AVR 2

[vjr0512]

We are finalizing the Diesel Generator specification and as per Client specification we have asked to consider redundant AVR for the DG set. Any significance on this requirement? Cummins do not have this feature.

Thanks to clarify.

 

[RRaghunath]

Redundant AVR helps maintain the generator output even if there is problem in one of the thyristors / firing circuits in the AVR.
If the DG set is for captive power plant and is important for maintaining the production it makes sense. If it is only to supply emergency or back up power, the generator doesn't run continuously and hence redundant AVR may not be of much significance.
Vendor standard offering may be with single AVR but if you insist the vendors should be able to provide redundant AVR. Of course, it also depends on the size of the DG set.

 

[FreddyNurk]

I'll admit, in my time working with high speed diesel power stations, I'd never seen a requirement for a redundant AVR.
I can't comment on larger medium speed units, although the one station I was involved with didn't have redundant AVRs either.
It might be prevalent at larger sizes than what I've seen though.

waross or catserveng will no doubt be able to provide further coverage.

 

[Hoxton]

The complexity of a twin AVR does not always result in greater availability...

 

[waross]

I had a customer who started by buying and installing a standby generator that was too small to run all his AC units.
After a year or so I installed a larger set for him.
He wanted the original standby set left in service as a backup to the back up.
I could have easily configured the smaller set to start automatically on the failure of both the larger set and the mains, but given the many modes of failure that would entail I opted for a manual transfer between the sets.
If the large set was out of service he could throw the manual transfer switch and curtail his use of AC in the event of a mains failure.
In your case I would order a second AVR as a spare and keep it available, on the shelf.
If a redundant AVR with automatic changeover is deemed absolutely necessary then it may be time to re-evaluate your needs. You may need a second complete redundant generator set and an N+1 setup.
I have seen very few isolated AVR failures.
Most AVR failures are a symptom of another problem. Replacing a failed AVR without investigating the cause will often result in a second failed AVR.
Reasons for AVR failure:
A faulty PMG.
A faulty brushless exciter.
Wiring issues. I have seen a nick in the insulation of a control wire lead to corrosion of the copper and eventual failure of a set.
A few other issues that would probably cause a redundant AVR to be called into service to replace a perfectly good AVR:
A failed brushless exciter that does't damage the AVR.
A failure of a rotating diode or suppressor.
A main field failure.
In regards to failed wiring due to compromised insulation and corrosion:
I saw that last Sunday night, all be it not on a generator.
Nicked insulation and corrosion led to the failure of the headlights on our hockey bus, late at night, 100 miles from home at minus 30 degrees C, and wind chill below minus 40 degrees.
A direct wire from the battery to the lights got us home safely. The problem was found and fixed a few days later.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

Since you made a comment about Cummins I'm assuming this is a high speed diesel up to about 4MW? A really good answer requires a bit more information about what exactly you're trying to do.

As far as I know, CAT, Cummins, Kohler, and most other genset manufacturers don't offer redundant AVR's as an option. I done a few systems on diesel engine driven units, in the 2-4MW range with redundant DECS200 AVR's. That regulator had an external tracking feature that allowed the "standby" AVR to be ready, while the Basler system worked pretty good, it was not "bumpless". I have not done a redundant system with the newer DECS250 but it appears the tracking and setup are the same.

Before you finalize your specification, think about what you're asking for. In reality very few modern AVR's just fail, the bulk of the failures I see are due to some other problem, usually in the generator. PMG, exciter and rotating rectifier problems seem most prevelant, rotor problems seem to be a repetative cause on older machines. Actually one of the most common failure modes I see on newer AVR's if the field output failing if the field circuit is opened under load.

On one site that had redundant AVR's (multiple 4MW units at a military installation), the primary AVR failed, switched to the backup and it failed, the unit finally tripping on Loss of Field. The actual root cause was several wires on the rotating rectifier assembly had come loose in operation (poor repair job). What was interesting was the voltage disruption that occured during the initial AVR failure, the backup AVR trying to recover the unit and then its failure. The consulting engineers reviewing the fault and impact to the system felt it may have been better to have just had the unit trip when the first AVR failed.

Also, in my own experience on "smaller" generator sets, redundant AVR systems are mostly PIA, they add a large amount of complexity to the excitation control that frankly over the years I never saw any real benefit to.

If you were doing a critical large turbine generator then the cost and complexity of a redundant AVR system may make a good case, just can see it making any real benefit of a modern standby generator.

My 2 cents, MikeL.

 

[RRaghunath]

vjr0512,
wouldn't you like to say some thing!!

 

[ScottyUK]

Even on the bigger machines redundant AVR's bring as many problems as they solve. The comments about rotor faults are equally valid on the big sets. On a station with eleven HV generators I think we lost an AVR once and that was due to an external fault on the machine. That's 110 machine-years in the period I was there.

Hold a spare AVR so you can get back up quickly - most small AVR's can be swapped in about half an hour.

 

[vjr0512]

Thanks to all for the responses. Actually it is a 2.5 MW DG set for a fertilizer plant and we have 3 machines of the same size to be integrated. This DG is not for captive power generation but for Emergency power (when the Gas turbine generator fails )for safe shutdown and during start up of the plant.

My query was whether we can technically convince client for not having a redundant AVR or the availability of the DG set does not get affected if we opt for non-redundant AVR.

Thanks once again for all the experts responded to the above query.

 

[waross]

It is not good practice to blindly replace a failed AVR without investigating and correcting the cause.
Almost all faults that will damage an AVR will then damage a redundant AVR. Thus double the time and cost of repairs plus a little more time waiting for replacement part.
"We have a redundant AVR so there is no need to stock a spare AVR, right?"
In most cases one AVR may serve as a spare for all of your machines.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

A good solution may be to suggest what we used to call a "qualified spare", on some critical applications we would purchase spare controllers, and during startup after settings were finalized, we would swap out the new spares with tested and programmed controllers.

You may also want to suggest that a proper maintenance program and product support plan gets put in place. Does the supplier of the equipment have experienced technicians within a reasonable distance to support the units if a failure occurs? Are critical spare parts stocked? Does the client have in house staff that could be trained and equipped to handle some diagnosis and repair if needed?

Also think about if that AVR or other control component is easy to replace if needed. A simple thing like screw terminals versus plug in connectors can make a parts swap much faster and reduce possibility of mistakes. I used to o a lot of service calls after someone had swapped a controller in a panic only to find some wires had gotten crossed during the repair. Is the OEM supplied AVR digital or analog? A digital AVR can have settings installed that usually don't require any adjustments or changes when installed. An analog AVR will have multiple potentiometers that will require setting after installation.

In the last few years I have retrofitted a number of units, both new and old, with the Basler DECS150. It has plug in terminal strips, making a swap if needed pretty easy, it has an auto tuning feature that actually works pretty well after some early problems, and it can be programmed without being installed, the processor will power thru the USB cable if there is no PMG or control power applied, which is really nice if you find settings have changed for some reason and you want to update the spare.

Just some additional thoughts, hope they were helpful, MikeL.