#rd Harmonics from Steam Turbine Generator 1

[sjvanhecke]

I have a problem i am trying to resolve. I have acustomer that has a 3MW 6900V steam turbine generator that had the stator rewound about 4 months ago. Since the gen. was reinstalled they are getting nuetral overcurrent trips. In monitoring the nuetral current, I have discovered that not only is there substantially more nuetral current then before, but it is mostly third harmonic current. There is some 9th harmonic, but very little. We have eliminated the Drives for 3 fans by monitoring the current with the drives off and back on. There are two Gas turbine generators also, that do not have this problem nor do they contribute to it. We know this by attaching a power recorder to the system and monitoring the currents at all conditions, and when the steam turbine is not running the neutral current goes away. So what I am wanting to know is, what can cause third harmonic current in a generator? I understand by doing some research that if third harmonic current is generated on the phases it will add on the neutral, but where does it come from???

 

[Shortstub]

Unless the generated voltage magnitudes are different, there will not be a neutral current.

 

[sngpl]

Try to measure the per phase winding resistance and also, if possible, the split phase winding resistance in order to make sure that all the phases have the same resistance. This is to see if there were any wrong connections made during the rewind.

 

[edison123]

Sngpl,

The poster mentioned in a later post that only the rotor was rewound and not the stator. So, that would rule out harmonics from originating due to stator winding. Also, just by measuring dc resistances, one cannot determine wrong stator winding connections. That would require ac phase balance test or dc polarity test.

 

[sjvanhecke]

To helpout a little bit there is a voltage imbalanceas mentioned in a previous post. I do realize this would cause neutral current, but why is it third harmonic?

 

[jbartos]

Suggestion: There should be one high-resistance system grounding at a time located at the generator neutral, if wye connected. If each generator powers its step up transformer, the high resistance system grounding will be at each generator. If generators are attached to one bus, then one generator only should have the high-resistance system grounding active at a time.
Reference:
Donald Beeman "Industrial Power Systems Handbook," 1st Edition, McGraw-Hill, 1955, page 373, Figure 6.21 Neutral Grounding by means of neutral bus and switchgear.

 

[jbartos]

Suggestion to sjvanhecke (Electrical) Nov 4, 2003 marked ///\\To helpout a little bit there is a voltage imbalanceas mentioned in a previous post. I do realize this would cause neutral current, but why is it third harmonic?
///See my first posting.\\\

 

[alehman]

sjvanhecke,
You still haven't clearly described your arrangement. Can you answer any of the other questions that have been posted?

Also, where is the "utility ground fault relay" that trips? Is this the same as the "neutral overcurrent relay" you described earlier? Is it on the STG lead or at the (assuming there is just one) utility transformer?

Does the neutral harmonic current exist with the combustion turbines off-line?

 

[electrical4160kv]

Use Fluke 46, check the phase angle, current, voltage and Harmonics. Then post again your measurement.

 

[alehman]

If the generators are on a common bus ahead of the utility transformer, if the neutrals are interconnected and if the problem only occurs when >1 generators is on line, the voltage imbalance you mentioned could cause circulating 3rd harmonic current. It would show up on the other generators also.

 

[HiSet]

First of all I am assuming that the neutral current is in fact third harmonic and not a zero sequence current as both type of current add up in the neutral circuit.Is it possible that you are overfluxing the utility transformer which then would generate fifth and third harmonic current.And third harmonic current by virtue of it's phase relationship add up in the neutral circuit.

 

[jbartos]

Suggestion to the previous posting: Please, notice that the zero sequence current is AC current and can have any harmonic content. It may also have a trace of DC component.

 

[imok2]

Not sure if this will help but take a look see

http://www.ee.sc.edu/classes/Fall02/elct751/RELAY_9.pdf

 

[jbartos]

Comment on the previous posting: The link is good for the generator system protection with respect to this thread posting.

 

[sjvanhecke]

We have been using a Dranetz BMI 4300 Power recorder to find out this data. There is other harmonic currents, but nowhere near the amount of third harmonics. It does infact show up on all generators when all three are paralleled, but when the gas turbines are running alone, most of the neutral current is gone. The grounding arrangement is the generators are connected to the nuetral bus and the HRG is on the low side of the GSU transformer. There is only one HRG.

 

[Shortstub]

Sjvanhecke,
You have yet to state a neutral current magnitude!

 

[sjvanhecke]

The nuetral current is about .8A secondary at 180Hz. The CT's are 400:5.

 

[Shortstub]

Sjvanhecke, some additional food for thought.

Since there is only a single element intentionally connecting the system's neutral and ground/earth, then two conditions must exist to cause a current to flow in the neutral... a source of voltage-magnitude unbalance, and a second connection between the system-phase conductors and ground/earth.

The former is caused by generation of 3rd harmonic voltages in the generator's stator winding. The latter condition is caused by the presence of phase-to-ground/earth capacitances, ie, cable, generator, transformer.

Why the stator produces 3rd harmonic after the rotor-fix, and not before, is of course open to speculation. If the harmonic voltage magnitudes are equal, there will not be a neutral current! Then, what are possible causes. To answer this: 1) measure the phase-to-phase voltages and construct the voltage trangle; and 2) determine the 3rd harmonic magnitudes in each phase. Of course, adding to the puzzle is the fact that only the rotor was worked on, not the stator. Three possibilities follow.

1) It is possible that stator-iron was damaged during the rotor installation.
2) The field current is somehow being modulated so that a time-varying magnetic field is produced. The tests above will show this!
3) If a 2-pole or 4-pole machine, then, presumably the field winding end-turns are fitted with an end-turn cover. I recall an incident, when, during field replacement, the rotor was unintentionally stored with supports under the end-turn cover... not the rotor shaft. The subsequent distortion to the cover escaped observation. In fact, it wasn't obvious until connected to the turbine and the resultant vibration was noted. Fortunately, it was never energized so I can't provide you with the electrical consequences. Perhaps, some of the designer folks can provide additional input.

Good Luck, hunting!

 

[jbartos]

Suggestion: The generators do have the third harmonic large. Apparently, this is somewhat inherent to their design. Also, a suitable design may mitigate the generator third harmonic somewhat. Visit

http://eepe.swan.ac.uk/upec/programme/abstracts/a505.pdf

for:
However,
third harmonic voltages vary widely due to the output variation of the generator’s active or reactive
power.

 

[edison123]

sjvanhecke
You mentioned

"It does infact show up on all generators when all three are paralleled, but when the gas turbines are running alone, most of the neutral current is gone. The grounding arrangement is the generators are connected to the nuetral bus and the HRG is on the low side of the GSU transformer. There is only one HRG."

Isn't it common practice to have individual neutral grounding (HRG or neutral grounding trafos)for each generator connected to grid ? The fact that the neutral currents are gone when only GT is running seems to indicate some probelm with this common neutral bus.

Just a suggestion.

 

[jbartos]

Suggestion: Generator system grounding should not create connections from each generator to the neutral ground bus if multiple generators are connected to one bus on their outputs and step up transformers are connected to this bus. However, if each generator has an individual connection to its step up transformer, then each generator should have its own system grounding protection to be properly protected against ground faults.