Underspeed vs under frequency protection for diesel driven alternator

[BK32]

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?

 

[FreddyNurk]

Underfrequency can't work if there's no AC present, whereas underspeed for diesel is usually fed by a magnetic pickup on the flywheel, thus is independent of any AVR or excitation issues. Some systems may not be able to detect a running machine on loss of voltage, depending on the controller present.

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.

 

[MatthewDB]

To answer the very last sentence, the field may be dependent on the cooling fan. If the field source is present without rotating, the field coil could burn out when the cooling fan is not moving air. In a brushless exciter scheme the exciter field could be at risk. In a fully self excited scheme this isn't a problem (PM exciter, self excited main or self excited exciter.)

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.

 

[waross]

Is this a very old machine?
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
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

waross, Basler's SR4, as far as I can recall, doesn't have any UFRO feature. The machines I worked on that were fitted with them were powered off the generator voltage (as opposed to using any PM that might have been fitted), so they ended up dropping the field on underfrequency events anyway. It was only two years ago that my former employer stopped buying them and moved on.

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.

 

[waross]

From the SR 4A manual:

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

 

[FreddyNurk]

waross, I don't believe that they did. To be honest, last time we looked at placing an order I don't remember seeing the option for it either. I knew everything else I'd seen had this feature as standard, and I also knew that the SR4 is not a new design (as also noted in the dates on your manual excerpt). I mentioned it as its only recently (last few years) that Basler had been pushing people onto their new digital AVRs (all of which, of course, do have this feature), and there had been considerable reluctance in certain circles to move away from the SR4, thus the model is still somewhat relevant.

 

[BK32]

Thanks all for your responses to my initial query.

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.

 

[ScottyUK]

Slightly different perspective, some of which might apply:

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.

 

[waross]

As you know, inductive reactance is proportional to frequency. If full voltage is maintained at lower frequencies, inductive loads such as transformers and motors will saturate magnetically. The impedance will drop and the current will increase. Burnout is rapid at under frequency with full voltage. On some of the older machines the AVR would burn out even faster than the motors and transformers.
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

 

[catserveng]

On the Basler SR family of regulators (SR4 and SR8) they didn't offer these regulators with any form of under-frequency protection or control, the UFOV was a separate module. I think the SR family of regulators, along with the SSR family are all now obsolete from Basler, with the AVC family being the analog replacement. The UFOV module went obsolete about 5 years ago, I had one on the shelf for a customer who finally used it last year, if it fails again it will be a retrofit for the regulator.

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,

http://www.gallois.be/ggmagazine_2006/gg_03_05_2006_102.pdf

On your Jeumont AVR, you can try here,

http://www.jeumontelectric.com/pages/uk/page.aspx?c=3&p=12,

but some of their older products were a mix of licensed products from Westinghouse and Schneider.

Hope that helps, Mike L.

 

[crshears]

Thanks for the ship's power supply link, Mike! Excellent stuff, and very useful to me for reasons that will be obvious to some...

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

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]