Technology and Development of 800 kV HVDC Applications

Bulk Power HVDC transmission schemes over distances of up to 2000 km are currently under planning for various large hydropower stations in India and China. Ultra high dc voltage (UHVDC) up to 800 kV is the preferred dc voltage level for these applications. Currently world-wide existing HVDC schemes are limited to maximum voltage levels of 500 kV to 600 kV. Therefore, the impact of increased steady state and transient voltage stresses on the design of the main equipment for UHVDC stations has to be carefully investigated. This paper focuses on specific design aspects for key UHVDC equipment and system to be taken into consideration. The state of art of HVDC equipment technology and its application in UHVDC system are described and discussed in detail along with some examples from research and development works. UHVDC APPLICATIONS In the last decades many HVDC projects have been realized for power rating of 2000 – 3000 MW at 500 kV voltage levels. The first HVDC project with voltage above 500 kV, the Cahora-Bassa between Mosambique and South Africa (533 kV, 1920 MW), was built almost 30 years ago. The second HVDC project with voltage above 500 kV, constructed about 20 year ago, is the Itaipu HVDC project (600 kV, 3150 MW). Several UHVDC projects, with preferred operating voltage in the range 750-800 kV have been identified in China and India with power transmission requirements up to about 6400 MW over a single bipolar dc line. The length of dc line may be up to 2000 km and even more. The motivations for choosing UHVDC are very similar to the ones for HVDC projects and are briefly outlined here: • Reduction of overall project cost compared to alternatives such as UHVAC • Transmitting power from remote hydro / thermal power stations over long lines where maintaining stable transmission may be difficult over long ac lines. • Conservation of right of way for transmission lines. Transmission of bulk power from narrow restricted right of way. • Controlled transmission of power with perhaps preplanned matching with expected load cycles. • Supporting the grid through frequency control and power modulation. • Transmission of power across asynchronous power systems. • Capability to provide flexible VAR support. The feasibility of UHVDC up to 800 kV has also been conducted by the international organizations such as IEEE and Cigre [1, 2]. All these working groups reported that the UHVDC system is technically feasible with some research and development efforts in few key areas.