Hi voltage power transmission

[thinfourth2]

Way back when i was at school i was taught that higher voltage is better for moving power.

To make this easy we use AC

Today i heard that they re now using high voltage DC on subsea lines from wind farms


So without the normal wind power will destroy everything

Why are they using DC instead of AC

[anonymous-user]

thinfourth2 said:

Way back when i was at school i was taught that higher voltage is better for moving power.

To make this easy we use AC

Today i heard that they re now using high voltage DC on subsea lines from wind farms


So without the normal wind power will destroy everything

Why are they using DC instead of AC

AC is easy when it is being generated by a "constant speed" source. i.e. you wind you generators and operate then at a nice constant fixed speed to spit out 50Hz AC. Everyone happy.


Now with wind power, the speed of the turbine and hence the generator depends upon the prevaling wind speed. Ah, now we have a problem. We either need an expensive and lossy varriable ratio gearbox, or we just rectify the output of the generator to DC, shove it around a bit as DC, collect it together inone place and use a large inverter to spit out our nice 50Hz AC waveform for dropping onto the Grid.

For efficiency reasons just about every windturbine over a couple of hundred watts will be using an actively commutated motor (rather than passively rectified (cheap but lossy)) simply deciding on a nice middling DC voltage suits all the changing conditions of wind generation.


Fundatentally, the efficiency of "high voltage" is actually one of "low current". Where wire and transmission systems have resisitive losses, and the power lost to heat is IsquaredR. So as current goes up your losses go up as the square. For a given power, the lowest current is achieved with the highest voltage. The reason for AC is really to enable cheap and simple passive transformers to be used to, er, transform between various voltages as required. DC voltage level transfers are much more difficult to do. So apure DC system has to compromise on it's peak transmission voltage, to a level that is still usable at the comsumer end (because no easy DC transformer exists), whereas an AC systen distribute at say 400KV, and then step down in stages to the 240V we use domestically.

[thinfourth2]

Max_Torque said:

Fundatentally, the efficiency of "high voltage" is actually one of "low current". Where wire and transmission systems have resisitive losses, and the power lost to heat is IsquaredR. So as current goes up your losses go up as the square. For a given power, the lowest current is achieved with the highest voltage. The reason for AC is really to enable cheap and simple passive transformers to be used to, er, transform between various voltages as required. DC voltage level transfers are much more difficult to do. So apure DC system has to compromise on it's peak transmission voltage, to a level that is still usable at the comsumer end (because no easy DC transformer exists), whereas an AC systen distribute at say 400KV, and then step down in stages to the 240V we use domestically.

So they are using DC as AC is cheaper and easier

[anonymous-user]

thinfourth2 said:

Max_Torque said:

Fundatentally, the efficiency of "high voltage" is actually one of "low current". Where wire and transmission systems have resisitive losses, and the power lost to heat is IsquaredR. So as current goes up your losses go up as the square. For a given power, the lowest current is achieved with the highest voltage. The reason for AC is really to enable cheap and simple passive transformers to be used to, er, transform between various voltages as required. DC voltage level transfers are much more difficult to do. So apure DC system has to compromise on it's peak transmission voltage, to a level that is still usable at the comsumer end (because no easy DC transformer exists), whereas an AC systen distribute at say 400KV, and then step down in stages to the 240V we use domestically.

So they are using DC as AC is cheaper and easier

They are using DC because that way the active rectifiers required for the variable speed wind driven generators are much much cheaper.

it goes:

varriable frequency AC - fixed DC - fixed frequency AC

with the DC bit just linking up the multiple generators which each have a dedicated recitifer (because they all will be turning, and hence commutating, at a slightly different speed). Then you jam all the dc bits together (same voltage so current is just added) and use one single socking great inverter to churn out your fixed frequency AC suitable for bashing back into the Grid.

[Flibble]

DC is also much cheaper to transmit long distance due to capacitive losses over long cables. This is particularly an issue for subsea cables as the water makes for much greater capacitive losses than air would.

Essentially any cable has a certain capacitance that rises as the length increases. With AC every time the waveform goes through zero and then back to peak you recharge the capacitor, losing power. With DC you only do it once as the cable is energised. For long underwater cables the losses are so great that converting AC to DC and transmitting then converting back to AC actually uses less power overall than transmitting AC.

Also an issue is that AC cables can't use large conductors very efficiently due to the skin effect, whereas DC cables can, making cable construction simpler.

[Wild Rumpus]

AC generation has to by synchronous (ie, all in phase and at the same speed). If you hook up a generator that is running at a different speed or out of phase with the main system then it won't work. With large steam-turbine sets it is pretty easy to get them running at exactly the same speed. It isn't easy when you are using an unpredicatable power source like the wind, hence the use of DC. Also, as already stated above, DC is beneficial for underground/undersea cable power transmission as it does not suffer from the same capacitance issues as DC.

[The Black Flash]

Every day's a school day. That's why I love PH

[FastNReliable]

HVDC has been used for bulk power transmission between countries for sometime. There are a number of current projects aiming to provide greater inter-connectivity offshore windfarms are amongst these. It's an interesting subject, power transmission is essentially all about reducing losses and providing stability.

The advantages of HVDC incluse:
It's more efficient when transmitting power over long distances
It requires fewer conductors than three phase HVAC
It can use the ground as a return conductor
2no. HVDC conductors provide a certain level of redundancy e.g. if one conductor fails the link can still operate at 1/2 power. HVAC requires 6no conductors to provide redundancy
HVAC typically uses more than 2no conductors/phase in order to reduce losses.
There are lower losses in HVDC compared to HVAC for the reasons already mentioned in this thread
HVDC is not frequency dependent therefore systems can be linked more readily (HVDC link to France, Northern Ireland
HVDC has no impact on Prospective Short Circuit levels so there is no need to upgrade other parts of the network.

There's a good Wikipedia page on HVDC links if you're interested.