Hi Everybody,
I work out in the Middle East in the land drilling-oil/gas industry on large mobile drilling rigs.
We use Caterpillar engine- generator sets, generally 5 per rig generating a 600V, 3 phase power supply, approx 1.8MVA per genset.
The neutral of the generators are not earthed and consequently we have no "automatic disconnection" of the supply system should there be a ground fault, unless it is above the magnitude of the operating current of the generator main breakers, which in some cases is 2500A per generator. All major machinery and SCR/rectification systems operate from the 600v AC Common bus.
We have a 480VAC MCC system, but again, the secondary of the transformer is delta(delta-delta transformers) and no neutral/earth connection.
We have a ground fault detection system which alerts us to ground faults both AC and DC, as some of the rigs also use DC traction motors to drive certain machinery, again heavy currents involved with some of them consuming up to 1600A per motor. (We regularly get ground faults due to the harsh and often damp conditions of drilling)
Excuse the ignorance, but it sometimes puzzles me as to why we still drive ground rods into the ground if there is no return path to the source for the fault current to return to.?
They are driven into the ground at various points around the installation and then bonded to the metal skids.
I can understand the bonding aspect and having all metal surfaces equi-potentally bonded to reduce eliminate touch/step voltages and electric shock if stepping from one platform to another.
The only reason I can see for driving earth rods in to the ground would be to equalize the ground potential to that of surrounding metal work and again reduce touch-step voltages in the area.
The main generator skids also has a main earth lead running from the generator metallic skids to what we call the sub base structure, which the main drilling rig sits on and extends deep into the ground.
With various study, some of which the content is from IET sits and courses, I have read about various myths of electrical current, and that it has no desire to go to earth and is not simply dissapated, so given what I have said above, what would be the reason that they do this?
Any ideas, thoughts, theories would be appreciated.
Best regards
Craig
