Pole Slip in Generator

[transient]

Guys !!
Happy Birthday , Marry Christmas and Happy New Year..he he he...

I want to discard all the crap from my mind and start afresh , as I am facing an issue with our clients ,on pole sliping of generator .
My understanding on this ,I want to explain with an example .
Although it is generator pole slipping , but i want to explain by transformer example.

Suppose I have two transformers connected to two different generators , now to run these two transformers in parallel , off course the upstream generators need to be in sync , another thing will be , these transformers need to be totally identical , i.e the vector group of the transformers should be the same .

If I take a reference axis X, Y , Z which are 120 deg apart , now if I place R1 , S1 , T1 coils (Transformer A) on the axis X,Y, Z the angle between X & R1 will be zero , the angle between Y and S1 will be zero , the angle between Z and T1 will be zero .

Now if I do the same with another transformer(Transformer B) which has R2 , S2 , T2 coils , then I am finding that X & R2 don't coincide but deviates by an angle of 10 degree , and also the other coils deviates with their respective axis by an angle of 10 degree.

Now when I close the breakers of these transformers on a common bus ( remember upper stream is in sync) then there will be some sparks , and may be oscillations in the transformers and may be the lifespan of the bus may reduce , also the desired voltage waveform will not be obtained from the bus .

Now with this concept lets jump back to POLE SLIPING.
Now the same phenomenon happens with two generators , due to pole slipping effect , the Pole (coil) inside the generator shifts slightly or may be significantly , due to sudden torque on one of the generator etc .

Lets assume that two generators are in sync and then one of the generator have its poles slipped , thus i think the waveforms of the two generators will go out of phase , depending on how much the Pole Slip have displaced the Poles (coils) and thus short circuit (sparking) on common bus bar will result .

Now I want to forgot what is there in my mind and want to listen , your theory of POLE SLIPING ?
and how can we detect it , and is there any safety function inside the SMART RELAYS to prevent pole slipping?
Regards
Transient

 

[gar]

131109-0913 EST

If you have two AC synchronous generators mechanically connected together and under no load are in perfect sync, have exactly the same output voltage, and the waveforms are pure sine waves, then the voltage difference between comparable phases is zero.

Next assume that one generator is 1/2 the size of the other, and thus the internal impedance is higher. There is still no load zero voltage difference between comparable phases.

Next connect the phases together, and apply a balanced resistive load. Both generators will supply output power flow. The distribution of power will be a function of the relative internal impedances. Now there is what you describe as pole slip when viewing the output voltage compared to some theoretical ideal internal source voltage. This phase shift is the same for both generators by the fact that the shafts are locked together, and the output terminals per phase are connected together.

Next provide a mechanical adjustment between the two generator shafts to create a phase shift between the ideal voltage sources. As this adjustment is changed there is some point where the smaller generator will have negative power flow and it becomes a motor supplying mechanical power input to the larger generator. There are internal power losses so this is not perpetual motion.

This is to stimulate your thought, and possibly others will get into the discussion.

.

 

[AdrianWint]

My understanding of pole slipping is different.

Pole slipping occurs inside the machine when the torque applied via the shaft is of sufficient magnitude to be able to overcome the magnetic forces which couple together the two rotating magnetic fields which exist inside the alternator.

When a generator is operating in sync with another source there will be an angle between the two magnetic fields - one on the rotor created by the exciter & one in the stator created by the external system. This is known as the load angle. As torque is applied to the shaft (and hence power is generated) these to fields tend to move apart by a few degrees. As the torque is increased the load angle will also increase. As it approaches 90 degrees there is a danger of pole slipping. When pole slipping occurs large currents flow from the external system into the machine in an attempt to re-establish the magnetic lock. The phases don't change nor does the sequence of them nor the angle between them.

Pole slipping causes great mechanical stress to the prime mover & electrical stress to the system - for these reasons it should be avoided. For each & every machine there is an 'operating chart' which gives the locus of acceptable real & reactive powers that the machine can produce & still be stable. Operating outside this locus may lead to pole slipping.

In the 'real' world pole slipping often occurs because the prime mover is grossly oversided with respect to the kW capabilities of the alternator.

 

[George_C]

Would you recommend any protective relay which would avoid pole slipping?

 

[Sahib]

Over speed relay?

 

[George_C]

Actually, I mean pole slip as mentioned above. It should prevent from pole slipping in a generator if the load was too big and the generator would not be able to deliver enough power.

 

[rcwilson]

Pole slipping is also called Out of Step. Most modern generator relays protect against Out of Step (ANSI Protection Function # 78) by monitoring the generator voltages and currents with an impedance function that trips when the apparent impedance (Z = V/I in vector format) as viewed from the generator terminals, has swung inside the generator.

The Schweitzer SEL 700G, GE Multilin G60, Basler BE1-11g and many other generator protection relays have function 78 that will trip on pole slip.

Pole slip is caused when the generator excitation reduces the magnetic field in the generator air gap below what is needed to maintain the torque the load is demanding. It is like draining the fluid out of a fluid coupling or an automatic transmission. At some point there is not enough electrical "fluid" (VARs) to keep from slipping gears. Since pole slip is related to under excitation and excitation controls VAR output, by monitoring the excitation or the VARs we can trip before, or as, pole slip occurs.

Under excitation protection (ANSI Protection Function 40) will protect against most out-of step events. Use the single function Basler BE1-40Q relay. All the relays above also have the 40 function.

The same protection is needed on a synchronous motor. Sometimes it is applied just as a negative VAR function. (Loss of excitation causes VARs to flow into the machine).

On utility generators, pole slip usually occurs during a disturbance as the generators in the power system are trying to stabilize the system after a portion has faulted and tripped. The rotor angles are swinging back and forth and might go beyond the 90 degree stable point.

 

[George_C]

Would it be also possible to use such a relay that would mesaure load angle of the generator? If I understand it correctly I could evaluate the load angle and imediately, when the load angle is bigger then 90? or some other predefined value the relay would trip.

 

[AdrianWint]

Have a look at some of the numerical protection relays. The MiCOM P343 from ALSTOM is specifically designed for generator protection & incorporates (amongst other elements) Pole slip protection.

(I have no commercial relationship to ALSTOM other than being a user of their kit)

Adrian

 

[Besoeker]

Have a look at some of the numerical protection relays. The MiCOM P343 from ALSTOM is specifically designed for generator protection & incorporates (amongst other elements) Pole slip protection.

(I have no commercial relationship to ALSTOM other than being a user of their kit)

Adrian

I thought they were now Converteam?
I dealt with Kidsgrove and Rugby when they were GEC, Cegelec, Alsthom, Alstom, and Converteam.

 

[AdrianWint]

I thought they were now Converteam?
I dealt with Kidsgrove and Rugby when they were GEC, Cegelec, Alsthom, Alstom, and Converteam.

Not quite. When the former AREVA business was carved up the MV & EHV side of the distribution business went (back) to ALSTOM. The Company is now known as ALSTOM T&D and is based is Stafford. I was there only last week so its definitely trading as ALSTOM (at least it is in the UK).

Adrian

 

[Besoeker]

Not quite. When the former AREVA business was carved up the MV & EHV side of the distribution business went (back) to ALSTOM. The Company is now known as ALSTOM T&D and is based is Stafford. I was there only last week so its definitely trading as ALSTOM (at least it is in the UK).

Adrian

Appreciate the information, thanks.
:thumbsup:

 

[aitzoloiarbide]

Slipping of the pole at 90 degrees?

Slipping of the pole at 90 degrees?

Hello to all;

I am analyzing the loss of a generator pole for my project. In theory the slipping of the pole is given when the load angle exceeds 90 degrees. However, I have an article that mentioned that the critical angle can vary depending on the active and reactive power. I wanted to know your opinion on this topic.

Thanks to all