Automotive/Alternator Frequecy

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Expert: - 4/29/2010


Question
I have Cummins gas engine and AVK alternator of 2150KW. at which frequency my genset should shed its load without tripping. e.g. two genset are running in parallel, due to some reason one genset trip, other gas engine could not take all load to avoid tripping so it mush shed some load, I want to know at what frequency it should shed its and and at what frequency genset should trip to save itself.

Answer
Two (or more) alternators will remain synchronised as long as their respective governors can track the changes in load and there is sufficient reserve on both generators to take additional load.  Load rejection will occur by a given machine when that machine reaches the limit of its output capability.  This all has to do with "droop setting" which is a ratio of change in frequency to change in load.  Utility droop settings are typically very low, reflecting the fact that most utilities are tied together on grids comprising up to several hundred generators, all synchronised onto what effectively is an "infinite bus", with very little perceptible change in frequency as generators are added and removed - 1 Hz per 40 GW is not uncommon for such systems.

Very small gensets, ie: 10kW and less, have droop figures closer to 1 Hz per 2 or 3 kW.

What the droop setting might be for YOUR gensets would best be gleaned from the documentation for each of them - I would strongly suggest you get this if you do not have it.  As to the overload trip point, or the point at which some load is selectively cut off, that would be a function of current-level or power-level detection and whatever protective-relating settings are specified for your gensets - again, the original documentation should provide this, although general current or power limit settings could be calculated and tried based on the unit ratings    

You speak of one of your two units having a rating of 2150 kW (about 2700 kVA at 0.8PF?) ... if the rating given is correct, that would be over 2 MW, or typical of the electrical output of a locomotive approaching the 4000 HP range.  That does, however, strike me as an unlikely power level for "gas" fueled engines, although the Cummins name does not suggest that this is gas ... unless these actually are stationary or semi-portable Diesels set up to burn a charge of natural gas mixed with the air. fired by a small amount of injected Diesel fuel, that is, eight or so percent of what would be injected for full Diesel compression-ignition operation.  That was not an uncommon practice for small regional power plants in regions with abundant natural gas, and was in use for some decades.

You might try some model-number searches on Google.  I find that Google searches work most productively if the search words and phrases used are terms and phrases that would be unique to the documents you are searching; ie: "generator droop setting" as an advanced-search phrase, along with manufacturer names and model numbers might get you what you need.

Good luck ... EGK  

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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