# Observations about Propagation, Attenuation, and Radiation of EMI from HVDC Converter Stations
## 4 Discussion
A converter station can be envisioned as a source of high-frequency energy (owing to these commutations), where this source is surrounded by a lossless shield (the valve hall) that redirects the flow of this energy. Rather than radiating directly away from the source, the valve hall walls redirect any radiated energy back into the interior until it eventually manages to escape via the conductors that pass through the walls. The energy could be said to be temporarily trapped within the hall. However, if a loss mechanism exists within the hall, it becomes possible to absorb this trapped energy before it has a chance to escape and radiate to the surrounding environment. It is shown here that adding electromagnetic absorbing material only in the vicinity of the ceiling of the valve hall can considerably reduce the EMI energy present in the valve hall before it exits via the conductors that penetrate its walls. As a result, the radiated EMI outside the valve hall is significantly reduced. It should be noted that this calculation is simply an illustration of what can be done. No attempt has been made to optimize either the electrical properties of the absorbing material, its geometry, or its location within the valve hall. This would be a fruitful endeavor. Note that the results presented here concerned a single \"commutation event\", and the focus was on the high-frequency radiation associated with that. No definitive claims were made about the precise field that might be measured for such an event. Rather, the goal was to better understand the degree to which the valve hall does or does not prevent the fields generated from such an event escaping to locations where their presence may be problematic. In practice, in a modern voltage source converter (VSC) station, thousands of commutations may be occurring in ways that have overlapping and interacting fields. The issue of multiple and interacting commutations will be the subject of future work. Given all the complexity of an HVDC station, although full-wave modeling is necessary, it is by no means sufficient. It can provide clues and insights into the behavior of various aspects of a converter station, but it cannot be trusted, by itself, to tell the full story. The base-level modeling of sufficiently complicated structures that mimic the relevant interacting components of an HVDC converter station needs to be validated by actual measurements. Having validated the model, some level of changing the components of the models to guide design is certainly reasonable. However, it would be completely unreasonable, as appears to have been the case in some previous works, to obtain results from a model and then assume that the introduction of a valve hall that was not previously included in the model will provide a further reduction of, for instance, 40 dB in the radiated fields. Conceptualization, R.G.O. ; Methodology, J.B.S. ; Software, J.B.S. ; Validation, J.B.S. and R.G.O. ; Investigation, R.G.O. ; Writing--original draft, J.B.S. and R.G.O. ; Writing--review and editing, J.B.S. and R.G.O. ; Project administration, R.G.O. All authors have read and agreed to the published version of the manuscript. This research was funded by the Electric Power Research Institute, Contract 10016675. Not applicable. The data supporting the conclusions of this article will be made available by the authors on request. The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. The following abbreviations are used in this manuscript: