EHV power lines underground - Costs vs Surface 3

[BPW363]

Does anyone have a comparison of costs and challenges for EHV distribution underground. I am interested in whether or not the lifecycle cost of a burried system is really much worse than for overhead lines.

The base case system would consist of 500kV lines delivering 1000MVA. The length underground would be 10km.

I know line burial is feasible but also wonder what the challenges would be if we opted to install the 500kV to 34.5kV transformers underground as well.

Fanciful?

 

[bacon4life]

AC cables are much more capacitive than overhead lines, so the 220 km line would also need a couple of intermediate substations with shunt reactors to absorb vars and control voltage.

Long cables are more likely to be DC, but that requires expensive converter stations at each end.

 

[Wildbairn]

Cheers Bacon4life
The reason I ask is that due to the cost of offshore windfarms and the underwater cable & connectors, and the issues over planning onshore wind-farms, whether it would be feasible, if slightly more expensive to replace OH transmission lines with underground cables, and put up wind turbines in series along the previous OH transmission line.
The example would be the Denny to Beauly line which is going to be upgraded to 440kv "super-pylons"
If wind turbines need substations to connect to the Grid and underground cables need substations etc. would this not provide some joined up thinking and local generation without scenic detriment etc?

 

[Marmite]

Aside from the massive difference in cost between 400kV transmission line and underground cable, Wind turbines tend to generate at 690V and have a step up transformer in or near the tower base to increase the voltage to say 11 or 33kV for collection. It would not be feasible to have a 400kV transformer at each turbine. 400kV transformers are enormously expensive. On top of that you would need to break in to the passing 400kV cable, which would require a substation with switchgear/terminations/ protection etc, which again would be prohibitively expensive.
Regards
Marmite

 

[Wildbairn]

So how do windfarms work? Surely each "wind-farm" has a transformer or substation and not one on each turbine?
And underground cabling have shunt reactors to absorb & control voltage, do turbines not work in series?

Additionally there are already a network of Transfer stations,along the route of the grid.

 

[Marmite]

Each turbine has a transformer to step up the generated voltage (690V) to the collection voltage (say 11kV). There will be a number of turbines daisy chained together onto an 11kv feeder back to the main intake substation. The main 11kv switchboard at the substation has a number of 11kV circuit breakers, each controlling a daisy chain of turbines. The main intake substation will have one or more transformers transforming 11kV upto the grid connection voltage. Normally a windfarm has a single intake substation at the grid connection point. The choice of connection voltage to the grid depends on the size of the windfarm and the economics of delivering the connection. As the voltage level increases the cost of plant and equipment increases sharply. Also at the 400kV and 275kV level the circuits take on a strategic national importance which means that the connection requirements are more onerous and expensive. If it was technically and economically feasible to tap wind turbines onto a 400kV cable every 300m or so like Christmas lights, the circuit would be taken out of service for a fault on any one of potentially 100 or so turbines. Losing 2MVA of generation could shutdown 2000MVA of transmission capacity.
Regards
Marmite

 

[Wildbairn]

Thanks Marmite, much clearer, sorry to be a pain, so is that why every windfarm I see has 2 or more turbines not working? Is this because if one stops another is booted up, so to speak, to maintain level of supply to the feeder station & grid. Surely it would be more sensible to tap into the Grid every 20-50km rather than 300m.

The reason for upgrading from 275kV to 440kv is to have remote generation from renewables. This of course becomes redundant if more is generated locally as this is the environmental best option.
I understand that underground cabling is more expensive, however as suggested in

https://online.tu-graz.ac.at/tug_online/voe_main2.getVollText?pDocumentNr=65933&pCurrPk=33553

if we are looking at a mainly rural area (Scotland) which relies heavily on tourism and the exporting of oil,gas, electricity & water to England & Europe this may be an investment worth making.
I suggest that a daisy chain of turbines long a line is the same as a daisy chain round a hill.

Thanks & Best Wishes
Wildbairn