The converter transformers are the heaviest equipment in a HVDC converter station. Single units can often have a total weight of 200 - 550 tons.
The converter transformer is an integral part of an HVDC system. High AC and DC voltages put specific requirements on the dielectric insulation. Non sinusoidal currents give rise to additional losses which are to be considered. Converter transformers connect the AC network to the thyristor valve bridge, and adjust the voltage on the valve side to a suitable level based on the DC voltage used for the transmission. The transformers can be of different design depending on the power to be transmitted, and possible transport limitations. The most common type is a single-phase-three-winding design. Three identical transformers are then needed per converter.
Converter transformer for Pacific Intertie, Sylmar Replacement Project, 621 MVA, 554 metric tons in the ABB factory and at the Sylmar station.
The converter transformer serves several functions:
- Supply of AC voltages in two separate circuits with a relative phase shift of 30 electrical degrees for reduction of low order harmonics, especially the 5th and 7th harmonics.
- Act as a galvanic barrier between the AC and DC systems to prevent the DC potential to enter the AC system.
- Reactive impedance in the AC supply to reduce short circuit currents and to control the rate of rise in valve current during commutation.
- Voltage transformation between the AC supply and the HVDC system.
- A fairly large tap range with small steps to give necessary adjustments in supply voltage.