Steam Turbine Overspeed upon Generator Breaker Open 1

[fsxn155]

Dear All

Recently while stopping a 55 MW Generator Drive Steam Turbine, as per normal shutdown procedure given by Turbine OEM, Turbine load was feduced to 4 MW and Generator Breaker Opened. However Turbine Speed reached Overspeed Trip limit but unfortunately Turbine couldn't be tripped due to malfucntion of trip system. We were able to identify the cause of Trip system malfunction as well as reason for Control System not being able to mantainn speed at set point during the event.

Being from mechanical background, I just want to know is it standard procedure to open the Generator Breaker at certain minimum load for some reason from electrical side (reverse power etc.) and if so why not directly trip/stop the Turbine at certain minimum load rather than opening the breaker. (I believe as Turbine can trip at any laod, there must be protectons avaialble to prevent reverse power)

Appreciate if anyone can share the shutdown procedures followed at other sites

Regards

 

[davidbeach]

The normal shutdown process for most steam turbines is to close all of the steam valves and then trip the generator breaker on reverse power. The reverse power condition proves that the valves actually closed.

A protection trip opens up the breaker and trusts that the steam valves all work. But for a normal shutdown there’s always time to deal with a valve issue before opening the breaker.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[fsxn155]

Dear David

Many tganks for the useful inputs. I request you to please share any reference document from OEM procedure as it would be extremely useful for me to propose the change
Thanks again for the feedback

 

[crshears]

See this thread:

https://www.eng-tips.com/viewthread.cfm?qid=518703

CR

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

 

[cranky108]

I would think closing the valves and waiting for the reverse power to trip the breaker, would be easer on the turbine and bearings.

 

[crshears]

From the thread referenced above:

"Sequential tripping" was employed on all eight of the 300 MW steam units in the first generating station I worked in. Its function essentially was to ensure that in situations where the trip request was not from operation of an electrical protection, a closed signal was received from all steam supply valves [plus open signals from the pair of units that had "release valves"] via limit switches before GSU breaker[s - ring bus configuration] tripping was initiated.

This means that once all steam admission valves were confirmed shut [ and, where applicable, release valves were confirmed open ] the unit breakers would trip. There was no actual trip from reverse power, as this protection typically incorporated a time delay, and sequential tripping does not include any intentional timed tripping.

CR

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

 

[davidbeach]

Reverse power can be set for zero delay or for many values greater than zero. And you don’t have to be particularly clever to cascade delays. So any non-negative delay is pretty much possible.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[wcaseyharman]

The General Electric large steam turbine technical material I have from the ‘80s (I posted a page in the referenced thread) has a comment that basically says the generator breaker should always be opened under reverse power conditions unless there’s an electrical fault.

All 5 of our steam turbines now open the breaker on shutdown via a reverse power relays. We did have a hitachi steam turbine that opened the breaker at 5 MW but during the relay upgrade we changed it. The scheme now trips the turbine at 5 MW and the breaker opens when all the stop valve limit switches are closed and the relay sees reverse power for a second.

 

[edison123]

Given that steam doesn't have the mass and inertia of water (hydraulic turbines), how would a small amount of steam due to valve passing would dangerously overspeed the generator on breaker opening?

In hydro machines, overspeed of 1.7 to 2.1 are designed for but I have seen a max of 1.2 only for stem turbines.

Muthu

http://www.edison.co.in

 

[crshears]

(H)ow would a small amount of steam due to valve passing dangerously overspeed the generator on breaker opening?

True that there's not a lot of mass or inertia there, edison, but the energy content is huge; with a machine not connected to the grid you'd be surprised how little steam it takes to cause an overspeed condition, hence the need for well maintained, freely operating, and frequently enough tested governors, both service and emergency.

CR

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

 

[wcaseyharman]

Hydros have something called runaway speed, where the speed of the wheel reaches the maximum velocity of the water and the turbine will no longer accelerate. For example, runaway speed on our 50MW 200 RPM is around 350 RPM. The generator and turbine are typically designed to withstand runaway speed.
Steam turbines have a much much higher runaway speed, and the turbines and generators are only designed for 110% - 120% of nominal speed. So the big difference is during a failure of the main speed control system when the turbine is disconnected from the grid, hydros will survive as they are designed to operate at runaway speeds, and steam turbines will fly apart, which is why there is so much emphasis around reverse power sequential breaker tripping - There are lots of stories of steam turbines blowing up on overspeed and sending turbine blades flying and killing people.

Hydros have a different susceptibility with wicket gate closing speeds, buts that for a different thread.

 

[cranky108]

Wicket gate? I don't think any of our hydros have wicket gates. Maybe gun deflectors or alternative needle valves.
Then again, the heads here are much higher.

 

[wcaseyharman]

You’re right, the wicket gate issue is only for a subset of hydros - Francis turbines (typically for medium head installations. I’m actually not sure about Kaplan susceptibility as we don’t have any).
But the comments about runaway speed are the same for all hydros as far as I know, and definitely include impulse type turbines (assuming Pelton type) like you have.

 

[waross]

Regardless of the type of flow control, the closing speed is critical.

On 17 August 2009, a turbine in the hydroelectric power station of the Sayano-Shushenskaya Dam near Sayanogorsk in Russia failed catastrophically, killing 75 people and severely damaging the plant. The turbine hall was flooded, and a section of its roof collapsed. All but one of the ten turbines in the hall were destroyed or damaged. The entire power output of the plant, totalling 6,400 megawatts, was lost, leading to widespread power outages in the area. An official report on the accident was released in October 2009.

Gates closed too rapidly.
Link

A November 2010, peer-reviewed article in International Water Power and Dam Construction suggested a previously unpublicized direct cause for the turbine failures: draft tube waterhammer.[20] The immediate cause of this is proposed to be simply the too-sudden closure of the turbine wicket (flow-control) gates. Too-rapid closure results in rupture of the liquid column as the local pressure downstream of the gates goes to vapor pressure. This frees the draft tube liquid to first surge towards the tailrace and then reverse, eventually slamming rapidly into the turbine with great force. Only such a phenomenon seems capable of producing the extremely sudden, extremely large, and extremely vertical force which is evident from photographic and verbal descriptions of the damage. From the article:

This hypothesis is that the explosion was caused by water column separation in the draft tubes of the destroyed units. This condition can readily be caused by a too-rapid wicket gate closure during unit load rejection. Adjustment of governor times to unsafe values to achieve fast response to operating load changes may have occurred in recent times in response to a need to improve grid frequency control. This, combined with compromised stud connections due to poor maintenance, can explain the extreme violence of this accident.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

I was referring to the valve not closing fully and a small amount of steam leak causing overspeed. Yes, sudden load throw with sticky valves could destroy the machine but don't they have bypass valves/rupturing diaphragms etc to vent out the steam for such a scenario? Yes, I have read about destroyed steam turbines due to valve malfunction/out of phase synchronization.

On a personal level, we were doing a 50 MW hydro rewind a few years ago with a platform mounted inside the stator and the turbine in place. One night, when we were not working, the stop logs failed, the turbine starting turning and was destroyed completely, It took months for the client to replace the lower bracket, bearings and the turbine.

Muthu

http://www.edison.co.in

 

[waross]

It's a matter of the energy required to spin an unloaded turbine generator compared to the energy content of the steam passing a leaking valve.
High pressure steam has a high energy content and it doesn't take much energy to spin an unloaded generator.
Reverse power indicates that there is not enough steam energy being input to drive the generator at even synchronous speed.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

I’ve only seen/heard of rupture disks on the condenser to prevent overpressure if the condenser cooling system fails. There can be main steam shutoff valves at the boiler but it seems those are either manual valves or motor operated valves which are very slow. The main stop valve is what the turbine relies on to limit over speed, which is typical for both gas and steam turbines.

One other thing to mention - the steam turbine is under a strong vacuum from the condenser, which makes windage losses of the turbine really low - Other than the generator rotor fan it doesn’t take much power from a leaky stop valve to accelerate the shaft.

As an example, steam turbines can motor at less than 1% of rated power. We actually had a little issue on a very recent relay replacement where our reverse power relay was set at -1 MW on a 40 MW steam turbine and the breaker never tripped so the operator had to manually trip the breaker. I think we had to change the reverse power trip on that turbine to -0.25 MW.

My point was hydros have a natural limit to overspeed based on maximum water velocity where steam from a steam turbine does not have a maximum velocity (it does, but it’s way way higher) so hydros can be built for maximum possible mechanical speed and sequential tripping is less important. Steam turbines are built for only 120% (ish) overspeed, but since they can spin much faster, even on a relatively small amount of steam, it becomes imperative to prevent any uncontrolled operation, and one of the best methods is sequential tripping of the generator breaker on reverse power.

Waross: that phenomena is what I was referring to, colloquially called “breaking the water column” in the industry. Properly setting the maximum gate closing time becomes critical when commissioning hydro turbine governors on Francis turbines. The OEM performed a hydraulic transient study for us and it was very interesting to see the tradeoff between maximum turbine overspeed versus penstock over pressure and draft tube vacuum.

 

[davidbeach]

High head hydros also (may) have bypass gates that open as the wicket gates close. That water column just keeps roaring along but as the water stops going into the turbine it flows out of the bypass discharge; the bypass discharge is much more energetic than the normal turbine discharge since there isn't the turbine extracting energy from the water. The stop valves at the top of the penstock then close, allowing the penstock to drain; that's a much slower process than closing the wicket gates.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[cranky108]

In run of the pipe applications, there is no valve at the top of the penstock. There is a ball valve before the turbine, which closes very slowly to prevent water hammer. So until it closes, the secondary needle valve, or deflectors must handle the bypass of the turbine.
Note that in some run of the pipe applications, there maybe more than one unit on the same pipe.