A
if your unit is on line producing 100 megawatts and the field breaker is opened accidentally, can it be closed back in or what action should be taken?
If ANYBODY's unit was on line producing 100 megawatts and the field breaker opened, accidentally or otherwise, the unit should have tripped, or at least the generator breaker should have opened (tripped).
The cause should have been Loss of Excitation, usually a device with ANSI number 41. There are other ways and relays/device numbers of detecting loss of field, but it's a serious condition and should lead immediately to at least a generator breaker opening (tripping) if not a unit trip.
If the unit continued to run and the generator rotor wasn't damaged due to inductive heating then ANYBODY would be very, Very, VERY lucky.
If the unit continued to run and the generator didn't at least 'slip a pole' and damage the load coupling, then ANYBODY is still very, Very, VERY lucky.
ANYBODY should be buying a lottery ticket.
What happened to your unit, anybody? Did it trip? Did the generator breaker open (trip)?
It would not generally be advisable to simply reclose the field breaker under the conditions you described (running at 100 MW).
The cause should have been Loss of Excitation, usually a device with ANSI number 41. There are other ways and relays/device numbers of detecting loss of field, but it's a serious condition and should lead immediately to at least a generator breaker opening (tripping) if not a unit trip.
If the unit continued to run and the generator rotor wasn't damaged due to inductive heating then ANYBODY would be very, Very, VERY lucky.
If the unit continued to run and the generator didn't at least 'slip a pole' and damage the load coupling, then ANYBODY is still very, Very, VERY lucky.
ANYBODY should be buying a lottery ticket.
What happened to your unit, anybody? Did it trip? Did the generator breaker open (trip)?
It would not generally be advisable to simply reclose the field breaker under the conditions you described (running at 100 MW).
A
anybody
the unit did trip opening generator main cb. operator asked the question and i was looking for a good explanation which you gave. thank you.
CSA... I agree with your admonition... tripping is necessary! However, I disagree with two of your views (one minor, the other more important):
o While the Field-Breaker Device Number is "41" (at least in the US) the Loss-of-Excitation (LoE) protective relay function is Device Number "40."
o More importantly, upon a LoE event the generator doesn't "slip a pole." Instead it operates as an induction generator if connected to a grid. If "island" operated, then voltage drops precipitously, causing other protective relay operation.
Whether immediate or delayed tripping is initiated depends on several factors. If a cylindrical-rotor machine, then instantaneous tripping is usually employed. If a salient-pole machine, connected to a stable grid, then, delayed tripping (in the order of seconds) is a reasonable course-of-action. Thus, insuring a more orderly shutdown!
In either case, I also agree that no human operator is fast enough to intervene, as one could to re-synchronize a synchronous motor after an accidental LoE event.
Regards, Phil Corso
o While the Field-Breaker Device Number is "41" (at least in the US) the Loss-of-Excitation (LoE) protective relay function is Device Number "40."
o More importantly, upon a LoE event the generator doesn't "slip a pole." Instead it operates as an induction generator if connected to a grid. If "island" operated, then voltage drops precipitously, causing other protective relay operation.
Whether immediate or delayed tripping is initiated depends on several factors. If a cylindrical-rotor machine, then instantaneous tripping is usually employed. If a salient-pole machine, connected to a stable grid, then, delayed tripping (in the order of seconds) is a reasonable course-of-action. Thus, insuring a more orderly shutdown!
In either case, I also agree that no human operator is fast enough to intervene, as one could to re-synchronize a synchronous motor after an accidental LoE event.
Regards, Phil Corso
G
Gen Eng
I have a transient recording where the excitation system failed (zero excitation) on a 768MVA cylindrical rotor machine where it tripped on pole slipping, regardless of the fact that is generally believed that it cannot pole slip due it running as induction generator. This machine is equipped with a rotating diode exciter. The protection that operated was situated in a ABB REG316 *4 relay.
It is believed that the oscillatory action of the rotor caused enough DC current to be generated via the rotating diodes to let it behave like a synchronous machine.
Because I do not have the shaft speed this cannot be conclusively proved.
It is believed that the oscillatory action of the rotor caused enough DC current to be generated via the rotating diodes to let it behave like a synchronous machine.
Because I do not have the shaft speed this cannot be conclusively proved.
Gen Eng,
It would seem that the generator has a "brushless" excitation system with some rotating excitation components--probably a stationary field to which variable DC is applied, and a rotating armature where AC is developed and applied to the rotating diodes that convert the AC to DC to apply to the cylindrical rotor.
Was there any physical damage to the coupling between the turbine and generator, or any heat damage to the cylindrical rotor causing damage or requiring repair? Generally, pole slippage results in very large mechanical forces due to repeated acceleration/deceleration which physically damage the coupling/coupling bolts.
Operation as an induction generator usually results in very large circulating currents in the rotor components (punchings; retaining rings (if made of magnetic material); etc.) that cause insulation to degrade shorts/grounds to occur. Also, the heating can result in expansion which causes interference between the rotor and the stator, causing damage to the stator.
Induction operation requires slip (difference in speed) but I wouldn't think that pole slipping which would result in repeated extremely fast rotation (acceleration) of the rotor and then extremely fast deceleration (stopping) of the rotor as it "catches up" with the rotating field of the stator would result in induced voltage that would somehow "get back" to the AC side of the rotating diodes.
How long did this event occur? Could it be that the pole slipping relay function simply operated first? When excitation is lost, there is a collapse of magnetic fields and it's not instantaneous. It's hard to understand how any induced voltage/currents in the generator could get "back" to the AC side of the rotating diodes.
Could it be that somehow the excitation being applied to the machine was intermittent?
Or that there was some residual magnetism in the stationary field of the brushless exciter that resulted in DC being applied to the rotating diodes, but that it was insufficient to maintain synchronous speed and resulted in pole slippage?
Finally, generator frequency is directly related to generator rotor speed. Do you have any high-speed data for the frequency at the time of the event?
It would seem that the generator has a "brushless" excitation system with some rotating excitation components--probably a stationary field to which variable DC is applied, and a rotating armature where AC is developed and applied to the rotating diodes that convert the AC to DC to apply to the cylindrical rotor.
Was there any physical damage to the coupling between the turbine and generator, or any heat damage to the cylindrical rotor causing damage or requiring repair? Generally, pole slippage results in very large mechanical forces due to repeated acceleration/deceleration which physically damage the coupling/coupling bolts.
Operation as an induction generator usually results in very large circulating currents in the rotor components (punchings; retaining rings (if made of magnetic material); etc.) that cause insulation to degrade shorts/grounds to occur. Also, the heating can result in expansion which causes interference between the rotor and the stator, causing damage to the stator.
Induction operation requires slip (difference in speed) but I wouldn't think that pole slipping which would result in repeated extremely fast rotation (acceleration) of the rotor and then extremely fast deceleration (stopping) of the rotor as it "catches up" with the rotating field of the stator would result in induced voltage that would somehow "get back" to the AC side of the rotating diodes.
How long did this event occur? Could it be that the pole slipping relay function simply operated first? When excitation is lost, there is a collapse of magnetic fields and it's not instantaneous. It's hard to understand how any induced voltage/currents in the generator could get "back" to the AC side of the rotating diodes.
Could it be that somehow the excitation being applied to the machine was intermittent?
Or that there was some residual magnetism in the stationary field of the brushless exciter that resulted in DC being applied to the rotating diodes, but that it was insufficient to maintain synchronous speed and resulted in pole slippage?
Finally, generator frequency is directly related to generator rotor speed. Do you have any high-speed data for the frequency at the time of the event?
P
PJK
Good Morning to all,
My unit was running on 62MW(GE-7EA), and gas pressure low alarmed, checked pressure was ok 18.85bar. but after 2 minutes, found unit was running on low p2 pressure around 12.5bar for 3 seconds. then unit trip on generator differential protection.
Whether if unit fails to trip on gas fuel pressure low (no back up fuel available), which protection will act next? or what may be the consequence after gas pressure low alarmed and unit not tripped?
My unit was running on 62MW(GE-7EA), and gas pressure low alarmed, checked pressure was ok 18.85bar. but after 2 minutes, found unit was running on low p2 pressure around 12.5bar for 3 seconds. then unit trip on generator differential protection.
Whether if unit fails to trip on gas fuel pressure low (no back up fuel available), which protection will act next? or what may be the consequence after gas pressure low alarmed and unit not tripped?
PJK,
The low gas fuel supply pressure alarm is <i>generally</i> detected by a pressure switch, the setting of which is defined in the unit Device Summary drawing. Sometimes, a pressure transducer is used for the indication--and there have even been cases where both have been used for indication and alarm on low pressure.
Does your site regularly experience low gas fuel supply pressure? Or just infrequently? Or almost never?
What was the grid frequency doing at the time of the event? Was it high? Low? Stable? Unstable?
"Differential protection" is not very descriptive. There is generator current differential protection (87G), which usually actuates a differential lockout (86G)--and there is often generator neutral differential protection (87N) which also actuates the differential lockout relay (86G). There are usually several other generator protective conditions which also actuate 86G. Some units have a transformer differential lockout (86T), which includes transformer differential relays (87T) and usually several other conditions.
So, without more information it's virtually impossible to comment on what may have happened.
If you can post the Process Alarm list, with time/date stamps for a few seconds prior to the trip and a few seconds after the trip, we might be able to help. (Depending on the type of operator interface, there may be an Alarm History file which can be exported to an ASCII text file, and then you can copy and paste the requested sections into your reply; unfortunately, control.com does not support attachments).
Without more information, that's about all that can be said.
The low gas fuel supply pressure alarm is <i>generally</i> detected by a pressure switch, the setting of which is defined in the unit Device Summary drawing. Sometimes, a pressure transducer is used for the indication--and there have even been cases where both have been used for indication and alarm on low pressure.
Does your site regularly experience low gas fuel supply pressure? Or just infrequently? Or almost never?
What was the grid frequency doing at the time of the event? Was it high? Low? Stable? Unstable?
"Differential protection" is not very descriptive. There is generator current differential protection (87G), which usually actuates a differential lockout (86G)--and there is often generator neutral differential protection (87N) which also actuates the differential lockout relay (86G). There are usually several other generator protective conditions which also actuate 86G. Some units have a transformer differential lockout (86T), which includes transformer differential relays (87T) and usually several other conditions.
So, without more information it's virtually impossible to comment on what may have happened.
If you can post the Process Alarm list, with time/date stamps for a few seconds prior to the trip and a few seconds after the trip, we might be able to help. (Depending on the type of operator interface, there may be an Alarm History file which can be exported to an ASCII text file, and then you can copy and paste the requested sections into your reply; unfortunately, control.com does not support attachments).
Without more information, that's about all that can be said.
P
pjk
Sure sir,
tomorrow i will post all the alarms what i have received. I need one clarification, as i mentioned before. if actually fuel pressure low and unit not tripped, then which protection will come into action to save the machine?
tomorrow i will post all the alarms what i have received. I need one clarification, as i mentioned before. if actually fuel pressure low and unit not tripped, then which protection will come into action to save the machine?
pjk,
No generator protection will protect the machine on low gas fuel supply pressure--unless your unit is extremely non-standard.
Possibly--<i>if the gas fuel supply pressure gets low enough for long enough</i>--reverse power will trip the machine, but you will see load decreasing as gas fuel supply pressure continues to drop, and you didn't mention anything about that.
No generator protection will protect the machine on low gas fuel supply pressure--unless your unit is extremely non-standard.
Possibly--<i>if the gas fuel supply pressure gets low enough for long enough</i>--reverse power will trip the machine, but you will see load decreasing as gas fuel supply pressure continues to drop, and you didn't mention anything about that.
