Tolerance band on Generator during emergency power situation 1

[ozmosis]

A tender specification from a well respected consultant recently passed my desk. Part of the requirements under the Variable Speed Drive specification was: "...the VSD's shall be capable of operation without damage for a period of four hours on a 415V +/-25% three phase power supply and any frequency between 25 and 75Hz to minimise potential problems when operating on emergency power supplies and during switching of supplies".
Has anybody seen a spec like this? Not being a generator man myself I'm not sure if gensets are capable of swinging to these extremes. Would the voltage and frequency regulators not shut the system down if this was to occur?
I can calculate the efect of the lower frequencies etc on the DC Link caps but I'm the sort of person to question specs that require products to go out of the normal International specifications. Or is this normal? I've never seen it in the 16 yrs in the VSD business.

 

[dpc]

Since the VFD is converting any incoming ac to dc anyway, it can probably handle a pretty wide range of incoming frequencies.

But I agree with you that it is unlikely that frequency excursions of this range would ever be encountered from a normal standby generator.

If the load on the VFD is critical, this spec might make sense. I would check with possible VFD suppliers to see if would give them any heartburn.

 

[ozmosis]

We are one of the potential VFD suppliers. As far as the heartburn counter then it's always fun and games with the specs our guys in the field pull in.
The higher frequencies are no problem, in fact probably better for the VFD in a way. It's the lower supply frequencies that cause the burn, not just in my chest but the DC link capacitors. The DC link voltage will drop due to the ripple thus the peak of the ripple current to the caps will create a lot of heat. The calculations to the VFD are no problem, it's the fact that somebody has written a specification with (apparently) no consideration to international specs that concerns me, and ends up with a lot of people spending a lot of time testing and calculating to meet a 'personal' specification. We spend zillions making sure our product meets all international specs and then we get the oddballs like this. However, as I mentioned, I'm not a genset man so this might be the norm and we've missed something somewhere.

 

[jghrist]

Manufacturer's tolerance on most equipment is ±0.5 Hz. Standard voltage tolerance on motors is ±10%. Steady-state voltage tolerance on computer equipment is 87%-106%. The VSD's may be only a small part of your customer's overall problem. I wonder if a similar spec is used for all other equipment.

Also, generators will be protected by under- and over-frequency relays. Induction types would usually only have a ±10% adjustment range. Microprocessor relays may have a range from 50-67 Hz on a 60 Hz system or 40-57 on a 50 Hz system.

If you quoted your standard voltage and frequency range and were considerably lower than competitors who quoted specialized equipment, the consultant may have a lot of explaining to do to his client as to why he has to spend more money.

 

[dpc]

For a standby generator running in an isolated island mode, I think a -10% for lower end of frequency range would be reasonable. Much below that would indicate a seriously overloaded generator or governor malfunction.

The generator usually has some type of underfrequency protection that would take it off line anyway below about 10% frequency dip anyway.

The spec may be a response to past problems running VFDs from standby generators. This has been an issue in many installations. One problem is noise from the drive causing problems with the generator voltage regulator. Another problem is the tendency for VFD to shut down for slight hiccups in incoming voltage and/or frequency. If you are trying to operate a water supply or sewage pump on an emergency generator, you don't appreciate the VFD deciding to shut down to protect its delicate self.

I would actually be much more concerned about VFD's ability to tolerate sustained undervoltage than I would be about underfrequency in this situation.

 

[edison123]

May be this guy misinterpreted "variable speed drive" applies for his "variable speed generator". Assuming it is a standard 50/60 Hz gen., at 75 Hz, the genset will fly apart let alone run for 4 hours.

 

[jbartos]

Suggestion to sed2developer (Electrical) Dec 4, 2003 marked ///\\A tender specification from a well respected consultant recently passed my desk. Part of the requirements under the Variable Speed Drive specification was: "...the VSD's shall be capable of operation without damage for a period of four hours on a 415V +/-25% three phase power supply and any frequency between 25 and 75Hz to minimise potential problems when operating on emergency power supplies and during switching of supplies".///Have several equipment manufacturers or one equipment manufacturer or equivalent been stated in the specification. This is a standard clause in AIA MasterSpec.\\\
Has anybody seen a spec like this? Not being a generator man myself I'm not sure if gensets are capable of swinging to these extremes.
///The voltage and frequency tolerances are excessive for mixed load. They may potentially suffice for heating elements, heat traces, regular owens, incandescent lights, rugged UPSs, etc.\\ Would the voltage and frequency regulators not shut the system down if this was to occur?
///It depends. Most of them will.\\I can calculate the efect of the lower frequencies etc on the DC Link caps but I'm the sort of person to question specs that require products to go out of the normal
///It depends how DC filters are engineered and designed.\\ International specifications. Or is this normal?
///It is not considered normal.\\ I've never seen it in the 16 yrs in the VSD business.
///I would request the manufacturer documentation to support the specification, which is often done.\\