Harmonic Voltage Distortion on Standby Generators

[X49]

I'm designing a system with a standby generator (~150kW), where most of the load will be 6 pulse VFD's. The generator will be sized to account for the additional heating in the alternator due to the harmonic currents.

My question is, what is an acceptable limit for harmonic voltage distortion. 5% THD seems to be a common limit for utilities, but this will be difficult to achieve on the generator. Is a higher level of THD acceptable for the relatively short periods when the standby generator will be running? Are there any relavent standards? I could install harmonic filters on all of the drives but I have heard of compatibility issues with gensets.

The loads are primarily VFD's, lighting, heating, and a PLC-based control system.

 

[ScusaMe]

An Harmonic Analysis of the system would be extremely beneficial to you as you ponder your questions.

Too, 'twould seem that a discussion with the manufacturer of the generator would be in order.

Might ask him how much harmonic distortion his generator and its Voltage Regulator can tolerate.

Then you might ask: How much voltage distortion can the connected loads tolerate and what consequence would say 3% or 5% etc. have upon their performance.

 

[X49]

On utility power, the distortion will be limited to around 2-3%.

On standby power, it will be between 7% and 11% depending on whether I add harmonic filters on the larger drives. The genset sizing software has a default upper limit of 11% so I assume this level is acceptable to the voltage regulator, but I will confirm.

As far as the loads, I shouldn't have to worry about the drives or the controls (on a UPS). The only other loads are fluorescent/HID Lighting , small HVAC motors, and convenience receptacles.

Is there a standard motor derating chart for voltage distortion?

 

[dpc]

Refer to IEEE 519. This will give you a common point to discuss with the generator supplier. They will be familiar with this standard.

I would make the harmonic distortion tolerance part of your engine-generator specification.

 

[iop995]

Harmonic limits are set by agency rules or in insulated network by loads type and/or user. Most of VFD don't care too much about harmonics level as input voltage is rectified to obtain DC bus for inverter. Maybe others load type to impose such limit.

 

[ScusaMe]

dpc [small]26 Jan 12 17:49[/small]

"Refer to IEEE 519. This will give you a common point to discuss with the generator supplier."

Hmmm. What section(s) of IEEE-519-1992 do you feel address harmonic distortion issues pertinent to AC Generators—particularly with regard to the concerns stated by X49 ?
Perhaps you are thinking of sections 6.2 and/or 7.3, which while mentioning AC machines in a general, non-specific treatment, do not address X49's concerns

I've read thru it now for the umteenth time and still can't seem to find where harmonic distortion associated with AC Generators is discussed.

Tnx