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Old 5th Mar 2019, 9:05 am   #1
Electronpusher0
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Default HVDC electricity transmission

In a previous thread someone mentioned:
"DC is used to connect between Britain and France to avoid synchronising the AC waveforms of the two countries."

This reminded me that I once visited the old converter station at Lydd in the 60s with the school, so I checked Wikipedia to remind myself.

Wikipedia states that 4 Mercury arc rectifiers were connected with anodes in parallel to rectify the a.c., however my recollection is that they used a total of 9 MARs, 3 per phase.
They were connected as three in parallel per phase to rectify the a.c. and were switched successively at different phase angles when used as an inverter (again 3 per phase).
Alongside the main building was several acres of high voltage (100kV) capacitors and inductors acting as a massive 50Hz filter.


Wikipedia also states that the system used a bi-wire system, to minimise the stray magnetic field and did not use earth to carry current.
I recall the person giving the talk about the station telling us that they did indeed use the earth on one occasion when they lost connection in one of the cables and ran successfully at reduced power until the cable was repaired.

Given that I am recalling a school visit some 50 years ago my memory may be faulty. Does anyone have any additional information?

Peter
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Old 5th Mar 2019, 8:00 pm   #2
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Default Re: HVDC electricity between France & UK

Also, can someone confirm that an 'inverter' gets its name from the fact that Mercury-Arc bulbs for producing AC from continuous current are mounted upside-down?
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Old 5th Mar 2019, 8:12 pm   #3
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Default Re: HVDC electricity between France & UK

I was told that the derivation was different and from an old catalogue from a major manufacturer.
It is pointed out that the companies rotary converters "may be run inverted, and will maintain AC supplies from accumulators, or from Direct current generating plant"
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Old 5th Mar 2019, 8:25 pm   #4
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Default Re: HVDC electricity between France & UK

Re operation of the cross channel HVDC link, AFAIK it is indeed normally operated as an insulated system, but can operate at half capacity with an earth return during faults.
It is very rare for the system to fail completely, but a sudden drop to half capacity has occurred quite a few times.
This can result in UK grid frequency going outside of permitted limits.

National grid are required to keep the frequency within the range of 49.5 cycles to 50.5 cycles. They almost always achieve this, but drops to below 49.5 cycles have occurred, often as a result of interconnector faults.
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Old 5th Mar 2019, 8:50 pm   #5
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Default Re: HVDC electricity between France & UK

Theres a great video on YouTube showing the construction of the NZ north to south island bipolar 500kV DC link. That can run monopolar and earth return too.
Also used MARs to rectify at the south island end and Ignitrons to commutate the DC back to AC at the north island. It was in the 60s, pylons taken to site by ex US army CCKWs, and Drotts to winch then assembled using series 2 Land-rovers with winches and a gin pole.
really worth a watch.

I had quite a collection of MARs at one point, even hosted a virtual online museum. been outdone by Ralf Jaeckel in Germany though.
Passed all but 2 on to others now, 2 is enough!!

There's also a HVDC line Scotland to North Wales, far more efficient than AC, far lower reactive losses.
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Old 5th Mar 2019, 11:42 pm   #6
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Default Re: HVDC electricity between France & UK

I get the impression that the devices are referred to as inverters simply on the grounds that they repeatedly invert the DC supply to produce an AC supply.
Modern units on interlinks have 4 switching devices per phase.
I have worked on 750W SMPSUs that had 4 IRF840s driving a single primary and one opto coupler. A little single phase one.
They got a bad reputation for going bang and when I started doing them I fixed this rubber job problem.
The IRF840s were all on one heat sink on each side with just two bolts and 4 insulators.
The holes in the heat sink were too small and the insulators used to crack.
The problem was much worse on 240V so us Brits had to sort it out with fully insulated FETs. My consensus was that the FETs had to be replaced anyway and ready insulated ones made the job quicker and saved drilling the heat sink out.
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Old 6th Mar 2019, 10:12 am   #7
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Default Re: HVDC electricity between France & UK

Quote:
Originally Posted by Megatron View Post
Theres a great video...
Totally off-topic and a left-field aside here.... Do you call yourself 'Megatron' because you once worked on LORAN transmitters?

Just curious.
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Old 6th Mar 2019, 8:53 pm   #8
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Default Re: HVDC electricity between France & UK

No its far more bizzare than that.
we have dogs who the kids always end up giving nick names as well as their proper names. Megan became Megatron Milly became Milli-volt and Blodwen became Blod Busbar.
hence Megatron was the first thing that came to mind
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Old 7th Mar 2019, 5:09 pm   #9
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Default Re: HVDC electricity between France & UK

I think the use of DC for the undersea cables, including the one to take Scottish power to the South of England where the AC at both ends of the link are in sync, is because the voltage which the cable insulation needs to withstand is lower. For an AC cable, the insulation has to withstand the peak voltage, but the instantaneous voltage is below that for almost all of each cycle. Not a big deal for a 240V mains cable, but it becomes a much bigger deal at lots of kV.

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Old 7th Mar 2019, 5:46 pm   #10
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Default Re: HVDC electricity between France & UK

I thought the reason for d.c. undersea cabling was that at a.c. the capacitive reactance of the cables (core-sheath) would be such that excessive current would be drawn over long lengths of cable. A.C. is used undersea, up to a point (Robin Rigg wind farm in the Solway Firth, for example).
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Old 7th Mar 2019, 7:18 pm   #11
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Default Re: HVDC electricity between France & UK

Quote:
Originally Posted by russell_w_b View Post
I thought the reason for d.c. undersea cabling was that at a.c. the capacitive reactance of the cables (core-sheath) would be such that excessive current would be drawn over long lengths of cable. A.C. is used undersea, up to a point (Robin Rigg wind farm in the Solway Firth, for example).
Yes, you could end up wasting half of the transmitted power on cable losses, due to a mixture of skin effect and reactive power losses, losses being (according to Wikipedia) 30-40% less than using AC.
Also since 3 phases are not required less cable is required and being not synchronized to the respective grids is less susceptible to failure.

Also impressive is the high voltage that can be used, so far up to 1100kV! has been achieved, imagine digging into that!

Last edited by dglcomp; 7th Mar 2019 at 7:25 pm.
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Old 8th Mar 2019, 9:51 am   #12
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Default Re: HVDC electricity between France & UK

DC Transmission links are quite an interesting subject in particular their impact upon one’s book of interesting knowledge they may wish to retain. Not least of these is the Cahora (previously) Cabora Bassa Dam 533kV link from Mozambique to Johannesburg.

One aspect is the presumed path taken by any earth fault currents back to the source. Perhaps even more interesting is the transmission of the link which is undertaken using separate pylons for the positive and negative lines. So what you may say?
Well in some cases it was observed that where the lines pass through cow fields that the cows would not graze between the lines. In some cases the distance between the lines is in excess of 1 mile. Do the cows know something we don’t?
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Old 8th Mar 2019, 10:01 am   #13
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Default Re: HVDC electricity between France & UK

Quote:
Originally Posted by Bookman View Post
In some cases the distance between the lines is in excess of 1 mile. Do the cows know something we don’t?
Well, at least with d.c. it's not a moo-ving electrical field. Something to do with potential gradient from leakage? With the transmission voltages involved, even a mile distant may cause ground potential differences to be detected by bare-footed animals.

Is this phenomena evident in all weather conditions?
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Old 8th Mar 2019, 4:01 pm   #14
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Default Re: HVDC electricity transmission

Thread title updated to reflect most of the thread.
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Old 9th Mar 2019, 8:39 am   #15
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Default Re: HVDC electricity between France & UK

Quote:
Originally Posted by russell_w_b View Post
Quote:
Originally Posted by Bookman View Post
In some cases the distance between the lines is in excess of 1 mile. Do the cows know something we don’t?
Well, at least with d.c. it's not a moo-ving electrical field. Something to do with potential gradient from leakage? With the transmission voltages involved, even a mile distant may cause ground potential differences to be detected by bare-footed animals.

Is this phenomena evident in all weather conditions?
Maybe it gives them the same feeling as an impending thunder storm, so they avoid the area as an instinctive response to avoid getting struck by lightning.
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Old 9th Mar 2019, 8:54 am   #16
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Default Re: HVDC electricity transmission

Wikipedia do state in their write ups that that the content can be updated if the content is deemed to be wrong as often what is stated is what someone has remembered incorrectly.
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Old 12th Mar 2019, 4:32 am   #17
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Default Re: HVDC electricity transmission

Quote:
Originally Posted by Megatron View Post
Theres a great video on YouTube showing the construction of the NZ north to south island bipolar 500kV DC link. That can run monopolar and earth return too.
Also used MARs to rectify at the south island end and Ignitrons to commutate the DC back to AC at the north island. It was in the 60s, pylons taken to site by ex US army CCKWs, and Drotts to winch then assembled using series 2 Land-rovers with winches and a gin pole.
really worth a watch.
Is that the video at: http://ipenz.org.nz/heritage/itemdetail.cfm?itemid=53?

There is more information at: http://www.techhistory.co.nz/Electri...ookstcable.htm.

Also, the wiki article looks to be not too bad, perhaps professionally written: https://en.wikipedia.org/wiki/HVDC_Inter-Island

These days the interisland link is two-way on both poles, ±350 kV and about 1400 MW; all solid-state.

I guess that in the early days, whenever it operated single-pole, it would have been the longest, highest voltage and highest capacity SWER link anywhere. One imagines that Lloyd Mandeno would have approved, but what he actually thought of it does not seem to have been recorded.

Benmore hydro at the south end is in and of itself quite impressive. The great breadth of the dam tends to disguise that fact that it is also reasonably high. Its expansive nature is quite the opposite of say Hoover which is much higher but horizontally very constricted. In the old days it was not difficult to get a tour of the NZED hydro stations simply by turning up and asking, and I saw quite a few that way. But in the Benmore case I tried that on a Sunday afternoon in 1982 January, and was told that it couldn’t be done, but that I was welcome to wander around the public area. Meanwhile the station got a call to host an “important” group at very short notice, and when they turned up, the guy in charge suggested that I could join that group's tour, so it worked out in the end. (I vaguely recall that I ended up asking most of the questions.)


Cheers,
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