21-04-2012, 12:52 PM
ADVANCED HVDC TECHNOLOGIES
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The HVDC Technology:
The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to DC (rectifier) at the transmitting end and from DC to AC (inverter) at the receiving end. There are three ways of achieving conversion:
Natural Commutated Converters.
Capacitor Commutated Converters (CCC)
Forced Commutated Converters.
Transmission medium: For bulk power transmission over land, the most frequent transmission medium used is the overhead line. This overhead line is normally bipolar, i.e. two conductors with different polarity. HVDC cables are normally used for submarine transmission. The most common types of cables are the solid and the oil-filled ones. The solid type is in many cases the most economic one. Its insulation consists of paper tapes impregnated with high viscosity oil. No length limitation exists for this type and designs are today available for depths of about 1000 m. The self-contained oil-filled cable is completely filled with low viscosity oil and always works under pressure. The maximum length for this cable type seems to be around 60 km.
The development of new power cable technologies has accelerated in recent years and today a new HVDC cable is available for HVDC underground OR submarine power transmission. The new HVDC cable is made of extruded polyethylene, and is used VSC based HVDC systems.
HVDC in the new Electrical Industry:
The question is often asked to when HVDC transmission should be chosen over an AC system. In the past, conventions were that HVDC was chosen when:
Large amounts of power (>500MW) needed to be transmitted over long distance (>500km);
Transmitting power under water;
Interconnecting two AC networks in an asynchronous manner.
HVDC systems remain the best economical and environmentally friendly option for the above conventional applications.
Rihand-Delhi HVDC Transmission:
National Thermal Power Corporation Limited built a 3000 MW coal-based thermal power station in the Sonebhadra District of Uttar Pradesh State. Part of the power from the Rihand complex is carried by the Rihand-Delhi HVDC transmission link, which has a rated capacity of 1500mw at + 500kv DC. Some of the power is transmitted via the existing parallel 400kv AC lines.
ADVANCED TECHNOLOGIES IN HVDC SYSTEMS:
The Electrical Power Research Institute (EPRI) continues to play a vital leadership role in the theoretical and experimental fronts in HVDC, AC/DC conversion equipment, and operation of HVDC systems. The EPRI High Voltage Laboratory in Lenox is a unique research and testing resource available to EPRI members. Lenox Laboratory has conducted pioneering research for a half a century, first under the direction of General Electrical and later a dedicated EPRI center.
Conclusion:
India has been a pioneer developer of HVDC since 1990 when the 1000 mw Rihand - Delhi line was commissioned in UP. Since then many 500 mw lines have come up. The 2000 mw Talcher - Kolar link is the biggest so far and spans four states: Orissa, Andhra Pradesh, Tamil Nadu and Karnataka. The project cost Rs.700 crores and was executed by Indians. The commissioning of the 200 MW, 200 KV National HVDC project has linked the 196 km. DC transmission line between Barsoor in Chhatisgarh and Lower Sileru in Andhra Pradesh.These facts should give us a measure of the little-known developmental works of very high calibre that are going on in India right now. We should be justly proud of this achievement