Disturbances in HVDC Classic transmissions

Frequencies between 100 Hz and up to say 3 kHz, i.e. harmonics within the audible range, can cause telephone interference to people close to the DC and AC lines coming from the converter station. The disturbance is then magnetically induced in the telephone cable (or wires) running at some distance from the high voltage line. In order to prevent this AC filters and DC filters that suppresses these frequencies are included in the station. (DC filters are normally unnecessary when the station is connected directly to a DC cable.) Telephone interference from HVDC stations are relatively rare. The cases that have occurred have mostly been due to ground faults in the telephone system.

If power line carrier communications (in the range from 20 - 40 kHz up to about 200 kHz) are used in the AC grid (or on the DC line) high frequency noise from the HVDC converter might cause interference. To prevent this, a PLC filter can be installed.
High frequency noise from the HVDC converter might also cause radio interference in the AM bands (150 kHz - 30 Mhz) in the vicinity of the converter station. FM radio, TV and mobile phones occupy higher frequencies and are not disturbed. The way to avoid radio interference is proper screening of the valve buildings (or outdoor valves). In addition small RI-filters are normally provided that take care of the RI noise that escape from the building via the AC and DC bushings.
The audible noise that a HVDC converter station emits to the surroundings comes mainly from the converter transformers, the valve cooling fans, the smoothing reactors and the AC and DC filters. There are a number of methods to mitigate the noise:

• orient disturbing equipment away from the most sensitive sound direction
• use of low noise level equipment
• screening or enclosing equipment

The owner always specifies the permissible audible noise level.