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Minus sequence and heat in synchonous motor rotor?
2

Minus sequence and heat in synchonous motor rotor?

Minus sequence and heat in synchonous motor rotor?

(OP)
I know about quite a few ways that a rotor can run too hot. Like repeated resynchs, too high excitation current, poor ventilation and such things. I can also understand that excessive minus sequence grid voltage can cause eddy currents in the rotor (saient poles). But, is it possible that a .6 percent minus component can do that?

That is what I heard as an explanation to accelerated aging of rotor coils.  I have seen .6 percent and more in other places. But no known problems with rotor heating. So, why on this particular 10 kV machine? Or is it just not so?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Minus sequence and heat in synchonous motor rotor?

On a turbo machine - i.e. a cylindrical rotor design - I wouldn't expect a severe problem from 0.6% negative sequence component. On a salient pole machine I am also inclined to say 'no' although I'm no expert on low speed synchronous machines. I'm no expert on turbo machines either, but I know a little more about them than salient pole types wink.

Is there anything on the nameplate which suggests this machine is being pushed to the limits? For example, what thermal class  is the insulation system, and what is the expected temperature rise? Big machines traditionally use a class F insulation but are designed to run at a class B rise, which is part of the reason why they last a long time and part of the reason why they can tolerate moderate NPS heating without damage. By contrast a class H machine running at a class H rise won't be as tolerant of NPS heating in the rotor.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Minus sequence and heat in synchonous motor rotor?

For synch generators, ANSI / NEMA MG-1 provides the guidance below:

Quote (NEMA MG-1 (2009)):


32.14 CONTINUOUS CURRENT UNBALANCE
A synchronous generator shall be capable of withstanding, without damage, the effects of a continuous current unbalance corresponding to a negative-phase sequence current I2 of the following values, providing the rated kVA is not exceeded and the maximum current does not exceed 105 percent of rated current in any phase. (Negative-phase-sequence current is expressed as a percentage of rated stator current.)

Type of Generator Permissible........Permissible I2 Percent
Salient pole With connected amortisseur winding....... 10
Salient pole With nonconnected amortisseur winding...... 8
Air-cooled cylindrical rotor..... 10

These values also express the negative-phase-sequence current capability at reduced generator kVA capabilities, as a percentage of the stator current corresponding to the reduced capability.

=====================================
(2B)+(2B)'  ?

RE: Minus sequence and heat in synchonous motor rotor?

The above section (32) of NEMA MG-1 covers generators 5MVA to 75MVA.

There is another section (33) which covers smaller "definite purpose" generators and which has an identical paragraph.

So I think, this would apply to all generators purchased to US standards, below 75MVA.

=====================================
(2B)+(2B)'  ?

RE: Minus sequence and heat in synchonous motor rotor?

(OP)
Thanks a lot! Exactly what I was asking for!

Pete, can I assume that a negative current sequence roughly corresponds to the negative voltage sequence? Or is there an impedance involved as a conversion factor?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Minus sequence and heat in synchonous motor rotor?

I do know that for induction motors, the negative sequence impedance is on the order of 0.1 to 0.2  of full-load positive sequence impedance and so when operating near full load the negative sequence currrent fraction (I2/I1) tends to be about 5 -10 times as high as the negative sequence voltage fraction (V2/V1).

I'm not sure about synchronous generators.  I'll poke around my references tonight if no-one else responds by then.
 

=====================================
(2B)+(2B)'  ?

RE: Minus sequence and heat in synchonous motor rotor?

By the way, at our plant the large motors and the generators are protected by a negative sequence current relay (46 relay).  We use it as a trip for the motors and as an alarm for the generator.  It might be interesting to investigate whether the generator in question has similar features and whether they have actuated.  

=====================================
(2B)+(2B)'  ?

RE: Minus sequence and heat in synchonous motor rotor?

(OP)
No. There's no negative sequence protection.

Is it a coincidence that the "amplification factor" is approximately the same as the starting current multiplier?

In my mental picture of the negative sequence voltage and current, it is a little like plug breaking a motor. The current then is also five to ten times higher than rated.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Minus sequence and heat in synchonous motor rotor?

You're right, it's not a coincidence that both multipliers are roughly the same for induction motors.

Both situations correspond to the impedance seen when slip is very high:  s=1 for locked rotor and s~2 for negative sequence.    As a rough approximation, the impedance is Z=j*w*L1+j*w*L2 for  both cases.   (neglects the term R2/s which is very small at these large slips, neglects small variations of L2 with frequency due to deep bar effect, magnetizing branch neglected for simplicity but doesn't change between the two cases).
 

=====================================
(2B)+(2B)'  ?

RE: Minus sequence and heat in synchonous motor rotor?

(OP)
Thanks. That clarifies it!

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Minus sequence and heat in synchonous motor rotor?

(OP)
Braking. Of course.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Minus sequence and heat in synchonous motor rotor?

If you're looking for an approximation: for a turbo machine the negative sequence reactance X2 is usually about the same value as the sub-transient reactance X''d, depending on whether an amortisseur winding is fitted. The amortisseur brings the value up a little to about 1.2x the X''d value.

I would guess a salient pole machine will have a similar relationship between X2 and X''d, although the values will be considerably higher than for a turbo machine (roughly double those for a turbo machine).

There's some discussion in the NPAG although it's aimed at big utility machines: http://www.fecime.org/referencias/npag/chap5-46-77.pdf. The table and equation on Page 54 almost certainly won't translate well to small machines. There's a table of typical values for different types of synchronous machines in http://www.ee.uidaho.edu/ee/power/jlaw/COURSES/ECE523/F07/HANDOUTS/SG/s21b.pdf although I can't vouch for its accuracy. Looks reasonable from a quick glance.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Minus sequence and heat in synchonous motor rotor?

(OP)
In this machine, there's no amortisseur winding - only the excitation winding. It is magnetized from a shaft generator with a rotating rectifier. So, the current induced in the winding is added to excitation current (rectified in the rectifier) and that, I think, ought to be seen by the AVR (or cos(phi) controller) so the external excitation control reduces excitation until all is back to normal. In that case, there shouldn,t be a problem?

There is a set of thyristors parallel to the rectifier and those thyristors are there to create a parallel path for current opposite the diode current (so the excitation winding acts as an amortisseur winding during start). The triggering voltage for tose thyristors is usually kept at 700-800 V. So it is not very likely that the negative sequence activates the thyristors. But, if it does, that would mean repeated out-of-sync and restarts. Had that once on a Siemens machine. This one is ABB. But same arrangement with diodes/thyristors.

I have not been given all the details. And I haven't seen the machine. The guy that told me about it has signed an NDA. Don't ask me why - but that is what he says.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Minus sequence and heat in synchonous motor rotor?

Hi Gunnar,

Not sure I follow your opening paragraph. Are you thinking that the NPS current turns up in the field winding? That's true to some extent, but the main problem is that the 2x line frequency current circulates in the rotor forging itself where it causes eddy current heating of the unlaminated rotor core.

Hadn't seen the problem you describe with thyristor false triggering causing a loss of sync. That's pretty serious! Have you any more details on that event?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Minus sequence and heat in synchonous motor rotor?

(OP)
I'm with you on eddy current in the forging. But since you mentioned amortisseur winding, I thought it was good to mention that the excitation is used to start the machine.
I didn't see that thyristor triggering problem myself. But a Siemens guy phoned me during comissioning and said that they had problems with a large heat pump drive. I asked what the triggering point was and it was quite low. He then set it to, I think, 800 V and the problem disappeared. Not sure if it was the induced EMF that caused the triggering, but that was what we agreed on. Then. The need for an explanation isn't always the same thing as the need for absolute truth.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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