Backup generator frequency variation 4

[alwaink]

For a diesel backup generator, what's the typical output frequency variation? I've read specs that say 60Hz +/- 5%. Is it typcially off by that much?

In my application, the load is very sensitive to frequency variation. The performance starts dropping when it's outside of 60Hz +/- 0.5Hz range. Any comments? Thanks.

 

[davidbeach]

Frequency excursions will typically be the result of load changes. If you need tight tolerance on frequency (who ever designed the load equipment should have taken their head out of the sand) your best bet would be to avoid having any motor loads on the generator.

 

[waross]

A typical small (15 KVA to 200KVA) diesel generator will run at 61.8 Hz. at no load and 60 Hz. at full load. This is based on 3% speed regulation and is very common. It is possible to get autogenous governors that will make a brief frequency excursion with load changes and return to 60 Hz.
respectfully

 

[itsmoked]

We use those isochronous add-on retrofit electronic governors (Woodward) and get only tiny brief frequency changes with 1/3 generating load changes.

http://www.woodward.com/pdf/ic/04106.pdf

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[catserveng]

There is actually two issues involved with generator set stability. One is "steady state" or how well the governing and engine dynamics maintain engine speed with no load changes, and "transient response" which is how well a unit responds to a load change.

Most manufacturers these days conform to ISO 8528 part 5, and will call out a class of performance they meet. ISO 3046 part 4 has guidelines for engine speed governing.

In reality, stability is a function of the integration of the governing and the engine mechanical response. A mechanically governed engine may have a fairly poor steady state stability of 3-5%, and some hydro-mechanical governors can be better than 1%. Electronic governors are usually much better, but rely on an electric or hydro-mechanical actuator to control the fuel system, and this usually reduces the stability somewhat, but in general, electronically governed units have better steady state and transient response than emchanically governed units. But the best electronic governor in the word can't make a poor mechanically responding engine any better. There are also the issues touched on a little in the previous posts such as droop or isochronus governor operation. And the newer engines have the governing imbedded into the engine ECM, and just because it is electronic doesn't make it the best electric power application governor.

A lot of facilities use really new and neat power conserving and highly efficient machines, and on utility they work just fine, because especially here in the US most utility power is very good by most anyone's standards (and yes, I'm sure there a few exceptions), but then a backup generator is installed, and usually it's selection is driven by price, not performance, and when the unit is tested, or worse just actually used in an outage situation, and things don't work in the plant, people start to wonder what is wrong with the generator.

A lot of available equipment has either a hardware (jumpers or switches) or software adjustments to allow for wider frequency and voltage tolerences depending on stability of source. If your equipment doesn't have that option, and you need a backup power source, then integrating some form of power conditioning or UPS will be required.

Any standalone generator will have frequency and voltage variation, both in "steady state" and in response to load change. How well it provides stable operation is a function of system integration, engine/generator/control selection, and installation.

These two links have some pretty good general info, and most manufacturers have more detailed information in their sizing and application guides,

http://www.cumminspower.com/

http://www/literature/technicalpapers/F-XXXX-SizingGensets.pdf

http://www.cumminspower.com/

http://www/literature/technicalpapers/F-1456-EconomicRiskStandbyPower.pdf

Now I'm not a Cummins guy, but sometimes their technical papers are very good, hope that helps.

 

[alwaink]

So is there some kind of frequency regulator that I can add to the output to make sure it stays within 60Hz +/- 0.5Hz?

I've looked at frequency converters that can take 47-63Hz input and output 60Hz +/- 0.1Hz. But those are really expensive (~$3,000).

Any other cheap (less than $500) alternative ways I can consider in my case? My load performance is very sensitive to frequencies outside of 60Hz +/- 0.5Hz.

Thanks.

 

[waross]

Do you do a lot of block loading or load dumping?
If you have a steady load, most generators will have no problem staying within 0.5Hz.
A generator using droop control will typically drop 3% in frequency from no load to full load.
If you have a steady load you should be able to adjust the speed/frequency to within the 59.5 Hz. 60.5 Hz. frequency band required.
If your frequency sensitive load is a small part of the total load you may consider a UPS or inverter to power the sensitive load.
Some UPS/inverters will accept a varying input frequency.
Some UPS/inverters will complain about off frequency conditions.
Some UPS/invertors themselves need a stable frequency.
This will be something to check if you choose a type of inverter.
BTW

I've read specs that say 60Hz +/- 5%.

That's a 10% variation. Are you looking at very small (below about 10 KW) generators? 10% is a pretty loose frequency tolerance for many generators.
respectfully

 

[catserveng]

A few more details might help before trying to get recommendations for a solution.

What size generator are you dealing with?

Is it an older unit or a newer unit?

What is it's rated engine RPM and what is the voltage level?

What kind of a governor system does it have, electronic or mechanical?

Is your problem mostly related to "steady state" or do you have varying loads that is causing the frequency variations?

What kind of voltage regulation do you have, constant voltage or volts/hertz?

Are there any apparent mechanical problems that may be contributing to your stability problems, like fuel and/or oil leaks? Did the engine perform better at one time then got worse?

If you're trying to fix a specific problem for a particular unit you'll need to answer these kinds of questions. If you're looking for general information on how diesel generators should work, try this site,

http://www.egsa.org

and get a copy of their On Site Power Generation Reference Book.

Hope that helps,

 

[alwaink]

Sorry about being ambiguous in my previous posts. I'm trying to have a backup system to keep 10 units of Sterling cooler (200W each) running. Each sterling cooler only requires about 100V/2A single phase.

So I'm only looking for a small generator, 2kW or so, not an industrial type of generator. Just something I can buy online or in a shop. The problem is that the way these coolers are designed, they work under tight frequency tolerance as I mentioned above, 60Hz +/- 0.5Hz. They would still work outside this tolerance, but the efficiency would drop exponentially. So within this freq range the loads are pretty stable.

With my limited knowledge in backup systems, looks like UPS would work best as far as frequency tolerance is concerned. However, the backup time is far less than a small diesel generator. That's why I'm trying to find out if a typical generator can stay within that frequency range.

So to answer some of the questions above...

-the loads are steady and small power requirement (2kW).
-frequency variation is not caused by varying loads but rather the way these sterling coolers are designed.
-I don't know how to answer the governor question. Would this be of concern since it's only 2kW?

I really appreciate the feedbacks above. Thanks.

 

[catserveng]

A small generator that would have the stability you need is likely going to be a real challenge, since smaller units in the size range you are looking at are usually air cooled and use spring, paddle and fan or liquid cooled using simple spring and flyweight governors, and they typically operate in droop as WAROSS pointed out. Even retrofitted with a small electronic governor most small units don't have good frequency stability, at least in my experience.

Technically your best solution would be a small generator with a small wide frequency input rotary UPS. We have built up some systems like this for military applications, but they are definately not in the price range you were targeting. You might also find a static UPS (battery type) that has enough input freqency tolerence to work with a small off the shelf contractor or homeowner type generator. APC has some industrial units we have used with small remote systems that have a good frequency input tolerance, for 60 Hz systems they accept input power from 54 to 66 Hz

Try PSCPower here

http://www.pscpower.com

 

[ScottyUK]

At that size level Honda have some little gennys which use an inverter to produce clean power from whatever the set produces internally. The frequency stability should be very good - it will almost certainly have either a quartz crystal or a ceramic resonator controlling it, so it will probably have better stability than the grid.

http://www.hondapowerequipment.com/gensup.asp

Search Google for [blue]Honda Inverter Generator[/blue] - there are dozens of suppliers, so there is likely to be one near you. I expect others make this type of machine too, but I like Honda's build quality. Over here their advertising tagline is "Wouldn't it be nice if everything just worked?". I wish they'd built our power station.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[itsmoked]

I'm with Scotty. There are also eBay dealers that sell inverter generators. I have friends who've gone that route and been very happy. Kipor 1000-6000w inverter generators.(They are bright yellow.)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[alwaink]

Thanks for all the comments. I will look into the Honda inverter generator.

 

[itsmoked]

The Hondas are really pricey! But they work really well too.
Some of them allow paralleling with a small synchronizing cable.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gpsolar]

We installed some backup inverter chargers connected to the grid and a stanby generator. The systems charges find from the grid but on the generator it does not. It is 220V / 50Hz. checks on the generator shows a voltage of 200 - 210V and frequency 53.3 Hz. A tripplite APSINT 3636 inverer does not accept the generator input and we change to a 7.5 KVA Sukam Inverter but this failed after sometime (the DSP fails). What is wrong ad what can we do?. The inverters have no frequency adjustment.

 

[sibeen]

You might also find a static UPS (battery type) that has enough input freqency tolerence to work with a small off the shelf contractor or homeowner type generator. APC has some industrial units we have used with small remote systems that have a good frequency input tolerance, for 60 Hz systems they accept input power from 54 to 66 Hz


The problem with this is that the UPS output frequency will follow the input frequency up to a certain, normally selectable, range. So, depending upon the UPS selected, you may find that the output frequency will follow the generator frequency and be well outside the tolerance of the loads (0.5Hz), so not in any way alleviating the problem. A better solution would be to use an inverter which doesn't try to synchronise its output to the incoming supply. As long as the inverter has reasonably tight frequency tolerances then everything should be OK.