23 Mar 2015
A substantial development of offshore wind farms is taking place in different parts of the world, yet the major issue is the technique used to transfer the energy from the wind farms to the grid. Two of the major technologies used are: High-Voltage DC known as HVDC and High-Voltage AC known as HVAC. The main concern of electrical power engineers is that the transferring of energy occurs with low losses. Taking that into consideration, the advantages and disadvantages of both technologies have been studied and compared in order to choose the most efficient scheme.
The HVDC system has been put in many power transmission projects, even though some disadvantages have been spotted. The components used in this system are as follows :
* AC and DC filters
* Converter Stations
* Smoothing reactor
* DC cable and return path
* Cooling devices
Converters in this system have been the element of inconvenience for several reasons. Some of these reasons is that converters are relatively high in price; moreover they need reactive power and have the potential to produce lots of harmonics. In addition, the HVDC system has a restricted ability when it comes to the handling of overloads. Furthermore, the most important negative aspect in HVDCs is the treatment of faults. Noting that the rated power of the recent circuit breakers tend not to be so high which in return introduces us to another problem, and this is that the current and voltage in a DC line can never traverse '0'. This however makes it tough to solve the fault, but a solution has been found and it is done by the help of the AC lines circuit breakers . On the other hand, HVDC systems are characterized by its admirable advantages. The latter, is considered to be uncomplicated when it comes to construction. This had a positive effect on the system since it allows it to be interconnected at different frequencies and causes the short circuits in the DC lines to become minimized. Moreover, there is no significant environmental effect since there will be no interference with the radio electromagnetic waves and the corona effect is supposed to be even less. Finally the DC system supplies an adaptable and a relatively fast power flow . The following figure shows the interconnection of offshore wind turbines with HVDC technology :
On the other hand, the HVAC system has almost the same advantages as the HVDC but its disadvantages are distinct. For instance, the underground HVAC cables have very low characteristic impedance and yet it needs a very high charging current. Moreover these cables are not able to pass on any load. This makes the DC cables more popular since the mentioned drawbacks are not part of their characteristics. On the other hand, the earth-return ability is available with HVDC transmission and not with the HVAC system. Major analysis has been made on both HVDC and HVAC and it was realized that distance plays a major role in the amplitude of losses. According to Brakelmann's theory, the allocation of current along the cable and the temperature it possesses play a major role in the calculation of losses. The formulae used during calculation of power losses in the submarine cables are as follows :
It was realized that the power produced by both technologies is almost the same, yet the HVDC could achieve marginally higher voltages per conductor. Moreover, the distance over which the cables are stretched has an influence on the losses. HVAC system have minimum losses along short distances ranged between 50 to 70 km, while as the distance increases above 70 km it is preferred to use HVDC. The following graph shows the technology to be used as the distance gets longer :
More factors are taken into consideration, such as the cost, consistency, quality and technical capability. Quality wise, HVDC Light technology has been introduced to the market and its advantages for connecting offshore wind farms to the grid have been remarkable. One of the main features that make the latter more suitable than HVAC is the minimized area and mass of the cables. This however permits the HVDC cables to function under maximum electric field stress. Moreover, HVDC cables are preferred to HVAC cables when it comes to installation at a very deep range, since the mass of HVAC cables are double that of the HVDC cables . Finally HVDC cables have been tested in systems of 150 KV and 400 MW were the result was positive . The following figure shows a pair of submarine and land Light cables used for offshore wind farm connections :
Building an offshore HVDC grid has been an idea that most of the European countries have been discussing. Not long ago these administrations and organisations have accepted it, yet taking into consideration the technical challenges within it. As mentioned above, the HVDC system is composed of two converter stations. The converters first used are the LCC (Line Commutated Converters), but a modern system has been introduced to the HVDC system and is called the VSC (Voltage Source Converters). The latter is considered better and more efficient than the LCC since it's made up of a smaller amount of components, the circuit is easier to analyze when faults occur, and the has lower losses. The following figure shows the difference in both circuits :
The challenge concerning these converters is that when an offshore grid has to be built both converters need to be used, since most of the submarine cables constructed are for LCC yet can function with VSC. The solution is to build a system composed of both converters and this is considered hard because of two major drawbacks: The extremely high cost of the converters and the losses. Moreover one of the tough challenges is the consistency of the system, and this leads us to the issue of the circuit breakers that have to be more efficient in order to ensure that the system is protected. Finally, the problem seems to be the funding because the technology is available [5, 6].
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