Thread: Underloading
View Single Post
Old 03-01-2013, 10:08 AM   #3
ksanders
Moderator Emeritus
 
ksanders's Avatar
 
City: SEWARD ALASKA
Vessel Name: DOS PECES
Vessel Model: BAYLINER 4788
Join Date: Feb 2011
Posts: 6,267
Thats a good article FF

As a big part of my life I work in the generator industry, covering everything from operations and maintenance to design engineering, to control system integration.

I'm not a trained diesel mechanic, my expertise is the electrical side.

What I have seen in the field is exactly what the article said. Most backup generators are underloaded. Fortunately most of these units do not get enough hours on them to make wet stacking as prevalent as it could be.

Part of the problem is a misunderstanding of the National Electrical Code requirements. Part of the problem is the variable and intermittant loads applied to generator sets, and part of the problem is simple operator error when setting up the weekly or bi-weekly test paramaters.

Lets dig into this further...

The NEC article 700 requires all automatic generator sets to be capable of supplying the full load of whatever it is protecting. This leads to mistakenly oversizing the generator set. Lets take a typical home for example.

If you want to run your entire house on backup generator then many electricians assume that you need a generator capable of producing 200 amps (for NEC compliance), which is the size of most main breakers. Thats a 50KW generator.

The problem is that you'll never draw 200 amps. You'll actually in many houses, depending on air conditioning loads rarely exceed 50 amps.

Its easy to see that this leads to a severe underloading condition. The trick is to properly size the generator and still comply with the NEC. The way to do this is to perform a load calculation on the home and buy a generator that fits those needs. That will often still result in some underloading but not nearly as much.

Now lets take a business user and their weekly generator test.

People incorrectly choose to exercise their generators without a load being applied. In many areas with pretty reliable power, the weekly test represents the vast majority of hours on the generator. This unloaded running time can and does lead to wet stacking.

The facility operators run the weekly test unloaded because they generally do not want to stress their equipment with the brief outage that occures as part of the switchover process.

What the operators fail to realize is that almost every major brand of Automatic Transfer Switch now has a feature called "phase monitoring" built in. What phase monitoring does is wait for the generator and the utility power feeds to come into sync of their power waveforms and then switch very quickly. This minimizes the outage time to a very quick blink of the lights, somewhere around 50 miliseconds. since the power sources are in sync, there is no stress to any equipment.

Again, great article, and thanks for posting it FF
__________________
Kevin Sanders
Bayliner 4788 Dos Peces
Seward, Alaska - La Paz, Baja California Sur
https://maps.findmespot.com/s/XLJZ#history/assets
ksanders is offline   Reply With Quote