What’s HVDC and how is it different from normal transmission?

Interesting article about long-distance power lines.

Tesla vs. Tesla: The Juice In Your Car Will Increasingly Come Through HVDC, Edison’s Preferred Current

◊ While it’s limited to about 800 kV, not dissimilar from AC, the way it’s constructed you effectively get double the voltage of AC.

◊ Underground and underwater lines don’t lose effectiveness for transmission compared to aboveground lines. No electromagnetic field is created by direct current to interact with other wires, the ground, or water.

◊ The wires can be arbitrarily thick because direct current doesn’t tend to flow along the surface.

◊ Direct current has no frequency, so it’s easy to connect two grids at different frequencies and use electronics to match frequency when it’s converted back to AC. HVDC is sometimes referred to as asynchronous transmission for this reason

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DC is also the in thing for ship’s propulsion these days:

This DC power line was built by ABB in 1954 and is till in use.

ABB, a major player in this space, built a submerged 96 kilometer HVDC transmission line between the Swedish mainland and an island.

China is going HVDC and UHV.


Basically if you need to connect 2 unsynchronized grids, span a body of water or transmit power extremely long distances it’s pretty useful.

and you can plug in a battery which are DC of course

When I started my career at sea (1960’s) the power on board ships were DC. Simple for speed control of winches, pumps etc., with small converters for things that required AC.

When I got on rigs in the 1970’s it was AC generators, with SCR to produce DC for the major Drilling Equipment. On a drillship we even had GE 753 DC motors for propulsion. (Interchangeable with the drilling equipment motors)

From the 1990s or so AC with variable frequency control became the norm, both for drilling equipment and propulsion motors.

Now some have realized the advantage of DC grid on board:

So we have come full circle in a little over 50 years.

in reality every 10 years it swings ac to dc and back if you are talking propulsion.
Whereas typical service pumps etc went ac and stayed there.
its either cheap to buy or cheap to maintain and these days its how much space does it take up

A HVDC link between UK and Belgium is in the process of construction:

A similar link between Denmark and UK is in preparation stage:

As is a link between Norway and UK:

When completed the power security and the possibility to shear peak production of renewable energy between the countries will be greatly enhanced.

Nobody wants to address torque?

I think that conversion to DC is for long distance transmission, not end use of DC The goal being to tie AC grids together over distance.

That is more feasible now because when the conversion is done with recent technology it is done with less loss then older systems.

Here are the two key points:

DC transformers plummeting in cost.

Mechanical breakers for DC were too slow while semiconductor breakers were fast enough but had 30% power loss. This has been hard to overcome, but it’s been licked recently with a new generation of hybrid breakers.

The T2 tanker I was second mate of was turbo electric DC. She had been jumboized by fitting a new hull forward of the pump room and the midship accomodation placed on the after castle and with a new funnel looked quite modern (1970) from a distance. We were in the Naptha trade and the electric cargo pumps just a flick of a switch to operate. The new portion of the hull was bigger than original and the cargo pumps capacity meant that shore leave was available.
I think that after the war some T2’s were used to generate power in Norway.

Yes there were three of them as far as I can recall. They were mediterranean moored just outside Drammen, with a converter station onshore to feed into the grid.

On my very first ship at sea (1959-60) I was Deckboy on a 16000 DWT tanker, the Polyrambler,(ex Bergestrand):
We did a refueling run for those T2 tankers from Isle of Grain to Drammen in the winter of 1960. (The first and only trip that vessel did to Norway)

With all due respect, I believe the T2 turbo-electrics were AC. The propulsion motor was a synchronous motor. The main (propulsion) alternator was 2300 VAC, The auxiliary generator was 440 VAC. A 2300/440 transformer was used to reduce voltage from main alternator.

There was a 115 VDC bus.

I stand correction.

Was that Sabine? Some of their “jumbo” T-2s were ridiculous looking, in particular the Colorado where they didn’t stack the forward house on the aft house, but placed it about 50’ in front of it.

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She was the Texaco Bombay. The previous 2nd mate had paid off sick and I got a pier head jump on my way home from completing a 6 month hitch on another vessel. I was onboard for about 2 weeks before a new 2nd mate joined and I continued on vacation.
They had been a very lucrative class to be in as they were running up to Saigon and a war bonus was paid. One of the T2’s was hit by a rocket which didn’t explode and ended up in the electrician’s accomodation. The Caltex tank farm was left alone and after the war the manager was revealed as a high ranking member of the Viet Cong. I think the rocket attack was a mistake.