Hybrid LCC/VSC HVDC systems

Line commutated converters (LCC) and voltage source converters (VSC) are the two technologies used for High Voltage Direct Current transmission (HVDC). Over the past 60 years more than 100 LCC-HVDC systems have been put into operation, with the transmission voltage reaching ±1100kV and transmission power capacity of up to 12GW. Meanwhile, more than 50 VSC-HVDC systems have been put into operation, of which the transmission voltage is up to ±800kV and the transmission capacity is up to 5GW. LCC-HVDC offers the advantages of larger transmission capacity, but generally has a large footprint which means large site area. The inverter station must be supported by an AC system with sufficient short circuit capacity as defined in broad terms by the short circuit ratio (SCR) to ensure reliable operation and satisfactory performance. The disturbances caused by commutation failure due to AC and DC disturbances, as well as the reactive power requirements of an LCC converter certainly have an impact on the connected AC system. On the other-hand VSC-HVDC does not suffer commutation failures, and the active power and reactive power can be controlled independently. These technical advantages enable the VSC-HVDC to ride through AC system faults and to control the reactive power to the connected AC system. To this end each have been utilized on a standalone basis.

The concept of combining the LCC and the VSC technologies in one HVDC link seems to be gaining popularity. It combines the LCC and VSC technology within one HVDC system. For example, in the Skagerrak4 HVDC project connecting Norway and Denmark, the LCC and VSC are used in different poles of the bipolar system; in the Luxi back-to-back project in China, the LCC and VSC converters are connected in parallel; in the application of power supply to offshore drilling platform, the LCC and VSC can be installed at the rectifier and inverter side, respectively, and in application of overhead line transmission (OHL), the LCC and VSC can be connected in series in one station. The hybrid HVDC system offers great technical value and application prospects since it combines the lower cost of LCC with the absence of commutation failure, and the flexible control of VSC. In this Brochure, the hybrid HVDC characterized by LCC at the sending end and VSC at the receiving end (referred as station level hybrid HVDC – point to point configuration) is systematically used as example.

Hybrid LCC/VSC HVDC Systems

The hybrid HVDC refers to an HVDC system that uses LCC and VSC together in one integrated HVDC system. From the perspective of composition, there are several possible "hybrid" configurations, including station level hybrid HVDC-hybrid infeed configuration, station level hybrid HVDC – point to point configuration, pole level hybrid HVDC, converter level hybrid HVDC, hybrid HVDC Grid etc.

This Brochure focuses on the station level hybrid HVDC – point to point configuration, whose typical diagram is shown in Figure 1 and the terminology hybrid HVDC refers to station level hybrid HVDC – point to point configuration, unless stated...