Problem of synchronous alternator 3

[bimbone]

Hello everyone,
Perhaps the title is not the best but here I will try to explain.
Of a synchronous machine from 1,5 MVA with the star center isolated from earth, and line voltage of 400 V its connected to the grid by the trasformer MV/LV with connections D/yn.
I saw the following fact: The PE conductor (yellow green) from the frame of synchronous machine goes to the star center of the transformer, then to the earth, has a 50 Hz to 40 A. My explanation is as follows, the synchronous machine having the frame connected to earth but not the star center, on that loop is colosed the capacitive current that arrive from coupling between capacity of the windings stator and frame,
And then close the center of the transformer.
According to you this maybe a plausible explanation?
Have some suggestion about it?
Maybe i must find the problem in the shaft currents or voltge?

 

[iop995]

In what conditions was measured this 50A current? Nominal load for alternator / no load / other situation?

 

[waross]

This sounds high for capacitive current on a machine that size..
A 50 Amp fault in a generator that size would be expected to quickly escalate into a total failure. An exception may be a ground fault close to the wye point in the alternator.
The anchor bolts and concrete pedestal may be providing a ground path and the current may be a neutral current from the distribution system.
Solutions:
1>: Check the alternator windings for grounds.
2>: Determine if the current is flowing from the alternator or from the ground via the anchor bolts. A Rogowski coil may be useful.

http://en.wikipedia.org/wiki/Rogowski_coil

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

 

[bimbone]

The measurement conditions were at the maximum load where i found 40 A on the wire PE from syncronous frame.
Moreover i saw the current from wire PE on syncronous frame increase if the power load incrases.
This suggests that the problem is the shaft current.
Agree that is not the capacitive current because it is too hight.
I think that shaft current or the current induced by the magnetic fild takes the loop with the lowest impedance therefore the copper wire PE, it has section of 240 mm2 the impedance of the wire for the meter is 0,0000801 Ohm/m. If the development is a few meters the PE loop has a small impedance then with few volt (shaft voltage) i have more current on the loop.
This friday i will do the test. I will put in series the small impedence with the PE from asyncronous machine so will reduce the current.
I will keep you informed

 

[waross]

Shaft current?? I understood that the path for shaft current was through the length of the shaft, through the bearing to the frame. through the frame to the other bearing and through that bearing back to the shaft. A ground connection would not be part of the loop.
Is the load well balanced? Have you verified that the generator star point is not grounded? I mean by physical inspection, not by looking at diagrams.

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

 

[iop995]

I think is an unbalance issue. 40A current it's about 1.84% of nominal alternator current, so a small imbalance in output voltage and or impedance may impose this current; also construction transformer imbalance (sure it's small differences in phase impedances) may conduct to this PE current. Considering frame being zero voltage reference and PE cable for 5m lenght, this lead to trafo star point potential ("imbalance") of 16mV...

 

[waross]

Check your alternator wye point.

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

 

[bimbone]

Hi warros,
Your explanation is correct, and it is true that a ground connection would not be part of the loop, but my system is a TN-S mode therefore the loop there is because the PE conductor is connect to asincronous frame and arrive from star poit of the trasformer. You must consider that protection cirquit PE is large.
I think that the current can has other pathways.
I also verified visually that the generator star point is not grounded.
The load is balace, all the energy is sold to the grid.
Unbalace issue is possible also small, but the star point of the syncronous is not grounded.

 

[rasevskii]

@bimbone:

How did you measure this 40A? Was it with a clamp ammeter or a digital meter with a clamp attachment? It can be actually harmonic current or stray harmonics from the excitation system...is this a brushless exciter with an electronic AVR? All kinds of odd currents can result from this kind of equipment. In a power station environment it is common to find all sorts of odd currents flowing around the earthing system, if you poke around with a clamp meter.

What was the reason for the original measurement? Is there actually a problem that appeared, or are you trying to find a problem when there isn't one...

See the remarks by waross...check the neutral point of the generator. Open up the covers if any and look inside...

Do a megger test of the stator winding if you have such equipment available.

Is this Diesel genset or what, is it old and dirty inside or new and clean. ??

Dont be blinded by science as an old field engineer once said a couple decades ago to us...

rasevskii

 

[rasevskii]

@bimbone:

If you megger the stator winding, make sure that voltage transformers, surge supressors, and capacitors are disconnected first. For a 400V machine do an initail meggering at 500V only.

rasevskii

 

[bimbone]

The genset is new. The syncronous is a Stamford and the prime mover unit is a GE Jenbacher is powered by biogas.
How did you measure this 40A? I measured the current with Fluke industrial scopemeter 123 and rogowki sensor,
I attached the photo the 1-ST signal is the current and 2-ND signal is the voltage between fase 1 and ground.
I'm looking for an explanation for the 40 A becasuse is the our custumer that wants an explanation.
I agree with you, rasevskii, because if i go on the other machines I find the same thing.
I just want to find an correct explanation for the customer.
However i have the megger therefore i check the stator winding.

Thanks for your attention

 

[waross]

Is it possible to measure the neutral current of the transformer.
Two possible conditions that may combine to cause such an issue.
A small voltage unbalance or phase angle error on the wye side of a wye:delta bank may cause heavy currents to circulate in the delta accompanied by heavy neutral currents on the wye side.
Grounding; Some grounding systems, particularly driven rods have a higher resistance than the foundation concrete and re-bar system.
If the alternator is looking at the delta side of the transformer the current may be primary side current returning through the foundations and anchor bolts.
We have 40 Amps returning from the alternator frame but is it returning to the transformer neutral or to ground?

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

 

[bimbone]

@WAROSS
The side trasformer that look at the grid is connected to the delta while the side trasformer that look at the syncronous machine is connected to the star with the its wye point is grounded.
The syncronous machine is connected to the star with the its wye point is not to the ground.
The frame of the syncronous machine is connected to the ground with three cable of the size 240 mm2 is over those cables that i have 40 amps.
There are other loops around the engine embedded because the machine is into a container of iron.
You can take a look at our applications, but i don't know if i can write the website of the company where i work.
I would not take a warning from administrators.
However i attach a photo of the cable of the syncronous machine, on the yellow/green cables there are 40 amps, you can view the wye poit disconnected to the ground.

 

[rasevskii]

@bimbone

Viewing the photo closely, the gen neutral in fact does not seem to be connected to anything. In that case you could remove the lower horizontal bar completely. It can be that there is a sneak connection to the frame somewhere. On the yellow-green earth lead I dont see why they used such a heavy cross section for just the frame ground. Maybe it is in fact a neutral conn.

If it is only a frame ground wire, then the genset skidmount and the container steelwork is also connected in, you might get current in that lead even if that genset is stopped. (and the others running).

rasevskii

 

[rasevskii]

@bimbone:

As a further thought, after verifying that there is actually no connection between the gen neutral bar and frame, then the reading you have taken of 40A (is this peak or RMS)? seems to be very distorted on the display, meaning a heavy harmonic content, due varios capacitive couplings between the stator windings and the frame and/or HF from the brushless exciter field (it is supplied by a thyristor system, after all) getting in. The AVR and brushless exciter system is voltage supplied from the stator, and may have a high ohmic ground connection somewhere (filters).

Probably nothing to worry about. I would question that 40A is the true value. Maybe a couple amps actually. Put away the rogowski and measure with an actual ammeter (mechanical) in series.

Dont be blinded ny science out at the landfill...

rasevskii

 

[waross]

I agree with rasevskii.
With that transformer connection, any voltage or phase angle distortion on either the primary or secondary will cause an offset of the transformer neutral from the symmetrical neutral position.
If the alternator neutral is connected the current flow between the shifted transformer neutral and the alternator neutral will be disproportionately large in respect of the voltage causing it. A very small voltage unbalance and/or phase angle error could easily cause such a current.
Is it possible to monitor the current leaving the transformer wye point?
However rasevskii may be closer to the real source of the problem.


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

 

[bimbone]

The section of the yellow/green conductor is that because i must to do it half size of the phases conductors. this method is used to observe the graph of security voltage/time, if I have a ground fault voltage divider between inpendence phases and impendece yellow/green the potential of the masses goes to that of yellow/green conductor.

Yes, in that syncronous the excitation is formed by small permanent magnet synchronous that power supply the AVR, The AVR give the current to an brushless therefore through the tiristors, the voltage is supplied from the main rotor.

Tha 40 ams is RMS

@WAROSS

Is it possible to monitor the current leaving the transformer wye point?
I don't know, because the sistem become a IT mode the synchronizer loses the reference and so i can not make the parallel with the grid,

thanks for the many tips

 

[7anoter4]

I agree with rasevskii : it has to be a current unbalance.
It could be produced by an EMF unbalance or Impedance unbalance-including Grid load unbalance.
The transformer no-load current may be unbalanced for a small unbalance of supplied voltages.
If the PE current will increase with the load the impedance unbalance is more likely.
But if it is only 2% may be neglected.

 

[DTR2011]

@ Bimbone

I believe what waross is asking is if you can measure the transformer neutral current while the unit is in normal operation. This can be accomplished with the clamp on CT.

I would expect that there is power metering for power sold to the utility and there would be metering PT and CT on HV side of transformer. You can check for voltage and current imbalance safely by making your connections on the secondaries of these instrument transformers (@ meter location). Perhaps even the power meter is digital and can give exact read out without further investigation.

I have done start up of these types of units at various locations where methane from garbage is burned and the smell there is not too pleasant, yes?

Another issue I ran into with this type of application was where the gen transformer was d/yn solid ground. It was at the end of a long feeder back to utility substation. The utility trafo was yn, solid ground, at same voltage level. What we had in that case was our trafo acting like a grounding bank for any imbalance on the line, which varied during the day. This was a bad design to begin with. Our solution was to impedance ground our trafo primary and change protections from 51N to a 59N scheme.

 

[waross]

Thanks smallgreek. That is what I have been trying to suggest.
The source may be the grid conditions and the transformer and not the generator. This condition would not vary as a result of the load on the generator but may (and probably would) vary with the load on the grid which may coincide with the generators picking up more load to respond to the grid loading.
If this condition exists a small unbalance will cause an unexpectedly high current. A high ground current originating from the transformer neutral wye point may be splitting with part going to ground directly and part going to ground via the frame and anchor bolts of the alternators. This condition would be shared with all alternators.
As I said, this is probably NOT the source of the issue here, but it may be desirable to rule this out.
You have a point where the conductor from the ground grid, the grounding conductor from the alternator frame and the transformer wye point meet. If you are able to measure the currents in the wye point conductor, the conductor to the ground grid and the grounding conductor to the alternator frame, you will easily confirm or reject this possible cause.

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