We are rehabilitating a Powerhouse composed of six Generating sets of 2250kVA, 11kV, 50HZ, 1000rpm.
When a power failure occurs, two generators are coupled "on start" (both breaker closes during start), taking immediately the essential load. Then, subsequent generators will be manually synchronized until full load is restored.
The Generators are isolated from the Mains by a changeover-interlocked system on the 11kV switchgear.
When the Mains return, the load is transferred back by means of a manual operation.
We recently overhauled one of the Alternators. The damage observed is mechanical. We suspect crash-synchro.
Is possible that during the transfer of the load to the Mains, since the operation is manual, a human error could provoke the crash-synchro.
We would like to confirm with somebody who has experienced this type of failure how luck like an alternator damaged by crash-synchro.
Pictures are available on request.
We are rehabilitating a Powerhouse composed of six Generating sets of 2250kVA, 11kV, 50HZ, 1000rpm.
When a power failure occurs, two generators are coupled "on start" (both breaker closes during start), taking immediately the essential load. Then, subsequent generators will be manually synchronized until full load is restored.
The Generators are isolated from the Mains by a changeover-interlocked system on the 11kV switchgear.
When the Mains return, the load is transferred back by means of a manual operation.
We recently overhauled one of the Alternators. The damage observed is mechanical. We suspect crash-synchro.
Is possible that during the transfer of the load to the Mains, since the operation is manual, a human error could provoke the crash-synchro.
We would like to confirm with somebody who has experienced this type of failure how luck like an alternator damaged by crash-synchro.
Pictures are available on request.
The manual loading is done activating one by one the non-essential feeders on the 11kV Distribution Board as follows:
a) Initial load 96A @ 11kV = 1827 KVA
b) Next feeder 35A @ 11 kV = 666 KVA
c) Next feeder 150 A @ 11 kV = 2855 KVA
The total load is 5347 KVA
A total of three generators is needed at full load.
Comment: The c) appears to be on the larger side. It may be needed to be reduced by the downstream tripped circuit breakers turning of a noticeable fraction of 2855kVA, perhaps to about a half. Also, the load block sequence should be from the larger one to the smaller one, which is obvious since the generators are more loaded by the cummulative load block effect.
From the sound of it, you are starting the two generators automatically on a power failure and connecting them to the essential service bus. What is not clear is whether you are synchronizing these two generators to each other - it almost sounds as though they are just being switched on to the bus as they start, which could be a source of your problem.
Mechanical damage due to faulty synchronizing could show up as some of the following -
- Coupling and/or gearbox damage, as applicable
- Rotor damage - shifted windings, shaft deformation
- Damage to hold-down bolts
- Damage to stator end turns - deformation, shifting
The above are extreme examples of worst-case scenarios. I have experience of a 65 MW unit that was "synchronized" 60 degrees out of phase - this unit suffered rotor winding shift that manifested itself in increased vibration levels depending on excitation current.
It would be good idea to install synchrocheck relaying to supervise any manual operations, and you should probably consider installing automatic synchronizing to remove any doubt.
Hope this helps.
Responding to peterb Dec 4, 2201 posting.
The sets are synchronized during running up of the sets. This is possible because the speed of the engines has been adjusted to approximately the same and for the fact that the engines receive the starting command at the same time. The command sent out for starting (Neamatic air start) is also sent out to the sets circuit breakers for switching on as soon as ignition speed is reached.
Special overvoltage protection has been incorporated to protect the rotating diodes against high circulating currents between the alternators during starting.
I'm not the expert here, but I fail to understand how the first two generators can be considered to be synchronized, just because they get simultaneous start signals. One engine could catch quicker or accelerate quicker and the other would be reverse powered (motorized generator) and dragged along until it was up to speed. I do know that a proper synchronization involves closely matching voltage and speed (the oncoming should be a couple percent higher to assume some load) then closing the breaker when the electrical phase angles match. I didn't see this in your description. I think the machines are being jerked into synchronization in one or two electrical cycles and your term "crash synchro" is appropriate.
Although it is unconventional, I don't think that the process as described by Nawao would result in any shock loading to either of the two generators.
This assumes that the breakers are closed to connect the two stator windings together BEFORE the voltage has built up to any appreciable level. Of course, if the run-up cycle is not precisely synchronized (and it can't be), one machine will be running ahead of the other after the voltage has built up. This will have the effect of motoring the "slow" generator - pulling it up to speed in step with the "fast" unit, but there will be no shock involved. As I said, unconventional, but it may work - does the whole process have the blessing of the genset manufacturer?.
I think that the mechanical damage is more likely the result of poor manual synch against the grid.
"The command sent out for starting (Neamatic air start) is also sent out to the sets circuit breakers for switching on as soon as ignition speed is reached."
This sounds as if the breakers are not closed prior to starting, but rather as (each) engine starts to fire. We need to get this statement defined. I interpreted this as one engine starts, closes its breaker, than the second reaches ignition speed and closes its breaker, resulting in an unsynchronized parallel. :-O
Responding to TheBlacksmith
Each engine closes his breaker as soon as reaches ignition speed. In that moment no appreciable level of voltage has built up.
Suggestion: At some point or convenient time, the loads can be disconnected from CBs and a timing diagram for CBs, switching their "would be load," could be obtained while measuring other parameters, e.g. voltages, rpms, etc.
Aftergoing through your statements and the pocedure for synchronisation
my sugestion is that, you need to modify circuit i such a way that
1) one machine is strated fisrt and the corresponding breaker is closed to connect it to the BUS
2)then second machine is strated and it is synchronised with the BUS
NB: it is suggested to use a check synch relay for the synchronisation.
