Seminar on H I G H V O LTA G E D . C . T R A N S M I S S I O N S Y S T E M www.fakengineer.com WHAT IS HVDC ? In a HVDC (High voltage D.C.) transmission system, electricity is taken from an AC power network, converted to DC in a converter station and transmitted to the receiving point by a transmission line (overhead) or cable(underground). It is then converted back to AC in another converter station and injected into the receiving AC network. HVDC enables the power flow to be controlled rapidly and accurately, and improves the performance, efficiency and economy of the connected AC networks. POINTS TO BE DISCUSSED Introduction. History of HVDC. HVDC goes global. India adopts HVDC. Why HVDC ? Advantages of HVDC over HVAC: • • • • • • • Disadvantages of HVDC. Cost of HVDC. Components of HVDC. Types of HVDC links. Application of HVDC. Recent advances in HVDC technology. Conclusion. INTRODUCTION • Now a days large blocks of power are needed to be transmitted over very long distances. •There arises some technical problems of transmitting power to such a long distance using A.C. •In the view of the drawbacks of a.c system the HVDC transmission has come into picture. •It is useful for bulk power transmission and interconnecting incompatible grids. History of HVDC Firstly concept was developed by a swiss engineer ‘Rene Thury’ during the end of 19th century. Earlier mercury-arc valves were used in Converters which later get replaced by Thyristor-valves. In 1941 a 60MW 115km ‘Elbe-project’ was designed in Berlin(Germany),but couldn’t be completed. Above equipments were moved to Soviet Union & in 1951, first HVDC link was established between Moscow and Kashira. However in 1954 first successful HVDC 20MW 96km commercial installation was done between the Gotland island and Sweden. This transformer converted the first HVDC transmission in Gotland. This transformer converted the first HVDC transmission in gotland HVDC goes Global Currently there are some more than 100 of HVDC systems across the whole world ,out of which 6 are in India. Some of them are as follows: -Between Sweden & Denmark. -Between south & north islands of New Zealand. -Between Italy & Sardinia. -Between Los Angeles & Oregon. -Between north African deserts & central Europe. -Between UK & western Europe. Proposal of a.c transmission across the English channel was abandoned and get replaced by HVDC system HVDC IN INDIA In India presently 6 HVDC systems are in operation. India- Pioneer developer of HVDC since 1990 with the first HVDC set up of 1000MW 814Km Rihand-Dadri line between U.P & Rajasthan. 2000MW 1440Km Talcher-Kolar link is biggest so far connecting four states-orissa,andhra pradesh,tamilnadu and karnataka. In march 2000,Power-grid(largest power transmission utility of India) awarded a new long distance HVDC transmission project to SIEMENS,Germany. More greater plans are cooking to add 1lakh MW of power by 2012 in which HVDC will play vital role to transmit these powers to all corners of India. Main data Commissioning year: 1990 Power rating: 1 500 MW No. of poles: 2 AC voltage: 400 kV (both ends) DC voltage: ±500 kV Length of overhead DC 814 km line: Main reason for Long distance, network choosing HVDC: stability FACTS & FIGURES Data of Rihand-Dadri line Data of Talcher-Kolar line 14th Feb 2003 1500MW Commissioned year No. of poles 2 Power rating 2000MW AC voltage 400KV(both ends) No. of poles 2 DC voltage 500KV No. of towers 5651 Length of overhead DC line 814Km Amount of steel 10,000 metric tone Amount of cement 80,000 metric tone Length of line 1400Km Cost of project Rs.700 crore Commissioned year 1990 Power rating Reason for choosing Long distance, HVDC Network stability. Comparison between the prices of AC & DC Transmission ADVANTAGES Advantages of HVDC can be classified as: Advantages of dc transmission Technical Advantages Economic Advantages Technical Advantages Reactive power requirement System stability Short Circuit Current Independent Control of ac system Fast change of energy flow Lesser Corona Loss and Radio interference Greater Reliability. •No limits in transmitted distance •Direction of power flow can be changed very quickly Economic advantages •DC lines and cables are cheaper than ac lines or cables. •The towers of the dc lines are narrower, simpler and cheaper compared to the towers of the ac lines. •Line losses in a dc line are lower than the losses in an ac lines. Types of DC links Monopolar Homopolar Bipolar Monopolar link Bipolar Link Incorporating HVDC into AC systems Two terminal DC link point to point transmission. Back to Back DC link DC line in Parallel with AC link. Multi-Terminal DC link. GROUND RETURN • Most dc transmission lines use ground return for reasons of economy and reliability Ground return are used by the monopolar and the bipolar link for carrying the return current. The ground path has a low resistance and, therefore low power loss as compared to a metallic conductor path provided the ground electrodes are properly designed. The resistance of the ground path is independent of the depth of the line. CONVERTER STATION EQUIPMENT Thyristor valves Converter Transformer DC Reactor Harmonics Filtering Equipment Control Equipment Reactive power compensation PROBLEMS The Design of grounding electrodes for low cost of installation and maintenance Location and screening of electrodes so that ground currents cause negligible electrolytic corrosion of buried and immersed metallic structures. ADVANCES RECENT GTO’s have come into use. Use of active ac and dc filters. Advanced fully digital control systems using optical fibers. CONCLUSION Recent studies indicate that HVDC systems are very reliable. The data collected from 31 utilities says that forced unavailability of energy due to the converter station is 1.62% The scheduled unavailability of energy is about 5.39%.