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HVDC is more economically feasible at longer distances
DC is used in extra high (EHV) and ultra high (UHV) power transmission (500,000V and up) for the following reasons: It is asynchronous. That is, becuase DC has no frequency, it can be used to connect two differnent systems without worrying about having to synchronize the systems. The DC connection is a open circuit to AC systems. This is important when it comes to system stability-having that isolation helps to separate two separate systems in the event of a electrical fault event, such as lightning or equipment failure. The DC circuit limits the fault's path, essentially protecting the unfaulted part of the system it is connected to. For long distances, it is more economical. DC transmission towers take up less space which means less land can be used for right of way. Since DC only needs two poles at a maximum (sometimes one pole is used, and the ground is used as a return), less wire is needed. Towers are also more economical since they dont' have to carry three separate wires. Less Losses. With a DC system, there are less losses-there is no reactive component to a DC circuit, so AC system related losses like capacitive, inductive and skin effect are nonexistant. Again becasue there is no AC, losses and other issues associated with electrical coupling/induced currents are not an issue. There are only resistive losses as opposed to AC, which has reactive and resistive losses. Easy to transfer large blocks of power. With DC, direction of power can be switched easier. This makes DC indispensable for areas where summer loadings and winter loadings are opposite-for example a state that is cooler in winter in one area and warmer in summer in another area. The DC circuit could be used to export power to the colder region in the winter from the warmer region, and from the cooler region to the warmer region in summer. Inverters are used to change the direction of power instantanously, as well as the amount of power transmitted. The disadvantages are: Cost. DC installations are very expensive, which is one reason why they don't really get used below transmission voltage level. A DC substation requires more equipment than an AC substation would require, partially becuase the conversion process requires equipment that will eliminate harmonics that are created. This is very inefficient for subtransmission and tranmission voltages below 500kV (500,000V). AC power is easier to step up and down. There is no such thing as a DC transformer, since transfomers require a magnetic field to work. DC current, since there is no frequency, cannot produce the induced current in the secondary coil of the transformer. Transformation is an essential part of the AC power system, so this inability is a very significant negative.
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using of DC for transmission is limited for sea
