Underspeed vs under frequency protection for diesel driven alternator
Underspeed vs under frequency protection for diesel driven alternator
(OP)
Have recently come across a generator with both under frequency and Underspeed protection. Both the under frequency and underspeed protection relays were connected to the generator ACB trip circuit. However the Underspeed relay was also connected to the "de-excitation" circuit. Once the speed of the alternator drive shaft dropped below the Underspeed relay threshold the Underspeed relay contact would energise a control relay which would result in the "shorting" of the main field winding and de-excite the machine.
My question is essentially what is the difference between the two protection schemes? Why does one de-excite the machine and not the other?
Also, assuming the Underspeed relay is bypassed, in what circumstances would the machine be at risk?
My question is essentially what is the difference between the two protection schemes? Why does one de-excite the machine and not the other?
Also, assuming the Underspeed relay is bypassed, in what circumstances would the machine be at risk?





RE: Underspeed vs under frequency protection for diesel driven alternator
Some AVRs will automatically drop the field if the frequency is too low (Stamford's MX341/321 units are some of them), whereas some of them will keep trying to excite the machine all the way down.
RE: Underspeed vs under frequency protection for diesel driven alternator
I would expect the frequencies to be different. The under-frequency relay is load and prime mover protection so it should be high. The under speed is lower where field protection is needed.
RE: Underspeed vs under frequency protection for diesel driven alternator
The historical issues with under speed were Automatic Voltage Regulator burn out at low frequencies, and stalling and/or voltage collapse due to motor starting loads.
This issue was addressed years ago when the Under Frequency Roll Off feature was added to AVRs.
UFRO made AVRs self protecting in the event of low frequency and maintains a close to optimum Volts per Hertz ratio so that inductive loads such as motors do not saturate. In the event that speed is pulled down by the starting of a large motor, UFRO aids speed recovery.
Under speed cooling issues are also avoided by the reduced field voltage when UFRO becomes active.
I see mostly generators under about 1.5 MW. It has been over thirty years since I encountered voltage regulators without UFRO, and the machines in question probably dated from the 1940s.
Catserveng may have insightful comments.
Bill
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"Why not the best?"
Jimmy Carter
RE: Underspeed vs under frequency protection for diesel driven alternator
To be honest I was wondering what issues were going to show up for such things, we didn't see any issues using the SR4 units, but the voltage disappeared pretty quick on underspeed events at any rate, something I expect was due to not using the PM option on the alternators.
RE: Underspeed vs under frequency protection for diesel driven alternator
First Printing: August 1971
Printed in USA
© 1997, Basler Electric Co., Highland, IL 62249
An excerpt:
Prolonged operation at speeds lower than normal can damage the voltage regulator and/or exciter
and generator field. If operation at reduced speed is essential, input power should be removed from
the regulator or an underfrequency overvoltage protection module (UFOV Module) should be added
to the system
Did your units have the UFOV module added?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Underspeed vs under frequency protection for diesel driven alternator
RE: Underspeed vs under frequency protection for diesel driven alternator
Waross to answer your question, the machine was installed circa 1985. It is installed on a auxiliary oiler replenishment ship. The DA is a 2MW shaft driven alternator (i.e. it is driven by a diesel engine via a reduction gearbox that also drives the main propulsion train on a ship.) The alternator is a 60Hz, four pole, brushless, indirect self excited compound machine.
In relation to the point made by MatthewDB above concerning the movement of air, I can confirm that the machine is cooled via a CACW system, with the fan mounted on the shaft to circulate the air via a sea water cooled heat exchanger through the alternator. On this basis alone, it appears as though the underspeed protection could have been introduced to prevent subsequent overheating of the field windings.
The AVR fitted is a Jeumont Schneider A.V.R. type EXCRT. There are no details at hand as to whether UFRO protection is provided? I believe Juemont Schneider no longer exist and I personally would not know whom to ask to find out further details?
Thanks for all your assistance.
RE: Underspeed vs under frequency protection for diesel driven alternator
On larger turbo-alternators underspeed is an engine protection element which opens the generator breaker, on the basis that it is better to have a load dump and allow the machine to recover to sync idle ready to reclose onto the system than it is to trip the engine and have to initiate a hot restart. Trips and starts are hard on the engine in terms of wear & tear. Under-frequency is typically a protection for the alternator and GSU transformer, both of which are susceptible to over-fluxing (Volt/Hz) damage. Typically the transformer get in trouble before the alternator. On a ship I'm sure there are also transformers and motors which would suffer similar problems, albeit a little smaller.
RE: Underspeed vs under frequency protection for diesel driven alternator
I am thinking that the under frequency trip is to protect the connected equipment from possible burnout due to under frequency.
The under speed trip may be to protect the AVR. I have had issues with frequency relays that were not reliable at reduced voltages so the under speed trip may also be a redundant safety trip in the event of issues with the under frequency relay and low voltage.
If your AVR has a three terminal strip or section labelled "Common", "50 Hz" and "60 Hz" that is probably the UFRO selection.
A typical arrangement for UFRO is for the voltage to start dropping when the frequency falls below 57 Hz (or 47 Hz on a 50 Hz machine)
But in an instance such as this we are guessing as to the intentions of the original designer.
Thanks for the information re the SR4 AVRs Freddy.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Underspeed vs under frequency protection for diesel driven alternator
I really don't remember seeing UFRO (or volts/Hz as it is sometimes called) on analog regulators until the SSR regulators came out in the early 1990's, can't speak to other manufacturers because at that time it was mostly Basler for us, with some sites using CAT regulators.
On shipboard generators that could run as a "shaft generator", underspeed protection was mandated by the regulatory agencies like DNV and ABS because if you are in the "cruise" mode and the vessel needs to make a sudden speed reduction for maneuvering or emergencies, there is no time to contact the engine room and shift operation of the generator. My understanding that the thought was it was easier to line up a shaft generator to a prime mover and restart than it was to deal with a failed voltage regulator due to running at too low of a speed. I had a DNV article about how they expected the control and protection of a shaft generator to work, but can't find it right now. As pointed out above, under-frequency protection doesn't seem reliable as speed drops, especially on the vintage controls you seem to have. I don't think it is common today as shipboard electrical controls have been highly automated as described here, www.gallois.be/ggmagazine_2006/gg_03_05_2006_102.p...
On your Jeumont AVR, you can try here, http://www.jeumontelectric.com/pages/uk/page.aspx?..., but some of their older products were a mix of licensed products from Westinghouse and Schneider.
Hope that helps, Mike L.
RE: Underspeed vs under frequency protection for diesel driven alternator
What a resource these fora are! It's like attending EngTips university or something, with unbelievably knowledgeable and experienced profs who are willing to share their wealth. Thanks to all of you.
CR
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