HVDC TRANSMISSION SYSTEMS
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02-04-2010, 03:23 PM
Electric power transmission was originally developed with direct current. The availability of transformers and the development and improvement of induction motors at the beginning of the 20th Century, led to greater appeal and use of a.c. transmission. Through research and development in Sweden at Allmana Svenska Electriska Aktiebolaget (ASEA), an improved multi-electrode grid controlled mercury arc valve for high powers and voltages was developed from 1929. Experimental plants were set up in the 1930â„¢s in Sweden and the USA to investigate the use of mercury arc valves in conversion processes for transmission and frequency changing. D.C. transmission (HVDC) now became practical when long distances were to be covered or where cables were required. The increase in need for electricity after the Second World War stimulated research, particularly in Sweden and in Russia. In 1950, a 116 km experimental transmission line was commissioned from Moscow to Kasira at 200 kV. The first commercial HVDC line built in 1954 was a 98 km submarine cable with ground return between the island of Gotland and the Swedish mainland. Thyristors were applied to d.c. transmission in the late 1960â„¢s and solid state valves became a reality. In 1969 d.c. link in Canada was awarded as the first application of solid state valves for HVDC transmission. Today, the highest functional d.c. voltage for d.c. transmission is +/- 600 kV for the 785 km transmission line. D.c. transmission is now an integral part of the delivery of electricity in many countries throughout the world.
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24-02-2011, 11:52 AM
BasisPrinciplesofHVDC.pdf (Size: 134.97 KB / Downloads: 755)
Electric power transmission was originally developed with direct current. The availability of transformers and the development and improvement of induction motors at the beginning of the 20th Century, led to greater appeal and use of a.c. transmission. Through research and development in Sweden at Allmana Svenska Electriska Aktiebolaget (ASEA), an improved multi-electrode grid controlled mercury arc valve for high powers and voltages was developed from 1929. Experimental plants were set up in the 1930’s in Sweden and the USA to investigate the use of mercury arc valves in conversion processes for transmission and frequency changing.
D.c. transmission now became practical when long distances were to be covered or where cables were required. The increase in need for electricity after the Second World War stimulated research, particularly in Sweden and in Russia. In 1950, a 116 km experimental transmission line was commissioned from Moscow to Kasira at 200 kV.
The first commercial HVDC line built in 1954 was a 98 km submarine cable with ground return between the island of Gotland and the Swedish mainland.
Thyristors were applied to d.c. transmission in the late 1960’s and solid state valves became a reality. In 1969, a contract for the Eel River d.c. link in Canada was awarded as the first application of sold state valves for HVDC transmission. Today, the highest functional d.c. voltage for d.c. transmission is +/- 600 kV for the 785 km transmission line of the Itaipu scheme in Brazil. D.c. transmission is now an integral part of the delivery of electricity in many countries throughout the world.
WHY USE DC TRANSMISSION?
The question is often asked, “Why use d.c. transmission?” One response is that losses are lower, but this is not correct. The level of losses is designed into a transmission system and is regulated by the size of conductor selected. D.c. and a.c. conductors, either as overhead transmission lines or submarine cables can have lower losses but at higher expense since the larger cross-sectional area will generally result in lower losses but cost more.
When converters are used for d.c. transmission in preference to a.c. transmission, it is generally by economic choice driven by one of the following reasons:
1. An overhead d.c. transmission line with its towers can be designed to be less costly per unit of length than an equivalent a.c. line designed to transmit the same level of electric power. However the d.c. converter stations at each end are more costly than the terminating stations of an a.c. line and so there is a breakeven distance above which the total cost of d.c. transmission is less than its a.c. transmission alternative. The d.c. transmission line can have a lower visual profile than an equivalent a.c. line and so contributes to a lower environmental impact. There are other environmental advantages to a d.c. transmission line through the electric and magnetic fields being d.c. instead of ac.
2. If transmission is by submarine or underground cable, the breakeven distance is much less than overhead transmission. It is not practical to consider a.c. cable systems exceeding 50 km but d.c. cable transmission systems are in service whose length is in the hundreds of kilometers and even distances of 600 km or greater have been considered feasible.
3. Some a.c. electric power systems are not synchronized to neighboring networks even though their physical distances between them is quite small. This occurs in Japan where half the country is a 60 hz network and the other is a 50 hz system. It is physically impossible to connect the two together by direct a.c. methods in order to exchange electric power between them. However, if a d.c. converter station is located in each system with an interconnecting d.c. link between them, it is possible to transfer the required power flow even though the a.c. systems so connected remain asynchronous.
The integral part of an HVDC power converter is the valve or valve arm. It may be noncontrollable
if constructed from one or more power diodes in series or controllable if
constructed from one or more thyristors in series. Figure 1 depicts the International
Electrotechnical Commission (IEC) graphical symbols for valves and bridges (1). The
standard bridge or converter connection is defined as a double-way connection
comprising six valves or valve arms which are connected as illustrated in Figure 2.
Electric power flowing between the HVDC valve group and the a.c. system is three
phase. When electric power flows into the d.c. valve group from the a.c. system then it is
considered a rectifier. If power flows from the d.c. valve group into the a.c. system, it is
an inverter. Each valve consists of many series connected thyristors in thyristor modules.
Figure 2 represents the electric circuit network depiction for the six pulse valve group
configuration. The six pulse valve group was usual when the valves were mercury arc.
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09-03-2011, 10:29 AM
SUNIT KUMAR SAHOO
SUNITS seminar.ppt (Size: 580.5 KB / Downloads: 485)
Why we need HVDC ?
Reactive power requirement
Short Circuit Current
Independent Control of ac system
Fast change of energy flow
Lesser Corona Loss and Radio interference
Comparison between the prices of AC & DC Transmission
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.
CONVERTER STATION EQUIPMENT
Harmonics Filtering Equipment
Reactive power compensation
• 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.
Typical tower structure and rights of way for alternative transmission system of 2000 MW capacity
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.
HVDC system requires a properly designed earth electrode at each station.
The electrode is situated at a safe distance (5 to 30 km) from the station.
The earth electrode at one of the station acts as a anode and at the other end acts as a cathode.
GTO’s have come into use.
Use of active ac and dc filters.
Advanced fully digital control systems using optical fibers.
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%.
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HVDC TRANSMISSION.pptx (Size: 1.14 MB / Downloads: 180)
HVDC transmission is widely recognized as being advantageous for long-distance, bulk power delivery, asynchronous interconnections and long submarine cable crossings. The number of HVDC project and implimentations committed or under consideration globally has increased in recent years reflecting a renewed interest in this mature technology. New converter designs have broadened the potential range of HVDC transmission to include applications for underground, offshore, economic replacement of reliability-must-run generation, and voltage stabilization.
HVDC transmission applications can be broken down into different basic categories. Although the rationale for selection of HVDC is often economic, there may be other reasons for its selection.
A. Long Distance Bulk Power Transmission
B. Cable Transmission
C. Asynchronous Ties
D. Offshore Transmission
E. Power Delivery to Large Urban Areas
LONG DISTANCE BULK POWER TRANSMISSION
HVDC transmission systems often provide a more economical alternative to ac transmission for long distance , bulk-power delivery from remote resources such as hydroelectric developments, mine-mouth power plants or large scale wind farms. Higher power transfers are possible over longer distances using fewer lines with HVDC transmission than with ac transmission. Bipolar HVDC lines are comparable to a double circuit ac line since they can operate at half power with one pole out of service but
require only one-third the insulated sets of conductors as a double circuit ac line.
Unlike the case for ac cables, there is no physical restriction limiting the distance or power level for HVDC
underground or submarine cables. Underground cables can be used on shared ROW with other utilities without impacting reliability concerns over use of common corridors. For underground or submarine cable systems there is considerable savings in installed cable costs and cost of losses when using HVDC transmission. Depending on the power level to be transmitted, these savings can offset the higher converter station costs at distances of 40 km or more.
With HVDC transmission systems, interconnections can be made between asynchronous networks for more economic or reliable system operation. The asynchronous interconnection allows interconnections of mutual benefit while providing a buffer between the two systems. Often these interconnections use back-to-back converters with no transmission line. Asynchronous HVDC links act as an effective “firewall” against propagation of cascading outages in one network from passing to another network.
Self-commutation, dynamic voltage control and black-start capability allow compact VSC HVDC transmission to serve isolated loads on islands or offshore production platforms over long distance submarine cables. This capability can eliminate the need for running expensive local generation or provide an outlet for offshore generation such as that from wind. The VSC converters can operate at variable frequency to more efficiently drive large compressor or pumping loads using high voltage motors.
POWER DELIVERY TO LARGE URBAN AREAS
Power supply for large cities depends on local generation
and power import capability. Local generation is often older and less efficient than newer units located remotely. Often, however, the older, less-efficient units located near the city center must be dispatched out-of-merit because they must be run for voltage support or reliability due to inadequate transmission. Air quality regulations may limit the availability of these units. New transmission into large cities is difficult to site due to right of way limitations and land use constraints.
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