrating its walls) merely serves to trap radiated fields; it is shown here that they reflect around the valve hall indefinitely. If conductors inside the valve hall that penetrate its walls are introduced into the model, the interior fields can and will eventually couple onto these conductors as a means of escape. Once a current is induced on a line, the valve hall does little to filter the current passing out of the hall. The result of this process is that the original pulse of EMI energy is spread over a significant amount of time. After it exits the valve hall, it is radiated to infinity. It is this radiation that is the subject of CIGRE guidelines.