Filterless line commutated converter for HVDC transmission

Conventionally, Line Commutated Converter (LCC) technology for High Voltage DC (HVDC) transmission incorporates AC filters to remove the harmonics generated from converter operation. Traditionally, the filters also provide reactive power support. However, AC filters and the associated switchgear are known to be bulky and this translates to increased CAPEX. This paper presents a new converter topology which incorporates a six-pulse thyristor bridge with a Voltage Source Converter (VSC) STATCOM valves connected at the transformer secondary terminals. The STATCOM valves provide AC current control at the transformer's secondary windings. Additionally, the STATCOM valves control the thyristor commutation process irrespective of secondary AC voltage. In order to provide distortion free current at the AC primary windings, two six-pulse converters are connected in a 12-pulse configuration, which provides distortion-free sinusoidal currents at the primary side. The topology therefore minimises the need for bulky AC filters and is also able to provide reactive power support to the AC network. The paper shows the analytical model, converter waveforms and a commercial benchmarking showing the potential of the proposed approach vs. conventional LCC technology.