HVDC converter station

Early HVDC systems used mercury-arc valves, but since the mid-1970s, solid state devices such as thyristors have been used.

Voltage sourced converters use insulated-gate bipolar transistors instead of thyristors, and these can provide power to a deenergized AC system.

Using a star-to-delta or "wye-delta" connection of the transformer windings, the converter can operate with 12 pulses for each cycle in the AC supply, which eliminates numerous harmonic current components.

The insulation of the transformer windings must be specially designed to withstand a large DC potential to earth.

Converter transformers can be built as large as 300 Mega volt amperes (MVA) as a single unit.

Filters are tuned to the expected harmonic frequencies and consist of series combinations of capacitors and inductors.

They consist of a coil which passes the load current, with a parallel capacitor to form a resonant circuit.

The three-phase alternating current switch gear of a converter station is similar to that of an AC substation.

Converter stations can generate serious levels of radio-frequency interference, so include design features to control these emissions.

As with all AC substations, oil from equipment must be prevented from contaminating ground water in case of a spill.

Substantial area may be required for overhead transmission lines, but can be reduced if underground cable is used.