The Western HVDC Link

It should be remembered that this scenario is nothing like just turning an mcb back on to see what happens. The voltage is 600kV DC and the normal load rating is 2,200MW. Bearing in mind the cost of the asset (the cable plus its two converter stations), and the possible effects of placing a direct short onto the supergrid network, it must be proved beyond reasonable doubt that no fault is present, before the cable and its associated substations are re-energised. As part of proving no fault exists, the exact reason  for the trip must be identified. 


Regards,


Alan.

OK, probably a silly question, but it's been nagging me. Why is the link d.c.? I can see the point when the two ends connect to two different grids that aren't synchronised (e.g. the UK and mainland Europe) but in this case it's just connecting Scotland with North Wales - so surely both ends are on the same grid and so naturally in sync with each other. In this case wouldn't an a.c. system have saved a lot of conversion at both ends?


  - Andy.

interesting figures, 

2.2GW/600kV implies  a circulating current of ~3.7kA and a load impedance of  some 170 ohms.

Also given the length, 385 000 metres under water and another 50km or so on land,  to make voltage drop sensible at a few %, the round loop cable resistance can only be single figure ohms, so not exactly thin.

Certainly considerable potential for a loud bang if it all gets released at once.


I understand the HVDC conversion is line commutated thyristor stacks, so the positive or negative lead of the  DC from the line is connected to each outbound grid phase in turn by turning on the thyristors in that path at a carefully timed moment, and then the state of the mains cycle on the outbound side defines the 'off' as the  part of the cycle when the net current falls to zero - this switch off method is the reason for the description as  "line commutated".  It can only be used to add current at the right times to an already running grid, and cannot be used as an 'islanded' source of power, which also has implication for how easily you can start it. Wave form quality i.e. harmonic content is also  poor as the current waveform is very  non-sinusoidal, and problems with both radiated and conducted  RF interference from such sites can be significant.

In days of old such things could also be done with modified mercury arc devices, though since about 1970 the thyristor/SCR has become dominant.

The whole thing is some quite remarkable feat of engineering, so it is understandable that start-up is cautious.

No such thing as a silly question! With a long transmission line, you get a phase shift along its length due to its reactance. This exact phase shift is a function of distance, as well as load. If youHave a second cable of a different length, it is likely that the difference in phase (even though the frequency is the same) will prevent you from being able to close the second end. The solution is a dc link, with the “inverter” in synchronism with the local grid connection point. 


Regards,


Alan.

We have a 106km long ac power cable that operates at 90kV. Once energised, it produces about 40MVAr of reactive power. I believe it is still the longest undersea ac power cable constructed. We had a number of technical reasons for wanting ac, otherwise a cable of that length would normally be dc. 


Refards,


Alan.

I presume this cable has been laid on top of the Manx cable as well as the other existing electric cables and gas pipes, is there any separation?


Also presumably it traps a section of the other existing cables on the sea floor and stops them from being lifted for maintenance?


Andy B.