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Operation of Synchronous Generator as Induction Generator.

Operation of Synchronous Generator as Induction Generator.

Operation of Synchronous Generator as Induction Generator.

(OP)
There is a small about 10 MW, 3000 RPM Generator coupled to a back pressure Turbine in our plant. One day, Excitation of the generator failed along with station DC power failure (supplied by 220 DC battery bank). Due to failure of DC power, no relay could the detect the loss of field / trip the generator. Even operation people could not trip the generator, as there was no DC power for that particular machine.

Since system was strong enough to supply the increased reactive power demand, there was no other disturbance in system e.g. voltage dip etc. After 8 minutes, machine copuld be isolated by tripping upstream breaker. Tripping of machine caused the  over speeding of Turbine but this whole incidence caused the tremendous damage to the Generator. Its rotor was flied off towards Turbine by breaking the coupling and shaft by about a meter. Fire also taken place in the plant. Machine was operating at full load even after the loss of excitation.

Can any one comment on this incident and have any idea about the capacity of machine to operate as Induction Generator? Trends of system parameters are available, if any one of you is interested.

RE: Operation of Synchronous Generator as Induction Generator.

In the incidence you described the damper winding of the generator operated as a squirrel cage, but I'm not able to calcluate up to which extend this is possible.

Regarding the protection used on your system there have been significant design flaws. Circuits for tripping should always be connected fail-safe. To my mind (I'm not a specialist in power generation equipment, but I'm working in an area where similar issues have to be considered) using a no voltage coil (a coil which trips the breaker if no voltage is applied)on the circuit breaker would have been mandatory. Furthermore stopping the turbine should have been initiated by similar means.


RE: Operation of Synchronous Generator as Induction Generator.

Stationary battery sets need to be treated with highest respect and attention.  "All your eggs are in one basket, so watch that basket!"  Fading battery sets can really get expensive, like $30-million in one crucial spot.  It's particularly embarrassing when you have to rely on a relay eighteen miles down the line to clear a circuit (and then in the mop-up you discover that you get only black tar out of the transformer oil-sample valve.)

RE: Operation of Synchronous Generator as Induction Generator.

Just to respond to electricuwe comments regarding fail-safe circuits.

In the U.S., the overwhelming majority of breaker control circuits are energize-to-trip.  They are not fail-safe.  In many cases,dual trip coils and redundant relaying is provided. This is why the dc control power circuits are so critical.  

Undervoltage release coils are rarely used at utility substations or generating (non-nuclear) facilities, at least in my experience.  For boiler protection, it is fairly common to use fail-safe systems, typically two out three voting logic, but this still is often followed by a one out of two energize-to-trip scheme.

Breakers are sometimes provided with capacitive-trip or other stored-energy devices to allow breaker tripping if dc is lost.   

I'd be interested in hearing if this is consistent with the experience of others.

RE: Operation of Synchronous Generator as Induction Generator.

I would suggest to apply breaker relay, in case the generator breaker woudln't operate, the time delay and relay sensing also used to operate the next level breaker.

Any comment?

RE: Operation of Synchronous Generator as Induction Generator.

Coming from a Field Engineering background I have seen some pretty nasty failures. Yours is right up there with the best.  All the Generator Circuit Breakers I have worked on have been energize to close and energize to trip.  I believe the reason “energize to trip” schemes are preferred is that a full load trip due to a loss of tripping voltage is also considered poor reliability.  This especially true in the Utility business where a nuisance trip could cause a cascading Generation failure or at least a power disturbance.  Because the trip circuit is so important, manufactures have implemented many ways to help insure proper operation.  Normally the Red and Green indicating lights are connected to the breaker in a very specific way.  The Green light (which indicates “OPEN”) is wired to a normally closed contact on the breaker and gets its power from the trip fuses.  The Red light (which indicates “CLOSED”) is wired in series with the trip coil in the breaker and gets its power from the trip fuses.  This design gives the following protection.  If the Green light is not “ON” do not close the breaker it might means that there is no tripping power.  With the Breaker closed the Red light being “ON” indicates that there is tripping power, and that there is a complete circuit through the trip coil of the Breaker. This same scheme can also be used for lockout relays, (wire the lockout trip coil in series with a white indicating light).  Unfortunately this design does not help insure that the Batteries and charging system are in good condition.  The Batteries are so important that a good maintenance program will check the voltage drop, specific gravity, and connections on each battery once a month.  A load test of the Battery Bank once every six months is also recommended.  This was a very tough lesson but one you will never forget

RE: Operation of Synchronous Generator as Induction Generator.

Words well spoken, electrageek.  Just make sure that a neat row of red lights may mean the battery set is doing OK, but no one noticed that all those red lights were slowly [but equally] dimming from a malfunctioning battery charger. All that "mop-up" overtime may or may not be welcome.  

RE: Operation of Synchronous Generator as Induction Generator.

To my mind its important to distinguish between fail-safe and redunant circuits.

For Fail-safe operation of the breaker tripping circuit a no voltage coil supplied from the the battery voltage and with NC-contacts of the protecive devices would have been fail safe (providing the protecive devices are fail safe themself). In case of a bad battery the power-station would fail to start operating which is less severe than the failure RAgrawal experienced.

Redunant operation is further step, but its without value if the circuit which has to trip the breaker is not fail safe.

RE: Operation of Synchronous Generator as Induction Generator.

Guys, fail safe as defined above is definitely not normally used for breaker tripping in the type of power systems that we are discussing here.  Electrageek has defined the purpose of those red lights, but there are some additional considerations - such as, the red lights should have a properly sized series resistor in circuit (so that if the bulb fails short circuited you don't trip the breaker).

The way that concerns about battery supply voltage is usually handled is to have an undervoltage monitor on the battery charger (which fires up an urgent annunciation point in case the battery voltage goes low.  The annunciation would be in a control room, where hopefully someone is awake to contact the service personnel.  Of course, the alarm level has to be selected high enough to allow time for corrective action and low enough to not be a nuisance.  Breaker trip circuit and protection supply monitoring are other options that would give early warning of impending trouble.
Dual trip coils and even dual batteries are used where there is an ultra-high security requirement - the cost does add up, though.

Back to RAgrawal's original concerns, conventional wisdom says that the generator itself should be able to withstand the loss of field condition for a fairly long time - the concern is usually the effect on the rest of the system, as the excitation for the generator will have to be supplied from the external system, with accompanying low voltage conditions and the risk of instability.  The excitation supply isn't normaly taken from the station battery - this supply is usually only used for field flashing.  Could this have been the root cause of the battery failure?

RE: Operation of Synchronous Generator as Induction Generator.

Wow, an outstanding discusssion.  One more brief mention of advantage of traditional energize-to-trip scheme: the coils don't have to be designed to carry the trip current continuously (whereas a deenergize to trip coil does)... results in more force margine available for the same coil (or ability to use a cheaper coil). Also continuously energized coils sometimes bind up due to baking of the insulation varnish which later deposits on the plunger and gums up the works... not very fail safe if it fails to reposition when deenergized.

RE: Operation of Synchronous Generator as Induction Generator.

Reverse power flow relay could be used to avoid such incidences

RE: Operation of Synchronous Generator as Induction Generator.

(OP)
freinds
thanks to all for your input but i think every one has mised the important question that for how much time a generator can withstand to oprate as induction generator. perterb has said that it can withstand for fairly long time whats exact time, because as far as i know it depends on saize n cooling media of generator. and time ranges from few seconds to 3-4 minutes, not more than this.

RE: Operation of Synchronous Generator as Induction Generator.

RAgrawal-
Tell us some more about the excitation system of this generator.  Is it brushless or brush type?
Sorry that I can/t give you a definitive answer re the permissible time for operation without excitation, that will have to come from the manufacturer.  There are recorded incidents of large generators operating for extended periods with the excitation off, the concern is more for system conditions.

On re-reading your original post, it sounds to me as though the damage to the machine is more consistent with unstable operation where the generator has fallen out of step with the external system. Out of step swings would place severe stress on the shaft, possibly leading to failure.  These swings would be accompanied by very noticeable voltage swings, which your post suggests were not very apparent.  With the loss of excitation, unstable operation is a distinct possibility, depending on system conditions and generator loading.

One more thought - the turbine overspeed trip occurred when the generator was isolated by operation of the upstream breakers.  As the unit was still supplying real power up to this time, this indicates that the shaft was intact up to then.  Is it possible that the shaft damage occurred due to the overspeed condition following the full load rejection?  Given the lack of voltage swings, it is a distinct possibility that this was the root cause of the shaft & coupling failure.

RE: Operation of Synchronous Generator as Induction Generator.

peterb - you're last thought makes a good point that the damage occurred only after the generator was electrically.  Apparently the overspeed trip did not operate soon enough and centrifigual force of a slightly unbalanced or resonant generator rotor while overspeeding caused the damage. It seems like some logical questions are:  
1 - Why didn't the overspeed trip act soon enough to prevent this?  
2 - What is different about this trip compared to a normal trip from full load that would prevent the overspeed trip from acting in time to prevent damage?

RE: Operation of Synchronous Generator as Induction Generator.

electricpete -
You've started a trend of thought here -
- Does the turbine overspeed trip provide direct fail-safe operation of the stop valve, or does it require the DC supply to operate?
- If there is a trip solenoid that operates the stop valve, and if this solenoid didn't operate because of the lack of DC supply, then the governor valve would try to control the speed.  The governor valve is not a stop valve and even when fully closed could possibly pass enough steam to overspeed the turbine.

RE: Operation of Synchronous Generator as Induction Generator.

We recently completed installation of a new steam turbine-generator to replace an older one that had been wrecked by overspeed condition.  

The limit switches on the main stop valve were home-brewed at this industrial facility.  Apparently there was something blocking the stop valve from completely seating, but the limit switch indicated it was closed.  The limit switch contact was used to trip the generator breaker.  

The control system thought the turbine was shut down, but there was enough steam leakage through the stop valve and "closed" governor valve to run the turbine up to 10,000 rpm or so.  At some point, everything came apart, including the generator.  I'm glad I wasn't there.  

 

RE: Operation of Synchronous Generator as Induction Generator.

dpc, I've never actually seen this happen but that's the classic reason why breaker tripping is interlocked via a reverse power or low forward power relay, for turbine mechanical or electrical non-emergency trips.  

RE: Operation of Synchronous Generator as Induction Generator.

peterb,

That would have been better, no doubt.  

Do you supervise the generator breaker control switch with the reverse power contact as well, or do you trust the operator?  

dpc

RE: Operation of Synchronous Generator as Induction Generator.

Good point, dpc; I haven't come across this usage, but it could be considered for a high-risk situation.  I guess the answer is that we just have to trust those pesky humans once in a while.  
Normally, you try to design the control board ergonomics so that the operator has to really think about it before executing the action.  One way of doing this is to make the breaker operation a two-step process, where one switch selects the breaker to be operated and the second is the traditional close/trip switch.  Different shaped /coloured handles are another approach.

RE: Operation of Synchronous Generator as Induction Generator.

(OP)
perterb

the excitation system of generator is brush type. AVR gets power through excitation transformer and feeds the field of exciter which intern feds to main fiels o generator.

As far as broken shaft is concern, so you are right, generator was supplying real power and that too rated untill explosion occurred.

solenoid trip of turbine could not be possible due to absence of DC power, it tripped mechanically. The setting of over speed trip was about 15 %.

RE: Operation of Synchronous Generator as Induction Generator.

dpc

I am surprised that a new turbine would come without a mechanical overspeed bolt.  Its a device that operates on centrifugal force, the faster you go the more force to throw the bolt outward from the shaft.  The bolt hits a lever that dumps the oil from the stop valve. Its amazing how accurate and reliable they are.  I thought all turbines had them. We normally test them after every overhaul and at least once a year. They can get sticky and miss-operate. The overspeed bolt is equal in importance to the batteries.  Without a mechanical overspeed bolt I would want a completely separate system for redundancy.

RAgrawal

I have been at sites where there has been no protection due to failed lockout relay coils, blown fuses, failed protective relays, and failed batteries.
The Generator's were still running and no one ever noticed. The problem with protective systems is that you don't need them very often and we tend to forget about them until its too late. Don't put off testing your protective systems.

The Generator capability curves usually give you the range of control for the Generator field. On the lagging Power Factor side you have the maximum field current that produces Vars to the grid. The limitation is when the rotor field current begin to overheat the rotor windings. On the opposite end (leading Power
Factor) is the minimum Generator field current, the Generator absorbs or uses Vars from the grid. As the field is reduced the Generator stator current increases because it needs Vars from the grid to make up for the Vars not being produce by the Generator field. The limitation is end turn heating of the Stator windings.  This is where your loss of excitation (40) relay should kick in. They normally do not look at the actual field current they look at the Generator PT's and CT's to calculate when the Generator has ventured into a dangerous excitation area. With most loss of excitation relays the point at which the relay operates and the timing of the relay are dependent on the Generator manufacturers design data.  They won't tell you where to set the relay, due to liability, but they do give you the heating factors needed to calculate the trip points. Many protective relay manufacturers (Beckwith, GE, Basler) have excellent instruction books, available on the web, that explain this a lot better then I can.

RE: Operation of Synchronous Generator as Induction Generator.

electrageek -
I fully agree with everything that you are saying, except to note that the problem arises when the stop valve does not fully close (as described by dpc).  This would mean that the mechanical overspeed could very well have dumped the oil, but the valve just did not close.  
If you have a full load rejection under these conditions, there will be a continuing overspeed condition.  This is why it is critical to monitor power output to interlock the trip, to ensure that there is not enough steam passing into the turbine to cause an overspeed when the breaker is opened.

RE: Operation of Synchronous Generator as Induction Generator.

I'm not much on turbine or generator controls, so having a hard time following the discussion.

electrageek - you say you're surprised they don't have overspeed bolt. But as I read the problem they did trip due to mechanical overspeed trip set at 115%.  Would bolt have provided a lower setpoint? why would it have been significant?

It still sounds to me like the damage was in fact done by overspeed condition (correct me if that doesn't make sense). That leads me to several questions which might explore why this would occur.

1 - Had the overspeed trip been tested? Do we know for a fact that it tripped at 115%?  Is 115% the proper setpoint provided by the manufacturer?
2 - Was there any abnormal vibration problem associated with this generator during normal operation prior to the event which might have limited its ability to withstand overspeed?  High imbalance?  abnormal degree of misalignment?
3 - Perhaps the abnormal operation prior to the overspeed trip caused some abnormal heating of rotor which might cause it to bow and therefore it could no longer withstand the same degree of overspeed as a healthy rotor?  To confirm that theory - Is there continuous monitoring of vibration which might indicated whether running speed vibration increased during the latter stages of induction motor generator operation but still prior to the trip?

And just to confirm one more time my basic understanding of the events (RAG):  Is it correct that the generator self-destructed at about the same time that the overspeed trip tripped which was almost immediately after the system breakers were opened?

RE: Operation of Synchronous Generator as Induction Generator.

electricpete

Actually I was responding to dpc about his turbine overspeed example on the stuck stop valve. He is correct that if the valve is really stuck open the speed will continue right on up to destruction.  The mechanical overspeed bolt will not be any better at shutting the valve. The same problem exists with non-return valves that stick open. They are the check valves that are suppose to prevent downstream extraction steam from being push back into the turbine and again causing an uncontrolled overspeed.

In reference to the original problem with RAG's turbine. He lost all excitation and normally would have tripped on a 40 relay function. I would think that someone could have grabbed a broom stick and reach in to trip the Generator Breaker manually.  There is usually a manual trip button located on the Breaker.  I wouldn't stand right in front of the Breaker.

RE: Operation of Synchronous Generator as Induction Generator.

electrageek - I'm starting to understand better now the scenario that has been discussed of leaking or stuck-open stop valve, which renders the overspeed trip useless even if it does actuate.

If that were the case, wouldn't the turbine overspeed also even after the generator destructed?  How was steam to the turbine ever stopped if the stop valve was stuck open?  (perhaps the governor valve?)

RE: Operation of Synchronous Generator as Induction Generator.

Electricpete -
What stopped the steam to the turbine?  If there wasn't an accessible manual valve, then the answer is - when the boiler was tripped and the steam supply was exhausted.  I saw this happen many years ago, where a turbine-driven compressor self destructed and exhausted all of the steam into the 13.8 kV switchgear located on the floor above.  That took many days and nights of cleanup to get the plant back up.  
This is obviously serious stuff and points to the need for coordinated design effort between electrical & mechanical disciplines and also for rigorous operating practice in testing and maintenance of stop valves and overspeed trips.

RE: Operation of Synchronous Generator as Induction Generator.

electricpete

The interesting thing about a steam turbine is that it take very little steam to keep the turbine running once it is moving.  The journal bearings, condensor vacuuum, and inertia of the system provides a very efficent low loss machine. Sometimes it is necessary to break vacuum just to get the machine to stop in a resonable time. Because of this Steam Turbines have a very low reverse power setting.  A 20MW Turbine may only pull 200KW of reverse power. It takes a very low range relay to reach this level. The Stop Valve is the primary protection for overspeed, the Govenor valves can also help but generally are not required to completely seal. Even a small leak can gradually cause the speed to go up and up and up. The thing is the amount of steam is fairly small so once something gives up is doesn't take much load to stop the machine.

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