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Diagnostic checks to identify the 640 kW EDG rotating diode failure

Diagnostic checks to identify the 640 kW EDG rotating diode failure

Diagnostic checks to identify the 640 kW EDG rotating diode failure

I have an Emergency Diesel Generator (EDG) rated 640 kW rating. It has an alternator rated 800 kVA, 415 V. Using soft starter, we have started a motor of 260 kW rating. When attempting to start the motor EDG tripped with many alarms/indications. One of the indication is the diode failure. However, after that, the 260 kW motor was taken out of service and the EDG was taken into service to feed other emergency loads. For the last two years it is working without any problem, but with around 200 kW load.

In this regard I would like to know is there any method to identify/establish the diode failure. Or can we consider the alarms came during the 260 kW motor starting as false alarm?

These are the rotating diodes for the exciters.

RE: Diagnostic checks to identify the 640 kW EDG rotating diode failure

That's a big motor for a DOL start on a set that size, it's not a surprise that the AVR objected.

You should be able to access the rotating rectifier at the non-drive end of the alternator without too much difficulty: the diodes are probably stud-type rectfiers so they're quite amenable to testing if you disconnect one leg of the diode. Use a multimeter with a diode test capability to check forward volt-drop and a 250V megger to check reverse blocking capability.

If they're puck-type rectifiers then the same tests apply but you need to test them with the puck under compression which can complicate things depending on how the connections are arranged, but that rectifier style is usually found on bigger machines. Big puck-type rectifiers sometimes give goofy results when tested with a multimeter because the die is barely biased into conduction by the tiny test current from a multimeter; it's usually better to test them with a DC source and a lamp.

RE: Diagnostic checks to identify the 640 kW EDG rotating diode failure

You already have a diode failure alarm / trip relay, which is a 24/7 protection. What else do you need?


RE: Diagnostic checks to identify the 640 kW EDG rotating diode failure

Thanks for the quick response.

Quote (You already have a diode failure alarm / trip relay, which is a 24/7 protection. What else do you need?)

I understand that the diode failure detection is based on the some approximate measurement of the AVR input voltage. A sort of ripple or pulse or harmonic measurement. So I don't now how far this indication is accurate.

The diode failure indication was noticed at the time of trip during the start up attempt of the largest motor. After that many times, the EDG was run, but the diode failure is not appearing not even a single time. This is why I have the question.

Also, I would like to know, what is the risk of continue operating the EDG with one diode blown off condition?
I believe only the capacity of the generator (with respect to the reactive power capability) would be impaired. Any other risk?
At the most, when the power demand exceeds, the EDG would trip.

Is my understanding correct?

RE: Diagnostic checks to identify the 640 kW EDG rotating diode failure

With one diode not working, it is a single phase bridge rectifier which means more exciter field current for the same load (which means your generator capacity will be limited due to the field current limiter kicking in) and more ripple content and harmonics in the generator voltage.

The large motor trip was probably triggered by the AVR rather than the diode failure relay, which is normally very reliable. Did you check the diodes after that trip and find them healthy?


RE: Diagnostic checks to identify the 640 kW EDG rotating diode failure

You said 260kW motor was started using soft starter, not DOL.
So,I am wondering about the harmonics that the soft starter might introduce in to the input power supply. This is from EDG and the input to AVR also is taken from the same supply thus the harmonics (due to Soft starter) could appear in the input power supply to AVR as well.
Could this be responsible for Diode fail alarm/trip!!
What did you do after the Diode failure trip appeared, any checks?
If it is false alarm (due to harmonics introduced by the soft starter),the solution may lie in introducing harmonic filters on the input side of soft starter.
Contact soft stater vendor and the EDG vendor. They generally have advice on such issues.

RE: Diagnostic checks to identify the 640 kW EDG rotating diode failure

It would be helpful to know what type generator and AVR you have, and what type Diode Fault protection system is being used.

Smaller generators have diode fault protection integral to the AVR, sometimes they can trip under certain load conditions depending on what settings are used. The factory default settings on those type protections usually work ok, but have found that in some cases we had to readjust settings with certain applications.

One of the first diagnostic checks I usually make on a generator is to measure the field volts and current, both at no load and at a load point like 50% rated kVA load to see if your actual and rated field volts and amps are close to the specified value.

This Stamford manual has a very good description of diode testing you may find useful, and works for just about any generator you'll run across in your size range,

Many newer generators use packaged diode assemblies, where some or all of the diodes are in a single module, their troubleshooting sometimes requires a specific procedure to get proper results.

This is from one AVR suppliers manual about the Diode Fault Monitor,
Exciter Diode Monitor
This setting determines the RMS value of ripple current in the field. If the RMS value exceeds the threshold for 5 seconds, a diode fault is detected.

If you could provide specific information on what generator, AVR and diode fault protection you have you may get a better answer.

Hope that helps, MikeL

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