I was watching a YT video recently where they said something along the lines of “AC current is too inefficient for long distance power transmission, so xyz project needs DC power” and then moved on. I (from my vague memories 20 years ago of school) thought that AC had less loss and inefficiencies. What am I missing/what did I tune out in school?
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Transmitting electricity at higher voltages is desirable, because it reduces losses due to electrical resistance. AC can have voltage converted with very efficient, very reliable and old technology (transformers).
AC has some disadvantages though.
AC connections have to be synchronised, and have to be kept synchronised otherwise any power lines connecting them will disconnect or be damaged. This requires a lot of planning and organisation and lots of powerlines. If you just need 1 or 2 power lines going from 1 grid region to another, DC has the advantage that it doesn’t have to be synchronised.
AC voltage is constantly changing – it goes up, peaks, goes down again, reverses, increases up to a reverse peak, goes back down again, reverses, etc. So, on average, the voltage is less than the peak. The same is true of the current. However, the wiring, transformers, insulators, etc. all have to be designed to be able to cope with the peak voltage. Because DC always operates at the peak, the resistive losses for the same maximum voltage are reduced by 50%.
AC power lines suffer from inductive and capacitative losses. These require expensive compensation equipment. Underground/underwater cables experience extreme capacitative losses – these losses increase as voltage increases. At 500 kV, even a 10 mile underground cable requires extreme amounts of compensation – which makes it just about viable under a city where you can tap off and install compensators every 2 or 3 miles. If you want a 20 mile underwater cable – that’s just not happening, unless you drop the voltage a lot (which drastically increases resistive losses).
The problem with DC is that the conversion to DC is incredibly expensive and also suffers some losses. However, if you have a very long line (e.g. 1000 miles) then the cost and losses of the converter stations is outweighed by the cost and efficiency savings of the line itself. If you have an underwater connection, then the breakeven point is much shorter (10-20 miles). If you need to connect to unsynchronised power grids, then a DC connection is a practical choice. You just build two converter stations in the same building and connect them with a 6 foot DC power line .
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