Hydro-generator field discharge resistor 2

[dpc]

Question for hydro engineers:

I am working with a client who has recently installed new voltage regulators on some old hydro generators. The exciters are rotating dc generators. As part of this retro-fit, they apparently removed old existing field breakers and some field discharge resistors.

They have been experiencing, it appears, high voltages at the generator main terminals after a unit trip (turbine will run up to probably 150 - 175% speed for a while, typical of most hydro units).

Assuming the new voltage regulator is properly shutting down on a generator trip, would there be enough residual flux in the exciter to create dc current sufficient to cause overvoltage in the unloaded ac generator during this overspeed condition?

 

[jbartos]

Suggestions marked ///\\dpc (Electrical) Mar 6, 2004
Question for hydro engineers:
///Please notice that this is Electric Power Engineering section of this Forum.\\I am working with a client who has recently installed new voltage regulators on some old hydro generators.
///This is really a good move.\\ The exciters are rotating dc generators.
///This is quite normal that dc generators are rotating. However, the exciter rectifiers may or may not rotate. Lets assume that they do rotate. There are no brushes.\\ As part of this retro-fit, they apparently removed old existing field breakers and some field discharge resistors.
///That is too bad since the field discharge resistors are usually kept. Generator exciter circuit breaker ANSI Device No. 41 has a normally closed contact that controls the discharge resistor function. See for example Reference:
Robert W. Smeaton "Switchgear and Control Handbook," Second Edition, McGraw-Hill Book Company, 1987,
Figure 6 "Generator and Exciter Control Panel with Diagram on page 1-6.\\They have been experiencing, it appears, high voltages at the generator main terminals after a unit trip (turbine will run up to probably 150 - 175% speed for a while, typical of most hydro units).
///Please, what seems to be a probable cause of the high voltages linked to? Does it developed after the existing old field breaker removal?\\Assuming the new voltage regulator is properly shutting down on a generator trip, would there be enough residual flux in the exciter to create dc current sufficient to cause overvoltage in the unloaded ac generator during this overspeed condition?
///Possibly. AVR is linked to the exciter.\\\

 

[edison123]

Hydro generators are typically operated with field discharge resistors because of their overspeed under full load trip and because field flux does not fall immediately upon field breaker trip. Flux drop is not as fast as speed rise. Without field discharge resistors, the generator + transformer windings could see voltages high enough to damage the windings.

jb,

DC exciters with diodes ! and brushless !!, what are you talking about ?

 

[dpc]

Thanks, edison. Do you have any advice on sizing the resistor? How long does it take for the voltage to decay? These are small units - under 2 MVA.

Is a contactor used to switch in the resistor when the unit trips?

 

[edison123]

Normally, field discharge resistance is equal to the generator main field resistance. This way, the back emf induced in the field winding cannot exceed twice the normal voltage. Yes, the field discharge is switched through a contactor, when the unit trips.

 

[edison123]

While the induced field voltage is a dielectric stress on the rotor winding, the resultant field current is more damaging since it allows the generated voltage (in stator) to persist and feed an winding internal fault.

Typical die down time of this induced field current may vary from a few seconds to even up to a minute for big machines.

 

[ScottyUK]

Hi dpc,

With a shaft-mounted rotating rectifier, a field discharge resistor has no effect. When the static excitation source is tripped, the current in the field continues to freewheel through the shaft mounted diodes until it decays according to the L/R time constant of the rotor.

If the unit was operating at a lagging PF, i.e. exporting VAr's onto the system, it will be over-excited. When the generator breaker opens, more-or-less simultaneously with the field breaker, the field current at that moment will be higher than that required to produce rated terminal voltage. Thus the terminal voltage of the unit will initially rise as you observe, then fall away as the field current decays.





-----------------------------------

Start each new day with a smile.

Get it over with.

 

[dpc]

Scotty,

These particular units are old and have a true dc generator as the exciter.

If this is related to the time constant of the exciter, I'm guessing the overvoltage is fairly brief?

 

[ScottyUK]

Hi dpc,

Yeah, I would suggest the time constant would be pretty short. What duration is the over-voltage condition? Anything longer than a few seconds would surprise me, but I'm used to the behaviour of turbo-alternators rather than slow-speed hydro units so I'll certainly give way to anyone with more specialist knowledge of big salient pole machines who disagrees with the above.

The manufacturer should be able to give you the time constant of the main field winding, and from that you could do a back-of-envelope calculation of how long the field would take to decay from the pre-trip level to a value below that required to produce rated terminal voltage. I think the dominant effect will be the main field TC rather than that of the exciter, which will be rather shorter.





-----------------------------------

Start each new day with a smile.

Get it over with.

 

[jbartos]

Comment on edison123 (Electrical) Mar 6, 2004 marked ///\\jb,
DC exciters with diodes ! and brushless !!, what are you talking about ?
///The dpc posting March 7, 2004:
""These particular units are old and have a true dc generator as the exciter."" clarified the DC exciter.
However, "rotating DC generator" is somewhat ambiguous term since AC generator with rotating rectifiers is also "rotating DC generator."\\