Offshore Power System Fault Level

[bhalfweeg]

I'm looking at possible designs for the distribution system of a new offshore installation with a maximum load of 70 MW supplied by 3 x LM6000 42 MW GTs. This immediately presents a problem of available switchgear fault and normal current ratings, with typical metalclad switchgear restricted to 11kV, 4000A, 50kA 3sec or 13.8 kV, 4000A, 40kA 3sec.

I've looked at the following fault level mitigation strategies:

1. fault limiting reactors
2. fault limiting fuses
3. step-up/step-down transformers
4. 22 kV generation and distribution
5. high-impedance generators

Since this is greenfield, I'm looking for more elegant solutions. Anyone come across such a scenario?

Cheers,

Brendan

 

[RajT]

The scenario of generating 3x 42 MW on a offshore platform is quite common by todays standards.
In your case the optimum design would be to select 13.8kV,60Hz as the gen voltage.

Option1
On the basis that you are allowing for all 3 sets to be paralleled,keep the gen Xd" high enough to give you a max fault of 50kA (with any HV motor contributions)
to allow specifying swuitchgear at say 50kA,1 sec.
From experience you are on the borderline of getting away with it without introducing a Is limiter but if you have to then so be it.
This option would also allow you to start 7-8MW m/c on DOL starters
Option 2
You would have the same arrangement as option1 but would ensure that only any 2 out of 3 sets would be on line at any time giving sc currents well within switchgear ratings.
The disadvantage is ofcourse the loss of production in the event of 1 set failing but note that the practice is common in that it is an acceptable allowance.

Option 3
Since the demand is 70MW, just run with 2 sets and all your problems are solved (and save some dosh)noting that LM6000 is a very reliable beast once bedded down.

I would forget about reactors,22kV gens, high imped gens, transformers as none of them will give you the degree of flexibility as you would get from the above options.

 

[mpparent]

I agree w/ rajt except for one thing,

I would go w/ three gen's...allowing n+1 redundancy. That way if one fails, you have a backup gen. Additionally, you have an easy out for maintenance, as you can have one engine down and still maintain capacity.

Mike

 

[jbartos]

Suggestion:
1. High impedance generator
2. 22kV generator bus and transformers
3. High impedance buses
4. Current limiting reactors
5. Current limiting fuses

 

[bhalfweeg]

The custom impedance generator sounds like a good option, but I've also heard some switchgear manufacturers have tested 50 kA switchgear to higher fault levels. Whether they're prepared to issue type test certificates is another question.

There is some opposition to a 60 Hz system, this is an installation in the UK sector of the North Sea, but that is a matter of perception rather than technical feasibility.

An n+1 philosophy was always the go, however the inflexibility of a 2oo3 generation interlock is not attractive, neither is a split bus arrangement.

Thanks for your suggestions, you've backed up the way I was already thinking.

cheers,

Brendan

 

[bigamp]

Check Siemens for switchgear. Their 8BK40 range of air insulated vacuum switchgear (if still available) has a 63kA withstand/break rating. The panels are 1100mm in width and weigh 2800kg each. No doubt very expensive.

Regards