# Ehv dc transmission

1. Oct 14, 2011

i have read that in high voltage transmission line dc is preffered on ac and i know all advantages and disadvantages but one main disadvantage that i cannot find was I^2*R losses which was the main reason why we use ac on the first place can someone please explain that?

2. Oct 14, 2011

### uart

You're mistaken, I^2*R losses are much the same for both AC and DC. Reducing I^2R losses is the reason for going to higher voltages in either case (both AC and DC).

I^2R losses are "series" losses. These losses decrease with the use of higher voltage due to the fact that you need less current for the same amount of power.

It's parallel losses that increase with with increasing voltage, however here there is much asymmetry between the AC and the DC case. This is because the shunt conductance that would cause parallel losses are typically not simple conductances, but rather are lossy dielectrics. Like a lossy capacitor this is an AC phenomenon. So the shunt losses are far greater for AC than they are for DC, that's the real advantage of DC at extremely high voltages.

Last edited: Oct 14, 2011
3. Oct 14, 2011

### vk6kro

For a given transmission line, if the DC current and the RMS current are the same, then the heat dissipation of the line will be the same for DC or AC.

So, this sets an upper limit on how much current the line can carry.

The voltage drop also depends on this current and the resistance of the wires.

Using a higher voltage (AC or DC) results in more total power being delivered and better efficiency.

Developing this higher voltage, and using it at the consumer end of the line, is currently easier using AC and transformers, but DC has some advantages, particularly if the transfer of power is between countries that use different electrical standards for domestic power supply.

4. Oct 14, 2011

### jim hardy

The transmission line can use cheaper insulators because there's no AC peak voltage in DC - 1000 kv is 1000kv not 1414kv.

By using earth for return they reduce # of wires and insulators , so the towers can be of lighter construction.

5. Oct 14, 2011

### vk6kro

In my country, AC transmission does not use the earth as a conductor. It is used for safety reasons, but does not normally carry a current.

There used to be some single wire transmission systems in remote areas, but these have been phased out for reasons of safety and reliability.

6. Oct 14, 2011

### jim hardy

""In my country, AC transmission does not use the earth as a conductor. It is used for safety reasons, but does not normally carry a current.""

same here. i was describing hvdc which in its real early days used earth for return. i assume it still does.

sorry about the confusion.

one wonders whether that earth return would make HVDC more vulnerable to those solar flare induced magnetic storms discussed in another thread... sure increases the area available for induction.

Last edited: Oct 15, 2011