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 FAULT & INFORM ME.
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 FAULT & INFORM ME.
