Anybody help with strange voltages and an alternator problem
Anybody help with strange voltages and an alternator problem
(OP)
We have an 400 Volt alternator driven by a CHP and connectect to an 11 KV busbar by way of a step up transformer. This setup has been working fine now for a few years. All of a sudden we are getting strange voltages indicated on the mains side side when the machine is at rest. When we run up the machine, we are getting voltage mis-match faults. The mains voltage is monitored between phases, and phase to earth and indicated at A on the diagram and displayed on the HMI. These voltage readings are when the generator is at rest and are unbalanced, ranging from about 370 Volts to 428 Volts, and absolutely silly voltages between phase and neutral. The only neutral we have is connected to the star point of the alternator. I think we are getting these strange readings, as we are measuring from the transformer phase to the generator neutral, so this may not be a bearing on the fault. We have found a couple of failed (open circuit) diodes on the exciter, and have replaced all twelve diodes, but the fault is still with us.
Appreciate if anyone can help on this as we have come to a dead end on what to check next. I am thinking that maybe the exciter has lost its magnetism. A drawing of the system is attached.
Appreciate if anyone can help on this as we have come to a dead end on what to check next. I am thinking that maybe the exciter has lost its magnetism. A drawing of the system is attached.





RE: Anybody help with strange voltages and an alternator problem
RE: Anybody help with strange voltages and an alternator problem
RE: Anybody help with strange voltages and an alternator problem
RE: Anybody help with strange voltages and an alternator problem
However, that doesn't explain the strange line to line voltages.
I would suspect that your meter has a calibration issue. Not shown on the diagram, but if you are using PTs it may be a failing PT.
Verify the voltages with an independent meter. With the generator off line the line to neutral voltages are unimportant but the line to line voltages are important.
If the primary neutral of the transformer is connected to the system neutral and/or grounded, Those voltages may indicate a voltage unbalance and/or a phase angle error on the primary. If you have access to the primary wye point conductor check for current when the generator is off line.
I suspect primary unbalance as the most likely cause of the strange voltages. This can happen when a voltage regulator bank upstream has a stuck tap changer on one phase.
Bill
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"Why not the best?"
Jimmy Carter
RE: Anybody help with strange voltages and an alternator problem
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Anybody help with strange voltages and an alternator problem
Suggestion: isolate the generator from the system, then run the machine up with the AVR disconnected and with a small variable DC power supply connected to the field terminals of the machine. Gently increase the field voltage until you make produce rated voltage on the main terminals. Carefully measure line-neutral and line-line voltages at the main terminals. I think you will find they are ok, in which case you likely have a problem with your instrument transformer connection.
RE: Anybody help with strange voltages and an alternator problem
I understood that the problem was on the bus with the generator off line.This is a classic case of a star:delta transformer with unequal voltages feeding the star side. This condition is accompanied by heavy circulating current in the delta. When the generator is running it corrects the problem. The generator phase currents will be unequal and will not reflect the balance of the local load current.
That is, the local load may be balanced while the generator currents may not be.
Bill
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"Why not the best?"
Jimmy Carter
RE: Anybody help with strange voltages and an alternator problem
Thanks - yes, I think you're right now I've re-read it.
Waqlevu,
Is the sync breaker a four-pole type? I wonder if the instrument is losing its connection to the system neutral when the generator breaker is open? A floating neutral at the measuring instrument can result in weird phase displacements and voltages. Could any auxiliary contacts be opening the instrument neutral?
RE: Anybody help with strange voltages and an alternator problem