The Siemens Power Transmission and Distribution Group (PTD; Erlangen, Germany) has been awarded an order by the Powergrid Corporation of India (PGCIL; New Delhi, India) to increase the capacity of the East-South long-distance high-voltage direct current transmission (HVDC) link in India.

Since 2003, the Siemens system has been transporting electrical power with low loss from the Talcher power plant complex in the state of Orissa in eastern India, located 1450 km (900 miles) to the south, to the industrial region around Bangalore. Presently, the system is designed for transmission of 2000 MW. After the upgrade is completed, it will be possible to make maximum use of the system's overload capacity of up to 2500 MW. In this way, power supplier PGCIL is catering to the growing demand for energy in the region around Bangalore. The order has a value of about 14 million euros and the project will run for 15 months.

To increase the capacity of the East-South link, the Siemens experts have developed a solution not usually used for HVDC systems. With the aid of a system known as Relative Aging Indication and Load Factor Limitation (RAI&LFL), it will be possible to use the overload capacity of the system more effectively without the need to install additional thyristors connected in series or in parallel to increase the dc transmission capacity.

Part one of the system, RAI, models the thermal characteristics of the power converter transformers and the smoothing reactors to ensure that the planned service life of the system at rated load is still attainable despite the increase in power. The aging curve of the transformers and the reactors is calculated on the basis of the modeling. Part two, LFL, computes the maximum possible load taking into account the oil temperature and ambient temperature and the availability of the cooling units. This allows power transmission of between 2000 MW and 2500 MW at a dc transmission voltage of ±500 kV for a substantially prolonged period due to the more efficient use of the overload characteristics of the HVDC system.

In addition to the RAI&LFL system, Siemens is also retrofitting several components to increase the performance of the system. These include new cooling units for the power converter transformers at both stations to dissipate more heat. Also included is forced cooling of the smoothing reactors for greater current carrying capacity and additional ac filter banks to cover increased reactive power. Further, new reactors for PLC filters, circuit breakers for the new ac filter banks, arresters and current transformers will also be installed. The open and closed-loop control concept and the protection concept will be adapted to the new operating mode of the HVDC transmission system as part of the upgrade, and the system's own auxiliary power supply will be augmented.