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Sustainable Energy System and HVDC Transmission


Published on Feb 12, 2016

Abstract

Sustainable energy generation like hydro, wind, sun and waves are normally not located close to the consumption. This fact makes transmission of the power as almost as essential as the generation. On 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 shorter distances by pipeline.

Gas is normally transported by pipeline but can also be transported by boat in liquid form. Of the sustainable resources only biomass can be transported by truck or train. Hydro, wind, sun and wave energy have to be transmitted as electricity.

Electricity has been used for more than 100 years mainly by alternating current. However, transmission of large amounts of power over long distances the use of high voltage direct current; HVDC has in many cases been found very economical and efficient.

The imperative of supplying energy at reasonable costs coupled with depletion reserves of non renewable energy sources has led to the establishment of remote generating station -predominantly fossil fuel fired thermal stations at pit head. Environmental conditions also sometimes dictate the sitting of power station at remote locations. Large hydro locations are invariably at distances of hundreds of kilometers from load centers. The need to economies on costly investments in generation reserves, sharing of benefits in utilizing variability in generation mixes and load patterns have given rise to interconnection of neighboring systems and development of large power grid.

SUSTAINABLE ENERGY SYSTEMS

The three important sustainable energy are hydropower, wind power, solar energy. In this section, the present and future situations of these energies are discussed.

1. HYDROPOWER

The by far most important generation of sustainable energy comes from hydropower. This has been the situation from the very beginning of electric generation. The result of this is that most hydropower close to consumption areas is already utilized.

A. Present situation

During the last 50 years many HVDC projects utilizing converters with thyristor valves have been constructed in order to transmit power from hydro plants.

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 thousand kilometers of lines transporting several thousand megawatts of power. This is the situation in China where hydro resources are located in the middle of the country and the consumption is in the east and in the south. Another country where long distance transmission has been discussed is Brazil where the affluences of the Amazon River have grand potentials of hydropower. Also here the main consumption areas are located several thousand kilometers away.

Another country is India where large hydro resources in Assam are only possible to explore if much of the power could be transmitted to other states several thousand kilometers away. Still another area of interest is Africa and the Congo River, which has the largest potential worldwide for hydropower. Also here the population situation is such that very long transmission is needed if exploration should be possible.

WIND POWER

The second most important sustainable type of energy where electricity is needed for the transportation is wind power.

A. Present situation

Until today most wind power has been small scale generation connected to the most nearby ac grid. This constitutes no problem as long as the wind power in feed is small compared to the total power installed in the network. Already a couple of projects have been constructed where the suitability of HVDC in conjunction with wind power.

B. Future situation

However, if wind shall constitute a more substantial part of the power in the network, big wind parks will have to be constructed. To construct such big parks on land will both meet big opposition as well as problems to find sites with good wind conditions. Most Probably new big sites will have to be constructed at shallow sites at the sea. As long as the distances are a few tenths of kilometers and the powers are up to some hundred megawatts, ac cable connections will be sufficient. But with increasing power and distance, connections will be more competitive.

In this case HVDC with voltage source converters seems to be best suited for the transmission.

3. SOLAR ENERGY

Solar energy is available all over the world in plenty. It is not utilized in large scale due to high cost solar panel and less efficiency.

A. Present situation

Today solar generation is not explored in a large scale, but the installed power is growing at a high rate each year.

B. Future situation

When the development of solar panels gets to certain efficiency and cost, solar energy might become a very important contribution to the electricity generation. As with other sustainable energy, suitable sites for solar plants are most certainly located in deserts where the efficiency will be the best and the land is not used for agriculture, forestry or urban settlement. In this case it becomes even more important to be able to transmit very large power of energy from sites with very good sunlight to consumption areas. The desert often has a very difficult environment for overhead lines this risk of salt contamination of the insulators.

In this case underground cables would be very advantageous. As was mentioned above, HVDC is very suited for cable transmission on land and with a further development of extruded cables there are possibilities to transmit very large amounts of power over long distances even to supply whole continents with power.

HVDC TRANSMISSION

Remote generation and system interconnections lead to the search for efficient power transmission at increasing power levels. The increase in voltage levels is not always feasible. The problem of AC transmission system has led to the development of DC transmission. However, as generation and utilization still remain at alternating current we require converter stations at both ends.

1. Basic elements

The basic elements of HVDC transmission system are:

1. Converters

2. Inverters

Converters are situated at the sending end of the system, which converts AC to DC .the converters are static – using high power thyristors connected in series to give the required voltage ratings.

The inverters are located et the receiving end and it convert DC to AC .the physical process of conversion is such that the same station can switch from rectifier to inverter by simple control action , thus facilitating power reversal.

2. Types of DC links

The DC links are classified into three types that are defined below:

1. Monopolar link

It consists of one conductor usually negative polarity and uses ground or sea return. some times metallic return is also used.












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