2011/10/28 at 7:53 am #10679adminKeymaster
Reliable HVDC Transmission by
Enhancing Sustainable Energy System
Sl no. Content details Page no. 1 Abstract 2 2 Index term 2 3 Introduction 3 4 What is HVDC 4 5 Voltage source converter 6 6 Advantage of VSC 7 7 Hydro power 9 8 Present sinario of hydropower 9 9 Future sinario of hydro power 10 10 Wind power 12 13 Present sinario o wind power 13 14 Got land VSC HVDC project 13 15 The tjaereborg project 15 16 Solar energy 17 17 Future sinario of sustainable energy sources 18 18 Conclusion 20 19 Refferances 21
Reliable HVDC Transmission by
Enhancing Sustainable Energy System
Article on the necessity of efficient electric transmission on electricity to make sustainable generation of energy possible in this respect high voltage direct current has important role.
Index Term –
sustainable energy, electric generation, electric transmission, HVDC, VSC, hydro power, wind power, solar power, cables, convertor.
Sustainable energy generation like hydro, wind, solar, wave, and etc arte normally not located close to the consumption.
This fact makes transmission of the power as most as essential as the generation.
Non sustainable ways of generating electricity does not normally have this problem. Coal can be transported by boat or train. oil is transported around the world by tankers and sorter distance by pipeline. Gas is normally transported by pipe line, but can also be transported by boat in liquid form of the sustainable only biomass can transported by
truck and train. hydro, wind, solar and wave energy have to be transmitted as electricity.
Electricity has been used for more than hundred years mainly by alternating current. However transmission of large amount of power over long distances the use of high voltage direct current .HVDC has in much case found very economical.
II.WHAT IS HVDC?
HVDC was first used commercially 50 years ago. since then a growing no of transmission has been constructed around the world/.
HVDC differ from high voltage AC current (HVAC). that the voltage not alternating 50 or 60 cycle per sec but is constant the advantage of HVDC is that long distance transmission is more efficient as there is no need the charge the capacitance of transmission line this is the alternating voltage. the drawback of HVDC is that one need more expensive terminal at the line ends.
The HVDC converter station is made up of number of equipment known from AC transmission scheme.iot also contain some special feature of which the most important one is the converter valves.
In the beginning the converters were equipped with mercury arc valve .later thyristor valve have been developed and has made the design of HVDC more flexible also increase the power that is possible to transfer.
HVDC has number of properties that make it different from AC transmission. Most important are
- The two can be connected to net works that are not synchronized or does not even have the same frequency.
- Power can be transmitted over very long distances without compensation for the reactive power.
- Reactive power is a power that does not add to the transmitted power, but is byproduct at AC transmission as the line or cable capacitance has to be charged 50 – 60 time/sac HVDC has the constant voltage it does generate reactive power. See also fig-2
- Only two conductors are needed (or even one conductor if ground or the sea is used as return) for HVDC compare two three conductor for AC.
Fig shows the power that is possible to transmit as function of the distance for AC cables of various voltage stress compare to HVDC With AC high power can be transmitted sort distance or low power long distances.
The same situation valid for lines, but here the effect is seen at longer distances.
- A. Voltage source converters
A new type of HVDC using transistors for the ac/dc conversion has been developed. By using the components that can not only switch on the current but also switch it off, making it possible to built voltage source convertors (VSC).
This type of convertors offers many advantages when it comes to transmission of power especially from sustainable energy systems.
VSC HVDC has the following additional advantages:
- Simultaneous control of both active and reactive power. THE ac – voltage can be controlled at both stations. See also figure 4.
- NO need for short circuit power for commutation. Can operate against even black networks.
- Can operate without communication between the stations
- Can operate from control the power continuously from one
- No change of voltage polarity when the power direction is changed. This makes easier to built multi terminal schemes
- Possibility to use robust and economical extruded cables for both land and sea.
- Small convertor that reduce the requirement
Figure 4 in a voltage source convertor active and reactive power can be controlled at the same time like in a synchronous convertor, but the control is much faster in the mille second range.
The by far most important of suitable energy comes from hydropower. This has been situation from the very beginning of electric generation. The result of this is that most hydropower close to consumption area is already utilized.
A. Present situation
During the last 50 year many HVDC projects utilizing convertors with thyristor valves have been constructed in order to transmit hydropower from hydro plants some of the most important these transmission are listed below.
B. Future situation
There is a tendency that new project are situated further away but also have more available power. We here talk about more than 1000 km of line transporting several thousand mega watt of power.
This is the situation in china where hydro resources are located in middle of the country and reconsumption is in east and in the south.
Another country where long distance transmission has been discussed in Brazil where the affluence of the Amazon River have grand potential of hydropower. Also here the main consumption areas are located several thousand km away.
Another country is India where large hydro resources in Assam are only possible to explore if much of power could be transmitted to other state several thousand km away.
Still another areas of interest if Africa and Congo River which the largest potential worldwide for hydropower. Also here population situation is such that very long transmission is needed if exploration should be possible.
Figure 4. The cost of transmitting 10000 MW of power over 2000 km. AC DC at various voltages is compared. All cost including cost of losses are included as we can seen 750 kV DC is the most cost efficient alternative. However this voltage level is still not in use.
IV. WIND POWER –
The second most important sustainable type where electricity is needed for transportation is wind power. Until today most wind power has been small scale generation connected to the most nearby ac grid. This constitute no problem as long as the wind power is feeder is small compare to the total power installed in the network . However , if wind shall constitute a more substantial power in the network , big wind park will have to be constructed .To construct such big part on land will both meet big opposition as well as problem to find sites with good wind condition most probablly new big site have to be constructed at shallow sites at the sea .
ASlong as the distance are a few km and the power are upto some 100 MG . AC cable connection will be sufficient but with incraesing power and distance DC connection will be more competitive . In this case HVDC with voltage source convertor mseems to be best situated for the transmission .
A . Present situation Already a couple of product have been constructed where stability of VSC HVDC in conjuction with wind power has been demonstrated
B . Gotland VSC HVDC project
The Swedish island of Gotland is situated in the middle of the Baltic sea an the island earlier got all its power though the HVDC – cables from the swedish mainland . Actually , Gotland had in 1954 the first commercial HVDCproject in the world .
As the wind conditions on the island are excallant a large number of wind power stations were built on the southern part of island . This made it possible toreduce the need for import of power However the ac – network on the island was not designed for feeding power from south to the center of island . Normally this could have been solve by constructing a new ac – line , but due to the crossing through a bird protection area it was judge to be less suitable with overhead lines . Instead of VSC HVDC system using underground extrude cables was built . This solved the problem to transmit the power and made very important contributions tostablize the voltage in the ac network on the whole island
Figure 5 . THE Gotland VSC HVDC project in the foreground . wind power in the background
Withthis reinforcement of the ac grid on the Gotland the wind power can continue to the expanded on the island . In fact , already today it happens that more power is produced by the wind mills on the island than what it consumed on the island . In fact , already today it happens that power is produced by the wind mills on the island that what is consume ( there are around 50000 inhabitants ) and the surplus is exported to the main land . so Gotland already today could be taken as an examp le of what might happens In the future more generally .
C . The Tjaereborg project
In Denmark wind power has grown more and more important over last decade . Today , Denmark is one of the country in the world with largest proportion of wind power . This has lead to problem to find new sites for new wind farms and locate them at the sea has attracted a lot of interest . Also VSC HVDC has interacted interest of several reason . Both for transmitting the power from sea to shore , but also to supply the wind generator with reactive power . Atmost wind generator assynchoronous machine they consume a lot of power and this can cause voltage stability problem in the generator end .
In the other end where the transmission line is connected to ac grid , there might also be problem in the short circuit power of that point is not very strong .
It is also favourable if one canadopt the frequency of wind generated to the speed of wind.
All this together led to the construction of the demonstration plant and the conjuction to asmall part of four wind generator . This plant has demostrated operation with variable frequency and serve as a demostrator for a bigger wind park far out at sea .
V . SOLAR ENERGY
Today solar energy generation is not explore in large scale , but the install power is growing at high rate each year . when the development of solar panels get to a certain effiency and cost , solar energy might become a very important contribution to the electricity as with other sustanable energy , suitable sites for solar plant are most certainly located in desert as shown in fig . below
Where the effiency will be best and land is not used for agricultuer , forestory or urban settlement .In this case it becomes even more important to be able to transmit a very large power of energy from site with very good sunlight to consumption areas . The desert often has a very difficult enviornment for overhead line this risk of salt cotamination of the insulator . In this case underground cables would be very advantageous as mention above , VSC HVDC development of extruded cables there possibilities to transmit vary large amout of power over long distance even to supply whole continent with power .
VI . FUTURE SCENARIO
If one look into the future fossile fuels will be more scare and more power will have to come from sustanable energy systems . this will pose new reqirement on not only generation of this power but also on transmission and storage of the energy as both wind and solar energy is not avaliable, when thye wind blows and when the sun shines and not when there is need for electricity in order to illustrate possible situation when there only sustainable generation a scenario has been made showing how Europe could be supply by electricity .in this scenario all the Europe is connected with very efficient transmission network with capability to transmit power all over Europe.the backbone of this network could be HVDC .power is supplied by hydropower (allready existing),wind power along the cost big bulk generation of solar energy from the Sahara dessert.
In the areas of sahara the energy that reach is 300 W/m2.with efficiency of 30 % and the covering 30% of the area of the solar pannels,the total installed power of Erope (700GW) need aspace of 26000 km2 .as the sahara has a surface of 10 million square km the power plant would only occ of the area.
Even if we still procedure most of our electric power by fossil fuels,there will come a day when this is no longer possible. Then sustainable energy generation will be needed.charateristics of such generation is that it is often located in very sparcly inhabitated areas and nomally very far from the place where the energy will be cosumed.here it will be very important to find efficient ways to trasport large amounts of electricity long distance.today,HVDC seems to be the best solution to this problem.
Paper from conference proceeding( published).
(1) Gunnar Asplund, kjell eriksson and kjell Svensson. “dc transmission based on voltage source convrter” GIGRE SC14 Colloquium in South africa 1997.
(2) Lars weimers.”HVDDC light – a new technology for better environment” .the IEEE Winter meeting in tampa,florida 1998.
(3) Anna – Karin Skytt , Per holmberg , Lars – Erik Juhlin , “ HVDC Ligth for connection of wind farms “ . Second international workshop on transmission network s for off shore wind farms Royal institute of technology , Stockholm ,Sweden , March 29-3-, 2001
(4) Nils horle , Asmund Maeland , kjell Eriksson , Tom Nestli, “ electrical supply for offshore installation made possible by use of VSC technology “ . Cigre 2002 Conferece , Paris , France , Aug 25 – 30 , 2002.2011/11/02 at 3:40 pm #12602LaurentParticipant
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