Rihand - Dadri

The Rihand-Dadri HVDC project is a high-voltage direct current transmission system in India. Commissioned in December 1991, it connects the Rihand Thermal Power Station in Uttar Pradesh to the Dadri substation near Delhi, spanning a distance of 814-816 kilometers.

Technical Specifications

The Rihand-Dadri HVDC link operates at ±500 kV DC voltage and has a power transmission capacity of 1,500 MW. It consists of a bipolar overhead transmission line with two poles. Each pole has a continuous power carrying capacity of 750 MW, with approximately 10% two-hour overload capability and 33% five-second overload capability.

The system includes converter stations at both ends - Rihand and Dadri. The Rihand terminal has converter transformers rated at 6 x 315 MVA, while the Dadri terminal has converter transformers rated at 6 x 305 MVA. The converters use 12-pulse Graetz bridge configuration with a total of 1,152 thyristors.

Key Components

The Rihand-Dadri HVDC system comprises several key components. Converter stations are located at Rihand and Dadri, which convert AC to DC and vice versa. The transmission line is an 814-816 km bipolar overhead line using quad Bersimis conductors. Smoothing reactors, specifically 180 mH, 500 kV DC air core (dry type) smoothing reactors, are used at both Rihand and Dadri terminals. For harmonic filtering, three banks of 230 MVAr each are employed as AC filters. The system also includes control and protection systems to ensure stable operation and protect the system from faults.

Special Features

The Rihand-Dadri HVDC system incorporates several advanced control features to enhance the reliability and performance of the integrated AC/DC bulk power transmission system. These include power modulation, frequency control, sub-synchronous resonance damping, reactive power control, and runback control. The system also provides reverse power flow capability, allowing bidirectional power transfer between the Eastern and Western parts of the Northern Grid.

Performance and Upgrades

Over the years, the Rihand-Dadri HVDC link has undergone several upgrades and refurbishments to maintain its reliability and efficiency. In 2021, Hitachi Energy successfully upgraded the Control & Protection system, valve cooling system, and Thyristor Control Units (TCUs).

The project has played an important role in evacuating power from the Rihand/Singrauli generation complex, which has a capacity of 3,000 MW. The HVDC link has been operating continuously at 1,500 MW, serving as a lifeline for power transmission in the region.

Environmental Considerations

The project has undergone environmental assessments to ensure compliance with regulations and minimize its impact on the surrounding areas. An Initial Environmental Examination (IEE) was conducted to describe the environmental conditions, potential impacts, and formulation of mitigation measures.

Fire Damage

On 24 June 1990, during the commissioning of the scheme, a complete quadrivalve of the Rihand converter station was destroyed, and the other two quadrivalves of the same pole badly damaged, by a fire which is believed to have started as a result of a loose connection on a grading capacitor. The fire was so intense that the valve hall was structurally damaged, and the affected converter was out of action for 18 months. Similar incidents on the Itaipu project in 1989 and the Sylmar Converter Station of the Pacific DC Intertie scheme in 1993 led to CIGRÉ publishing guidelines on the design of thyristor valves in order to reduce fire risks.