Information on the operating status of the HVDC transmission
line was extracted from records of the HVDC converter
station. Typically the line was operated for only a portion of the
day and de-energized at night.We excluded from these analyses
periods when the dc line was operating, as well as 5 minutes before
the line was switched on and 30 minutes after the line was
switched off. The latter exclusions were to account for synchronization
errors between the line voltage records and the data loggers
and to allow ions produced by the HVDC line to dissipate.