Generator Rotor Earth Fault in 600 MW Steam Generator

Believe the earth fault relay, and refer to the original equipment manufacturer's operation and maintenance manual under the section called "troubleshooting". If a suitable solution cannot be found there, contact the manufacturer directly by telephone.

Thanks for Reply, Actually we got the IR value of 0 ohms between Rotor Shaft and Slip rings. So, there is not doubt over Earth fault Relay.

By the by, Are you from Anpara 'C'?

Long ago, may be about 30 years back I was passing through Anpara, I saw a huge board written APSEB site for 3100 MW thermal power Plant.

Yes, I am in Anpara "C".

No, Iam From MP.

.....So, Again we have run the generator and when the turbine speed was coasting up at about of 300 RPM , we found that the 1-3 HZ Rotor earth fault relay was acted and it is not getting re-seted. when we have checked the IR value in this condition found of 0 mega ohms. Now we are not getting any fault. Kindly suggest.

Suggestion 1

Decide whether you do or don't have a fault.

Suggestion 2

Ask your technical director to actually give you the resources to do your job and to stop diverting the maintenance budget.

Suggestion 3

Don't let anyone there know who you work for until the contemporary massive blunder has been forgotten.

Pandabinod;

"The generator was running in 300 MW and tripped due to some system disturbance in Electrical fault like Differential protection. Just after tripping of generator we are found there is 1-3 HZ Rotor earth fault protection."

Your description of the trip event coupled with earth fault relay event(s) resulting in generator "lockout" and the different values in Megohms being observed when the rotor is on turning gear indicates you have suffered serious rotor conductor failure. Most likely the wedging and/or binding has become loose and damage has been done to the winding insulation allowing contact between the conductor(s) and the rotor laminations. If the windings are not secure, the rotation of the rotor coupled with the weight of the conductors will allow different amounts of pressure to be applied to the contact point thereby resulting in random values of impedance when referenced to earth.

This is a high critical safety risk not only to the machine but also to all personnel working in the immediate area. You need to hire a competent generator service company to perform a thorough electrical test of the generator before attempting a re-start of the machine.

Because of the 600MW size of the unit I am assuming it is Hydrogen cooled which if ignited by the electrical arc/spark from an earth contact can result in catastrophic (explosive) failure of the generator.

If this should occurr, the damage will be extensive, is unbelievable when observed, and will cause damage to periphial structure and adjacent equipment.

You need to be extremely cautious and careful with this machine and you really need expert advice and help. Now!

Hydrogen Cooled?! In my view it should have blown long time ago!

The fuel-to-air ratio has to reach the exact proportion in order for the hydrogen to ignite.

This has happened way too many times due to poor maintenance & operations practices and is fueled by ignorance of the cooling system operation and inherent risk . The most recent event I am familar with was over in the Arab Immigrate area in late 2011 wherein a 350 MW unit failed and took out everything around it from the basement up to the roof and both units on each side. Amazingly there was not any fatalities other than the equipment. This particular event was triggered by the mechanical failure of the "jack" shaft between the prime mover and the generator.

Below is a link with good information as to the mechanics of the cooling system.

I have some pictures taken at the site that will send chills down your back if you think about what could have happened.

http://search.yahoo.com/r/_ylt=A0oG7uAFBhhQKRgAgYpXNyoA;_ylu=X3oDMTE1dTZma2ZnBHNlYwNzcgRwb3MDMgRjb2xvA2FjMgR2dGlkA01TWTAwM18xMjU-/SIG=12f8t6eqt/EXP=1343780485/**http%3a//en.wikipedia.org/wiki/Hydrogen-cooled_turbogenerator

More reason not to temper with such installations if not qualified by the OEM! Immagine if some fault has developped and these signs should be telling the maintenance people to call the big guys before going any further with experimenting/testing!

Thanks for reply. Yes our generator of Hydrogen cooled. but we are getting variable IR value in diffrent position of Rotor when it is running in turning gear.

Are you hoping that somebody will tell you that something isn't rubbing?

Denial is not a river in Africa......deal with the problem, start taking things apart slowly and looking really carefully.

This type of rotor earth fault is called "flying rotor earth fault". This indicates that there is contact of rotor conductor and rotor body due to:

1. loose conducting particle making contact at certain position of the rotor and moving to some other position at other position of rotor. This position may change from time to time (as the particle is loose).

2. The slot insulation damaged and changes position at different speed / position of rotor.

One thing you could attempt to burn the fault by running the rotor at speed where earth fault appears and stays. You may get this condition even at stand still position. Apply small variable voltage between one slip ring and rotor body. You may reach rotor body through rotor earthing brush. Increase the voltage till loose particle burns due to passage of current. If you are lucky, your rotor may become healthy else the fault will become permanent. Then you need to get it attended by OEM or other agency competent to take repair work.

Do not run the generator with this type of fault as second earth fault may prove to be catastrophic.

Dear sir

Can u please tell what are the faults you got in the rotor after that or what action u had taken ?