1. HVDC TRANSMISSION SYSTEM

2. CONTENTS: HISTORY. INTRODUCTION. WHY PREFERS. HOW DOES IT WORKS. ADVANTAGES/DISADVANTAGES HVDC FOR RENEWABLE SOURCES. CONCLUSION 3. History. Firstly HVDC b/w Swedish and Gotland in 1954. In North America, total HVDC transmission capacity in 1987 was 14,000 MW. 4. INTRODUCTION HVDC stands for. for long distance transmission. requires converter stations at each end of the line. can interconnect diff power systems( i.e 50 Hz and 60 Hz) 5. HVAC ADVANTAGES Voltage transformation. Easy conversion into mechanical. energy and vice versa. LIMITATIONS Long distance transmission Difficult to use cables, already at 100 km high reactive power consumption 6. NEEDS OF HVDC The losses which occurs in the systems are at all the stages i.e. , at generation , transmission & distribution level. The losses at transmission level can be greatly reduced by HVDC transmission. 7. WHY TO PREFER HVDC THAN HVAC? Long distance transmission 5 times more energy transmits than AC(same lines) Less losses (no inductance, capacitance). Cost of transmission medium & land is low. Maintenance & operation cost is low. Initial cost is high but overall cost is low than ac. 8. COMPARISON B/W AC & DC TRANSMISSION COST. 9. HVDC TECHNOLOGY If DC is required to be used for transmission and since our primary source of power is A.C, The following are three basic steps:- 1. Convert AC into DC (rectifier) 2. Transmit DC 3. Convert DC into AC ( inverter) 10. HOW IT WORKS ? 11. COMPONENTS OF HVDC TRANSMISSION SYSTEMS Converters Smoothing reactors Harmonic filters Reactive power supplies Electrodes DC lines AC circuit breakers 12. CONVERTERS Perform AC/DC (rectifier) and DC/AC (inverter) conversion consist of bridges and transformers DC SMOOTHING REACTORS Decrease harmonics in voltages and currents in DC lines. Prevent current from being discontinuous. AC HARMONIC FILTERS Used to reduce harmonics (in voltages and currents) caused by converters which generate harmonics, Hence prevent from interference with any comm system. 13. REACTIVE POWER SUPPLIES Converter may consume reactive pwr of abt 50% / more of active pwr. Reactive power is, therefore, provided near converter. For a strong AC power system, this reactive power is provided by a shunt capacitor. ELECTRODES Used to provide connection to the earth for neutral. DC LINES They may be overhead lines or cables. DC lines are very similar to AC lines. CIRCUIT BREAKERS Used to break cct if fault occurs in the transformer and for taking the DC link out of service. 14. 6 PULSE RECTIFIER 15. 6 PULSE RECTIFIER WAVEFORM 16. 6 PULSE RECTIFIER 17. INVERTER OPERATION. 18. CONTROL OF HVDC SYSTEMS Objectives of Control Efficient and stable operation. Maximum flexibility of power control without compromising the safety of equipment. Implementation and their performance during normal and abnormal system conditions. 19. BASIC MEANS OF CONTROL This can be accomplished by: Controlling firing angles of the rectifier and inverter. Controlling alpha is done by amount of gate current. Lesser the angle, more stable the signal. 20. BASIC PRINCIPLES OF CONTROL Direct current from the rectifier to the inverter Power at the rectifier terminal Power at the inverter terminal cilcr doidor d RRR VV I coscos ddrdr IVP 2 dLdrddidi IRPIVP Schematic diagram of control 21. POWER FLOW DIRECTION Decrease voltage at station B or increase voltage at station A, power flows from A to B. 22. Power reversal is obtained by reversal of polarity of both direction. 23. APPLICATION BASED HVDC TRANSMISSION TYPES Unique solution to connect asynchronous systems or grids with different frequencies. 24. Most economical solution to transmit electrical energy over distance more than 600km. 25. Alternative to submarine transmission, also economical for shorter distances i.e 10 km or more. 26. FOR 10 KM 27. Types of DC links Monopolar Bipolar Homopolar 28. MONOPOLAR LINKS It uses one conductor . Return path by ground/water. Due to ive polarity, no corona effect occurs. 29. BIPOLAR LINKS Has two conductors. Junction b/w the conductors is grounded. Can carry half the rated load, if fault occurs in one pole. 30. HOMOPOLAR LINKS Has two conductors polarity( usually negative.) The return path for such a system is through ground. 31. ADVANTAGES 32. TECHNICAL ADVANTAGES System stability less Corona Loss and no skin effect Greater Reliability. 33. ECONOMIC ADVANTAGES Trans Lines => less lines & less meterial required, cheaper. Towers => narrower, simpler and cheaper Line losses => less increases efficiency. Earlier lines can be used. 34. DISADVANTAGES Power loss in conversion, switching and control. Expensive inverters. Per kilometer cost reduces if lines are of fairly of large distances. 35. HVDC FOR RENEWABLE ENERGY SOURCES. 36. EXAMPLES OF PROJECTS 1) Gotland HVDC Capacity: 50 MW Length: 70 km In operation since 1999 Requirements additional wind power, 90 MW minimized environmental impact 37. GOTLAND PROJECT 38. HVDC PLUS Known as IGBT TECHNOLOGY IGBT can be turned on and off in controlled manner. Switching up to 200 levels making it efficient, giving more smooth ac signal. 39. HVDC PLUS TECHNOLOGY. 40. HVDC IN PAK 1000 MW from Taftan to Quetta through a 500 KV,HVDC transmission line, under discussion (from IRAN). Proposed 500 MW from India through a 400 KV AC transmission line and a back-to- back HVDC converter in Pakistan, estimated construction time 24-30 months 41. CONCLUSION Recent studies indicate that HVDC systems are very reliable. Very large investments in e.g in China and India shows that high-voltage direct current will very important in the future, especially in big, new- industries countries. 42. Thank you!