Low Forward vs Reverse Power Protection Synchronous Generator

[Shark96]

For a steam driven turbo-generator, which protection is better for normal shut down procedure: Reverse power or low forward. The generator manual does specifically say to shut down the unit on low forward. Are there any disadvantages to using low forward tripping during normal shut down?

 

[davidbeach]

For a steam turbine you should go reverse power. If you get to reverse power you know that all of the steam valves are closed. At low forward power you might still have some small amount of steam making its way in; if that's the case it might be enough to overspeed the unit once it is disconnected from the power system.

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]

Another issue is on start-up, if the kicker pulse is not enough to push the unit above the low-forward power setting, it can trip off.
The other issue is the turbo can over heat if it is run too long without steam.

 

[Shark96]

Thank you guys. Much appreciated.

 

[crshears]

If underload protection were to be used, it would be a simple matter to install a pushbutton to energize a time-hold relay solenoid, the normally closed contact of which would open and block the underload trip for a minute or so until the governor speeder gear could be adjusted so as to ensure there's sufficient load placed on the unit before the timer times out.

I heard anecdotal stories of nasty operational issues that arose because reverse power relays proved unreliable; I like the idea of using an underload protection instead.

CR

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

 

[Arslan19]

We have recently faced some outages of our 7.6MVA Steam Turbine Generator on actuation of Reverse Power Protection. Below are some details:

1. Protection Relay SIEMENS SIPROTEC 7UM62 is installed at our Generator Breaker, 02 stages of Reverse Power Protection are enabled in it.
Setpoint: -119.3 kW (-1.57% of 7.6MVA)
Time Delay (with Trip SOV operation): 50 ms
Time Delay (without Trip SOV operation): 5 sec
2. As Steam Turbine Control is entirely a local PLC based (ComAP/intellisys) which also provides some generator protections, so another Reverse power stage with following settings in enabled in that
Setpoint: -182.4kW
Time Delay: 02 sec
3. Hydraulic governor system is installed on this Steam Turbine which usually cause hunting issues leading to load fluctuation and ultimately tripping the turbine on Reverse Power Protection from ComAP controller as its time delay is lesser that Siprotec 7UM62 (without Trip SOV operation)
4. Moreover, even normal stop initiated by Operators lead to reverse power from Siprotec 7UM62bwith 50ms delay.

My question here is; whether it’s advisable to allow the machine to wait for reverse power to actually happen (both for governor hunting issue OR manual stop? Shouldn't we trip the machine before it actually experiences reverse power?
We can activate low forward power function in relay, if advisable?

 

[wcaseyharman]

I have some literature from General Electric for large steam turbine operation. In the documentation they state in large bold letters that except for a major electrical fault on the generator the generator breaker should always be opened under reverse power conditions to prevent the possibility of catastrophic overspeed. This was also the recommendation of the insurance company.
Typically it seems like the time delay for reverse power is 1 second or so for a normal shutdown and 3-10 seconds on a real reverse power event.

 

[Arslan19]

Can you kindly share the relvant GE literature if possible?
Is it possible for us to generate a turbine trip in addition to generator breaker opening for low forward power protection? This will ensure that prime mover is also isolated and ensure unnecessary stress caused by reverse power?

 

[dpc]

The reason for reverse power supervision of tripping is to make very sure that the steam valves are closed. In some cases, the valves fail to fully close, even with indication of fully closed. If the generator breaker is opened with even minimal steam flow, a catastrophic turb-gen overspeed can occur, leading to complete destruction of the unit and danger to personnel. The generator can operate in a reverse power situation for quite a while (within its capability curve). The protection is for the LP turbine which can overheat without sufficient steam flow. It is common to have both a low forward power trip (generally with a longer time delay) and a reverse power trip. The reverse power trip generally supervises the manual opening of the field breaker or the generator breaker, but not an emergency trip.

You need to distinguish between reverse power supervision of manual or non-critical tripping, and actual reverse power tripping. Generally two different functions with different settings. Reverse power and low forward TRIPS with overly sensitive settings can definitely be a nuisance. Operators generally know to immediately pick up some load after synchronization. The pickup settings for these functions are generally extremely low compared to the machine ratings, so CT/PT accuracy at these low power levels must be considered.

 

[wcaseyharman]

GE, and every other reference I have including the excellent generator protection book by Donald Reimert did not recommend any low forward power trips for steam turbines. The only trips that were recommended not supervised by reverse power were electrical relay trips.

I’ll see if I can’t scan some and attach.

 

[waross]

generator breaker should always be opened under reverse power conditions to prevent the possibility of catastrophic overspeed.
I know more about generators than about steam turbines BUT:
I have difficulty seeing how a synchronous machine with the breaker closed may go overspeed on reverse power.
Reverse power or forward power, the speed is locked to synchronous speed by the grid.
After the breaker is opened, I would expect a machine under reverse power conditions to slow down, not speed up.
Of course with any steam flow, all bets are off and overspeed is not only possible but likely.

generator breaker should always be opened under reverse power conditions to prevent the possibility of catastrophic overspeed.

I wonder if this is a typo or a misquote?
Am I about to learn something new here?

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

 

[wcaseyharman]

I used “opened” as the action/process of opening or tripping, not as the state of being open. Hopefully that clears up the confusion.

 

[waross]

Not really.

generator breaker should always be opened

Not much ambiguity here.

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

 

[davidbeach]

If all steam valves fully close, the generator will motor the turbine, reverse power, and it's safe to open the breaker. The quote is correct, but only applies for normal shutdowns. When the unit it tripped to clear an electrical fault the breaker is opened and trust is placed in the steam valves to actually close. While past performance is no guarantee of future performance, if the steam valves properly close every time there's an opportunity to verify their operation then it's assumed they'll also do so.

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]

Sorry, I am not sure then what the question is - as others have also said, anytime the breaker is opened it needs to be done under reverse power to prevent the steam turbine from going into overspeed if the stop values fail to fully close. As you say generator/turbine will not overspeed when connected to the grid (with the exception of out of step conditions or unintentional islands), generally this is only a concern on shutdown.

I spent a bit of time redoing the logic for a 100MW steam turbine two years ago where they tripped the breaker on low forward power, and subsequently tripped the turbine. Even worse, if the breaker didn’t trip in a certain amount of time they would operate a lockout relay to trip the breaker, defeating the low forward power supervision. My coworker and I redid all of that - now the turbine trips at 5MW, then after 1 second of reverse power (and the stop valve limit switches are closed) the breaker opens on the #1 reverse power setting. The #2 reverse power setting has I think a 5 second delay. So far no problems.

As an aside, the reverse power relay is why it’s recommended to synchronize with a synchro scope turning clockwise (gen faster than the grid). On old hydro electric generators with no reverse power protection you could synchronize either faster or slower, it didn’t matter.

 

[Arslan19]

I really appreciate everyone's opinion here. So, summing up the topic with below comments:
"Steam Turbine Generator should always trip (open the Generator Circuit Breaker) after actuation of reverse power under normal stop and loss of prime mover (either due to governor malfunction or actual stop valve closure). So, we should rule out the option to activate low forward power protection"

Now, keeping in view the current settings (mentioned herewith below), is it advisable to reconsider the settings? Also, its important to note that its an old AEG KANIS G20 Model Steam Turbine with Brush Alternator. Both are operational since 1985 or so.

1. Protection Relay SIEMENS SIPROTEC 7UM62 is installed at our Generator Breaker, 02 stages of Reverse Power Protection are enabled in it.
Setpoint: -119.3 kW (-1.57% of 7.6MVA)
Time Delay (with Trip SOV operation): 50 ms
Time Delay (without Trip SOV operation): 5 sec
2. As Steam Turbine Control is entirely a local PLC based (ComAP/intellisys) which also provides some generator protections, so another Reverse power stage with following settings in enabled in that
Setpoint: -182.4kW
Time Delay: 02 sec

As this machine is quite old, I have made its capability curve using MATLAB, you may have a quick look at it to check it the settings are well within safer zone

 

[wcaseyharman]

I attempted to attach a picture of the GE paper that discusses “sequential tripping” of steam turbine generator breakers.

The settings you provided seem reasonable to me, except for the 50ms seems a little short, I’d probably put it at half a second or a second to add a little security to the element.

 

[Arslan19]

Thank you so much.
I have another query emerging in my mind now that few Electrical Relay Protections (like unbalance protection, overload protection etc.) are only meant to open Generator Circuit Breaker and operate a lockout relay (86G1). They don't send a trip signal to turbine. As these protection functions don't require detailed troubleshooting, generator breaker can be closed again in minimum possible time as turbine keeps on FSNL.

Isn't there the risk of overspeed under such scenarios?

I have seen similar tripping matrix in GE Gas Turbine (Frame V) to only trip generator breaker on few less critical electrical protections instead of tripping the turbine as well.

 

[waross]

Let's look at the consequences.
Defined;
Steam trip: Steam valve closed, no steam flow.
Breaker trip: Breaker open.
Condition; Steam trip, breaker closed. Result, reverse power, eventual overheating of turbine.
Condition; Breaker trip, steam valve not completely closed. Result. Overspeed, likely catastrophic.
Given the extreme difficulty of over-speeding a generator when the breaker is closed, and the very little steam flow required to cause catastrophic overspeed damage if the breaker is open, I would resist any scheme that opens the breaker before zero steam flow is proven.
Do any turbine schemes include a "vent to atmosphere" to ensure zero steam flow through the turbine before the breaker is opened?

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

 

[wcaseyharman]

Some of our older gas turbines had “trip to speed to load” lockout relays for certain trips. We removed all of them - we had no reason to immediately re-synchronize, and the plant staff generally had no idea what the different lockouts meant. It would often be several hours to get qualified staff to the site for troubleshooting and event review anyway, so we did away with those schemes. An electrical trip generally trips the turbine, generator breaker and exciter. We went with differentiating between lockout and non-lockout trips (non-lockout trip would allow an immediate restart, the problem is outside the plant). Over/under frequency trips are non-lockout trips, for example.