Powerflex 7000 and Generator Compatibility Issues

[superd66]

Has anyone experienced compatibility / sizing issues when useing the powerflex 7000 and a backup genertor system that operates in the island mode only??

History: Recent plant renovations included installing 4 Powerflex 7000 drives driving loads from (2)500 (1)700 and (1) 800hp. Back up generator power supplied via Detroit Diesel 2MW genset 7M4374. There is no line / generator / utility side harmonic filters / traps or inductive loadbanks on the oem supplied system.
This station is unmanned and needs to operate with no onsite operator action.

When operating against the utility with any pump combination everything is fine.

When operating the 2 500hp drives on the generator everything is fine.

However when we try and start the 700hp unit when the (2) 500hp units are already running the generator begins to oscillate and become unstable and unless operator action is taken to decrease the load, if no action is taken the generator breaker trips the station out leaving us with no power. The 700hp unit can run by itself on the generator. The 800hp cannot run with any pump combination on the generator it can run by itself.

PF with these drives is leading, spec'd to be unity to .95 lagging.

Design / specs required the ability to run any combination of 3 pumps

What we've tried:

AB has tried resizing the caps / inductors on the drives. to no avail.

Tried installing a 450 kvar inductive load bank that helped with the pf and some harmonics however it didnot affect the runnability issues.

Design eng / contractor / generator / AB, are all pointing the blame to somebody else and not coming up with an answer.

Design eng. recommends putting in differnt drive mfg. as AB is the only mfg useing caps / inductors on the front end and they believe this causing a resonance issue

AB recommends upsizing generator.

AB has stated from the begining that they were unaware that the system would be operating on a generator and if they new they would have recommended sizing the generator for 200% station load.

Has anyone else ever experinecd this, heard the 200% recommendation have real world exp with this problem?

Sorry for the long post
Thanks
Darrin

 

[davidbeach]

200% sounds pragmatic, 250% or 300% might be better. Why in the world would someone have tried to use such an undersized generator for such a harmonic rich load? The generator AVR is probably unable to deal with the harmonics. I'd point at the design engineer as he would be the one that knew the whole story and who should have known better.

 

[Alex68]

Are the generator oscillations related to frequency or voltage?
Have you installed a fault recorder? At least inside protections?
did you tried starting first the 700hp drive and then the 2 500hp ones?

If the problem is due to harmonic distortion, you could solve the problem installing a passive harmonic filter.
Analysing the voltage wave distortion, we could easily size the filter. So a COMTRADE file is required.

 

[ScottyUK]

Assuming that the drives have a standard 6-pulse rectifer at the front end, this is typical of the kind of load which makes life awkward for the generator's AVR. Usually it is a UPS load rather than a group of big inverters, but the problems are certainly similar. There are different types of AVR which have different abilities to deal with distorted loads. The designer should have discussed this with the supplier.

The designer should also have declared the type of load to the generator supplier and contractually specified that the generator should be capable of supplying the declared load with, for example, less than 1% frequency deviation and less than 5% voltage THD, or whatever is acceptable for your other loads. If the designer hasn't written this into the specification issued to the generator supplier then he is at fault if the generator and loads have an adverse effect on each other.

It is entirely possible that a good AVR is badly tuned, and that this is source of the problem. A lot of generator field techs and commissioning guys don't fully understand the adjustments available on the 'black box' that a modern AVR often is, and don't make best use of the available settings. You will need probably help from the OEM to do set up the AVR to optimum behaviour, but if this project is descending into a contractual mess then they may well prefer to help rather than get involved in litigation.

As Alex68 has suggested, passive filters are one option. Active filters are another possibility, but it may be more economical to replace the AVR module with a type designed for this type of load.

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I don't suffer from insanity. I enjoy it...

 

[ozmosis]

superd66
There's an interesting article written by Mirus International about using harmonic filters on generator supplies. We have used the Lineator on numerous occaisions, not on generators mind, but their advice has always been pretty good.

http://www.mirusinternational.com/pages/faq.html

faq 14: Is the LINEATOR™ suitable for generator applications?