Viking Link Interconnector

As this forum is normally the font of all wisdom I wondered if someone can answer this non engineering question.

A friend who lives in New Zealand mentioned that the power transmitted by the electricity interconnector between Picton in the South Island and Wellington in the North Island was DC current. The power arrives at Picton as AC current, is converted to DC, transmitted to Wellington and then converted back to AC.

Recently I read about the Viking Link Interconnector http://viking-link.com/your-questions/ which states that high voltage DC is an efficient method of transmitting electricity.

With the power losses associated with converting the current twice, what are the other factors that make this an efficient system compared with transmitting AC current?

Thanks

Jon

Well that passed a little time reading up on that quite interesting subject.

It seems that there are less losses in transmitting DC power as it doesn't have a frequency compared with AC. These losses increase with the distance.

One factor which I believe could be important s that the DC link provides a system isolator between the two AC systems. This allows, for example, the North and South islands to be out of phase with each other but still pass power between each other.

Other things are that the DC cabling can be smaller than the AC especially on overhead cabling therefore towers can be smaller, cheaper, too.

don't know about ur particular case

but I was tought only to use AC current over long distances because of the voltage drop......

what super conductor can do in the future who knows but I think they are also AC as well.......

BUT super conductors are just a thing for the lab at the mo......

High voltage is the secret of efficient power transfer. As power cables are of fixed resistance (ohms), and waste in Watts is Amps² x Ohms, it pays to reduce cable losses by reducing amps and raising volts.

For example, when 1000W is to be transferred across a 1ohm cable:

• Transferring at 1V is a disaster because power dissipated in the cable would be 1000A*1000A*1Ω = 1MW. Far more energy is wasted heating the wire than reaches the other end.
• 10000V at 0.1A up the same wire is much better - same maths, but low current means only 0.01W is wasted heating the cable.

Increasing and decreasing 50Hz or 60Hz voltage is easily done with a transformer. Although robust devices, they're a little wasteful - between 2% and 5% of total power will be wasted as heat in the transformer. Nonetheless transformers are a good way of distributing high voltage AC power up to, say, 400km.

AC transmission smacks into a big problem over really long distances because power cables have reactance as well as ohmic resistance. When volts rise from zero during the AC cycle the cable charges up as a capacitor, and then discharges on the negative cycle. In the same way, the cable's self-inductance will store energy as magnetism, and convert it back to moving electrons on the opposite cycle. A power cable's resistance to AC is much higher than it's DC resistance. Cables will even radiate power like an antenna, dumping expensive energy into space. A very long cable connected to a 10000V 50Hz generator could be completely dead at the far end.

To avoid the problem interconnectors transfer power as high voltage DC. Cable capacitance and reactance have no effect on steady DC currents, so the wiring doesn't work against itself as AC does.

High voltage DC is converted to AC electronically, not mechanically. The electronics are more efficient than transformers, so even less energy is wasted. Interconnects are mostly used for long distance links between countries rather than internally because their installation is considerably more expensive than ordinary AC switch gear. Their financial purpose seems as important as providing supply - interconnects allow unwanted surplus electricity in one country to be sold cheap to another and vice versa. Flow direction varies with the market, rather than as a direct result of consumer activity.

Dave

The simple answer is that with A/C, at the same voltage as the DC, the current reduces to zero 100 times a second at 50Hz.

edited to add(before the pedant does):  that is the peak A/C voltage, not the RMS value.

Edited By not done it yet on 09/08/2020 16:35:34

Because of the distributed inductance and capacitance of a long cable, with ac a distance is reached where the current carrying capacity of the cable is wholly taken by the charging current. You can,t get any power out at the end. Charging only occurs once, not every 10ms, with a dc cable and from then on power can be transmitted.

Dave

Off topic but triggered by the mention of financial aspects above. I can see a dozen or so wind turbines from my house and on Saturday once the fog lifted and we actually got a bit of sun I noticed that not one was turning. Late evening I strolled out to observe the sunset and then noticed they were all spinning. A nice balance of wind and solar sources.

With AC, the peak voltage is 1.4 times the RMS voltage. The insulators have to be sized to withstand the peak. So for a given line, it can potentially transfer 40% more power with DC. As mentioned by others above, reactance effects also become a problem with long AC lines. Synchronising large grids and keeping them stable can also be a problem and breaking them up with a few DC links can make life easier.

Ac was favoured initially because of the difficulty in the early days of changing the voltage with DC. This would have required rotary machines with commutators, and would not be very successful for high voltages. Such machines were used for DC supplies for trams, trains, and trolley busses, but the voltages there were not as high as would be desired for long distance transmission.

DC transmission became practical when high voltage mercury vapour filled switching devices became available. More recently solid state devices are used. These use banks of devices, thyristors generally I believe, in series and parallel to get the voltage and current capability needed.

If I recall correctly, the Cook strait scheme mentioned in the first post actually uses DC all the way from Roxburgh to Haywoods, so that the line for about 3/4 of the length of the South Island is DC. There is also a bit of land transmission at the North Island end to reach Haywards. Initially the scheme used two conductors, at plus and minus 250kV to ground, but with the possibility of running with only on conductor in use. Later upgrades increased the working voltage, I think at one stage only on one side. I'm not sure what the working conditions are now. Originally power could only go one way, but it can now go in either direction.

Submarine power cables over long distances are all DC, the capacitance between the conductor and the surrounding sheath would be too high for AC to be workable.

John

so, what about the UK connection to Europe.....same for the islands o the North....?

short distance = AC.....?

.all very interesting....keep it up....

DC connectors don't radiate which is a plus.

regards Martin

Edited By Martin Kyte on 10/08/2020 08:46:39

Edited By Martin Kyte on 10/08/2020 08:46:57

Short distance, but underwater, which increases the capacitance of the cable compared with well-separated wires strung in air between pylons.

The pros and cons must be more complicated than this thread implies because the longest 3-phase AC undersea cable in the world is between the Isle of Man and Blackpool, 104km at 90kV. The HVDC cross-channel connector between Sellindge and Calais is shorter (73km) but at 2000MW it carries much more power that the IoM cable. It operates at ±270kV to keep the current low. My guess is the IoM uses relatively little current, and it was cheaper to put up with the losses. Too expensive to run a lossy 2000MW cable, so the UK/France link was upgraded.

Seems early underwater systems did in ships' magnetic compasses. No wonder the English Channel is more dangerous than the Bermuda Triangle!

Dave

Latest interconnector on its way, Norway to UK, all that lovely hydro power.

The French interconnector is actually 4 separate cables, each of 500MW. Two were damaged by a dragged anchor, in storms a couple, or so, of years ago IIRC and were out of commission for a while.

Another advantage of DC is that the UK grid does not have to be synchronised with the connecting grid.  UK or european grid failures can be kept isolated.🙂

Edited By not done it yet on 10/08/2020 09:55:42

Lower breakdown voltage requirements.

regards Martin

As said high voltage transmission of power is more effective and DC has lower losses. Once electronic converters became more economic DC transfer has many advantages.

One issue with AC that has not been mentioned is Skin Effect. This is where the alternating current causes a magnetic field that repels the electrons towards the outside of the conductor (Left Hand Rule). This is increses with frequency so is mostly seen in high frequency circuits but is significant at 50 and 60Hz at high power levels.
Basically the cener of the conductor is not carrying significant current (electrons). This means it's wasted material. In high power radio circuts it is so significant that they use copper tube with silver plating with no loss over solid bar. It's also why they can use steel strength members in the center of copper or aluminium AC HV power cables with little additional loss.

Robert G8RPI.

D/C will be much similar, but without any eddy current losses? Electrons are all negatively charged so repel each other. Unless lightning is A/C, the ‘current to the outside’ works with both A/C and D/C (think Faraday cages and perhaps millions of volts, car/air passengers In lightning strikes) and excavator drivers who get tangled with electric cables are all safe while inside their ‘cage’. It’s just the more changes of direction cause more losses

Thanks for all the answers.

I now have a better understanding of the subject.

As I said, this forum is the font of all knowledge!

Take care

Jon