Co Gen Trips on Neutral Current

[nickoliver]

Hello,
A job I did last year suddenly started having problems. It is a 600KW Co Generation unit at a hospital. The operating voltage is 480V. Everything worked fine for months and now it seems to be tripping on a "Neutral Current Fault". I don't have any more detail as I haven't been back to the site but everyone involved is stumped. Any idea what could cause this and what the resolution might be?

 

[ccjersey]

Some new equipment in the hospital? Anyone put a clamp or recorder on the neutral yet?

 

[itsmoked]

Or new construction?

Bad connection in the neutral sensing?

 

[power2020]

It seems solid earth and no isolation tranformer between utility supply and transformer. Check for any earth fault in system usually in lighting and also check neutral over current relay settings.

 

[stanlsimon]

One possible source of neutral current in rotating equipment is an internal short to ground. Is the generator bonded neutral to ground?

 

[nickoliver]

I don't have any new info other than some voltage and current readings the operators recorded. The internal short to ground is an interesting point. During the commissioning process the generator controls start up man closed the breaker 180 deg out of sinc. The generator just about lifted off it's mounts and blew some fuses in a 1200A switch. The alternator was inspected and we were told it was ok. Could this happening now be a result of this previous trauma? It happened about 5 months ago.

I don't believe there is any new equipment i the hospital and I have not been to the site to check any connections.

 

[waross]

Does the load vary, or the generator go on and off line?
The previous trauma may quite well have done damage that left the machine susceptable to further damage by thermal cycling.
I saw a coupling blown off of a 350 KW 1200 RPM by an out of sync. closure.
yours

 

[stanlsimon]

If this generator is serving large motors, there is supposed to be a time delay on the transfer switch so that the motor slows way down before the switch makes.

Is this generator feeding the critical branch of the hospital? I wouldn't want to be under the knife in a O.R.
with a generator tripping out.

 

[mohanlal]

Just check whether the loads are balanced by measuring the phase current and monitor neutral current in the system. Also check neutral current high trip set.
Subramanian