Over current problems

D

Thread Starter

donnapa14

We have a CAT 3508 genset, 4160V, 3 phase that always trips on timed neutral over current. Sometimes it will run for an hour or so, other times it will run for several hours before it trips on timed neutral over current. What could cause the over current condition?
 
Donnapa14...

when you say neutral overcurrent are you referring to the connection from neutral-to-ground (earth?)

1) What are line-currents?

2) What are phase-to-phase voltages?

3) What are phase-to-neutral voltages?

4) What is type of load?

Regards,
Phil Corso
 
@Phil Corso,

I am referring to the generator's overcurrent protective relay.

The line currents go as high as 75 amps, voltage is maintained at 4160V. I also noticed that the power factor goes down to 0.70 lagging before the generator trips. We don't monitor the phase to neutral voltage though.

Our load are mostly lighting and air-conditioning.
 
Donnapa14... typically, a 4160V, 3-ph, 3-wire, system does not have a Neutral conductor. Thus, the neutral-current you cited is derived from a residual connection of the three-line current CTs.

The 75A magnitude is indicative of a severe unbalance in the 3-line currents, i.e, usually a level considered to be very dangerous for the generator!

Can you provide the 3-phase or line-current measurements?

Regards,
Phil
 
Phil,

With 2 units online running parallel, the usual per phase current draw is from a low of 50A to a high of 64A. That's a load of about 300Kw to 400Kw per unit. Btw, our gensets are CAT 3508, 565Kw, 4160V, 3 phase, 90A, SR-4 Generators.

Regards,
Don
 
Donnapa14... since your setup now consists of paralleled generators the situation becomes more complex!

Can you furnish a simple Single-Line-Diagram showing the protective relaying of both generators? Even a hand-drawn sketch will do!

Phil
 
Phil... here's my email address: donnapa14 [@] yahoo [.] com. Can you furnish me your email address so I can send you the single line diagram?

> Can you furnish a simple Single-Line-Diagram showing the
> protective relaying of both generators? Even a hand-drawn sketch will do!
 
Don... thanks for the SLD. The three line-current CTs are wye-connected, and the 51N is responding to line-current unbalance.

To confirm my hypothesis use the Ammeter and Voltmeter switches to read the three Phase-to-Phase Voltages, the three Phase-to-Neutral Voltages, the three Line-Amperes, and the Ground current!

Regards,
Phil
 
Phil,

The unit in question is now out of service for a top-end overhaul. We'll have to wait until the overhaul is done.

I'll keep in touch. Thanks and good day.

Don
 
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