Anybody help with strange voltages and an alternator problem

[Waqalevu]

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.

 

[Waqalevu]

Below is a better drawing of the system, showing L1 and the neutral. L2 & L3 has been omitted for clarity. I have also added photo 1 which shows the HMI when the generator is at rest. When I disconnect the neutral at B or C, the voltages return to normal as shown in photo 2.

 

[Waqalevu]

Photo 1

 

[Waqalevu]

Updated diagram

 

[waross]

When the sync breaker is open, the delta transformer winding loses its ground reference. The ground path is now a combination of insulation leakage current and capacitive current. The line to ground voltages are a reflection of the insulation resistance and capacitance of the three phases.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

The the cause of the problem may be on a feeder 5, 10, or more miles from your plant.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I don't think you have a problem on the field side of the alternator. Loss of diodes effectively reduces the gain of the AVR but it shouldn't result in mis-matched voltages on the main terminals.

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.

 

[waross]

Hi Scotty:
I understood that the problem was on the bus with the generator off line.

All of a sudden we are getting strange voltages indicated on the mains side side when the machine is at rest.

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
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Hi Bill,

Thanks - yes, I think you're right now I've re-read it. I noticed the comment about replacing the shaft-mounted rectifier and responded to that but I'm pretty sure the rectfier isn't part of the problem. I'm fairly sure this is going to be a measurement problem rather than a problem on the power circuit.

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?

 

[squeeky]

What changed? Has someone laid a new cable or wires near the metering wires? Have you lost the earth screen on the power cables with a bad gland connection. Looking at the imbalnace it is 0.1% That is very small. 40v on a 400v system. Have you lost your reference earth and thus your neutral point is floating with ground currents. This relates to Waross comment on miles away. With the unbalance showning on the mains side with nothing on the bus or alternator it is definately a reference problem. Is your mains unbalanced and you have zero phase sequence coming to your delta side and circulating and thus inducing a voltage?