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#rd Harmonics from Steam Turbine Generator

#rd Harmonics from Steam Turbine Generator

#rd Harmonics from Steam Turbine Generator

(OP)
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???

RE: #rd Harmonics from Steam Turbine Generator

Suggestion: Rotating machinery, motors and generators, are known to have 3rd harmonic, 5th harmonic, 9th harmonic, etc. content by their principle of operation and design.

RE: #rd Harmonics from Steam Turbine Generator

If they rewound the unit using a different coil configuration than the original, it could change the harmonic content of the generator output. 2/3 pitch for example minimizes 3rd harmonic content.

Different winding configurations also have different impedances to various harmonics. If you have harmonic producing loads, the harmonic currents may change as a result of changed generator impedance at various harmonic frequencies.

See also
Thread238-29247
Thread238-17696

RE: #rd Harmonics from Steam Turbine Generator

The 'pitch' referred to is a technique of adjusting the position of the windings of the generator - usually in the rotor -(which in turn determines the orientation of the magnetic fields. The objective is to try tonot have the  the rotating magnetic field of the rotor at intercept teh stator poles at exactly 90 degrees. If they do, a somewhat square looking voltage wave is generated in the stator windings. The fourier series of this square wave is rich in odd order harmonics, and the triplen harmonics (3rd, 9th, 15th, etc) will sum in the neutral, possible leading to false your trips. The high harmonic current flows might be a result of a low impedance path in your power system for the harmonics - possibly a grounded wye connected capacitor bank someplace??.

One technique to supress this might be to use a small reactor to ground the neutral, or a small reactor shunted by a low resistance to create a 'low pass filter' in the neutral. Be careful to research this carefully, as you don't want to create high voltage on the neutral connection in the event of a ground fault on the power system, or a system resonance condition..

RE: #rd Harmonics from Steam Turbine Generator

You really should consider using a relay with 3rd harmonic rejection for this application.

There are plenty of relays about that have this feature.

RE: #rd Harmonics from Steam Turbine Generator

I assume yours is a 2 pole generator. In such machines, the stator winding pitch is 2/3rd of full pitch to eliminate (repeat eliminate) third harmonics in stator winding. If your rewinder changed this 2/3rd pitch, it would account for the third harmonics in the neutral. Check with your rewinder whether any such winding pitch change was done.

RE: #rd Harmonics from Steam Turbine Generator

(OP)
Thanks for all of the great information that everyone has given, this helps out alot.  It sounds like this might be a pitch problem.  Do you think that the pitch could have been changed when the rewind was done?  can this be verified by some sort of testing?  

RE: #rd Harmonics from Steam Turbine Generator

Other than physically opening the machine and checking the stator winding pitch, your rewinder is the best source of information.

RE: #rd Harmonics from Steam Turbine Generator

(OP)
Upon more in depth research, only the rotor was rewound, not the stator.  My assumption is that the pitch is only changed in the stator.  Where else could this be coming from?

RE: #rd Harmonics from Steam Turbine Generator

How balanced are the loads phase-to-phase....?
Have you taken voltage and current readings phase-to-phase ?

If so, could you share with us the numbers...
Thanks

RE: #rd Harmonics from Steam Turbine Generator

Winding pitch has to do with the stator only. Rewinding the rotor only should not effect generated harmonics or impedance. Did you measure the neutral harmonics prior to rewinding?

RE: #rd Harmonics from Steam Turbine Generator

Suggestion: The following is a list of harmonic components developed in a cage machine:
1. Stator:
1.1. Stator m.m.f.
1.2. Rotor m.m.f.
1.3. Slot ripple modulated by 1.1.
1.4. Saturation, giving slot-ripple sidebands
2. Rotor:
2.1. Slip-frequency currents representing the main-flux fundamental
2.2. Components associated with 1.1.; slot-ripple due to stator teeth
2.3. Saturation as in 1.4.
2.4. Currents induced by harmonic fluxes
  

RE: #rd Harmonics from Steam Turbine Generator

(OP)
As far as the balance of the loads, we have a slight imbalance of about maybe 30 volts primary and about 64 Amps on the primary, which is about where or nuetral overcurrent relay is set, but it is my assumption that the meter is showing the harmonic current in the phase also.  From what I have been reading if there is any harmonic content in the phases, it will show up on the nuetral.  I know how to eliminate it, with a line reactor, but the customer wants to know where it is coming from.

RE: #rd Harmonics from Steam Turbine Generator

Neutral current can only exist if there is another source/connection elsewhere in the system?  Inadvertent or otherwise!

Does "neutral" tripping occur when only the generator is connected to the load distribution bus?

Are ground/earth fault relays also tripping on any of the downstream relays?

Is ground/earth fault current limited by a neutral resistor?

Where is ground/earth fault CT located?

Are GT-generators also grounded/earthed?

RE: #rd Harmonics from Steam Turbine Generator

It would be helpful to have some deatil of the earthing arrangement. Is the neutral of each generator separately earthed?  Is there a common neutral earthing point that serves all generators?  Is the earthing connection(s) solid, or through resistors or reactors?  Are all generators earthed the same way?

Are the customer's loads connected 3-wire or 4-wire?

RE: #rd Harmonics from Steam Turbine Generator

(OP)
Again, thanks for all of your responses.  Here is a little more information.  This is an actual generating plant that is connected to the grid.  There really is no "load bus".  The only load is an MCC that is used to run the generators.  We have eliminated the possibilty that it is something associated with the generator that changes with load by turning off the three draft fans that are associated with the steamer.  The "neutral" tripping does only occur when it is conected to the grid and at full load.  The third harmonic current is linear in relation to the load.  As the generator increases MW output the third harmonic current increases.  There are no downstream relays.  The relay that trips is the utility ground fault relay.  The plant (low side) of the transformer has a high reistance ground to the nuetral.  The CT is located on the nuetral bus upstream of the generator but before the GSU transformer.  It seems as though that each generator is tied to the nuetral bus which has a HRG at the transformer.  I hope this helps.

RE: #rd Harmonics from Steam Turbine Generator

Please provide additional detail on setup, including the following:

1)  How are the STG and GTG sets connected to the grid?  Via a synchronizing bus?  Thru one transformer?  More than one?

2)  Is (are) the GSU transformer(s) wye-connected on the 6900V side of the GSU transformer(s)?  With corresponding neutral resistors?

3)  Also what are the min/max neutral current magnitudes coincident with the STG set loading?

4)  What are STG phase-to-phase voltages when disconnected from the GSU transformer?  That is, before syncronized to other sources?

RE: #rd Harmonics from Steam Turbine Generator

Are you having this problem when only one of the generators is working and the other isolated from the bus?. If so I think the problem is with the grounding since both generators are grounded separately and the harmonic current circulates.If this is the problem, you can use a common neutral grounding for the generators when running in parallel.

RE: #rd Harmonics from Steam Turbine Generator

It certainly appears that the harmonics are due to the way the generator has been rewound. However at this point I may add that there could be two major external sources of harmonics; 1- excitation system, 2- station battery charger because both have rectification equipment.

Is it possible to run the generator with its exciter powered from an external source i.e. station common services? Was the excitation system also up graded at the time of the rewind?

I think you should also try to eliminate the effect of excitation equipment before proceeding further?

RE: #rd Harmonics from Steam Turbine Generator

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

RE: #rd Harmonics from Steam Turbine Generator

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.

RE: #rd Harmonics from Steam Turbine Generator

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.

RE: #rd Harmonics from Steam Turbine Generator

(OP)
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?

RE: #rd Harmonics from Steam Turbine Generator

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.

RE: #rd Harmonics from Steam Turbine Generator

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.\\\

RE: #rd Harmonics from Steam Turbine Generator

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?

RE: #rd Harmonics from Steam Turbine Generator

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

RE: #rd Harmonics from Steam Turbine Generator

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.

RE: #rd Harmonics from Steam Turbine Generator

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.

RE: #rd Harmonics from Steam Turbine Generator

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.

RE: #rd Harmonics from Steam Turbine Generator

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

RE: #rd Harmonics from Steam Turbine Generator

(OP)
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.

RE: #rd Harmonics from Steam Turbine Generator

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

RE: #rd Harmonics from Steam Turbine Generator

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

RE: #rd Harmonics from Steam Turbine Generator

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!

RE: #rd Harmonics from Steam Turbine Generator

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

RE: #rd Harmonics from Steam Turbine Generator

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.

RE: #rd Harmonics from Steam Turbine Generator

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.

RE: #rd Harmonics from Steam Turbine Generator

We have the same problem with an 11kv 1.5MW Diesel driven generator, it was originally installed as a standby generator at a large residential complex for essential supplies only.
We installed the required synchronising equipment to generate back into the utility company to reduce peak billing.
It synchronised OK but when load was increased the generator neutral earth fault relay operated.
The neutral current was measured at up to 10Amps through the neutral earthing resistor (68ohms).
Harmonic analysing equipment showed that this 10Amps was 95% 3rd harmonic with a little bit of 9th.
The neutral earth fault relay was an Alstom MCGG which does not have harmonic restraint, we replaced it with an Alstom KCGG which does. Problem solved !!! Not quite, the generator doesn't trip any more but the 3rd harmonic current is still a problem, the neutral earthing resistor is getting hot and smoking.
It is believed that the generator windings are wound as suggested in a previous posting, and is just inherrant to the generator design (Being made in France doesn't help).

RE: #rd Harmonics from Steam Turbine Generator


Based on ANSI/IEEE Std 32-1972 …Neutral Grounding Devices the resistor has assigned a time rating.  It is possible that it is intended for short-time duty and not continuous or extended duty.  That characteristic should be reflected on the equipment nameplate.

An additional note in the standard is: “Where there is a third harmonic component of current, it shall not exceed 15 percent of the rated continuous duty current.”
  

RE: #rd Harmonics from Steam Turbine Generator

bunnypig,

One possible answer is to use a Y/D transformer to trap the triplen harmonics in the delta winding. This would be a typical configuration for a large generators with a step-up generator transformer to the grid, connecting the generator to the LV delta.

Is your generator actually generating 11kV, or is it stepped up for connection? 1.5MW seems a small rating for an 11kV set. If you have a transformer, what is its configuration?

RE: #rd Harmonics from Steam Turbine Generator

ScottyUK,

Yes the the generator is 11kv, not stepped up. The powers that be in a multilevelled government organisation have decided to put their money into a reactor on the generator neutral to filter out the 3rd harmonic.

RE: #rd Harmonics from Steam Turbine Generator

Suggestion: The resistor power rating is supposed to have a large design margin in the resistor power rating since the resistor increases its resistance with increasing heat which leads to the higher voltage drop across the transformer primary, which serves as the current transformer primary rather than the voltage transformer primary.

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