Q - Spurious HV genset earth currents source

[genRman]

Could anyone provide enlightenment on a problem I am facing with a Gas engine driven 11kV genset?
We have a gas engine, direct coupled (frame close coupled) to a 11kV 2/3rds pitch 3250kVA generator.
Connection is in start with a floating neutral.
Customers load lead connections are made in single steel wire armoured with the armour being connected at the switch end to ground only.
The set base frame is split in 2 halfs, one for the generator, the other for the engine. The generator base frame is connected to the common earth bus by a 50mmsq cable. There is cross earth bonding between the engine to base frame ( engine & gen on rubber AVM's), gen to base frame, engine base frame to generator base frame, engine to control panel & from control panel back to the common earth bus.

Under no load operation ( even with the load leads disconnected) we see a current flowing in the earth conductor, aproximately 6.5 to 15 amps.
Measure that with a slightly higher impeadance instrument & that value drops off quicky to somthing like 1.7A
The current is at 50Hz, & varies with the change of speed of the genset. The current amplitude varies with the change in generator terminal voltage.

There is significant stator field flux around the generator frame and a differential between drive end and non drive end of the generator in the regions of 182mV in open circuit condition.

My initial thoughts were capacitive coupling between the generator HV winding the surrounding metal, generating a potential gradient, but can't balance that against the positive current flow through so many different paths
Any bright ideas people?

 

[DougMSOE]

Just thinking out loud.
To generate a current you need
magnetic flux
conductor
relative motion between the two.
You said that you changed the RPM and the frequency changed of this problem current. Change the excitation on the field and see what happens.

 

[DanDel]

Highly likely that it is circulating ground current caused by induction from the generator. The generator has a floating neutral and therefore no real reference to ground.
What types of instruments are you using to measure this current(you refer to using a 'slightly higher impedance instrument')? A clamp-on in typical for current measurement.

 

[busbar]

1-4% of nameplate current is probably of no significant concern, except for periodic checks to see that it does not evolve into something more serious.

What is the purpose of isolating stator-neutral-to-ground connection at starting? What happens to the subject current measurement at closing? Is there a resistor involved?

Would you elaborate on "slightly higher impeadance [current] instrument"?

 

[jbartos]

Suggestion: Contact the generator manufacturer for the generator capacitive charge current value. The current normally enters the system grounding calculation, either medium resistance or high resistance system grounding calculations.