# Observations about Propagation, Attenuation, and Radiation of EMI from HVDC Converter Stations
### Previous Research  
Given the purpose of this paper, the following discussion of previous research identifies the relevance (or not) of each paper in determining the impact of valve hall shielding or other aspects of reducing EMI. The earliest reported research on this topic is that of [PERSON] and [PERSON] in 1962 [4]. The next was the work reported by [PERSON] in 1972 [5]. There, measurements were reported of EMI in the switchyard and along transmission lines. In addition, the attenuation of a valve hall was measured before equipment was installed. From [5], \"The effectiveness of the valve hall screen at Celilo was verified before equipment was installed in the valve hall. A noise generator, essentially a vertical electric dipole, was erected in the valve hall and the noise level measured as a function of distance from the source. The dip in the profile at the wall screen was taken as a measure of the attenuation... In the frequency range from 0.15 to 5 MHz an attenuation of 40 to 45 dB was measured\". Soon after this, [PERSON] and [PERSON] reported their work on calculating the EMI from HVDC converter stations [6]. ([PERSON] also publishes under the names [PERSON] and [PERSON].) They discussed the source of EMI as the transient change in voltage across the terminals of a valve. Then, they calculated the high-frequency transient currents in the converter station using the appropriate high-frequency equivalent circuits for different elements of the network. Comments were made about the effectiveness of inductive filters (15 to 20 dB reduction). These currents were then used to calculate the EMI fields in the vicinity of the station. In the 1980s, a large EPRI project was conducted to better understand the EMI from HVDC converter stations [7; 8; 9; 10]. In this project, a scale model of an HVDC converter station was developed and used for measurements. Further, an EMI prediction program was developed using wideband circuit equivalents as components [8]. One relevant result was that [9] \"the study showed that if the smoothing reactor is in the HVDC bus, it does attenuate the RF signal on the HVDC bus significantly\". There was no attempt to model the shielding of the valve hall. In 1989, [PERSON] and [PERSON] [11], rather than producing a full analysis of a converter station, applied full-wave analysis using the method of moments to evaluate the effectiveness of traditional shielding and filtering techniques. This is a useful reference for at least two reasons: (1) specific high-frequency equivalent circuits of components are identified and (2) the results could potentially be used to identify the relative effectiveness of shielding and filtering. It was shown that there are situations in which the radiated field in a particular direction may actually be enhanced because of a shielding enclosure [11]. This stands in contrast, for example, to the blanket statements in [5; 12] that indicate a valve hall will provide at least 40 dB of reduced radiation. The statements in [5; 12] are based on measurements and the authors do not provide a full-wave analysis of an electromagnetic environment representative of an actual valve hall. Later, [PERSON], [PERSON], and [PERSON] reported measurements of the electromagnetic environment, but no modeling [13]. In 1999, [PERSON] et al. built a 1/400 scale model of an HVDC converter station and analyzed it using the method of moments in the frequency range 200-1000 MHz [14]. Included in the model were busses, the equivalent circuits of devices such as DC reactors, the valve hall, and other buildings. No details were provided about the meshing or about how the openings in the valve hall through which the busses passed were (if they were) modeled; however, the calculated and measured fields were reasonably close. In 2006, [PERSON] et al. provided useful information about background noise as well as the characteristics of different sources of EMI attributable to HVDC converters stations [3]. These include coronas and sparking, as well as commutation noise.