Offshore Earthing at 11kV - Wave Energy

[RayA]

I have read some of the post regarding the earthing of ships whilst at sea, however I have a question of a slightly differnt nature which I can't seem to find an answer to.

My company are building a 300kW floating wave energy device which will connect to grid via a 500m 3phase 11kV cable. The generator is a 415V 12 pole induction machine controlled through a regenerative drive and then stepped up to 11kV onboard.

The structure of the device is made of steel. This is why I have some questions about earthing.

Should The cable will be connected to earth onshore. This could be bonded to earth onboard and hence to the rest of the steel structure?

What happens under fault conditions?

The sea and structure provide a great earth, but what about electrolytic corrosion?

With the inverter earthed onto the structure, will this adversely effect sensitive onboard instrumentation?

By earthing at land and by the structure to sea is there a possibility of earth loops?

Hoping some out there can shed some light on this interesting problem.

Many thanks
Ray

 

[jbartos]

Suggestions marked ///\\Should The cable will be connected to earth onshore.
///A separate ground conductor can be run to connect the floating steel structure to the shore ground.\\ This could be bonded to earth onboard and hence to the
rest of the steel structure?
///The above-mentioned ground connected cable at shore and onboard will equalize the potential between the shore ground and onboard steel structure.\\
What happens under fault conditions?
///It depends on the system grounding method. A high resistance grounding might be considered. The ground fault will be alarmed.\\
The sea and structure provide a great earth, but what about electrolytic corrosion?
///This is more on the DC side rather than AC side. Some electrolytic protection may be needed since there is always some small DC current flowing between the shore and onboard steel structure.\\
With the inverter earthed onto the structure, will this adversely effect sensitive onboard instrumentation?
///Yes and no, depending on the inverter EMI and EMC.\\
By earthing at land and by the structure to sea is there a possibility of earth loops?
///Yes, since there will be a different impedance of ground cable in comparison to the ground path impedance through earth and water.\\