Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations cowski on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
generator rotor earth fault 5
- Thread starter electric
- Start date
-
4
- #2
Generator Ground Faults- Part I
I will have to write this in two parts since I will be leaving to take my son to his Lacrosse game before I can finish what I want to say.
I assume the question is from someone outside the U.S. where the common termininology is to use earth faults rather than ground faults. In any event whether you use earth or ground the problems are the same.
One of the most commonly misunderstood abnormal conitions on a generator is a ground condition in the field (rotor) winding. While there is general understanding of the concept of grounds in the generator field circuit, this understanding is most oftern distorted by fears and concerns of what will happen if the ground stays on the generator system. Some operators want to trip the machine immediately and effect repair, while other more knowledgeable operators believe that the machine can be operated indefinitely with a ground without any undue concern.
Generator field circuits are operated from an excitation source which is undgrounded. Since the excitation source is ungrounded, the occurrence of the first ground does not cause any damage nor does it effect the operation of the generator in any way. However the first ground fault does set the stage for what could potentially cause a very serious condition-the second ground fault.
When a second ground fault occurs, a portion of the field winding is bypassed and this short circuiting of a portion of the field winding produces an unbalance in the air gap flux between the stator and rotor. This uneveness in the flux produces an unbalanced magnetic force on the rotor. Depending on the amount of the field winding which has been bypassed, the unbalanced force can be sufficient to spring the rotor shaft and make it eccentric. GE reports that there are on record occurrences where the resulting vibration has broken bearing pedestals and allowed the rotor to grind against the stator and this caused extensive damage and long duration outages. Now remember that this happened when there were two ground faults and it will not happen with only one.
I have at hand the information of a 40 MVA,3600 RPM 13.8 KV machine which has approximately 500 turns (250 turns per pole). The excitation source for this machine is static and operates at 250 volts D-C. This means that the voltage per turn on this machine is .5 volts per turn when the machine is operated near full load conditions. Therefore durning normal operation there is very little voltage stress on the winding insulation beteween the adjacent turns in the field winding. When a ground fault occurs, it is generally due to long term thermal degradation of the insulation system or contamination of the insullation from oil, carbon dust in the case of a direct connected exciter and a poor sealing system between the exciter and rotor winding or the intrusion of water from the cooling system. I have seen all of these result in genreator ground faults.
Since the occurence of a second ground fault can produce extreme vibration and quick damage, a knowledgeable individual with full understanding of the consequences shuld be consulted on what to do. Trip the macchine or continue to run. I have always elected to continue to operate the machine and examine the static exciter to determine if the failure is in the excitation system. I should add that this generator was equipped with a vibration monitoring system on the bearings and an alarm and trip settings were operational
I will have to write this in two parts since I will be leaving to take my son to his Lacrosse game before I can finish what I want to say.
I assume the question is from someone outside the U.S. where the common termininology is to use earth faults rather than ground faults. In any event whether you use earth or ground the problems are the same.
One of the most commonly misunderstood abnormal conitions on a generator is a ground condition in the field (rotor) winding. While there is general understanding of the concept of grounds in the generator field circuit, this understanding is most oftern distorted by fears and concerns of what will happen if the ground stays on the generator system. Some operators want to trip the machine immediately and effect repair, while other more knowledgeable operators believe that the machine can be operated indefinitely with a ground without any undue concern.
Generator field circuits are operated from an excitation source which is undgrounded. Since the excitation source is ungrounded, the occurrence of the first ground does not cause any damage nor does it effect the operation of the generator in any way. However the first ground fault does set the stage for what could potentially cause a very serious condition-the second ground fault.
When a second ground fault occurs, a portion of the field winding is bypassed and this short circuiting of a portion of the field winding produces an unbalance in the air gap flux between the stator and rotor. This uneveness in the flux produces an unbalanced magnetic force on the rotor. Depending on the amount of the field winding which has been bypassed, the unbalanced force can be sufficient to spring the rotor shaft and make it eccentric. GE reports that there are on record occurrences where the resulting vibration has broken bearing pedestals and allowed the rotor to grind against the stator and this caused extensive damage and long duration outages. Now remember that this happened when there were two ground faults and it will not happen with only one.
I have at hand the information of a 40 MVA,3600 RPM 13.8 KV machine which has approximately 500 turns (250 turns per pole). The excitation source for this machine is static and operates at 250 volts D-C. This means that the voltage per turn on this machine is .5 volts per turn when the machine is operated near full load conditions. Therefore durning normal operation there is very little voltage stress on the winding insulation beteween the adjacent turns in the field winding. When a ground fault occurs, it is generally due to long term thermal degradation of the insulation system or contamination of the insullation from oil, carbon dust in the case of a direct connected exciter and a poor sealing system between the exciter and rotor winding or the intrusion of water from the cooling system. I have seen all of these result in genreator ground faults.
Since the occurence of a second ground fault can produce extreme vibration and quick damage, a knowledgeable individual with full understanding of the consequences shuld be consulted on what to do. Trip the macchine or continue to run. I have always elected to continue to operate the machine and examine the static exciter to determine if the failure is in the excitation system. I should add that this generator was equipped with a vibration monitoring system on the bearings and an alarm and trip settings were operational
Generator Ground Faults- Part I
I will have to write this in two parts since I will be leaving to take my son to his Lacrosse game before I can finish what I want to say.
I assume the question is from someone outside the U.S. where the common termininology is to use earth faults rather than ground faults. In any event whether you use earth or ground the problems are the same.
One of the most commonly misunderstood abnormal conitions on a generator is a ground condition in the field (rotor) winding. While there is general understanding of the concept of grounds in the generator field circuit, this understanding is most oftern distorted by fears and concerns of what will happen if the ground stays on the generator system. Some operators want to trip the machine immediately and effect repair, while other more knowledgeable operators believe that the machine can be operated indefinitely with a ground without any undue concern.
Generator field circuits are operated from an excitation source which is undgrounded. Since the excitation source is ungrounded, the occurrence of the first ground does not cause any damage nor does it effect the operation of the generator in any way. However the first ground fault does set the stage for what could potentially cause a very serious condition-the second ground fault.
When a second ground fault occurs, a portion of the field winding is bypassed and this short circuiting of a portion of the field winding produces an unbalance in the air gap flux between the stator and rotor. This uneveness in the flux produces an unbalanced magnetic force on the rotor. Depending on the amount of the field winding which has been bypassed, the unbalanced force can be sufficient to spring the rotor shaft and make it eccentric. GE reports that there are on record occurrences where the resulting vibration has broken bearing pedestals and allowed the rotor to grind against the stator and this caused extensive damage and long duration outages. Now remember that this happened when there were two ground faults and it will not happen with only one. In general the larger the amout of the field winding that is bypassed, the greater the vibration and the quicker the failure.
I have at hand the information of a 40 MVA,3600 RPM 13.8 KV machine which has approximately 500 turns in the field circuit(250 turns per pole). The excitation source for this machine is static and operates at 250 volts D-C. This means that the voltage per turn on this machine is .5 volts per turn when the machine is operated near full load conditions. Therefore durning normal operation there is very little voltage stress on the winding insulation beteween the adjacent turns in the field winding. When a ground fault occurs, it is generally due to long term thermal degradation of the insulation system or contamination of the insulation from oil, carbon dust in the case of a direct connected exciter and a poor sealing system between the exciter and rotor winding or the intrusion of water from the cooling system. I have seen all of these result in genreator ground faults.
Since the occurence of a second ground fault can produce extreme vibration and quick damage, a knowledgeable individual with full understanding of the consequences shuld be consulted on what to do. Trip the machine or continue to run. I have always elected to continue to operate the machine and examine the static exciter to determine if the failure is in the excitation system. I should add that this generator was equipped with a vibration monitoring system on the bearings and an alarm and trip settings were operational. In other words we had a built in protective system for a second ground fault.
Time to go- I will finish later tonight and give you a method of testing the rotor circuit to determine if any turn-turn shorts are present. Turn-turn failure will eventually involve ground. I will also discuss the Field Ground Relay
I will have to write this in two parts since I will be leaving to take my son to his Lacrosse game before I can finish what I want to say.
I assume the question is from someone outside the U.S. where the common termininology is to use earth faults rather than ground faults. In any event whether you use earth or ground the problems are the same.
One of the most commonly misunderstood abnormal conitions on a generator is a ground condition in the field (rotor) winding. While there is general understanding of the concept of grounds in the generator field circuit, this understanding is most oftern distorted by fears and concerns of what will happen if the ground stays on the generator system. Some operators want to trip the machine immediately and effect repair, while other more knowledgeable operators believe that the machine can be operated indefinitely with a ground without any undue concern.
Generator field circuits are operated from an excitation source which is undgrounded. Since the excitation source is ungrounded, the occurrence of the first ground does not cause any damage nor does it effect the operation of the generator in any way. However the first ground fault does set the stage for what could potentially cause a very serious condition-the second ground fault.
When a second ground fault occurs, a portion of the field winding is bypassed and this short circuiting of a portion of the field winding produces an unbalance in the air gap flux between the stator and rotor. This uneveness in the flux produces an unbalanced magnetic force on the rotor. Depending on the amount of the field winding which has been bypassed, the unbalanced force can be sufficient to spring the rotor shaft and make it eccentric. GE reports that there are on record occurrences where the resulting vibration has broken bearing pedestals and allowed the rotor to grind against the stator and this caused extensive damage and long duration outages. Now remember that this happened when there were two ground faults and it will not happen with only one. In general the larger the amout of the field winding that is bypassed, the greater the vibration and the quicker the failure.
I have at hand the information of a 40 MVA,3600 RPM 13.8 KV machine which has approximately 500 turns in the field circuit(250 turns per pole). The excitation source for this machine is static and operates at 250 volts D-C. This means that the voltage per turn on this machine is .5 volts per turn when the machine is operated near full load conditions. Therefore durning normal operation there is very little voltage stress on the winding insulation beteween the adjacent turns in the field winding. When a ground fault occurs, it is generally due to long term thermal degradation of the insulation system or contamination of the insulation from oil, carbon dust in the case of a direct connected exciter and a poor sealing system between the exciter and rotor winding or the intrusion of water from the cooling system. I have seen all of these result in genreator ground faults.
Since the occurence of a second ground fault can produce extreme vibration and quick damage, a knowledgeable individual with full understanding of the consequences shuld be consulted on what to do. Trip the machine or continue to run. I have always elected to continue to operate the machine and examine the static exciter to determine if the failure is in the excitation system. I should add that this generator was equipped with a vibration monitoring system on the bearings and an alarm and trip settings were operational. In other words we had a built in protective system for a second ground fault.
Time to go- I will finish later tonight and give you a method of testing the rotor circuit to determine if any turn-turn shorts are present. Turn-turn failure will eventually involve ground. I will also discuss the Field Ground Relay
Part I continued
Generator Rotor Ground Faults
I note that I titled the initial post Generator Ground Faults and of course it should have been Generator Rotor Ground Faults since nothing in the material referred to any other winding except the rotor.
When practical, it is standard practice to install a 64 relay on the machine to monitor the condition of the rotor circuit to ensure that no indavertent grounds are present. When the machine is equipped with collector rings, the installation of a field ground relay is strait forward and for static excitation systems, this relay can be included as a part of the excitation package. This is not the case when the generator is equipped with a Brushless excitation system. Some manufacturers do offer a ground detection system for these installations, but I have found that each manufacturer applies this detection in a different manner and some will not consider offering this feature.
I suggest that if the machine is large or important, you should consider the installation of a vibration monitoring system which will alarm/trip the machine at high levels of vibration. Specific limits and set points should always be provided by the machine manufacturer.
Rotor Ground Faults-Part II
How Do You Find The Ground?
It is necessary to isolate the field winding from the excitation source. Make an insulation resistance test from the rotor winding to the rotor steel. A zero or low reading is conformation that a ground exists. The exact location of the ground is difficult to predict, but on round rotor designs I have always found the problem to be under the retaining ring. This is the portion of the winding which is cannot be restrained by factory installed rotor wedges, the portion of the winding that can flex the most, and in general the portion of the winding that is most susceptible to damage from the rotational forces of operation.
A word of warning,if you have to send the rotor out for a rewind you had better order the new windings in advance. After you have confirmed that a rotor gound exists, you had better get a purchase order for a new winding and then go back on line with a ground fault until the materials are at hand to effect a rewind. Better count on 7-10 days for this effort and do a lot of planning with the repair and rigging experts to get the unit in and out in a safe manner. Removal of a round rotor unit should always be supervised by an individual completely familiar with your specific unit. When the unit is out of the generator, close up the unit with appropriate material to keep contaminants out and install some type of heat to maintain the stator winding above ambient temperature.
Annual Test On The Rotor Winding
One of the conditions I worry about in the rotor winding are turn-turn shorts which do not involve the ground circuit. It is known that round rotor field windings are susceptible to failure in the turn to turn insulation system. In general turn-turn failures are associated withthe initial construction and are corrected before the unit is shipped. Most manufactures conduct a series of tests after each coil assembly is pressed into the rotor steel. These test consists of insulation resistance tests as the rotor progresses through the manufacturing process so that shorted turns can be corrected as early as possible and with the least amount of schedule and cost impact.
One of the test is the A-C Impedance test on the rotor coil assemblies. The machine I mentioned earlier which had 500 turns in the rotor winding and these turns were combined into 6 coil groups per pole. The reason an A-C test is used is that shorted turn DC effects are magnified approximatley ten fold by A-C measurements. Aproximately 6-7 sets( depends on the number of coil assemblies) of A-C Impedance tests are conducted as the machine goes manufacturing. The final test is conducted after the machine is at rated speed and after final balance. This impedance is corrected to a nominal value and is available to the customer-all you have to do is ask for the information. A test at standstill is also made and this is the test data you need. You need the voltage at which the test was made and the A-C current which results from this test. The standstill factory test is the benchmark you will use in the field to confirm that you have or do not have turn-turn shorts.
In the field it is necessary to disconnect the excitation system and have a variable A-C voltage source and install a 5-10 amp meter in series with the source. For the machine I mentioned above the A-C test voltage was 100 volts and the expected current was 1.3 amps when the rotor was at ambient temperature ( 25 deg. C). A 5 watt variac should be sufficient. Corrections have to be made for the ambient temperature. The Z value for this machine was 76.9 ohms.
If the field test results differ by more than 10 % from the factory tests, you have a reason to be concerned. It is time to call in and or consult with the factory experts.
I should add that the remedy for a turn-turn short is the same as fixing a rotor ground-it's a big problem and it's time to get out the checkbook.
You should not extrapolate the 100 volt rotor impedance test to all manufacturers-this test just applies to GE machines. I know that Brush (Great Britian) recommmended 36 volts and 4.2 amps and a Z= 8.6 ohms for a unit that was rated 57 MVA, 13.2 KV and 3600 RPM.
I hope this answers your questions concerning rotor earth faults.
Generator Rotor Ground Faults
I note that I titled the initial post Generator Ground Faults and of course it should have been Generator Rotor Ground Faults since nothing in the material referred to any other winding except the rotor.
When practical, it is standard practice to install a 64 relay on the machine to monitor the condition of the rotor circuit to ensure that no indavertent grounds are present. When the machine is equipped with collector rings, the installation of a field ground relay is strait forward and for static excitation systems, this relay can be included as a part of the excitation package. This is not the case when the generator is equipped with a Brushless excitation system. Some manufacturers do offer a ground detection system for these installations, but I have found that each manufacturer applies this detection in a different manner and some will not consider offering this feature.
I suggest that if the machine is large or important, you should consider the installation of a vibration monitoring system which will alarm/trip the machine at high levels of vibration. Specific limits and set points should always be provided by the machine manufacturer.
Rotor Ground Faults-Part II
How Do You Find The Ground?
It is necessary to isolate the field winding from the excitation source. Make an insulation resistance test from the rotor winding to the rotor steel. A zero or low reading is conformation that a ground exists. The exact location of the ground is difficult to predict, but on round rotor designs I have always found the problem to be under the retaining ring. This is the portion of the winding which is cannot be restrained by factory installed rotor wedges, the portion of the winding that can flex the most, and in general the portion of the winding that is most susceptible to damage from the rotational forces of operation.
A word of warning,if you have to send the rotor out for a rewind you had better order the new windings in advance. After you have confirmed that a rotor gound exists, you had better get a purchase order for a new winding and then go back on line with a ground fault until the materials are at hand to effect a rewind. Better count on 7-10 days for this effort and do a lot of planning with the repair and rigging experts to get the unit in and out in a safe manner. Removal of a round rotor unit should always be supervised by an individual completely familiar with your specific unit. When the unit is out of the generator, close up the unit with appropriate material to keep contaminants out and install some type of heat to maintain the stator winding above ambient temperature.
Annual Test On The Rotor Winding
One of the conditions I worry about in the rotor winding are turn-turn shorts which do not involve the ground circuit. It is known that round rotor field windings are susceptible to failure in the turn to turn insulation system. In general turn-turn failures are associated withthe initial construction and are corrected before the unit is shipped. Most manufactures conduct a series of tests after each coil assembly is pressed into the rotor steel. These test consists of insulation resistance tests as the rotor progresses through the manufacturing process so that shorted turns can be corrected as early as possible and with the least amount of schedule and cost impact.
One of the test is the A-C Impedance test on the rotor coil assemblies. The machine I mentioned earlier which had 500 turns in the rotor winding and these turns were combined into 6 coil groups per pole. The reason an A-C test is used is that shorted turn DC effects are magnified approximatley ten fold by A-C measurements. Aproximately 6-7 sets( depends on the number of coil assemblies) of A-C Impedance tests are conducted as the machine goes manufacturing. The final test is conducted after the machine is at rated speed and after final balance. This impedance is corrected to a nominal value and is available to the customer-all you have to do is ask for the information. A test at standstill is also made and this is the test data you need. You need the voltage at which the test was made and the A-C current which results from this test. The standstill factory test is the benchmark you will use in the field to confirm that you have or do not have turn-turn shorts.
In the field it is necessary to disconnect the excitation system and have a variable A-C voltage source and install a 5-10 amp meter in series with the source. For the machine I mentioned above the A-C test voltage was 100 volts and the expected current was 1.3 amps when the rotor was at ambient temperature ( 25 deg. C). A 5 watt variac should be sufficient. Corrections have to be made for the ambient temperature. The Z value for this machine was 76.9 ohms.
If the field test results differ by more than 10 % from the factory tests, you have a reason to be concerned. It is time to call in and or consult with the factory experts.
I should add that the remedy for a turn-turn short is the same as fixing a rotor ground-it's a big problem and it's time to get out the checkbook.
You should not extrapolate the 100 volt rotor impedance test to all manufacturers-this test just applies to GE machines. I know that Brush (Great Britian) recommmended 36 volts and 4.2 amps and a Z= 8.6 ohms for a unit that was rated 57 MVA, 13.2 KV and 3600 RPM.
I hope this answers your questions concerning rotor earth faults.
-
1
- #5
jbartos
Electrical
- Jan 15, 2000
- 6,440
Suggestion: Industry standards might also get some consideration, e.g.
0112-1996 IEEE Standard Test Procedure for Polyphase Induction Motors and Generators 1996
Instructions are given for conducting and reporting the more
generally applicable and acceptable tests to determine the
performance characteristics of polyphase induction motors and
generators. Electrical measurements, performance testing,
temperature tests, and miscellaneous tests are covered.
0112-1996 IEEE Standard Test Procedure for Polyphase Induction Motors and Generators 1996
Instructions are given for conducting and reporting the more
generally applicable and acceptable tests to determine the
performance characteristics of polyphase induction motors and
generators. Electrical measurements, performance testing,
temperature tests, and miscellaneous tests are covered.
jack6238,
I found your discussion very interesting & although a 'new member' i have concidered the problem of rotor turn to turn short circuits.
Have you come across instrumentation to detect shorted turns?
I've looked at LCR meters with Q factor, but trials have shoen poor discrimination of a 2 turn short ( in a rotor of 800 turns)
I've also concidered airgap flux detection, but since I deal with salient pole generators, my machines don't have the airgap clearance nessacery to get a flux probe in!
any suggestions?
I found your discussion very interesting & although a 'new member' i have concidered the problem of rotor turn to turn short circuits.
Have you come across instrumentation to detect shorted turns?
I've looked at LCR meters with Q factor, but trials have shoen poor discrimination of a 2 turn short ( in a rotor of 800 turns)
I've also concidered airgap flux detection, but since I deal with salient pole generators, my machines don't have the airgap clearance nessacery to get a flux probe in!
any suggestions?
jbartos
Electrical
- Jan 15, 2000
- 6,440
Suggestions:
1. Only a very accurate LCR meter could potentially detect the turn to turn fault.
2. It appears that 2 turn-to-turn fault could stay if there is not arcing, heat development and further deterioration. The voltage discrepancy or unbalance could be adjusted downstream of the generator, e.g. by the voltage regulation or balancing.
1. Only a very accurate LCR meter could potentially detect the turn to turn fault.
2. It appears that 2 turn-to-turn fault could stay if there is not arcing, heat development and further deterioration. The voltage discrepancy or unbalance could be adjusted downstream of the generator, e.g. by the voltage regulation or balancing.
motorspert
Electrical
There is a device called an RSO (Recurrent surge oscillogram)tester which can be used on turbo type rotors to check for faults. i think this is a UK developed tool not common overseas. it measures the transient response to a square wave
For sure. I wonder if it is worth salvaging some of the content of his posts and making them in to a FAQ? He took a lot of time to compose his replies and it would be a shame for that effort to go to waste.
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
- Status
Similar threads
- Question
- Locked
- Question
- Locked
- Question