Underfrequency protection of steam turbine 1

[Ysaac]

Steam turbine blades are designed and tuned for operation at a rated frequency rotation. So, under and overfrequency protection, especially underfrequency, are used to prevent blade ressonance and fatigue damage in the turbine when it´s runinng with load (system conected) out of frequency range. So, once in this condition the turbine should be removed from system.

Well, imagine that a fault in the system results in islanding of plant and the speed regulator was not able to regulate and the frequency of plant reach a value considered critical (underfrequency caused by a overshoot of overfrequency) to a turbine: 100% or 70% i.e loaded.

My question is: Since that the plant at this moment is providing energy to its auxiliar service, should I trip the turbine considering it is operating in the same critical frequency value? In other words, is the operating without load and out of frequency range as dangerous as the operating with load and out of frequency range?

 

[GTstartup]

I don't think the load is relevant. Underfrequency is a protective relay function and not a turbine controls function and I have never seen a 81 relay interlocked with actual load.

The sequence normally goes like this:

Stage 1 - Alarm
Stage 2 - Trip Yard Breaker
Stage 3 - Trip Generator breaker
Stage 4 - Trip Turbine

 

[slavag]

GTstartup.
Im agree with you, except small remarks.
But..maybe topikstarter ask about big industrial generator and ask about some decoupling system. Here possible some evalution, but not on the turbine trip level, 47-47.5Hz ( for 50Hz systems)

 

[Ysaac]

Well, let me explain better...

I have read that in order to avoid blade ressonace and it´s efects, a steam turbine operating conected to a system must be removed from there if a underfrequency condition is present (for example 58 Hz in a 60Hz system). The books also says that these effects are cumulative.

My question is: in case of a turbine/generator NOT conected to system, providing energy only to its auxiliar service (10% of total load), should the turbine be triped to avoid blade ressonance and it´s efects if the frequency fall to 58Hz i.e ?

 

[RRaghunath]

Ysaac,
Under frequency trip is always time delayed in the order of a few seconds, do you think the speed governor is not capable of bringing the speed back to rated within that period! It is quite unlikely.

Check the governor mode selected. The governor should switch from droop mode to speed control (isochronous) mode the moment islanding takes place. If this is not already happening, you may need to modify the control circuit suitably.

Trust the above is helpful.

 

[GTstartup]

If the turbine reaches the speed at which you have determined is dangerous, it should be tripped regardless of the load it is supplying (with proper time delay - allowed level and time curves are shown in ANSI C37.106. The turbine trip should be the last of a series of attempts to restore the frequency by shedding load at progressively lower frequency settings.

Having said this, underfrequency protection is one of those things of which your grid authority (WECC, NERC etc or equivalent) may have strict rules.

 

[hacksaw]

blade resonance is an entirely different matter than the mains frequency.

believe you need to be more specific about the generator control settings versus the turbine trip conditions.

once the generator is tied to the buss it will hold onto the buss frequency

if it is a free standing generator then frequency is controlled by turbine speed

 

[cranky108]

Most under frequency relays have an under voltage dropout.
This being the case I would trip on under frequency when ever the frequency setting was reached (Yes I had a time delay set).

The turban should be near operating speed before the field is flashed, and after the field is flashed it was fair game for the under frequency relay.

 

[rbulsara]

Plus if you have under frequency conditions at 10% load, you have some other serious issues with mechanical and fuel systems.

Rafiq Bulsara

http://www.srengineersct.com

 

[byrdj]

the utility size steam turbine controls i comissioned prior to 1990 did not have "iso mode". so in a sitiation where the unit was only carring house load, say following a breaker opening event, the droop governor would normaly have the speed faster than rated. I don't do excitation, but it seems if the v/hz is in limits the generator

As hacksaw mentioned, turbine bucket resonace is a serios matter to large lp turbines where rated speed might be a narrow margin between two resonances. the manufactor of the turbine (or buckets) calculates what the expected life would be if operated in an off rated speed zone. I don't recall exact values but like 90 minutes near with 10 minutes for a zone prior to the do not operate zone. these were not wired for auto trip, only operator call.

the effectes of bucket resonace is not a function of steam flow. as a though, low flow would be a worse situation than with higher flow that would possible surpress the building of the resonace magnatiude.

even no load operation operation should be avoided if it could stimulated the higher bucket resonace. for example the high speed warming hold was not a constaint speed.

if i recall, the accumulative timers only count if the generator is excited, but the effects on the turbine buckets still occure

so yes, operation at any load is accumalitive on expected bucket life.

as a side note, some of the non US units i commissioned actually accumalated off frequency time while on the grid. the grid had swings into the resonace bands. the solution was a redesign LSB to allow a larger band of operation

 

[Ysaac]

Thanks Everybody, Especially to byrdj !!

 

[byrdj]

interesting discussion today related to bucket resonase induced failure that was contributed to torsianal vibration setup by jerky barring gear operation

 

[racookpe1978]

Resonance problems in operating large turbines usually only come up in frequency ranges far, far under the operating 3600 (US 60 Hz generated output) normal operating range. Resonances would be )for example) seen at 1350 rpm, 2100, 2800 or 3200 rpm starting up, but the units are designed to be stable & resonance-free near their 3600 rpm normal speed. At 3480 rpm (58 Hz), your operating conditions are so far from standard, and therefore so transient, that bucket problems are the least of your worries for the few seconds you be at that range.

As noted above, when on-line to the grid, you WILL be at 60 Hz - never down to 58 Hz - and so the on-line 3480 rpm conditions don't apply.

If you were stand-alone (plant loads only) for some reason and produsing your own power, your own equipment is not running properly nor stably, so you cannot stay at 58 Hz for more than a few seconds anyway. In an emergency facing large area blackouts where you are the only power available, you still would have no reason to stay at 58 Hz.

A smaller deviation (maybe to 59.8 Hz) during stand-alone conditions can't really be justified either.

 

[byrdj]

rotor critical speeds and bucket resonaces are not the same.
where as rated speed my be above or below 2nd critical. the rated speed stimulus for bucket resonace my be between the 11th and 12th. (i believe the diagram that shows this is called the Cambell) imagine all those buckets atached to the rotor ar like tuning forks. the fequency the bucket rings at is its resonace. if not tuned properly, this resonace results in cyclic fatigue in a relatively short time

as also discussed today, the rotor torsianal vibration can become a major problem in try to tune buckets

 

[ScottyUK]

racooke,

You assume that the grid will be at 60Hz, or 50Hz, but it's entirely possible and likely under contingency conditions that the grid frequency can be well away from nominal.

In the UK, which is a 50Hz country, the grid operator requires our machines to be capable of operating continuously down to 47.5Hz and to be able to deliver power for a short period at 47Hz. The last thing the grid operator needs when the system frequency is down and they are trying to recover it is for more generation to be lost as this will lead to a cascade and possibly outright collapse of the grid. I expect the US transmission system operator will have similar requirements.

47Hz is nearly at resonance on some steam sets so there is often a unit lifetime penalty incurred if the frequency dips that low for more than a second or two. Gas turbine prime movers can not maintain output at such low frequencies because the compressor discharge pressure drops away so quickly. Output can be maintained by over-firing the engine but this has a massive effect on hot parts life.


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If we learn from our mistakes I'm getting a great education!

 

[racookpe1978]

Thank you for the good (operational) feedback! [Ya mean mah turbines don't always run perfectly right at da speed where dere supposed to? Darn. Dabgunit. Shucks. Shoot. 8<) ]

 

[ScottyUK]

Yep, dats wot ah mean.


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If we learn from our mistakes I'm getting a great education!

 

[thewizardofgaz]

I am in the middle of this right now with a 250 Mw steam unit. The fatigue damage is load related. It is virtually non-existant at zero load as evidenced by the fact the vendors allow you to do overspeed tests without any limitations. How much does load affect it? Don't know. I do know though that for both our units we have sequential tripping, turbine stop valves close first and then the genrator breakers open on generator reverse power. This is of course for a mechanical trip and not an electrical fault.

 

[ScottyUK]

If that is your tripping scheme then be aware that the reverse power relay can be confused if you trip the set mechanically and the generator still has a heavy reactive load on it. Small phase errors in the instrument transformers can, and do, give the illusion of forward power when the set is in fact motoring so the reverse power relay will not trip. I would consider an alternative tripping signal in addition to the relay. As a suggestion, something like: hydraulic power to the MSVs and governors de-latched AND MSVs confirmed closed initiates a trip signal to the gen breaker. Adapt to suit your machine!


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If we learn from our mistakes I'm getting a great education!