Generator reverse power question

[scott88]

Hi,

Please could you help with a theoretical dilemma I am having;

For the purpose of the question, imagine a three phase Ac generator with a rotor that is excited via a static excitation system and the dc supply fed via brushgear and slip rings. If the brushgear is removed, the excitation equipment electrically isolated, all back energisation protection has failed, and the generator was back fed, would it be possible for the generator to act as an induction motor and turn?

My thoughts are not because of the rotor winding construction and that the winding would be seen as an open circuit due to excitation equipment isolated?

Thanks,

 

[waross]

If the prime mover is still driving the generator it may still supply power to the grid working as an induction generator. To supply power as an induction generator the governor will have to drive the generator at above synchronous speed. The generator will draw reactive power from the grid for excitation.
In the absence of a squirrel cage winding rotor current will be induced in the rotor itself. This is not very efficient and the rotor may be damaged by overheating. Again, depending on the governor settings the governor will be driving the set at above or at synchronous speed and so will not develop any torque. Think induction motor overdriven at synchronous speed.
In the event that both the prime mover and the excitation fail, the generator will probably act as an induction mtor and drive the prime mover. The rotor may be damaged by the heating and many types of prime movers do not like to be over driven.
I say may a lot as a generator may not develop nearly as much torque as a similarly sized induction motor. An induction motor depends on the slip frequency to induce the rotor current. Governor action may or may not allow enough speed difference to develop an effective slip frequency.
It depends.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scott88]

Hi,

I think my question is not very clear.

I would like to know if the brush gear is removed so the rotor winding would be open circuit, and the generator circuit breaker was closed, is it possible to motor the prime mover? I am currently thinking not because it would not be possible to self excite the rotor as an induction motor would because the winding is open circuit.

Thanks,

 

[waross]

It is unlikely to start as an induction motor.
Once turning the generator may work as an induction motor, but an induction motor depends on the slip frequency to develop the rotor current and runs at below synchronous speed.
A synchronous motor has a squirrel cage winding and a laminated rotor.
Many generators do not have a squirrel cage winding and the rotors may not be laminated.
If the prime motor fails the rotor current may be induced in the rotor steel and the generator may act as an induction motor and drive the prime mover. The speed will be below synchronous speed. Remember the slip frequency.
Re-read my original post please.
A field failure will not have a direct effect on the governor. The prime mover will continue to drive the generator at or above synchronous speed. The generator will not drive the prime mover unless and until the prime mover speed drops below synchronous speed and develops a slip frequency.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scott88]

Hi,

Thanks for the responses.

So if the prime mover and generator were stationary, the rotor winding open circuit (Excitation isolated, brushes removed), my understanding is that because the rotor winding is open circuit, it would not be possible if the stator was re-energised to turn the prime mover, thus acting as an induction motor?

For the generator to act as an induction motor it would need to be turning and/or have a rotor winding complete circuit. Without a rotor winding circuit, it is not possible for a current to flow? So if it was a squirrel cage rotor with just the horizontal conductor bars with no shorting rings at either end and the iron core, this would be the same instance and from staionary if the stator was energised it would not turn? Is this correct?

Thanks

 

[davidbeach]

Damper bars on the rotor may act as a squirrel cage and allow it to start as an induction motor.

 

[scott88]

Thanks for all of the answers, I appreciate the responses.

If anyone else has a definite answer or knows of any studies into the effects on a generator at standstill with an open circuit rotor and back energising the stator please let me know. I am interested to know if the rotor would move and if so how would this be possible with induced voltage but no current flow or magnetic field.

 

[ScottyUK]

Some discussion here from Beckwith. The fact that most modern generator relays have a dedicated relay function (27/50) for "accidental energisation", "inadvertent energisation" or some similar name must indicate that these events occur with reasonable frequency.

I expect that on a machine without a amortisseur winding on the rotor the main effect will be a massive lagging load (machine consuming VARs) and severe heating of the rotor due to eddy currents at line frequency circulating in the unlaminated forging. The rotor will likely produce some torque, even without an amortisseur winding, simply by virtue of induced current circulating in the rotor forging. This torque might be sufficient to accelerate a steam turbine already on barring gear and with bearing lift oil established, but the fault should be detected and cleared before it starts to have any significant effect.

 

[davidbeach]

must indicate that these events occur with reasonable frequency

either that or it is so devastating when it does happen and so relatively easy to protect against that it's a rather common function.

 

[byrdj]

Please forgive my lack of knowledge of the electrical terms that are being used to discribe the generator. my comments my not be applicable but I have seen a steam turbine generator, prior to comissioning where the generator breaker was accidently closed and the steam turbine was rolled from turning gear to a couple hundred RPMs after a couple minutes.

the damage that occurred to the generator rotor (field) was due to circulating current on the rotor's surface arcing accross the retaining wedges. the repairing generator specilist implied this was fairly common and I think they said it would not have been as severe if the no excitation motoring had occured with the unit near rated speed as the torque/surface current would be less

 

[LionelHutz]

The times I've seen synchronous motors energized with open fields the field has arced over in the slip rings and at that point the field winding is shorted again.

Assuming there is no failure, I would expect some motoring torque to be produced and the generator will turn if this torque is enough to overcome any friction in the system. In other words, it may turn but there isn't enough dat available to answer.

 

[ScottyUK]

Agree with byrdj on the cause of the problems and the likely behaviour of the machine.

Solid forged rotors are very susceptible to damage from induced currents, hence the existence of negative phase sequence relays to protect them. An energise-at-standstill event produces a similar conditions with similar detrimental effects.

 

[squeeky]

Local terms in different countries have different meanings. I'm on today to seek a thread to apologise for a mix up. Not this one. Generators and alternators. Here is South Africa, alternators are often called generators. Hopefully I'm correct in assuming that this is an alternator. Our systems also have loss of excitation protection as well as the two on the breaker described above. Luckily we have never experienced too many failures.

Breaker of alternator failed to open when the plant blacked out due to a Utility failure. This was not picked up. When the power was restored, they closed the in coming breaker. Because the alternator breaker was in, but the system off, and it was connected. The incoming supply tripped on over current. The rotor was damaged and had to be rewound.

A turning and running alternator lost it's excitation. (Prior to loss of excitation protection protection) and the machine started to motor. With a very light load (turbine regulator valve almost closed due to low load ), the machine started to run away. Luckily we we have three sets of over speed protection which tripped the ESV. Emergency shut off valve.

I have been told of machines that destroy them selves due to mechanical forces on over speed due to loss of excitation.

 

[waross]

I have no doubt that machines have been destroyed by overspeed due to loss of excitation. However they were not motoring. They were driven overspeed by the steam turbine.
If a generator motors, it acts as either a synchronous motor or as an induction motor. Neither will exceed synchronous speed.
On loss of excitation, the generator becomes an induction generator. The steam turbine, absent external overspeed protection, may drive it over synchronous speed.
But, an induction motor depends on the currents in the rotor induced by the stator windings. The squirrel cage winding on the rotor of an induction motor provides a low impedance path for the rotor currents. A squirrel cage type damper winding on a synchronous alternator will often provide a suitable path for the rotor currents but not all alternators have a damper winding.
In the absence of a damper winding the induced currents must flow in the surface of the rotor. The impedance is higher than the impedance of a damper winding. As a result the current is less and the counter torque is less by a square factor.
If the torque produced by a slightly open steam valve is greater than the counter torque of the un-excited alternator the alternator will run away.
A DC generator may run away due to motoring upon loss of field.
An AC alternator will never motor at a speed greater than synchronous speed.
The driving torque must exceed the counter torque for an alternator to "run away".

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[milestone625]

It should have some amortisseur or damper winding, and the rotor current will be induced.
Even without excitation, the generator will run as induction motor.
Since the Rotor is not laminated, it will cause serious heating on the surface of rotor .
It would damage the rotor in short time

 

[bartb102]

I know of one hydroelectric generator whose breaker failed to open when an unknown fault occured while shutting down for other service. Fault caused 86G lockout relay trip which turns off exciter & is supposed to trip breaker (energy storage device had failed). As unit did not complete its stop within an allowed time the draft tube gates were closed, removing all motive power from the turbine. Generator continued to rotate at sync speed. Operator finally called asking assistance- we determined that breaker was still closed. He manually tripped the breaker allowing the unit to stop rotating. End result was a $20,000 power bill for energy and demand charge. Luckily no damage to generator, only to operator's nerves.