#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
RE: #rd Harmonics from Steam Turbine Generator
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
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
There are plenty of relays about that have this feature.
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
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
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
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
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
RE: #rd Harmonics from Steam Turbine Generator
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
Are the customer's loads connected 3-wire or 4-wire?
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
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
RE: #rd Harmonics from Steam Turbine Generator
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
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
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
RE: #rd Harmonics from Steam Turbine Generator
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
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
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
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
Not sure if this will help but take a look see
http://www.ee.sc.edu/classes/Fall02/elct751/RELAY_9.pdf
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
You have yet to state a neutral current magnitude!
RE: #rd Harmonics from Steam Turbine Generator
RE: #rd Harmonics from Steam Turbine Generator
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
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
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
RE: #rd Harmonics from Steam Turbine Generator
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
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
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
http://www.iaei.org/magazine/03_a/lowenstein/magazine_0...
for more info on the third harmonic filtering
RE: #rd Harmonics from Steam Turbine Generator