![• In 1886, William Stanley, an associate of Westinghouse,
developed and tested a commercially
practical transformer and ac distribution system for 150 lamps
at Great Barrington, Massachusetts.
With the development of polyphase systems by Nikola Tesla,
the ac system became even more attractive.
• By 1888, Tesla held several patents on ac
motors, generators, transformers, and transmission systems.
Westinghouse bought the patents to these early inventions,
and they formed the basis of the present-day ac systems.
• AC vs DC [Tesla vs Edison]
• In the 1890s, there was considerable controversy over
whether the electric utility industry should be standardized on
dc or ac.
• There were passionate arguments between Edison, who
advocated dc, and Westinghouse, who favoured ac.
•](https://image.slidesharecdn.com/unit1basicsofhvdctransmission-240319071834-45023f45/85/Chaper-4-Unit-1-Basics-of-HVDC-Transmission-ppt-5-320.jpg)
Chaper 4 Unit 1 Basics of HVDC Transmission.ppt
- 1. EEU725A HVDC AND FACTS • Introduction to HVDC: • Introduction of DC Power transmission technology – Comparison of AC and DC transmission, Application and Description of DC transmission system, Planning for HVDC transmission, Modern trends in DC transmission, Types of HVDC Systems. • Analysis of HVDC Converters: • Pulse Number-Choice of converter configuration, simplified analysis of Gratez circuit, 12- pulse converter based HVDC systems and their characteristics, Control of Converters. • Harmonics and Filters: • Introduction – Generation of Harmonics, Design of AC filters and DC filters, HVDC light and HVDC PLUS (Power Universal Link), Series and Parallel operation of converters. •
- 2. EEU725A HVDC AND FACTS • Introduction to FACTS: • The concept of flexible AC transmission – reactive power control in electrical power transmission lines, uncompensated transmission line, Introduction to FACTS devices and its importance in transmission Network, Introduction to basic types of FACTS controllers, Comparison of HVDC and FACTS. • FACTS Controllers: • Principles of series and shunt compensation, description of static var compensators (SVC), thyristor controlled series compensators (TCSC), static phase shifters (SPS), static synchronous series compensator (SSSC), STATCOM.
- 3. The history of the evolution of electric power system • The commercial use of electricity began in the late 1870s when arc lamps were used for lighthouse illumination and street lighting. • First Complete Electric Power System – 1882 • The first complete electric power system (comprising a generator, cable, fuse, meter, and loads) was built by Thomas Alva Edison – the historic Pearl Street Station in New York City which began operation in September 1882. • This was a dc system consisting of a steam-engine-driven dc generator supplying power to 59 customers within an area roughly 1.5 km in radius. • The load, which consisted entirely of incandescent lamps, was supplied at • 110 V through an underground cable system.
- 4. Introduction of AC Systems – 1886 • In spite of the initial widespread use of dc systems, they were almost completely superseded by ac systems. By 1886, the limitations of dc systems were becoming increasingly apparent. They could deliver power only a short distance from the generators. • To keep transmission power losses and voltage drops to acceptable levels, voltage levels had to be high for long- distance power transmission. Such high voltages were not acceptable for generation and consumption of power; therefore, a convenient means for voltage transformation became a necessity. • The development of the transformer and ac transmission by L. Gaulard and J.D. Gibbs of Paris, France, led to ac electric power systems. George Westinghouse secured rights to these developments in the United States.
- 5. • In 1886, William Stanley, an associate of Westinghouse, developed and tested a commercially practical transformer and ac distribution system for 150 lamps at Great Barrington, Massachusetts. With the development of polyphase systems by Nikola Tesla, the ac system became even more attractive. • By 1888, Tesla held several patents on ac motors, generators, transformers, and transmission systems. Westinghouse bought the patents to these early inventions, and they formed the basis of the present-day ac systems. • AC vs DC [Tesla vs Edison] • In the 1890s, there was considerable controversy over whether the electric utility industry should be standardized on dc or ac. • There were passionate arguments between Edison, who advocated dc, and Westinghouse, who favoured ac. •
- 6. • This posed a problem for interconnection. Eventually 60 Hz was adopted as standard in North America, although many other countries use 50 Hz. • The increasing need for transmitting larger amounts of power over longer distances created an incentive to use progressively higher voltage levels. • Early AC Systems • The early ac systems used 12, 44, and 60 kV (RMS line-to-line). • This rose to 165 kV in 1922, 220 1W in 1923, 287 kV in 1935, 330 kV in 1953, and 500 kV in 1965. • Hydro Quebec energized its first 735 kV in 1966, and 765 kV was introduced in the United States in 1969. • To avoid the proliferation of an unlimited number of voltages, the industry has standardized voltage levels. • The standards are 115, 138, 161, and 230 kV for the high voltage (HV) class, and 345, 500 and 765 kV for the extra-high voltage (EHV) class.
- 7. The limitation of HVAC transmission system • Reactive Power Loss: ... • Stability: ... • Current Carrying Capacity: ... • Skin and Ferranti Effect: • Power Flow Control is not Possible: ... • Requires less space compared to ac for same voltage rating and size. ... • Ground Can be used as return conductor. ... • Less corona loss and radio interference.
- 8. Constraints of AC Transmission P distance Thermal Limit of conductors DC Line AC line Voltage and Stability Constraints SIL CONSTRAINTS FOR LONG DISTANCE TRANSMISSION
- 9. SIL -Surge Impedance Loading is the connected load in transmission line for which reactive power generated is equal to reactive power consumed i.e. the flow of reactive power is zero. There is an exact balance between reactive power generation and consumption Surge Impedance Loading is a very essential parameter when it comes to the study of power systems as it is used in the prediction of maximum loading capacity of transmission lines. following equation Capacitive VAR = Inductive VAR V2 Where, V = Phase voltage I = Line Current Xc = Capacitive reactance per phase XL = Inductive reactance per phase Upon simplifying Where,f = Frequency of the system L = Inductance per unit length of the line l = Length of the line Hence we get, V/I=Sq Root L/C=Zs
- 10. • Reactive Power Loss:- inductances and the capacitance in the transmission lines and also there are so, many elements that is, corresponding to the reactive power loss will be one of the concern in AC system. • The Ferranti effect if the system is system is lightly loaded or unloaded then receiving end voltage is higher than the sending end voltage, to control the Ferranti effect We normally use the reactors. We have the three types of reactors one is your line reactors, bus reactors and the tertiary reactors, • Skin effect • if the two conductors are there and the AC current is flowing. So, due to the effect of current and flux in first coil it will be affecting another coil and thereby; it will change the reactance and the impedance of the line.
- 11. • The power flow control; • the actual AC system, it is not possible to control, it depends upon the current will follow depending upon the impedance seen by the current • Stability the stability is one of the concerns in your AC system, if your system is highly loaded though that system is more prone to the instability. • Current carrying capability 1. cannot have long cables or long transmission lines due to the huge charging, and especially in the extra high voltage system 2. cannot load the line fully to their thermal limit, 3. cannot have a longer distance cable. ( Due to the excessive charging it is not possible to have a long distance cable and so, that we have to go for the DC cables in that cases the skin effect and the Ferranti effect is also prominent in the AC).
- 12. Why HVDC ? • Direct current : Roll along the line ; opposing force friction (electrical resistance ) • AC current will struggle against inertia in the line (100times/sec)- current inertia – inductance-reactive power
- 13. Major advantage of HVDC In the HVDC transmission system all these four problems are eliminated, and that becomes advantage of the DC transmission system means, • There is no reactive power loss at all as reactive L and C has no impact on that then • There is no stability concern, because there is no Power angle delta. •The stability concern does not arise and thereby we can load our transmission lines up to it is thermal limit.
- 14. Lower losses explained ~ AC Load Generator L L L L R R R R C C C Inductors (L) counteract changes of the line current Inductive losses Capacitors (C) counteract changes of the line voltage Capacitive losses Current and voltage changes polarity 50 or 60 times per second ! Resistors cause resistive losses and can not be avoided DC = Direct Current flows without any changes in Current and Voltage thereby eliminating the Reactive Losses The ideal world Reality Transmission line equivalent, simplified
- 15. HVDC Transmission Systems – 1950s • With the development of mercury arc valves in the early 1950s, high voltage dc (HVDC) transmission systems became economical in special situations. • The HVDC transmission is attractive for transmission of large blocks of power over long distances. The cross-over point beyond which dc transmission may become a competitive alternative to ac transmission is around 500 km for overhead lines and 50 km for underground or submarine cables.
- 16. HVDC Transmission System DC generation has problem of commutation and insulation, so HVDC cannot be generate. Also, voltage transformation is not possible with ease and efficiency in DC by transformer like device. In AC form electricity can be generated and step up by transformers. For HVDC transmission it is converted into DC with the help of rectifier. The DC power will flow through the overhead lines. At the user end, this DC has to be converted into AC by an inverter placed at the receiving end. Then this AC is stepped down to safe utility voltage using transformers.
- 17. Milestones in HVDC • The first modern commercial application of HVDC transmission occurred in 1954 when the Swedish mainland and the island of Gotland were interconnected by a 96 km submarine cable. • With the advent of thyristor valve converters, HVDC transmission became even more attractive. The first application of an HVDC system using thyristor valves was at Eel River in 1972 – a back-to-back scheme providing an asynchronous tie between the power systems of Quebec and New Brunswick.
- 18. • With the cost and size of conversion equipment decreasing and its reliability increasing, there has been a steady increase in the use of HVDC transmission. • Interconnection of neighbouring utilities usually leads to improved system security and economy of operation. Improved security results from the mutual emergency assistance that the utilities can provide. Improved economy results from the need for less generating reserve capacity on each system.
- 19. • In addition, the interconnection permits the utilities to make economy transfers and thus take advantage of the most economical sources of power. • These benefits have been recognized from the beginning and interconnections continue to grow. • Almost all the utilities in the United States and Canada are now part of one interconnected system. The result is a very large system of enormous complexity. • The design of such a system and its secure operation are indeed challenging problems.
- 20. HISTORICAL BACKGROUND ☺1880- DC at low voltage levels (Thomas Alva Edison ) ☺AC system- Higher voltage levels (Nikola Tesla) ☺“war of currents” ☺1882-1930- Thury systems ☺1901-Hewitt’s mercury-vapour rectifier (HVDC Born) ☺1929- Uno Lamm’s mercury arc valves (Father of HVDC) ☺1945 – Commercial HVDC system in Berlin ☺1954 –First commercial HVDC( 96 km sea cable, 20 MW, Sweden mainland and the island of Gotland) ☺1960- Thyristor based valve technology ☺1967 –First tested in the Gotland transmission ☺1972 –Introduced on a larger scale in Canada(320 MW)
- 21. HISTORICAL BACKGROUND(CONT..) ☺ First microcomputer based control equipment for HVDC in 1979 ☺ Highest DC transmission voltage (+ 600kV, 3150MW) in Itaipu, Brazil, 1984 ☺ First DC Active filter in1994 ☺ HVDC 2000 developed by M/S. ABB-1995 ☺ First CCC in Argentina – Brazil inter connection in 1998 ☺ First VSC for transmission in Gotland Sweden in 1999 ☺ +800kV,6400MW UHVDC Xiangjiaba-Shanghai-2010 *REAL HISTORY
- 24. Advantages of HVDC 1. The HVDC link is a asynchronous connection between two AC stations i.e., the transmission of power is independent of sending or receiving end frequencies. Hence, it allows power transmission between AC networks with different frequencies or networks, which cannot be synchronized, for other reasons. 2. Inductive and capacitive parameters do not limit the transmission capacity or the maximum length of a DC overhead line or cable. 3. For a long cable connection, e.g. beyond 40 km, HVDC will in most cases offer the only technical solution because of the high charging current of an AC cable. This is of particular interest for transmission across open sea or into large cities where a DC cable may provide the only possible solution. 4. It provides accurate, efficient and fast control of the active power flow. 5. A lesser number of conductors and insulators are required thereby reducing the cost and losses of the overall system.
- 25. 6. It requires less phase to phase and ground to ground clearance. So their towers are less costly and cheaper. 7. An HVDC system does not contribute to the short circuit current of the interconnected AC system. Lesser corona loss is less as compared to HVAC transmission lines of similar power. 8. The bipolar HVDC system uses earth return. If any fault occurs in one pole, the other pole with ‘earth returns’ behaves like an independent circuit. This results in a more flexible system. 9. It does not generate or absorb any reactive power. So, there is no need for reactive power compensation. 10. The conductor cross section is fully utilized because there is no skin effect and proximity effect. 11. Fast modulation of DC transmission power can be used to damp power oscillations in an AC grid and thus improve the system stability .
- 26. * RUN UP FUNCTION
- 27. Economics of Long Distance HVDC • Assume: same insulator characteristics for AC and DC and based on peak voltage, and AC line operating at the same current level (!) 2 * V * I <> √3 * V * I * cos f For a given power: DC requires less ROW, cheaper towers, less conductor, insulator costs. but Terminal Equipment is costly • Operational Costs : losses : AC > DC
- 28. DC terminal equipment are very costly compare to AC stations. The DC line cost and loss cost curve is not as steep as the AC curve because of considerably lower line costs per kilometer. For long AC lines the cost of intermediate reactive power compensation has to be taken into account. The break-even distance is in the range of 500 to 800 km depending on a number of technical and commercial factors.
- 29. 29 General Cost Structure of HVDC Terminal Station 20 % Valves 16 % Xmers 5 % F&I 10 % Engg. 8 % E.T.C. 10 % Other Eqpt. 7 % Control system 10 % AC Filter 14 % Civil
- 30. Bulk power transmission with lower line losses as no reactive power transfer takes place Connect two asynchronous system. Provides system stability. Controllability of power flow at high speed. Additional flexibility in grid operation Firewall against grid disturbances 30 ADVANTAGES OF HVDC TRANSMISSION
- 31. ADVANTAGES OF HVDC (CONTD..) • No skin/proximity effect • Distance is not limited by stability point of view • Corona effect less significant • Environmental benefits • Less right of way for transmission lines • No contribution to short circuit level of the ac bus connected.
- 32. ADVANTAGES OF HVDC (CONTD..) Exact power flow control Interconnected systems maintain their autonomy Disturbances in one system are not propagated to the other Efficient use of generating capacity Stability control
- 33. Lower losses HVDC 2x500 kV
- 34. Assuptions for a comparison : - Total distance: 800 km - Span length: 500 m Number of towers: Average weight: Total weight of steel: 765 kV AC Transmission Line Three Lines 1600 pcs 7500 kg 3 x 1600 x 7500 = 36.000 tons ± 600 kV DC Transmission Line Two Bipoles 1600 pcs 5000 kg 2 x 1600 x 5000 = 16.000 tons DC towers versus AC towers Less than half the amount of steel !
- 35. Right Of Way (ROW) DC HVDC HVAC with FACTS Conventional HVAC
- 36. 400 MW AC 2000 MW DC Comparison of AC & DC Transmission Line Corridor
- 37. AC transmission line require large corridors DC line transmitting as much power requires fewer towers HVDC conserves forests and saves land Transmission line corridor with HVDC Cable HVDC cables
- 38. Long Distance Transmission : AC vs DC • DC : Power Flow is controlled (modulation for stability enhancement, fault current limiting in DC lines is also possible) • Ground return possible in DC due to relatively low ground impedance for extended periods. Buried metallic structures may pose problems due to corrosion. • Transformation of voltage level for utilization of not possible without converter station.
- 39. Assuptions for a comparison : - Total distance: 800 km - Span length: 500 m Number of towers: Average weight: Total weight of steel: 765 kV AC Transmission Line Three Lines 1600 pcs 7500 kg 3 x 1600 x 7500 = 36.000 tons ± 600 kV DC Transmission Line Two Bipoles 1600 pcs 5000 kg 2 x 1600 x 5000 = 16.000 tons DC towers versus AC towers Less than half the amount of steel !
- 40. Investment Costs Distance AC Terminal costs Total AC cost DC terminal Costs Total DC Cost Variables - Cost of Land - Cost of Materials - Cost of Labour - Time to Market Permits etc. Investment costs versus distance Critical Distance
- 41. Limitations of HVDC • High cost-Conversion equipments • Converter Station Size- Large • Need of ancillary components- Filters,PE,Thermal M.S. • High cost of DC circuit breakers • Inability to use transformers to change voltage levels • Generation of harmonics • Complexity of control • Link charging problem during grid failure. • SCR must be higher than 3. • Converters consumes reactive power
- 42. Limitation of HVDC transmission: 1. Converter stations needed to connect DC link to AC power grids are very expensive, more complex and have small overload capacity. 2. In contrast to AC systems, designing and operating multi-terminal HVDC systems is complex. 3. Converter substations generate current and voltage harmonics, consume reactive power. As a result, it is necessary to install expensive filter- compensation units and reactive power compensation units. 4. Grounding HVDC transmission involves a complex and difficult installation, as it is necessary to construct a reliable and permanent contact to the Earth for proper operation and to eliminate the possible creation of a dangerous “step voltage.” 5. Converters consumes reactive power
- 44. APPLICATIONS OF HVDC • Long undersea cable links (> 50 km) • Long overhead lines (> 500 km) • Interconnection of different grids or networks • Combinations of the above • Increasing existing grid utilization. • Integration of generation( conventional/non- conventional)
- 45. U1 sin(a1) U2 sin(a2) X (~ distance) ) sin( 2 1 2 1 a a X U U P 30 2 1 a a (to maintain transient stability) HVDC U1 sin(a1) U2 sin(a2) Power flow independent from system angles DC DC I U P N DC I I 0 HVAC Interconnection with HVDC
- 47. AC transmission principles P E 1 E 2 X sin d X ~ ~ E1 d = E2 0 Power Direction R Ud2 Ud1 E1 d P Ud1 (Ud1 -Ud2 ) R = E2 0 ~ ~ DC transmission principles HVDC Control
- 49. •High voltage direct current (HVDC) power systems use High voltage D.C. (500KV or 800KV) for transmission of bulk power over long distances. •HVDC transmission systems is more costly, but it gives improved efficiency, stability, reliability, and transmission capacity. •The transmission of electricity in the form of HVDC over long distances by means of submarine cables or overhead transmission line is preferred over EHVAC transmission. HVDC has advantage of cost, losses and many other factors. •The high-capacity (800-kV, 6,000-MW) HVDC bipole line is from Bishwanath Chariali in Assam to Agra in Uttar Pradesh through Alipurduar in West Bengal. •The high-voltage corridor would facilitate transfer of 24,000 MW from future generation projects in the north-eastern region and Bhutan to the northern states. •This line is built by Power Grid Corporation of India at an investment of Rs 12,000 crore