#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???

 

[jbartos]

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.

 

[alehman]

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

 

[rconnett]

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..

 

[DiscoP]

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

There are plenty of relays about that have this feature.

 

[edison123]

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.

 

[sjvanhecke]

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?

 

[edison123]

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

 

[sjvanhecke]

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?

 

[jOmega]

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

 

[jbartos]

Suggestion: Visit

http://www.iemfg.com/prods/dwnlds/240-600vacSpecs.pdf

http://edgewebhosting.net/40rnl.pdf

for: Main Stator 2/3 pitch and one slot skewing to minimize voltage harmonics
Also, Reference:
M. G. Say "Alternating Current Machines," John Wiley & Sons, 1978, Section: Sewed Slots on page 106

 

[alehman]

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?

 

[jbartos]

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

 

[sjvanhecke]

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.

 

[Shortstub]

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?

 

[alehman]

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?

 

[sjvanhecke]

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.

 

[Shortstub]

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?

 

[Rarichan]

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.

 

[sngpl]

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?