Generator voltage unbalance

[kokimkd]

Hello to every one.

My name is Kosta,and I`m new in the forum. I`m working as maintenance engineer,in company for building and maintenance of small hydro powerplants. I have this problem: 3 phase generator 1100 kVA, 0,4 kV when it`s starting and there is no load, with circuit breaker open ( not on grid ) the voltage are very unbalanced (208, 230 and 258 volts ). When the generator is synchronized and it`s on grid all three voltages are same (240 240 240 ). So my question is: Does this mean that generator windings are damaged? I haven`t measure them because we don`t have micro om meter. Is anyone more experienced have this problem or know what`s the problem?

 

[waross]

If the windings were damaged you probably would have seen smoke by now.
The most likely cause is metering errors.
There are a lot of possible causes of metering error. Put a scope on the outputs and look for distorted wave forms and harmonic content.
Another possibility is that the voltages are being read correctly and the set does have a problem. If that is the case the readings may indicate a phase angle error. This is not likely. One cause of a phase angle error could be a gross mistake in winding the set with one of the windings placed in the wrong slots. This is highly unlikely and probably physically difficult. This would result in a lot of reactive current when online.
You may have an uneven air gap. If the set works well connected to the grid don't worry.
To check:
If you have a KVAr or KVARHr meter look for excess reactive current when online. Look for reactive current that can not be completely cancelled by adjustment of the excitation.
While connected to the grid or local network, watch the currents on the three phases while adjusting the excitation. You should be able to find a setting where all three currents are minimum. If the phases show minimum current at different excitation settings then you may have phase angle errors. That said, I doubt that the set passed inspection and testing with any gross winding mistakes.

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

 

[crshears]

Hello Kosta, welcome to the forums.

As to your original post [OP}:

What Bill said, plus:

Since a grid is typically a finitely huge bus, as soon as a unit is connected to it the grid will, if properly balanced, force all winding voltages to the same value, which is what seems to be happening in your case.

If there is in fact an issue with the unit I would expect to see the voltage imbalances when the unit is at speed but off line to translate into current imbalances once the unit is synchronized, which is what I'm getting out of Bill's last paragraph.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Thank you crshears.

If there is in fact an issue with the unit I would expect to see the voltage imbalances when the unit is at speed but off line to translate into current imbalances once the unit is synchronized,

The extra current will be mostly reactive current. Reactive currents can be cancelled by adjusting the excitation or voltage adjustment.
If there is a fundamental frequency voltage unbalance it is unlikely that the reactive currents on each phase may be cancelled with the same excitation or voltage setting.

NOTE: Reactive currents are at 90 electrical degrees to real currents and may be obscured by heavy load currents. It is best to try the excitation adjustment with as little load as possible, but still connected to the grid.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iceworm]

kosta --
Adding to the excellent posts from Bill and CR-
The symptoms, unbalanced voltages when disconnected from the grid, is classic for the gen neutral is not bonded at the gen and you are measuring Line to Ground, or Line to bus neutral. The gen neutral is floating.

That being said, the Line to Line voltages are farther out than I would have expected.(Using 120 deg separation, I get 379V, 404V, 422V) I'd curious what the L-L voltages are when disconnected from the grid.

The suggestion to measure the line currents is excellent. As mentioned, if they are even, its good.

You do need some equipment. A power analyzer about now would be good. Be nice to see exactly what the negative sequence is when running.

Curiosity question: The machine likely has winding RTDs. Can you get to the readings and see if it is running hotter than it has in the past?

ice

Harmless flakes working together can unleash an avalanche of destruction