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Posted By Topic: Where HVDC is used

ppaw1965
Jul 30 2018 23:02

Where LVDC isn’t suitable.
   

SteveH
Jul 31 2018 17:15

For 631 Kilometers from Benmore in the South Island to Haywood Substation in the North Island.
   

toyoto
Jul 31 2018 21:41

from memory its because the cable is a lot cheaper, don\'t quote me on that though
   

evanh
Jul 31 2018 23:26

It think it\'s all about stability at great lengths. The conversion process doesn\'t require synchronising the two ends to each other.

But, because of the obvious difficulty with voltage conversion, DC is only gets used in special cases like this.


   

evanh
Jul 31 2018 23:31

Excuse my typo\'s:

I think it\'s all about stability at great lengths. The conversion process doesn\'t require synchronising the two ends to each other.

But, because of the obvious difficulty with voltage conversion, HVDC only gets used in special cases like this.


   

evanh
Aug 01 2018 05:51

Pylons don\'t transmit. Generators and transformers transmit.

The electrical transmission travels along electrical wires (conductors) that are hung from the pylons.


Direction can be view in two ways:

1) As 50 Hz current that moves back and forward in even amounts. Net movement is ideally zero.

2) As power transmitted from generators to consumers.

This probably won\'t help - The two views are not independent. The above current is proportional to the consumer demand. Current combined with the set voltage, gives you the power being consumed.


   

evanh
Aug 01 2018 06:10

There is many analogies. In this case, you could think of AC current flow like using a handsaw. Where you saw back and forth until the branch is sawn off. The saw didn\'t go anywhere, but work was still achieved.

Energy was transferred/transmitted from you to the cut in the tree.

   

dbuckley
Aug 01 2018 10:42

There are two reasons HVDC is used.

The first is to connect two AC grids together when they are not synchronized. Where this happens the two converter stations are usually next to each other. This is sometimes called \"back to back\" HVDC.

As an example, in the USA, there are a number of separate grids, with names like (from Wikipedia) The Eastern Interconnection, The Western Interconnection, the Texas Interconnection, the Quebec Interconnection, and the Alaska Interconnection. There are a number of HVDC links connecting these grids.

The other use of HVDC is where it is uneconomic or impossible to use conventional AC transport.

The trouble with AC transmission is that the conductors form a capacitor with the soil we stand on. Because the polarity of an AC line changes 100 times a second, this capacitor is forever charging and then discharging and then charging in the opposite polarity and then discharging back to zero. All that charging and discharging requires current, and that is current that doesn\'t get delivered to the load at the other end of the line.

This is beautifully illustrated by holding up a fluorescent tube under a power line at night and noting the tube lights. This is because one is standing in the air that is the dielectric between the two plates of the capacitor, namely the overhead wire and the ground.

The upshot of this is that AC lines leak power, and as line length increases, the losses mount, until the point where there is no power left to deliver.

And it is worse with undersea cables; the capacitance to ground is many times higher with undersea cables, so the length of an undersea AC cable is very limited.

Which is why the New Zealand HVDC link was required to join the two island\'s grids, it just couldn\'t be done with AC. So we got what was, at the time, the longest, highest power HVDC link anywhere.

Designing AC power lines is quite tricky; there are I2R losses from current flowing; the losses go up by the square of the current, so for a given power carrying capability, one wants to have the lowest current flowing, so this pushes one towards a higher voltage. But a higher voltage line has greater losses through capacitance to ground, so there is a balance to be struck. Not to mention the economic balance, heavier cables for more current at lower resistive loses, versus cables needing to be higher off the ground as the voltage goes up.

HVDC just has I2R losses, no capacitive losses, so that part of the balance is much simpler.
   

AlecK
Aug 01 2018 12:19

Probably the most concise yet clearest explanation I\'ve ever seen.
Thanks.
   

evanh
Aug 01 2018 14:36

Yes, I\'ll remember that.

You know what, I actually thought the fluorescent trick would also happen under a HVDC line. I never though of that particular leakage as a capacitive action ...


   

pluto
Aug 01 2018 16:18

Another important reason for HVDC, was the peak voltage that the undersea cable between north and south islands would have to withstand. In DC is the DC voltage value + a safety margin.

In AC it is the peak value of the AC voltage + a safety margin.

In AC the use of a 3 core cable would may have been necessary and transformers at each end of the cables to do the voltage change all with additional losses.
   

gregwires
Aug 01 2018 16:55

Not that 250,000V DC is trivial to contain. it was an engineering feat at the time. The link is quite interesting if you want to know more about it.
http://ipenz.org.nz/heritage/documents/Williams%20et.%20al.%20NZE%20Vol.21%20Iss.4,%20pp.145-160%20(2%20MB).pdf
   

SteveH
Aug 01 2018 19:34

\"Which is why the New Zealand HVDC link was required to join the two island\'s grids, it just couldn\'t be done with AC. So we got what was, at the time, the longest, highest power HVDC link anywhere.\"

What is the situation today, is there a longer one now?


\"Probably the most concise yet clearest explanation I\'ve ever seen.
Thanks.\"

I agree Aleck, well constructed reply DB, Bravo Zulu
   

ppaw1965
Aug 01 2018 22:15

Seems it’s the way of the future. Didn’t realise how common it is these days.
https://www.powermag.com/a-new-record-for-the-longest-transmission-link/
   

dbuckley
Aug 01 2018 22:20

Thanks for the kind words.

SteveH asks \"What is the situation today, is there a longer one now?\"

Hell yeah. Wikipedia provides a nice list of HVDC projects, https://en.wikipedia.org/wiki/List_of_HVDC_projects - China has some whoppers. Allegedly under construction is the the Xinjiang - Anhui line, planned to move 10GW across 3,300 KM, at 1.1MV. That\'s about five times longer than the inter-island link, and shifts about ten times the power.

I\'m going to stick my neck out here and venture that the converter stations for that line are going to be really, really loud. The converter stations use thyristors to convert AC to DC and back again, and they chop up the mains something rotten, like a bad stage lighting dimmer, but a lot bigger. The chopped up power then goes into a transformer.

Most of the \"stuff\" in the switchyard of a converter station is filters, to remove the harmonics generated by the thyristor chopping process, without which the overhead lines would operate like transmitting antenna and blat everything for miles around.


https://en.wikipedia.org/wiki/List_of_HVDC_projects
   

dbuckley
Aug 03 2018 11:47


> The generator in hydro dam generate AC because transformer work in AC only so how to make high voltage DC from it

Good question: the same way AC is always converted to DC, with a rectifier. Here\'s a video from Transpower that explains the principle:

https://www.youtube.com/watch?v=WVI8Z7p_rdY

As the video notes at the end, in reality HVDC systems use twelve pulse rectification rather than six pulse, and this requires using two different types of transformers, a delta/delta and a delta/wye.

For HVDC, these are very specialist transformers, and for the inter-island link, it was these transformers becoming unmaintainable and irreplaceable, as no-one has made transformers suitable for mercury-arc systems for a very long time, and that was the catalyst for the upgrade. And a very nice upgrade it has been too.

I should also note that these rectifiers can be run \"backwards\" as inverters, to generate an AC waveform.
https://www.youtube.com/watch?v=WVI8Z7p_rdY
   

evanh
Sep 17 2018 02:17

I got a little surprise yesterday when I popped over to the HVDC link west of Darfield, little town called Whitecliffs, and held up a fluorescent tube underneath and it darn well lit up!

There was also a notable fizzing sound above the nearby willow trees, so I guess it isn\'t a very smooth DC and therefore doesn\'t entirely prove the causation of leakage.

   

mickydnz
Sep 29 2018 08:50

The DC going through the HVDC line isn\'t perfectly constant. There is a ripple at the characteristic harmonics of 600Hz, 1200Hz, 1800Hz and so on (magnitude decreases for higher orders) due to the 12 pulse converter. These harmonics are filtered at the converter stations by the smoothing reactors and DC filters but some ripple still gets through! I\'m guessing it\'s the 600Hz component which is lighting your tube!

HVDC is the way of the future for the reasons outlined above. Eventually we will just have an HVDC grid with power electronic DC/DC converters instead of transformers.
   

evanh
Sep 29 2018 12:10

Yeah, I was guessing the ripple could still be an explanation for the leakage. But it\'s far from a certainty in my mind after that result.


Regarding replacing transformers with DC-DC converters - I don\'t think so!

For voltage conversion, DC-DC converters still have the transformer in them. Or, if there is no concern for balancing/isolating the two voltages then, at least a big ass inductor or two is needed along with all the rest of the oversized components that would go to make up a switchmode converter. That\'s a whole lot of complications being added at every conversion point.

The main reason why switchmodes are used in appliances is for eliminating the loses of the linear regulators of old. No such advantage can be gained here.

Unless there is a compelling efficiency reason to use a different frequency in the conversion transformer then DC-DC does not make sense for handling the various voltages of electricity distribution.

   

evanh
Sep 29 2018 16:07

Off the top of my head, HVAC has 11kV, 33kV, 66kV, 110kV, 220kV.