Generator Reverse Power Trip on synchronizing 9

[edison123]

Trying to commission a 11 KV, 2.5 MW steam turbine driven generator. The generator recently rewound and tested OK. Phase sequence also checked ok.

On synchronizing, the generator trips on reverse power every time. The turbine engineer says the turbine, the governor and the steam valve are ok.

I noted that on closing the breaker, the power factor goes haywire and the Woodwards Multifunction Relay reads about 100 amps in all 3 phases but the power is reading (-) 358 KW and the excitation is reduced to zero. (I will post two photos of the control board meters - before and after synch)in the following posts.

The AVR is Basler. I think the AVR is malfunctioning. Any other possible reasons for the RPR to act every time ?

Muthu

http://www.edison.co.in

 

[edison123]

Before synch photo

Muthu
www.edison.co.in

 

[edison123]

After Synch photo

Muthu
www.edison.co.in

 

[rasevskii]

For edison123-

Well it went all bonkers when the CB was closed...The excitation went to zero and the governor must have closed the throttle valve too..We cant read anything on the digital gadget there, but check first--

-The governor system is trying to to pick up a small loading immediately when the CB is closed, in other words the synscope must be rotating very slowly clockwise at breaker closure. I assume there is actually a synchroscope somewhere.

-That the incoming and running voltages were properly matched before CB closure.

-That the AVR is set so that a slight positive var flow will result at CB closure. It looks like it was set the other way in that the AVR saw too high a voltage at CB closure and ran the excitation to zero.

We see that the stator current went to a high value, indicating a bad mismatch, and the power went to negative kw.

-see that the AVR cross current compensation is connected properly. It may be reversed, meaning that it will go immediately unstable. Try shorting out that CT temporarily.

The governor system has to have a problem in that it is closing the throttle valve immediately. It can be that a load feedback is reversed. You said Woodward, they have something like that. Often called Load Pulse I think. Try disabling that.

There are two problems at the same time it seems. Possibly integrated into one controller?

Try synchronizing manually. See what happens to the steam valve, it should not go closed immediately.

It can be that PT and CT connections are wrong and that vars and watts are not being measured correctly.

The metering looks correct. Is that on the same PTs and CTs as the control?

regards, rasevskii

 

[edison123]

Thanks rasevskii. I will be at the site tomorrow. Will check out the points you mentioned. That cross current compensation looks interesting.


Muthu

http://www.edison.co.in

 

[rasevskii]

The AVR cross-current compensation is usually a CT input connected in the center phase. That would be the yellow phase in the British system. It may be in the wrong phase, reversed, or both..

That still would not explain why the governor is closing the steam valve, it can be that there is a load feedback that is in some way wrong.

regards, rasevskii

 

[ScottyUK]

Difficult to say without knowing a bit more about the governor control loop, but if it is a phasing problem at the instrument transformers then you may be getting some very strange results from the power transducers which may in turn be forcing the governor closed. That would be a possible explanation for the behaviour of both the AVR and the governor.


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

 

[rasevskii]

It seems to be some kind of comprehensive Woodward control system, possibly. I just spent some time on the net to see what Woodward is offering currently. Should we say "oversophistication" ??

At least the metering on the panel was sensibly designed. One glance and the whole picture is evident.

Let us know what you find. There must be some wrong connection from the PTs and CTs to the Woodward so it is not measuring KW actually. Possibly someone damaged something in the electronics using a Megger on the wiring, not uncommon.

regards, rasevskii

 

[cranky108]

Probally a dumb question, but what is you reverse power time delay?

I haven't found any text books that talk about this, but it's important.

 

[waross]

Did I read that correctly?
I see 10.29 kV on the square LCD monitor before closing and 10.84 kV after closing.
Jack up the set voltage. It looks like the grid voltage is above the set voltage. Once the breaker is closed the grid is feeding 18.84 kV to a generator outputting only 10.29 kV. The AVR sees the grid voltage and shuts the excitation down trying to drop the voltage. Then the grid excites the generator as if it were an induction generator. The result is a bad power factor and about 358 kW of losses in the stator caused by the exciting current.
Run the set voltage up to 10.84 kV or more. That should get you close enough to avoid a reverse power trip. If it still trips, be guided by the voltage shown after closing and set the voltage up a little more.
The root of the problem is most likely metering errors. The meter showing the grid voltage does not match the meter showing the gen-set voltage.
The old school fix is to bump up the voltage until you can sync without a reverse power trip and then either adjust the meter calibrations until they match, or get out the magic marker and put a mark and/or instructions on the face of the voltmeter.
The more accepted method may be to pull the PTs and Voltmeters and have the PTs checked and replaced if faulty and have the meters calibrated to the same standard.
The set voltage should be at or above grid voltage when closing the breaker. Yours voltage seems to be about 5% low when closing.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

The AVR expert is at site and has been going at it for about 8 hours now. Still no dice. I have communicated to him your suggestions and hopefully one of them will work out. Will keep you posted.

Muthu

http://www.edison.co.in

 

[rasevskii]

It should not take 8 hours to find the problems there.

An expert should have it sorted out within the first hour or less. Unless it is a complete mess of course.

If you send an attachment to us on the forum showing the panel connections in a pdf or CAD format we may be able to pinpoint the fix to the situation.

I assume that would allowed by the client and the forum.

Is this an upgrade to an old plant? Sounds like that. Are there other generators working on the same busbar? Were the existing old PTs and CTs checked out or changed as suggested by others?

Is this a Basler digital AVR? What versions of Woodward equipments are used there?

regards, rasevskii

 

[GTstartup]

Voltage too low when synchronzing is not an AVR problem (per se) it is a synchronizing problem. The AVR goes to a fixed setpoint on initial excitation and then the synchronizer adjusts then closes the breaker. You could set the AVR initial SP to just above the grid voltage but if the grid voltage is higher next time you synch you will be in the same situation. You could also set the synchronizer to only synch on Ugen>UNet. This will cause longer synchrizing times.

That being said I agree that 5% allowable delta seems too high. I would set it at 3% and see what happens. Athough I haven't yet wrapped my head around why leading reactive power surge would cause a reverse power trip.

 

[ZapSib]

I understand there is reverse active power trip ( P=-358 kW)
Should be governor issue rather than AVR.
I belive that synchronizing shall issue an increase frequency command right after closing in order to avoid the P<- trip.
Some product have the means available, just need to activate it.

Rgds.

 

[waross]

Hi GTstartup;
Reverse power usually indicates a motoring condition, but not always. As I read the meters the AVR is removing the excitation in a futile attempt to drop the grid voltage. Once the AVR shuts down the grid excites the generator as if it were an induction generator. (Probably not all that good for the rotor.) There will be a grid supplied exciting current in the generator and this current will cause I[sup]2[/sup]R losses. These losses are supplied by power flowing from the grid into the machine. Hence a reverse power trip without a motoring condition.
Is the AVR in voltage control mode?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rasevskii]

Another consequence of the generator in motoring condition without excitation as Waross has mentioned as being not good for the rotor, may be failed diodes in the rotating rectifier assembly due the high induced voltage in the rotor poles. Usually there us a overvoltage thyristor in the assembly that fires and shorts out the rectifier under such transients, but this has limited energy capability and only protects on short time transients.

If that has happened, on the next start up it will not be possible to get full stator voltage, and the exciter field will be overloaded on a sustained basis.

In this case the diode fail protection will sense the sustained exciter field overcurrent and trip the excitation off after a time delay.

Let us hope that diode fail protection exists and is activated.

The assumption is that this is a brushless exciter...

regards, rasevskii

 

[ZapSib]

In case grid voltage is above generator output voltage, after CB closure, generator should drawn reactive power from grid isn't it?

 

[GTstartup]

Warcross, I have tested dozens of underexcitaion limiters on line and even loss of excitation relays (on occasion) and I have never had a reverse power relay trip first, modern reverse power relays can tell the difference between active and reactive power. I agree with Zapsib - sounds like the unit is not picking up load either due to a governor problem or a long shot is a synchronizing issue. i.e. synchronizing with positive slip, generator speed < system.

 

[waross]

I agree, reactive power should not trip a reverse power relay.
However, the reactive current causes I[sup]2[/sup]R losses in the generator. Those losses are real power and that power (-358 kW) will be seen by the reverse power relay.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

GTstartup,

It's not such a big problem on modern low-burden digital relays but it is a documented problem with electro-mechanical relays. The problem exists as a result of phase shifts through the instrument transformers. On a gas turbine the motoring load of the compressor is so large that it would be difficult to set a relay so badly as to not trip the set, but a steam turbine spinning in vacuum has a very small motoring load relative to the size of the set and that can make relay setting tricky and / or cause misoperation. The problem is worse with larger sets because their motoring load is a smaller fraction of the set rating than on a smaller machine. Hydrogen-cooled sets - which tend to be large ones - have lower windage losses than an equivalent air-cooled machine.


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