Rotor Earth Fault Of 500mw Tg

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bangal

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We have 500mw generator (thermal) having DAVR-type excitation.For generator rotor earth fault protection,we feed 1HZ,20v square wave(d.c.) through slip ring-1 (insulated from shaft) in which the star point of armature winding of main exciter is connected. Rotor earth fault relay is connected accross slip ring-1 & another slip ring(electically connected with the rotor shaft) & compare the out-put(if any) with the said input ,then generate alarm signal if resistance come down to 40kohm & generate trip signal if resistance come down to 4 kohm.

On 23rd june , the said generator got trip on rotor earth fault.We meggered all the possible cable/path,values found more than 100Mohm.We shynochronised the generator after changing the E/F-relay. Generator again got tripped on ROTOR EARTH fault after 07 hours of run.We open the exciter chamber & cleaned thoroughly all the parts & parcel of exciter,meggered all the possible cable/path,values found ok.Nothing found abnormal except some carbon particles(due to carbon brush rubbing) at the slip-rings area.One point to mention that as carbon brush were rubbed out to much,it is not contiously touching the respective slip-ring.
The different parameters are as follows:-
The parameters before tripping -
1. Exciter current= 60 amps (for last 15 minutes).
2. Cold/hot gas temperature of generator =35?C /52 ?C
3. Drew point /purity of hydrogen gas = -8.9 ?C / 98.4%.
4. Seal oil DP = 1.72 ksc.
5. PW level before / after tripping = 233.52 / 233.52 mm.
6. Conductivity of PW after main filter = 2.1 ?s /cm.
7. Cold / hot air temperature of exciter = 35.6 ?c / 52.2 ?c.
8. Load / MVAR = 499 Mw / 79 MVAR.
Post trip checks parameters:-

1. IR of control cable from GRP end > 1 GΩ.
2. IR of rotor from exciter J.B > 90 MΩ.
3. Diode wheel ?FUSE? check through stroboscope: OK.

After opening of exciter chamber following checking?s are done :-
1.Diode wheel fuse cartridge physically check: OK.
1. Generator rotor resistance = 69.5 milliohms .
2. IR across slip rings after through cleaning of all accessible parts > 75 MΩ.
3. IR between brush holders (static) & earth > 1.2M Ω.
4. IR between earth & +VE, -VE connecting bolts > 80 M Ω.
5. Rotor earth fault relay MRR-1 has been checked with standard resistance & found satisfactory operation.

PLZ. ANALYSE THE THE CAUSE OF EARTH FAULT & INFORM ME.
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
bangal said:
We have 500mw generator (thermal) having DAVR-type excitation.For generator rotor earth fault protection,we feed 1HZ,20v square wave(d.c.) through slip ring-1 (insulated from shaft) in which the star point of armature winding of main exciter is connected. Rotor earth fault relay is connected accross slip ring-1 & another slip ring(electically connected with the rotor shaft) & compare the out-put(if any) with the said input ,then generate alarm signal if resistance come down to 40kohm & generate trip signal if resistance come down to 4 kohm.

On 23rd june , the said generator got trip on rotor earth fault.We meggered all the possible cable/path,values found more than 100Mohm.We shynochronised the generator after changing the E/F-relay. Generator again got tripped on ROTOR EARTH fault after 07 hours of run.We open the exciter chamber & cleaned thoroughly all the parts & parcel of exciter,meggered all the possible cable/path,values found ok.Nothing found abnormal except some carbon particles(due to carbon brush rubbing) at the slip-rings area.One point to mention that as carbon brush were rubbed out to much,it is not contiously touching the respective slip-ring.
The different parameters are as follows:-
The parameters before tripping -
1. Exciter current= 60 amps (for last 15 minutes).
2. Cold/hot gas temperature of generator =35?C /52 ?C
3. Drew point /purity of hydrogen gas = -8.9 ?C / 98.4%.
4. Seal oil DP = 1.72 ksc.
5. PW level before / after tripping = 233.52 / 233.52 mm.
6. Conductivity of PW after main filter = 2.1 ?s /cm.
7. Cold / hot air temperature of exciter = 35.6 ?c / 52.2 ?c.
8. Load / MVAR = 499 Mw / 79 MVAR.
Post trip checks parameters:-

1. IR of control cable from GRP end > 1 GΩ.
2. IR of rotor from exciter J.B > 90 MΩ.
3. Diode wheel ?FUSE? check through stroboscope: OK.

After opening of exciter chamber following checking?s are done :-
1.Diode wheel fuse cartridge physically check: OK.
1. Generator rotor resistance = 69.5 milliohms .
2. IR across slip rings after through cleaning of all accessible parts > 75 MΩ.
3. IR between brush holders (static) & earth > 1.2M Ω.
4. IR between earth & +VE, -VE connecting bolts > 80 M Ω.
5. Rotor earth fault relay MRR-1 has been checked with standard resistance & found satisfactory operation.

PLZ. ANALYSE THE THE CAUSE OF EARTH FAULT & INFORM ME.
Click to expand...

My consulting fees for this work would be $3200/8 hour day. I require door-to-door transportation of my choosing. My hotel accomodations are to be at 4* or better. Lucky for you, I don't drink.

Let me know when would you like to schedule my visit.:grin:
 
quogueelectric

quogueelectric

Senior Member
Location
new york
weressl said:
My consulting fees for this work would be $3200/8 hour day. I require door-to-door transportation of my choosing. My hotel accomodations are to be at 4* or better. Lucky for you, I don't drink.

Let me know when would you like to schedule my visit.:grin:
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I will do it for 3000 and I will require blue label I am not a plumber from the money pit.
 
quogueelectric

quogueelectric

Senior Member
Location
new york
quogueelectric said:
I will do it for 3000 and I will require blue label I am not a plumber from the money pit.
Click to expand...
Is this a GE 4700000kva #180x322? What is your steam initial pressure??Oh and of course temp?? 2400psig/1000*F?? WHat is the temp on the stator winding and field resistor?? 54C/64C?? Please read instructions GEK-5538 before going any further.
 
mivey

mivey

Senior Member
Pierre C Belarge said:
Maybe he meant a 500 kilowatt generator?
Click to expand...
As for this part, I believe you are correct.

[edit: maybe it was notation misused. We normally think of M as mega but it can mean thousand like when used in MMBtu. But why did he not then use MMΩ ?]
 
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mivey

mivey

Senior Member
quogueelectric said:
Is this a GE 4700000kva #180x322? What is your steam initial pressure??Oh and of course temp?? 2400psig/1000*F?? WHat is the temp on the stator winding and field resistor?? 54C/64C?? Please read instructions GEK-5538 before going any further.
Click to expand...
If we can pick on Pierre, why not quogue?

4,700,000kva? Do you work on these often?
 
W

whillis

Member
Location
Vancouver, BC
Size issues aside, this is an interesting thought experiment. If I understand the initial post correctly, the rotor has a ground fault system that consists of a slip ring connected to the star point of the rotor and another slip ring connected to the rotor shaft. If the resistance between these slip rings falls to 4k ohm or less then the ground fault trips.
Assuming the description above is accurate, then the obvious reason for the repeating fault is a bad winding making intermittent contact with the rotor structure. If the fault only happened when the unit was operating under load, what test could be set up to prove/disprove the bad winding theory?
 
wirenut1980

wirenut1980

Senior Member
Location
Plainfield, IN
I think the OP meant 500 megawatt generator...utility generation.

I know little to none about how to troubleshoot generators of that size, but one thing to check is to make sure the alarm you are getting is really happening, or if the alarm circuit is malfunctioning.

Can you monitor input and output of the square wave to see if maybe you are getting some noise or other interference on that circuit?
 
quogueelectric

quogueelectric

Senior Member
Location
new york
mivey said:
If we can pick on Pierre, why not quogue?

4,700,000kva? Do you work on these often?
Click to expand...
Not often but enough to know that you dont know and if anything I posted is out of normal operating range of these parameters PLEASE!! correct me.
 
mivey

mivey

Senior Member
quogueelectric said:
Not often but enough to know that you dont know and if anything I posted is out of normal operating range of these parameters PLEASE!! correct me.
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Well, it is big enough to power 3.88 time machines.:smile:

Many base load power plant units are in the 1-1.5 GW range. I'll say you have never worked on a 4.7 GW unit, much less not often.

As for the OP, it would not be unusual for a utility to have a 500 MW turbine but a unit this size would probably have folks from GE or Siemens or such helping out instead of us. That is why I am not sure about the units he used.
 
quogueelectric

quogueelectric

Senior Member
Location
new york
mivey said:
Well, it is big enough to power 3.88 time machines.:smile:

Many base load power plant units are in the 1-1.5 GW range. I'll say you have never worked on a 4.7 GW unit, much less not often.

As for the OP, it would not be unusual for a utility to have a 500 MW turbine but a unit this size would probably have folks from GE or Siemens or such helping out instead of us. That is why I am not sure about the units he used.
Click to expand...
The good folks from GE hire electricians to do the work god bless them. OOPS slipped a zero only 4 zeros sorry for typo
 
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